Solid oxide electrochemical cell with cr-getter using praseodymium

Abstract

Apparatus is disclosed comprising a solid oxide electrochemical cell having an oxygen supply inlet and a cell exhaust outlet. The apparatus has a getter material containing praseodymium, the getter material being situated between the oxygen supply inlet and cell exhaust outlet. The apparatus may have a getter unit containing a getter material comprising praseodymium. Also disclosed are porous monoliths comprising a ceramic material that may be used as getter materials or supports, and methods of forming such porous monoliths and coating porous ceramic monoliths with a getter material.

Description

[0001] APPARATUS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to apparatus comprising solid oxide electrochemical cells, porous monoliths for use in such apparatus, methods of forming porous monoliths, and methods of coating porous monoliths for use in the apparatus.

[0004] BACKGROUND OF THE INVENTION

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

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

[0007] A solid oxide fuel cell (SOFC) generates electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based) and the device is generally ceramic-based, using an oxygen-ion conducting metal-oxide containing ceramic as its electrolyte. Many ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) have useful ion conductivities at temperatures in excess of 450 °C or 500 °C (for ceriumoxide based electrolytes) or 650 °C (for zirconium oxide-based ceramics), so SOFCs tend to operate at elevated temperatures. The fuel electrode, electrolyte and air (oxygen) electrode of an SOC may each be formed of one or more layers to optimise operation. A solid oxide electrolyser cell (SOEC) may have the same or a similar structure as an SOFC, but may operate in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide.

[0008] In operation, the electrolyte of the SOFC conducts oxygen ions from an oxygen electrode to an anode located on opposite sides of the electrolyte. A fuel contacts the anode (usually known as the “fuel electrode”) and an oxidant, such as air or an oxygen rich fluid, contacts the oxygen electrode. Conventional ceramic-supported (e.g. anode-supported) SOCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOCs have been developed which have the active cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOCs and can be sealed using conventional metal welding techniques.

[0009] Applicant’s WO-A-2015 / 136295 discloses metal-supported SOFCs in which the metal support plate has a porous region surrounded by a non-porous region with the active layers being deposited upon the porous region so that gases may pass through the pores from one side of the metal support plate to the opposite side to access the active layers coated thereon. The porous region comprises small apertures (holes drilled through the metal foil substrate) extending through the support plate.

[0010] WO-A-2016 / 128721 discloses an interconnect for a low temperature solid oxide fuel cell, in particular an interconnect comprising a chromium oxide layer (chromium (III) oxide / chromia).

[0011] Components including metal supports, interconnects and other components in SOFCs, SOECs, and other devices may be formed of SOC-specific materials including steels. There is a desire to use materials containing chromium, that may be lower cost or may have other beneficial properties. It has, however, been observed that materials (for example metal alloys) may exhibit chromium volatility. Volatile chromium compounds may cause problems during manufacture and may poison SOC electrodes during operation leading to loss of electrochemical activity. There have been attempts to coat components (e.g. with alumina, CoCe or rare earth materials) to reduce the chromium problem For example, W0-A- 2023 / 175353 discloses a method for producing a praseodymium and / or terbium coated chromium-containing component.

[0012] Although such methods are successful, there is a need to provide alternate method of reducing or preventing problems associated with chromium in SOCs.

[0013] It is an aim of the present invention to address this need. SUMMARY OF THE INVENTION

[0014] The present invention accordingly provides, in a first aspect, apparatus comprising a solid oxide electrochemical cell comprising an oxygen electrode, an electrolyte, and a fuel electrode; an oxygen supply inlet and a cell exhaust outlet; and a getter material comprising praseodymium, the getter material being situated between the oxygen supply inlet and cell exhaust outlet.

[0015] Preferably, the getter material may be situated after (e.g. downstream of) the component at higher or highest temperature because this may be the location of the highest amount of volatile chromium. For example, the getter material may be situated after a heat exchanger (for e.g. fuel cells or in fuel cell mode).

[0016] The apparatus may further comprise an oxygen conduit defining an oxygen supply path between the oxygen supply inlet and the oxygen electrode.

[0017] The getter material may form a getter material layer on a part of the oxygen conduit in the oxygen supply path.

[0018] The oxygen electrode may comprise an oxygen electrode bulk layer and an oxygen electrode active layer which may be located between the oxygen electrode bulk layer and the electrolyte.

[0019] The getter material may be situated between the oxygen supply inlet and the oxygen electrode active layer.

[0020] The oxygen electrode may comprise an oxygen electrode bulk layer directly or indirectly in contact with the oxygen active electrode layer.

[0021] Optionally, in one embodiment, the oxygen electrode bulk layer may comprise the getter material. The getter material in the oxygen electrode bulk layer may comprise praseodymium cobalt nickelate (PCN), praseodymium strontium cobaltite (PSC) and / or praseodymium strontium cobalt ferrite (PSCF). Thus, for example, the getter material may be selected from Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.sSro.sCoCh, Pro.6Sro.4Coo.2Feo.sO(3-5), or mixtures thereof. Preferably the getter material comprises PSC.

[0022] The oxygen electrode may comprise an oxygen electrode interfacial layer optionally located between the oxygen electrode active layer and the oxygen electrode bulk layer. The getter material may be situated between the oxygen supply inlet and the oxygen electrode interfacial layer. Alternatively (or additionally), the getter material may be situated between the oxygen supply inlet and the oxygen electrode bulk layer. Thus, the getter material may be located (e.g. as an additional layer) on the surface of the oxygen electrode bulk layer, or as part of the interfacial layer (e.g. incorporated into a layer between the active and bulk electrode).

[0023] The solid oxide electrochemical cell may comprise other layers and components. Thus, the solid oxide electrochemical cell may further comprise at least one electrolyte layer directly or indirectly in contact with the oxygen electrode active layer. The at least one electrolyte layer may comprise an electron blocking layer. The at least one electrolyte layer may comprise a main electrolyte layer. Optionally, the electron blocking layer may be located between the main electrolyte layer and the oxygen electrode active layer.

[0024] Generally, the solid oxide electrochemical cell may further comprise at least one layer of a fuel electrode, wherein the one layer of the fuel electrode is in contact (direct or indirect) with the main electrolyte layer.

[0025] In some embodiments, the getter material may be contained in a getter unit.

[0026] Thus, in a second aspect, the present invention provides apparatus comprising a solid oxide electrochemical cell comprising an oxygen electrode, an electrolyte, and a fuel electrode, an oxygen supply inlet and a cell exhaust outlet, and a getter unit containing a getter material comprising praseodymium, the getter unit being situated between the oxygen supply inlet and cell exhaust outlet.

[0027] The getter unit may be located in the oxygen supply path. Preferably, the getter unit may be located in the oxygen supply path after the part of the oxygen supply path at the highest temperature; more preferably the part of the oxygen supply path at a temperature of 500° C or higher.

[0028] The apparatus may further comprise an interconnect in electrical connection with the oxygen electrode. The interconnect may comprise raised features (e.g. dimples) contacting the oxygen electrode and defining a gas volume on the oxygen electrode.

[0029] The apparatus may further comprise a contact paste electrically connecting the interconnect and the oxygen electrode. Getter material may be incorporated in the contact paste.

[0030] The getter material layer may be located on the surface of the interconnect.

[0031] The getter material layer may be located between the raised features of the interconnect.

[0032] The getter material may further comprise strontium which is advantageous because this may act as a getter for sulphur. The getter material may be selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite (PSCF), praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.eO(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), praseodymium doped ceria, praseodymium strontium manganese, praseodymium strontium cobaltite (PSC), Pro.sSro.sCoCh (PSC551), a material of composition Pr(i-a)LnaO(2-5), wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm, 6 is the degree of oxygen deficiency, and 0.01 < a < 0.4, or mixtures thereof.

[0033] The getter unit may comprise a getter support and the getter material may be on or in the getter support. In some embodiments, the getter support may comprise a porous monolith (e.g. honeycomb). In other embodiments, the getter unit may comprise a packed bed of high surface area pellets comprising the getter material. The support may comprise alumina, alumina silicate, alumina mixture, and / or zirconia. The getter support may comprise a metal organic framework (MOF) material. The metal organic framework (MOF) material may comprise ZIF-8, UiO-66, MIL-101, and / or HKUST-1, or mixtures thereof.

[0034] Generally, the apparatus may comprise chromium containing materials which may act as a source of the Cr on which the getter material acts. Thus, the apparatus may comprise at least one component comprising a metallic alloy comprising chromium. The component may comprise a ferrous alloy comprising chromium. The component may comprise stainless steel. The alloy may comprise 11% wt Cr or greater; optionally 15%wt Cr or greater; optionally 17%wt Cr or greater; optionally 19%wt Cr or greater. The component may comprise, for example, an interconnect, a spacer, a metal plate, or a substrate, pipe fitting, pipe, heat exchanger, a valve component.

[0035] In the apparatus, the solid oxide electrochemical cell may be an electrolytic cell, an oxygen separator, a sensor or a fuel cell.

[0036] The apparatus may comprise a stack of electrochemical cells, wherein each electrochemical cell is as set out herein.

[0037] One type of getter material (or a support for getter material) for a getter unit may be a porous monolith.

[0038] A porous monolith may be a ceramic structure having a plurality of channels or holes separated by thin walls extending in the monolith structure. The channels may be round, square, or hexagonal, and the hole density may vary. In a monolith, the relatively large surface area of the channels / holes facilitates adsorption of e.g. volatile Cr, and the open nature of the channels and holes may reduce resistance to gas flow. A porous monolith may be formed by extruding a water-based paste of powdered raw materials (e.g. alumina, talc, clay, and silica). The extrusions may be dried, cut to length, and calcined at temperatures e.g. above 1400°C.

[0039] The getter unit may comprise one unit or two or more units (e.g. 2, 3, 4, 5, 6 or more units). For example, the getter unit may comprise multiple units. The getter unit may comprise multiple units that may be the same or different, or may comprise a combination of types of getter unit (for example monolith and packed bed).

[0040] Thus, in a third aspect, there is provided a porous monolith comprising a material selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite (PSCF), praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro.sSro.sCoCh (PSC551), a material of composition Pr(i-a)LnaO(2-5), or mixtures thereof, and at least one reinforcing agent, wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm, 6 is the degree of oxygen deficiency, and 0.01 < a < 0.4, wherein the reinforcing agent is selected from clay, silica, alumina and mixtures thereof.

[0041] Such a monolith may be formed by extrusion.

[0042] Thus, in a fourth aspect, there is provided, a method of forming a porous monolith comprising a ceramic material selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite (PSCF), praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro.sSro.sCoCh (PSC551), a ceramic material of composition Pr(i-a)LnaO(2-5), or mixtures thereof, the method comprising: providing a paste comprising the ceramic material, at least one reinforcing agent and optionally a plasticiser, a binder, and / or a ceramic bulking agent, extruding the paste through a die to form a green extruded ceramic body, drying the green extruded ceramic body, sintering the green extruded ceramic body at a suitable temperature to obtain a predetermined porosity, wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm, 6 is the degree of oxygen deficiency, and 0.01 < a < 0.4.

[0043] In other aspects, the getter unit may comprise a support (that may be a ceramic monolith or another support (e.g. particulate support) that is coated e.g by a washcoat or impregnation method, a liquid coating method in which a dissolved or dispersed getter material contacts the support thereby depositing at least a portion of the dissolved / dispersed getter material on the support and / or impregnating getter material in voids and / or pores of the support, with subsequent optional drying and / or optional further heating.

[0044] Thus, in a fifth aspect, there is provided a method of coating a porous ceramic monolith with a getter material, the method comprising liquid coating a porous ceramic monolith with a liquid containing a solvent and a getter material comprising praseodymium, optionally drying, and optionally heating the coated porous monolith.

[0045] The invention in its various aspects is advantageous because it may provide protection to components and devices (including electrochemical cells) from contamination by chromium that may evaporate from components (including stainless steel components) at higher temperature and which may otherwise react to form a stable chromate phase over the active surface of the components (e.g. over electrodes in electrochemical cells).

[0046] Definitions

[0047] In this specification, the terms “rare earth metal” or “rare earth element” refer to metals selected from Y, Sc, and lanthanoid.

[0048] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71.

[0049] The term "dopant" as used herein is not intended to be restricted to a maximum percentage of elements, ions or compounds added to chemical structures. Similarly, the term "doping" is intended to mean the addition of a certain amount of elements, ions or compounds to a material. It is not limited to a maximum quantity of material, after which, further addition of material no longer constitutes doping. Metal oxides including mixed metal oxides may exhibit non-stoichiometry with the oxide being deficient in oxygen. In this specification, 6 indicates the degree of oxygen deficiency of a material. 6 may vary depending on the environment and history of the material. As would be understood by the skilled person, values of 6 are usually small. 6 may be 0.25 or lower, optionally 0.2 or lower, optionally < 0.15.

[0050] The term “source of’ an element, compound or other material refers to a material comprising the element, compound or other material whether or not chemically bonded in the source. The source of the element, compound or other material may be an elemental source (e.g. Ln, Ni or O2) or may be in the form of a compound or mixture comprising the element, compound or other material including one or more of those elements, compounds or materials.

[0051] “Oxidant electrode,” “oxygen electrode” or “air electrode” and “fuel electrode” are used herein and may be used interchangeably to refer to cathodes (oxygen electrodes) and anodes, respectively, of e.g. solid oxide fuel cells. Hence, in the context of fuel cells “cathode” may be used inter changeably with “oxygen electrode,” and “anode” may be used interchangeably with “fuel electrode”.

[0052] The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0053] It will be understood that “attached” and “on” refer to direct or indirect attachment and positioning, respectively.

[0054] It will be understood that each layer referred to in this specification may be comprised of multiple sub-layers (and those sub-layers may have varying compositions).

[0055] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to tubular or planar cells unless the context otherwise requires. Electrochemical cell units may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-500 fuel cell units in a stack, with the individual fuel cell units arranged electrically in series.

[0056] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have a similar or the same structure and reference to electrochemical cells may refer (unless the context suggests otherwise) to any of these types of cell. The cell may be based upon a solid oxide electrolyte, optionally a metal-supported solid oxide cell. In fuel cell mode, a fuel contacts the anode (fuel electrode) and an oxidant, such as air or an oxygenrich fluid, contacts the cathode (oxygen electrode), so in fuel cell mode operation, the oxygen electrode will be the cathode. A solid oxide electrolyser cell (SOEC) may have the same or similar structure as an SOFC, but is essentially the SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide by using the anode, cathode, and solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen.

[0057] The term “flow path” is used to define fluid flow paths between various components, and thus it is also to be understood that those components are in fluid flow communication with one another.

[0058] The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0059] Furthermore, although all aspects of the invention or disclosure preferably “comprise” the features described in relation to that aspect, it is specifically envisaged that they may “consist” or “consist essentially” of those features outlined in the claims.

[0060] The invention will now be described with reference to the accompanying figures and examples.

[0061] BRIEF DESCRIPTION OF THE FIGURES

[0062] Figure 1 shows a schematic of a fuel cell apparatus illustrating fluid flow paths.

[0063] Figure 2 shows a schematic cross section of a solid electrochemical cell unit in accordance with the disclosure.

[0064] Figure 3 shows (a) Nano SIMS depth profile of praseodymium for part of a solid oxide electrochemical cell with no Pr added to the oxygen electrode bulk layer; (b) Nano SIMS depth profile of chromium for part of the solid oxide electrochemical cell of Fig 3(a); (c) Nano SIMS depth profile of praseodymium for part of a solid oxide electrochemical cell with Pr added to the oxygen electrode bulk layer; and (d) Nano SIMS depth profile of chromium for part of the solid oxide electrochemical cell of Fig 3(a). Figure 4 shows an individual value plot from analysis of variance (ANOVA) analysis from a cell pass-off test (CPOT) showing the voltage change from open circuit at an applied current of 8 A in fuel cell mode for various Pr-containing bulk oxygen electrode layers subjected to ex-situ accelerated chromium poisoning, compared to fresh unpoisoned cells and a standard cell without Pr in the bulk oxygen electrode layer after accelerated ex-situ poisoning. The lower the voltage change the higher the performance of the cell.

[0065] Figure 5 shows a scatterplot of the voltage change under an 8A fuel cell load as a function of the praseodymium loading mass per cell subjected to ex-situ accelerated chromium poisoning. The lower the value the better the performance of the cell; for Pr-loadings greater than 80mg the cell performance is largely unaffected by chromium poisoning.

[0066] Figure 6 shows an individual value plot of cell voltages of cells containing various Pr loadings in the bulk oxygen electrode layer after accelerated ex-situ chromium poisoning compared to poisoned standard cells containing no Pr and fresh unpoisoned standard cells. The voltages were recorded at a temperature of 610°C and an applied current of 14.5A in fuel cell mode. The higher the voltage the better the cell performance.

[0067] Figure 7 shows a schematic of an electrolyser cell system illustrating fluid flow paths.

[0068] DETAILED DESCRIPTION OF THE INVENTION

[0069] Referring to Figure 1, fuel cell system 10 is a solid oxide fuel cell (SOFC) system. Fuel cell stack 20 is a metal-supported SOFC fuel cell stack, as taught in WO-A-2015 / 004419. Fuel cell system 10 has a steady state IkW electric output from fuel cell stack 20, and comprises 121 metal-supported SOFC fuel cells 30. Each fuel cell 30 has an anode side 40, electrolyte layer 50, and cathode side 60. Each fuel cell layer in the fuel cell stack is separated by an electrically conducting gas impermeable metal interconnect plate (interconnector) (not shown). Fuel cell stack endplates and compression means (not shown) are also provided.

[0070] Reference herein to fuel cell 30 is to the full set of 121 fuel cells 30.

[0071] Electrical load L is placed across fuel cell 30.

[0072] Fuel cell stack anode inlet 41 is in fluid flow communication with fuel cell anode inlet 41 A for the flow of anode inlet gas to the anode side 40 of fuel cell 30. Fuel cell anode outlet 42A is in fluid flow communication with fuel cell stack anode off-gas outlet 42 for the flow of anode off-gas.

[0073] Fuel cell stack cathode inlet 61 is in fluid flow communication with fuel cell cathode inlet 61 A for the flow of cathode inlet gas to the cathode side 60 of fuel cell 30. In the flow line there is a getter unit 290 containing getter material comprising praseodymium to act as a chromium getter. Fuel cell cathode outlet 62A is in fluid flow communication with fuel cell stack cathode off-gas outlet 62 for the flow of cathode off-gas. Generally, cathode inlet 61 is an oxygen supply inlet 61.

[0074] Steam reformer 70 comprises reformer inlet 71 for anode inlet gas and reformer outlet 72 for exhausting anode inlet gas.

[0075] Tail-gas burner 80 is in fluid flow communication with fuel cell stack anode and cathode offgas outlets 42, 62 and has a tail gas burner exhaust 81, anode off-gas inlet 82 and cathode offgas inlet 83. Tail-gas burner 80 defines a fluid flow path from fuel cell stack anode and cathode off-gas outlets 42, 62 to tail-gas burner exhaust 81, and is configured for burning anode and cathode off-gases and producing a tail-gas burner off-gas.

[0076] An anode inlet gas fluid flow path A is defined from fuel source 90 to evaporator 100 to steam reformer 70 to fuel cell stack anode inlet 41 to fuel cell anode inlet 41 A, i.e. the components are in fluid flow communication with one another.

[0077] An anode off-gas fluid flow path B is defined from fuel cell anode outlet 42A to fuel cell stack anode off-gas outlet 42 to anode off-gas heat exchanger 110 (HX-AOG) to condenser heat exchanger 120 to separator 130 to anode off-gas inlet 82 of tail-gas burner 80.

[0078] Main cathode inlet gas flow path 230 and air bypass inlet gas flow path 240 have a number of common components and share a common flow path in a number of places, marked as cathode inlet gas fluid flow path C.

[0079] Main cathode inlet gas flow path 230 is defined from oxidant inlet 140 to blower 210 to valve / separator 220 to anode off-gas heat exchanger 110 to air pre-heater heat exchanger 150 (HX-APH) to reformer heat exchanger 160 (HX-Ref) to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A. Air bypass inlet gas flow path 240 is defined from oxidant inlet 140 to blower 210 to valve / separator 220 to air bypass inlet 190 to reformer heat exchanger 160 to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A.

[0080] Valve / separator 220 is controlled by control means 200 so as to split the flow of inlet air between main cathode inlet gas flow path 230 and air bypass inlet gas flow path 240.

[0081] Thus, the air bypass inlet gas flow path 240 bypasses anode off-gas heat exchanger 110 and air pre-heater heat exchanger 150.

[0082] In this embodiment, the common parts of gas flow paths 230 and 240 (cathode inlet gas fluid flow path C) are therefore (a) oxidant inlet 140 to blower 210 to valve / separator 220, and (b) reformer heat exchanger 160 to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A.

[0083] A cathode off-gas fluid flow path D is defined from fuel cell cathode outlet 62A to fuel cell stack cathode off-gas outlet 62 to cathode off-gas inlet 83 of tail-gas burner 80.

[0084] A tail-gas burner off-gas fluid flow path E is defined from tail gas burner exhaust 81 to air pre-heater heat exchanger 150 to evaporator heat exchanger 170 (HX-Evap) to fuel cell system exhaust 180 (i.e. cell exhaust outlet 180).

[0085] Anode off-gas heat exchanger 110 is in fluid flow communication with (i) fuel cell stack anode off-gas outlet 42 (i.e. with fuel cell anode outlet 42A) and tail-gas burner anode off-gas inlet 82, and (ii) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61 A), and is arranged for exchanging heat between anode off- gas from fuel cell stack 20 and cathode inlet gas to fuel cell stack 20.

[0086] Air pre-heater heat exchanger 150 is in fluid flow communication with (i) tail-gas burner exhaust 81 and fuel cell system exhaust 180, and (ii) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61A), and is arranged for exchanging heat between tail-gas burner 81 off-gas and cathode inlet gas to fuel cell stack 20.

[0087] Reformer heat exchanger 160 is a parallel-flow heat exchanger and is in fluid flow communication with (i) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61 A), and (ii) fuel source 90 and fuel cell stack anode inlet 41 (i.e. with fuel cell anode inlet 41 A), and is arranged for exchanging heat between cathode inlet gas and anode inlet gas. Evaporator 100 has a fuel inlet 101 for anode inlet gas from fuel source 90, a water inlet 102 for water from water supply 103, and an evaporator exhaust 104 for exhausting anode inlet gas from evaporator 100, and is located in the anode inlet gas fluid flow path between fuel source 90 and steam reformer 70. Evaporator 100 additionally comprises evaporator heat exchanger 170 located in the tail-gas burner off- gas fluid flow path E between air pre-heater heat exchanger 150 and fuel cell system exhaust 180.

[0088] Evaporator heat exchanger 170 is in fluid flow communication with (i) tail-gas burner exhaust 81 and fuel cell system exhaust 180, and (ii) fuel source 90 and water supply 103 and fuel cell stack anode inlet 41 (i.e. with fuel cell anode inlet 41 A), and is arranged to exchange heat between tail-gas burner off-gas and anode inlet gas and water, generating a steam fuel mix for the anode inlet gas to steam reformer 70.

[0089] Condenser heat exchanger 120 is in fluid flow communication with (i) fuel cell stack anode off-gas outlet 42 (i.e. with fuel cell anode outlet 42A) and tail-gas burner anode off-gas inlet 82, and (ii) cooling circuit 121, and is arranged for exchanging heat between anode off-gas from fuel cell stack 20 and a cooling fluid in cooling circuit 121.

[0090] Separator 130 is located in the anode off-gas fluid flow path between condenser heat exchanger 120 and tail-gas burner 80, and has a separator condensate outlet 131, and is adapted to separate condensate from the anode off-gas fluid flow path, and exhaust the condensate via the condensate outlet 131.

[0091] Control means 200 is connected to fuel cell stack cathode inlet gas temperature sensor Tl, fuel cell stack cathode off-gas temperature sensor T2, blower 210 and valve / separator 220. Control means 200 is configured to maintain the temperature determined by temperature sensors Tl and T2 at or about a desired temperature during steady-state operation of the fuel cell system.

[0092] Control means 200 is adapted to operate two independent control loops which operate upon the cathode inlet gas passing through the cathode inlet gas fluid flow path C.

[0093] In the first control loop, the heating of cathode inlet gas is controlled. In the second control loop, the mass flow rate of cathode inlet gas is controlled.

[0094] In use, fuel cell system 10 goes through three phases: start-up, steady state, and shutdown. In the start-up phase, fuel cell stack 20 is cold (or at least below its steady-state operational temperature), and therefore must be heated in order to achieve an operational state. In the steady state phase, fuel cell stack 20 is maintained at operational temperature, as determined by the sensors T1 and T2. Electricity is generated and used by load L across fuel cell 30. Temperatures detected by temperature sensors T1 and T2 will vary, and control means 200 varies the inlet air mass flow rate and the splitting of air between flow paths 230 and 240 accordingly. In the shutdown phase, electrical power is no longer required from fuel cell system 10, and a controlled shutdown sequence is initiated. Power demand from fuel cell stack 20 is reduced to zero and the temperature set point for fuel cell stack air inlet T1 is reduced, while the air flow rate from the blower 210 is increased.

[0095] The apparatus of the fuel cell system 10 may have a getter unit (not shown) comprising getter material comprising praseodymium, the getter unit situated between the oxygen supply and cell exhaust outlet

[0096] Figure 2 shows, schematically and not to scale (for reasons of clarity), a cross section of a solid electrochemical fuel cell unit 302. A substrate, formed by a ferritic stainless steel metal support 304, is plate-like with a peripheral, non-porous region 306 and a central, porous region 308 where holes have been drilled (e.g. laser-drilled) through the metal support 304. A barrier layer (not shown) to reduce corrosion may be located on the surface of the metal support 304 (on one or both sides thereof). A layer of a fuel electrode layer 310 formed of e.g. Ni:CGO (Ni: cerium gadolinium oxide) is located on the porous region 308 of the metal support 304. An electrolyte layer 312 of rare earth doped ceria (RE=Y, Sc or any Ln) of thickness 4 pm or greater (optionally 6 pm to 12 pm) is located on the fuel electrode layer 310. The electrolyte layer 312 may surround the fuel electrode layer 310 to reduce or prevent gas leaking from the fuel side (or volume) 320 to the air side (or volume) 318 or vice versa. The electrolyte layer 312 may further overlap at least part of the non-porous region 306 of the substrate / support 304. The electrolyte may comprise an interlayer 314 of rare earth (RE) stabilised zirconia (RE = Y, Sc or any Ln, e.g. Yb) of thickness 0.5 pm or greater (e.g. 1 pm to 4 pm) is located on the electrolyte layer 312. The fuel cell unit 302 has an oxygen electrode 316 located on the interlayer 314. The oxygen electrode layer 316 may be formed of one or more layers of an electrically conductive ceramic material, for example an oxygen electrode active layer contacting the interlayer 314, an oxygen electrode bulk layer contacting the interconnect 324 and an interfacial layer between the oxygen electrode bulk layer and the oxygen electrode active layer. Fuel cell unit 302 as illustrated in Figure 2 further includes an interconnect 324 (that may also be connected to another fuel cell unit (not shown) in the stack. The interconnect 324 may be a metal sheet pressed or formed to provide the contact features 328 and flanged perimeter 326 which allows the interconnect 324 to contact the support of a second fuel cell unit (above, not shown) at the periphery of both components and they may be sealingly attached to one another around that periphery, for example by welding. The interconnect 324 is provided with contact features 328 (e.g. dimples) which extend and contacts the surface of the oxygen electrode 316, to provide electrical contact (there may be contact paste, not shown, between the interconnect dimples 328 and the oxygen electrode 316) and also forming an oxygen fluid volume 318 bounded by the oxygen electrode 316 and the interconnect 324. The interconnect 24 has a getter coating layer 330 comprising a getter material comprising praseodymium on the surface facing the oxygen electrode 316.

[0097] Oxygen enters the cell by means of the oxygen fluid flow path 332 from the oxygen supply inlet (not shown), and first enters the oxygen fluid volume in the vicinity of the leading edge 334 of the cell.

[0098] In alternate embodiments the interconnect 324 may instead or additionally have getter material comprising praseodymium as a deposit or elsewhere on the interconnect and / or on the leading edge 334 of the cell (e.g. as an inlet strip) between the oxygen inlet and the fuel cell. Additionally or alternatively, getter material comprising praseodymium may be incorporated in the contact paste, on or between the raised features (e.g. dimples 328) of the interconnect.

[0099] Additionally or alternatively, the oxygen electrode bulk layer may comprise a getter material. The getter material in the oxygen electrode bulk layer may e.g. be selected from praseodymium cobalt nickelate (PCN), praseodymium strontium cobaltite (PSC), praseodymium strontium cobalt ferrite praseodymium cobalt nickelate (PCN) (PCN60), praseodymium strontium cobaltite (PSC), praseodymium strontium cobalt ferrite (PSCF), or mixtures thereof, optionally be selected from Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5) (PSCF), Pro.sSro.sCoCh (PSC), or mixtures thereof.

[0100] Getter material may be printed as layer in other areas of the cells as required (e.g. on the oxygen electrode).

[0101] Figure 7 shows a schematic of an electrolyser cell system illustrating fluid flow paths. The fluid flow path for the oxygen electrode(s) (i.e. anode in electrolyser mode) enters an optional heat exchanger 403, an optional inlet heater 404 (that may operate using appropriate means such as electrical or combustion heating) and then through an external getter unit 402, that contains a getter material comprising praseodymium and optional support, to the oxygen side of a stack of electrolyser cells 405. Gas flow from the oxygen side may flow through the optional heat exchanger 403.

[0102] Fluid flow on the fuel side passes through a heat exchanger 407, an inlet heater 406 (that may operate using appropriate means such as electrical or combustion heating) and to the fuel side of the electrolyser stack 405. Gas flow from the fuel side flows through the heat exchanger 407. The getter material in the getter unit may, for example, contain getter material such as praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite, praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5) (PSCF), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro.sSro.sCoCh (PSC551), or a material of composition Pr(i-a)LnaO(2-5), Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), Pro.sSro.sCoCh, or mixtures thereof.

[0103] Embodiments of the invention are described further below.

[0104] Getter material comprising praseodymium has the great advantage of capturing chromium to minimise the chromium content in the air being delivered to the cathode.

[0105] Pr is a particularly advantageous to act as a Cr getter, because it may form PrCrCh.

[0106] The praseodymium-based getter may be located as a separate component external to parts of the SOC, e.g. as an external getter (e.g. getter unit).

[0107] For example: as alumina honeycomb (or similar support) with a Pr-based washcoat; an extruded high surface area PSC monolith / honeycomb; PSC beads / grains / pellets in a packed- bed; high surface area bead / grain / pellets coated with a Pr washcoat or other similar method (nanoparticle impregnation), in a packed bed.

[0108] The getter material may comprise both of Pr- and Sr-containing species to create a co-getter capable of removing both chrome and sulphur species from the incoming air stream. PrCrCh is a Cr(III) compound (non-hazardous) as opposed to a Cr(VI) (hazardous), so this this would allow the used getter to be disposed of with less Health and Safety (HSE) risk (and reduced cost).

[0109] One embodiment may comprise an oxygen electrode bulk layer comprising praseodymium analogues of some lanthanum -based perovskites. For example Pro.99Coo.4Nio.eO(3-5) (PCN60), and / or Pro.6Sro.4Coo.2Feo.80(3-5), These materials may be highly effective at protecting the oxygen electrode active layer from chromium poisoning and be chemically compatible with other oxygen electrode layers.

[0110] Pr-based material incorporation in contact paste as a Cr-getter

[0111] Interconnects may exhibit poor resistance against breakaway oxidation as a result volatile gaseous Cr (VI) species can come out from interconnects and incorporate / poison the oxygen electrode.

[0112] To mitigate this issue, Pr-based getter material may be incorporated in contact paste ink (PrOx / PSC etc).

[0113] Advantages of this would include Pr-based material would be working as a getter for Cr and help to protect the oxygen electrode from poisoning. Pr and Sr -containing coatings would also act as a getter for SO2 in air stream.

[0114] Low pressure drop chromium getter

[0115] Praseodymium oxide and mixed oxides containing PrOx may be excellent chromium getters due the high thermodynamic affinity between praseodymium and chromium, trapping gaseous Cr(VI) species as PrCrCh, with the chromium captured as non-toxic Cr(III). One possible application for this is removing gaseous Cr(VI) species from air streams upstream of an SOC stack, or indeed a hot exhaust stream in order to meet environmental regulations.

[0116] One possible embodiment of this is to extrude a honeycomb monolith made predominantly of the PSmOlO material, possibly in combination with clays, silica fibres or other reinforcing agents. This would involve making a paste of the ceramic powder(s) in water with plasticisers and binders as required, extruding the paste through a die using a screw extruder, drying the green ceramic body and then firing it at a high enough temperature to sinter it whilst retaining significant porosity. This monolith could then be inserted into a pipe within a system as required, capturing airborne chromium whilst providing very low pressure drop.

[0117] Leading Edge Cathode Poison Strip - Lifetime Mitigation

[0118] Deposition of a sacrificial layer of material on the leading edge of the cell, upstream of the active area, to absorb advected poisons / contaminants in the air stream before the reach the cell active area chemistry. This would be a mitigation to improve the lifetime of the cell.

[0119] Cell getter external to cell

[0120] Poisons play a critical role in the degradation behaviour of the cell. Most of the poisons enter through the air stream causing a moving front. Sacrificial getter layers may be printed in different areas of the cell.

[0121] For example, an inlet strip with a Pr containing (optionally Sr containing ) material may act as Cr and SO2 getter could be printed as a small strip ahead of the active area to reduce the impact of the moving front.

[0122] For example, a Pr containing (optionally Sr containing ) material may be printed in between dimples in the valleys of an interconnect or on top of the oxygen electrode bulk layer.

[0123] Examples

[0124] Samples of inks containing ceramics (Lao.99Coo.4Nio.60(3-5); LCN60) and PSC as Cr getter material were prepared and applied as an oxygen electrode bulk layers (cathode bulk layers, CBLs) in solid oxide electrochemical fuel cells using standard electrolyte and anode.

[0125] Inks were prepared by milling a slurry of PSC with a suitable amount of a modified polyurethane wetting agent / dispersant (60wt% in butyl acetate / methoxypropylacetate), then adding a slurry of LCN60, polyolefin-based defoamer, a solution of a modified polyurethane wetting agent / dispersant, and a thermoplastic, polyvinyl butyral resin binder. The mixture was further milled.

[0126] Standard solid oxide electrochemical cells were prepared with a PSC / CGO cathode active layer (CAL), a zirconia electron blocking layer, a ceria electrolyte, and an anode, the cell being supported on a stainless steel porous substrate. Either a layer of LCN60 or a PSC / LCN60 layer with varying wt% Pr, were printed on the cathode active layer to form a cathode bulk layer. The Pr containing cathode bulk layers in the examples were as indicated in Table 1. Each Pr containing cathode bulk layer was double printed to achieve the appropriate thickness (about 40 microns) and target Pr loading.

[0127] Table 1 : Pr containing cathode bulk layers, wt % Pr and target Pr mass loading.

[0128] Differences between the open circuit voltage (OCV) and power values of the cells with Pr- containing cathode bulk layers did not seem to be significant compared with standard cathode bulk layers.

[0129] Ex-situ chromium testing was conducted on the cathode bulk layers using a method as follows.

[0130] Chromium Testing

[0131] A chromium-containing ink containing approximately 65wt% chromium (III) oxide dispersed in a mixture of solvent, binder, dispersant and defoamer was prepared.

[0132] A layer of the chromium-containing ink was printed on the cathode bulk layer of each sample to be tested and the printed sample was aged at 600°C in 2 litres per minute flowing wet air for 144 hours.

[0133] The samples were tested by undergoing a Standard CPOT (cell pass-off test) and a full AC Impedance spectroscopy (ACIS) sweep. The effect of Cr poisoning can be determined by analysing the 8A power curve that occurs as part of the CPOT. By calculating the absolute voltage change during the power curve, the effect of chromium poisoning can be determined by comparison with a Standard CBL and a Cr-poisoned Standard CBL.

[0134] The samples were tested in both a standard CPOT which provides a snapshot of performance at one temperature and a full performance characterisation sweep as a function of temperature and current density, including AC impedance spectroscopy (ACIS) at each point. In the CPOT test fuel cell performance is measured at a temperature of 610°C by applying a load of 8 A (equivalent to a current density of 102 mA / cm2) with the stack fuelled with simulated steam reformed natural gas with a stearmcarbon ratio of 2.5 and 50% external hydrocarbon conversion, at a fuel utilisation of 60%. Under these conditions the exothermic cell electrochemical reaction is balanced by the endothermic internal methane steam reforming reaction, leading to a largely isothermal stack where the performance of individual cells is unaffected by internal temperature gradients. The performance metric is the drop in cell voltage as a result of applying the 8A load, with the lower the voltage drop the lower the internal resistance of the cell and the better the performance.

[0135] The full performance characterisation sweep operates on a similar principle but across a range of stack temperatures between 500°C and 650°C, with current densities of up to 225mA / cm2, at a fuel utilisation of 75%, with the same fuel composition as the CPOT test.

[0136] Results and Characterisation

[0137] Figure 3(a) shows Nano SIMS depth profile of praseodymium for part of the oxygen electrode bulk layer 342, oxygen electrode active layer 344, and part of the electrolyte 346 of a solid oxide electrochemical cell with no Pr added to the oxygen electrode bulk layer, for ex- situ chromium poisoned electrode, showing delineation between oxygen electrode active layer 344 (also known as cathode active layer, CAL in fuel cells) and oxygen electrode bulk layer 342 (also known as cathode bulk layer, CBL in fuel cells). Figure 3(b) shows Nano SIMS depth profile of chromium for part of the oxygen electrode bulk layer 348, oxygen electrode active layer 350, and part of the electrolyte 352 of the solid oxide electrochemical cell of Fig 3(a). Figure 3(c) shows Nano SIMS depth profile of praseodymium for part of the oxygen electrode bulk layer 354, oxygen electrode active layer 356, and part of the electrolyte 358 of a solid oxide electrochemical cell with Pr added to the oxygen electrode bulk layer, for ex-situ chromium poisoned electrode, showing delineation between oxygen electrode active layer 356 (also known as cathode active layer, CAL in fuel cells) and oxygen electrode bulk layer 354 (also known as cathode bulk layer, CBL in fuel cells). Figure 3(d) shows Nano SIMS depth profile of chromium for part of the oxygen electrode bulk layer 360, oxygen electrode active layer 362, and part of the electrolyte 364 of the solid oxide electrochemical cell of Fig 3(a), showing that the cells with Pr in the oxygen electrode bulk layer 360 have chromium preferentially retained in the oxygen electrode bulk layer 360, and reducing or avoiding potential Cr-poisoning of the oxygen electrode active layer 362.

[0138] Figure 4 shows an individual value plot of ANOVA analysis from a CPOT test showing the voltage change / drop from open circuit at an applied current of 8A in fuel cell mode for various Pr-containing bulk oxygen electrode layers subjected to ex-situ accelerated chromium poisoning, compared to fresh unpoisoned cells and cells without Pr in the bulk oxygen electrode layer after accelerated ex-situ poisoning. The lower the voltage change the higher the performance of the cell.

[0139] Figure 5 shows a scatterplot of the voltage change at 8A from the CPOT test as a function of Pr loading in the bulk oxygen electrode layer for ex-situ chromium poisoned cells. For any Pr loading greater than 80mg the performance is essentially indistinguishable from a fresh unpoisoned cell.

[0140] Figure 6 shows individual value plot from ANOVA analysis of cell voltage from a representative point from the full performance characterisation sweep, in this case a stack temperature of 610°C and a current of 14.5A, equivalent to a current density of 185mA / cm2. Various Pr-containing oxygen electrode bulk layers are compared with freshly manufactured (unpoisoned) standard cells and a poisoned cell with no Pr in the oxygen electrode bulk layer. The higher the voltage the better the performance of the cell.

[0141] The results indicate that 25, 20, and 15 wt% Pr containing getter materials are essentially the same as the fresh, unpoisoned CBLs, suggesting that they offer an acceptable level of protection (and getter function) against Cr. Such materials would therefore be excellent getter materials either in layers in electrochemical cells or as getter materials in other parts of the cell or in getter units.

[0142] Reference signs:

[0143] 10 - fuel cell system

[0144] 20 - fuel cell stack 30 - fuel cell

[0145] 41 - anode side fuel cell stack anode inlet

[0146] 41 A - fuel cell anode inlet

[0147] 42 - fuel cell stack anode off-gas outlet

[0148] 42A - fuel cell anode outlet

[0149] 50 - electrolyte layer

[0150] 60 - cathode side

[0151] 61 - fuel cell stack cathode inlet

[0152] 61 A - fuel cell cathode inlet

[0153] 62 - fuel cell stack cathode off-gas outlet

[0154] 62A - fuel cell cathode outlet

[0155] 70 - steam reformer

[0156] 71 - reformer inlet

[0157] 72 - reformer outlet

[0158] 80 - tail-gas burner

[0159] 81 - tail-gas burner exhaust

[0160] 82 - anode off-gas inlet

[0161] 83 - cathode off-gas inlet

[0162] 90 - fuel source

[0163] 100 - evaporator

[0164] 101 - fuel inlet

[0165] 102 - water inlet

[0166] 103 - water supply

[0167] 104 - evaporator exhaust 110 - anode off-gas heat exchanger

[0168] 120 - condenser heat exchanger

[0169] 121 - cooling circuit

[0170] 130 - separator

[0171] 131 - separator condensate outlet

[0172] 140 - oxidant inlet

[0173] 150 - air pre-heater heat exchanger

[0174] 160 - reformer heat exchanger

[0175] 161 - reformer heat exchanger oxidant inlet

[0176] 162 - reformer heat exchanger oxidant outlet

[0177] 170 - evaporator heat exchanger

[0178] 180 - fuel cell system exhaust

[0179] 190 - air bypass inlet

[0180] 200 - control means

[0181] 210 - blower

[0182] 220 - valve / separator

[0183] 230 - main cathode inlet gas flow path

[0184] 240 - air bypass inlet gas flow path

[0185] 250 - tail gas system

[0186] 260 - air bypass inlet gas flow path

[0187] 290- getter unit

[0188] A - anode inlet gas fluid flow path B - anode off-gas fluid flow path

[0189] C - cathode inlet gas fluid flow path D - cathode off-gas fluid flow path E - tail-gas burner off-gas fluid flow path

[0190] G - reformer cathode off-gas fluid flow path L - electrical load

[0191] T1 - fuel cell stack cathode inlet gas temperature sensor

[0192] T2 - fuel cell stack cathode off-gas temperature sensor

[0193] T3 - fuel cell stack anode inlet gas temperature sensor

[0194] 302 fuel cell unit

[0195] 304 support

[0196] 306 non-porous region of support

[0197] 308 porous region of support

[0198] 310 fuel electrode

[0199] 312 electrolyte

[0200] 314 interlayer

[0201] 316 oxygen electrode

[0202] 318 oxygen fluid volume

[0203] 320 fuel side fuel fluid volume

[0204] 324 interconnect

[0205] 326 flanged perimeter (of interconnect)

[0206] 328 contact features (dimples)

[0207] 330 getter coating layer

[0208] 332 oxygen fluid flow path

[0209] 334 leading edge of cell

[0210] 342 oxygen electrode bulk layer

[0211] 344 oxygen electrode active layer 346 electrolyte

[0212] 348 oxygen electrode bulk layer

[0213] 350 oxygen electrode active layer

[0214] 352 electrolyte

[0215] 354 oxygen electrode bulk layer

[0216] 356 oxygen electrode active layer

[0217] 358 electrolyte

[0218] 360 oxygen electrode bulk layer

[0219] 362 oxygen electrode active layer

[0220] 364 electrolyte

[0221] 402 External getter unit

[0222] 403 Optional Heat exchanger

[0223] 404 Optional Inlet heater,

[0224] 405 Electrolyser stack

[0225] 406 Inlet heater,

[0226] 407 Heat exchanger

[0227] All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

Claims1. Apparatus comprising a solid oxide electrochemical cell comprising an oxygen electrode, an electrolyte, and a fuel electrode; an oxygen supply inlet and a cell exhaust outlet; and a getter material comprising praseodymium, the getter material being situated between the oxygen supply inlet and cell exhaust outlet.

2. Apparatus as claimed in claim 1, wherein the apparatus further comprises an oxygen conduit defining an oxygen supply path between the oxygen supply inlet and the oxygen electrode.

3. Apparatus as claimed in claim 2, wherein the getter material forms a getter material layer on a part of the oxygen conduit in the oxygen supply path.

4. Apparatus as claimed in any one of the preceding claims, wherein the oxygen electrode comprises an oxygen electrode bulk layer and an oxygen electrode active layer located between the oxygen electrode bulk layer and the electrolyte.

5. Apparatus as claimed in claim 4, wherein the getter material is situated between the oxygen supply inlet and the oxygen electrode active layer.

6. Apparatus as claimed in either claim 4 or claim 5, wherein the oxygen electrode bulk layer comprises the getter material.

7. Apparatus as claimed in claim 6, wherein the getter material in the oxygen electrode bulk layer is selected from praseodymium cobalt nickelate (PCN), praseodymium strontium cobaltite (PSC), praseodymium strontium cobalt ferrite (PSCF), or mixtures thereof.

8. Apparatus as claimed in any one of the preceding claims, wherein the getter material is situated between the oxygen supply inlet and the oxygen electrode, optionally the oxygen electrode bulk layer.

9. Apparatus as claimed in any one of the preceding claims, wherein the getter material is contained in a getter unit.

10. Apparatus comprising a solid oxide electrochemical cell comprising an oxygen electrode, an electrolyte, and a fuel electrode, an oxygen supply inlet and a cell exhaust outlet, and a getter unit containing a getter material comprising praseodymium, the getter unit being situated between the oxygen supply inlet and cell exhaust outlet.

11. Apparatus as claimed in either claim 9 or claim 10, wherein the getter unit is located in the oxygen supply path.

12. Apparatus as claimed in any one of the preceding claims, wherein the apparatus further comprises an interconnect in electrical connection with the oxygen electrode.

13. Apparatus as claimed in claim 12, wherein the interconnect comprises raised features contacting the oxygen electrode and defining a gas volume on the oxygen electrode.

14. Apparatus as claimed either claim 12 or claim 13, further comprising a contact paste electrically connecting the interconnect and the oxygen electrode.

15. Apparatus as claimed in claim 14, wherein the getter material is incorporated in the contact paste.

16. Apparatus as claimed in either claim 14 or claim 15, wherein getter material layer is located on the surface of the interconnect.

17. Apparatus as claimed in any one of the preceding claims 12 to 16, wherein the getter material layer is located between the raised features of the interconnect.

18. Apparatus as claimed in any one of the preceding claims, wherein the getter material is selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite (PSCF), praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.eSnuCoojFeo.sOo-s), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro Sro CoCh (PSC551), a material of composition Pr(i-a)Lna0(2-5), wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm, 6 is the degree of oxygen deficiency, and 0.01 < a < 0.4, or mixtures thereof.

19. Apparatus as claimed in any one of the preceding claims 9 to 18, wherein the getter unit comprises a getter support and the getter material on or in the getter support.

20. Apparatus as claimed in claim 19, wherein the getter support comprises a porous monolith, optionally a porous honeycomb monolith.

21. Apparatus as claimed in claim 19, wherein the getter support comprises a metal organic framework (MOF) material, optionally comprising ZIF-8, UiO-66, MIL-101, and / or HKUST-1, or mixtures thereof.

22. Apparatus as claimed in any one of claims 19 to 21, wherein the getter support comprises a packed bed of high surface area pellets.

23. Apparatus as claimed in any one of claims 19 to 22, wherein the getter support comprises alumina, alumina silicate, alumina mixture, zirconia, or mixtures thereof.

24. A porous monolith comprising a material selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite, praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro.sSro.sCoCh (PSC551), a material of composition Pr(i-a)LnaO(2-5), or mixtures thereof, and at least one reinforcing agent, wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm,6 is the degree of oxygen deficiency, and0.01 < a < 0.4, wherein the reinforcing agent is selected from clay, silica, alumina and / or mixture thereof.

25. A method of forming a porous monolith comprising a ceramic material selected from praseodymium oxide, praseodymium cobalt nickelate (PCN), praseodymium strontium cobalt ferrite, praseodymium cobaltite, praseodymium ferrite, praseodymium nickel ferrite, Pro.99Coo.4Nio.60(3-5) (PCN60), Pro.6Sro.4Coo.2Feo.sO(3-5), praseodymium strontium cobaltite (PSC), praseodymium doped ceria, praseodymium strontium manganese, Pro.sSro.sCoCh (PSC551), a ceramic material of composition Pr(i-a)LnaO(2-5), or mixtures thereof, the method comprising:Providing a paste comprising the ceramic material, at least one reinforcing agent, and optionally a plasticiser, a binder, and / or a ceramic bulking agent,Extruding the paste through a die to form a green extruded ceramic body,Drying the green extruded ceramic body,Sintering the green extruded ceramic body at a suitable temperature to obtain a predetermined porosity, wherein Ln is selected from at least one rare earth metal, optionally Ln is selected from Nd, Sm, Eu, Gd, preferably Gd or Sm (PSmO), more preferably Sm, S is the degree of oxygen deficiency, and0.01 < a < 0.4.

26. A method of coating a porous ceramic monolith with a getter material, the method comprising liquid coating a porous ceramic monolith with a liquid containing a solvent and a getter material comprising praseodymium, optionally drying, and optionally heating the coated porous monolith.

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