Electrochemical cell

Abstract

An electrochemical cell comprising a first electrolyte layer comprising a material having a composition Ce(1-x) LnxO(2-0.5x-δ). Ln is selected from at least one rare earth metal excluding Ce, 0.22 ≤ x ≤ 0.45, and δ is the degree of oxygen deficiency.

Description

[0001] ELECTROCHEMICAL CELL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to electrochemical cells, to stacks of such electrochemical cells, to methods of producing such electrochemical cells, to electrolysis systems comprising such electrochemical cells and to methods of operating such electrochemical cells in electrolysis mode.

[0004] BACKGROUND OF THE INVENTION

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

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

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

[0008] A solid oxide fuel cell (SOFC) generates electrical energy through the electrochemical oxidation of a fuel gas. In operation, the electrolyte of the SOFC conducts oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. A fuel, for example a fuel derived from the reforming of a hydrocarbon or alcohol, contacts the anode (usually known as the “fuel electrode”) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (usually known as the “oxygen electrode”).

[0009] A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC but is, in practice, an SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of a fuel gas generating useful products. A fuel, for example, water and / or carbon dioxide and / or NO2, contacts the cathode (usually known as the “fuel electrode”) and a potential is applied across the cell resulting in electrolysis of the fuel.

[0010] Conventional ceramic-supported (e.g. anode-supported) SOCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOCs have recently been developed which have the electrochemically active component layers supported on a metal support / substrate. In these cells, the electrochemically active layers may be thin since they only perform an electrochemical function: that is to say, the electrochemically active 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.

[0011] Applicant’s earlier patent applications WO-A-2002 / 35628, WO-A-2009 / 090419 and WO-A- 2015 / 136295 disclose metal-supported SOCs in which the electrochemically active layers (or active fuel cell component layers) comprise anode, electrolyte and cathode layers deposited (e.g. as thin coatings / films) on, and supported by, a metal substrate support plate (e.g. foil).

[0012] There have been attempts to lower manufacturing cost, to increase reliability and increase the efficiency of SOFCs and SOECs. Unfortunately, some materials may be less stable under certain conditions. In particular, some electrolyte materials may partially reduce when subjected to relatively high voltages during electrolysis, or when exposed to a reducing fuel cell fuel gas (especially at higher operating temperatures). Partial reduction may result in the materials exhibiting mixed ionic / electronic conductivity, reducing operating efficiency.

[0013] There is a need, therefore, to provide electrochemical cells with improved electrolyte systems.

[0014] It is an aim of the present invention to address such a need.

[0015] SUMMARY OF THE INVENTION

[0016] The present invention accordingly provides, in a first aspect, an electrochemical cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5), wherein Ln is selected from at least one rare earth metal excluding Ce, 0.22 < x < 0.45, and 6 is the degree of oxygen deficiency. It was previously thought that a relatively high level of doping in ceria (e.g. of 20 cation % and above) would lead to unacceptably low ionic conductivity for an electrolyte at the usual temperatures of operation. However, the present inventors have discovered that relatively high levels of doping in ceria surprisingly have the effect of less partial reduction of doped ceria when subjected to relatively high voltages / current densities during electrolysis, or when exposed to a reducing, fuel cell fuel gas (especially at higher operating temperatures). This provides the advantages of less failure of SOCs due to ceria reduction which allows a longer life for SOCs and higher useable current density in SOECs.

[0017] The rare earth metal may be selected from a lanthanoid, Sc, Y and mixtures thereof.

[0018] Suitably, Ln is not (i.e. may exclude) Pr.

[0019] Ln excludes Ce.

[0020] The rare earth metal Ln may be selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Sc, Y and mixtures thereof. The rare earth metal may be selected from La, Sm, Gd, and Yb. Preferably, the rare earth metal may be selected from Gd, and / or Sm; more preferably Gd.

[0021] Suitably, the amount of Ln in the material of the first electrolyte layer may be 23 cation% or higher (i.e. 0.23 < x), optionally 24 cation% or higher (0.24 < x), optionally 25 cation% or higher (0.25 < x), optionally 26 cation% or higher (0.26 < x), optionally 27 cation% or higher (0.27 < x), optionally 28 cation % or higher (0.28 < x).

[0022] Suitably, the amount of Ln in the material of the first electrolyte layer may be 40 cation% or lower (i.e. x < 0.40), optionally 35 cation % or lower (x < 0.35), optionally 32 cation % or lower (x < 0.32).

[0023] Thus, preferably, 0.23 < x < 0.40, optionally 0.24 < x < 0.35, optionally 0.25 < x < 0.35, optionally 0.26 < x < 0.35, optionally 0.27 < x < 0.35, optionally 0.28 < x < 0.32. Generally, about 30 cation % in the material of the first electrolyte layer may be advantageous and provides a compromise between ionic conductivity and reducibility (i.e. x may be 0.30).

[0024] In some applications, x may be 0.30 or higher. Thus, optionally 0.31 < x, 0.32 < x, 0.35 < x. Optionally x < 0.4, x < 0.40, x < 0.38.

[0025] Optionally 0.31 < x < 0.40, 0.32 < x < 0.4, 0.32 < x < 0.38.

[0026] 6 may vary depending on the environment and history of the material of the first electrolyte layer. In the material of the first electrolyte layer, 6 may be 0.25 or lower, optionally 6 may be 0.2 or lower, optionally 6 may be 0.15 or lower.

[0027] 6 may have a lower limit of 0.0001 or higher, optionally 0.001 or higher, optionally 0.005 or higher, optionally 0.01 or higher, optionally 0.05 or higher.

[0028] The first electrolyte layer may be relatively thin yet still be effective. Thus, the first electrolyte layer may have a thickness of 17 pm or lower, optionally 15 pm or lower, optionally 13 pm or lower, optionally 12 pm or lower.

[0029] The first electrolyte layer may have a thickness of 3 pm or higher, optionally 4 pm or higher, optionally 5 pm or higher, optionally 6 pm or higher, optionally 7 pm or higher.

[0030] Thus, the first electrolyte layer may have a thickness in the range 3 pm to 17 pm, optionally in the range 4 pm to 15 pm, optionally in the range 4 pm to 13 pm, optionally in the range 5 pm to 12 pm, optionally in the range 6 pm to 12 pm.

[0031] The electrochemical cell may comprise two or more layers of electrolyte. Thus, the first electrolyte layer may comprise a bulk electrolyte layer and the electrochemical cell may further comprise a second electrolyte layer. Optionally the second electrolyte layer may comprise an electron blocking layer. This is advantageous because such a layer may reduce or avoid problems associated with residual mixed ionic / electronic conduction in the components of the cell.

[0032] The second electrolyte layer may comprise zirconia, optionally rare earth doped zirconia. Optionally, the second electrolyte layer may comprise zirconia doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln), optionally the second electrolyte layer may comprise 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.

[0033] The second electrolyte layer, e.g. of doped zirconia, advantageously provides a substantially electronically insulating layer.

[0034] The thickness of the second electrolyte layer may be 0.5 pm or higher, optionally 1 pm or higher, optionally 2 pm or higher.

[0035] The thickness of the second electrolyte layer may be 5 pm or lower, optionally 4 pm or lower, optionally 3 pm or lower. Thus, the thickness of the second electrolyte layer may be in the range 0.5 gm to 5 gm, optionally 0.5 pm to 4 pm; optionally 0.5 pm to 3 pm; or optionally 0.5 pm to 2 pm.

[0036] The electrochemical cell may further comprise a first electrode and a second electrode. Optionally, the first electrolyte layer, and optional second electrolyte layer, may be located between the first electrode and the second electrode.

[0037] The first electrode may comprise a fuel electrode and the second electrode may comprise an oxygen electrode.

[0038] Alternatively, the first electrode may comprise an oxygen electrode and the second electrode may comprise a fuel electrode.

[0039] The oxygen electrode may comprise one or more layers, and therefore may be a multilayer oxygen electrode system which may provide additional and / or improved properties for the electrochemical cell. For example, the oxygen electrode may be a two-layer, or three-layer system. Generally, each layer of the oxygen electrode system may be the same or different and, if different, may be formed of different materials and may have different properties and uses in the oxygen electrode as a whole.

[0040] A first oxygen electrode layer may comprise doped ceria, optionally the first oxygen electrode layer may comprise ceria gadolinium oxide (CGO) with dopant of Gd of 15 cation% or lower, optionally 10 cation% or lower.

[0041] The first oxygen electrode layer may comprise a material that is electrically conductive, optionally the first oxygen electrode layer may comprise a material that is an electrically conductive ceramic material. The first oxygen electrode layer may comprise a perovskite material.

[0042] For example, the first oxygen electrode layer may comprise a strontium containing material, optionally selected from rare earth strontium cobaltite; rare earth strontium ferrite, rare-earth strontium cobalt ferrite, rare earth containing strontium manganite, or mixtures thereof.

[0043] For example, the (first) oxygen electrode (layer) 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.

[0044] 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<q<0.3; 0.03<q<0.3; 0.04<q<0.3; 0.05<q<0.3; 0.05<q<0.27; 0.05<q<0.25; 0.05<q<0.25; or 0.05<q<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.

[0045] 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.

[0046] Advantageously, the Pr / Ln111material may have a cubic crystalline structure; preferably a fluorite crystalline structure. The first oxygen electrode layer 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.

[0047] 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.

[0048] The first oxygen electrode layer may consist essentially of a first electrode material (e.g. a perovskite material). Optionally, the first oxygen electrode layer may comprise a composite layer comprising a first electrode material and at least one further material. The further material may comprise, for example, doped ceria or doped zirconia or mixtures thereof.

[0049] Doped ceria may comprise cerium gadolinium oxide (CGO) with dopant of Gd of 15 cation% or lower, optionally 10 cation% or lower. Thus, the first electrode layer may comprise a mixture of e.g. a perovskite material (e.g. rare earth containing strontium cobaltite) and doped ceria (e.g. CGO) with dopant of Gd of 15 cation% or lower, optionally 10 cation% or lower.

[0050] The first oxygen electrode layer may have a thickness in the range 1 pm to 7 pm, optionally 1 pm to 6 pm; 1 pm to 5 pm; 1 to 4 pm or about 3 pm. The first oxygen electrode layer may be directly or indirectly in contact with the electrolyte. The first oxygen electrode layer may be between the electrolyte and the second oxygen electrode layer.

[0051] The oxygen electrode may comprise a second oxygen electrode layer (e.g. a bulk oxygen electrode layer) directly or indirectly in contact with the first oxygen electrode layer. The material of the second oxygen electrode layer may be electrically conductive, optionally the material of the second oxygen electrode layer may be an electrically conductive ceramic material. The material of the second oxygen electrode layer may be selected from a perovskite material, for example 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.

[0052] The second oxygen electrode layer may be a composite layer further comprising at least one additional second electrode layer material, optionally wherein the second oxygen electrode layer material may comprise doped ceria (e.g. CGO) with dopant of Gd of 15 cation% or lower, optionally 10 cation% or lower, and / or may comprise a strontium-containing material, optionally selected from rare earth strontium cobaltite; rare earth strontium ferrite, rare-earth strontium cobalt ferrite; wherein the rare earth component may optionally be Pr, La, Gd and / or Sm.

[0053] The second oxygen electrode layer may have a thickness in the range 10 pm to 80 pm; 15 pm to 75 pm; 17 pm to 73 pm; 20 pm to 70 pm; 20 pm to 65 pm; 20 pm to 60 pm; 25 pm to 55 pm; 30 pm to 50 pm; or 35 pm to 45 pm.

[0054] Generally, the second electrolyte layer may be located between the first electrolyte layer and the oxygen electrode.

[0055] Optionally, the electrochemical cell may further comprise an interlayer between the second electrolyte layer and the oxygen electrode to act as a diffusion barrier layer reducing or preventing the diffusion of components of e.g. the oxygen electrode into other layers of the electrolyte (for example into the second electrolyte layer). For example, where Sr is present in the oxygen electrode, and the second electrolyte layer comprises zirconia, the interlayer may form a diffusion barrier reducing or preventing diffusion of Sr into the zirconia layer and thereby reducing or preventing the reaction between Sr and Zr which may form an insulating layer of strontium zirconate (with consequent increase in resistance of the electrochemical cell). The interlayer may comprise doped ceria, optionally selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC, or CGO), praseodymium doped ceria (PDC), samaria-gadolinia doped ceria (SGDC) and mixtures thereof. The interlayer may comprise doped ceria (e.g. CGO) with dopant of 15 cation% or lower, optionally a dopant of Gd of 15 cation% or lower.

[0056] The electrochemical cell may further comprise a CGO interfacial layer of cerium gadolinium oxide located between the first electrolyte layer and the fuel electrode layer, wherein the CGO interfacial layer is doped with 15 cation% or lower Gd, optionally 12 cation% or lower Gd, optionally 10 cation% or lower Gd. The CGO interfacial layer may be doped with 1 cation% or higher Gd, optionally 2 cation% or higher Gd, optionally 5 cation% or higher Gd.

[0057] The fuel electrode may comprise a transition metal CGO cermet, optionally nickel CGO cermet.

[0058] The fuel electrode may comprise doped ceria of formula Ce(i-y)LnnyO(2-o.5y-5), wherein y <0.15. optionally < 0.12, optionally < 0.1, and Ln11is selected from at least one rare earth metal, optionally Gd.

[0059] Preferably, y<x, i.e. the dopant level in the doped ceria of the fuel electrode is preferably lower than the dopant level in the first electrolyte layer.

[0060] 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.

[0061] The fuel electrode may have one or more layers.

[0062] The electrochemical cell may further comprise a porous metal support, wherein at least a first layer of the first electrode is on a first surface of the porous metal support, a first electrolyte layer is on the first electrode, and at least a first layer of a second electrode in on the first electrolyte layer.

[0063] The porous metal support may comprise a barrier layer on at least one surface thereof, optionally wherein the barrier layer comprises CGO. The porous metal support may comprise a porous steel support, optionally a stainless steel support.

[0064] The metal support may have a porous region surrounded by a non-porous region with the electrochemically active layers being deposited upon the porous region so that gases may pass through the pores of the porous region from one side of the metal support to the opposite side to access the active layers coated thereon. The porous region may comprise small apertures (holes drilled through the metal foil substrate, also referred to as pores) extending through the support plate. Thus, 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.

[0065] Thus, in a further aspect, the present invention may provide an electrochemical cell comprising: a porous metal support, at least one layer of a first electrode on the porous metal support, a first electrolyte layer on the at least one layer of the first electrode, the first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5), wherein Ln is selected from at least one rare earth metal, 0.20 < x < 0.45, and 6 is the degree of oxygen deficiency.

[0066] The electrochemically active area may be disposed on the porous region of the support on a first surface of the support plate and may further comprise an interconnect (e.g. a metallic, particularly a stainless steel interconnect) (directly or indirectly) attached and electrically connected to (e.g. to a second surface of) the support plate.

[0067] The electrochemical cell may comprise an electrolyser cell, an oxygen separator, a sensor or a fuel cell. The invention is particularly advantageous for electrolyser cells.

[0068] Thus, in one further aspect the invention provides an electrolyser cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5), wherein Ln is selected from at least one rare earth metal, 0.20 < x < 0.45, and 6 is the degree of oxygen deficiency. Optionally, 0.22 < x < 0.45.

[0069] In one additional aspect, there is provided an electrochemical cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5), wherein Ln is selected from at least one rare earth metal excluding Ce, 0.20 < x < 0.45, and 6 is the degree of oxygen deficiency, optionally as modified by and / or having the optional features of the first aspect. Electrochemical cells may be arranged in a stack of electrochemical cell units, electrically connected in series. In such stacks electrical connection between cell units may rely on touchcontact of an optional interconnect of one electrochemical cell unit with an electrode of another electrochemical cell unit.

[0070] Thus, in a second aspect, the present invention provides, a stack of electrochemical cells, wherein each electrochemical cell is according to the first aspect.

[0071] The first electrolyte layer may be produced by depositing suitable precursor materials.

[0072] Thus, in a third aspect, the present invention provides a method of producing an electrochemical cell, the method comprising: applying a first electrolyte layer composition comprising a source of Ce and a source of Ln on to a substrate, optionally having deposited on the substrate at least one electrochemically active layer, optionally drying, and optionally sintering the first electrolyte layer; thereby forming a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5), wherein Ln is selected from at least one rare earth metal, 0.22 < x < 0.45, and 6 is the degree of oxygen deficiency.

[0073] Applying the first electrolyte layer composition may be by printing, optionally roller printing, jet printing or screen-printing, or by spraying (for example atomised spraying), or by vapor deposition such as using chemical vapor deposition (CVD), or using physical vapor deposition (PVD). The first electrolyte layer may be composed of one or more sublayers (each formed by e.g. repeating the step of applying a first electrolyte layer composition and optionally drying and optionally sintering), for example 1 to 6 sublayers, optionally 1 to 4 sublayers, optionally 1 to 3 sublayers, optionally 1 to 2 sublayers. Thus, applying a first electrolyte layer may comprise applying one or multiple sublayers of the first electrolyte layer.

[0074] Sintering may be performed at a temperature in the range 750 °C to 900 °C, preferably from 810 °C to 900 °C, preferably from 820 °C to 890 °C. Sintering may be performed a nonreducing environment. Sintering may be performed in an air atmosphere.

[0075] The first electrolyte layer, first electrode layer and / or other layers of the electrochemical cell may be pressed, optionally isostatically pressed, before and / or during sintering to improve adhesion and other properties.

[0076] In a fourth aspect, the present invention provides an electrolysis system comprising an electrochemical cell according to the first aspect. In a further aspect, the present invention provides a method of operating an electrochemical cell in electrolysis mode, the method comprising providing an electrochemical cell according to the first aspect, contacting the electrochemical cell with a fuel gas to be subject to electrolysis, and applying a potential to the electrochemical cell. This aspect may additionally or alternatively provide a method of operating a stack of electrochemical cells in electrolysis mode, the method comprising providing a stack of electrochemical cells (each cell in the stack according to the first aspect), contacting each electrochemical cell with a fuel gas to be subject to electrolysis, and applying a potential to each electrochemical cell.

[0077] The fuel gas may comprise, for example, steam and / or CO2 and / or NO2.

[0078] The method may be conducted at an operating temperatures of e.g. 450°C to 650°C.

[0079] In a further aspect, there is provided an electrochemical cell comprising a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-5); and a second electrolyte layer comprising a rare earth doped zirconia. In this aspect, the second electrolyte layer is in contact with the first electrolyte layer, Ln is selected from at least one rare earth metal excluding Ce, 0.20 < x < 0.45, and 6 is the degree of oxygen deficiency. A relatively high Ln content in the first electrolyte layer leads to reduced interdiffusion between the first and second electrolyte layers and in turn a lower ohmic resistance of the cell unit. Features of the preceding aspects may be combined with the present aspect.

[0080] Definitions

[0081] In this specification, the terms “rare earth metal” or “rare earth element” refer to metals selected from Y, Sc, and lanthanoid.

[0082] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71.

[0083] 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.

[0084] The term "perovskite structure" as used herein refers to a network of chemically bonded crystal structures which have a generally perovskite (ABX3) structure. Use of the term “perovskite structure” is not intended to mean that materials as disclosed herein have a single, uniform crystal structure throughout the entire structure.

[0085] The term "solid oxide cell" (SOC) is intended to encompass both solid oxide fuel cells (SOFCs) and solid oxide electrolyser cells (SOECs).

[0086] The term "cation percent" or "cation percentage" (abbreviated herein to "cation %" or “% cation”) refers to the percentage of cations with respect to the composition.

[0087] In this specification, the number of significant figures for a value (e.g. x < 0.4, x < 0.40) indicates the uncertainty in the value according to usual scientific convention.

[0088] 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.

[0089] 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, Gd, 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.

[0090] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to tubular or planar cells. Electrochemical cell units may be tubular or planar in configuration. Planar electrochemical cell units may be arranged overlying one another in a stack arrangement, for example 100-500 cell units in a stack, with the individual cell units arranged electrically in series.

[0091] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have similar structures and reference to electrochemical cells may refer (unless the context suggests otherwise) to any of these types of cell.

[0092] “Oxygen electrode” or “oxidant electrode” or “air electrode” and “fuel electrode” are used herein and may be used interchangeably to refer to cathodes and anodes respectively of SOFCs or anodes and cathodes respectively of SOECs.

[0093] The cells described herein include metal supported cells where the layers of the cell are supported by a metallic substrate, but the invention also encompasses anode supported, electrolyte supported or cathode supported cells where the respective layer provides the structural support for all the other layers coated thereon.

[0094] Electrochemical cells as encompassed by the invention may comprise a) two planar components welded together with fluid volume in between (e.g. substrate with electrochemical layers and interconnector (separate plate)); b) three planar components welded together with fluid volume in between (e.g. substrate with electrochemical layers and interconnector (separate plate) and spacer providing fluid volume or access thereto).

[0095] 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.

[0096] 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.

[0097] It will be understood that “attached” and “on” refer to direct or indirect attachment and positioning, respectively. In particular, unless the context otherwise suggests, reference to a particular layer of an electrochemical cell being “on” another layer encompasses both the layers being in contact and having an intermediate layer between them.

[0098] It will be understood that each layer may be comprised of multiple sub-layers (and those sublayers may have varying compositions).

[0099] It will be understood that layers of the electrochemical cell may be composite layers comprise a mixture of two or more materials.

[0100] The invention will now be described with reference to accompanying figures and examples.

[0101] BRIEF DESCRIPTION OF THE FIGURES

[0102] Figure 1 shows a schematic cross section of an electrochemical cell in accordance with the disclosure.

[0103] Figure 2 shows area specific resistance (ASR) in SOFC conditions as a function of temperature for electrochemical cells having a first electrolyte layer (ELI) with varying levels of Ln (Gd) dopant. Figure 3 is a graph showing SecASRp (polarisation resistance) in SOFC conditions as a function of temperature for electrochemical cells having a first electrolyte layer (EPl) with varying levels of Ln (Gd) dopant.

[0104] Figure 4 is a graph showing open circuit voltage as a function of temperature for electrochemical cells having a first electrolyte layer (EPl) with varying levels of Ln (Gd) dopant.

[0105] Figure 5 shows normalised area specific resistance (ASR) measured in SOEC as a function of temperature for electrochemical cells having a first electrolyte layer with varying levels of Ln (Gd) dopant.

[0106] Figure 6 shows normalised ohmic resistance (ASRs) measured in SOEC as a function of temperature for electrochemical cells having a first electrolyte layer with varying levels of Ln (Gd) dopant.

[0107] Figure 7 shows a scanning electron micrograph (SEM) of the cross section of an SOEC cell after testing to failure.

[0108] DETAILED DESCRIPTION OF THE INVENTION

[0109] Figure 1 shows, schematically and not to scale (for reasons of clarity), a cross section of an electrochemical cell 2.

[0110] 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, e.g. of CGOIO; i.e. of cerium gadolinium oxide with 10 cation % Gd) to reduce corrosion may be located on the surface of the metal support 4 (on one or both sides thereof).

[0111] A fuel electrode 10 is located on the porous region 8 of the metal support 4. The fuel electrode 10 may be formed of e.g. Ni:CGO (Ni: cerium gadolinium oxide).

[0112] An optional interfacial layer 12 of CGOIO is located on the fuel electrode 10.

[0113] A first electrolyte layer 14 of rare earth doped ceria (RE= Ln of 22cation% or higher) of thickness 2 pm or greater (optionally 6 pm to 12 pm) is located on the interfacial layer 12. The first electrolyte layer 14 may surround the fuel electrode 10 and interfacial layer 12 to reduce or prevent gas leaking from the fuel side 26 to the air side 24 or vice versa. The first electrolyte layer 12 may further overlap at least part of the non-porous region 6 of the substrate / support 4.

[0114] A second, electron blocking electrolyte layer 16 of rare earth (RE) stabilised zirconia (RE = Sc or Ln, e.g. Y or Yb) of thickness 0.5 pm or greater (e.g. 1 pm to 4 pm) is located on the first electrolyte layer 14.

[0115] An interlayer 18 of CGO (doped with 15 cation% or lower Gd) of thickness 100 nm to 1 pm is located on the second electrolyte layer 16, and is in direct contact with the second electrolyte layer 16. The interlayer 18 acts to reduce or prevent diffusion of e.g. Sr from the oxygen electrode 20,22 into the second electrolyte layer 16.

[0116] The electrochemical cell 2 has an oxygen electrode 20, 22 comprising a first oxygen electrode layer 20 of, for example, rare earth strontium cobaltite / ceria-gadolinia oxide (e.g. RE SC / CGO) or rare earth doped praseodymia (optionally with CGO) located on the interlayer 18 and directly in contact with the interlayer 18. The oxygen electrode has a second oxygen electrode layer 22 of an electrically conductive perovskite material.

[0117] Examples

[0118] In the following examples, electrochemical cells were made, generally having the structure as illustrated in Figure 1 with the first electrolyte layer having varying levels of Ln dopant.

[0119] CGO40 ELI + CGO10 BL refers to a two part electrolyte layer of CGO40 and CGO10.

[0120] The electrochemical cells were made and placed in 15 cell stacks and tested in both SOFC and SOEC modes.

[0121] AC impedance spectroscopy was conducted to allow ohmic resistance to be separated from polarisation resistance.

[0122] For SOFC conditions the cells were fuelled by simulated steam reformed natural gas at a composition equivalent to a thermodynamic equilibrium of 545°C with a stearmcarbon ratio of 2.5, under the conditions 133 mAcm'2, 75% fuel utilisation, 545°C reformate equilibrium.

[0123] The SOEC conditions involved running the stack in electrolysis mode at three temperatures: 510°C, 530°C, and 550°C. At each temperature an IV curve was run until the stack on average reached the thermoneutral voltage (about 1.28 V / cell), at which point there is no net thermal input or output from the electrochemical reaction. The cells are fed with a mixture of 95% steam and 5% hydrogen, and run to 75% oxygen utilisation (transmission of oxygen across the cell units from the fuel side (supply of oxygen in fuel - steam) to the oxygen side).

[0124] Results are shown in Figures 2 to 6.

[0125] In the results, first electrolyte layers having higher than 20cation% dopant showed advantageous properties of reduced high temperature electronic leakage which more than compensates for possibly higher electrolyte resistance at lower temperatures. Differences in electrolyte resistance are much less significant at higher temperature. Such electrolyte layers are advantageous for electrochemical cells, in particular in electrolysis mode.

[0126] Figure 2 shows area specific resistance (ASRs) in SOFC conditions as a function of temperature for electrochemical cells having a first electrolyte layer (ELI) with varying levels of Gd dopant. At relatively low temperatures, ASRs follows the pattern CGOIO < CGO20 ~ CGO30 < CGO40 ~ CGO40 ELI + CGOIO BL. High temperature values for all variants converge to very similar values.

[0127] Figure 3 is a graph showing SecASRp (polarisation resistance) in SOFC conditions as a function of temperature for electrochemical cells having a first electrolyte layer (EPl) with varying levels of Gd dopant. At relatively low temperatures SecASRp also follows the pattern consistent with ionic conductivity of the electrolyte CG010<CG020<CG030<<CG040 ~ CGO40 ELI + CGOIO BL. At temperatures higher than around 570°C, the SecASRp values of the samples become much more similar.

[0128] Figure 4 is a graph showing open circuit voltage (OCV) as a function of temperature for electrochemical cells having a first electrolyte layer (EPl) with varying levels of Gd dopant. The OCV results are consistent with reduced levels of electronic leakage in materials with higher levels of dopant (e.g. CGO30 c / CGOlO) as OCV falls more slowly with increasing temperature, and the CGOIO and CGO30 curves converge at the higher temperatures.

[0129] Figure 5 shows normalised area specific resistance (ASR) measured in SOEC as a function of temperature for electrochemical cells having a first electrolyte layer with varying levels of Gd dopant measured in SOEC conditions at 75% oxygen utilisation and thermoneutral voltage (approx. 1.28V / cell). Electrolytes containing CGO30 layer adjacent to the fuel electrode compare favourably with CGOIO electrolytes. Cell performance is broadly comparable. Figure 6 shows normalised ohmic resistance (ASRs) measured in SOEC as a function of temperature for electrochemical cells having a first electrolyte layer with varying levels of Gd dopant measured in SOEC conditions and thermoneutral voltage (approx. 1.28V / cell). Electrolytes containing CGO30 layer adjacent to the fuel electrode compare favourably with CGOIO electrolytes. Cell performance is broadly comparable.

[0130] Results in Figures 5 and 6, under SOEC conditions are consistent with results of tests under SOFC conditions in Figures 2 to 4.

[0131] Figure 7 shows an SEM cross section of an SOEC cell intentionally tested to destruction by applying a high voltage to the cell (>2V). The SOEC cell has a two part first electrolyte layer of CGO30 and CGO 10. The SEM shows a fuel electrode 31, CGO30 electrolyte layer 32, CGOIO electrolyte layer 33, thin zirconia electron blocking layer 34 and oxygen electrode 35. After testing to destruction, there is extensive cracking of the CGOIO layer of the electrolyte induced through tensile stresses from reduction due to high voltage. However, this cracking does not extend into the CGO30 layer showing very enhanced robustness to highly reducing conditions in SOEC mode of materials with higher levels of dopant.

[0132] In some cases a lower ohmic resistance of cell units with a rare earth (e.g., Gd) doping level in the first electrolyte layer 14 of greater than 10% may be observed, such cell units also comprising a second, electron blocking, electrolyte layer 16 of rare earth (RE) stabilised zirconia. This may be due to reduced interdiffusion with the second, electron blocking, electrolyte layer 16 of rare earth (RE) stabilised zirconia, said interdiffusion may result in a partially ionically insulating Ce-Zr composite at the boundary between the first electrolyte layer and the second electrolyte layer. A rare earth dopant level in the first electrolyte of 20% or above (e.g., up to 45%) reduces the amount of Ce in the first electrolyte thereby reducing the scale of interdiffusion layer formation.

[0133] REFERENCE NUMERALS

[0134] 2 electrochemical cell

[0135] 4 metal support

[0136] 6 non-porous region of metal support

[0137] 8 porous region of metal support

[0138] 10 fuel electrode 12 CGO interfacial layer

[0139] 14 first electrolyte layer

[0140] 16 second, electron blocking electrolyte layer

[0141] 18 interlayer

[0142] 20 first oxygen electrode layer

[0143] 22 second oxygen electrode layer

[0144] 24 oxygen side

[0145] 26 fuel side

[0146] 31 fuel electrode

[0147] 32 CGO30 electrolyte layer

[0148] 33 CGO 10 electrolyte layer

[0149] 34 zirconia electron blocking layer

[0150] 35 oxygen el ectrode .

[0151] 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 electrochemical cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x-6), wherein Ln is selected from at least one rare earth metal excluding Ce,0.22 < x < 0.45, and6 is the degree of oxygen deficiency.

2. An electrochemical cell as claimed in claim 1, wherein the rare earth metal is selected from Gd and Sm; preferably Gd.

3. An electrochemical cell as claimed in either claim 1 or claim 2, wherein 0.23 < x < 0.40, optionally 0.24 < x < 0.35, optionally 0.25 < x < 0.35, optionally 0.28 < x < 0.32.

4. An electrochemical cell as claimed in any one of the preceding claims, wherein the first electrolyte layer has a thickness of 17 pm or lower, optionally 15 pm or lower, optionally 13 pm or lower, optionally 12 pm or lower.

5. An electrochemical cell as claimed in any one of the preceding claims, wherein the first electrolyte layer has a thickness of 3 pm or higher, optionally 4 pm or higher, optionally 5 pm or higher, optionally 6 pm or higher, optionally 7 pm or higher.

6. An electrochemical cell as claimed in any one of the preceding claims, wherein the first electrolyte layer has a thickness in the range 3 pm to 17 pm, optionally in the range 4 pm to 15 pm, optionally in the range 4 pm to 13 pm, optionally in the range 5 pm to 12 pm, optionally in the range 6 pm to 12 pm.

7. An electrochemical cell as claimed in any one of the preceding claims, further comprising a second electrolyte layer, optionally wherein the second electrolyte layer comprises an electron blocking layer.

8. An electrochemical cell as claimed in claim 7, wherein the second electrolyte layer comprises zirconia, optionally rare earth doped zirconia, optionally wherein the second electrolyte layer comprises zirconia doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln), optionally wherein the second electrolyte layer 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.

9. An electrochemical cell as claimed in either claim 7 or claim 8, wherein the thickness of the second electrolyte layer is 0.5 pm or higher, optionally 1 pm or higher, optionally 2 pm or higher.

10. An electrochemical cell as claimed in any one of claims 7 to 9, wherein the thickness of the second electrolyte layer is 5 pm or lower, optionally 4 pm or lower, optionally 3 pm or lower.

11. An electrochemical cell as claimed in any one of the preceding claims, further comprising a first electrode and a second electrode.

12. An electrochemical cell as claimed in any one of the preceding claims, wherein the first electrode comprises a fuel electrode and the second electrode comprises an oxygen electrode.

13. An electrochemical cell as claimed in any one of claims 1 to 11, wherein the first electrode comprises an oxygen electrode and the second electrode comprises a fuel electrode.

14. An electrochemical cell as claimed in either claim 12 or claim 13, wherein the second electrolyte layer is located between the first electrolyte layer and the oxygen electrode.

15. An electrochemical cell as claimed in any one of claims 12 to 14, further comprising a CGO interfacial layer of cerium gadolinium oxide located between the first electrolyte layer and the fuel electrode layer, wherein the CGO interfacial layer is doped with 15 cation% or lower Gd, optionally 12 cation% or lower Gd, optionally 10 cation% or lower Gd.

16. An electrochemical cell as claimed in any one of claims 12 to 15, wherein the fuel electrode comprises a transition metal CGO cermet, optionally nickel CGO cermet.

17. An electrochemical cell as claimed in claim 16, wherein the fuel electrode comprises doped ceria of formula Ce(i-y)LnnyO(2-o.5y-5), wherein y <0.

15. optionally < 0.12, optionally < 0.1, and Ln11is selected from at least one rare earth metal, optionally Gd.

18. An electrochemical cell as claimed in any one of claims 11 to 17, wherein the oxygen electrode comprises 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, optionally Gd or Sm, optionally Sm, 6 is the degree of oxygen deficiency, and 0.01 < q < 0.4.

19. An electrochemical cell as claimed in any one of claims 11 to 18, further comprising a porous metal support, wherein at least a first layer of the first electrode is on a first surface of the porous metal support, a first electrolyte layer is on the first electrode, and at least a first layer of a second electrode in on the first electrolyte layer.

20. An electrochemical cell as claimed in any one of the preceding claims, wherein the electrochemical cell comprises an electrolyser cell, an oxygen separator, a sensor or a fuel cell.

21. An electrochemical cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x- 6), wherein Ln is selected from at least one rare earth metal excluding Ce,0.20 < x < 0.45, and6 is the degree of oxygen deficiency, optionally as modified by and / or having the features of any one of claims 2 to 20.

22. A stack of electrochemical cells, wherein each electrochemical cell is as claimed in any one of the preceding claims.

23. A method of producing an electrochemical cell, the method comprising applying a first electrolyte layer composition comprising a source of Ce and a source of Ln on to a substrate, optionally having deposited on the substrate at least one electrochemically active layer, wherein Ln is selected from at least one rare earth metal excluding Ce, optionally drying, and optionally sintering the first electrolyte layer; thereby forming a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-0.5x-5), wherein Ln is selected from at least one rare earth metal,0.22 < x < 0.45, and6 is the degree of oxygen deficiency.

24. An electrolysis system comprising an electrochemical cell as claimed in any one of claims 1 to 20.

25. A method of operating an electrochemical cell in electrolysis mode, the method comprising providing an electrochemical cell as claimed in any one of claims 1 to 21 or providing a stack of electrochemical cells as claimed in claim 22, contacting the or each electrochemical cell with a fuel gas to be subject to electrolysis, and applying a potential to the or each electrochemical cell.

26. An electrochemical cell comprising: a first electrolyte layer comprising a material having a composition Ce(i-x) LnxO(2-o.5x- 5>; and a second electrolyte layer comprising a rare earth doped zirconia, wherein: the second electrolyte layer is in contact with the first electrolyte layer,Ln is selected from at least one rare earth metal excluding Ce,0.20 < x < 0.45, and6 is the degree of oxygen deficiency.

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