Electrode and electrochemical cell

Abstract

An electrode for an electrochemical cell. The electrode comprises at least a first layer comprising a first electrode composition comprising Pr (1-x) LnxO (2-0.5 x-delta) and a lithium source. Ln is selected from at least one rare earth metal selected from the group consisting of La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof. Delta is oxygen deficit degree, 0.01 < = x < = 0.4. An electrochemical cell comprising said electrode and a stack of 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 comprises small pores (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 in fact is a SOFC operated in reverse or in regeneration 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 which has 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 polymeric combustion method for use as an anode component in SOFC devices.

[0010] Reasons for doped praseodymium oxides have been investigated, independent of SOC. For example, Zollner et al. (J. Crystal Growth, vol. 355, issue 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, issues 6-7, 1999, pp. 831-839) disclose a study of the catalytic oxidation performance of Ce-Pr mixed oxides. Simona Somacescu et al. (J. Nanoparticle Research; vol. 14, issue 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 in situ modified colloidal particles in HREM.

[0011] Tuller et al. (Energy Environ Sci, 2022, 15, 4038) describe the reactivation of chromium oxide poisoning of oxygen exchange kinetics in ceria-based solid oxides Pr 0.1 Ce 0.9 O 2-δ oxide poisoning in chromium oxide.

[0012] 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, issue 34, pp. 15538-15544 describe Pr6O 11 Electrocatalysts for the oxygen reduction reaction 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-din combination with (Pr,Nd)2Ni04(PNNO) to improve the activity and phase stability of PNNO for use as a cathode for solid oxide fuel cells. Biswas, R et al (1997) Journal of Materials Science Letters. 16. 1089-1091 discloses Pr 1- x La x O 2-δ Preparation, structure and electrical conductivity of (Pr0.8Ce0.2)0.8Fe0.2Mn0.8O3-δ (x = 0.05, 0.1, 0.2). Zhu et al, Advanced Materials Research, Volume 1065-1069, (2014), Pages 1921-1925 discloses Ce 0.8 Pr 0.2-x Nd x O 2-δ Preparation and properties of (Pr0.8Ce0.2)0.8Fe0.2Mn0.8O3-δ (x = 0.02, 0.05, 0.1). WO-A-2006 / 106334 Al describes a solid oxide fuel cell (SOFC) in which the cathode material comprises a doped material having a perovskite structure, which can include praseodymium. The structure has the conventional notation ABX3, in which cerium is substituted into the "B" site.

[0013] However, there remains a need to provide electrode materials having suitable properties for use in electrochemical cells.

[0014] It is an object of the present invention to address this need. SUMMARY

[0015] Accordingly, the present invention provides in a first aspect an electrode for an electrochemical cell, the electrode comprising at least a first layer comprising a first electrode composition, the first electrode composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source,

[0016] wherein Ln is selected from at least one rare earth metal, the rare earth metal being selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof,

[0017] δ is the degree of oxygen deficiency, and

[0018] 0.01 < x < 0.4.

[0019] comprising Pr (1-x) Ln x O (2-0.5x-δ)The first electrode composition is highly advantageous because the applicants have surprisingly found that it increases the average cell potential, especially at lower temperatures, and can act as a sintering aid. The Pr (1-x) Ln x O (2-0.5x-δ) It can also mitigate the effects of chromium poisoning during SOC use, and it can also (without wishing to be bound) increase the surface reaction rate at the air electrode / electrolyte interface.

[0020] The first electrode material can be a mixed material. However, under certain conditions (for example during or after sintering or in use), there can be solid solution (for example of Li in RE-doped praseodymium oxide) in the composition or there can be lithium oxide (or other lithium compound, for example lithium carbonate) particles or nanoparticles. Thus, the first electrode material with the overall composition can be one or more phases or a mixed material, and different parts of the electrode can have different local compositions or mixed materials or phases.

[0021] δ can vary depending on the environment and history of the first electrode material. In many oxidising environments containing praseodymium oxides, praseodymium is in thermodynamic equilibrium between its +3 and +4 oxidation states, depending on the temperature and oxygen partial pressure. When Pr 4+ is reduced to Pr 3+ Oxygen vacancies are created by the reduction of praseodymium. Oxygen vacancies caused by the reduction of praseodymium are known as extrinsic vacancies. Using the notation, the equilibrium can be represented as:

[0022]

[0023] where Vo" is an oxygen vacancy.

[0024] In the first electrode material, δ can be 0.25 or lower, suitably 0.2 or lower, more suitably <0.15.

[0025] The lower limit for δ can be 0.0001, optionally 0.001, optionally 0.005, optionally 0.01, optionally 0.05.

[0026] The addition of (for example trivalent) dopant cations to praseodymium oxide creates intrinsic oxygen vacancies in the structure. In the first electrode material, the rare earth metals can suitably act as dopants.

[0027] Suitably, the upper limit for lithium in the composition can be 7 cation %, optionally 6 cation %, optionally 5 cation %.

[0028] Suitably, the lower limit for lithium in the composition can be 0.001 cation %, optionally 0.01 cation %, optionally 0.1 cation %, optionally 1 cation %.

[0029] Cation % means atomic percent (equivalent to mole percent) based on cations in the first electrode composition (e.g. cations Pr, Li, Ln).

[0030] Lithium can be present in the first electrode composition in an amount of 0.001 to 5 cation %. Optionally, lithium can be present in the first electrode composition in an amount of 0.01 to 5 cation %.

[0031] Suitably, the lithium source can be present in the first electrode composition in an amount of 0.1 to 5 cation %. More suitably, the lithium source can be present in the first electrode composition in an amount of 0.2 to 5 cation %, 0.3 to 5 cation %, 0.5 to 4 cation %, 0.5 to 3 cation %.

[0032] The rare earth metal can be selected from the lanthanide series, Sc, Y and mixtures thereof, excluding cerium.

[0033] 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; optionally from La, Sm, Gd and Yb; preferably Nd, Sm, Eu, Gd, more preferably Gd or Sm, most preferably Sm.

[0034] As used in the present specification, Ln represents a dopant, thus Ln does not include Pr.

[0035] Praseodymium oxides represent a somewhat varied 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.

[0036] 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.

[0037] In the first electrode material, x can be selected to achieve a balance between oxygen vacancy concentration and ionic 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.

[0038] Suitably, 0.02 < x < 0.25.

[0039] Thus, suitably, the first electrode material can comprise Pr 0.9 Ln 0.1 O (1.95-δ) , Pr 0.85 Ln 0.15 O (1.925-δ) , Pr 0.8 Ln 0.2 O (1.9-δ) or mixtures thereof; and a lithium source.

[0040] The first layer of the electrode can consist essentially of the first electrode material.

[0041] 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 according to the formula Zr (1-x) Y x O (2-0.5xδ) where 0 < x < 0.2.

[0042] Thus, the first layer can comprise 20 wt% or more of the first electrode material; optionally 25 wt% or more of the first electrode material; optionally 30 wt% or more of the first electrode material; optionally 35 wt% or more of the first electrode material; optionally 40 wt% or more of the first electrode material; optionally 45 wt% or more of the first electrode material; optionally 50 wt% or more of the first electrode material; optionally 55 wt% or more of the first electrode material; optionally 60 wt% or more of the first electrode material.

[0043] 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.

[0044] The lithium source can include any suitable lithium compound, such as lithium oxide, lithium hydroxide, lithium salts (e.g. lithium nitrate, lithium carbonate), lithium salts of organic acids (e.g. lithium citrate, lithium acetate or lithium oxalate) and / or lithium as a dopant.

[0045] 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.

[0046] Thus, the electrode can comprise at least a second layer, the second layer comprising a second electrode material. Optionally, the second electrode material can be electrically conductive, optionally can be an electrically conductive ceramic material.

[0047] The second layer can have a thickness in the range of 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.

[0048] 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 first electrode material of a composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source, the second layer comprising a second electrode material; wherein Ln is selected from at least one rare earth metal from the group consisting of La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, δ is the oxygen deficiency, 0.01 < x < 0.4.

[0049] As one 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 electrical conductivity (i.e. electronic conductivity) than the first layer, and can thus act as a current collector.

[0050] 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.

[0051] The second layer (e.g. air electrode body layer) can advantageously be formed from or include a second electrode material which is electrically conductive, 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 body layer, is typically porous (as is the first layer), to allow good interaction with oxygen at the air side of the cell. The second layer (e.g. air electrode body layer) can have a lower electrocatalytic activity than the first layer (which, as noted above, can have a high electrocatalytic activity).

[0052] The second electrode material can comprise an electronically conductive ceramic material, optionally having a perovskite structure ABX3.

[0053] 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.

[0054] Optionally, the second layer can be a composite layer further comprising at least one further second electrode material. The further 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.

[0055] 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.

[0056] The electrode can further comprise a third layer, which can comprise a third electrode material.

[0057] 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 desired.

[0058] 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.

[0059] Additionally or alternatively, the third electrode material can comprise a strontium containing material, optionally selected from strontium cobaltite; strontium ferrite, strontium cobalt ferrite; wherein the rare earth component can optionally be Pr, La, Gd and / or Sm; preferably Pr.

[0060] Optionally, the third electrode material can comprise a mixture of strontium cobaltite or strontium ferrite and rare earth doped ceria (REDC). A particularly suitable third electrode material can comprise a 60:40 weight ratio mixture of praseodymium strontium cobaltite (e.g. PSC 551 : Pr 0.5 Sr 0.5 CoO3) and CGO.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] The electrode of the first aspect or the second aspect can be an air electrode.

[0066] The electrode of the first aspect or the second aspect can be an air electrode in an electrochemical cell, for example an air electrode in an SOC, an SOFC or an SOEC.

[0067] Accordingly, in a third aspect, the present application therefore provides an electrochemical cell comprising an electrode according to any of the preceding aspects; optionally further comprising one or more of an electrolyte, a second electrode and a substrate. The second electrode can be a second fuel electrode.

[0068] 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.

[0069] 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-cerium oxide co-stabilized zirconia (ScCeSZ), scandium-yttrium oxide co-stabilized zirconia (ScYSZ) and mixtures thereof.

[0070] The electrochemical cell can further comprise a substrate; optionally a metallic substrate, preferably a steel substrate. The substrate can be porous.

[0071] 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 powder deposited film, which is then sintered to form a porous substrate. Reference herein to a metallic substrate or porous steel plate can refer to either of these.

[0072] The electrochemical cell can be an electrolysis cell, an oxygen separator, a sensor or a fuel cell, preferably a solid oxide electrochemical cell (which can be a fuel cell SOFC or an electrolysis cell SOEC).

[0073] 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 regenerative mode to effect electrolysis of water and / or carbon dioxide by using a solid oxide electrolyte to produce hydrogen and / or carbon monoxide and oxygen.

[0074] The electrodes are printed or otherwise applied to a substrate onto which the layers are deposited.

[0075] 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 at least a first layer comprising a first electrode composition, the first electrode composition comprising Pr (1-x) Ln x O (2-0.5x-δ)and a lithium source, optionally drying at least the first layer, and optionally sintering at least the first layer; thereby forming an electrode; wherein Ln is selected from at least one rare earth metal from the group consisting of La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, δ is the oxygen deficiency, 0.01 < x < 0.4.

[0076] This is advantageous because the applicant has surprisingly found that the presence of a lithium source results in a greatly improved sintering which can result in a greatly improved average cell voltage in an electrochemical cell, such as a SOFC.

[0077] Optionally, the method can further comprise applying a material to the substrate to form at least one electrolyte layer, applying the first electrode composition on the electrolyte layer to form an air electrode layer, optionally drying the air electrode layer, and co-sintering the electrolyte layer and the air electrode layer.

[0078] The air electrode layer (e.g. 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 sequentially laid down as green layers (and optionally pressed). The at least one electrolyte layer (there can be other electrolyte layers) can be a layer comprising zirconia (e.g. an electron blocking layer). Co-sintering is very advantageous because it allows production in fewer steps.

[0079] 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.

[0080] In the method, the first electrode composition can comprise Pr (1-x) Ln x O (2-0.5x-δ) and a lithium salt, optionally lithium nitrate, lithium carbonate, lithium citrate, lithium acetate and / or lithium oxalate, preferably lithium nitrate.

[0081] The method of preparing the material of the first electrode composition can comprise the steps of:

[0082] dissolving or dispersing Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source, optionally dissolving or dispersing a lithium source (e.g. lithium compound) in a solvent, preferably an alcohol;

[0083] optionally drying; and

[0084] calcining the Li-containing Pr (1-x) Ln x O (2-0.5x-δ) (e.g. at a temperature of 450 °C to 600 °C) to produce the material of the first electrode composition (e.g. by decomposing the lithium salt to produce lithium oxide).

[0085] 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.

[0086] In a fifth aspect, the present application provides a composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source, wherein Ln is selected from at least one rare earth metal selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, and δ is the degree of oxygen deficiency, 0.01 < x < 0.4.

[0087] In a sixth aspect, the present application provides a composition comprising Pr (1-x) Sm x O (2-0.5x-δ) and a lithium source, wherein δ is the degree of oxygen deficiency, 0.01 < x < 0.4.

[0088] In a seventh aspect, the present application provides a stack of electrochemical cells according to the third aspect.

[0089] Definitions

[0090] In this specification, the terms "lanthanoid" and "lanthanide" are used interchangeably and refer to the chemical elements having atomic numbers 57-71.

[0091] 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.

[0092] 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 that allow chemical bonds to form more easily between them.

[0093] The term "solid oxide cell" (SOC) is intended to include both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC).

[0094] The term "cation percentage" or "cation percentage" (abbreviated herein as "cation %" or "% cation") refers to the percentage of cations relative to the composition.

[0095] The term "source" of an element, compound, or other material refers to a material that contains the element, compound, or other material, whether or not chemically bonded in the source. The source of an element, compound, or other material can be an elemental source (e.g., Ln, Sm, Pr, or O2) or can be in the form of a compound or mixture containing an element (e.g., lithium nitrate), a compound, or other material that includes one or more of these elements, compounds, or materials.

[0096] In this specification, references to electrochemical cells, SOC, SOFC, and SOEC may refer to tubular or planar cells. Electrochemical cell units may be tubular or planar. Planar fuel cell units may be arranged in a stacked arrangement, for example, 100-200 fuel cell units are arranged in a stack, wherein the individual fuel cell units are arranged electrically in series.

[0097] The electrochemical cell may be a fuel cell, a reversible fuel cell, or an electrolyzer cell. Generally, these cells may have the same structure, and reference to an electrochemical cell may refer (unless the context implies otherwise) to any of these types of cells.

[0098] "Oxidant electrode" or "air electrode" and "fuel electrode" are used herein, and due to potential confusion between fuel cells or electrolyzer cells, may be used interchangeably to refer to the cathode and anode, respectively, of a SOFC.

[0099] Although cells are described in this specification in which the fuel electrode (eg, anode) is placed first on the substrate, the present invention also encompasses cells in which the air electrode is placed first on the substrate.

[0100] The cells described herein include metal-supported cells, where the layers of the cell are supported by a metal substrate, but the invention also encompasses anode-supported, electrolyte-supported, or cathode-supported cells, where each layer provides structural support for all other layers coated thereon.

[0101] The electrochemical cells encompassed by the present invention may include:

[0102] a) Two planar components welded together with a fluid volume in between (e.g. substrate with electrochemical layer and interconnector (separate plates))

[0103] b) Three planar components are welded together with a fluid volume in between (eg a substrate with electrochemical layers and interconnectors (separate plates), and spacers providing the fluid volume).

[0104] As those skilled in the art will appreciate, the various features of an aspect of the disclosure as described herein may, if desired, be used in combination with any other features in the same or other aspects of the disclosure, with appropriate modification.

[0105] Furthermore, while all aspects of the invention or disclosure preferably "comprise" the features described in relation to that aspect, it is specifically contemplated that they may "consist of" or "consist essentially of" those features outlined in the claims.

[0106] The present invention will now be described with reference to the accompanying drawings and examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0107] Figure 1 Shown is a scanning electron micrograph (SEM) cross-section of a SOFC having a cathode active layer (CAL) according to materials of the present disclosure.

[0108] Figure 2 for Figure 1 Scanning electron micrograph (SEM) cross-section of a SOFC showing details of the CAL and adjacent layers.

[0109] Figure 3 The average cell potential of a cell having a CAL comprising a 2% lithium ion battery according to the present disclosure and sintered at 870° C. (at 133 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions) and normalized to standard cells in the same stack.

[0110] Figure 4 The average cell potential as a function of temperature for cells having a CAL comprising a 2% lithium ion battery according to the present disclosure and sintered at 870° C. (at 225 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions) and normalized to standard cells in the same stack. DETAILED DESCRIPTION

[0111] Figure 1 A SOC is shown including a cathode active layer (CAL) 30 comprising Ln-doped praseodymium oxide (PLnO) containing a lithium source (eg, Li-containing PSmO 10 ).

[0112] exist Figure 1In this case, the SOC layer is a bulk air electrode layer (CBL) 10, a ReSC / CGO interfacial air electrode layer 20, an air electrode active layer (CAL) 30 comprising a composition according to the present disclosure (e.g. a lithium source and PrLnO, e.g. 2LiPSmO10), a doped ceria barrier layer 35, a zirconia electronic barrier layer 40, a CGO electrolyte layer 50, and a fuel electrode 60. The fuel electrode 60 is supported on a metal substrate (not shown).

[0113] The metal substrate can be a metal, in particular a steel, more particularly a ferritic stainless steel substrate, typically a foil substrate.

[0114] The CAL 30 comprises a material according to the present disclosure. The compositions of the other layers are of a type known to those skilled in the art, as are the methods of manufacture and application. Reference can be made, for example, to WO 2009 / 090419 A2, which discusses methods for applying these types of layers as well as exemplary compositions to metal substrates, and in particular stainless steel substrates. The layers, including the air electrode layers, can exhibit good adhesion and / or can be isostatically pressed to further improve adhesion.

[0115] Figure 2 A scanning electron micrograph (SEM) cross-section is shown, which shows in more detail Figure 1 the CAL 30 of the SOFC. The microstructure is generally uniform throughout the main portion of the CAL 30, and has good adhesion to the adjacent layers. The porosity and grain size are uniform across the depth of the CAL 30, with densification occurring at the interface with the interfacial air electrode layer of ReSC / CGO.

[0116] Figure 3 A plot is shown of the average cell potential as a function of temperature for a cell with a CAL comprising a material according to the present disclosure (curve 4) sintered at 870°C (at 133 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions) compared to a standard cell in the same stack (curve 5). The performance of the Li-containing PSmO10 CAL cell is much better than the standard cell, particularly at low temperatures.

[0117] Figure 4 A plot is shown of the average cell potential as a function of temperature for a cell with a CAL comprising a material according to the present disclosure (curve 6) sintered at 870°C (at 225 mA cm -2 , 75% Uf, 545°C reforming equilibrium conditions) compared to a standard cell in the same stack (curve 7). The Li-containing PSmO10 CAL has a very high average cell voltage under these conditions at 570°C of 0.867 V.

[0118] To investigate the working stack (in this case operating in SOFC mode), a fuel mixture of natural gas partially simulating external steam reforming was supplied to the stack 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 or 225 mA cm -2 The stack temperature was varied by controlling the furnace temperature at which the test was conducted.

[0119] The following general methods were used to synthesize the Li-containing praseodymium oxide powder of the invention (Examples 1, 2, 3), to synthesize a printing ink using the Li-containing praseodymium powder (Example 4), and to print a CAL using the ink (Example 5).

[0120] Example 1 : Synthesis of rare earth (RE) doped praseodymium oxide powder

[0121] Solution preparation

[0122] A stoichiometric mixture of praseodymium nitrate hexahydrate and the desired rare earth dopant nitrate was dissolved in deionized (DI) water to give a solution molarity of 0.4 M.

[0123] In a separate container under the fume hood, oxalic acid dihydrate was dissolved in the same volume of DI water used to dissolve the nitrate salts to give a molar ratio of oxalic acid to nitrate of 1.7 (slightly in excess of the stoichiometric requirement of 1.5 to ensure all metal ions precipitate).

[0124] Once the oxalic acid was completely dissolved, concentrated ammonium hydroxide solution was added while monitoring the pH until the acid was neutralized (pH 7), leaving an ammonium oxalate solution.

[0125] Precipitation

[0126] While the mixture was being vigorously stirred, the nitrate solution was added to the ammonium oxalate solution to give a light green precipitate of insoluble praseodymium plus dopant oxalate salts.

[0127] Filtration

[0128] A Buchner funnel with high strength filter paper and aquarium pump was prepared. With the pump running, the precipitate mixture was poured onto the filter and allowed sufficient time to pass until most of the supernatant solution was removed, leaving a precipitated cake on the filter paper.

[0129] Washing

[0130] The precipitate was washed 3 times with DI water and once with ethanol.

[0131] Drying

[0132] The wet cake was transferred from the funnel to a suitable container and dried in an oven at 70 °C overnight.

[0133] Crushing

[0134] The dry precipitate pieces were pulverized using a pestle and mortar and the resulting powder was transferred to an alumina crucible.

[0135] Example 2: Alternative synthesis method of rare earth doped praseodymium oxide powder

[0136] Solution preparation

[0137] A stoichiometric mixture of praseodymium nitrate hexahydrate and the desired rare earth dopant nitrate was dissolved in deionized (DI) water to give a 0.15 M solution molarity.

[0138] In a separate container under the fume hood, a concentrated ammonium hydroxide solution was diluted in deionized water to give a 0.45 M solution in an equal volume to the nitrate solution.

[0139] Precipitation

[0140] The nitrate solution was added to the ammonium hydroxide solution while the mixture was vigorously stirred to give a light green gelatinous precipitate of insoluble praseodymium plus dopant hydroxides.

[0141] Filtration

[0142] A Buchner funnel with high strength filter paper and aquarium pump was prepared. With the pump running, the precipitate mixture was poured onto the filter and allowed sufficient time to pass until most of the supernatant solution was removed leaving a precipitated cake on the filter paper.

[0143] Washing

[0144] The precipitate was washed 3 times with DI water and once with ethanol.

[0145] Drying

[0146] The wet cake was transferred from the funnel to a suitable container and dried in an oven at 70 °C overnight.

[0147] Crushing

[0148] The dry precipitate pieces were pulverized using a pestle and mortar and the resulting powder was transferred to an alumina crucible.

[0149] Example 3: Penetration of lithium source in rare earth doped praseodymium oxide powder

[0150] Co-precipitated RE-doped praseodymium oxide powder samples were obtained for use in subsequent procedures. The samples were contacted with a lithium source according to the following method.

[0151] A 10% Sm praseodymium oxide (Pr 0.9 Ln 0.1 O (1.95-δ) , “PSmO10”) sample was processed to produce two compositions with 1 and 2 cation% lithium.

[0152] A 0.5M lithium nitrate solution in ethanol was slowly added to the PSmO10 powder using a high precision pipette, ensuring that the powder did not become visibly wet again. The resulting powder was then mixed in a pestle and mortar.

[0153] The Li-containing PSmO10 powder was oven dried and then calcined at 500°C for 2 hours to decompose the lithium nitrate to lithium oxide.

[0154] Samples of PSmO10, PSmO10 containing 1 cation% Li and PSmO10 containing 2 cation% Li were calcined in a tube furnace at 850°C for 1 hour to simulate cathode firing.

[0155] Pellets made from the same powders were uniaxially pressed and simultaneously fired to evaluate sintering.

[0156] The sintering shrinkage was evaluated and the results are shown in Table 1 below.

[0157] Table 1. Sintering shrinkage evaluation.

[0158] Lithium doping level ]]> ​ Linear shrinkage rate / % after firing at 850°C ]]> ​ No doping 0.77 1 cation % 0.85 2 cations % 2.69

[0159] Example 4: Synthesis of printable ink

[0160] Dispersion and milling of lithium containing rare earth doped praseodymium oxide powder

[0161] Li-containing RE-doped praseodymium oxide powder prepared as described in Examples 1 to 3 was weighed and mixed with a carrier, a dispersant and an anti-foaming agent to form a slurry containing about 46wt% of the target amount of powder.

[0162] The slurry was transferred to a basket mill, to which was also added a slurry of twice the weight of 1mm YSZ milling media.

[0163] The slurry was milled at about 7000rpm until a D90<0.9pm was achieved. The particle size distribution can be measured using a Malvern 2000 laser diffraction particle size analyser.

[0164] The slurry was then removed from the basket mill.

[0165] Ink manufacturing

[0166] The dispersion milled Li-containing RE-doped praseodymium oxide powder slurry prepared in the previous section was transferred to a high shear disperser (HSD) beaker and placed on the HSD.

[0167] The binder powder was weighed in an amount equivalent to 2.5-3.5 wt% of the finished ink.

[0168] The binder was added to the slurry being actively dispersed on the HSD.

[0169] The ink was left on the HSD until the binder was completely dissolved in the ink.

[0170] The ink was transferred to a three-roll mill (TRM) for final homogenization and 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 larger than 5 μm were left in the finished ink.

[0171] Example 5: Printing of ink and formation of active layer

[0172] The substrate for printing included an electrolyte layer deposited on a metal supported SOFC. The ink was screen printed in a single pass using an automated screen printer onto the electrolyte layer of the metal supported SOFC. It was then dried in a drying oven. The ink solids content and screen mesh combination were chosen to give a thin print of about 3 μm. After the addition of the CBL, the layer was then sintered with the CBL to form the CAL at a temperature of 820 to 870 °C. After sintering, X-ray diffraction and BET analysis were repeated. After sintering, the crystallite size increased slightly and the BET surface area decreased, but the crystal structure did not change. The layer was still composed of a single phase with a cubic fluorite structure.

[0173] Example 6: SOFC cell with air electrode CAL using Li containing PrLnO.

[0174] The Li-containing RE-doped praseodymium oxide powder slurry described herein and exemplified in Examples 1-4 above had comparable or better performance than the standard.

[0175] A SOFC air electrode was prepared consisting of three layers. The three-layer electrode advantageously reduced the impact of chromium contamination (praseodymium oxide can react with chromium oxide to form a perovskite) and ensured better adhesion between the bulk and active layers.

[0176] The three layers of the electrode are the LCN60 bulk layer which provides excellent stability and thermal expansion matching to the rest of the cell, the rare earth strontium cobaltate / CGO interface composite layer and the 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 sulphur will react with the rare earth strontium cobaltate / cobalt ferrite before reaching the strontium free active layer which can be susceptible to poisoning by chromium. The interface layer has a similar thermal coefficient to the air electrode bulk layer. This protects the active layer from degradation (which is not affected by water vapour, carbon dioxide or sulphur dioxide)

[0177] The air electrode is made by screen printing in three layers, a thin layer (about 3 microns) of the first electrode material (e.g. 2 cation % Li PSmO10), a thin layer (about 3 microns) of the rare earth strontium cobaltate / CGO (e.g. ReSC / CGO 1060:40; where "Re" means rare earth) and finally a thicker (about 40 microns) bulk layer (LCN60).

[0178] Optionally, these layers can be sintered and isostatically pressed or uniaxially pressed to increase their green density and then finally sintered at 800-900°C in air to form the finished air electrode.

[0179] 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, 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.

[0180] The stack can be run under the described conditions with air flow on the air side and a simulated steam reformed natural gas fuel on the fuel side.

[0181] All publications mentioned in the above specification are herein incorporated by reference. While the illustrative embodiments of the application disclosed herein are presented in terms of specific embodiments, it is appreciated that the application is not limited to these precise embodiments and that various changes and modifications can be made therein without departing from the scope of the present application as defined in the following claims and their equivalents.

Claims

1. An electrode for an electrochemical cell, the electrode comprising at least a first layer, the first layer comprising a first electrode composition, the first electrode composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source. wherein Ln is selected from at least one rare earth metal selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, δ is the degree of oxygen deficiency, and 0.01≤x≤0.4。 2. The electrode of claim 1, wherein the lithium source is present in the first electrode composition in an amount of 0.001 to 5 cation %.

3. The electrode of claim 2, wherein the lithium source is present in the first electrode composition in an amount of 0.01 to 5 cation %.

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

5. The electrode of any one of the preceding claims, wherein 0.02 < x < 0.

25.

6. The electrode of any one of the preceding claims, wherein the first electrode composition comprises Pr 0.9 Ln 0.1 O (1.95-δ) , Pr 0.85 Ln 0.15 O (1.925-δ) , Pr 0.8 Ln 0.2 O (1.9-δ) or mixtures thereof, and a lithium source.

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

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

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

10. The electrode of any one of the preceding claims, wherein the electrode is an air electrode.

11. The electrode of any one of the preceding claims, wherein the lithium source comprises lithium oxide, lithium hydroxide, a lithium salt, a lithium salt of an organic acid, and / or lithium as a dopant.

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

13. The electrochemical cell of claim 12, wherein the electrochemical cell further comprises an electrolyte, and the electrolyte comprises 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.

14. The electrochemical cell of claim 12 or 13, 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.

15. A method of manufacturing an electrode for an electrochemical cell, the method comprising: providing a substrate, optionally having layers comprising a fuel electrode and an electrolyte deposited thereon, applying at least a first layer, the first layer comprising a first electrode composition, the first electrode composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source, optionally drying at least the first layer, and optionally sintering the at least first layer; thereby forming an electrode; wherein Ln is selected from at least one rare earth metal selected from La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and mixtures thereof, δ is the degree of oxygen deficiency, and 0.01≤x≤0.4。 16. The method of claim 15, wherein the method further comprises: applying a material to the substrate to form at least one electrolyte layer, applying the first electrode composition on the electrolyte layer to form an air electrode layer, optionally drying the air electrode layer, and co-sintering the electrolyte layer and the air electrode layer.

17. The method of claim 15 or 16, wherein the sintering or co-sintering is performed at a temperature in the range of 750 °C to 900 °C.

18. The method of any one of claims 15-17, wherein the first electrode composition comprises Pr (1-x) Ln x O (2-0.5x-δ) and a lithium salt, preferably lithium nitrate.

19. The method of any one of claims 15-18, further comprising the step of preparing the first electrode composition by mixing Pr (1-x) Ln x O (2-0.5x-δ) with a lithium source, optionally a lithium source dissolved or dispersed in a solvent, optionally an alcohol.

20. A composition comprising Pr (1-x) Ln x O (2-0.5x-δ) and a lithium source. wherein Ln is selected from at least one rare earth metal selected from the group consisting 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, and 0.01≤x≤0.4。 21. A composition comprising Pr (1-x) Sm x O (2-0.5x-δ) and a source of lithium. wherein δ is the degree of oxygen deficiency, and 0.01≤x≤0.4。 22. A stack of electrochemical cells according to any one of claims 12 to 14.

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