Solid oxide electrolysers and assemblies

Abstract

A multi-layered structure for an electrolyser comprising: an electrolyte 304.2; a permeable electrode 304.4; the permeable electrode and electrolyte 304.2 having a common interface to form a reaction

Description

BACKGROUND

[0001] Given climate change, the world is looking for alternative means of delivering stored energy to power cars, aeroplanes, etc in the form of e-fuels that enable the practical movement of large amounts of renewable energy.

[0002] An electrolyser is an electrochemical device for efficient conversion of renewable power to transportable e-fuels such as hydrogen, methanol, ammonia and e-SAF. Solid oxide electrolyser cells (SOECs) are a core enabling technology for future sustainable energy systems, in particular, so-called green energy systems.

[0003] Electrolysers to date have been excessively large, making collocation with renewable resources very challenging. Additionally, low temperature electrolysers have lacked efficiency or needed expensive Iridium based catalysts while high temperature electrolysers have lacked durability as a result of operating at high current densities aimed at reducing cost while balancing their endothermic operation with internal heat production.

[0004] SOECs have yet to be realised that provide commercially attractive alternatives to the above. BRIEF INTRODUCTION OF THE DRAWINGS

[0005] Example electrolysers and systems will be described with reference to the accompanying drawings in which

[0006] Figure 1 shows a hierarchy of components of electrolysers and assemblies;

[0007] Figure 2 illustrates a legend relating to the entities shown in figure 1;

[0008] Figures 3A to 3D depict a number of symmetric laminated sheets;

[0009] Figure 4 illustrates a symmetric laminated sheet of figure 3;

[0010] Figure 5 shows an anode cell, a cathode cell and a pair of chemical heating cells;

[0011] Figure 6 depicts a view of an expanded stack in different expansion states;

[0012] Figure 7 illustrates a view of further stages in fabricating a stack;

[0013] Figure 8 depicts a view of a further stage of manufacturing a stack;

[0014] Figure 9 illustrates a view of a stack;

[0015] Figure 10 shows a view of a stack assembly;

[0016] Figures 14A and 14B illustrate arrays of stack assemblies in plan view;

[0017] Figure 23 shows a graph of variation of power density with cell size;

[0018] Figure 24 depicts a graph of variation of thermal Biot number with cell size;

[0019] Figure 25 illustrates a graph of variation of thermal Peclet number for flow through a permeable electrode with cell size;

[0020] Figure 26A shows a graph of variation of single cell thickness with cell size for a predetermined pressure drop;

[0021] Figure 26B shows the graph of variation of single cell thickness with cell size for a predetermined pressure drop of Figure 26A together with graph values;

[0022] Figure 27A depicts a graph of variation of the ratio of convective velocity to mass transfer coefficient with cell size; and

[0023] Figure 27B depicts the graph of variation of the ratio of convective velocity to mass transfer coefficient with cell size of Figure 27A together with graph values. DETAILED DESCRIPTION

[0024] Referring to Figure 1, there is shown a view 100 of a hierarchy for electrolysers and systems according to examples. The hierarchy comprises a number of solid oxide sheets 102. The sheets 102 relate to a cathode half-cell 104, an anode half-cell 106, a dense insulator 108 and a chemical heating half-cell 110. The cathode half-cell 104 comprises a cathode electrolyte 104.1, a permeable cathode electrode 104.2 and a permeable cathode current collector 104.4. The anode half-cell 106 comprises an anode electrolyte 106.1, a permeable anode electrode 106.2 and an anode current collector 106.4. The dense insulator 108 comprises a dense insulating material such as Alumina or Magnesium Aluminate (spinel), Magnesia and Magnesia with Magnesium Aluminate Spinel (MMA), or Aluminium Nitride. The chemical heating half-cell 110 comprises a chemical heating half-cell permeable conductor 110.1 and a permeable heating cell catalyst 110.2.

[0025] Each of the above are examples of multi-layered structures for an electrolyser. Each of the multi-layered structures can comprise substantially planar layers. Although the examples described herein use layers in the form of sheets, that is, substantially planar material, examples are not limited thereto. Examples can be realised in which material shaped other than substantially planar such as, for example, planar material that is curved in at least one dimension and, optionally, in two dimensions to present an arcuate surface or a curved surface. Furthermore, the material can be shaped as strips.

[0026] Pairs of cathode half-cells 104 can be combined to produce a cathode cell, which will be described with reference to Figure 2 below.

[0027] Similarly, pairs of anode half-cells 106 can be combined to produce an anode cell, which will also be described below with reference to Figure 2.

[0028] Also, pairs of chemical heating half-cells 110 can be combined to produce a chemical heating cell, which will be described below with reference to Figure 2.

[0029] An electrolyser comprises a set of cathode cells comprising at least one cathode cell and a set of anode cells comprising at least one anode cell. Examples can be realised in which an electrolyser stack comprises a set of cathode cells comprising at least one cathode cell, a set of anode cells comprising at least one anode cell, and a set of chemical heating cells comprising at least one chemical heating cell. The combined electrolyser stacks perform overall electrolysis reactions 2H2O -» 2H2 + O2 and / or 2CO2 -» 2CO + O2 when supplied with electrical work. Additionally, where CO is present with H2O or CO2 is present with H2 or at least any three of the four CO, CO2, H2O and H2 are present the water gas shift reaction CO + H2O -» CO2 + H2 will typically be active in the forward or reverse direction according to the relative concentrations of the species and the temperature as many different surfaces are capable of catalysing this reaction at the operating temperature of SOECs such as, for example, between 500°C to 900°C, preferably, between 600°C and 830°C. Typically, this enables the production of CO from CO2 through reverse water gas shift in the cathode even when the kinetics of the cathode electrochemical reaction CO2 + 2 e" CO + O2" are relatively poor. Operation of the electrolyser much below 800 °C may be unattractive if CO2 or CO are to be present on the cathode as this will increase the chances of creating carbon depositing conditions within the cathode.

[0030] Figure 1 also shows, within the hierarchy, a stack 112. The stack 112 comprises a set of cathode cells, a set of anode cells, a set of dense insulator layers 108 and a set of chemical heating cells. Each of the sets can comprise at least one layer or cell within their respective sets of a respective cell type. Examples can be realised in which the set of cathode cells comprises a number of cathode cells, the set of anode cells comprises a number of anode cells, the set of dense insulator layers comprises a number of dense insulator layers, and the set of chemical heating cells comprises a number of chemical heating cells. In the example shown in figure 1, the set of chemical heating cells comprises at least one, preferably two, chemical heating cells and the set of dense insulator layers comprises at least one, preferably two, dense insulator layers according to how many chemical heating cells are within the set of chemical heating cells.

[0031] Herein the terms “convection” and other terms derived from that stem such as, for example, “convectively”, refer to, or comprise the bulk motion of a fluid mixture at a respective mean mixture velocity. This can be contrasted with diffusion which describes the difference between the velocity of a respective gas species and the mixture velocity such as, the respective mean mixture velocity.

[0032] Convective heat transfer refers to heat transfer by forced convection which can be, or is, defined as transport of heat from one point to another in a fluid as a result of macroscopic motions of the fluid resulting from an applied external force such as, for example, a fan, pump, mixer, or the like. Furthermore, as used herein, the term “pathway” refers to a gas species pathway within a porous material. Still further, the terms “dense layer” and “impermeable dense layer” are used synonymously to mean impermeable to the gas species used or encountered such as, for example, at least one, or more than one, of the following taken jointly and severally in any and all permutations: air, hydrogen, carbon monoxide, carbon dioxide, steam and possibly other unreacted fuel precursor species or reaction products. The term “dense insulator layer” refers to such a “dense layer” or “impermeable dense layer” that is also an electrical insulator.

[0033] Therefore, examples of a multi-layered structure for an electrolyser can be realised. The multi-layered structure can comprise:

[0034] an electrolyte; such as, for example, a planar electrolyte;

[0035] a permeable electrode; such as, for example, a planar permeable electrode;

[0036] an inert permeable barrier;

[0037] the electrode and electrolyte having a common interface to form a reaction region; the common interface can be a planar common interface;

[0038] the permeable electrode providing a permeable electrode reactant pathway through the electrode to present a reactant at the common interface; the permeable electrode reactant pathway being fed by a permeable reactant pathway, and

[0039] the inert permeable barrier comprising an inert permeable barrier reactant pathway to at least one, or both, of:

[0040] provide an effective depth greater than a physical depth of the inert permeable barrier to at least reduce, preferably, eliminate, diffusion of any gas species within the inert permeable barrier reactant pathway to form a convectively dominant reactant flow regime within the inert permeable barrier, or

[0041] host a convectively dominant reactant flow regime within the inert permeable barrier;

[0042] wherein the inert permeable barrier reactant pathway is coupled to the permeable reactant pathway.

[0043] At least one, or both, of: the permeable cathode electrode and the permeable anode electrode are examples of such a permeable electrode. At least one, or both, of: the cathode electrolyte 104.1 and anode electrolyte 106.1 are examples of such an electrolyte.

[0044] The example stack 112 depicted in Figure 1 comprises four anode cells and three cathode cells. The cathode cells are disposed between respective anode cells. Examples can be realised in which the cathode cells are interdigitated with the anode cells.

[0045] Examples can be realised in which the stack 112 also comprises a set of two dense insulator layers 108 having the sets of cathode cells and anode cells positioned in between the dense insulator layers 108 and a pair of chemical heating cells 110 positioned adjacent to respective dense insulator layers.

[0046] The stack 112 comprises a set of barrier layers 112.1. In the example depicted in Figure 1, the stack comprises at least a barrier layer over respective inlets 113 to the cathode cells. Each barrier layer in the set of barrier layers 112.1 is formed from an inert permeable oxide or material. An “inert permeable barrier layer” and an “inert permeable barrier” are each examples of a barrier layer or of a barrier. The terms barrier and barrier layer are used synonymously. It will be appreciated that the inert permeable barriers are perpendicular relative to the other layers.

[0047] Each cathode cell has a cathode cell inlet 113 and a cathode cell outlet (not shown in figure 1). The cathode current collectors of each cathode cell are electrically coupled to one another via a cell to cell cathode current collector 114. The cathode current collectors of each cell can be arranged to provide the permeable cathode reactant pathways.

[0048] Each anode cell has a respective anode cell inlet 115 and a respective anode cell outlet (not shown in figure 1). The anode current collectors of each anode cell are coupled to a cell to cell current collector 116. The anode current collectors of each cell can be arranged to provide the permeable anode reactant pathways.

[0049] Examples can be realised in which the inert permeable barrier layer 112.1 also extends to cover one or more sides of any chemical heating cells within the set of chemical heating cells, which can follow from facilitating ease of assembly or fabrication.

[0050] Examples can be realised in which the stack additionally comprises a set of at least one additional barrier layer. In the example depicted in figure 8, the set of at least one additional barrier layer comprises a plurality of additional barrier layers. Examples can be realised in which the set of additional barrier layers comprises one or more than one, taken jointly and severally in any and all permutations, of:

[0051] - an upper barrier layer 804 associated with at least one, or both, of the upper chemical heating cell and the upper most anode cell within the core of the stack,

[0052] - a barrier layer 806.1 disposed adjacent to the lower most chemical heating cell inlet and outlet, and

[0053] - a barrier layer 806.2 disposed on the opposite side of the chemical heating cell relative to barrier layer 806.1. The set of at least one additional barrier layer is arranged to at least reduce, or prevent, creepage electrical discharge.

[0054] It can be seen that each chemical heating cell has an inlet 120. Each chemical heating cell also has a respective outlet 122.

[0055] Next in the hierarchy 100 shown in Figure 1 is a stack assembly 118. The stack assembly comprises a set of stacks 112. In the example shown, the set of stacks comprises four instances 124.1 to 124.4 of the above-described stack 112. The stacks 124.1 to 124.4 are positioned adjacent to one another and oriented so that respective chemical heating cell inlets 120 are fed by a centrally disposed chemical heating channel 126 with a chemical heating fluid.

[0056] Disposed at each outlet 122 of each chemical heating cell of each stack 124.1 to 124.4 is a respective chemical heating outlet channel. In the example depicted, two chemical heating outlet channels 128.1 and 128.2 are provided. The chemical heating outlet channels 128.1 to 128.2 are used to carry at least one, or both, of: excess chemical heating fluid or reacted chemical heating mixture associated with or otherwise derived from the chemical heating fluid. Examples can be realised in which the chemical heating fluid can comprise at least one, or both, of: carbon monoxide and carbon dioxide in a mixture with hydrogen and steam, and in which the heating is a methanation reaction. Although the examples depicted in figure 1, and in figures 10 and / or 14 below, show a particular arrangement of chemical heating supply flow channels and chemical heating outlet channels, examples are not limited to such an arrangement. Examples can be realised in which some other arrangement of chemical heating supply flow channels and chemical heating outlet channels is realised. For instance, examples can be realised in which the roles of the chemical heating supply flow channels and the chemical heating outlet channels are reversed such that the centrally disposed channel 126 / 1012 forms a chemical heating outlet channel and the outwardly disposed channels 128.1 / 1014 to 128.2 / 1016 form chemical heating supply flow channels. Still further examples can be realised in which the chemical heating supply flow and chemical heating outlet channels are disposed diagonally opposite one another at respective corners or edges of a chemical heating cell. Other chemical heating channel arrangements will be described with reference to figure 14 below. The chemical heating supply flow channel 126 / 1012 and chemical heating outlet channels 128.1 / 1014 to 128.2 / 1016 are permeable to supply the chemical heating mixture to, and to allow the chemical heating mixture to egress from, the respective channels. Therefore, other than in regions supporting such supply and egress, the chemical heating supply flow channel and chemical heating outlet channels are sealed, that is, are rendered impermeable to the chemical heating ingress or egress, as well as being impermeable to other gas flows.

[0057] The notation used for flows depicted herein is a name followed by a plus or minus sign. The name indicates the type of flow and the plus or minus sign indicates whether the flow is supply or outlet respectively. For example, ‘Heating+’ indicates a supply chemical heating flow, ‘Heating-’ indicates an outlet chemical heating flow, fuel+ indicates a fuel formation side supply flow, fuel- indicates a fuel formation side outlet flow, air+ indicates an oxygen evolution side supply flow and air- indicates an oxygen evolution side outlet flow.

[0058] Still referring to the stack assembly, a set of pairs of adjacent stacks share a fuel supply channel. As indicated, adjacent stacks share common fuel supply channels. In the example stack assembly 118 shown in Figure 1, a first set of adjacent stacks 124.1 and 124.2 share a common fuel supply channel 130.1. A second set of adjacent stacks 124.3 and 124.4 also share a common fuel supply channel 130.2.

[0059] Additionally, adjacent sets of stacks share an air or sweep gas supply channel. In the example stack assembly 118 shown in Figure 1, a first set of stacks 124.1 and 124.4 share a common air or sweep gas supply channel 132.1. A second set of stacks comprising stacks 124.2 and 124.3 also share a common sweep gas supply channel 132.2.

[0060] Also visible in the stack assembly 118 of Figure 1 is a set of respective fuel outlet channels 134.1 to 134.4 and a set of air or oxygen evolution side outlet channels 136.1 and 136.2. Each air or oxygen evolution side outlet channel accommodates a set of air outlet flows. In the example depicted, a first air or oxygen evolution side outlet channel 136.1 carries two air or sweep gas outlet flows and a second air or sweep gas outlet channel 136.2 carries two air or sweep gas outlet flows.

[0061] The fuel outlet flow is carried by respective fuel outlet channels 134.1 to 134.4 that are defined by a set of fuel impermeable barriers. In the example depicted, two sets of such fuel impermeable barriers are indicated. A first set of fuel impermeable barriers comprises a substantially planar fuel impermeable barrier 139.1 and a pair of fuel / reactant and sweep gas impermeable end barriers 139.11. A second set of fuel impermeable barriers comprises a substantially planar fuel impermeable barrier 139.2 and a pair of fuel / reactant and sweep gas impermeable end barriers 139.21.

[0062] The stack 112 also comprises a set of one or more than one interconnect 137. An interconnect 137 is used to electrically couple parts of a stack, in particular, an interconnect is used to facilitate an electrical connection between a stack and respective chemical heating cells such as an electrical connection between a chemical heating cell and a cathode at one end of the stack, such as the bottom of the stack, and an anode and a chemical heating cell at the other end of the stack such as at the top of the stack. The chemical heating cells form the electrical connections between adjacent stacks, via respective interconnects, such that, for example, longitudinally adjacent cells are connected together electrically in series. It will be appreciated that current flows from the anodes of one stack, via the chemical heating cells, into the cathodes of an adjacent stack. The stacks can be disposed at least one, or both, of: longitudinally adjacent to one another or transversely adjacent to one another. Although examples have been described in which longitudinally adjacent cells are connected together electrically in series, other electrical arrangements can be realised according to performance requirements. Examples can be realised in which the adjacent cells are connected at least one, or both, of: electrically in series and electrically in parallel. Connecting adjacent cells electrically in series and / or in parallel will influence at least one, or both, of: voltage input and current supply.

[0063] Continuing to refer to Figure 1, there is shown an array 138 of stack assemblies 118. The array of stack assembles comprises a set of stack assembles 118 arranged in a column 140. Each column 140 of stack assembles can comprise at least one, or more than one, stack assembly 118. The array of stack assembles can comprise a set 142 of columns 140 of stack assembles 118. Examples can be realised in which each column of the set 142 of columns 140 of stack assembles 118 is offset relative to an adjacent column of stack assembles 118, or in which stack assemblies 118 in columns and rows are aligned with one another, as depicted in Figure 1 and Figure 14.

[0064] It can be appreciated that adjacent stack assembles within the array 138 of stack assemblies 118 share common air or sweep gas outlet channels. In contrast, in the example depicted in Figure 1, adjacent stack assembles within the same column 140 of stack assemblies do not share reactant fuel outlet channels with one another.

[0065] Figure 1 also depicts a set or block 144 of arrays 138 of stack assemblies 118. The set or block 144 of stack assemblies 118 can comprise at least one, or more than one, array 138 of stack assemblies 118. In the example shown, the set or block of stack assemblies 144 comprises five arrays of stack assemblies 118; namely arrays 146.1 146.2 146.3 146.4 146.5.

[0066] Referring to figure 2, there is shown a view 200 of a legend relating to the entities described above with reference to, and as shown in, figure 1.

[0067] The stack 112 of figure 1 and the sheets are colour coded. The stack is made from a number of green state sheets 202 and a set of inks 204. The set of green state sheets 202 comprises a cathode sheet 206, a cathode current collector sheet 208, an anode sheet 210, an anode current collector sheet 212, an electrolyte sheet 214, a catalyst sheet 216, a permeable conductor sheet 218, and a dense insulator sheet 220. The set of inks 204 comprises a cathode current collector ink 222, an anode current collector ink 224, an electrolyte ink 226, an interconnect ink 228, and an inert permeable barrier ink 230.

[0068] The cathode sheet 206, cathode current collector sheet 208 and electrolyte sheet 214 are used to form the above-described cathode half-cell 104. Therefore, the cathode green state sheet 206 can be used to form the above-described cathode electrode 104.2, the cathode current collector green state sheet 208 can be used to form the above-described current collector 104.4 and the electrolyte green state sheet 214 can be used to form the above-described electrolyte 104.1 of the cathode half-cell 104.

[0069] The electrolyte green state sheet 214 can be used to form the above-described electrolyte 106.1 of the anode half-cell 106, the anode electrode green state sheet 210 can be used to form the above-described anode electrode 106.2, and the anode current collector green state sheet 212 can be used to form the above-described anode current collector 106.4. Examples can be realised in which the anode electrolyte layer is optional.

[0070] The above-described chemical heating half-cell 110 can be formed from the catalyst green state sheet 216 and a permeable conductor green state sheet 218.

[0071] The dense insulator layer 108 can be formed from the dense insulator green state sheet 220.

[0072] The cathode current collector 114 of the stack can be formed from the cathode current collector ink 222.

[0073] The anode current collector 116 of the above-described stack can be formed from the anode current collector ink 224.

[0074] The electrolyte ink 226 can be used to form seals to prevent gas egress in, from, or through, a material. However, alternative examples can be realised using any gas impermeable barrier to constrain gas flows such as, for example, at least one, or more than one, of: reactants, oxygen evolution side flows, dilutants, chemical heating fluids, taken jointly and severally in any and all permutations. Using the electrolyte ink 226 as such a sealant has the benefit of there being a good or acceptable coefficient of thermal expansion (CTE) match with the materials used to fabricate the layers and stacks according to the examples. Alternatives to the electrolyte ink 226 could be an electrolyte that is doped to become less conducting or non-conducting, or a glass ceramic, or other ceramics that tolerate the operating temperatures of the examples.

[0075] The above-described interconnect 137 can be formed using the interconnect ink 228.

[0076] The above-described permeable barrier 112.1 can be formed using the inert permeable barrier ink 230.

[0077] Referring to figure 3A, there is shown a view 300A of a number of symmetric laminated sheets. A cathode lamination 302 is used to form a cathode cell from two cathode half-cells. The two cathode half-cells can be formed using the above-described cathode half-cell 104. The cathode lamination 302 comprises a pair of electrolyte layers 302.2, a pair of cathode layers 302.4 and a cathode current collector layer 302.6. Although relatively thin layers have been used to fabricate the example half-cathode cells and half-anode cells, examples are not limited to such arrangements. Examples can be realised in which relatively thicker layers are used to realise the layers of the multi-layered structures. Any such relatively thicker layers can be formed using multiple relatively thin layers to provide greater control over manufacturing processes to avoid problems such as, for example, problems during drying, like cracking. Examples of relatively thin layers would comprise layers having a thickness of 100 microns and below.

[0078] An anode lamination 304 is used to form an anode cell from two anode half-cells. The two anode half-cells can be formed using the above-described anode half-cell 106. The anode cell lamination 304 comprises a pair of electrolyte layers 304.2, a pair of anode layers 304.4, and an anode current collect layer 304.6.

[0079] A chemical heating lamination 306 is used to form a chemical heating cell from two chemical heating half-cells. The two chemical heating half-cells can be formed using the abovedescribed half-cell 110. An example chemical heating lamination or cell 306 comprises a pair of permeable conductor layers 306.2 and a catalyst layer 306.4 disposed between the permeable conductor layers 306.2. Examples can be realised in which the conductor layers can be solid rather than permeable.

[0080] The chemical heating cell 306 comprises, or provides, at least one permeable (porous, tortuous) chemical heating catalyst layer (comprising a chemical heating catalyst to provide a chemical heating catalyst layer pathway for a chemical heating flow in a chemical heating flow direction) that is defined by T2 rf2

[0081] ‘ch fch^ch

[0082] which relates the geometric parameters of the chemical heating catalyst and the abovementioned pathway to the operational parameters of the chemical heating cell, and

[0083] a chemical heating cell inert permeable chemical heating barrier to host a chemical heating cell inert permeable barrier chemical heating pathway, which will be referred to, and is an example, of a “chemical heating cell inert permeable barrier pathway” herein, over at least one, or both, of: a chemical heating catalyst layer inlet or outlet; each being defined by

[0084] dchbdchTchb tch£chb

[0085] which relates the geometric parameters of the chemical heating cell inert permeable barrier and its associated pathway to the operational parameters of the chemical heating cell and barriers,

[0086] where

[0087] zch is the permeable chemical heating catalyst layer pathway tortuosity in the chemical heating flow direction,

[0088] zchb is the chemical heating cell inert permeable barrier pathway tortuosity in the chemical heating flow direction,

[0089] sch is the permeable chemical heating catalyst layer porosity,

[0090] schb is the chemical heating cell inert permeable barrier porosity,

[0091] dchb is the chemical heating cell inert permeable barrier depth in the chemical heating flow direction,

[0092] dch is the permeable chemical heating catalyst layer depth in the chemical heating flow direction,

[0093] tch is the permeable chemical heating catalyst layer thickness,

[0094] fch is the ratio of permeable chemical heating catalyst layer pathway pore size to permeable chemical heating catalyst layer thickness, tch.

[0095] Examples of the relationship between the geometric parameters of both of the permeable chemical heating catalyst and the abovementioned pathway and the operational parameters of the chemical heating cell are given in Table 3.

[0096] Examples of relationship between the geometric parameters of the chemical heating cell inert permeable barrier and its associated pathway to the operational parameters of the chemical heating cell and barriers are given in Table 3.

[0097] Examples can be realised in which the chemical heating cell also comprises at least one, or more than one, of the following characteristics, taken jointly and severally in any and all permutations:

[0098] - the permeable chemical heating catalyst layer thickness, tch, is arranged to provide a balance between reaction time, contact via a catalyst or reaction contact surface, in-plane sheet electrical conductance and in-plane thermal conduction, which follows from increasing the permeable chemical heating catalyst layer 306.4 thickness, with ensuring a compact design providing increased power density, which follows from decreasing permeable chemical heating catalyst layer 306.4 thickness;

[0099] - the permeable chemical heating catalyst layer thickness, tch, has a range of 5 / im to 1000 urn, optionally 5 / im to 700 / im, and, preferably, 10 / im to 300 / mw, t2

[00100] - the ratio —influences the balance between an additional, preferably, smallest, £chfch flow resistance provided by the permeable chemical heating catalyst layer material while providing structural strength and in-plane sheet electrical conductance and in-plane thermal conduction; t2

[00101] - the ratio —has a range of 1 to 500, optionally, 2 to 50; £chfch

[00102] - the permeable chemical heating catalyst layer depth, dch, is selected according to manufacturing capabilities to balance cell handling capabilities, since below a given lower depth limit handling cells would be difficult, and power density, which influences an upper limit of cell thickness since large cell depths would require cell thickness to increase dramatically to provide in-plane thermal conduction as well as in-plane sheet electrical conductance and manage pressure drops, which would adversely lead to a lower power density overall;

[00103] - the permeable chemical heating catalyst layer depth, dch, has a range of 0.25 mm to 40 mm, optionally, 0.25 mm to 20 mm and, preferably, of 5 mm to 20 mm;

[00104] - the ratio dchbTchb js associated with a range of effective depths in the flow direction £chb of the chemical heating cell inert permeable barrier pathway to influence, that is, limit, at least reduce, or eliminate, diffusion effects, as described in paragraphs

[00197] to

[00239] ;

[00105] - the ratio dchbTchb has a range of 1 to 1000 mm, optionally, 5 to 200 mm and, £chb preferably 5 to 100 mm, and more preferably less than 100 mm;

[00106] - the ratio, fch, of permeable chemical heating catalyst layer pathway pore size to permeable chemical heating catalyst layer thickness, tch, is selected to balance flow resistance with providing structural strength and in-plane sheet electrical conductance to influence or constrain electrical resistive losses;

[00107] - the ratio, fch, of permeable chemical heating catalyst layer pathway pore size to permeable chemical heating catalyst layer thickness, tch, has a range of 0.02 to 1, and, preferably, 0.25 to 0.75;

[00108] - the permeable chemical heating catalyst layer pathway tortuosity, zch, is selected to balance flow resistance and catalyst or reaction contact surface against structural strength and in-plane sheet electrical conductance of at least the chemical heating catalyst;

[00109] - the permeable chemical heating catalyst layer pathway tortuosity, zch, has a range of 1 to 3, optionally, 1 to 2.5, and, preferably, 1 to 2;

[00110] - the chemical heating cell inert permeable barrier pathway tortuosity, zchb, is arranged to increase the effective depth of the inert permeable barrier layer to at least reduce, and, preferably, eliminate, diffusion of any gas species present in the inert permeable barrier;

[00111] - the chemical heating cell inert permeable barrier pathway tortuosity, zchb, has a range of 1 to 10, preferably, 1 to 5;

[00112] -the permeable chemical heating catalyst layer porosity, ech, is selected to balance flow resistance against structural strength and in-plane sheet electrical conductance of the chemical heating catalyst;

[00113] - the permeable chemical heating catalyst layer porosity, ech, has a range of 0.1 to 0.9, and, preferably, 0.5 to 0.8;

[00114] - the chemical heating cell inert permeable barrier porosity, echb, has a lower bound to accommodate examples that use an orifice plate or foraminate plate and an upper bound to accommodate higher tortuosity materials^

[00115] - the chemical heating cell inert permeable barrier porosity, echb, has a range of 0.01 to 0.5, and, preferably, 0.05 to 0.5;

[00116] - the chemical heating cell inert permeable barrier depth, dchb, can be set to achieve a balance, at a lower limit, between an increase in effective depth to limit diffusion, as described with reference to paragraphs

[00197] to

[00239] , and so cannot be too thin, with, at an upper limit, constraining the size of the inert permeable barrier such that it has a volume smaller than an associated electrolyser, which influences overall power density;

[00117] - the chemical heating cell inert permeable barrier depth, dchb, is greater than or equal to 0.01 mm, optionally 0.01 mm to 5mm and, preferably, 0.1 mm to 2 mm; r2 d2

[00118] - the ratio ch 2ch3 , which relates the geometric parameters of the permeable £ch fc^ch chemical heating catalyst layer and its associated pathway to the operational parameters of the chemical heating cell, has a range of 1e6 / m to 5e12 / m, optionally, 1e8 / m to 5e11 / m and, preferably, 1e9 / m to 1e11 / m;

[00119] - the ratio dchbdchTcbb, relates the geometric parameters of the chemical tch£chb heating cell inert permeable barrier and its associated pathway to the operational parameters of the chemical heating cell and barriers, has a range of 0.1 m to 250 m, optionally, 0.5 m to 50 m and, preferably, 1 m to 25 m.

[00120] In the examples described herein, the chemical heating cell inert permeable barrier can be realised using, for example, a permeable wall of at least one, or both, of: a chemical heating fluid supply channel and a chemical heating outlet channel, as described with reference to figure 10.

[00121] A dense insulator layer 308 is formed using a dense insulator material. The dense insulator material can be, for example, at least one: Alumina or Magnesium Aluminate (Spinel), Magnesia and Magnesia with Magnesium Aluminate Spinel (MMA), or Aluminium Nitride.

[00122] Referring to figure 3B. 1, there is shown a view 300B of a cathode cell 302 together with a barrier according to an example. The cathode cell is shown as comprising a reactant flow 302.8. The reactant flow 302.8 is fed to the inert permeable barrier 302.10. The inert permeable barrier 302.10 provides an inert permeable barrier reactant pathway 302.12. The inert permeable barrier reactant pathway 302.12 is arranged to at least reduce, and, preferably, eliminate, diffusion effects of any gas species within the inert permeable barrier before the interface 302.13 between the end of the inert permeable barrier 302.12 and cathode cell 302. The inert permeable barrier reactant pathway 302.12 is arranged to feed a permeable reactant pathway 302.14 via the permeable reactant pathway inlet 302.16. The permeable reactant pathway 302.14 is carried by a suitably permeable structure 302.6 as described herein. Examples can be realised in which the suitably permeable structure 302.6 is a current collector, such as, for example, a cathode current collector. Examples can be realised, however, in which the suitably permeable structure 302.6 is made from the same material as the cathode electrode and, therefore, effectively eliminated or replaced. Still further examples can be realised in which the suitably permeable structure is a heterogeneous structure comprising multiple layers or entities, as opposed to being a single homogeneous structure. Yet further examples can be realised in which the suitably permeable structure 302.6 is not primarily intended to conduct current and, therefore, is non-conductive or is minimally conductive with the primary function being to support gas flow. However, providing a permeable gas flow pathway that is also conductive is more space efficient and improves power density. The inert permeable barrier 302.10 ensures that a convectively dominant flow regime prevails within the inert permeable barrier. The permeable reactant pathway 302.14 makes the reactant 302.8 available to the cathode electrodes 302.4, which form part of a permeable cathode electrode reactant pathway 302.18. The permeable cathode electrode reactant pathway 302.18 supplies the reactant 302.8 to the cathode cell reaction region 302.20, which is the interface between the cathode electrode 302.4 and the cathode electrolyte 302.2. Within the cathode reaction region, SOEC cathode reactions such as H2O + 2 e" H2 + O2" and CO2 + 2 e" CO + O2" are performed with the oxygen ions then propagating through the electrolyte to the anode where they engage in anode electrolysis reactions described below. Electrons associated with these reactions are supplied through the cathode current collector and are driven by the emf applied to the electrolyser through the combined connections to the cathode and anode current collectors. Additionally, where CO is present with H2O or CO2 is present with H2 or at least any three of the four CO, CO2, H2O and H2 are present the water gas shift reaction CO + H2O -» CO2 + H2 will typically be active in the forward or reverse direction according to the relative concentrations of the species and the temperature as many different surfaces are capable of catalysing this reaction at the operating temperature of SOECs. Typically, this enables the production of CO from CO2 through reverse water gas shift in the cathode even when the kinetics of the cathode electrochemical reaction CO2 + 2 e" CO + O2"are relatively poor. Operation of the electrolyser much below 800 °C may be unattractive if CO2 or CO are to be present on the cathode as this will increase the chances of creating unwanted carbon depositing conditions within the cathode. Unused reactant and reaction product 302.22 are output from the cell 302 via a permeable reactant pathway outlet 302.24.

[00123] It should be born in mind that references to the propagation of O2" ions are a convenient shorthand for the transport of oxygen vacancies in the solid state lattice of the electrolyte and ionic conduction materials in the electrodes that are associated with charge transport that is associated with these vacancies.

[00124] Although not shown in the view 300B, the additional current collectors 114 can applied over inlet 302.16 and outlet 302.24 of the cathode cell 302 prior to applying the inert permeable barrier 302.10. Current is supplied to the cathode cell 302 by these additional current collectors 114 and flows horizontally (or in-plane) to supply electrical current to the cathode electrodes 302.4 and the reaction regions 302.20 where the electrochemical reactions take place. The resistive or ohmic loss associated with this current flow is, therefore, dependent on the inplane sheet electrical conductance of the electrodes 302.4 and current collector 302.6 in addition to the horizontal distance over which current travels to reach the electrodes from where it is supplied at the cell inlet 302.16 and outlet 302.24 as well as the magnitude of the current supplied.

[00125] Referring to figure 3B.2, there is shown a sectional view of the cathode cell described above with reference to figure 3B.1 showing dimensions of the various layers.

[00126] The cathode cell has the following characteristics:

[00127] - the inert permeable barrier 302.10 has a depth of db in the reactant flow direction,

[00128] - the permeable cathode electrode 302.4 has a depth in the reactant flow direction of de,

[00129] - the permeable cathode current conductor 302.6 has a depth in the reactant flow direction of de,

[00130] - the pair of electrolytes 302.2 also have a depth in the reactant flow direction of de >

[00131] - the permeable cathode electrode has a thickness denoted by tan,

[00132] - the permeable cathode current conductor 302.6 has a thickness of te,

[00133] - the electrolytes 302.2 both have a thickness of tei, and

[00134] - the inert permeable barrier layer 302.10 has a thickness of tb, which is the same as the overall cathode cell thickness taco

[00135] Referring to figure 3C.1, there is shown a view 300C of an anode cell 304. The anode cell is shown as comprising a reactant flow 304.8. The reactant or sweeping flow 304.8 is fed to a permeable reactant pathway 304.14 via a permeable reactant pathway inlet 304.16. The permeable reactant pathway 304.14 is carried by a suitably permeable structure 304.6 as described herein. Examples can be realised in which the suitably permeable structure 304.6 is a current collector, such as, for example, an anode current collector. Examples can be realised, however, in which the suitably permeable structure 304.6 is made from the same material as the anode electrode and, therefore, effectively eliminated or replaced. Still further examples can be realised in which the suitably permeable structure is a heterogeneous structure comprising multiple layers or entities, as opposed to being a single homogeneous structure. Still further examples can be realised in which the suitably permeable structure 304.6 is not electrically conductive or not highly conductive thereby reducing or eliminating its function as a current collector. The permeable reactant pathway 304.14 makes the reactant 304.8 available to the anode electrodes 304.4, which form part of a permeable anode electrode reactant pathway 304.18. The permeable anode electrode reactant pathway 304.18 supplies the reactant 304.8 to the reaction region 304.20. The reaction region 304.20 is formed at the interface between the anode electrodes 304.4 and the electrolytes 304.2. Within the anode electrodes, SOEC anode reactions such as 2O2" O2 + 4e" are performed with the oxygen ions supplied through the electrolyte from the anode where they were created in cathode electrolysis reactions described above. Electrons associated with these reactions are removed through the anode current collector and are driven by the emf applied to the electrolyser through the combined connections to the cathode and anode current collectors. Unused reactant and reaction product 304.22 are output from the cell 304 via a permeable reactant pathway outlet 304.24.

[00136] Although not shown in the figure 300C, the additional current collectors 116 are applied over the inlet 304.16 and outlet 304.24 of the anode cell 304. Current is thus collected from the anode electrodes 304.4 and the reaction regions 304.20 where the electrochemical reactions take place and flows horizontally (or in-plane) toward these additional collectors to be removed from the anode cell. The resistive or ohmic loss associated with this current flow is therefore dependent on the in-plane sheet electrical conductance of the electrodes 304.4 and current collector 304.6 in addition to the horizontal distance over which current must travel from the electrodes to reach the inlet 304.16 and outlet 304.24 where it is removed from the cell as well as the magnitude of the current collected.

[00137] Referring to figure 3C.2, there is shown a sectional view of the anode cell described above with reference to figure 3C.1 showing dimensions of the various layers.

[00138] The anode cell has the following characteristics:

[00139] - the permeable anode electrode 304.4 has a width in the oxygen evolution side flow direction of wc,

[00140] - the permeable anode current conductor 304.6 has a width in the oxygen evolution side flow direction of wc,

[00141] - the pair of electrolytes 304.2 also have a width in the oxygen evolution side flow direction of wc,

[00142] - the permeable anode electrode has a thickness denoted by tca,

[00143] - the electrolytes 304.2 have a thickness tei

[00144] - the permeable anode current conductor 304.6 has a thickness of tc, and

[00145] - the overall cell has a thickness of tcco.

[00146] Referring to figure 3D.1, there is shown a view300D of a chemical heating cell 306 together with barriers according to an example. The chemical heating cell is shown as comprising a chemical heating flow 306.8. The chemical heating flow 306.8 is fed to a chemical heating cell inert permeable barrier 306.10. The inert permeable barrier 306.10 provides a chemical heating cell inert permeable barrier pathway 306.12. The chemical heating cell inert permeable barrier pathway 306.12 is arranged to at least reduce, and, preferably, eliminate, diffusion effects of any gas species within the inert permeable barrier before the interface 306.13 between the end of the inert permeable barrier 306.10 and the chemical heating cell 306. The chemical heating cell inert permeable barrier pathway 306.12 is arranged to feed a permeable chemical heating cell pathway 306.14 via a permeable chemical heating cell pathway inlet 306.16. The permeable chemical heating cell pathway 306.14 is carried by a suitably permeable structure 306.4 as described herein that forms the reaction region 306.20. Examples can be realised in which the suitably permeable structure 306.4 is a permeable chemical heating catalyst. Still further examples can be realised in which the suitably permeable structure is a heterogeneous structure comprising multiple layers or entities, as opposed to being a single homogeneous structure. Further examples can be realised in which the permeable conductor 306.2 is solid or almost solid (to support electrical conduction if the chemical heating catalyst 306.4 does not conduct electricity sufficiently) as it is not involved in supporting chemical reactions. The chemical heating fluid enters the chemical heating cell via the permeable chemical heating cell pathway inlet 306.16 and leaves the chemical heating cell via a permeable chemical heating cell pathway outlet 306.24 where the chemical heating passes through a second chemical heating cell inert permeable barrier 306.10 disposed at the permeable chemical heating cell pathway outlet 306.24. The chemical heating catalyst supports the methanation reactions 3H2+CO —> CH4+H2O , 4H2+CO2 —> CH4+ 2H2O and it, or other surfaces, may catalyse the shift reaction CO + H2O -» CO2 + H2 that can increase the fraction of the mixture that can methanate. Methanation reactions for providing heat are most effectively exploited if the electrolyser is operated at temperatures of roughly 600 °C, which help favour methanation reactions in the forward direction. Other reactions may also be used to deposit heat into the heating cells including partial oxidation of hydrocarbon fuels e.g. CH4 + 0.5 O2 —> CO + 2H2. Unused reactant(s) and reaction product(s) 306.22 are output from the cell 306 via the permeable chemical heating cell pathway outlet 306.24 via a second chemical heating cell inert permeable barrier pathway 306.12 within the second chemical heating cell inert permeable barrier 306.10. It will be appreciated the chemical heating fluid is a reactant and that the chemical heating outlet is a reaction product. The chemical heating mixture can then be reconditioned in a heat absorbing chemical reaction involving the chemical heating fluid (not shown) flowing through a set of chemical heat absorbing cells that indirectly take in the heat ultimately used to provide heat to the electrochemical reactions within the adjacent cathode and anode cells of the core of the stack. The reconditioning uses the endothermic steam methane reforming reaction CH4 + H2O -» CO + 3H2 which will progress in the forward direction provided the mixture is raised in temperature by preferably around 150 to 200 °C to say 800 °C.

[00147] Although not shown in the figure 3D.1, current flows horizontally (or in-plane) through the chemical heating cell to facilitate the electrical connections between adjacent stacks via respective interconnects. The resistive or ohmic loss associated with this current flow is, therefore, dependent on the in-plane sheet electrical conductance of the permeable chemical heating catalyst 306.4 and permeable conductor 306.2 in addition to the horizontal distance over which current travels between the respective interconnects as well as the magnitude of the current supplied.

[00148] Therefore, examples provide a multi-layered structure for an electrolyser comprising:

[00149] an electrolyte;

[00150] a permeable cathode electrode;

[00151] an inert permeable barrier;

[00152] the permeable cathode electrode and electrolyte having a common interface to form a reaction region;

[00153] the permeable cathode electrode providing a permeable cathode electrode reactant pathway through the electrode to present a reactant at the common interface; the permeable cathode electrode reactant pathway being fed by a permeable cathode reactant pathway that is hosted by a permeable cathode reactant pathway structure, and

[00154] the inert permeable barrier comprising an inert permeable barrier reactant pathway to at least one, or both, of:

[00155] provide an effective depth greater than a physical depth of the inert permeable barrier to at least reduce, preferably, eliminate, diffusion of any gas species within the inert permeable barrier reactant pathway to form a convectively dominant reactant flow regime within the inert permeable barrier, or

[00156] to host a convectively dominant reactant flow regime within the inert permeable barrier,

[00157] wherein the inert permeable barrier reactant pathway is coupled to the permeable cathode reactant pathway.

[00158] The permeable cathode reactant pathway and a permeable cathode reactant pathway structure are related, or defined, by

[00159] ke Je

[00160] which relates the geometric parameters of the permeable cathode reactant pathway and its associated pathway structure to the electrolyser operating parameters, and

[00161] the inert permeable barrier at the cathode reactant pathway inlet comprises an inert permeable barrier reactant pathway defined by roO162l te^b ’

[00163] which relates the geometric parameters of the inert permeable barrier and its associated pathway to the electrolyser and associated barrier operating parameters (such as, for example in Table 1),

[00164] where

[00165] Te is the permeable cathode reactant pathway tortuosity in the reactant flow direction,

[00166] zb is the inert permeable barrier reactant pathway tortuosity in the reactant flow direction,

[00167] se is permeable cathode reactant pathway structure porosity,

[00168] sb is the inert permeable barrier porosity,

[00169] de is the permeable cathode reactant pathway structure depth in the reactant flow direction,

[00170] db is the inert permeable barrier depth in the reactant flow direction,

[00171] te is the permeable cathode reactant pathway structure thickness,

[00172] f, is the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te.

[00173] Examples of the relationship between the geometric parameters of the permeable cathode reactant pathway and its associated pathway structure to the electrolyser operating parameters are given in Table 1.

[00174] Examples of the relationship between the geometric parameters of the inert permeable barrier and its associated pathway to the electrolyser and associated barrier operating parameters are given in Table 1.

[00175] Examples can be realised in which the multi-layered structure also comprises at least one, or more than one, of the following characteristics taken jointly and severally in any and all permutations:

[00176] - the permeable cathode reactant pathway structure thickness, te , has a predetermined thickness arranged to balance cell packing density, or power density, following from a relatively smaller permeable reactant pathway structure thickness, and in-plane electrical sheet conductance, following from a relatively larger permeable reactant pathway structure thickness to reduce resistive losses;

[00177] - the permeable cathode reactant pathway structure thickness, te, has a range of 5 [im to 1000 [im, optionally, 5 / im to 500 / im and, preferably, 10 / im to 150 / im, T2

[00178] - the ratio —e— influences the balance between an additional, preferably, smallest, £efe flow resistance provided by the permeable reactant pathway structure while providing structural strength and in-plane electrical sheet conductance to balance resistive losses; T2

[00179] - the ratio—e— has a range of 1 to 500, optionally, 2 to 50, which represents the £efe additional flow resistance through the permeable cathode reactant pathway as compared to a straight pipe of diameter te,

[00180] - the permeable cathode reactant pathway structure depth, de, is selected according to manufacturing capabilities to balance cell handling capabilities, since below a given lower depth limit handling cells would be difficult, and power density, which influences an upper limit of cell thickness since large cell depths would require cell thickness to increase dramatically to account for ohmic losses and pressure drops, which would adversely lead to a lower power density overall;

[00181] - the permeable cathode electrode reactant pathway structure depth, de, has a range of 0.25 mm to 40 mm, optionally, 0.25 mm to 20 mm and, preferably, of 5 mm to 20 mm,

[00182] - the ratio is associated with a range of effective depths in the flow direction of £b the inert permeable barrier pathway to influence, that is, to limit, at least to reduce, or to eliminate, diffusion effects, as described below in paragraphs

[00197] to

[00239] ;

[00183] - the ratio has a range of 2 to 2500 mm, optionally, 10 to 500 mm and, £b preferably 50 to 250 mm, and more preferably, less than 200mm,

[00184] - the ratio, fe, of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te, is selected to balance flow resistance with providing structural strength and in-plane electrical sheet conductance to influence or constrain electrical resistive losses;

[00185] - the ratio, fe, of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te, has a range of 0.02 to 1, and, preferably, 0.25 to 0.75,

[00186] - the permeable cathode reactant pathway tortuosity, ze, is arranged to at least reduce, preferably minimise, flow resistance, and, therefore, pressure drops across the permeable cathode reactant pathway;

[00187] - the permeable cathode reactant pathway tortuosity, ze, has a range of 1 to 3, optionally, 1 to 2.5, and preferably 1 to 2,

[00188] - the inert permeable barrier pathway tortuosity, zb, is arranged to at least reduce, and, preferably, minimize, diffusion effects by increasing the effective diffusion depth;

[00189] - the inert permeable barrier pathway tortuosity, zb, has a range of 1 to 10, optionally, 1 to 5,

[00190] - the permeable cathode reactant pathway structure porosity, se, is selected to balance flow resistance against structural strength and in-plane sheet electrical conductance of the permeable cathode reactant pathway structure;

[00191] - the permeable cathode reactant pathway structure porosity, se, has a range of 0.1 to 0.9, and, preferably, 0.5 to 0.8,

[00192] the inert permeable barrier porosity, sb, has a lower bound to accommodate an orifice or foraminate plate example and an upper bound to accommodate higher tortuosity materials

[00193] -the inert permeable barrier porosity, sb, has a range of 0.01 to 0.5, and preferably, 0.05 to 0.5,

[00194] - the inert permeable barrier depth, db, can be set to achieve a balance, at a lower limit, between an increase in effective depth to limit diffusion, as described below with reference to paragraphs

[00197] to

[00239] , and so cannot be too thin, with, at an upper limit, constraining size of the inert permeable barrier such that it has a volume smaller than an associated electrolyser, which influences overall power density;

[00195] - the inert permeable barrier depth, db, is greater or equal to 0.01 mm, optionally 0.01 mm to 5 mm and, preferably, 0.1 mm to 2 mm; r2 d2

[00196] - the ratio —which relates the geometric parameters of the permeable cathode £e fe te reactant pathway and its associated pathway structure to the electrolyser operating parameters, has a range of 5e6 / m to 5e13 / m, optionally, 5e8 / m to 5e12 / m and, preferably 1e10 / m to 1e12 / m;

[00197] - the ratio dbdeTbi which relates the geometric parameters of the inert permeable te£b barrier and its associated pathway to the electrolyser and barrier operating parameters, has a range of 0.2m to 1000m, optionally, 1m to 200m, and preferably 10m to 100m.

[00198] The characteristics of the permeable cathode or anode reactant pathways and the characteristics of the inert permeable barriers of the examples described herein are such that the effective depth, in the reactant flow directions, for diffusion for the inert permeable barrier pathway is significantly increased relative to the geometrical depth of the inert permeable barrier layer and, hence, diffusion of gas species is at least reduced, and, preferably, minimised. Consequently, flow splitting between permeable reactant pathways is then primarily influenced through convective flow parameters, which are, in turn, driven by pressure difference and, hence, geometry, resulting in a convectively dominated flow regime that essentially precludes adverse flow splitting caused by variations in species concentrations driven by chemical and electrochemical electrode processes.

[00199] The convective flow parameters comprise the relative resistances of the permeable reactant pathway and barrier layers, which are higher than those of an open plenum feeding the gas species to the permeable reactant pathway and barrier layers thereby resulting in substantially uniform gas species flow into the permeable reactant pathways and barriers layers. T2

[00200] Effective depth for diffusion is the physical depth scaled by where tx is the tortuosity experienced by a gas species within a given structure or material indicated by the subscript, x, and sx is the porosity experienced by a gas species within the given structure or material, x, so that it becomes a measure that describes a relative length-based scale factor that e r2 reduces diffusion. Permeability is defined as that is, as the reciprocal of A A generic subscript p has been used in the claims to claim a structure applicable to the cathode, anode, barrier and heating cells. In the description, the generic subscript p has been replaced with subscripts associated with the respective structures; namely, the cathode, anode, barrier and heating cells.

[00201] As described and used here, the terms “porosity” and “tortuosity” refers to the characteristics of a gas species pathway of or through a material. “Porosity” is defined as the volume fraction of connected void spaces in a porous material, i.e. the volume fraction of the gas pathway relative to the total volume of the material. “Tortuosity” is defined as the ratio of the extended length of the gas pathway pores between two points due to their circuitous paths relative to the straight-line distance between those points. References to at least one, or both, of: porosity and tortuosity of a material are references to “porosity” and “tortuosity” defined above.

[00202] The porosity and tortuosity characteristics of a pathway through a permeable material are often determined by making comparisons between experimental measurements and the theoretical predictions for the flow of a multi-component gas mixture in idealised geometry, such as for example along a straight capillary tube of fixed pore diameter. Where, as described above, the porosity and tortuosity act as an appropriate scale factor to convert between theoretical predictions and experimental measurement. The effective or mean pore diameter of a permeable material can also be determined experimentally either from flow measurements or by some other means. Accordingly, the term “pore size”, as used herein, is defined as the foregoing “mean pore diameter”. Furthermore, references to the thermal and electrical conductivity of a permeable structure refers to the effective material properties taking into account the volume fraction of gas pathways and solid phases where it is expected that the solid phase makes a more significant contribution. Therefore, for example, at least one, or both, of: thermal conductivity and electrical conductivity of a highly porous material will be less than the thermal conductivity or electrical conductivity of a less porous material.

[00203] The Mass Peclet number, Pem, describes the ratio of convective mass transport to diffusive mass transport

[00204] Pem =

[00205] where

[00206] u is the convective gas velocity

[00207] D is a mass diffusion coefficient

[00208] L is a suitable length scale over which the diffusion is occurring; the length being a distance or depth in a respective flow direction,

[00209] When applied to an inert permeable barrier according to the examples described herein the Mass Peclet number, Pem, becomes

[00210] Pem=^

[00211] which shows that a desired value of Pem can be achieved by adjusting the effective depth of the inert permeable barrier as described above. The examples provided and £b claimed herein aim to increase the value of Pem.

[00212] Heat transfer within and between the electrolyser structures of the examples described herein, such as the permeable reactant pathway, is dominated by conduction such that the heat required by the electrochemical reactions can be provided by the heating reactions within the chemical heating cells.

[00213] The molar rate of reactant (steam or carbon dioxide) consumption r per unit area of the cathode electrode (302.4) I electrolyte (302.2) interface is given by

[00214] r = — Equation (1) 2F

[00215] where

[00216] i is the current density at the interface, and

[00217] F is Faraday’s constant.

[00218] The convective velocity u averaged over the cross-sectional area of the permeable cathode reactant pathway and its associated structure required to supply the reactant consumption rate r per unit area for a double sided cathode cell with two electrode / electrolyte interfaces is given by

[00219] u = Equation (2) FU fCte

[00220] where

[00221] c is the molar gas density,

[00222] de is the permeable cathode reactant pathway structure depth in the reactant flow direction,

[00223] te is the permeable cathode reactant pathway structure thickness, and

[00224] Uf is a utilization factor indicating the proportion of the supplied reactant flow that is consumed.

[00225] The pressure drop Ap across the depth de of the permeable cathode reactant pathway structure in the reactant flow direction is given by

[00226] &p=^-^ude Equation (3) £e tefe

[00227] where

[00228] fe is the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te,

[00229] p is the reactant dynamic viscosity,

[00230] Te is the permeable cathode reactant pathway tortuosity in the reactant flow direction, and

[00231] Se is permeable cathode reactant pathway structure porosity.

[00232] Substituting for u from above and re-arranging, the geometric properties of the permeable cathode reactant pathway and permeable cathode reactant pathway structure are related, or defined, by

[00233] = Equation (4)

[00234] which further relates these geometric properties to the operational parameters of the cathode cell where in addition to the terms described above

[00235]

[00236]

[00237]

[00238] operating p is the cathode cell operating pressure, Tis the cathode cell operating temperature, and R is the universal gas constant. In the above equation (4), the left-hand side captures the key choices made in an electrolyser while the right-hand side describes the geometry with which the electrolyser is constructed. Once the electrolyser has been constructed, the parameters on the left must be traded to maintain the same overall value. For example, if current density were decreased, for instance, to support temporary operation with reduced renewable power availability, other parameters on the left must be adjusted to compensate to maintain the same overall value. On the left-hand side, the first factor captures pressure corrected for temperature to effectively provide a molar density that describes how dense the flows are that are put through the electrolyser on a molar basis that can, in turn, be related to the current. The second factor frpU f describes the pressure drop experienced per unit current density at which the electrolyser is operated noting that steam or carbon dioxide utilisation effectively acts as a correction on the pressure drop experienced per unit current density at which the electrolyser is operated that drives increasing the pressure drop if steam or carbon dioxide utilisation is decreased. The final factor —^—scales the combination of the first two factors to be able to relate this to the expression on the right-hand side that captures the key relationships between the geometrical dimensions of the flow path feeding the cell.

[00239] It follows that, for example, achieving a high volumetric power density which can be realised by lowering te would also necessitate decreasing the depth of the cell de otherwise unrealistic operating conditions such as, for example, excessive pressure drop will be incurred on the left hand side of equation (4).

[00240] Further details and ranges for all the geometric properties and operational parameters used in the expression above are given in Table 1.

[00241] Examples also provide a multi-layered structure electrolyser comprising:

[00242] an electrolyte;

[00243] a permeable anode electrode;

[00244] the permeable anode electrode and electrolyte having a common interface to form a reaction region;

[00245] the permeable anode electrode providing a permeable anode electrode reactant pathway through the electrode to present a reactant at the common interface; the permeable anode electrode reactant pathway being fed by a permeable anode reactant pathway.

[00246] The permeable anode reactant pathway is hosted by a permeable anode reactant pathway structure, which are related, or defined, by

[00248] which relates the geometric parameters of the permeable anode reactant pathway and its associated pathway structure to the electrolyser operating parameters (such as, for example in Table 2),

[00249] where

[00250] tc is the permeable anode reactant pathway tortuosity in an oxygen evolution side flow f direction,

[00251] sc is the permeable anode reactant pathway structure porosity,

[00252] wc is the permeable anode reactant pathway structure width in the sweep gas flow direction,

[00253] tc is the permeable anode reactant pathway structure thickness,

[00254] fc is the ratio of permeable anode reactant pathway pore size to permeable anode reactant pathway structure thickness, tc.

[00255] Examples of the relationship between the geometric parameters of the permeable anode reactant pathway and its associated pathway structure to the electrolyser operating parameters are given in Table 2.

[00256] It will be appreciated that the anode reactant comprises a sweep gas such as, for example, air or steam that does not strictly react with the electrode, instead performing a function of diluting evolved oxygen to avoid buildup of excessive oxygen concentrations that would harm materials and be a significant challenge to safety engineering. However, it may also be a reacting fuel species, often referred to as an anode assist fuel, such as a tail gas from a fuel synthesis unit. The anode reactions then include H2 + O2-^ H2O + 2e". In this situation, where CO is present with H2O or CO2 is present with H2 or at least any three of the four CO, CO2, H2O and H2 are present the water gas shift reaction CO + H2O -» CO2 + H2 will typically be active in the forward direction replacing hydrogen as it is consumed in the anode and indirectly enabling exploitation of fuel energy present in CO. The environment in an SOEC anode is less predisposed to carbon formation than that at the cathode and it may be possible to operate anodes in the presence of CO and CO2 down to approximately 600 °C although higher temperatures up to 800 °C would reduce this risk. It will be appreciated that the depth of a cathode cell and the width of an anode cell are correlated, as are the width and depth of the other stack components. Examples can be realised in which the depth of a cathode cell and the width of an anode cell are the same.

[00257] Examples can be realised in which the multi-layered structure also comprises at least one, or more than one, of the following characteristics taken jointly and severally in any and all permutations:

[00258] - the permeable anode reactant pathway structure thickness, tc , has a predetermined thickness arranged to balance cell packing density, or power density, following from a relatively smaller permeable reactant pathway structure thickness, and in-plane sheet electrical conductance, following from a relatively larger permeable reactant pathway structure thickness to reduce electrical resistive losses;

[00259] - the permeable anode reactant pathway structure thickness, tc, has a range of 10 [im to 1000 [im, optionally, 10 / im to 600 / im and, preferably, 20 / im to 250 iinr, the range of permeable anode reactant pathway structure thickness, tc, is greater than the range of the permeable cathode reactant pathway structure thickness, te, since the anode provides a greater gas flow capacity and lower material electrical and thermal conductivities; T2

[00260] - the ratio —4 influences the balance between an additional, preferably, smallest, £cfc flow resistance provided by the permeable anode reactant pathway material while providing structural strength and in-plane sheet electrical conductance to balance electrical resistive losses; T2

[00261] - the ratio —c— has a range of 1 to 500, optionally, 2 to 50, £cfc

[00262] - the permeable anode reactant pathway structure width, wc, is selected according to manufacturing capabilities to balance cell handling capabilities, since below a given lower width limit handling cells would be difficult, and power density, which influences an upper limit of cell thickness since large cell widths would require cell thickness to increase dramatically to account for ohmic losses and pressure drops, which would adversely lead to a lower power density overall;

[00263] - the permeable anode reactant pathway structure width, wc, has a range of 0.25 mm to 40 mm, optionally, 0.25 mm to 20 mm and, preferably, of 5 mm to 20 mm,

[00264] - the ratio, fc, of permeable anode reactant pathway pore size to permeable anode reactant pathway structure thickness, tc, is selected to balance flow resistance with providing structural strength and in-plane sheet electrical conductance to influence or constrain electrical resistive losses;

[00265] - the ratio, fc, of permeable anode reactant pathway pore size to permeable anode reactant pathway structure thickness, tc, has a range of 0.02 to 1, and, preferably, 0.25 to 0.75,

[00266] - the permeable anode reactant pathway tortuosity, tc, is arranged to at least reduce flow resistance, and, therefore, pressure drops across the permeable anode reactant pathway;

[00267] - the permeable anode reactant pathway tortuosity, tc, has a range of 1 to 3, optionally, 1 to 2.5 and, preferably, 1 to 2;

[00268] - the permeable anode reactant pathway structure porosity, ec, is selected to balance flow resistance against structural strength and in-plane sheet electrical conductance of the permeable anode reactant pathway structure;

[00269] - the permeable anode reactant pathway structure porosity, ec, has a range of 0.1 to 0.9, and, preferably, 0.5 to 0.8; 2 2

[00270] - the ratio which relates the geometric parameters of the permeable anode £c fc reactant pathway and its associated pathway structure to the electrolyser operating parameters, has a range of 1e6 / m to 5e12 / m, optionally, 1e8 / m to 5e11 / m and, preferably 1e9 / m to 2e11 / m.

[00271] Accordingly, examples can be realised that provide a multi-layered structure for an electrolyser; the multi-layered structure comprising:

[00272] a planar electrolyte;

[00281]

[00282]

[00283]

[00284]

[00285]

[00286]

[00287]

[00288]

[00289]

[00290]

[00291]

[00292]

[00293]

[00273] a planar permeable electrode;

[00274] an inert permeable barrier;

[00275] the electrode and electrolyte having a common planar interface to form a reaction region;

[00276] the permeable electrode providing a permeable electrode reactant pathway through the electrode to present a reactant at the common planar interface; the planar permeable electrode reactant pathway being fed by a permeable reactant pathway, and

[00277] the inert permeable barrier comprises an inert permeable barrier reactant pathway,

[00278] wherein the inert permeable barrier reactant pathway feeds the permeable reactant pathway,

[00279] wherein:

[00280] the planar permeable reactant pathway is hosted by a permeable reactant pathway structure, which are related, or defined, by r2 d2 tp Up fp tp and the inert permeable barrier reactant pathway is defined by dfydpT^ tp£b where tp is the permeable reactant pathway tortuosity in the reactant flow direction, zb is the inert permeable barrier pathway tortuosity in the reactant flow direction, £p is the permeable reactant pathway structure porosity, sb is the inert permeable barrier porosity, dp is the permeable reactant pathway structure depth in the reactant flow direction, db is the inert permeable barrier depth in the reactant flow direction, tp is the permeable reactant pathway structure thickness, fp is the ratio of permeable reactant pathway pore size to permeable reactant pathway structure thickness, tp.

[00294] The multi-layered structure can further comprise an electrode current collector to collect current from the planar electrode and to provide the permeable reactant pathway structure. The above-described cathode current collector 104.4 and the above-described anode current collector 106.4 are examples of such an electrode current collector.

[00295] Referring to figure 3D.2, there is shown a sectional view of the chemical heating cell described above with reference to figure 3D.1 showing dimensions of the various layers.

[00296] The chemical heating cell has the following characteristics:

[00297] - the pair of permeable conductors 306.2 have a depth of dch,

[00298] - the permeable chemical heating catalyst layer 306.4 has a depth in the chemical heating flow direction of dch,

[00299] - the chemical heating cell inert permeable barrier 306.10 has a depth in the chemical heating flow direction of dchb,

[00300] - the pair of permeable conductors have a thickness of tpc,

[00301] - the permeable chemical heating catalyst layer has a thickness denoted by tch, and

[00302] - the chemical heating cell inert permeable barrier 306.10 has a thickness of tChb which is the same as the overall chemical heating cell thickness of tChco.

[00303] Referring to figure 4, there is shown a view 400 of an initial firing 402 of the abovedescribed laminated sheets 403 coupled with cutting 404 of the above-described laminated sheets. It can be appreciated that any, that is, one or more than one, or all, of the above-described laminated sheets 302-308 each have adjacent corners removed. The adjacent removed corners create substantially rectangular facets 406 and 408. The facets 406 and 408 are used to form inlets and outlets for the chemical heating fluids used within the electrolyser in the chemical heating cells.

[00304] Referring to Figure 5, there is shown a view 500 of an anode cell 502, a cathode cell 503 and a chemical heating cell 506. The anode cell 502 is an example of any anode cell described herein such as, for example, anode cell 304. The cathode cell 503 is an example of any cathode cell described herein such as, for example, cathode cell 302. The chemical heating cell 506 is an example of any chemical heating cell described herein such as, for example, chemical heating cell 306.

[00305] The anode cell 502 is formed from an initially fired cut laminated sheet such as the laminated sheet 403 described above with reference to Figure 4. The facets 406 and 408 presented by the removed corners and their selected adjacent edges 508 and 510 are sealed with an impermeable sealant. The impermeable sealant provides a barrier against sweep gas egress from the sealed edges 504, 506, 508 and 510. Examples can be realised in which the edges are sealed using an electrolyte dip or ink 511 that soaks into the edges 504 to 510 of the anode cell 502. The remaining edges 512, 514 of the anode cell 502 remain undipped and, therefore, provide edge facets for supporting inlets and outlets for the anode cell 502. The inlets and outlets are examples of the permeable reactant pathway inlet 304.16 and permeable reactant pathway outlet 304.24. The dip or ink 511 can be realised using for example ink 226.

[00306] Similarly, the cathode cell 503 has a plurality of sealed edges 504.1 to 504.4 that are impermeable to a reactant intended to be carried within the cathode cell 503. Again, facets 504.1 and 504.2 presented by the cut corners and edges 504.3 and 504.4 are sealed with the same electrolyte dip or ink 504.5 to 504.8. The sealed edges define a pair of edge apertures 504.10, 504.12 to allow ingress and egress of reactant through the cathode cell 503. These inlets and outlets are examples of the permeable reactant pathway inlet 302.16 and permeable reactant pathway outlet 302.24.

[00307] The chemical heating cell 506 similarly has four sealed edges 508’ to 514’. A pair of the sealed edges 510’, 514’ are sealed using the same electrolyte dip or ink to thereby form chemical heating impermeable barriers 516 and 518, as described above with reference to the cathode cell 503 and the anode cell 502. A further pair of the sealed edges 508’ and 512’ are sealed using an interconnect dip or ink 228 to thereby form chemical heating impermeable barriers 520 and 522. The dipped or inked edges define a pair of apertures 524 and 526. The pair of apertures 524 and 526 are formed from the facets 406 and 408 presented by removing the corners of the initially fired cut laminated sheet described above with reference to Figure 4. The apertures 524 and 526 define ingress and egress apertures, or inlets and outlets, for a chemical heating fluid carried by the chemical heating cell 506. The inlets and outlets are examples of the permeable chemical heating cell pathway inlet 306.16 and permeable chemical heating cell pathway outlet 306.24.

[00308] It will be noted that two chemical heating cells are presented; one for the bottom of a stack, labelled Bottom Chemical Heating Cell 506A, and one for the top of a stack, labelled Top Chemical Heating Cell 506B. The Top and Bottom Chemical heating cells are identical, but for the sealants. In the Top Chemical Heating Cell, a pair of the sealed edges 510’, 514’ are sealed using the same interconnect dip or ink to thereby form chemical heating impermeable barriers 520 and 522. A further pair of the sealed edges 508’, 512’ are sealed using the same electrolyte dip or ink to thereby form a chemical heating impermeable barriers 516 and 518, as described above with reference to the cathode cell 503 and the anode cell 502. In the Bottom Chemical Heating Cell, a pair of the sealed edges 508’, 512’ are sealed using the same interconnect dip or ink to thereby form chemical heating impermeable barriers 520 and 522. A further pair of the sealed edges 510’, 514’ are sealed using the same electrolyte dip or ink to thereby form chemical heating impermeable barriers 516 and 518, as described above with reference to the cathode cell 503 and the anode cell 502.

[00309] The electrolyte dip or ink can be realised using the above-described electrolyte ink 226. The interconnect dip or ink used to realise the impermeable barriers 520, 522 can be the above-described interconnect ink 228.

[00310] Referring to Figure 6, there is shown a view 600 of a pair of expanded stacks 602 and 604. The pair 602 and 604 of stacks are identical but are shown in different states of assembly or expansion.

[00311] Prior to assembly, the dips or inks applied, as described with reference to Figure 5, are dried. It can be appreciated that the stacks 602 and 604 comprise a set of chemical heating cells, a set of cathode cells, a set of anode cells, and a set of dense insulators. Each cathode cell of the set of cathode cells is formed from a pair of cathode half-cells. Each anode cell of the set of anode cells is formed from a pair of anode half-cells. Each chemical heating cell of the set of chemical heating cells is formed from a pair of chemical heating half-cells. In the example illustrated, the set of anode cells comprises four anode cells 606 to 612, the set of cathode cells comprises three cathode cells 614 to 618, and the set of chemical heating cells comprises two chemical heating cells 626 to 628. The cathode cells are disposed in between respective pairs of anode cells. Such an anode / cathode / anode cell configuration aims to ensure that all cathodes are double sided. Having such a double-sided arrangement supports establishing a uniform fuel flow regime. The sets of anode cells 606 to 612 and cathode cells 614 to 618 form a core 620 of repeat units. In the example shown, a repeat unit comprises a cathode cell / anode cell pair.

[00312] Although the example depicted in Figure 6 illustrates a central core 620 comprising three repeat units, examples are not limited to such an arrangement. Examples can be realised in which some other number of repeat units is used. For example, a central core 620 comprising one or more than one repeat unit can be realised. Furthermore, examples can be realised in which a plurality of repeat units are used to realise the central core 620. Examples can be implemented in which ten or more repeat units are used to realise the central core 620.

[00313] Dense insulator layers 622 and 624 are disposed either side of, and outwardly relative to the outer most anode cells 606 and 612. The dense insulator layers 622 and 624 can be realised using the above dense insulator green state sheets 220. The set of chemical heating cells can comprise the pair of chemical heating cells 626 and 628 that are positioned next to the dense insulator layers 622, 624.

[00314] The facets of the cathode cells and anode cells presented by the truncated corners are aligned with one another and are aligned with the apertures 524, 526 of the chemical heating cells 626, 628. It will also be noted that the corners of the dense insulator layers 622, 624 have been similarly removed.

[00315] Referring to the partially assembled / partially expanded righthand view 604 of the stack, it can be appreciated that a tuple is formed that comprises the first anode cell 606, the dense insulator layer 622 and the upper most heating cell 626. Similarly, a further tuple is formed that comprises the lower most dense insulator layer 624 and the lower most chemical heating cell 628. Also shown in figure 6 is a set of axes, or coordinate directions, 630. The coordinate directions 630 comprise three axes. A first axis 632 shows cell depth, or a reactant or fuel side flow direction. A second axis 634 shows cell width, or a sweep gas or air side flow direction. A third axis 636 shows a cell thickness. It will be appreciated that the flow directions of the cathode and anode cells are perpendicular relative to one another within respective parallel planes.

[00316] Referring to Figure 7, there is shown a view 700 of further stages in fabricating a stack 702. The stack 702 is an example of the above-described stack 112, or any other electrolyser stack described herein.

[00317] Figure 7 shows three additional views 704 to 708 of the stack 702 in different states of assembly.

[00318] Referring to the first view 704, current collectors are applied to respective portions of the stack. A cathode current collector 704.1 is applied using a dip or using a cathode current collector dip or ink. The cathode current collector dip or ink can be realised using the abovedescribed cathode current collector ink 222. The cathode current collector dip or ink 704.1 is arranged to span the cathode current collectors of each of the cathode cells 614, 616, 618, which electrically couples the cathode current collectors of the core 620. A second cathode current collector, identical to 704.1, is applied to the opposite side of the stack (not shown).

[00319] Similarly, respective further cathode current collector dips or inks 704.2 are applied to two sides of the lower tuple (only one of which is shown) of the dense insulator layer 624 and chemical heating cell 628.

[00320] The anode current collectors of each of the anode cells are electrically coupled by applying an anode current collector dip or ink 704.3 to the anode current collectors of each of the anode cells. The anode current collector dip or ink can be realised using the above-described anode current collector ink 224. The anode current collector dip or ink 704.3 is arranged to span the anode current collectors of each of the anode cells 608, 610, 612, which electrically couples the anode current collectors of the core 620.

[00321] Similarly, a further anode current collector dip or ink 704.4 is applied to the upper tuple comprising the anode cell 606, the insulating layer 622 and the upper most chemical heating cell 626 to electrically couple the upper most anode cell 606 to the interconnect 520, 522 of the Top chemical heating cell 626.

[00322] Referring to the second view 706 of the partially assembled stack, it can be appreciated that the core 620 has been placed adjacent to the upper tuple.

[00323] In the third view 708 of the stack, a still further layer of anode current collector dip or ink 710 has been applied to couple the anode current collector layers 704.3 and 704.4 of the core 620 and upper tuple respectively.

[00324] Similarly, in view 702, the lower tuple has been positioned adjacent to the core 620 to thereby couple the applied cathode current collector layers 704.1 and 704.2 and, therefore, couple the cathode cells to the interconnects 520 and 522 at the bottom of the chemical heating cell 628.

[00325] Referring to Figure 8, there is shown a view 800 of the next stage of manufacture of the stack. An inert permeable barrier layer 802 is applied to at least one side of the stack to form an inert permeable barrier over the inlet apertures of the cathode cells. The inert permeable barrier layer 802 is an example of any of the above-described barrier layers.

[00326] The inert permeable barrier layer 802 can also at least reduce, or prevent, creepage electrical discharge at high altitude following loss of pressure by increasing length of the interface or pathway over which creepage electrical discharge must occur before becoming problematical. The inert permeable barrier layer 802 can be realised using a dip or ink such as the above-described permeable barrier ink 230 or in any other way.

[00327] Examples can be realised in which a further inert permeable barrier layer 804 is also added to the upper tuple. The further inert permeable barrier layer 804 can also at least reduce, or prevent, creepage electrical discharge at high altitude.

[00328] Similarly, still further permeable barriers 806.1 and 806.2 can be added to both sides of the lower tuple below the anode current collectors, again, to at least reduce, or prevent, creepage electrical discharge at high altitude.

[00329] Therefore, examples can be realised that comprise at least the first permeable barrier layer 802 over the inlets to the cathode cells with or without either, or both, of the further permeable barriers 804 and 806.

[00330] Figure 8 also shows an example depicting alternative arrangements of the inert permeable barrier layer 802 for examples in which the inlet and outlets of the cathode cells are reversed. The inert permeable barrier layer 802 is in position, again, over the inlets to the cathode electrodes.

[00331] Referring to Figure 9, there is shown a view 900 of a stack 902. The stack 902 can be any of the electrolyser stacks described and / or claimed herein.

[00332] Also shown in Figure 9 is a first sectional view 904 through the stack along line FF. The first sectional view 904 shows the flow of sweep gas through the stack. It will be appreciated that the first sectional view 904 depicts sectional views through two stacks 902. The two stacks are longitudinally arranged relative to one another along the same central axis (not shown). The sweep gas flow comprises a sweep gas supply flow 906 and a sweep gas outlet flow 908. The sweep gas supply flow 906 feeds a sweep gas such as, for example, steam or air, into the inlets 910 of the anode cells of the stacks. The sweep gas flows through the permeable reactant pathway 304.14 of each anode cell. The sweep gas leaves each anode cell via a respective outlet 912 where sweep gas along with evolved oxygen forms part of the sweep gas outlet flow 908.

[00333] The sweep gas supply 906 and outlet 908 flows are oriented in the same direction, which is vertically in the example depicted in figure 9. Arranging for the sweep gas or air flows 906 and 908 to be in the same direction allows the flows to vent without having a manifold or the like as part of a housing, which would otherwise constrain or have to take into account thermal expansion of the stacks. The sweep gas supply and outlet flows being in the same direction facilitate establishing a more uniform temperature regime that has less temperature variation in the longitudinal direction of the stacks. Still further, ensuring that the sweep gas is vented at one end of the stack and that reactant is collected at the other end of the stack ensures that the reactant and sweep gas are kept separate, which prevents them mixing and combusting. Any such combusting would generate heat that would introduce undesirable local temperature variations.

[00334] In realising the examples, it will be appreciated that imperfections in the upper and lower faces of the stacks might need to be accommodated since those faces might not be perfectly flat. An electrically conductive paste 914 can be used to accommodate such imperfections when stacking the stacks. Examples can be realised in which the electrically conductive paste 914 additionally, or alternatively, provides a gas tight seal to seal the outermost permeable conductor layer such as, for example, when the latter is not solid or is otherwise permeable or compromised.

[00335] The view 900 of Figure 9 also shows a further sectional view 918 through the pair of stacks along line AA. The pair of stacks is identical to the stack 902 depicted in Figure 9.

[00336] The further sectional view 918 shows an associated reactant supply flow 920 and a fuel mixture and / or reactant outlet flow 922. The reactant supply flow 920 passes through an inert permeable barrier layer 924. The inert permeable barrier 924 is an example of the abovedescribed inert permeable barrier layer 302.10 or 802. The reactant then enters and flows through the permeable cathode cells via respective permeable reactant pathway inlets 926 and leaves the permeable cathode cell via respective permeable reactant pathway outlets 928. The foregoing inlets 926 and outlets 928 are examples of the permeable reactant pathways inlet 302.16 and permeable reactant pathway outlets 302.24. Any spent reactant, produced fuel and / or unspent reactant is recovered via the reactant outlet flow 922 where product fuel or fuels can be separated or used in someway. The reactant outlet flow 922 is an example of the unused reactant outlet and reaction product flow 302.22.

[00337] Referring to figure 10, there is shown a view 1000 of a stack assembly 1002. The stack assembly is an example of the above stack assembly shown in and described with reference to figure 1.

[00338] The stack assembly 1002 comprises a set of stacks 1004 to 1010. In the example shown, the set of stacks comprises four stacks 1004 to 1010 of any of the above-described stacks such as, for example, stack 112. The stacks 1004 to 1010 are positioned adjacent to one another and oriented so that respective chemical heating cell catalyst layer inlets are fed by a centrally disposed chemical heating channel 1012.

[00339] Disposed at each outlet of each chemical heating cell of each stack 1004 to 1010 is a respective chemical heating outlet channel 1014 to 1016. The chemical heating outlet channels 1014 to 1016 are used to carry at least one, or both, of excess chemical heating fluid or chemical heating mixture produced as a consequence of a chemical heating (not shown) flowing through the set of chemical heating cells 306.

[00340] Still referring to the stack assembly 1002, a set of pairs of adjacent stacks share a fuel supply channel. The notation used for flows depicted herein is a flow name followed by a “+’ or sign. The name indicates the type of flow and the “+’ or ‘-‘sign indicates whether the flow is a supply flow or an outlet flow. For example, ‘heating+’ indicates a supply chemical heating flow, ‘heating-’ indicates an outlet chemical heating flow, fuel+ indicates a fuel supply flow, fuel-indicates an outlet fuel side flow, air+ indicates an air supply flow and air- indicates air side outlet flow.

[00341] As indicated, adjacent stacks share common fuel supply channels. In the example stack assembly 1002 shown in Figure 10, a first set of adjacent stacks comprising first 1004 and second 1006 stacks share a common fuel supply channel 1022. A second set of adjacent stacks comprising stacks 1008 and 1010 also share a common fuel supply channel 1024. Both fuel supply channels 1022 and 1024 are sealed at one end by the central chemical heating supply channel 1012 and at the opposite end by dense barriers 1062 and 1060 respectively.

[00342] Additionally, adjacent sets of stacks share an air or sweep gas supply channel. In the example stack assembly 1002 shown in Figure 10, a first set of stacks comprising first 1004 and fourth 1010 stacks share a common air supply channel 1026. A second set of stacks comprising second 1006 and third 1008 stacks also share a common air supply channel 1028.

[00343] Also visible in the stack assembly 1002 of Figure 10 are sets of fuel outlet channels 1030 to 1036 and sets of air outlet channels 1038 and 1040. The fuel outlet channels 1030 to 1036 are arranged to carry fuel outlet flows 1040.1. The fuel outlet flows 1040.1 are examples of the above-described fuel outlet flows 922. The fuel outlet flows can comprise at least one, or more than one, of: excess or unused reactant, product fuel or mixtures resulting from the reactions of these species; the foregoing being taken jointly and severally in any and all permutations.

[00344] The fuel outlet flow, fuel-, is carried by respective fuel outlet flow channels 1030 to 1036 that are defined by a set of fuel impermeable barriers. In the example depicted, two sets of such fuel impermeable barriers are indicated. A first set of fuel impermeable barriers comprises a substantially planar fuel impermeable barrier 1042 and a pair of fuel / reactant and sweep gas impermeable barriers 1044 and 1046. A second set of fuel or reactant and sweep gas impermeable barriers also comprises a substantially planar fuel impermeable barrier 1048 and a pair of fuel / reactant and sweep gas impermeable barriers 1050 and 1052.

[00345] Referring to figure 14A, there is shown a view 1400A of an array or set 1401 of stack assemblies. Each stack assembly 1402 to 1418 in the array 1401 can be realised using any of the stack assemblies described above, or herein.

[00346] The stack assemblies 1402 to 1418 are arranged in an ordered manner relative to one another. Examples can be realised in which stack assemblies of a set of stack assemblies are arranged linearly relative to one another as shown in the central column 1420 of figure 14A. Examples can be realised in which such columns of stack assemblies, such as columns 1420 to 1424 are also arranged in an aligned manner, that is, the stack assemblies in rows are aligned and the stack assemblies in columns are aligned. However, examples can also be realised in which stack assemblies of adjacent columns are offset relative to one another. The stack assemblies in the array can be arranged in any manner that aims to improve, that is, increase, the packing density of the stack assemblies that, in turn, improves the power density of the stack assemblies.

[00347] The above-described air outlet channels can be realised as a shared or common sweep gas outlet channel 1426 as depicted in figure 14A. The gaps between stack assemblies accommodate thermal expansion of the stack assemblies.

[00348] Figure 14A shows the various flows and flow directions using the following:

[00349] - reactant or fuel supply and outlet flows are indicated using ‘F+’ and ‘F-‘, which represent flows out of and into the page respectively in this example;

[00350] - sweep gas or air supply and outlet flows are indicated using ‘A+’ and ‘A-‘, which represent flows out of the page respectively in both instance in this example; and

[00351] - chemical heating supply and outlet flows are indicated using ‘H+’ and ‘H-‘, which represent flows out of and into the page respectively.

[00352] Although examples have been described in which the flow directions are as indicated in figure 14A, and any of the other figures, examples can be realised in which other flow directions are used. For instance, examples can be realised in which at least one, or both, of: the flow directions are reversed or in which the flow functions are reversed, that is, supply flows become outlet flows and visa-versa. Further examples can be realised in which different combinations of supply and outlet channels can be used. For example, a chemical heating outlet channel could be disposed diagonally opposite the central chemical heating supply channel.

[00353] Referring to figure 14B, there is shown a view 1400B of an array or set 1401 of stack assemblies. Each stack assembly 1402 to 1418 in the array 1401 can be realised using any of the stack assemblies described above, or herein. Reference numerals common to figures 1400A and 1400B refer to the same entities.

[00354] It can be seen that the repeating unit of the stack assemblies comprises a set of stacks 1428 to 1442, a set of fuel supply channels 1444 to 1450, a set of fuel outlet channels 1452 to 1462, a set of sweep gas supply channels 1464 to 1470, a set of sweep gas outlet channels 1426, a set of chemical heating supply channels 1472 to 1474, and a set of chemical heating outlet channels 1476 to 1480. In the example depicted, the set of stacks comprises nxm stacks, where n>1 and m>1. In the particular example shown in figure 1400B, n=4 and m=2. However, examples are not limited to such values of n and m. Examples can be realised in which other values of n and m are used.

[00355] Table 1 below shows a summary of parameters associated with a cathode cell according to any of the example cathode cells described herein. For each parameter, where appropriate, upper and lower bounds are given together with a typical or mid-point in the form of “Max", “Min", and “Mid’.

[00356] TABLE 1: Cathode Cell Parameters Component Cathode Cell Permeable Cathode Reactant pathway and structure (e) Inert Permeable Barrier and pathway (b) Notes Based on molar flow of H2O. Required at inlet only for cathode electrode Ratio P^pFUf r| dj RT32pl ~ £e } Pe^FpD^ dbderb RTi = te£b <m) Operational Point Min Mid Max Min Mid Max Pressure p (bar) 5 10 25 5 10 25 Pressure Drop Ap (mbar) 20 50 300 5.4 9.2 6.2 Dynamic Viscosity / / (Pa.s) 4e-05 3e-05 2e-05 Universal Gas Constant R (J / mol / K) 8.3145 8.3145 Faraday’s constant F (C / mol) 96485 96485 Current Density i (A / cm2) 0.3 0.2 0.1 0.3 0.2 0.1 Temperature T (°C) 900 800 580 900 800 580 Fuel Utilization Uf 0.33 0.9 0.95 0.33 0.9 0.95 Fuel flow rate per cm2 of cathode cell area (with 2 electrolyte interfaces) (mol / s / cm2) 9.4e-06 2.3e-06 1.1e-06 9.4e-06 2.3e-06 1.1e-06 Mass Peclet Number Pe r 5 20 50 Diffusion Coefficient D (m2 / s) 8e-04 4e-4 1e-4 Ratio Value 8.5e8 (1 / m) 2.5e10(1 / m) 1.5e12 (1 / m) 2.3 (m) 35 (m) 156 (m) Ratio Range 5e8 5e12 (1 / m), optionally 1e10 <<1 ei2 (1 / m) 1 <PemFPDuf <20Q RTi ' 7 ’ optionally 10 <PemFpDUf <100 (m) RTi Pathway Tortuosity t 1<Te<2 1< <5 Porosity e 0.5< ee<0.8 0.05< Eb <0.5 Depth d 0.5 <de <2(cm) db see below Pathway pore size ratio fe 0.25 <fe <0.75 Non-dimensional terms 2 2 1 <-^7 <500, optionally 2 <-^7 <50 £efe £efe t2 20<—<500, 2 2 1.25<—<8, optionally 2<^<8 £e £e t2 optionally 20<^<200 Permeable Reactant Pathway Structure Thickness ^(tim) Barrier Depth db (mm) Ratio Value 8.5e8 (1 / m) 2.5e10(1 / m) 1.5e12(1 / m) 2.3 (m) 35 (m) 156 (m) Non-dimensional term ci, 20 500 20 500 20 500 T2 de=0.5 (cm), ^=2.22 £efe 40 13 3 0.9 0.04 4.6 0.2 5.2 0.2 T2 de=0.5 (cm), ^=128 £efe 156 50 13 3.6 0.1 17.6 0.7 20.1 0.8 T2 de=2 (cm), ^=2.22 £efe 102 33 8 0.6 0.02 2.9 0.1 3.3 0.1 T2 de =2 (cm), =128 £eJe 392 127 32 2.3 0.1 11.1 0.4 12.6 0.5 Range 5 <te <500 (p.m), optionally 10 <tfl<150 (p.m) 0.01< db<5 (mm), optionally 0.1 <db<2 (mm) Effective Pathway Depth sssssssssssssssssssss^ -“(cm) ^(cm) T2 de =0.5 (cm), — =1.25 £e 0.625 1.9 9.1 10.4 T2 de =0.5 (cm), —=8 £e 4 7.2 35.3 40.1 T2 de=2 (cm), -=1.25 £e 2.5 1.2 5.8 6.5 r2 de=2(cm),— =8 £e 16 4.5 22.2 25.3 Range d r2 0.5 <^<15 (cm), £e d r2 optionally 0.5 <<10(cm), £e 1 <^<50 (cm), Eb optionally 5 <<25 (cm) £h Barrier to Cell Pathway depth ratio sssssssssssssssssssssssss T2 de =0.5 (cm), — =1.25 £e 3.0 14.6 16.6 T2 de =0.5 (cm), — =8 £e 1.8 8.8 10.0 T2 de=2 (cm), -=1.25 £e 0.5 2.3 2.6 T2 de=2(cm),— =8 £e 0.3 1.4 1.6 Range 1 20, optionally 2 <^ / ^ <10 Eb / Ee Sb / Ee

[00357] The inert permeable barrier pressure drops given in the tables herein are calculations and not prescribed operating parameters. These calculations have been provided for illustrative purposes and were obtained for a nominal inert permeable barrier pathway pore size of 100 microns. Neither the inert permeable barrier pressure drop nor the inert permeable barrier pathway pore size strongly affect the primary function of any of the inert permeable barriers described herein, which is to at least reduce, or eliminate, diffusion effects. Hence either term can be prescribed independently to meet design specifications or match other requirements as necessary.

[00358]

[00359] The parameter values given in all of the tables described herein are preferred parameter values and / or parameter value ranges. Examples can be realised in which wider parameter value ranges can be used, as described below with respect to the methods of operation and operating parameter spaces.

[00360] Table 2 below shows a summary of parameters associated with a permeable anode reactant pathway and structure, which hosts, for example, the permeable reactant pathway 304.14 according to any of the anode cells described herein. For each parameter, where appropriate, upper and lower bounds are given together with a typical or mid-point in the form of “Max", “Min", and “Mid’.

[00361] TABLE 2: Anode Cell Parameters Component Permeable Anode Reactant Pathway and Structure (c) Notes Here nitrogen is assumed as the sweep gas with a flow such that oxygen concentration on exit is close to that in air Ratio p^pFUg _ rg 2.5RT32pi~ } Operational Point Min Mid Max Pressure p (bar) 5 10 25 Pressure Drop Ap (mbar) 20 50 300 Dynamic Viscosity / / (Pa.s) 4.5e-05 4.25e-05 4e-05 Universal Gas Constant R (J / mol / K) 8.3145 Faraday’s constant F (C / mol) 96485 Current Density i (A / cm2) 0.3 0.2 0.1 Temperature T (°C) 900 800 580 Air Utilization Ua 0.25 0.5 0.75 Air flow rate per cm2 of anode cell area with 2 electrolyte interfaces (mol / s / cm2) 3.1e-05 1e-05 3.5e-06 Ratio Value 2.3e8(1 / m) 4.0e9(1 / m) 2.4e11(1 / m) Ratio Range 1e8< p*pFUa <5e11 (1 / m), 2.5RT32(li ' ' optionally 1e9 <p&pFUa <2e11 (1 / m) r J 2.5RT32(li ' ' Pathway Tortuosity tc 1<TC <2 Porosity sc 0.5 <£c <0.8 Width wc 0.5 <wc<2(cm) Pathway pore size ratio fc 0.25 <£<0.75 Non-dimensional terms 2 2 1 <<500, optionally 2 <^ <50 £cfc £cfc 2 2 1.25<—<8, optionally 5<^<8 Sr Sr Permeable Anode Reactant Pathway Thickness tc (urn) Ratio Value 2.3e8 (1 / m) 4.0e9(1 / m) 2.4e11(1 / m) T2 wc =0.5 (cm),-^7 =2.22 £cfc 62 24 6 T2 wc =0.5 (cm), ^7 =128 £cfc 241 93 24 T2 wc =2 (cm), =2.22 £dc 157 61 16 T2 wc=2 (cm), ^=128 £clc 607 235 60 Range 10 <tc <600 (pm), optionally 20 <tc <250 (pm)

[00362] The parameter values given in table 2 described herein are preferred parameter values and / or parameter value ranges. Examples can be realised in which wider parameter value ranges can be used, as described below with respect to the methods of operation and operating parameter spaces.

[00363] Table 3 below shows a summary of parameters associated with a chemical heating cell according to any of the example chemical heating cells described herein. For each parameter, where appropriate, upper and lower bounds are given together with a typical or mid-point in the form of “Max", “Min", and “Mid’.

[00364] TABLE 3: Chemical Heating Cell Parameters Component Chemical heating Cell Permeable chemical heating catalyst layer and pathway (ch) Chemical Heating Cell Inert Permeable Barrier and pathway (chb) Notes Assuming a stack containing a predetermined number, a, of cathode cells that operates at maximum current density of 0.3A / cm2, each chemical heating cell requires approximately the same chemical heating flow rate as the fuel supplied to all of the predetermined number of cathode cells to manage the heat required by the stack. Chemical heating utilization Uh >1 when chemical heating requirement is less than the maximum that could be delivered. Required at inlet and outlet of chemical heating catalyst layer Ratio P^pFUh = d?b aRT32pi Schfc^ch P&mFpDUft ^-chbdcc^chb aRTl tch^chb Operational Point Min Mid Max Min Mid Max Pressure p (bar) 5 10 25 5 10 25 Pressure Drop Ap (mbar) 20 50 300 5.4 9.2 6.2 Dynamic Viscosity p (Pa.s) 4e-05 3e-05 2e-05 Universal Gas Constant R (J / mol / K) 8.3145 8.3145 Faraday’s constant F (C / mol) 96485 96485 Current Density i (A / cm2) 0.3 0.2 0.1 0.3 0.2 0.1 Temperature T (°C) 900 800 580 900 800 580 Chemical heating fluid Utilization Uh 1 2 5 1 2 5 Chemical heating flow rate per cm2 of chemical heating cell area (mol / s / cm2) 6.2e-05 2.1e-05 4.2e-06 6.2e-05 2.1e-05 4.2e-06 Mass Peclet Number Pem 5 20 50 Diffusion Coefficient D (m2 / s) 8e-04 4e-4 1e-4 Ratio Value 1.3e8 (1 / m) 2.8e9(1 / m) 4.0e11(1 / m) 0.35 (m) 3.9 (m) 41.1 (m) Ratio Range 1e8 5e11 (1 / m), aRT32[ii ' ' optionally 1e9 <p&pFUh <ie11 (1 / m) r J aRT32(ii ' ' 0.5 <PemFpDUh <50 (m), optionally 1 h <25 (m) Pathway Tortuosity t 1<Tch <2 <5 Porosity e 0.5< Ecfl <0.8 0.05< Ecflb <0.5 Depth d 0.5 <dcfl <2(cm) dcflb see below Pathway pore size ratio fch 0.25 <fch <0.75 Non-dimensional terms t2 1 <<500, £chfch t2 optionally 2 <—< 50 £chfch 2 2 1 25<—<8, optionally 5<—<8 £<:h ^ch t2 20<^<500, £chb t2 optionally 20<-^<200 Echb Permeable chemical heating catalyst layer thickness tch (^im) Barrier Depth dchb (mm) Ratio Value 1.3e8 (1 / m) 2.8e9 (1 / m) 4.0e11 (1 / m) 0.35 (m) 3.9 (m) 41.1 (m) Non-dimensional t2 term £cb.b 20 500 20 500 20 500 dch =0.5 (cm), t2 =2.22 Bchfch 76 27 5 0.26 0.01 1.06 0.04 2.13 0.09 dch =0.5 (cm), t2 -^-=128 Bchfch 292 104 20 1.02 0.04 4.08 0.16 8.24 0.33 dch =2 (cm), t2 -^-=2.22 Bchfch 190 68 13 0.17 0.01 0.67 0.03 1.34 0.05 t2 dch =2 (cm), ch v h edJ?h = 128 736 263 51 0.64 0.03 2.57 0.10 5.19 0.21 Range 5 <tch <700 (gm), optionally 10 <tch <300 (gm) 0.01 <dchb <5 (mm), optionally 0.1 <dchb <2 (mm) Effective Pathway Depth ^^(cm) ^^(cm) t2 dch=0.5 (cm), =1.25 0.625 0.5 2.1 4.3 t2 dch=0.5 (cm), =8 4 2.0 8.2 16.5 t2 dch=2 (cm), =1.25 2.5 0.3 1.3 2.7 t2 dch=2 (cm), ^=8 16 1.3 5.1 10.4 Range 0.5 <^4 <15 (cm) £ch optionally 0.5 <<10(cm), £ch 0.5 dchbT2M <20 (cm), £chb optionally 0.5 <dchbTchb <10 (cm) £chb Effective Barrier to Cell Pathway depth ratio t2 dch=0.5 (cm), =1.25 0.8 3.4 6.8 t2 dch=0.5 (cm), =8 0.5 2.0 4.1 t2 dch=2 (cm), =1.25 0.1 0.5 1.1 t2 dch=2 (cm), ^=8 0.1 0.3 0.6 Range 0.1 <dchbT2chb optiona||y Q 5 <dchb^chb / ^Ch^h< 7 £chb / £ch £chb / £ch

[00365] The values in table 3 have been determined assuming that the predetermined number, a, of cathode cells is 20. The parameter values given in table 3 described herein are preferred parameter values and / or parameter value ranges. Examples can be realised in which wider parameter value ranges can be used, as described below with respect to the methods of operation and operating parameter spaces.

[00366] Figure 23 shows a graph 2300 of variations in stack assembly power density (W / cm3) 2302 with cell size (cm) 2304 for a number of plots at corresponding operating pressures at the inlets to the cells for two types of structure; namely, interdigitated electrolysers according to the examples described herein and planar electrolysers at 100 mbar pressure drops across the electrolyser cathodes and anodes at a current density of 0.3 A / cm2.

[00367] A first set of plots relate to planar electrolysers. The first set of plots comprises six plots. A first plot 2306 shows a rapid fall off in power density from almost 10 W / cm3 at about 0.5 cm cell size to almost 1.0 W / cm3 for cell sizes of about 10 cm to 14 cm and beyond for a planar electrolyser. A second plot 2308 shows a rapid fall off in power density from almost 15 W / cm3 at about 0.5 cm cell size to almost 2.5 W / cm3 for cell sizes at, and beyond, about 10 cm for a planar electrolyser. A third plot 2310 shows a rapid fall off in power density from almost 17 W / cm3 at about 0.5 cm cell size to almost 4 W / cm3 for cell sizes at, and beyond, about 10 cm for a planar electrolyser. A fourth plot 2312 shows a rapid fall off in power density from almost 18 W / cm3 at about 0.5 cm cell size to almost 4.5 W / cm3 for cell sizes at, and beyond, about 10 cm for a planar electrolyser. A fifth plot 2314 shows a rapid fall off in power density from almost 20 W / cm3 at about 0.5 cm cell size to almost 5 W / cm3 for cell sizes at, and beyond, about 10 cm for a planar electrolyser. A sixth plot 2316 shows a rapid fall off in power density from just over 20 W / cm3 at about 0.5 cm cell size to just over 5 W / cm3 for cell sizes at, and beyond, about 10 cm for a planar electrolyser.

[00368] A second set of plots relate to interdigitated electrolysers. The second set of plots comprises four plots 2318 to 2324. A first plot 2318 of the second set shows a rapid fall off in power density from almost 12-13 W / cm3 at about 0.5 cm cell size to <1 W / cm3 for cell sizes of about 10 cm to 14 cm and beyond for an interdigitated electrolyser. A second plot 2320 of the second set shows a rapid fall off in power density from almost 18 W / cm3 at about 0.5 cm cell size to <1 W / cm3 for cell sizes at, and beyond, about 10 cm for an interdigitated electrolyser. A third plot 2322 of the second set shows a rapid fall off in power density from almost 22 W / cm3 at about 0.5 cm cell size to <1 W / cm3 for cell sizes at, and beyond, about 10 cm for an interdigitated electrolyser. A fourth plot 2324 of the second set shows a rapid fall off in power density from almost 22.5 W / cm3 at about 0.5 cm cell size to <1 W / cm3 for cell sizes at, and beyond, about 10 cm for an interdigitated electrolyser.

[00369] The graph 2300 of figure 23 also depicts a number of regions of interest 2326 to 2330. A first region of interest 2326 is associated with the performance of the example electrolysers described herein. It can be appreciated that very high stack assembly power densities can be realised for a range of cells sizes spanning about 0.5 cm to 2 cm, preferably, 0.5 cm to 1 cm. A second region of interest 2328 is associated with the performance of currently known commercially available solid oxide electrolyser stacks. It can be appreciated that very low power densities are realised for a range of cells sizes from about 10 cm upwards. Therefore, the performance of examples described herein is significantly better than prior art solid oxide electrolysers, which are pursuing a strategy of improving performance such as, for example, improving power density, by operating large electrolysers (>10cm) at higher current densities. A third region of interest 2330 is associated with the second set of electrolysers and is associated with prior art Proton-Exchange Membrane (PEM) electrolysers. Again, although the power densities are marginally better than those associated with the prior art solid oxide electrolysers, the power densities are a small percentage of the power densities achievable with the example electrolysers described herein, as depicted by the first region of interest. It should be noted that current SOEC electrolysers are typically operated at current densities twice that shown, however the same trend with reduced cell size resulting in much greater power per cubic centimetre would be visible. It will be appreciated that the cell size described with reference to figure 23 is interchangeable with the cell width and depth described with reference to examples herein.

[00370] Referring to figure 24, there is shown a graph 2400 showing a set of plots of variation of thermal Biot number 2402 with cell size 2404 for a single cathode cell within an assembly operated over a range of pressures from 1 bar to 25 bar. The electrolyser assembly comprises a set of electrolyser stacks 20 cm high delivering 0.3 A / cm2. The set of electrolysers comprises a single cathode cell with sufficient fuel flowing over the inlet and outlet edges to supply a stack of cells that is 20cm in height.

[00371] The thermal Biot number (Bi) describes the ratio of convective heat transfer (at the surface of a body) to internal heat transfer by conduction (within the volume of the body) and is defined for the example of a cathode cell described herein by

[00372] Bi = ^de

[00373] where

[00374] h is the heat transfer coefficient at the inlet or outlet of a cathode cell (from gases flowing in the gas supply and outlet plenums or channels)

[00375] k is the effective thermal conductivity of the cathode cell (accounting for solid and gaseous portions)

[00376] de is the cathode cell depth in the reactant flow direction.

[00377] When the thermal Biot number (Bi) « 1 solid thermal conduction is dominant over the heat transfer processes at the cell inlet and outlet and the temperature variations within the cell are minimized.

[00378] The set of plots comprises six plots 2406 to 2416. All plots 2406 to 2416 of the set of plots show an initially rapidly increasing thermal Biot number with cell size that progressively becomes relatively insensitive to increasing cell size beyond a cell size of about 10 cm. Figure 24 shows a set of regions of interest or operation 2418 and 2420. A first region of interest or operation 2418 is associated with example electrolysers described and claimed herein. The thermal Biot numbers are substantially confined to between about 0.04 and 0.8 for corresponding cell sizes of 0.5 cm to 2 cm. A second region of interest or operation 2420 is an estimate of the performance associated with prior art solid oxide electrolysers. The respective thermal Biot numbers are substantially greater than 1 and are almost or are over 10 for prior art SOFC cell sizes of 10 cm and greater. It will be appreciated that the cell size described with reference to figure 24 is interchangeable with the cell depth described with reference to examples herein and a plot showing the same relationship for cell width would have similar characteristics. The dramatic reduction in Biot number afforded by the current invention manifests itself in stacks where thermal gradients within the stack are effectively shorted out by the cell materials allowing stack current loadings to be altered rapidly without generating significant stresses within the stack. This rapid load following capability has until now only been possible in low temperature electrolysers, particularly PEM based units which have been able to commercially exploit this feature as a saleable capability for grid support. It also confers advantages in being able to have an electrolyser respond to variable availability of renewable power resources such as wind and solar without or with reduced electricity storage requirement.

[00379] Referring to figure 25, there is shown a graph 2500 showing a set of plots of variation of thermal Peclet number, Pet, 2502 with cell size 2504 fora single cathode cell operated over a range of pressures from 1 bar to 25 bar and at 0.3 A / cm2.

[00380] The thermal Peclet number, Pet, describes the ratio of thermal convective transport to conductive transport in the direction of a gas flow. For the example of a permeable cathode cell described herein the thermal Peclet number is defined by

[00381] Pet =

[00382] where:

[00383] de is the cathode cell depth in the reactant flow direction.

[00384] u is the averaged gas velocity

[00385] p is the gas density

[00386] Cp is the gas specific heat capacity

[00387] k is the effective thermal conductivity of the permeable cathode cell (accounting for solid and gaseous portions)

[00388] When the thermal Peclet number Pet « 1, solid thermal conduction is dominant over thermal convection within the cell and the temperature variations within the cell are minimized.

[00389] The set of plots comprises six plots 2506 to 2516. All plots 2506 to 2516 of the set of plots show an initially rapidly increasing Peclet number with cell size that progressively becomes relatively insensitive to increasing cell size beyond a cell size of about 10 cm. Figure 25 shows a set of regions of interest or operation 2518 and 2520. A first region of interest or operation 2518 is associated with example electrolysers described and claimed herein. The Peclet numbers are substantially confined to between about 0.4 and 2 for corresponding cell sizes of 0.5 cm to 2 cm. A second region of interest 2520 is shown in figure 25 that is associated with prior art electrolysers. The trend in the art is to increase output by increasing the electrolyser size and, consequently, the Peclet number, as depicted by the arrow 2522. It will be appreciated that the cell size described with reference to figure 25 is interchangeable with the cell depth described with reference to examples herein and a plot showing the same relationship for cell width would have similar characteristics.

[00390] The dramatic reduction in thermal Peclet number afforded by the examples described and / or claimed herein manifests itself in stacks where thermal gradients within the stack are effectively shorted out by the cell materials allowing stack current loadings to be altered rapidly without generating significant stresses within the stack. This rapid load following capability has until now only been possible in low temperature electrolysers, particularly PEM based units which have been able to commercially exploit this feature as a saleable capability for grid support. The examples described and / or claimed herein, also confer advantages in being able to have an electrolyser respond to variable availability of renewable power resources such as wind and solar without or with reduced electricity storage requirement.

[00391] Figure 26A shows a graph 2600A of the variations in single electrolyser thickness (one cathode half-cell and one anode half-cell) (microns) 2602 with cell size (cm) 2604 for two sets of plots at respective pressure drops across the electrolysers; namely, 100 mbar and 1000 mbar at a current density of 0.3 A / cm2.

[00392] A first set of plots relate to a pressure drop of 100 mbar. The first set of plots comprises six plots. A first plot 2606 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 90 microns to 300 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 1 bar. A second plot 2608 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 70 microns to 190 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 5 bar. A third plot 2610 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 60 microns to 160 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 10 bar. A fourth plot 2612 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 58 microns to 140 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 15 bar. A fifth plot 2614 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 56 microns to 130 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 20 bar. A sixth plot 2616 shows a rapid rise in required single cell thickness to maintain a required pressure drop of 100 mbar across the electrolyser from about 54 microns to 125 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 25 bar.

[00393] A second set of plots relate to a pressure drop of 1000 mbar. The second set of plots comprises six plots 2618 to 2624, modelled using the same single electrolyser as the first set of plots. A first plot 2618 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 31 microns to 75 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 1 bar. A second plot 2620 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 25 microns to 55 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 5 bar. A third plot 2622 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 24 microns to 46 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 10 bar. A fourth plot 2624 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 24 microns to 45 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 15 bar. A fifth plot 2626 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 22 microns to 42 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 20 bar. A sixth plot 2628 of the second set of plots shows a very slow rise in required single cell thickness to maintain a required pressure drop of 1000 mbar across the electrolyser from about 22 microns to 40 microns as cell size increases over a range of 0.5 cm to 5 cm at a reactant supply flow pressure of 25 bar. Figure 26B depicts sets of data points associated with the graphs of figure 26A.

[00394] The graph 2600 of figure 26A also depicts a number of regions of interest 2630 to 2634.

[00395] A first region of interest 2630 is associated with the performance of the example electrolysers described herein. It can be appreciated that relatively low cell thicknesses can be used for cell sizes spanning about 0.5 cm to 2 cm, to meet pressure drop requirements without incurring unacceptable in-plane ohmic losses, which are described below for an interdigitated cell. It will be appreciated that the cell size described with reference to figure 26 is interchangeable with the cell width and depth described with reference to examples herein.

[00396] A second region of interest 2632 is associated with a region where the Ohmic losses associated with a single interdigitated cell having a respective single cell thickness and a respective cell size are tolerable or acceptable. The aforementioned ohmic losses are inversely proportional to the cell thickness (which reduces electrical resistance by increasing the in-plane cross-sectional area available to conduct electricity) and proportional to the cell size squared (which firstly acts to increase electrical resistance by increasing the in-plane conduction flow path length and secondly acts to increase the total magnitude of the in-plane current flow). Thus, increasing cell size requires a corresponding quadratic increase in cell thickness to maintain ohmic losses. As outlined above by the first region of interest 2630, the examples described herein are designed to operate in the part of the second region of interest 2632 where cell size is sufficiently small that ohmic loss considerations do not require cell thicknesses to be increased significantly beyond those required to meet pressure drop requirements.

[00397] A third region of interest or performance 2634 is associated with a region where the Ohmic losses associated with a single cell having a respective single cell thickness and a respective cell size are intolerable or unacceptable. The in-plane ohmic losses in the third region 2634 are too great to sustain acceptable performance for interdigitated cells such as those described herein.

[00398] The division between the second and third regions of interest or performance is defined by an Ohmic loss transition line 2636.

[00399] Referring to figure 27A, there is shown a view 2700A of a graph of the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2702 with cell size (cathode cell depth) 2704 for a cathode cell described herein in the absence of the inert permeable barrier. The modelling or simulation results depicted in figure 27A were derived for a single cathode cell with flow over the inlet and outlet edges sufficient for supplying a set of stacks 20cm high. The mass transfer coefficient h describes the enhanced diffusion that can occur at a cell or layer wall / interface due to concentration gradients and boundary layer effects. Therefore, in this example, u / h is used to describe the ratio of convective mass transport to diffusive mass transport at the entrance to a cathode cell in the absence of an inert permeable barrier and is similar to the mass Peclet number, Pem, defined for the inert permeable barrier itself. As mentioned earlier, it is desirable that this ratio u / h »1 such that convection is the dominant transport mechanism at the inlet to a cell.

[00400] A first set of plots is established for a current density of 0.3 A / cm2. The first set of plots comprises six 2706 to 2716.

[00401] A first plot 2706 depicts the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to a cathode cell with cell size at a pressure of 1 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to a cathode cell varies from circa 0.27 to 1.64. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 1 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to a cathode cell 2702 varies from 0.27 to circa 0.69 according to examples.

[00402] A second plot 2708 the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 5 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.47 to 2.80. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 5 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2708 varies from 0.47 to circa 1.18.

[00403] A third plot 2710 shows the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 15 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.67 to 4.04. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 15 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2710 varies from 0.67 to circa 1.71.

[00404] A fourth plot 2712 illustrates the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 10 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.59 to 3.53. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 10 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2712 varies from 0.59 to circa 1.49.

[00405] A fifth plot 2714 depicts the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 20 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.74 to 4.45. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 20 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2714 varies from 0.74 to circa 1.88.

[00406] A sixth plot 2716 illustrates the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 25 bar for a current density of 0.3A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.8 to 4.80. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 25 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2716 varies from 0.8 to circa 2.03.

[00407] A second set of plots is established for a current density of 0.6 A / cm2. The second set of plots comprises five 2718 to 2726.

[00408] A first plot 2718 depicts the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 1 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.55 to 3.28. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 1 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2718 varies from 0.55 to circa 1.39. Therefore, it will be appreciated that the ratio u / h is only an issue at small current densities and small cell sizes where the small value of the ratio u / h means that diffusive mass transport processes are significant and that an inert permeable barrier is required to have the desired effect of at least reducing, and, preferably, minimising diffusion effects and, thereby, facilitating a convectively dominant flow regime. This convective flow regime enables uniform supply of substantially identical mixtures across different cells and prevents position of a cell within the stack influencing the composition received when working with attractively small supply plenums.

[00409] A second plot 2720 the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 5 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 0.93 to 5.61. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 5 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2720 varies from 0.93 to circa 2.37.

[00410] A third plot 2722 shows the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 10 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 1.18 to 7.07. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 10 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2722 varies from 1.18 to circa 2.98.

[00411] A fourth plot 2724 illustrates the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 15 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 1.35 to 8.09. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 15 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2724 varies from 1.35 to circa 3.42.

[00412] A fifth plot 2726 depicts the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 20 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 1.48 to 8.90. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 20 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2726 varies from 1.48 to circa 3.76.

[00413] A sixth plot 2728 illustrates the variation in the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell with cell size at a pressure of 25 bar for a current density of 0.6A / cm2. It can be seen that the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell varies from circa 1.59 to 9.59. However, example cells size range from 0.5 cm to 2 cm for the electrolysers described and claimed herein. Therefore, at 25 bar, the ratio of convective velocity to mass transfer coefficient (u / h) at entrance to the cathode cell 2728 varies from 1.59 to circa 4.05.

[00414] Figure 27B depicts the graph of variation in the ratio of convective velocity to mass transfer coefficient (u / h) with cell size of Figure 27A together with graph end point values.

[00415] Although the above examples have been described with reference to using inks and dipping processes to deposit such inks, examples are not limited to such arrangements. Examples can be realised in which such layers are deposited or formed using some other techniques such as, for example, inkjet printing, additive manufacturing, spraying or some other deposition technique. For example, the inert permeable barrier can be deposited or created using ink jet printing technologies. Furthermore, although the above examples have been described with referencing to dipping the stack in inks to create the inert permeable barriers, examples can be realised in which the inert permeable barrier is fabricated separately from the remainder of the stack and assembled into the desired configuration. Therefore, a prefabricated permeable barrier such as, for example, barrier 802 can be used realise the above examples, subject to there being a gas-tight seal that prevents gas circumventing the inert permeable barrier. For example, if the inert permeable barrier is fabricated from a material having a high sintering temperature that could be problematical to one or more other aspects of a stack, such a barrier can be fabricated separately and assembled with the stack at a later stage.

[00416] Referring to figures 7 to 9, although the stacks have been described and shown with current collector layers deposited on the side of the stacks that have particular dimensions and positions, examples are not limited to such arrangements. Examples can be realised in which the positions and widths of the current collector layers deposited on the side of the stacks are different to those presently depicted. Varying at least one, or both, of the widths and positions of the anode current collectors and the cathode current collectors can be realised to influence the respective electrical resistances of those layers. For example, the current collector layers can be dimensioned to balance the resistances of those layers, or to achieve relative respective conductivities according to performance requirements. Examples can, therefore, be realised in which the cathode current collector layers are relatively elongate and centrally positioned on a face of the stack. Examples can be realised in which the current collectors have a width that is commensurate with the width of a side face of the stack, subject to current leakage or shorting to an adjacent layer at the corners of a stack. Therefore, examples can be realised in which the anode current collector has a width that is commensurate with the full width of the side face of the stack bearing the anode cell inlets and the cathode current collector is a centrally disposed significantly narrower width strip, which would allow the electrical resistances of the anode current collector and the cathode current collector to be balanced such that there was no bias towards the cathode or anode.

[00417]

[00418]

[00419]

[00420]

[00421]

[00422]

[00423]

[00424]

[00425]

[00426]

[00427]

[00428]

[00429]

[00430]

[00431]

[00432] Material Definitions The above examples can be realised using the following materials: YSZ Yttria Stabilized Zirconia ScSZ Scandia doped zirconia Ni Nickel LSM Lanthanum Strontium Manganate LSCF Lanthanum Strontium Cobalt Ferrite LSTNC Lanthanum doped Strontium Titanate NCAL Lithiated transition metal oxide MgAhCL Spinel AI2O3 Alumina Rh Rhodium BCY Barium Cerium-Yttria (Proton conductor) Pt Platinum

[00433] Pd Palladium

[00434] SDC Samarium doped ceria

[00435] GDC Gadolinium doped ceria

[00436]

[00437] Material Selections: Micro-stack

[00438] Component Material - Preferred Material - Alternative Cathode Permeable YSZ-Ni Permeable Pt-YSZ, Permeable Ni-NCAL, Permeable Ni-SDC Cathode Current Collector Permeable Ni Anode Permeable YSZ-LSM Permeable YSZ-LSCF, Permeable LSCF-GDC Anode Current Collector Permeable LSM Permeable LSCF Electrolyte Dense YSZ Dense SDC Dense ScSZ Dense SDC / NCAL Chemical Heating Catalyst Permeable AhOa-Ni Permeable AI2O3-Rh Permeable or Non-permeable Conductor Permeable Ni Interconnect Pt / Pd Permeable Barrier Permeable MgAI2O4 Fuel processing catalyst for anode assist (optional) H2 Fuel: N / A NG Fuel: Permeable AI2O3-Rh NH3 Fuel: Permeable AI2O3-Ni Fuel polishing catalyst for anode assist (optional) H2 Fuel: N / A NG Fuel: Permeable AI2O3-Rh NH3 Fuel: Permeable BCY Dense Insulator Dense MgAI2O4 (possibly laminated with YSZ to prevent reactions with Ni) Pipes Permeable MgAI2O4 (sealed with glass to prevent leaks) Dense Barrier Dense MgAI2O4

[00439]

[00440] Although the above conductor has been listed as being permeable, examples are not limited to such an arrangement. Examples can be realised in which the conductor is not permeable such as, for example, a solid conductor.

[00441] Accordingly, the examples described herein provide solid oxide electrolysers that are fabricated using solid oxide materials or using materials that become solid oxide materials. Therefore, the multi-layered structures and materials, multi-layered electrolysers, cells, electrodes, anodes, half-cells, stacks, stack assemblies, heating cells, described and claimed comprise, or are, solid oxide multi-layered structures and materials, multi-layered electrolysers, cells, electrodes, anodes, half-cells, stacks, stack assemblies and heating cells.

[00442] The above examples, and the claims below, refer to a reactant. Nevertheless, the term reactant can cover both fuel species and sweep gas species. The term sweep gas is used to describe a flow that dilutes evolved oxygen to avoid build-up of excessive oxygen concentrations that are harmful to materials and detrimental to overall safety engineering of any surrounding system. To simplify descriptions, the term reactant can also comprise a dilutant and includes the use of sweep gases on the anode intended to reduce the concentration of evolved oxygen.

[00443] Although the examples of anode cells have been described without reference to having an accompanying inert permeable barrier layer, examples are not limited to such arrangements. Examples can be realised in which the anode cells have one or more than one inert permeable anode cell barrier. The one or more than one inert permeable anode cell barrier can be disposed over, or positioned in relation to, at least one, or both, of: a permeable reactant pathway inlet or a permeable reactant pathway outlet. Examples of such a permeable reactant pathway inlet or a permeable reactant pathway outlet are the above-described permeable reactant pathway inlet 304.16 and permeable reactant pathway outlet 304.24. The inert permeable anode cell barriers can have the dimensions and characteristics of any of the inert permeable barriers described herein.

[00444] Although examples have been described comprising at least one or more than one inert permeable barrier, examples are not limited to such arrangements. Examples can be realised in which one or more of the various elements of the electrolyser do not have an associated set of inert permeable barriers; such an associated set of inert permeable barriers comprises one inert permeable barrier or multiple inert permeable barriers. For instance, examples can be realised in which at least one, or more than one, of the following, taken jointly and severally in all permutations, do not have such a respective set of inert permeable barriers:

[00445] - the cathode cell,

[00446] - the anode cell, and

[00447] - the chemical heating cell.

[00448] An optional fuel polishing catalyst layer may be deposited over the inlets to the anode cells. Furthermore, a fuel processing catalyst layer may be deposited over the optional fuel polishing layer and upstream in flow from the polishing layer. The fuel polishing layer and the fuel processing catalyst layer reduce, and preferably eliminate, adverse effects within the cells such as, for example, nitriding or carbon deposition within at least one, or both, of: the anode electrodes or anode current collectors when the anode is fed with a gas capable of electrochemically reacting with oxygen ions being transported to the anode so that the voltage required to operate the electrolyser is lowered and free oxygen gas is no longer evolved at the anode. This method of operating an electrolyser is described as ‘anode assist’. For example, the fuel polishing layer and the fuel processing catalyst layer are arranged to ensure that a reactant, such as, for example, ammonia, is fully decomposed into hydrogen and nitrogen prior to entering the anodes. Another example would be where hydrocarbon rich tail gas from a fuel synthesis reactor is fed to the anode and the fuel processing and polishing layer perform pre-reforming or steam reforming functions that remove species likely to harm the anode such as hydrocarbon species containing carbon-carbon bonds that have much greater predisposition to form carbon on anodes containing Nickel.

[00449] Examples can be realised in which an inert permeable barrier layer, similar in function to that previously described at the entrance to the cathode, can be deposited over at least one, or both where present, of: the fuel polishing catalyst layer and the fuel processing catalyst layer so that diffusion of reaction products from the electrode, fuel processing layer and fuel polishing layer into the inlet manifold supplying these components is reduced or substantially eliminated and a convectively dominated flow regime is maintained in the way that flow splits between the adjacent anode cells with equal flow and identical composition.

[00450] It is noted that the reactant supplied to the cathode side of the electrolyser is referred to as fuel although there may be cases where on inlet there is no fuel in the mixture, only water and / or carbon dioxide and fuel is then formed as the flow passes through the cathode, however, for reasons of material compatibility and to ensure stable electrochemistry, examples can be realised in which at least a small proportion of at least one, or both, of: hydrogen or carbon monoxide will be present in the gas flow supplied to the cathode. Although examples have been realised that use chemical heating, that is, heating due to chemical reactions, as a source of heat, examples are not limited thereto. Examples can be realised in which an electrical conductor is operated as an electrical heater in response to an applied current and voltage. Therefore, for instance, examples can be realised in which heating is provided by applying an electrical current to an electrical conductor as an alternative to the chemical heating cell permeable heating catalyst layer 306.4.

[00451] Although the above examples have been described with reference to using inks or dips in the manufacturing process, examples are not limited to such arrangements. Examples can be realised in which one or more than one feature or layer of the cells and multi-layered structures described herein are manufactured in some other way such as, for example, using additive manufacturing techniques like 3D printing.

[00452] The examples have been described with reference to cells having layers and, in particular, with reference to cells comprising a layer or layers; each of which is formed as a single or unitary layer such as, for example, the methanation catalyst layer, an electrolyte layer, a dense layer, an insulating layer, the electrical conductors, and the like. However, examples are not limited to such single or unitary layers. Examples can be realised in which a layer is realised using a plurality of layers. The plurality of layers can comprise two or more layers. The benefit of using such a plurality of layers is that a layer can develop a pin hole or through vias that effectively provides a short circuit or direct path for a gas species or current. A way to mitigate against adverse effects of such unintended pins holes or through vias is to use multiple layers since the probability that such multiple layers comprise pin holes or through vias that are aligned with pinholes or through vias of an immediately adjacent layer is so small as to be almost zero.

[00453] In the examples described herein, layers that do not have a defined thickness range for hosting a permeable pathway, such as, for example, electrolyte layers, cathode electrodes, anode electrodes, dense layer, and sealing inks or dips, taken jointly and severally in any and all permutations, are relatively thin so as not to unnecessarily increase unneeded volume or bulk. Examples can be realised in which such relatively thin layers have a thickness of 20 / im or less, preferably, 10 / im or less. Current collectors deposited on the outside of the stack could, however, be thicker such as, for example, being 500 / im or less such as, for example, being 100 pm or less.

[00454] The thicknesses of the cathode cell and anode cell are governed by their operating current density, which dictates the magnitude of the in-plane gas and current flows. The cells are designed to balance being sufficiently thick to provide sufficient sheet electrical conductance to reduce ohmic losses with having a large enough pore size to reduce pressure drops whilst also being thin enough to ensure a high power density.

[00455] The thickness of the chemical heating cells is governed by similar requirements to above. However, the magnitude of the gas and current flows associated with the chemical heating cell operation depends on how many electrolysers are grouped together in a stack. Within a stack, the cells are electrically connected in parallel. Therefore, increasing the number of electrolysers increases the overall current flow, that is, current varies as a function of the number of electrolysers connected in parallel. The stacks are connected together in series. Therefore, increasing the number of stacks, increases the overall voltage, that is, voltage varies with the number of stacks connected in series.

[00456] Historically depth in the flow direction for electrolysers and compact reactors such as reformers has been kept high to allow manufacturers to make larger areas of electrolyser in a single operation and at times to increase flow rates to improve reactant supply and mass transfer at interfaces.

[00457] Large depth in the flow direction, however, creates both the need for more reactant supply as a larger area is being serviced as well as increasing the distance over which wall friction must be overcome. Consequently, extending depth in the flow direction has a significant impact on the need for passage thickness to avoid excessive pressure drop. Increased thickness directly reduces the number of active surface areas that can be packed into a unit volume and consequently developers have sought to increase current density within the cell to compensate for lost packing density. Such an approach, however, exacerbates the problem as increasing current density further increases the need for reactant supply and lowers efficiency with a consequent increase in the amount of heat that must be supplied. In the examples described herein, the surface is defined as, or comprises, the interior surface presented by pores within permeable materials

[00458] The examples described and claimed here take a completely opposite approach by giving effect to at least one, or more, of the following taken jointly and severally in any and all permutations:

[00459] - depth in the flow direction being reduced,

[00460] - thickness of flow passages being reduced, that is, lowered, and

[00461] - allowing tighter packing of electrolyser active areas.

[00462] The foregoing alleviates the need for high current densities and this, in turn, improves stack and system efficiency allowing endothermic operation in which waste heat from external processes can be converted to chemical energy in lieu of some of the power that would otherwise be needed. The lower current densities improve choice of materials and electrode durability. The examples described and claimed herein are counter to the technological direction of the art.

[00463] A still further benefit of reducing depth in flow direction in electrolysers is in plane current collection, which encounters greater resistances when cell depth is increased that, in turn, decreases voltage and efficiency.

[00464] It will be appreciated that the various entities of the examples described above are symmetrical. For example, the cells are symmetrical. Also, the stacks are symmetrical, as well as the arrangements of stacks being symmetrical. For example, referring to any and all of the above figures, there are multiple lines of symmetry, which have been marked using a dashed line together with SS or SS with a number of superscripts such as, for example, S1S1, S1S2, S3S3, etc. It will be appreciated that only some of the symmetries have been identified to preserve the clarity of the drawings. Other symmetries are self-evident even though those additional symmetries have not been specifically identified.

[00465] The symmetrical nature of various aspects of the examples facilitates manufacturing by reducing the tendency of the layers or structures tending to bend or otherwise deform in the absence of symmetries. Furthermore, the layers can be fabricated using carefully matched co-sinterable materials that will also contribute to reducing stresses and strains within the layers and structures to at least reduce, or preferably, prevent, the tendency to bend.

[00466] Still further, it will be appreciated that the diagrams are schematical for the purposes of illustrations. For example, while figure 9 depicts a ‘stack’, the dimensions of the layers involved are such that a stack might have a width of 1cm and a depth of 1cm and have a height of 2mm, even in examples that use multiple layers such as, for example 20 layers.

[00467] Referring, for example, to figure 10, it can be appreciated that the channels such as, for example, the chemical heating supply flow channels 126 / 1012 and heating outlet flow channels 128.1 / 1014 to 128.2 / 1016 are realised as external manifolds, that is, the manifolds are external to the electrolysers and electrolyser stacks. However, examples are not limited to such arrangements. Examples can be realised in which such manifolds are internal manifolds, that is, the manifolds are formed within, or as an integral part of, the electrolyser or electrolyser stack. Any of the above-described supply and outlet channels are examples of manifolds. Accordingly, example multi-layered structures, cells, stacks, half-cells, arrays, and system comprise at least one, or both, of: one or more than one external manifold and one or more than one internal manifold. Examples can be realised that use a combination of: one or more than one external manifold and one or more than one internal manifold.

[00468]

[00469] Methods of operation and operating parameter spaces

[00470]

[00471] The above-described cells, structures and systems can be operated across, or within, a defined operating space comprising a plurality of variables. There now follows a description of the operating parameters for each of the foregoing.

[00472] Cathode cells

[00473] Referring to the cathode cells, examples can be realised that provide a method of operating an electrolyser, the method comprising

[00474] maintaining an operating environment within the electrolyser having an operating parameter space defined by:

[00475] — = c, L J l T 32(1R

[00476] where

[00477] p is the absolute pressure,

[00478] Ap is pressure drop across the electrolyser (in the reactant flow direction)

[00479] F is Faraday’s constant,

[00480] Uf is a utilisation factor indicating the proportion of the supplied reactant consumed by the electrolyser,

[00481] R is the gas constant,

[00482] T is the temperature,

[00483] p is the reactant gas dynamic viscosity,

[00484] i is the current density (^), and

[00485] c is a constant.

[00486] The operating parameter space of the foregoing method of operation can be defined by, or comprises, any one or more than one of the following taken jointly and severally in any and all permutations:

[00487] c has a value in the range 5e6 / m to 5e13 / m, optionally, 5e8 / m to 5e12 / m and, preferably 1e10 / m to 1e12 / m,

[00488] p , the absolute pressure, has a range of 1 bar to 50 bar, preferably 5 bar to 25 bar,

[00489] Ap , the pressure drop, has a range of 10 mbar to 1000 mbar, preferably 20mbar to 300mbar,

[00490] , the utilisation factor, has a value in the range of 0.1 to 1.0, preferably 0.33 to 0.95,

[00491] T , the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C,

[00492] p the reactant gas dynamic viscosity, has a value in the range 5e-5 Pa.s to 1e-5 Pa.s, preferably, 4e-5 Pa.s to 2e-5 Pa.s,

[00493] i, the current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2.

[00494] Examples can be realised in which the above methods of operation, and operating parameter space, are related to the geometric parameters in which = c = where the geometric parameters comprise:

[00495] Te is the permeable cathode reactant pathway tortuosity in the reactant flow direction,

[00496] se is permeable cathode reactant pathway structure porosity,

[00497] de is the permeable cathode reactant pathway structure depth in the reactant flow direction,

[00498] te is the permeable cathode reactant pathway structure thickness,

[00499] fe is the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te.

[00500] Examples can be realised in which one or more than one of the above geometric parameters are defined by the following taken jointly and severally in any and all permutations:

[00501] Te, the permeable cathode reactant pathway tortuosity in the reactant flow direction, has a range of 1 <te <3, optionally, 1 <ve <2.5, and, preferably, 1 <Te <2,

[00502] se, the permeable cathode reactant pathway structure porosity, has a range of 0.1 <Ee <0.9 and preferably 0.5< Ee <0.8,

[00503] de, the permeable cathode reactant pathway structure depth in the reactant flow direction, has a range of 0.25< de <40 (mm), optionally, 0.25< de <20 (mm), and, preferably 5< de <20 (mm),

[00504] te, the permeable cathode reactant pathway structure thickness, has a range of 5 <te <1000 (p.m), optionally, 5 <te <500 (p.m) and, preferably, 10 <te <150 (p.m),

[00505] fe, the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te, has a range of 0.02<fe <1.0, and, preferably, 0.25<fe <0.75, and / or

[00506] in which p^pFUf = c = — -^-(m'1) has a range of 5e6 / m to 5e13 / m, optionally, V'3 2jtz l 5e8 / m to 5e12 / m and, preferably 1e10 / m to 1e12 / m .

[00507] The foregoing methods of operation can additionally comprise maintaining an operating environment within an inert permeable barrier having a parameter space defined by

[00508] f = C1

[00509] where

[00510] Pem is the mass Peclet number,

[00511] F is Faraday’s constant,

[00512] p is the absolute pressure,

[00513] D is a mass diffusion coefficient of a reactant gas species,

[00514] Uf is the utilisation factor indicating the proportion of the supplied reactant consumed by the electrolyser,

[00515] R is the gas constant,

[00516] T is the temperature, and

[00517] i is the current density (^).

[00518] The expression PemFpDUf _ C1 describes the effectiveness of the barrier to achieve a desired mass Peclet number.

[00519] Examples can be realised which the parameter space for maintaining an operating environment within an inert permeable barrier comprises one or more than one of the following taken jointly and severally in any and all permutations:

[00520] = ^(m), has a range of 0.2 m to 1000 m, optionally, 1 m to 200 m and, preferably, 10 m to 100 m,

[00521] Pem, the mass Peclet number, has a value in the range of 2 to 500, preferably, 5 to 50,

[00522] p , the absolute pressure, has a value of in the range of 1 bar to 50 bar, preferably, 5 bar to 25 bar,

[00523] D, the mass diffusion coefficient of a reactant gas species, has a value in the range of 1e-3 m2 / s to 1e-4 m2 / s, preferably 8e-4 m2 / s to 1e-4 m2 / s,

[00524] Uf, the utilisation factor, has a value in the range of 0.1 to 1.0, preferably 0.33 to 0.95,

[00525] T, the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C,

[00526] i, the current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2,

[00527] Examples can be realised in which the above methods of operation, and operating parameter space, associated with the inert permeable barrier are related to geometric parameters . P&mFpDUf dbdeTi . . , by-----;—J- = q = b (m), wherein

[00528] db is the inert permeable barrier depth in the reactant flow direction,

[00529] zb is the inert permeable barrier reactant pathway tortuosity in the reactant flow direction,

[00530] sb is the inert permeable barrier porosity,

[00531] de is the permeable cathode reactant pathway structure depth in the reactant flow direction, and

[00532] te is the permeable cathode reactant pathway structure thickness.

[00533] Examples can be realised in which the geometric parameters associated with the inert permeable barrier are defined by one or more than one of the following taken jointly and severally in any and all permutations:

[00534] db, the inert permeable barrier depth in the reactant flow direction, has a value in the range of db>0.01 (mm), optionally, 0.01 <db<5 (mm) and, preferably, 0.1 <db<2 (mm).,

[00535] zb, the inert permeable barrier reactant pathway tortuosity in the reactant flow direction, has a value in the range of 1 <zb< 10, optionally, 1 <tb <5

[00536] Eb, the inert permeable barrier porosity, has a value in the range of 0.01 <sb <0.5, preferably, 0.05< Eb <0.5,

[00537] de, the permeable cathode reactant pathway structure depth in the reactant flow direction, has a range of 0.25< de <40 (mm), optionally, 0.25< de <20 (mm), and, preferably 5< de <20 (mm),

[00538] te, the permeable cathode reactant pathway structure thickness, has a value in the range of 5 <te <1000 (p.m), optionally, 5 <te <500 (p.m) and, preferably, 10 <te <150 (p.m).

[00539] As indicated above, the parameter values given in Table 1: Cathode Cell Parameters described herein are preferred parameter values and / or parameter value ranges. However, examples can be realised in which the above-described wider parameter value ranges can be used jointly and severally in any and all permutations.

[00540] Anode cells

[00541] Referring to the anode cells, examples provide a method of operating an electrolyser, the method comprising maintaining an operating environment within the electrolyser having an operating parameter space defined by: [005421 —- F = c L J l T2.5R32H ca

[00543] where

[00544] p is the absolute pressure,

[00545] is the pressure drop across the electrolyser (in the sweep gas flow direction)

[00546] F is Faraday’s constant,

[00547] Ua is a utilisation factor proportional to the concentration of oxygen evolved into the sweep gas on exit and scaled so that it has a predetermined value of, for example, value 1 when evolved oxygen reaches a predetermined threshold such as, for example, 20% ,

[00548] R is the gas constant,

[00549] T is the temperature,

[00550] p is the sweep gas dynamic viscosity,

[00551] i is the current density (^), and

[00552] cca is a constant.

[00553] Anode operating parameters

[00554] The operating parameter space of the foregoing method of operation can be defined by, or comprises, any one or more than one of the following taken jointly and severally in any and all permutations:

[00555] cca has a value in the range 1e6 / m to 5e12 / m, optionally, 1e8 / m to 5e11 / m and, preferably 1e9 / m to 2e11 / m,

[00556] p , the absolute pressure, has a range of 1 bar to 50 bar, preferably 5 bar to 25 bar,

[00557] , the pressure drop, has a range of 10 mbar to 1000 mbar, preferably 20mbar to 300mbar,

[00558] Ua , the utilization factor, has a value in the range of 0.1 to 1.0, preferably 0.25 to 0.75,

[00559] T the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C,

[00560] p, the sweep gas dynamic viscosity, has a value in the range 5e-5 Pa.s to 1e-5 Pa.s, preferably, 5e-5 Pa.s to 4e-5 Pa.s,

[00561] i the current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2.

[00562] Examples can be realised in which the above methods of operation, and operating parameter space, are related to anode geometric parameters in which —-— = Cca = r r ar ; 7 2.5 / 532^ ca 2 2 777^), where Jc Lc

[00563] tc is the permeable anode reactant pathway tortuosity in the sweep gas flow direction,

[00564] sc is permeable anode reactant pathway structure porosity,

[00565] wc is the permeable anode reactant pathway structure width in the sweep gas flow direction,

[00566] tc is the permeable anode reactant pathway structure thickness,

[00567] fc is the ratio of permeable anode reactant pathway pore size to permeable anode reactant pathway structure thickness, tc.

[00568] Examples can be realised in which one or more than one of the above anode geometric parameters are defined by the following taken jointly and severally in any and all permutations:

[00569] tc, the permeable anode reactant pathway tortuosity in the sweep gas flow direction, has a range of 1< tc <3, optionally, 1 <tc <2.5, and, preferably, 1 <tc <2,

[00570] sc, the permeable anode reactant pathway structure porosity, has a range of 0.1 <£c <0.9 and preferably 0.5< £c <0.8,

[00571] wc, the permeable anode reactant pathway structure width in the sweep gas flow direction, has a range of 0.25< wc <40 (mm), optionally, 0.25< wc <20 (mm), and, preferably 5< wc <20 (mm),

[00572] tc, the permeable anode reactant pathway structure thickness, has a range of 10 < tc <1000 (p.m), optionally, 10 <tc <600 (p.m) and, preferably, 20 <tc <250 (p.m), and / or

[00573] fc, the ratio of permeable anode reactant pathway pore size to permeable anode reactant pathway structure thickness, tc, has a range of 0.02 <fc<1.0, and, preferably, 0.25 <fc <0.75.

[00574] As indicated above, the parameter values given in table 2 described herein are preferred parameter values and / or parameter value ranges. However, examples can be realised in which the above-described wider parameter value ranges can be used jointly and severally in any and all permutations.

[00575] Chemical Heating Cell

[00576] Referring to the chemical heating cells, examples can be realised in that provide a method of operating a chemical heating cell, the method comprising maintaining an operating environment within the chemical heating cell having an operating parameter space defined by:

[00577] ^pUh F =cch, L J T i aR32(i cn

[00578] where

[00579] p is the absolute pressure,

[00580] Ap is pressure drop across the chemical heating cell in the reactant flow direction

[00581] F is Faraday’s constant,

[00582] Uh is a utilisation factor describing the supplied flow rate of reactant (chemical heating) as a proportion of the electrolyser stack fuel side supply (where Uh>1 indicates a reactant undersupply when a low level of heating is required or the associated electrolyser stack is less than the predetermined number of cathode cells), the electrolyser stack fuel side supply can be associated with a predetermined number, a, of cathode cells such as, for example, 20 cathode cells,

[00583] R is the gas constant,

[00584] T is the temperature,

[00585] p is the reactant gas dynamic viscosity,

[00586] i is the adjacent electrolyser stack current density (^),

[00587] cch is a constant and

[00588] a is the number of cathode cells to which the heating cell provides heat.

[00589] The operating parameter space of the foregoing method of operating a chemical heating cell can be defined by, or comprises, any one or more than one of the following taken jointly and severally in any and all permutations:

[00590] cch has a value in the range 1e6 / m to 5e12 / m, optionally, 1e8 / m to 5e11 / m and, preferably 1e9 / m to 1e11 / m scaled according to a,

[00591] p , the absolute pressure, has a range of 1 bar to 50 bar, preferably 5 bar to 25 bar,

[00592] Ap ,the pressure drop, has a range of 10 mbarto 1000 mbar, preferably 20mbar to 300mbar,

[00593] Uh ,the utilisation factor, has a value in the range of 0.5 to 10, preferably 1 to 5,

[00594] T ,the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C,

[00595] p , the chemical heating gas dynamic viscosity, has a value in the range 5e-5 Pa.s to 1e-5 Pa.s, preferably, 4e-5 Pa.s to 2e-5 Pa.s, and / or

[00596] i ,the adjacent electrolyser stack current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2.

[00597] Examples can be realised in which the above methods of operating a chemical heating cell, and chemical heating cell operating parameter space, are related to geometric parameters of the chemical heating cell in which ^pUh F— = Tch ^ch (m~1) where T i aR32n sch f^ch

[00598] zch is the permeable chemical heating catalyst layer pathway tortuosity in the reactant flow direction,

[00599] sch is permeable chemical heating catalyst layer porosity,

[00600] dch is the permeable chemical heating catalyst layer depth in the reactant flow direction,

[00601] tch is the permeable chemical heating catalyst layer thickness,

[00602] fch is the ratio of permeable chemical heating catalyst layer pathway pore size to permeable chemical heating catalyst layer thickness, tch, and

[00603] a is the number of cathode cells to which the heating cell provides heat.

[00604] Examples can be realised in which one or more than one of the above chemical heating cell geometric parameters are defined by the following taken jointly and severally in any and all permutations:

[00605] zch, the permeable chemical heating catalyst layer pathway tortuosity in the reactant flow direction, has a range of 1< vch <3, optionally, 1 <vch <2.5, and, preferably, 1 <vch <2,

[00606] sch, the permeable chemical heating catalyst layer porosity, has a range of 0.1< £cfl <0.9 and preferably 0.5< £cfl <0.8,

[00607] dch, the permeable chemical heating catalyst layer depth in the reactant flow direction, has a range of 0.25< dch<40 (mm), optionally, 0.25< dch<2Q (mm), and, preferably 5< dcfl <20 (mm),

[00608] tch, the permeable chemical heating catalyst layer thickness, has a range of 5 <tch <1000 (p.m), optionally 5 <tch <700 (p.m), and, preferably 10 <tch <300 (pm),

[00609] fch, the ratio of permeable chemical heating catalyst layer pathway pore size to permeable chemical heating catalyst layer thickness, tch, has a range of 0.02< fch <1.0, and, preferably, 0.25<fcfl <0.75, and / or

[00610] —£— = cch = Tch (nr1) has a range of 1e6 / m to 5e12 / m, optionally, T i aRUp. £chfchtch 1e8 / m to 5e11 / m and, preferably 1e9 / m to 1e11 / m.

[00611] The foregoing methods of operating a chemical heating cell can additionally comprise: maintaining an operating environment within a chemical heating cell inert permeable barrier having a chemical heating cell parameter space defined by

[00612] Cchb

[00613] where

[00614] Pem is the mass Peclet number,

[00615] F is Faraday’s constant,

[00616] p is the absolute pressure,

[00617] D is a mass diffusion coefficient of a reactant gas species,

[00618] Uh is a utilisation factor describing the supplied flow rate of reactant (chemical heating) as a proportion of the electrolyser stack fuel supply,

[00619] R is the gas constant,

[00620] T is the temperature,

[00621] i is the adjacent electrolyser stack current density (^), and

[00622] a is the number of cathode cells to which the heating cell provides heat.

[00623] Examples can be realised which the chemical heating cell inert permeable barrier parameter space for maintaining an operating environment within the chemical heating cell inert permeable barrier comprises one or more than one of the following taken jointly and severally in any and all permutations:

[00624] PemaRTiUh = Cchb(m)> has a range of 0.1 m to 250 m, optionally, 0.5 m to 50 m and, preferably, 1 m to 25 m,

[00625] Pem, the mass Peclet number, has a value in the range of 2 to 500, preferably, 5 to 50,

[00626] p , the absolute pressure, has a value of in the range of 1 bar to 50 bar, preferably, 5 bar to 25 bar,

[00627] D, the mass diffusion coefficient of a reactant gas species, has a value in the range of 1e-3 m2 / s to 1e-4 m2 / s, preferably 8e-4 m2 / s to 1e-4 m2 / s,

[00628] Uh, the utilisation factor, has a value in the range of 0.5 to 10, preferably 1 to 5,

[00629] T, the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C,

[00630] i, the adjacent electrolyser stack current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2, and a is the number of cathode cells to which the heating cell provides heat

[00631] Examples can be realised in which the above methods of operating a chemical heating cell inert permeable barrier, and chemical heating cell inert permeable barrier operating parameter space, associated with the chemical heating cell inert permeable barrier are related to geometric parameters by

[00632] ^mFpDUh = = dchbdch^chb wherein L J aRTi cM} tchEchb V

[00633] dchb is the chemical heating cell inert permeable barrier depth in the reactant flow direction,

[00634] zchb is the chemical heating cell inert permeable barrier pathway tortuosity in the reactant flow direction,

[00635] schb is the chemical heating cell inert permeable barrier porosity,

[00636] dch is the permeable chemical heating catalyst layer depth in the reactant flow direction, and

[00637] tch is the permeable chemical heating catalyst layer thickness,

[00638] a is the number of cathode cells to which the heating cell provides heat.

[00639] Examples can be realised in which the chemical heating cell inert permeable barrier geometric parameters associated with the chemical heating cell inert permeable barrier are defined by one or more than one of the following taken jointly and severally in any and all permutations:

[00640] dchb, the inert permeable barrier depth in the reactant flow direction, has a value in the range of dchb >0.01 (mm), optionally, 0.01 <dchb <5 (mm) and, preferably, 0.1 <dchb <2 (mm).

[00641] zchb, the inert permeable barrier pathway tortuosity in the reactant flow direction, has a value in the range of 1 <vchb <10, optionally, 1 <vchb <5,

[00642] schb , the inert permeable barrier porosity, has a value in the range of 0.01 <£chb <0.5, preferably, 0.05 <£chb <0.5,

[00643] dch, the permeable chemical heating catalyst layer depth in the reactant flow direction, has a value in the range of 0.25< dch <40 (mm), optionally, 0.25< dch <20 (mm), and, preferably 5< dch <20 (mm),

[00644] tch, the permeable chemical heating catalyst layer thickness, has a value in the range of 5 <tch <1000 (p.m), optionally 5 <tch <700 (p.m), and, preferably 10 <tch <300 (p.m).

[00645] As indicated above, the parameter values given in Table 3: Chemical Heating Cell Parameters described herein are preferred parameter values and / or parameter value ranges. However, examples can be realised in which the above-described wider parameter value ranges can be used jointly and severally in any and all permutations.

[00646] Examples can be realised in which a stack could be built with the inert barrier layer being formed from other components of the stack such as, for example, the walls of a partially sealed porous tube forming part of the flow distribution network, which could also serve as a structural support.

[00647] Examples could also be created in which there is no porous barrier on the fuel side of the cells. However, it should be borne in mind that the electrolysers described and claimed herein operate at a scale where diffusion is significant so that ensuring uniformity of supply from cell to cell requires more than just consideration of pressures and geometry. Compositions and diffusional effects must also be considered. Examples with no porous barrier layer would suffer a significant compromise in performance arising from variations in composition due to significant diffusion between the cells and the supply manifold even where pressure drops and overall gas flow rates are uniform.

[00648] For examples of stack that have a cracking or reforming reaction ahead of the electrolyser electrochemistry, for example if using ammonia or hydrocarbon fuels for anode assist, there would be significant diffusion of reactant into the cell and product out of the cell and into the manifold supplying the cells and this would differ between cells close to fuel supply and those further away, which would cause significant variations in the at least one or more of: supplied fuel composition, limiting cell performance and increasing temperature variations taken jointly and severally in any and all permutations.

[00649] Even in stacks where there are no additional reactions beyond the cathode electrochemical reactions within the cathode cells, the absence of a porous barrier layer would allow products of electrochemical reactions within the current collector and fuel electrodes to diffuse significantly back into the manifolds feeding the cells. Since these manifolds feed the cells sequentially, there will be significant variations in fuel composition at the inlet to each cell even if there was excellent uniformity of at least one of: current density, catalytic material activity and channel dimensions cell to cell taken jointly and severally in any and all permutations. This would make it very challenging to accurately match current density to fuel supply to allow maximisation of inlet steam utilisation. However, such sub-optimal examples without the inert permeable barrier could nevertheless be realised subject to accepting a reduction in achievable efficiency, power density and more challenging stress distributions.

Claims

1. A multi-layered structure for an electrolyser comprising:a. an electrolyte;b. a permeable electrode;c. the permeable electrode and electrolyte having a common interface to form a reaction region;the permeable electrode providing a permeable electrode reactant pathway through the electrode to present a reactant at the common interface; the permeable electrode reactant pathway being fed by a permeable reactant pathway hosted by a permeable reactant pathway structure, in whichd. the permeable reactant pathway and the permeable reactant pathway structure are related, or defined, byt2 d2i. —7 / 5 = c, which relates the geometric parameters of the permeable £p fp tpreactant pathway and its associated pathway structure to the electrolyser operating parameters,e. wherei. tp is the permeable reactant pathway tortuosity in the reactant flow direction,ii. sp is the permeable reactant pathway structure porosity,iii. dp is the permeable reactant pathway structure depth in the reactant flow direction,iv. tp is the permeable reactant pathway structure thickness,v. fp is the ratio of permeable reactant pathway pore size to permeable reactant pathway structure thickness, tp, andvi. c is a constant defined by the electrolyser operating parameters.

2. The multi-layered structure of claim 1, in which the permeable reactant pathway structure thickness, tp, has a predetermined thickness arranged to balance cell packing density, or power density, following from a relatively smaller permeable reactant pathway structure thickness, and in-plane electrical sheet conductance, following from a relatively larger permeable reactant pathway structure thickness, to reduce resistive losses, and / orin which the electrode is a cathode electrode and in which the permeable reactant pathway structure thickness, tp, has a range of 5 / mi to 1000 i^m, optionally, 5 / im to 500 [im and, preferably, 10 / im to 150 pm.T23. The multi-layered structure of any preceding claim, in which the ratio -75- is arranged to£pjpinfluence the balance between an additional flow resistance provided by the permeable reactant pathway while providing structural strength and in-plane electrical sheet conductance to balance resistive losses, and / orT2in which the electrode is a cathode electrode and in which the ratio —77 has a range of 1 £pfpto 500, optionally, 2 to 50.

4. The multi-layered structure of any preceding claim, in which the permeable reactant pathway structure depth, dp, is selected to balance cell handling capabilities and power density, and / orin which the electrode is a cathode electrode and in which the permeable reactant pathway structure depth, dp, has a range of 0.25 mm to 40 mm, optionally, 0.25 mm to 20 mm and, preferably, of 5 mm to 20 mm.

5. The multi-layered structure of any preceding claim, in which the ratio, fp, of permeable reactant pathway pore size to permeable reactant pathway structure thickness, tp, is selected to balance flow resistance with providing structural strength and in-plane electrical sheet conductance to influence or constrain electrical resistive losses, and / orin which the electrode is a cathode electrode and the ratio, fp, of permeable reactant pathway pore size to permeable reactant pathway structure thickness, tp, has a range of 0.02 to 1, and, preferably, 0.25 to 0.75.

6. The multi-layered structure of any preceding claim, in which the permeable reactant pathway tortuosity, tp, is arranged to at least reduce, preferably minimise, at least one, or both, of: flow resistance and pressure drop across the permeable reactant pathway, and / or in which the electrode is a cathode electrode and in which the permeable reactant pathway tortuosity, tp, has a range of 1 to 3, optionally, 1 to 2.5, and, preferably, 1 to 2.

7. The multi-layered structure of any preceding claim, in which the permeable reactant pathway structure porosity, sp, is selected to balance flow resistance against structural strength and in-plane sheet electrical conductance of the permeable reactant pathway structure, and / orin which the electrode is a cathode electrode and in which the permeable reactant pathway structure porosity, sp, has a range of 0.1 to 0.9, preferably, 0.5 to 0.8.

8. The multi-layered structure of any preceding claim in which the electrode is a cathode and t2 d2the ratio — which relates the geometric parameters of the permeable cathode reactant Jp tppathway and its associated pathway structure to the electrolyser operating parameters has a range of 5e6 / m to 5e13 / m, optionally, 5e8 / m to 5e12 / m and, preferably 1e10 / m to 1e12 / m.Barrier9. The multi-layered structure of any preceding claim comprising:f. an inert permeable barrier;g. the inert permeable barrier comprises an inert permeable barrier reactant pathway to at least one, or both, of:i. provide an effective depth greater than a physical depth of the inert permeable barrier to at least reduce, preferably, eliminate, diffusion of any gas species within the inert permeable barrier reactant pathway to form a convectively dominant reactant flow regime within the inert permeable barrier; orii. host a convectively dominant reactant flow regime within the inert permeable barrier,h. wherein the inert permeable barrier reactant pathway feeds the permeable reactant pathway;in whichi. the inert permeable barrier reactant pathway and inert permeable barrier are related, or defined, byi. which relates the geometric parameters of the inert permeable tp £bbarrier and its associated pathway to the electrolyser and barrier operating parameters,whereii. zb is the inert permeable barrier reactant pathway tortuosity in the reactant flow direction,iii. Eb is the inert permeable barrier porosity,iv. db is the inert permeable barrier depth in the reactant flow direction.

10. The multi-layered structure of claim 9, in which the ratio associated with a range of effective depths in the flow direction of the inert permeable barrier pathway, is selected to at least to reduce, or to eliminate, diffusion effects of any gas species, and / orin which the electrode is a cathode electrode and in which the ratio has a range of 2to 2500 mm, optionally, 10 to 500 mm and, preferably 50 to 250 mm, and more preferably less than 250mm11. The multi-layered structure of any of claims 9 to 10, in which the inert permeable barrier porosity, sb, has a lower bound to accommodate an orifice orforaminate plate example and an upper bound to accommodate higher tortuosity materials, and / orin which the electrode is a cathode electrode and in which the inert permeable barrier porosity, sb, has a range of 0.01 to 0.5, preferably, 0.05 to 0.5.

12. The multi-layered structure of any of claims 9 to 11, in which the inert permeable barrier pathway tortuosity, xb, is arranged to at least reduce, and, preferably, minimize, diffusion effects of any gas species by increasing an effective diffusion depth, and / orin which the electrode is a cathode electrode and in which the inert permeable barrier pathway tortuosity, rb, has a range of 1 to 10, optionally, 1 to 5.

13. The multi-layered structure of any of claims 9 to 12, in which the inert permeable barrier depth, db, is selected to achieve a balance, at a lower limit, between an increase in effective depth to limit diffusion of any gas species, with, at an upper limit, constraining size of the inert permeable barrier such that it has a volume smaller than an associated electrolyser, and / orin which the electrode is a cathode electrode and in which the inert permeable barrier depth, db, is greater or equal 0.01 mm, optionally 0.01 mm to 5mm and, preferably, 0.1 mm to 2 mm.

14. The multi-layered structure of any of claims 9 to 13, in which the electrode is a cathode andthe ratio which relates the geometric parameters of the inert permeable barrier and tp£bits associated pathway to the electrolyser and barrier operating parameters has a range of 0.2m to 1000m, optionally, 1m to 200m and, preferably 10m to 100m.

15. The multi-layered structure of any preceding claim, further comprising a permeable electrode current collector to conduct current from the electrode; optionally, the permeable electrode current collector forms the permeable reactant pathway structure to host the permeable reactant pathway.Cathode operating parameters16. The multi-layered structure of any preceding claim, in which electrolyser operating parameters comprise a cathode operating parameter space defined byapUtp F _i T32pR ’a. wherei. p is the absolute pressure,ii. Ap is pressure drop across the electrolyser (in the reactant flow direction) iii. F is Faraday’s constant,iv. Uf is a utilisation factor indicating the proportion of the supplied reactant consumed by the electrolyser,v. R is the gas constant,vi. T is the temperature,vii. p is the reactant gas dynamic viscosity,viii. i is the current density (-^), andix. c is the constant.

17. The multi-layered structure of claim 16, in whicha. c has a value in the range 5e6 / m to 5e13 / m, optionally, 5e8 / m to 5e12 / m and, preferably, 1e10 / m to 1e12 / m.

18. The multi-layered structure of either of claims 16 and 17, in whicha. p , the absolute pressure, has a range of 1 bar to 50 bar, preferably 5 bar to 25 bar.

19. The multi-layered structure of any of claims 16 to 18, in whicha. Ap , the pressure drop, has a range of 10 mbar to 1000 mbar, preferably 20mbar to 300mbar.

20. The multi-layered structure of any of claims 16 to 19, in whicha. Uf , the utilisation factor, has a value in the range of 0.1 to 1.0, preferably 0.33 to 0.95.

21. The multi-layered structure of any of claims 16 to 20, in whicha. T , the temperature, has a value in the range of 930°C to 450°C, preferably 900°C to 580°C.

22. The multi-layered structure of any of claims 16 to 21, in whicha. p the reactant gas dynamic viscosity, has a value in the range 5e-5 Pa.s to 1e-5 Pa.s, preferably, 4e-5 Pa.s to 2e-5 Pa.s.

23. The multi-layered structure of any of claims 16 to 22, in whicha. i , the current density (^), has a value in the range 100000 A / m2 to 300 A / m2, optionally, 6000 A / m2 to 300 A / m2 and, preferably, 3000 A / m2 to 1000 A / m2.

24. The multi-layered structure of any of claims 16 to 23, in which ----= c = —=} } l T 32pR Zpfptp(m-1), wherea. tp = re is the permeable cathode reactant pathway tortuosity in the reactant flow direction,b. sp = se is permeable cathode reactant pathway structure porosity,c. dp = de is the permeable cathode reactant pathway structure depth in the reactant flow direction,d. tp = te is the permeable cathode reactant pathway structure thickness, e. fp = fp is the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, tp.Geometric parameters for cathode25. The multi-layered structure of claim 24, in whicha. re, the permeable cathode reactant pathway tortuosity in the reactant flow direction, has a range of 1< re <3, optionally, 1 <t6 <2.5, and, preferably, 1 <Te <2.

26. The multi-layered structure of any preceding claim, in whicha. se, the permeable cathode reactant pathway structure porosity, has a range of 0.1<£e <0.9 and preferably 0.5< £e <0.8.

27. The multi-layered structure of any of preceding claim, in whicha. de the permeable cathode reactant pathway structure depth in the reactant flow direction, has a range of 0.25<de<40 (mm), optionally, 0.25< de <20 (mm), and, preferably 5< de <20 (mm).

28. The multi-layered structure of any of any preceding claim, in whicha. te, the permeable cathode reactant pathway structure thickness, has a range of 5 <te <1000 (pm), optionally, 5 <te <500 (pm) and, preferably 10 <te <150 (pm).

29. The multi-layered structure of any preceding claim, in whicha. fe, the ratio of permeable cathode reactant pathway pore size to permeable cathode reactant pathway structure thickness, te, has a range of 0.02< fe <1.0, and, preferably, 0.25< / e <0.75.

30. The multi-layered structure of any claim 16 to 29, in which-----L = c = —-^(m-1) has a range of 5e6 / m to 5e13 / m, optionally, 5e8 / m to 5e12 / m and, preferably 1e10 / m to 1e12 / m.

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