Electrode and electrochemical cell

Abstract

An electrode for an electrochemical cell. The electrode comprises at least a first layer comprising a first electrode material, the first electrode material being a composition Ppy: Pr (1-(x + y)) LnxO (2-0.5 x-delta). Ln is selected from at least one rare earth metal, delta is oxygen deficit degree, y is greater than or equal to 0.0001 and less than or equal to 0.05, and x is greater than or equal to 0.01 and less than or An electrochemical cell comprising said electrode and a stack of said electrochemical cells, a method for producing said electrode and said composition.

Description

TECHNICAL FIELD

[0001] The present invention relates to an electrode for an electrochemical cell, an electrochemical cell comprising such an electrode, a method of producing such an electrochemical cell and a material for use in such an electrode. BACKGROUND

[0002] Electrochemical cells formed from oxide layers (commonly referred to as solid oxide cells: SOC) can be used as fuel cell or electrolyser cells.

[0003] SOC fuel cell units generate electricity using an electrochemical conversion process of an oxidising fuel. SOC fuel cell units can also or instead operate as regenerative fuel cell (or reverse fuel cell) units (commonly referred to as solid oxide electrolyser fuel cell units), for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide.

[0004] Solid oxide fuel cells (SOFCs) generate electrical energy through the electrochemical oxidation of a fuel gas (typically hydrogen-based), and the devices are typically ceramic-based, using a ceramic containing oxygen-ion conducting metal oxide as its electrolyte. Many ceramic oxygen-ion conductors (e.g. doped zirconia or doped ceria) have useful ionic conductivity at temperatures in excess of 500°C (for ceria-based electrolytes) or 650°C (for zirconia-based ceramics), so SOFCs tend to operate at elevated temperatures.

[0005] In operation, the electrolyte of a SOFC conducts oxygen ions from the cathode to the anode, which is located on the opposite side of the electrolyte. A fuel, for example a fuel resulting from the reforming of a hydrocarbon or alcohol, contacts the anode (commonly referred to as the “fuel electrode”), and an oxidant, for example air or an oxygen-rich fluid, contacts the cathode (commonly referred to as the “air electrode”). Conventional ceramic-supported (e.g. anode-supported) SOFCs have low mechanical strength and are prone to cracking. Therefore, metal-supported SOFCs have recently been developed, which have layers of active fuel cell components supported on a metal substrate. In these cells, the ceramic layers can be very thin, as they only perform an electrochemical function: that is, the ceramic layers are not self-supporting, but are thin coatings / films laid on and supported by the metal substrate. Such metal-supported SOFC stacks are more robust, less costly, have better thermal characteristics than ceramic-supported SOFCs, and can be sealed using conventional metal welding techniques.

[0006] Applicant’s earlier patent application WO-A-2015 / 136295 discloses a metal-supported SOFC in which the electrochemically active layer (or active fuel cell component layer) comprises an anode layer, an electrolyte layer and a cathode layer each deposited (e.g. as a thin coating / film) on and supported by a metal support plate (e.g. a foil). The metal support plate has a porous region surrounded by a non-porous region, with the active layers deposited on the porous region so that gas can pass from one side of the metal support plate through the pores to the opposite side to access the active layers coated thereon. The porous region includes small holes (holes drilled through the metal foil substrate) which extend through the support plate overlying the anode (or cathode, depending on the orientation of the electrochemically active layer).

[0007] A solid oxide electrolysis cell (SOEC) can have the same structure as a SOFC but is in fact a SOFC operated in reverse or in a regenerative mode to effect electrolysis of water and / or carbon dioxide.

[0008] The fuel electrode, electrolyte and air electrode of an SOC can each be formed of one or more layers to optimise operation. Effective air electrode materials allow oxygen diffusion to the air electrode / electrolyte interface and have a similar coefficient of thermal expansion to the electrolyte. Practical air electrode materials often have a perovskite structure ABX3, where A and B are different metal ions (there can be more than one A and B metal ion), X can be O. The air electrode in some SOFCs can be formed of an active layer close to the electrolyte with high activity for the electrochemical reduction of oxygen and a bulk layer which can be a metal conductor. There are many known cathode materials.

[0009] Cruz Pacheco et al (J. Phys: Conference Series, Volume 687, Issue 1, 2016) disclose the synthesis of praseodymium-doped ceria by a polycombustion method for use as an anode component in SOFC devices.

[0010] Reasons for doped praseodymium oxides have been investigated, unrelated to SOCs. For example, Zollner et al. (J. Crystal Growth, vol. 355, no. 1, 2012, pp. 159-165) disclose the stoichiometry-structure relationship of cerium-doped praseodymium oxide films epitaxially grown on Si(111). Knath et al. (Journal of the European Ceramic Society, vol. 19, no. 6-7, 1999, p. 831. Popescu Ione et al. (Applied Catalysis A: General, vol. 578, 2019, p. 30-39) disclose a study of catalytic oxidation performance of Ce-Pr mixed oxides. Simona Somacescu et al. (J. Nanoparticle Research; vol. 14, no. 6, 2012, pp. 1-17) disclose CePrO structure, morphology, surface chemistry and catalytic performance. Kang et al. (J. Alloys and Compounds, vol. 207-208, 1994, pp. 420-423) disclose structure and structural defects of colloidal particles modified in situ in HREM.

[0011] US-B-6,117,582 describes a cathode composition for a solid oxide fuel cell, with a cathode made of a transition metal perovskite such as PrCoO3 or praseodymium submanganite. US-A-2017 / 149067 discloses fuel cells and cathodes which can contain nickelate compounds such as Pr2NiO4. Nikitch, C et al., International Journal of Hydrogen Energy, September 2016, vol. 41, no. 34, pp. 15538-15544 describe Pr6O 11 Electrocatalysts for oxygen reduction reactions and their use as cathodes in SOFCs. CN-A-106057641 discloses La, Nd and Gd doped Pr semiconducting oxides. Wang et al. 2017 232 nd ECS Meet, Abstr, (MA2017-02 / 39 / 1730) discloses Pr 1-x Nd x O 2-d In combination with (Pr,Nd)2NiO4(PNNO) to improve the activity and phase stability of PNNO for use as a cathode for a solid oxide fuel cell. Biswas, R et al. (1997) Journal of Materials Science Letters. 16. 1089-1091 disclose Pr 1- x La x O 2-δ(x = 0.05, 0.1, 0.2) preparation, structure and conductivity. Zhu et al., Advanced Materials Research, Vol. 1065-1069, (2014), pp. 1921-1925 disclosed Ce 0.8 Pr 0.2-x Nd x O 2-δ Preparation and properties of (x = 0.02, 0.05, 0.1). WO-A-2006 / 106334A1 describes a solid oxide fuel cell (SOFC) in which the cathode material includes a dopant material having a perovskite structure, which may include praseodymium. This structure has the conventional notation ABX3, with cerium substituted into the "B" site.

[0012] However, there remains a need to provide electrode materials having suitable properties for use in electrochemical cells.

[0013] The present invention aims to address this need. Summary of the Invention

[0014] Therefore, the present invention provides, in a first aspect, an electrode for an electrochemical cell, said electrode comprising at least a first layer comprising a composition Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ) The first electrode material, wherein Ln is selected from at least one rare earth metal, δ is the degree of oxygen deficiency, 0.0001≤y≤0.05, and 0.01≤x≤0.4.

[0015] This is very advantageous because the presence of Pd reduces the polarization resistance of oxygen reduction / evolution and lowers the area specific resistance of the electrode, significantly improving the SOC efficiency using this electrode.

[0016] The first electrode material can generally be a single phase. However, under certain conditions (e.g., during or after sintering or during use), a solid solution (e.g., a solid solution of Pd in ​​RE-doped praseodymium oxide) or PdO (or Pd) nanoparticles may be present in the composition. Thus, the first electrode material having an overall composition can be a single phase or a mixed material, and different portions of the electrode can have different local compositions or mixed materials or phases.

[0017] δ can vary depending on the environment and history of the first electrode material. In many oxidizing environments containing praseodymium oxides, praseodymium is in thermodynamic equilibrium between its +3 and +4 oxidation states, which depends on temperature and oxygen partial pressure. 4+ Reduction to Pr 3+ When , oxygen vacancies are generated. Oxygen vacancies caused by praseodymium reduction are called extrinsic vacancies. The balance can be expressed as:

[0018]

[0019] where Vo" is an oxygen vacancy.

[0020] In the first electrode material, δ can be 0.25 or lower, suitably 0.2 or lower, more suitably <0.15.

[0021] The lower limit of δ can be 0.0001, optionally 0.001, optionally 0.005, optionally 0.01, optionally 0.05.

[0022] The addition of (e.g. trivalent) dopant cations to praseodymium oxide creates intrinsic oxygen vacancies in the structure. In the first electrode material, the rare earth metal can suitably act as a dopant.

[0023] Pd can be present in the composition, such that the lower limit of y can optionally be 0.001, optionally 0.005 (equivalent to 0.5 atomic % based on cations in the composition), optionally 0.01 (equivalent to 1 atomic % based on cations in the composition).

[0024] Suitably, the upper limit of Pd in the composition can be 0.05 (equivalent to 5 atomic % of cations in the composition), optionally 0.04 (equivalent to 4 atomic % of cations in the composition), optionally 0.03 (equivalent to 3 atomic % of cations in the composition).

[0025] Suitably, Y can be: 0.001 < y < 0.05, optionally 0.001 < y < 0.03.

[0026] The rare earth metal can be selected from the lanthanide series, Sc, Y and mixtures thereof.

[0027] Suitably, the rare earth metal is not cerium.

[0028] Suitably, the rare earth metal can be selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof. More suitably, the rare earth metal can be selected from La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof. More suitably, the rare earth metal can be selected from La, Nd, Sm, Eu, Gd and Yb; preferably Nd, Sm, Eu, Gd, more preferably Gd or Sm, most preferably Sm.

[0029] Most suitably, the rare earth metal can be selected from La, Sm, Gd and Yb; preferably Sm.

[0030] As used in the present specification, Ln represents a dopant, thus Ln does not include Pr.

[0031] Pr oxide represents a somewhat variable phase system. Single phase Pr02is usually formed in pure oxygen and at elevated pressures (>20000 kPa). Among the different oxides, Pr60 11 is particularly stable. At ambient temperature and pressure, Pr60 11 adopts a cubic fluorite structure in which the praseodymium ions in Pr60 11 are in mixed valence states of Pr(III) and Pr(IV) with external oxygen vacancies, which promote oxygen ion conduction and are believed (without wishing to be bound) to provide catalytic activity.

[0032] Advantageously, the presence of the rare earth metal dopant in the first electrode material can result in the formation of additional intrinsic oxygen vacancies and can stabilise the cubic fluorite structure of the material.

[0033] In the first electrode material, x can be selected to achieve a balance between oxygen vacancy concentration and ion mobility, for example 0.02 to 0.25. Advantageously, x can 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, x can be 0.08 to 0.2 or 0.08 to 0.12, more suitably x can be about 0.1 ; about 0.15; or about 0.2.

[0034] Suitably, 0.02 < x < 0.25.

[0035] Thus, suitably, the first electrode material can be a composition Pd y :Pr 0.9-y Ln 0.1 O (1.95-δ) , Pd y :Pr 0.85-y Ln 0.15 O (1.925-δ) , Pd y :Pr 0.8-y Ln 0.2 O (1.9-δ) or mixtures thereof; wherein Ln is La, Sm, Gd or Yb; preferably Sm.

[0036] The first layer of the electrode can consist essentially of the first electrode material. Optionally, the first layer can comprise a composite layer comprising the first electrode material and at least one further material. The further material can comprise, for example, doped ceria or doped zirconia or mixtures thereof. The doped ceria can comprise cerium gadolinium oxide (CGO). The doped zirconia can be a solid solution of the formula Zr (1-x) Y x O (2-0.5xδ) where 0 < x < 0.2.

[0037] Thus, the first layer can comprise 20% or more by weight of the first electrode material; optionally 25% or more by weight of the first electrode material; optionally 30% or more by weight of the first electrode material; optionally 35% or more by weight of the first electrode material; optionally 40% or more by weight of the first electrode material; optionally 45% or more by weight of the first electrode material; optionally 50% or more by weight of the first electrode material; optionally 55% or more by weight of the first electrode material; optionally 60% or more by weight of the first electrode material.

[0038] The first layer can 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.

[0039] The electrode can be a multi-layer electrode system, which provides additional and / or improved performance to the electrochemical cell. For example, the electrode can be a two-layer, three-layer, four-layer or five-layer system, or can have more than five layers. Typically, each layer of the electrode system can be the same or different, and if different, can be formed of different materials and can have different properties and purposes in the electrode system as a whole.

[0040] Thus, the electrode can comprise at least a second layer comprising a second electrode material. Optionally, the second electrode material can be electrically conductive, optionally can be an electrically conductive ceramic material.

[0041] The second layer can 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.

[0042] In a second aspect, the present application thus provides an electrode for an electrochemical cell, the electrode comprising at least a first layer and at least a second layer, the first layer comprising a composition Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ)a first electrode material, the second layer comprising a second electrode material; wherein Ln is selected from at least one rare earth metal of La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, δ is the degree of oxygen deficiency, 0.0001 < y < 0.0.5, and 0.01 < x < 0.4.

[0043] As an example, the first and second layers of the electrode system can comprise a first layer as described above for use as an air electrode active layer (also referred to in SOFCs as a cathode active layer, CAL), and a second layer as an air electrode bulk layer (also referred to in SOFCs as a cathode bulk layer, CBL), respectively. The air electrode bulk layer can have more electronic conductivity (i.e. electronic conductivity) than the first layer, and can thus act as a current collector.

[0044] The first layer can be located next to an electrolyte (which itself can be an electrolyte system consisting of multiple layers), with an intermediate layer (e.g. another layer of the electrode) between the first layer of the electrode and the electrolyte, or wherein the first layer directly contacts (i.e. is directly adjacent to) the electrolyte layer.

[0045] The second layer (e.g. air electrode bulk layer) can advantageously be formed from or comprise an electrically conductive second electrode material, e.g. which can be a metallic conductor at the operating temperature of the electrochemical cell, and which can have a relatively high electronic conductivity at those temperatures. The second layer material is preferably chemically and mechanically stable. The second layer, e.g. air electrode bulk layer, is typically porous (as is the first layer), to allow good interaction with oxygen on the air side of the cell. The second layer (e.g. air electrode bulk layer) can have a lower electrocatalytic activity than the first layer (which, as described above, can have a high electrocatalytic activity).

[0046] The second electrode material can comprise an electronically conductive ceramic material, optionally having a perovskite structure ABX3.

[0047] Suitable second electrode materials include lanthanum cobaltate, lanthanum nickel ferrite, La 0.99 Co 0.4 Ni 0.6 O (3-δ) (LCN60) and mixtures thereof.

[0048] Optionally, the second layer can be a composite layer further comprising at least one additional second electrode material. The additional electrode material can comprise a strontium-containing material, optionally selected from rare earth strontium cobaltate; rare earth strontium ferrite, rare earth strontium cobalt ferrite; the rare earth component can optionally be Pr, La, Gd and / or Sm, preferably Pr.

[0049] Optionally, the composite second electrode layer can comprise 60 wt% or more of the second electrode material; optionally 65 wt% or more of the second electrode material; optionally 70 wt% or more of the second electrode material; optionally 75 wt% or more of the second electrode material.

[0050] The electrode can further comprise a third layer, which can comprise a third electrode material.

[0051] To improve adhesion between the first electrode layer and the second electrode layer, the third layer can optionally be located between the first layer and the second layer, if required.

[0052] Optionally, the third electrode material can comprise an oxygen ion conductor. The oxygen ion conductor can preferably comprise doped ceria, or doped zirconia or mixtures thereof. The doped ceria can preferably comprise cerium gadolinium oxide (CGO), which is a solid solution having the formula Ce (1-x) Gd x O (2-0.5x-δ) where 0 < x < 0.5. The doped zirconia can be a solid solution having the formula Zr (1-x) Y x O (2-0.5xδ) where 0 < x < 0.2.

[0053] Additionally or alternatively, the third electrode material can comprise a strontium containing material, optionally selected from the group consisting of: rare earth strontium cobaltite; rare earth strontium ferrite, rare earth strontium cobalt ferrite; wherein the rare earth component can optionally be Pr, La, Gd and / or Sm; preferably Pr.

[0054] Optionally, the third electrode material can comprise a mixture of rare earth strontium hibonite or rare earth strontium ferrite and rare earth doped ceria (REDC). A particularly suitable third electrode material can comprise a mixture of praseodymium strontium cobaltite (e.g. PSC 551 : Pr 0.5 Sr 0.5 CoO3) and CGO in a 60:40 wt% ratio.

[0055] The third electrode material can facilitate good adhesion between the first and second electrode layers and can reduce any reaction between the second electrode material (e.g. LCN60) and the first electrode material under battery conditions, which can result in the formation of secondary phases, which can result in poorer adhesion and potentially increased ohmic resistance.

[0056] Furthermore, the third electrode layer can act as a poison getter for the first electrode layer, as contaminants in the battery can react with the third electrode material (e.g. containing strontium cobaltite / cobalt ferrite) before contacting the first electrode layer. This advantageously protects the first electrode material and layer from degradation. These contaminants can include chromium, silicon and sulphur from SO2 in the air.

[0057] The third layer can have a thickness in the range of 1 pm to 5 pm; 1 pm to 4 pm; 2 pm to 5 pm; or 2 pm to 4 pm.

[0058] During sintering, the electrode layers (e.g. the first electrode layer, the second electrode layer and / or the third electrode layer) can be pressed (optionally isostatic pressed) to improve adhesion and other properties.

[0059] The electrode of the first aspect or the second aspect can be an air electrode.

[0060] The electrode of the first aspect or the second aspect can be an air electrode in an electrochemical cell, such as an SOC, SOFC or SOEC.

[0061] Accordingly, in a third aspect, the present application therefore provides an electrochemical cell comprising an electrode according to any of the preceding claims; optionally further comprising one or more of an electrolyte, a second electrode and a substrate. The second electrode can be a second fuel electrode.

[0062] The electrolyte can comprise at least one electrolyte layer comprising doped ceria, optionally selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), praseodymium-doped ceria (PDC), samarium-gadolinium-doped ceria (SGDC) and mixtures thereof.

[0063] The electrolyte can comprise at least one electrolyte layer comprising zirconia, optionally selected from scandium oxide stabilized zirconia (ScSZ), yttrium oxide stabilized zirconia (YSZ), ytterbium oxide stabilized zirconia (YbSZ), scandium oxide cerium oxide co-stabilized zirconia (ScCeSZ), scandium oxide yttrium oxide co-stabilized zirconia (ScYSZ) and mixtures thereof.

[0064] The electrochemical cell can further comprise a substrate; optionally a metallic substrate, preferably a steel substrate. The substrate can be porous.

[0065] The metallic substrate can be a metal foil (i.e. a solid metal) in which openings are provided. This has the advantage that the porosity can be adjusted and positioned in specific areas of the substrate. Alternatively or additionally, the metallic substrate can have an intrinsic porosity (e.g. isotropic porosity), for example by tape casting a film of powder, which is then sintered to form a porous substrate. Reference herein to a metallic substrate or a porous steel plate can refer to either of these.

[0066] The electrochemical cell can be an electrolysis cell, an oxygen separator, a sensor or a fuel cell, or an electrolysis cell, preferably a SOFC.

[0067] In fuel cell mode, a fuel contacts the anode (fuel electrode), and an oxidant such as air or an oxygen-rich fluid contacts the cathode (air electrode), so in fuel cell mode operation, the air electrode will be the cathode. A solid oxide electrolysis cell (SOEC) can have the same structure as a SOFC, but is essentially a SOFC operated in reverse or in a regenerative mode to effect electrolysis of water and / or carbon dioxide by using the solid oxide electrolyte to produce hydrogen and / or carbon monoxide and oxygen.

[0068] The electrode of the first or second aspect is printed or otherwise applied to a substrate onto which the layers are deposited.

[0069] In a fourth aspect, the present application thus provides a method of manufacturing an electrode for an electrochemical cell, the method comprising: providing a substrate, optionally depositing layers comprising a fuel electrode and an electrolyte thereon, applying an electrode composition comprising a source of Pd, Pr and Ln to the substrate to form an air electrode layer, wherein Ln is selected from at least one rare earth metal, optionally drying, and optionally sintering the air electrode layer; thereby forming the electrode.

[0070] Optionally, the method can further comprise applying a material to the substrate to form at least one electrolyte layer, applying the electrode composition on the electrolyte layer to form the air electrode layer, optionally drying, and co-sintering the electrolyte layer and the air electrode layer.

[0071] The air electrode layer (e.g. the active air electrode layer CAL) can be co-sintered (i.e. co-sintered) with an underlying electrolyte material layer, wherein these two layers have been laid down sequentially as green layers (and optionally pressed). The at least one electrolyte layer (there can be further electrolyte layers) can be a layer comprising zirconia (e.g. an electron blocking layer). Co-sintering is highly advantageous as it allows production in fewer steps.

[0072] The sintering or co-sintering can be carried out at a temperature in the range of 750 °C to 900 °C, preferably 790 °C to 900 °C. The sintering can be carried out in an air atmosphere.

[0073] In the method, the electrode composition can comprise a co-precipitate of a palladium salt, a praseodymium salt and an Ln salt. The method can comprise the step of forming the electrode composition by providing a mixture of a palladium salt, a praseodymium salt and an Ln salt and co-precipitating the electrode composition.

[0074] The method of making the composition can comprise:

[0075] (a) preparing a first solution comprising a soluble Pd salt (preferably Pd nitrate), a soluble Pr salt (preferably Pr nitrate) and a soluble Ln salt (preferably an Ln nitrate);

[0076] (b) mixing the first solution of (a) with a second solution capable of reacting with the salt of (a) (e.g. a basic solution of ammonium hydroxide) to form an insoluble precipitate, wherein the insoluble precipitate can (subsequently) thermally decompose;

[0077] (c) calcining the insoluble precipitate to decompose the insoluble precipitate and produce a material according to the first aspect of the application.

[0078] The method of forming an electrode comprising at least a first electrode layer can comprise the steps of providing a suitable dispersion in a carrier of a first electrode layer material, applying a coating of the dispersion to a substrate; and sintering the coating to form the air electrode.

[0079] In a fifth aspect, the present application provides a material of composition Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ) wherein Ln is selected from at least one rare earth metal, δ is the degree of oxygen deficiency, 0.0001 < y < 0.0.5, 0.01 < x < 0.4.

[0080] In a sixth aspect, the present application provides a material of composition Pd y :Pr (1-(x+y)) Sm x O (2-0.5x-δ) wherein δ is the degree of oxygen deficiency, 0.0001 < y < 0.0.5, 0.01 < x < 0.4.

[0081] Definitions

[0082] In this specification, the terms "lanthanoid" and "lanthanide" are used interchangeably and refer to the chemical elements having atomic numbers 57-71.

[0083] The term "dopant" as used herein is not intended to be limited to the maximum percentage of an element, ion or compound added to a chemical structure. Similarly, the term "doping" is intended to mean the addition of an amount of an element, ion or compound to a material. It is not limited to the maximum amount of the material after which further additions of the material no longer constitute doping.

[0084] The term "perovskite structure" as used herein refers to a single network of chemically bonded crystal structures having a general perovskite (ABX3) structure. This does not mean that the single network needs to have a single, uniform crystal structure throughout the structure. However, when different crystal structures appear between different regions of the network, it is often the case that these regions have complementary structures, allowing chemical bonds to form more easily between them.

[0085] The term "solid oxide cell" (SOC) is intended to include both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC).

[0086] The term "atomic percent" or "atomic percentage" (abbreviated herein as "atom %") refers to the atomic percentage with respect to a given dopant site, in the case of Pd, refers to the atomic percentage calculated as Pd cations in the composition.

[0087] The term "source" of an element, compound, or other material refers to a material that includes the element, compound, or other material, whether or not it is chemically bonded in the source. The source of an element, compound, or other material can be a source of the element (e.g., Ln, Sm, Pr, or O2) or can be in the form of a compound or mixture of elements, compounds, or other materials that includes one or more of these elements, compounds, or materials.

[0088] In this specification, references to electrochemical cells, SOCs, SOFCs, and SOECs can refer to tubular or planar cells. The electrochemical cell units can be in a tubular or planar configuration. Planar fuel cell units can be arranged in a stacked arrangement on top of each other, for example, 100-200 fuel cell units in a stacked arrangement, with individual fuel cell units arranged in electrical series.

[0089] The electrochemical cells can be fuel cells, reversible fuel cells, or electrolysis cells. In general, these cells can have the same structure, and a reference to an electrochemical cell can refer to any of these types of cells (unless the context implies otherwise).

[0090] "Oxidant electrode" or "air electrode" and "fuel electrode" are used herein and, due to potential confusion between fuel cells or electrolysis cells, are used interchangeably to refer to the cathode and anode, respectively, of a SOFC.

[0091] Although fuel cells are described in this specification in which the fuel electrode (e.g., anode) is placed first on the substrate, the present invention also includes cells in which the air electrode is placed first on the substrate.

[0092] The cells described herein include metal-supported cells, in which the layers of the cell are supported by a metal substrate, but the present invention also includes anode-supported, electrolyte-supported, or cathode-supported cells, in which a layer provides structural support for all other layers coated thereon.

[0093] The electrochemical cells encompass by the present invention can include:

[0094] a) two planar components welded together with a fluid volume in between (e.g., a substrate with electrochemical layers and an interconnector (separate plate))

[0095] b) three planar components welded together with a fluid volume in between (e.g., a substrate with an electrochemical layer and interconnect (separate plates), and a spacer providing the fluid volume).

[0096] As will be appreciated by those skilled in the art, the various features of the aspects of the disclosure as described herein can be used in combination with any other features of the same or other aspects of the disclosure, if desired, with appropriate modifications, as will be appreciated by those skilled in the art.

[0097] Furthermore, although all aspects of the application or disclosure preferably "comprise" the features described in relation to that aspect, it is specifically contemplated that they can "consist" or "consist essentially of those features outlined in the claims.

[0098] The application will now be described with reference to the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0099] Figure 1 Scanning electron microscope (SEM) cross-section showing a portion of a SOFC including an air electrode (cathode) active layer (CAL) comprising a material according to the disclosure.

[0100] Figure 2 Area specific resistance (ASR) of a cell with a CAL as a function of temperature (at 133 mA cm -2 , 75% fuel utilization (Uf) operating on simulated steam reformed natural gas with a thermodynamic equilibrium of 545°C), the CAL comprising a material according to the disclosure sintered at different temperatures normalized to a standard cell in the same stack.

[0101] Figure 3 Area specific resistance (ASR) of a cell with a CAL as a function of temperature (at 225 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions), the CAL comprising a material according to the disclosure sintered at different temperatures normalized to a standard cell in the same stack.

[0102] Figure 4 Series resistance (Rs) from an alternating current impedance spectrum (ACIS) of a cell with a CAL as a function of temperature (at 133 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions), the CAL comprising a material according to the disclosure sintered at different temperatures normalized to a standard cell in the same stack.

[0103] Figure 5 Serpentine polarization resistance (SecRp) from an alternating current impedance spectrum (ACIS) of a cell with a CAL as a function of temperature (at 133 mA cm -2, 75% Uf, 545°C reforming equilibrium conditions), CALs comprising materials according to the present disclosure sintered at different temperatures, normalized to a standard cell in the same stack.

[0104] Figure 6 XRD patterns (Cu Kα radiation) are shown for undoped PSmO10 in comparison with 2PdPSmO10 calcined at 700°C and 820°C. DETAILED DESCRIPTION

[0105] Figure 1 An SOC is shown, comprising a cathode comprising a 2 atomic % (based on cations) Pd 10% Sm doped praseodymium oxide (2PdPSmO) cathode active layer (CAL) 1, a thin doped ceria buffer layer 2 and a zirconia electron blocking layer 3 (forming part of the electrolyte, not shown).

[0106] Although not shown, the SOC in Figure 1 may be deposited on the surface of a metal substrate, such as a metal, in particular a ferritic stainless steel substrate, more particularly a foil substrate.

[0107] The CAL 1 comprises a material according to the present disclosure. The compositions of the other layers are of a type known to the skilled person, as are the methods of manufacture and application. Reference can be made, for example, to WO 2009 / 090419 A2, which discusses methods for applying layers of these types and exemplary compositions to metal substrates, in particular stainless steel substrates. The layers, including the air electrode layer, can show good adhesion and / or can be isostatically pressed to further improve adhesion.

[0108] Figure 2 The normalized ASR of the CAL of 2PdPSmO is shown as a function of temperature, where the 2PdPSmO was sintered at 800°C (curve 4), 820°C (curve 5) or 850°C (curve 6). The ASR of the 2PdPSmO cells is consistently lower than the standard cell, with evidence of a trend for improved performance as the sintering temperature of the cathode is increased. The performance is normalized so that the ASR of the standard cell is 100 at each temperature, so a value lower than 100 means that the ASR is lower and the cell performance is improved.

[0109] Figure 3 The normalized ASR of the cell with the CAL of 2PdPSmO is shown as a function of ACIS scan temperature (at a current density of 0.1 A cm-2 Figure 2 in the higher current density of 0.2 A cm-2, where the 2PdPSmO was sintered at 800°C (curve 7), 820°C (curve 8) or 850°C (curve 9).

[0110] Similarly, the ASR of the 2PdPSmO cell was consistently lower than the standard cell.

[0111] Figure 4 The normalized series resistance (Rs) of the cells with CAL having 2PdPSmO is shown as a function of temperature (at 0.7V Figure 2 under the same conditions, where the 2PdPSmO was sintered at 800°C (curve 10), 820°C (curve 11) or 850°C (curve 12). There is evidence that even at lower temperatures, the Rs of the 2PdPSmO cell has a tendency to be lower than the standard cell, and also that the higher the sintering temperature of the cathode, the better the performance. Without wishing to be bound, this can be consistent with an improved CAL electrolyte interface phase of 2PdPSmO, which is a standard cell with a reduced resistance of ion transport at higher temperatures sintered in the cell.

[0112] To investigate the measurement of ASR, Rs and polarization resistance (in this case working in SOFC mode), the stack was supplied with a fuel mixture of natural gas partially simulating external steam reforming at a flow rate such that 75% of the oxidizable fuel was consumed by electrochemical reactions within the stack. Air was supplied to the air electrode side of the cell stack at a flow rate well in excess of the stoichiometric requirement for oxygen to minimize internal temperature gradients. The constant current density was 133 mA cm -2 The stack temperature was varied by controlling the furnace temperature at which the test was conducted.

[0113] At each temperature, once the cell stack reached thermal equilibrium, the impedance of all 17 cells was measured using an AC impedance spectrum. This technique allows the internal cell impedance to be separated into ohmic (non-frequency varying) and non-ohmic components. The electrochemical impedance of the air electrode belongs to the non-ohmic part of the impedance, hereafter referred to as the polarization resistance. In a complete fuel cell, it is not normally possible to separate the contribution of the air electrode from the fuel electrode, so the polarization resistance is the polarization resistance of the whole cell. In this case, the polarization resistance is calculated from the open circuit voltage drop minus the voltage drop due to ohmic resistance (which does not vary much with applied current at a given temperature). This is referred to as the secant polarization resistance (SecRp), which is different from the polarization resistance value directly from the ACIS measurement, which is essentially the local gradient of the current-voltage (IV) curve, which can be very non-linear at high fuel utilization. It has been found that the SecRp measurement is better at distinguishing changes in electrode performance. The quoted values are normalized to the value of a cell with a standard air electrode in the same stack, so that the normalized value is 100, and are all averages of at least three cells. Since the fuel electrode and the external environment of the cell are the same, any differences in polarization resistance can be attributed to changes in the electrochemical activity of the air electrode in the reduction of oxygen.

[0114] These curves show that the material according to the present disclosure is useful as an electrode (air electrode) material.

[0115] Figure 5 The SecRp of the cell with CAL of 2PdPSmO is shown as a function of the ACIS temperature (in the case of Figure 2 The 2PdPSmO CAL cells exhibited lower SecRp than the standard cells (under the same conditions), with 2PdPSmO sintered at 800°C (curve 13), 820°C (curve 14), or 850°C (curve 15). The cells with the highest CAL sintering temperature performed best at most temperatures. These materials are used as electrode (air electrode) materials in accordance with the present disclosure.

[0116] Figure 6 Shown are XRD patterns (Cu K-alpha radiation) of undoped PSmO10 (pink and brown lines) compared with 2PdPSmO10 calcined at 700°C (green line) and 820°C (orange line).

[0117] There are no significant differences between the patterns except for the apparent improvement in crystallinity of the sample calcined at 820°C. Within the sensitivity range of XRD, no reflections indicating the presence of palladium or palladium oxide as a second phase were observed.

[0118] The following are Examples 1-4, a general method for synthesizing Pd:rare earth doped praseodymium oxide according to the present invention (Example 1), synthesizing a printing ink using the Pd doped praseodymium powder (Example 2), and printing a CAL using the ink (Example 3).

[0119] Example 1 : Preparation of Pd:RE doped praseodymium oxide powder

[0120] Solution preparation

[0121] A stoichiometric mixture of praseodymium nitrate hexahydrate and the desired rare earth dopant nitrate and Pd nitrate was dissolved in deionized (DI) water to give a 0.15 M solution molarity.

[0122] In a separate container under a fume hood, the concentrated ammonium hydroxide solution was diluted in deionized water to give an equal volume of 0.45 M solution to the nitrate solution.

[0123] Precipitation

[0124] While the mixture was vigorously stirred, the nitrate solution was added to the ammonium hydroxide solution to obtain a light green gelatinous precipitate of insoluble praseodymium dopant hydroxide.

[0125] Separation

[0126] The precipitate was separated from the supernatant by centrifugation and then washed with deionized water and ethanol in centrifuge bottles.

[0127] Washing

[0128] The precipitate was washed 3 times with DI water and once with ethanol.

[0129] Drying

[0130] The wet cake was transferred from the funnel to a suitable container and dried in an oven at 70°C overnight.

[0131] Crushing

[0132] The dried precipitate pieces were pulverized using a laboratory blender and the resulting powder was transferred to an alumina crucible.

[0133] Calcination

[0134] The pulverized precipitate was transferred to an alumina crucible which was placed in a suitable furnace and heated in air to a temperature of 650°C to decompose the hydroxide precipitate to the desired mixed oxide.

[0135] Example 2: Synthesis of printable ink

[0136] Dispersion and milling of Pd:RE doped praseodymium oxide powder

[0137] The Pd:RE doped praseodymium oxide powder prepared as described in Example 1 was weighed and mixed with a carrier, dispersant and defoamer to form a slurry containing about 46 wt% of the target amount of powder.

[0138] The slurry was transferred to a basket mill to which was also added a slurry of twice the weight of 1 mm YSZ milling media.

[0139] The slurry was milled at about 7000 rpm until a D90 < 0.9 μm was achieved. The particle size distribution can be measured using a Malvern 2000 laser diffraction particle size analyser.

[0140] The slurry was then removed from the basket mill.

[0141] Ink manufacture

[0142] The dispersed milled Pd:RE doped praseodymium oxide powder slurry prepared in the previous section was transferred to a high shear disperser (HSD) pot and placed on the HSD.

[0143] An amount of binder powder equivalent to 2.5-3.5 wt% of the finished ink was weighed.

[0144] The binder is added to the slurry that is actively dispersed on the HSD.

[0145] The ink remains on the HSD until the binder is completely dissolved in the ink.

[0146] The ink is transferred to a three-roll mill (TRM) for final homogenization and is passed through the mill four times with a front nip of 5 μm to ensure complete homogenization of the binder into the ink and no particles greater than 5 μm remain in the finished ink.

[0147] Example 2: Printing of ink and formation of active layer

[0148] The substrate for printing includes an electrolyte layer deposited on a metal supported SOFC. The ink is screen printed in a single pass using an automated screen printer onto the electrolyte layer of the metal supported SOFC. It is then dried in a drying oven. The ink solids content and screen mesh combination are chosen to give a thin print of about 3 μm. After the addition of the CBL, the layer is then sintered with the CBL at a temperature of 800 to 870 °C to form the CAL. After sintering, the X-ray diffraction and BET analysis are repeated. After sintering, the crystallite size increases slightly and the BET surface area decreases, but the crystal structure does not change. The layer still consists of a single phase with a cubic fluorite structure.

[0149] Example 4: SOFC cell with air electrode CAL using Pd:PrLnO.

[0150] The Pd:RE doped praseodymium oxide powder slurries described herein and exemplified in Examples 1-3 above have comparable or better performance than standard.

[0151] An SOFC air electrode is prepared that consists of three layers. The three layer electrode advantageously reduces the impact of chromium contamination (praseodymium oxide can react with chromium oxide to form a perovskite) and ensures better adhesion between the bulk layer and the active layer.

[0152] The three layers of the electrode are an LCN60 bulk layer that provides excellent stability and thermal expansion matching to the rest of the cell, an interface composite layer, and a catalytically active layer of rare earth doped praseodymium oxide. The interface layer both ensures good adhesion between the active layer and the bulk and acts as a poison getter for the active layer, as poisons such as chromium and sulfur will react with the rare earth strontium cobaltate / cobalt ferrite before reaching the strontium free active layer, which can be susceptible to chromium poisoning. The interface layer has a similar thermal coefficient to the air electrode bulk layer. This protects the active layer from degradation (it is not affected by water vapor, carbon dioxide, or sulfur dioxide)

[0153] The air electrode is made by screen printing three layers, a thin layer (about 3 microns) of the first electrode material (2PdPSmO10), a thin layer (about 3 microns) of rare earth strontium cobaltite / CGO (e.g. ReSC / CGO 60:40; where "Re" means rare earth), and finally a thicker (about 40 microns) bulk layer (LCN 60).

[0154] Optionally, these layers can be sintered and isostatically or uniaxially pressed to increase their green density, and then finally sintered at 800-850°C in air to form the finished air electrode.

[0155] The described air electrode is provided in a standard metal supported SOFC, and incorporated in a stack of 17 cells. For each cell, the anode is ceria-nickel metal ceramic, and the electrolyte includes CGO with a doped zirconia electronic blocking layer. The CAL can be in direct contact with the zirconia electronic blocking layer, or a layer of e.g. CGO can be interposed between the active layer and the zirconia electronic blocking layer.

[0156] Under the described conditions, the stack can be operated with air flow on the air side and a fuel of simulated steam reformed natural gas on the fuel side.

[0157] All publications mentioned in the above specification are herein incorporated by reference. While the specification has been described in detail with respect to specific illustrative embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the application defined by the appended claims and their equivalents.

Claims

1. An electrode for an electrochemical cell, comprising at least a first layer comprising a composition of Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ) The first electrode material, wherein Ln is selected from at least one rare earth metal, δ is the degree of hypoxia, 0.0001≤y≤0.05, and 0.01≤x≤0.4。 The electrode according to claim 1 , wherein 0.001≤y≤0.

05. The electrode according to claim 2 , wherein 0.001≤y≤0.

03.

4. An electrode according to any one of the preceding claims, wherein the rare earth metal is selected from La, Sm, Gd and Yb; preferably Sm.

5. The electrode according to any one of the preceding claims, wherein 0.02≤x≤0.

25.

6. The electrode according to any one of the preceding claims, wherein the first electrode material is a composition Pd y :Pr 0.9-y Ln 0.1 O (1.95-δ) 、Pd y :Pr 0.85-y Ln 0.15 O (1.925-δ) 、Pd y :Pr 0.8-y Ln 0.2 O (1.9-δ) or a mixture thereof; wherein Ln is La, Sm, Gd or Yb; preferably Sm.

7. An electrode according to any one of the preceding claims, wherein the first layer comprises 20 wt% or more of the first electrode material; optionally 30 wt% or more of the first electrode material; optionally 40 wt% or more of the first electrode material; optionally 55 wt% or more of the first electrode material.

8. The electrode according to any one of the preceding claims, wherein the first layer has a thickness in the range of 1 μm to 7 μm.

9. The electrode according to any of the preceding claims, wherein the electrode comprises at least a second layer comprising a second electrode material.

10. An electrode for an electrochemical cell, said electrode comprising at least a first layer, said first layer comprising a composition Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ) a first electrode material, and the electrode comprises at least a second layer, the second layer comprising a second electrode material; wherein Ln is selected from at least one rare earth metal, δ is the degree of hypoxia, 0.0001≤y≤0.05, and 0.01≤x≤0.4。 11. The electrode according to any one of the preceding claims, wherein the electrode is an air electrode.

12. An electrochemical cell comprising an electrode according to any one of the preceding claims; optionally further comprising one or more of an electrolyte, a second electrode and a substrate.

13. The electrochemical cell of claim 12, wherein the electrolyte comprises at least one electrolyte layer comprising doped ceria, the doped ceria optionally being selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), praseodymium-doped ceria (PDC), samarium-gadolinium-doped ceria (SGDC), and mixtures thereof.

14. An electrochemical cell according to claim 12 or claim 13, wherein the electrolyte comprises at least one electrolyte layer comprising zirconium oxide, optionally selected from scandia-stabilized zirconium oxide (ScSZ), yttria-stabilized zirconium oxide (YSZ), ytterbium oxide-stabilized zirconium oxide (YbSZ), scandia-ceria co-stabilized zirconium oxide (ScCeSZ), scandia-yttria co-stabilized zirconium oxide (ScYSZ), and mixtures thereof.

15. The electrochemical cell according to any one of the preceding claims 12 to 14, further comprising a substrate; optionally a metal substrate, preferably a steel substrate.

16. The electrochemical cell of any preceding claim 12 to 15, wherein the electrochemical cell is an electrolysis cell, an oxygen separator, a sensor, or a fuel cell, and optionally wherein the electrochemical cell comprises a solid oxide electrochemical cell.

17. A stack of electrochemical cells according to any one of claims 12 to 16.

18. A method of making an electrode for an electrochemical cell, the method comprising: providing a substrate, optionally with deposited thereon a layer comprising a fuel electrode and an electrolyte, applying an electrode composition comprising Pd, Pr and Ln sources to the substrate to form an air electrode layer, wherein Ln is selected from at least one rare earth metal, optionally drying at least the first layer, and optionally sintering at least the first layer; Thereby, the electrode according to any one of claims 1 to 11 is formed.

19. The method according to claim 18, wherein the method further comprises: applying a material to the substrate to form at least one electrolyte layer, applying an electrode composition onto the electrolyte layer to form an air electrode layer, optionally drying at least the air electrode layer, and The electrolyte layer and the air electrode layer are co-sintered.

20. The method according to claim 18 or claim 19, wherein the sintering or co-sintering is carried out at a temperature of 790°C or higher.

21. The method of any one of preceding claims 18 to 20, wherein the electrode composition comprises a co-precipitate of a palladium salt, a praseodymium salt, and a Ln salt.

22. The method of any one of claims 18 to 21 further comprising the step of forming the electrode composition by providing a mixture of a palladium salt, a praseodymium salt, and a Ln salt and co-precipitating the electrode composition.

23. A composition Pd y :Pr (1-(x+y)) Ln x O (2-0.5x-δ) Materials, wherein Ln is selected from at least one rare earth metal, δ is the degree of hypoxia, 0.0001≤y≤0.0.5, and 0.01≤x≤0.4。 24. A composition Pd y :Pr (1-(x+y)) Sm x O (2-0.5x-δ) Materials, where δ is the degree of hypoxia, 0.0001≤y≤0.05, and 0.01≤x≤0.4。

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