Electrolyser cell unit

Abstract

An electrolyser cell unit, a method of producing an electrolyser cell unit, and a method of operating an electrolyser cell unit. The electrolyser cell unit comprising a support plate having a porous region and an electrochemically active area on the porous region. The electrochemically active area comprising an oxygen electrode, an electrolyte, an interlayer, and a fuel electrode. The oxygen electrode is disposed between the support plate and the electrolyte.

Description

[0001] ELECTROLYSER CELL UNIT

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to electrolyser cell units, to stacks of electrolyser cell units, and to methods of producing such electrolyser cell units.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrochemical cells formed of oxide layers (often known as solid oxide cells: SOC) may be used as fuel cells or electrolyser / electrolysis cells.

[0006] SOC fuel cell units produce electricity using an electrochemical conversion process that oxidises fuel. SOC cell units can also, or instead, operate as regenerative fuel cell (or reverse fuel cell) units, often known as solid oxide electrolyser fuel cell units, for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide.

[0007] SOC units are generally ceramic-based, using an oxygen-ion conducting metal-oxide- containing ceramic as an electrolyte. Many ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) have useful ion conductivities at temperatures in excess of 500°C (for cerium-oxide based electrolytes) or 650°C (for zirconium oxidebased ceramics), so SOCs tend to operate at elevated temperatures.

[0008] A solid oxide electrolyser cell (SOEC) is a device to achieve the electrolysis of water and / or carbon dioxide. In operation, the electrolyte of the SOEC conducts oxygen ions between a cathode and an anode located on opposite sides of the electrolyte. A fuel, for example steam, contacts the fuel electrode and a potential is applied across the cell so that oxygen evolves from the oxygen electrode and hydrogen is produced at the fuel electrode.

[0009] Conventional ceramic-supported SOCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOCs have recently been developed which have the active fuel cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOCs and can be sealed using conventional metal welding techniques.

[0010] Applicant’s earlier patent application WO-A-2002 / 35628 discloses fuel cells, and in particular intermediate-temperature solid oxide fuel cells (IT-SOFCs) which are typically used in stacks to generate a power output of from 1 to 100 kW and find application as local power generators. WO-A-2009 / 090419 discloses methods for the deposition of ceramic films on ceramic or metallic surfaces, such as films of stabilised zirconia and doped ceria such as CGO (cerium gadolinium oxide), useful in the manufacture of high and intermediate temperature operating fuel cells including solid oxide fuel cells (SOFC). WO-A-2015 / 136295 discloses metal-supported SOFCs in which the electrochemically active layer (or active fuel cell component layer) comprises fuel electrode (anode), electrolyte and air electrode (cathode) layers respectively deposited (e.g. as thin coatings / films) on, and supported by, a metal support plate (e.g. foil). The metal support plate has a porous region surrounded by a non-porous region with the active layers being deposited upon the porous region so that gases may pass through the pores from one side of the metal support plate to the opposite side to contact the active layers coated thereon. The porous region comprises discrete apertures extending through the support plate.

[0011] SOFCs are sometimes used in reverse, i.e., in electrolyser mode. However, this may lead to problems and operation that is less efficient and more problematic than would be desired.

[0012] There is a need, therefore, to provide an improved electrolyser (SOEC) unit.

[0013] It is an aim of the present invention to address such a need.

[0014] SUMMARY OF THE INVENTION

[0015] The present invention accordingly provides, in a first aspect, an electrolyser cell unit comprising: a support plate having a porous region, and an electrochemically active area on the porous region, the electrochemically active area comprising an oxygen electrode, an electrolyte, an interlayer and a fuel electrode, wherein the oxygen electrode is disposed between the support plate and the electrolyte.

[0016] Advantages of this invention include improved gas flow management on both the fuel and oxygen sides of the cell unit. The electrode nearest to the support plate has relatively restricted fluidic communication due to said communication being via the porous region. In contrast, fluidic communication with the electrode furthest from the support plate is not restricted by the porous region. An electrolyser cell unit is supplied with fuel at the fuel electrode and generates product at both electrodes: for example hydrogen at the fuel electrode and oxygen at the oxygen electrode where the fuel is steam. As a result, it is advantageous to position the oxygen electrode nearest to the support plate since the only species communicating therewith is oxygen generated at the oxygen electrode, and this is the only species which need pass through the porous region. The fuel electrode, furthest from the support plate is encumbered by lesser - or no - restriction to access of the fuel (a larger molecule than the products of the electrolysis reaction, and so further benefit in this species not needing to pass through the porous region) and product generated thereat. In some cases, the oxygen electrode may be in fluidic communication, via the porous region, with a first fluid volume enclosed within a cell repeat unit.

[0017] Since fuel need not pass through the porous region to reach the fuel electrode, said electrode may be more robust in use.

[0018] The interlayer may be adapted to block electron transport. Some electrolytes, for example ceria-based electrolytes, may become mixed ionic conductors at operating temperature, thereby allowing electrons to pass through the electrolyte. This is undesirable since electron transport through the electrolyte reduces performance thereof, and so it is advantageous to provide an interlayer to act as an electron-blocking layer. The interlayer may be an electrolyte layer, in other words it is preferably dense and ionically conducting.

[0019] The interlayer may comprise zirconia, optionally the interlayer may comprise rare earth doped zirconia.

[0020] Thus, the interlayer may comprise zirconia doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln).

[0021] The rare earth doped zirconia may be selected from scandia stabilised zirconia (ScSZ), yttria stabilised zirconia (YSZ), scandia ceria co-stabilised zirconia (ScCeSZ), ytterbia stabilised zirconia (YbSZ), scandia yttria co-stabilised zirconia (ScYSZ) and mixtures thereof.

[0022] Doped zirconia may be a solid solution which may be of formula Zr(i-x)MxO(2-o.5x-5) where 0<x<0.2, 6 is the degree of oxygen deficiency, and M is a rare earth element (M = Sc, Y, Ln, Yb or a mixture). The dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally in the range 6 to 12 atom %.

[0023] Where the dopant is Y, the Y dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally about 8 atom%. Where the dopant is Sc, the Sc dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally about 10 atom%. Where the dopant is Yb, the Yb dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally about 8 atom%.

[0024] The interlayer may be formed from multiple sub-layers.

[0025] The interlayer may have a thickness of 0.5 pm or greater. The interlayer may have a thickness of 1 pm or greater, optionally 2 pm or greater.

[0026] The interlayer may have a thickness of 5 pm or lower. Optionally, the interlayer may have a thickness of 4 pm or lower, or 3 pm or lower.

[0027] Thus, the interlayer may have a thickness in the range 0.5 pm to 5 pm, a thickness in the range 1 pm to 4 pm, or a thickness in the range 2 pm to 3 pm

[0028] Generally, thicknesses of the interlayer in these ranges are advantageous because they result in low or very low electrical (i.e., electron) conductivity. The optimum thickness is a tradeoff between electronic leakage (which may start to occur when the layer is thinner) and ionic resistance (which tends to increase when the layer is thicker).

[0029] The interlayer may advantageously act to block electronic leak currents by virtue of being a pure oxide ion conductor.

[0030] The interlayer is preferably dense enough to reduce diffusion of fuel side gasses, e.g., reducing gas (e.g. hydrogen) to the electrolyte. Thus, preferably the interlayer has low porosity or is not substantially porous (although it may have some closed porosity). Providing a dense layer (and reducing porosity) may be achieved by careful selection of particle size, sintering aids and temperature profile. Producing a thin, dense interlayer may be easier on the electrolyte than on an electrode.

[0031] The interlayer may be disposed between the electrolyte and the fuel electrode. In an electrolyser cell, it is advantageous to position the interlayer on the fuel electrode side of the electrolyte to block electrons from flowing into the electrolyte, thereby improving efficiency of the cell unit. An alternative may be to position the interlayer between the oxygen electrode and electrolyte, to block electrons from leaving the electrolyte, but this may be less efficient than the former arrangement.

[0032] Deposition of a good quality interlayer may be easier on the electrolyte (than on an electrode), as a result the interlayer thickness may be reduced, leading to reduced ionic resistance and improved efficiency.

[0033] Thus, the interlayer may be deposited on the electrolyte. The electrolyte may be a dense layer, and deposition thereon may be eased by its density. As a result, thickness of the interlayer may be reduced in comparison to positioning of interlayers on electrodes - which may be less dense than electrolytes.

[0034] The electrolyser cell unit may comprise a doped ceria electrolyte layer disposed between the interlayer and the fuel electrode. The doped ceria electrolyte layer may lower interfacial resistance between the electrolyte and the fuel electrode and may lead to increased electrochemical activity of the fuel electrode due to material match between said layers.

[0035] The electrolyte may comprise at least one electrolyte layer comprising doped ceria, optionally wherein the electrolyte comprises at least one electrolyte layer comprising rare earth doped ceria.

[0036] The rare earth doped ceria may comprise ceria doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln).

[0037] The rare earth doped ceria may be selected from samarium-doped ceria (SDC), gadolinium- doped ceria (GDC), praseodymium doped ceria (PDC), samaria- gadolinia doped ceria (SGDC) and mixtures thereof. Gd and Sm are advantageous and may yield higher ionic conductivities.

[0038] The dopant concentration in the ceria may be in the range 45 atom % or lower, optionally 40 atom % or lower, optionally 20 atom % or lower.

[0039] The dopant concentration in the ceria may be 3 atom % or higher, optionally 5 atom % or higher, optionally 10 atom % or higher.

[0040] Thus, the dopant concentration in the ceria may be in the range 3 to 45 atom %, optionally 5 to 40 atom %, optionally 10 to 20 atom %.

[0041] The electrolyte may have a thickness of 17 pm or lower. Optionally, the electrolyte may have a thickness of 15 pm or lower, optionally 12 pm or lower. The electrolyte may have a thickness of 4 pm or greater. Optionally, the electrolyte may have a thickness of 5 pm or greater, 6 pm or greater or 7 pm or greater.

[0042] Thus, the electrolyte may have a thickness in the range 4 pm to 17 pm, a thickness in the range 5 pm to 15 pm, or a thickness in the range 6 pm to 12 pm.

[0043] The electrolyte has a primary purpose of facilitating oxygen ion diffusion from one electrode to another and may advantageously provide a mechanically stable layer having very low or no gas permeability and having ionic resistance that is as low as possible.

[0044] The advantage of the use of a doped ceria electrolyte includes relatively low temperature operation (but relatively good ionic conductivity).

[0045] The oxygen electrode may be in direct contact with the electrolyte.

[0046] The fuel electrode may comprise at least one layer comprising doped ceria, optionally at least one layer of the fuel electrode may comprise ceria gadolinium oxide (CGO). The fuel electrode may be a cermet, which may be Zr or Ce-based, and / or may comprise a material as found in the electrolyte or interlayer. As a result, electrical conductivity of the fuel electrode may be greater than if the electrode were a ceramic. This may enable improved electrical connection to the fuel electrode (outermost electrode relative to the support plate), said electrical connection typically via interfacial contact. The greater electrical conductivity means that losses in the electrical connection may be reduced relative to a lower conductivity electrode.

[0047] The fuel electrode may comprise at least one layer comprising a source of transition metal or metal oxide. The transition metal (or oxide thereof) may be one or more of nickel, iron, cobalt, or copper, preferably nickel.

[0048] Thus, the fuel electrode may comprise at least one layer comprising a source of nickel, optionally the source of nickel may comprise nickel oxide.

[0049] The fuel electrode may comprise at least one layer comprising a transition metal (including metal oxide) CGO cermet.

[0050] The fuel electrode may comprise at least one layer comprising nickel CGO cermet.

[0051] The fuel electrode may have a thickness of 3 pm or higher, optionally 5 pm or higher, optionally 10 pm or higher, optionally 15 pm or higher. The fuel electrode may have a thickness of 60 pm or lower, 50 pm or lower, optionally 45 pm or lower, optionally 40 pm or lower, optionally 35 pm or lower, optionally 30 pm or lower, optionally 25 pm or lower, optionally 20 pm or lower optionally 15 pm or lower. Thus, the fuel electrode may have a thickness in the range 3 pm to 60 pm, 5 pm to 50 pm, optionally 15 pm to 25 pm.

[0052] In use, the fuel electrode may be contacted by a fuel gas that may comprise, for example, steam and / or CO2 and / or NO2.

[0053] The oxygen electrode may comprise a material that is electrically conductive, optionally the oxygen electrode may comprise a material that is an electrically conductive ceramic material.

[0054] The oxygen electrode may comprise a material that is selected from lanthanum cobaltite, lanthanum ferrite, lanthanum nickel ferrite, Lao.99Coo.4Nio.60(3-5) (LCN60), praseodymium strontium cobaltite, praseodymium doped ceria, lanthanum strontium manganese, lanthanum strontium cobaltite and mixtures thereof.

[0055] In the formula for Lao.99Coo.4Nio.60(3-5) (LCN60), 6 indicates the degree of oxygen deficiency.

[0056] The oxygen electrode may comprise a Pr / Ln111material of composition Pr(i-q)LnniqO(2-5), wherein Ln11is selected from at least one rare earth metal, optionally Ln111is selected from La, Nd, Sm, Eu, Gd, preferably Gd or Sm, more preferably Sm, 6 is the degree of oxygen deficiency, and 0.01 < q < 0.4. Such a Pr / Ln111material has excellent activity and other properties and does not need to contain alkaline earth metal oxides (e.g. strontium oxide). Alkaline earth metal oxides (e.g. Sr) may be problematic in electrochemical cells because they may react, in particular, with zirconia-based electrolytes.

[0057] In the Pr / Ln111material, q may be selected to achieve a balance between oxygen vacancy concentration and ion-mobility e.g. 0.02 to 0.25. Advantageously, q may be in the range 0.02<x<0.3; 0.03<x<0.3; 0.04<x<0.3; 0.05<x<0.3; 0.05<x<0.27; 0.05<x<0.25; 0.05<x<0.25; or 0.05<x<0.3. Suitably, q may be from 0.08 to 0.2 or 0.08 to 0.12, more suitably q may be about 0.1; about 0.15; or about 0.2.

[0058] Suitably the Pr / Ln111material may be of formula Pro.9Lno.iO(i.95-5), Pro.85Lno.i50(i.925-5), Pro.8Lno.20(i.9-5) or mixtures thereof; wherein Ln is selected from one or more of La, Nd, Sm, Eu, Gd, or Yb; preferably Sm.

[0059] Advantageously, the Pr / Ln111material may have a cubic crystalline structure; preferably a fluorite crystalline structure. The oxygen electrode (or a layer thereof) may essentially comprise or consist of a single phase having a cubic fluorite structure, or of two or more phases having a cubic fluorite structure. The fluorite structure of the Pr / Ln111material may also be advantageous because it may provide good interfacial interaction with an electrolyte layer comprising a material with a fluorite structure.

[0060] Suitably the Pr / Ln111material may be in direct contact with the electrolyte. In such cases, the resulting cells may exhibit improved electronic leakage, in turn improving performance particularly at high temperatures.

[0061] The oxygen electrode may comprise or further comprise doped ceria, optionally the oxygen electrode may comprise or further comprise doped ceria gadolinium oxide (CGO).

[0062] The oxygen electrode may further comprise a material that is also comprised in the electrolyte (e.g., Ce, Zr).

[0063] The oxygen electrode may comprise multiple sub-layers. For example, the oxygen electrode may comprise a bulk electrode and an active electrode (each of which may comprise multiple sub-layers). In other cases, there may only be an active electrode. Either or both of the active and bulk (where present) electrodes may comprise Pr. If a bulk oxygen electrode is present, it may comprise a Cr or S getter.

[0064] The support plate may comprise a metal support plate, optionally the metal support plate may comprise a steel support plate, optionally a stainless steel support plate. The support plate may also be referred to as a substrate.

[0065] The porous region of the support plate may comprise drilled holes through the support plate in the porous region, optionally may comprise laser drilled holes through the support plate in the porous region. Alternatively, the porous region may comprise an inherently porous substrate / support plate, which may be attached to a peripheral non-porous support.

[0066] Optionally, the support plate may comprise a barrier layer on at least one surface thereof. The barrier layer may be a layer to prevent / reduce corrosion of the support by oxygen.

[0067] In electrolyser cell units according to the invention, the electrochemically active area may be disposed on the porous region on a first surface of the support plate and the electrolyser cell unit may further comprise an interconnect (directly or indirectly) attached and electrically connected to a second surface of the support plate. This has the advantage that the interconnect (of an adjacent cell unit) may contact the fuel electrode, which is usually more conductive than for example an oxygen electrode. The interconnect may be attached by welding.

[0068] Electrolyser cell units according to the first aspect may be arranged in a stack of electrolyser cell units, electrically connected in series. In such cases, electrical connection between cell units may rely on touch-contact of an interconnect with the electrode furthest from the support plate, i.e., touch-contact with the fuel electrode of the cell unit of the first aspect. Resistive losses associated with this contact between interconnect and fuel electrode may be lower than between interconnect and oxygen electrode.

[0069] Thus, in a second aspect, the present invention provides, a stack of electrolyser cell units, wherein each electrolyser cell unit is according to the first aspect.

[0070] The layers of the oxygen electrode, electrolyte layer, interlayer, and fuel electrode layer may be deposited sequentially on the support plate by any suitable method. Each of these layers may be comprised of multiple sub-layers (sub-layers may have varying compositions).

[0071] Thus, in a third aspect, the present invention provides a method of producing an electrolyser cell unit, the method comprising: providing a support plate having a porous region, applying an oxygen electrode material on the support plate over the porous region (e.g., at least over the top of the porous region), to form an oxygen electrode, applying an electrolyte material on the oxygen electrode to form an electrolyte, applying an interlayer material to form an interlayer, and applying a fuel electrode material to form a fuel electrode, thereby forming an electrochemically active area on the porous region.

[0072] Applying the interlayer material to form an interlayer may comprise applying the interlayer material on the electrolyte to form the interlayer. Furthermore, applying the fuel electrode material to form a fuel electrode may comprise applying the fuel electrode material on the interlayer to form the fuel electrode.

[0073] Applying the oxygen electrode material, the electrolyte material, the interlayer material, and the fuel electrode may be by printing, optionally roller printing, jet printing or screenprinting, or by spraying (for example atomised spraying), or by vapor deposition such as using chemical vapor deposition (CVD), or using physical vapor deposition (PVD). Specifically, applying the interlayer material may be by a solution deposition-based process, for example jetting or spraying. Such solution deposition is advantageous in forming a dense thin layer in a process-efficient manner. Sintering may occur after deposition of one or more layers and may be performed at a temperature in the range 750 °C to 1100 °C, preferably from 800 °C to 970 °C. Sintering may be performed in an air atmosphere.

[0074] In a fourth aspect, the present invention provides a method of operating an electrolyser cell unit, the method comprising providing an electrolyser cell unit according to the first aspect, contacting the fuel electrode of the electrolyser cell unit with a fuel to be subject to electrolysis, and applying a potential across the electrodes of the electrolyser cell unit.

[0075] The fuel to be subject to electrolysis may comprise steam and / or CO2 and / or NO2.

[0076] The electrolyser cell may be operated at a temperature in the range 400 to 700 °C, preferably 500 to 600 °C.

[0077] Definitions

[0078] In this specification, the terms “rare earth metal” or “rare earth element” refer to metals selected from Y, Sc, and lanthanoid.

[0079] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71.

[0080] The term "dopant" as used herein is not intended to be restricted to a maximum percentage of elements, ions or compounds added to chemical structures. Similarly, the term "doping" is intended to mean the addition of a certain amount of elements, ions or compounds to a material. It is not limited to a maximum quantity of material, after which, further addition of material no longer constitutes doping.

[0081] Metal oxides including mixed metal oxides may exhibit non-stoichiometry with the oxide being deficient in oxygen. In this specification, 6 indicates the degree of oxygen deficiency of a material. 6 may vary depending on the environment and history of the material. As would be understood by the skilled person, values of 6 are usually small. Thus, 6 may be 0.25 or lower, suitably 0.2 or lower and more suitably < 0.15. 6 may have a lower limit of 0.0001, optionally 0.001, optionally 0.005, optionally 0.01, optionally 0.05. The term “source of’ an element, compound or other material refers to a material comprising the element, compound or other material whether or not chemically bonded in the source. The source of the element, compound or other material may be an elemental source (e.g. Ln, Ni or O2) or may be in the form of a compound or mixture comprising the element, compound or other material including one or more of those elements, compounds or materials.

[0082] In this specification references to electrolyser cell, SOC, and SOEC or electrolyser cell unit may refer to tubular or planar cells unless the context otherwise requires.

[0083] Electrolyser cell units may be tubular or planar in configuration. Planar cell units may be arranged overlying one another in a stack arrangement, for example 100-400 cell units in a stack, with the individual cell units arranged electrically in series. References to “a stack of electrolyser cells” therefore refer to a plurality of electrolyser cells units arranged electrically in series.

[0084] “Oxidant electrode,” “oxygen electrode” or “air electrode” and “fuel electrode” are used herein and may be used interchangeably to refer to anodes and cathodes, respectively of SOECs.

[0085] Electrolyser cells as encompassed by the invention may comprise two planar components in a spaced arrangement with a fluid volume in between. The two planar components may be a substrate / support plate with electrochemical layers and an interconnect (separate plate), which may be attached (directly or indirectly, e.g., by welding) to the support plate around the periphery of both components. One or both components may have a flanged perimeter to space a central area of each component from the other in order to provide the fluid volume therebetween.

[0086] Further planar components may be provided between the support plate and interconnect. In some cases, a fluid distribution element may be provided between the support plate and interconnect surrounding fluid ports provided through each component. The fluid distribution element may have corresponding fluid ports with necks for fluidic communication with the fluid volume - to supply fluid to or exhaust fluid from the fluid volume. In some cases, typically alternative to the flanged perimeter, a spacer plate may be provided between the support plate and interconnect to space said components from one another, thereby providing the fluid volume. The spacer plate may have an open central area and a closed periphery (like a window frame). In such cases, the support plate, spacer plate, and interconnect may be sealingly attached to one another around their periphery, e.g., by welding through the three components.

[0087] The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0088] Furthermore, although all aspects of the invention or disclosure preferably “comprise” the features described in relation to that aspect, it is specifically envisaged that they may “consist” or “consist essentially” of those features outlined in the claims.

[0089] It will be understood that “attached” and “on” refer to direct or indirect attachment and positioning, respectively.

[0090] It will be understood that each layer may be comprised of multiple sub-layers (and those sublayers may have varying compositions).

[0091] The invention will now be described with reference to the accompanying figures.

[0092] BRIEF DESCRIPTION OF THE FIGURES

[0093] Figure 1 shows a schematic cross section of an electrolyser cell unit in accordance with the disclosure.

[0094] Figure 2 shows a schematic cross section of an electrolyser cell unit as in Figure 1 including an interconnect.

[0095] Figure 3 shows a schematic cross section of a stack of two electrolyser cell units each including an interconnect as in Figure 2.

[0096] Figure 4 is a scanning electron microscope image of a partial cross section through an electrolyser cell in accordance with the disclosure.

[0097] DETAILED DESCRIPTION OF THE INVENTION

[0098] The present disclosure relates to electrolyser cell units with the oxygen (also known as oxidant or air) electrode deposited on to a porous metal substrate formed as a metal support plate and the fuel electrode on top of the electrolyte. The electrolyser cell units of the invention improve gas flow management on both the fuel and oxygen sides.

[0099] Access to / from the oxygen electrode is only needed for transport of oxygen generated in the electrolysis reaction (e.g., when the electrolyte is an oxygen ion conducting electrolyte). Such oxygen transport is not typically a rate limiting step in operation of electrolyser cell units. In contrast, fluidic communication with the fuel electrode is required for supply of fuel and transport of a product (e.g., hydrogen) of the electrolysis reaction. The electrolyser cell units described herein improve gas flow management to / from the cell units by positioning the oxygen electrode nearest to the support plate, accessible via the porous region which allows fluidic communication through the same, but which nonetheless forms a restriction to gas produced at the electrode. However, given that the electrode is the oxygen electrode, lower flow rates are required than if the electrode were the fuel electrode. This may allow, for example, support plates to be manufactured with fewer laser drilled holes than in other configurations, while allowing sufficient fluidic communication.

[0100] The support plate and an interconnect may be attached to one another to form a repeat unit. In some examples, the porous region is in fluidic communication with a fluid volume enclosed by the repeat unit. In use, oxygen would be generated at the oxygen electrode and transported, via the porous region to the fluid volume enclosed by the repeat unit. In a stack of such cell units, the cell units are arranged with space therebetween to form a second fluid volume for fuel and product of the electrolysis rection (e.g., hydrogen) at the fuel electrode. Because the fuel electrode is not the nearest electrode to the support plate, access to and from the fuel electrode is over a much greater surface area than in prior arrangements thereby improving access and rate of reaction.

[0101] Operation of an electrolyser unit can be limited by gas flow issues if the flow of fuel (e.g. steam) and product (e.g., hydrogen) is restricted, especially at high current densities. Flow of fuel (e.g. steam) and product (e.g. hydrogen) through the porous region of the support plate may be restricted and this would lead to higher area specific resistance and less efficient operation. Since the fuel electrode is on the top of the active layers in the present disclosure, this restriction does not occur.

[0102] The configuration means that the fuel electrode (e.g. of Ni: CGO) is not structurally confined (it may be towards or on the top of the electrochemically active layers) which means fuel minimal availability constraints may be relaxed while reducing likelihood of fuel starved regions, therefore the fuel electrode may exhibit improved redox stability and improved activity.

[0103] The fuel electrode may overlap past the edge of the porous region and over the non-porous region of the support plate. Nonetheless, the overlapping fuel electrode will be accessible to fuel, maximising active area and protecting the fuel electrode against oxidation.

[0104] The interlayer (e.g., an electron blocking interlayer), since it may be deposited on the fuel side of the cell unit (for example between the electrolyte and the fuel electrode), reduces the tendency for a ceria based electrolyte to be electrically reduced under the higher voltages applicable in electrolysis (compared to fuel cells). This addresses a problem that may occur when ceria is reduced and becomes more electrically (i.e. electron) conductive. Further, such an interlayer may be deposited on a densified electrolyte which is a cheaper and / or easier deposition process than deposition on an electrode. This may further allow the interlayer to be reduced in thickness, improving efficiency.

[0105] Corrosion issues with electrical connection are reduced. Dimples on an interconnect typically form a contact between a cell unit and an outermost electrode of a neighbouring cell unit. The side of the interconnect exposed to oxygen is typically provided with a coating to reduce corrosion of the interconnect. Said coatings may sometimes crack around the dimples. Corrosion may result when electrical current is routed through the cracked oxygen-side coating, such corrosion may have a greater electrical resistivity than the coating and may lead to oxygen penetrating further into the interconnect, in turn causing more corrosion. However, the cell units of the present invention reduce this mechanism since electrical current may be routed around the fluid volume enclosed by the cell unit - i.e., around the oxygen-containing volume - through the periphery of the cell unit (typically a weld between support plate and interconnect). As a result, limited current may flow through the air-side coating of the interconnect.

[0106] Repeat units may contact and electrically connect via outward dimples on the fuel side of the interconnect (of a first cell unit) and the fuel electrode (of a second, neighbouring cell unit). Accordingly, this contact is made to a better electronic conductor (fuel electrode - cermet, rather than oxygen electrode - ceramic) than prior cell units. This may reduce the number (density) of such dimples needed, further increasing fluidic access to / from the fuel electrode. In any case, contact resistance between cell units is reduced, thereby improving efficiency. Figure 1 shows, schematically and not to scale (for reasons of clarity), a cross section of an electrolyser cell unit 2. A substrate, formed by a ferritic stainless steel metal support 4, is plate-like with a peripheral, non-porous region 6 and a central, porous region 8 where holes have been drilled (e.g. laser-drilled) through the metal support 4. A barrier layer (not shown) to reduce corrosion may be located on the surface of the metal support 4 (on one or both sides thereof). A layer of an oxygen electrode layer 10 of an electrically conductive ceramic material is located on the porous region 8 of the metal support 4. An electrolyte layer 12 of rare earth doped ceria (RE=Y, Sc or any Ln) of thickness 4 pm or greater (optionally 6 pm to 12 pm) is located on the oxygen electrode layer 10. The electrolyte layer 12 may surround the oxygen electrode layer 10 to reduce or prevent gas leaking from the oxygen side (or volume) 20 to the fuel side (or volume) 18 or vice versa. The electrolyte layer 12 may further overlap at least part of the non-porous region 6 of the substrate / support 4. An interlayer 14 of rare earth (RE) stabilised zirconia (RE = Y, Sc or any Ln, e.g. Yb) of thickness 0.5 pm or greater (e.g. 1 pm to 4 pm) is located on the electrolyte layer 14. The electrolyser cell unit 2 has a fuel electrode 16 located on the interlayer 14. The fuel electrode layer 16 may be formed of e.g. Ni:CGO (Ni: cerium gadolinium oxide). The interlayer and / or the fuel electrode may overlap the non-porous region 6 of the substrate / support 4. In some cases, a further doped ceria electrolyte layer may be disposed between the interlayer and the fuel electrode.

[0107] Figure 2 shows, schematically and not to scale (for reasons of clarity), a cross section of an electrolyser cell unit 22. Electrolyser cell unit 22 is similar to electrolyser cell unit 2 of Fig. 1, but further includes an interconnect 24. The oxygen electrode layer 10 is provided on a first side of the substrate / support 4. In this case, a first side of the interconnect 24 faces a second side of the support 4 in a spaced, opposed relationship to provide an enclosed (first) fluid volume 20a therebetween. The enclosed fluid volume 20a is in fluidic communication with the oxygen electrode 10 via the porous region 8, and so the enclosed fluid volume 20a is for oxygen (and optional sweep gas). One or more fluid ports (not shown) may be provided in the interconnect 24 and / or support 4 (non-porous region 6 thereof) for fluidic communication with the enclosed fluid volume 20a. The interconnect 24 may be provided with contact features (e.g., dimples, not shown) which protrude into the enclosed fluid volume 20a and contact the second side of the support 4 to maintain the spacing between interconnect 24 and support 4. In this case, the interconnect 24 is provided with a flanged perimeter 26, which allows the interconnect 24 to contact the support 4 (non-porous region 6 thereof) at the periphery of both components. Said components, interconnect 24 and support 4, are sealingly attached to one another around that periphery, for example by welding. In this example the interconnect 24 is provided with contact features 28 (e.g. dimples) which extend away from the support 4, toward a neighbouring cell unit (as will be further described with reference to Fig. 3, below), to provide a space (between the contact features) which serves as a second fluid volume bounded by the fuel electrode 16 - for fuel and product of the electrolysis reaction generated at the fuel electrode 16. The interconnect 24 may be a metal sheet pressed or formed to provide the contact features 28 and flanged perimeter 26. The interconnect 24 may have a protective coating on one or both sides thereof.

[0108] Figure 3, shows, schematically and not to scale (for reasons of clarity), a cross section of a stack 32 of first electrolyser cell unit 22a and second electrolyser cell unit 22b. Each electrolyser cell unit 22a, 22b in the stack 32 is an electrolyser cell unit 22 as shown in Fig. 2.

[0109] As in Figures 1 and 2, the oxygen electrode layer 10 of each electrolyser cell unit 22a, 22b is provided on a first side of its respective support 4. A first side of each interconnect 24 faces a second side of the support 4 of each electrolyser cell unit 22a, 22b in a spaced, opposed relationship to provide respective enclosed (first) fluid volumes 20a therebetween. Each enclosed fluid volume 20a is in fluidic communication with the respective oxygen electrode 10 of its electrolyser cell unit 22a, 22b via the porous region 8 of the support 4, and so the enclosed fluid volume 20a is for oxygen (and optional sweep gas). One or more fluid ports (not shown) may be provided in each interconnect 24 and / or support 4 (non-porous region 6 thereof) for fluidic communication with the enclosed fluid volume 20a. Each interconnect 24 may be provided with contact features (e.g., dimples, not shown) which protrude into the respective enclosed fluid volume 20a and contact the second side of the support 4 to maintain the spacing between interconnect 24 and support 4. The flanged perimeter 26 at the periphery of each interconnect 24 contacts and is sealingly attached to the periphery of its respective support 4, for example by welding.

[0110] Each interconnect 24 is provided with contact features 28 (e.g., dimples) which extend away from the support 4, toward a neighbouring cell unit. Thus, in the embodiment illustrated in Figure 3, the contact features 28 of the interconnect 24 of the first electrolyser cell unit 22a extend towards the second electrolyser cell unit 22b, contacting the fuel electrode layer 16 of the second electrolyser cell unit 22b thereby forming a space between the contact features 28 and the fuel electrode 16 which serves as a second fluid volume 18a. The contact features 28 of the first electrolyser cell unit 22a provide electrical connection between the first electrolyser cell unit 22a and the fuel electrode layer 16 of the second electrolyser cell unit 22b. The second fluid volume 18a formed between the neighbouring electrolyser cell units 22a, 22b of the stack 32 is in fluidic communication with the fuel electrode 16 of the second electrolyser cell unit 22b, and so the second fluid volume 20a is for fuel and the product(s) of the electrolysis reaction generated at the fuel electrode 16 of the second electrolyser cell unit 22b.

[0111] Figure 4 is a scanning electron microscope image of a partial cross section through an electrolyser cell unit. Fig. 4 shows an oxygen electrode layer 10 (in this case formed from two sub-layers as apparent from the change in porosity, the active oxygen electrode layer closest to the electrolyte and comprising a Pro.9Smo.iO(i.95-5) and CGO composite and a bulk oxygen electrode layer comprising LCN60 closest to the support plate 4). The oxygen electrode layer is located on a porous region of the (e.g., metal) support plate 4, in this case with a barrier layer 5 therebetween. The barrier layer 5 is configured to reduce corrosion in the cell unit, and may be a layer comprising CGO. Fig. 4 further shows an electrolyte layer 12 (comprising CGO), interlayer 14, and fuel electrode layer 16 (comprising Ni:CGO). The oxygen electrode layer 10 is disposed between the electrolyte layer 12 and the support plate 4. The interlayer 14 is disposed between the electrolyte 12 and the fuel electrode 16. The interlayer 14 is of rare earth (RE) stabilised zirconia (in this case comprising YbSZ) and is located on the electrolyte layer 12. A further doped ceria electrolyte layer 15 (e.g., comprising CGO) is disposed between the interlayer 14 and the fuel electrode 16, to lower interfacial resistance between the electrolyte and the fuel electrode and may lead to increased electrochemical activity of the fuel electrode due to material match between said layers. As is apparent from the image in Fig. 4, the interlayer is dense with low porosity and thickness, therefore well adapted to block electron transport to the electrolyte from the fuel electrode, thereby improving efficiency of the cell unit. Further, the low porosity leads to blocking of fuel side gasses from transport to the electrolyte. The cell unit of Fig. 4 had an open circuit voltage of 1.28V, which in comparable conditions (e.g., 530 degrees C, with hydrogen fuel) is comparable to or higher than prior art cell units in which the fuel electrode is disposed between the electrolyte and support plate.

[0112] Reference Numerals

[0113] 2 electrolyser cell unit 4 support

[0114] 5 barrier layer

[0115] 6 non-porous region of support

[0116] 8 porous region of support

[0117] 10 first, oxygen electrode layer

[0118] 12 electrolyte layer

[0119] 14 interlayer

[0120] 15 further doped ceria electrolyte layer

[0121] 16 second, fuel electrode layer

[0122] 18 fuel side

[0123] 18a second fluid volume

[0124] 20 oxygen side

[0125] 20a first fluid volume

[0126] 22 electrolyser cell unit

[0127] 22a first electrolyser cell unit

[0128] 22b second electrolyser cell unit

[0129] 24 interconnect

[0130] 26 flanged perimeter (of interconnect)

[0131] 28 contact features (dimples)

[0132] 32 stack of electrolyser cell units

[0133] All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

Claims1. An electrolyser cell unit comprising: a support plate having a porous region, and an electrochemically active area on the porous region, the electrochemically active area comprising an oxygen electrode, an electrolyte, an interlayer, and a fuel electrode, wherein the oxygen electrode is disposed between the support plate and the electrolyte.

2. An electrolyser cell unit as claimed in claim 1, wherein the interlayer comprises zirconia, optionally wherein the interlayer comprises rare earth doped zirconia.

3. An electrolyser cell unit as claimed in claim 2, wherein the interlayer comprises zirconia doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln), optionally wherein the interlayer comprises a material selected from scandia stabilised zirconia (ScSZ), yttria stabilised zirconia (YSZ), ytterbia stabilised zirconia (YbSZ), scandia ceria co-stabilised zirconia (ScCeSZ), scandia yttria co-stabilised zirconia (ScYSZ) and mixtures thereof.

4. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the interlayer is disposed between the electrolyte and the fuel electrode.

5. An electrolyser cell unit as claimed in claim 4, further comprising a doped ceria electrolyte layer disposed between the interlayer and the fuel electrode.

6. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the electrolyte comprises at least one electrolyte layer comprising doped ceria, optionally wherein the electrolyte comprises at least one electrolyte layer comprising rare earth doped ceria.

7. An electrolyser cell unit as claimed in claim 6, wherein the electrolyte comprises at least one electrolyte layer comprising a material selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC also referred to as ceria gadolinium oxide, CGO),praseodymium doped ceria (PDC), samaria-gadolinia doped ceria (SGDC) and mixtures thereof.

8. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the oxygen electrode is in direct contact with the electrolyte.

9. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the fuel electrode comprises at least one layer comprising doped ceria, optionally wherein at least one layer of the fuel electrode comprises ceria gadolinium oxide (CGO).

10. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the fuel electrode comprises at least one layer comprising a source of nickel, optionally wherein the source of nickel comprises nickel oxide.

11. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the fuel electrode comprises at least one layer comprising a transition metal CGO cermet.

12. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the fuel electrode has a thickness of 3 pm or higher, optionally 5 pm or higher, optionally 10 pm or higher, optionally 15 pm or higher.

13. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the fuel electrode has a thickness of 50 pm or lower, optionally 45 pm or lower, optionally 40 pm or lower, optionally 35 pm or lower.

14. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the oxygen electrode comprises a material that is electrically conductive, optionally wherein the oxygen electrode comprises a material that is an electrically conductive ceramic material.

15. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the oxygen electrode comprises a material that is selected from lanthanum cobaltite, lanthanum ferrite, lanthanum nickel ferrite, Lao.99Coo.4Nio.eO(3-5) (LCN60), praseodymium strontium cobaltite, praseodymium doped ceria, lanthanum strontium manganese, lanthanum strontium cobaltite and mixtures thereof.

16. An electrolyser cell unit as claimed in claim 15, wherein the oxygen electrode further comprises doped ceria, optionally wherein the oxygen electrode further comprises doped ceria gadolinium oxide (CGO).

17. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the support plate comprises a metal support plate, optionally wherein the metal support plate comprises a steel support plate, optionally a stainless steel support plate.

18. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the porous region of the support plate comprises drilled holes through the support plate in the porous region, optionally comprises laser drilled holes through the support plate in the porous region.

19. An electrolyser cell unit as claimed in any one of the preceding claims, wherein the electrochemically active area is disposed on the porous region on a first surface of the support plate and wherein the electrolyser cell unit further comprises an interconnect attached and electrically connected to a second surface of the support plate.

20. A stack of electrolyser cell units, wherein each electrolyser cell unit is as claimed in any one of the preceding claims.

21. A method of producing an electrolyser cell unit, the method comprising providing a support plate having a porous region, applying an oxygen electrode material on the support plate over the porous region, to form an oxygen electrode, applying an electrolyte material on the oxygen electrode to form an electrolyte, applying an interlayer material to form an interlayer, and applying a fuel electrode material to form a fuel electrode, thereby forming an electrochemically active area on the porous region.

22. A method as claimed in claim 21, wherein applying the interlayer material to form an interlayer comprises applying the interlayer material on the electrolyte to form the interlayer.

23. A method as claimed in either claim 21 or claim 22, wherein applying the oxygen electrode material, the electrolyte material, the interlayer material, and the fuel electrode is by printing, optionally roller printing, jet printing or screen-printing, or by spraying, or by vapour deposition, or using physical vapour deposition (PVD).

24. A method of operating an electrolyser cell unit, the method comprising providing an electrolyser cell unit as claimed in any one of claims 1 to 19, contacting the fuel electrode of the electrolyser cell unit with a fuel to be subject to electrolysis, and applying a potential across the electrodes of the electrolyser cell unit.

25. A method as claimed in claim 24, wherein the fuel to be subject to electrolysis comprises steam and / or CO2 and / or NO2.

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