Improved fuel electrodes for solid oxide cells

Abstract

A porous electrode layer for a solid oxide electrolysis cell (SOEC) is provided, said electrode layer being a composite comprising dispersed nickel particles and stabilised zirconia particles, said composite further comprising at least one transition metal (TM) from Groups 3 – 12 as dopant. The transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001:1 – 0.25:1, or 0.0001:1 – 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni). A powder mixture, suitable for producing the porous electrode layer, and an SOEC comprising said layer are also provided.

Description

[0001] IMPROVED FUEL ELECTRODES FOR SOLID OXIDE CELLS

[0002] TECHNICAL FIELD

[0003] The present invention relates to a solid oxide cell (SOC) comprising a porous electrode layer as fuel electrode, said electrode layer being a composite comprising dispersed nickel particles and stabilised zirconia particles, said composite further comprising at least one transition metal (TM) from Groups 3 - 12 as dopant. The transition metal (TM) is present in said porous electrode layer in low amount relative to the nickel (Ni). A method for manufacturing a solid oxide cell (SOC) is also provided.

[0004] BACKGROUND

[0005] State-of-the-art electrodes for solid state electrochemical cells, such as oxide ion conducting or proton conducting solid oxide fuel cells and electrolysis cells, have a three-dimensional porous microstructure to obtain high electrochemical reaction rates (also known as high performance). The reaction sites are locations where electrons, oxygen ions and chemical reactants and products can be transported to / from, and hence where the chemical reactions occur. Therefore, these types of porous electrodes must be comprised of materials with electronic and ionic conductivities and they must have sufficient porosity to allow diffusion of the reactants and products to and from reactions sites.

[0006] To provide the electronic, ionic and gas pathways, electrodes are typically mixed particle composites composed of necked particles, which may be fabricated by cheap wet-ceramic techniques such as tape-casting, spraying, screen-printing and laminating, followed by sintering. One example is the composite (also known as cermet) of nickel (Ni) particles and yttria-stabilized zirconia (YSZ) particles. The Ni-YSZ composite is for instance used as fuel electrode in oxide ion conducting cells, and the Ni provides electronic pathways, the YSZ provides ionic pathways and the reaction sites are the three-phase boundaries (sometimes referred to as Triple Phase Boundaries) between Ni, YSZ and the gas phase. The Ni-YSZ type electrode has been optimized significantly in the past decade.

[0007] Degradation of the Ni / YSZ fuel electrode of a solid oxide electrolysis cell has a negative effect on the lifetime of the cell; especially the observed Ni migration affects the lifetime. Ni migration is closely linked to adhesion, interfaces and morphology in the Ni-YSZ electrode structure. To avoid or at least minimize the Ni-migration the standard solution will be to operate the cell under less harsh conditions, i.e. go to lower current density (while still operating under thermoneutral conditions). Even though this means a lower H2production rate, it can be needed to minimize the Ni-migration and ensure a long enough life-time at sufficiently low cell resistance.

[0008] Publications in this field include Blaszczak et al, Int. J. Hydrogen Energy 2022, W02017 / 029350, JP 2014 026720 and US2021 / 237642.

[0009] SUMMARY

[0010] It has been found by the present inventors that high performance and surprisingly long-term durability of the fuel electrode of SOECs can be achieved by doping the porous electrode layer which makes up this electrode with a transition metal.

[0011] Therefore, in a first aspect, a solid oxide cell (SOC) is provided, said SOC comprising - in order - a porous electrode layer as fuel electrode, a solid-oxide electrolyte layer; optionally a barrier layer; and an oxy-electrode layer, wherein said porous electrode layer is a composite comprising dispersed nickel particles and stabilised zirconia particles, and said composite further comprising at least one transition metal (TM) from Groups 3 - 12 as dopant, preferably wherein said transition metal is selected from Co, Cu, Fe, Mn, Mo or W or mixtures thereof, and wherein said transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni).

[0012] A method is also provided for manufacturing a solid oxide cell (SOC) as defined herein, said method comprising the steps of: providing a powder mixture for a porous electrode layer, said powder mixture comprising : a. nickel precursor powder, b. stabilised zirconia powder and c. dopant precursor, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, in which said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, and wherein - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder, optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form a layer on said substrate, exposing said layer to sintering in oxidising conditions to form an electrode precursor layer, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode of the solid oxide cell.

[0013] A further method for manufacturing a solid oxide cell (SOC) is provided, said method comprising the steps of: providing a powder mixture for a porous electrode layer comprising (a) nickel precursor powder, and (b) stabilised zirconia powder, and (c) a dopant precursor solution, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, in which said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form an electrode precursor layer on said substrate, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, impregnating said electrode layer with the dopant precursor solution (c), optionally, exposing the electrode precursor layer to thermal treatment in reducing conditions, such that said transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni), and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode of the solid oxide cell.

[0014] A process is provided for the electrolysis of a fuel stream to a product stream in the solid- state electrolysis cell (SOEC) when the solid oxide cell (SOC) is a solid oxide electrolysis cell (SOEC), as described herein, said process comprising the steps of: feeding said fuel stream to the fuel electrode side of said SOEC, feeding aflush stream to the oxy-electrode layer of said SOEC, and applying a voltage across said SOEC, so as to electrolyse said fuel stream to a product stream.

[0015] Further details of the technology are provided in the enclosed dependent claims and examples.

[0016] FIGURES

[0017] Figure 1 shows the fuel electrode charge transfer resistance (Rctr) as a function of time during galvanostatic testing, for an undoped Ni:YSZ cell (tested for 1000 h) and a Fe-doped (Nio.s5Feo.15) :YSZ cell.

[0018] Figure 2 shows the fuel electrode charge transfer resistance (Rctr) as a function of time during galvanostatic testing, for an undoped Ni:YSZ cell (tested for 1000 h) and a Co-doped (Ni0.85Co0.15) :YSZ cell.

[0019] DETAILED DISCLOSURE

[0020] The invention provides a solid oxide electrolysis cell (SOEC) showing high performance as well as surprisingly long-term durability.

[0021] A solid oxide cell (SOC) is provided. The SOC comprises - in order - a porous electrode layer as fuel electrode, a solid-oxide electrolyte layer; optionally a barrier layer; and an oxy- electrode layer. The solid oxide cell (SOC) may be a solid oxide electrolysis cell (SOEC) or a solid oxide fuel cell (SOFC).

[0022] A solid oxide cell (SOC) is an electrochemical conversion device generally including a fuel electrode, an oxy electrode, and a solid electrolyte separating the fuel electrode and the oxy electrode and allowing oxygen ions, to pass through it. The SOCs generally also include contact layers to increase the in-plane electrical conductivity and provide improved electrical contact between adjacent SOCs when arranged in stacks. Adjacent SOCs are generally separated by interconnect layers, also referred to as interconnects. SOC stacks may further be arranged in modules.

[0023] The term "fuel side" refers to the side of the SOC that comprises the fuel electrode and where a feed gas is converted into a product gas (in electrolysis mode) and the term "oxy side" refers to the side of the SOC that comprises the oxy-electrode where oxygen is consumed or formed (depending on whether the SOC is operated as a fuel cell or electrolysis cell, respectively). For the avoidance of doubt, it is noted that the fuel electrode functions as the cathode (reduction process) during electrolysis operation. Correspondingly, the oxy electrode functions as the anode (oxidation process) during electrolysis operation.

[0024] Interconnects generally serve as a gas barrier to separate the fuel and oxy sides of adjacent SOECs, and at the same time they enable current conduction between the adjacent cells, i.e. between a fuel electrode of one cell and an oxy electrode of a neighbouring cell. Further, interconnects are normally provided with a plurality of flow paths for the passage of gas on both sides of the interconnect.

[0025] The porous electrode layer is a composite comprising dispersed nickel particles and stabilised zirconia particles. By the term "composite" as used herein is meant a material made of two or more materials with different physical or chemical properties, such as a composite of a ceramic material and a metallic material (also known as a cermet). An example of a composite is NiO-YSZ, and an example of a cermet is the Ni-YSZ cermet. The composite forms the three-dimensional porous microstructure of the porous electrode layer. The composite may be referred to as the solid phase of the porous electrode layer, and the nonsolid phase make up the pores (or the three-dimensional porous microstructure) of the porous electrode layer. The performance and stability of the composite comprising nickel particles, and stabilised zirconia particles, may also depend on the composite mixture, and the chemical composition of the stabilised zirconia. The stabilised zirconia may be selected from the group consisting of yttria-stabilised zirconia, scandia-stabilised zirconia, yttria- and scandia co-stabilised zirconia, magnesia-stabilised zirconia, calcia-stabilised zirconia, ceria- stabilised zirconia, scandia- and ceria co-stabilised zirconia or a combination thereof. Particular yttria-stabilised zirconias of interest are 8YSZ (8 mol% Y2O3 stabilised ZrO2), 4YSZ (4 mol% Y2O3 stabilised ZrO2) and 3YSZ (3 mol% Y2O3 stabilised ZrO2). In general an 8 mol% stabilised zirconia (ZrO2) is suitable for solid oxide fuel electrode layers.

[0026] The nickel, the dopant and the stabilised zirconia may in an embodiment comprise the solid phase of the composite. Different ratios between the nickel and the stabilised zirconia may be present in the composite. In an embodiment, the porous electrode layer comprises a nickel volume fraction of the solid phase of >60, >50, >45, >40, >30, or >20 vol%, most preferably >30 vol%. The upper limit being 70 vol%.

[0027] In embodiments, the volume ratio between doped nickel and stabilised zirconia is suitably between 20:80 - 70:30 or 20:80 - 50 :50, preferably 40:60 - 60:40, e.g. 70:30, 60:40, 50:50, 45 :55, 40 :60, 30:70, or 20:80, most preferably 40:60.

[0028] Suitably, the porosity in the porous electrode layer is below 33, 30, or 29 vol%, more preferably below 26 or 24 vol%, and most preferably below 23, 22, 21, 18, 15, or 13 vol%. In one aspect, the pores are essentially or exclusively generated from the volume created by reducing a corresponding metal oxide to the metal particles. The porous electrode layer must have enough porosity for reactant and product to flow through without creating a too high pressure difference. The lower limit for porosity depends on the three dimensional structure of the pores and of the flow required through the cell, but in practice the lower limit may be set at 8, 10 or 12 vol% as measured by image analysis, such as from SEM analysis using Image! for determination of the porosity.

[0029] The composite comprises at least one transition metal (TM) from Groups 3 - 12 as dopant. Preferred transition metals are selected from Co, Cu, Fe, Mn, Mo or W or mixtures thereof. The amount of dopant is relatively low; i.e. the transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 -0.25: 1, 0.0001 : 1 - 0.1 : 1, 0.0005: 1 - 0.05: 1, 0.001 : 1 - 0.01 : 1, 0.01 : 1 - 0.1 : 1, or 0.05: 1 - 0.25: 1, expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni).

[0030] The molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) in the solid phase may be determined by any suitable technique, such as inductively-coupled plasma-optical emission spectrometry (ICP-OES); or image analysis (Scanning Electron Microscopy, SEM or Transmission Electron Microscopy, TEM) combined with Energy Dispersive Spectroscopy (EDS, such as X-ray EDS). Preferably, the molar ratio may be determined via elemental analysis, e.g. inductively-coupled plasma-optical emission spectrometry ICP-OES. ICP-OES is an analytical technique that utilizes an inductively coupled plasma and optical emission spectroscopy to determine the elemental composition of a sample. ICP-OES is recognized for its great ability to detect a wide range of elements simultaneously and provide accurate results.

[0031] The volume of the non-solid phase (the pores) may be determined by any suitable technique for measuring pore volume divided by total volume of the sample. Suitable techniques for measuring pore volume are e.g. Mercury Intrusion Porosimetry, Gas Pycnometry or Image Analysis.

[0032] The composition of the solid phase may be defined by volume fractions of the components in the solid phase. A volume fraction of a component in the solid phase of the porous electrode layer may be determined from a determination of the mass fraction or of the molar fraction of the component in the solid phase combined with the relevant volumetric densities. The masses of each component (metallic or ceramic) may e.g. be determined by the same methods as suggested above for determining the molar fractions of TM :Ni. Preferably, the volume fraction of a component within the solid phase is calculated using the mass of the component determined by ICP-OES and the known volumetric density of the component. As used herein, the volume fractions are calculated for electrodes in the reduced state, without accounting for the additional porosity formed upon NiO, or (Ni,TM)Oxide, reduction. The same goes for volume ratios.

[0033] In the composite the stabilised zirconia particles is understood to provide a ceramic ionconducting phase, and dispersed in such a way to form a fine, and well-dispersed microstructure that hinders migration and coarsening of the metallic phase e.g. Ni phase, when the composite is operated as an electrode, whereby high performance and long-term durability is obtained. In particular, high performance and long-term durability of an electrode at high current density (i.e. high fuel production rate for electrolysis cells) can be obtained.

[0034] It has previously been shown that Ni migration correlates with Ni-wetting angle and the gradient in pO2 (in turn; fuel electrode overpotential). For state-of-the-art fuel electrodes a fuel electrode overpotential above roughly 200 mV will lead to Ni migration and thereby irreversible degradation of the electrode, and in turn the SOEC cell. A correlation exists between Ni-wetting angle and the threshold (in terms of pO2 gradient) for when Ni migration is initiated.

[0035] The present inventors have discovered that the Ni wetting angle can be manipulated by dopants in the fuel electrode and the electrode precursor (that is the powder used for electrode slurry preparation). This, in turn, leads to reduced Ni migration in the electrode, and improved lifetime.

[0036] By modifying the Ni-wetting angle by well-designed doping, SOE cells could then be designed to cope with e.g. 300mV or even 400 mV of fuel electrode overpotential before detrimental Ni migration threshold is met and severe degradation of the fuel electrode starts. A particular powder mixture according to the invention comprises or consists of: a. 45-75 weight %, preferably 55-67 weight %, more preferably 59-62 weight % nickel oxide powder, b. 25-55 weight %, preferably 30-45 weight %, more preferably 35-40 weight % yttria-stabilized zirconia (YSZ) and c. dopant precursor, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, wherein said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof.

[0037] As previously - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder.

[0038] A more preferred powder mixture comprises: a. 59-62 weight % nickel oxide powder, b. 35-40 weight % yttria-stabilized zirconia (YSZ) and c. dopant precursor, wherein said dopant comprises or consists of a transition metal (TM) precursor, wherein said transition metal (TM) is a metal from selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, wherein - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder.

[0039] The transition metal precursor in the powder mixture is suitably in the form of the transition metal oxide or the transition metal carbonate or the transition metal nitrate, preferably the transition metal oxide. The dopant precursor may consist of the transition metal precursor. The dopant precursor may comprise the transition metal precursor and further components e.g. suitable additives, a solvent etc.

[0040] In the powder mixture, the characteristic size of the particle size distribution is di0between 0.03-0.07 pm, d5o between 0.08-0.4 pm, d90between 0.4-1.2 pm, more preferably di0 between 0.04-0.06 pm, d5o between 0.1-0.3 pm, d90between 0.9-1.1 pm, and most preferably di0= 0.05 pm, d5o = 0.2 pm, and d90= 1 pm.

[0041] In one aspect, the transition metal in the porous electrode layer, or the powder mixture may be Co, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0042] In another aspect, the transition metal in the porous electrode layer, or the powder mixture may be Cu, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0043] In another aspect, the transition metal in the porous electrode layer, or the powder mixture may be Fe, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0044] In another aspect, the transition metal in the porous electrode layer, or the powder mixture may be Mn, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0045] In another aspect, the transition metal in the porous electrode layer, or the powder mixture may be Mo, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0046] In another aspect, the transition metal in the porous electrode layer, or the powder mixture may be W, and may be present in an amount of between 0.0001:1 -0.25:1, or 0.0001:1 - 0.1:1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0047] Preferred amounts of the transition metal in the porous electrode layer, or the powder mixture, are between 0.0005:1 - 0.05:1, such as 0.001:1 - 0.01:1, preferably XX - XX expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

[0048] Methods

[0049] A method for manufacturing a solid oxide cell (SOC) is further provided, said method comprising the steps of: providing a powder mixture for a porous electrode layer, said powder mixture comprising: a. nickel precursor powder, b. stabilised zirconia powder and c. dopant precursor, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, in which said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, and wherein - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 -0.25: 1, 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder, optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form a layer on said substrate, exposing said layer to sintering in oxidising conditions to form an electrode precursor layer, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode.

[0050] In some embodiments the above method of providing a powder mixture comprises a first prior step of forming a nickel-transition metal (Ni-TM) precursor powder, and wherein - in said Ni-TM precursor powder - the transition metal (TM) is present in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1 expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder, and a second prior step of mixing the Ni-TM precursor powder with the stabilised zirconia powder to provide the powder mixture. This has an advantage of ensuring the nickel and the transition metal are homogenically dispersed within the metal phase. Thus they are available at the electrochemically active sites (triple phase boundaries) in the final electrode microstructure. When the nickel precursor powder is doped with the dopant in a first prior step and a second prior step, the nickel-transition metal (Ni-TM) precursor powder may be referred to as doped nickel precursor powder and the nickel phase of the porous electrode layer may similarly be referred to as doped nickel or doped nickel phase.

[0051] The first prior step of forming a nickel-transition metal (Ni-TM) precursor powder may further comprise impregnating the nickel precursor powder with the dopant precursor in an aqueous solution and subsequently performing a pre-calcination of the impregnated nickel precursor powder at 400-1000 degC to provide the powder mixture. The pre-calcination functions to induce formation of a single phase solid solution of Ni and transition metal (NiO and and the TM nitrate) and ensures together with the first prior step that the nickel and the transition metal are homogenically dispersed. Thus they available at the electrochemically active sites in the final electrode microstructure.

[0052] A second method involves impregnation of a transition metal dopant into an electrode layer. Therefore, a method for manufacturing a solid oxide cell (SOC) is provided, said method comprising the steps of: providing a powder mixture for a porous electrode layer comprising (a) nickel precursor powder, and (b) stabilised zirconia powder, optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form an electrode precursor layer on said substrate, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, impregnating said electrode layer with transition metal or transition metal precursor, optionally, exposing the electrode precursor layer to thermal treatment in reducing conditions, such that said transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 -0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni), and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode.

[0053] It is believed that doping with transition metal can be used to optimize the Ni-wetting angle.

[0054] The quantification of distribution of wetting angles in the fuel electrode structure can be obtained via 3D-reconstruction of the electrode structure conducted via focused ion beam milling and SEM imaging (also known as FIB-SEM) followed by quantitative image analysis of the SEM images. This will lead to quantification of the electrode structure with a resolution in the nano-scale and at the same time investigating of the distribution and dispersion of the added dopant in the porous structure can be obtained.

[0055] A process is also provided for the electrolysis of a fuel stream to a product stream in the solid oxide cell (SOC) according to any one of the above embodiments, said SOC comprising - in order - a porous electrode layer as fuel electrode, a solid-oxide electrolyte layer; optionally a barrier layer; and an oxy-electrode layer, when the solid oxide cell (SOC) is a solid oxide electrolysis cell (SOEC), as described herein, said process comprising the steps of feeding said fuel stream to the fuel electrode side of said SOEC, optionally feeding a flush stream to the oxy-electrode layer of said SOEC, and applying a voltage across said SOEC, so as to electrolyse said fuel stream to a product stream.

[0056] In said process, the fuel stream may be a water-rich stream and the product stream is thus a hydrogen-rich stream, or the fuel stream may be a CO2-rich stream and the product stream is thus a CO-rich stream or the fuel stream is a water-rich and CO2-rich stream and the product stream is a hydrogen-rich and CO-rich stream.

[0057] The present invention has been described with reference to a number of embodiments. However, the skilled person is able to select and combine various embodiments within the scope of the invention, which is defined by the appended claims. All documents referenced herein are incorporated by reference.

[0058] Example 1 : Manufacturing and testing of a solid oxide cell SOC with a (Fe-doped Ni) :8YSZ 46:54 vol.% composite.

[0059] The first step comprises individual tape-casting of four layers on a polymeric support.

[0060] Layer 1 : Conventional fuel electrode support,

[0061] Layer 2: Doped Fuel electrode as disclosed herein,

[0062] Layer 3: Conventional electrolyte

[0063] Layer 4: Conventional barrier layer. The suspensions (also referred to as slurries) for tape-casting are manufactured by means of ball milling oxide powders in ethanol, with suitable additives, e.g. polyethylene glycol (PEG) as Plasticizer.

[0064] Layer 1 : The layer 1 suspension comprised as solids approx. 47 vol% yttria stabilized zirconia (3YSZ) and approx. 53 vol% NiO (relative to total solid loading) which was tape-casted. The porosity of layer 1 was about 30% after tape-casting, sintering and reduction. The green thickness was in the range of 300 pm.

[0065] Layer 2 : . Fe-doping of the NiO powder was achieved by multiple wet impregnations of NiO using Fe(NO3)3.9H2O precursor solution, followed by pre-calcination of this mixture to 400°C to remove the nitrate, resulting in a (Nio.85Fe0.i5)Ox(Ni-TM) precursor powder (Fe:Ni~0.18: l). This precursor powder was mixed with an yttria stabilised zirconia powder (8YSZ) to obtain a powder mixture. After ball milling, the powder mixture had median particle sizes in submicrometer range, i.e. 0.1 - 1 pm. Milled, suspended powder mixture was formed into a slurry which comprised as solids approx. 41 vol% stabilised zirconia (8YSZ) and approx. 59 vol % Fe-doped Ni-based oxide (doped nickel oxide: stabilised zirconia ~41 :59), where the (Ni-TM) precursor powder had a ratio of Fe:Ni of 0.15:0.85 (~0.18: l). The slurry was tape- casted on a substrate. After sintering and reduction, an electrode layer of a (Nio.85Feo.i5) :8YSZ composite having a volume ratio of doped nickel stabilised zirconia in the porous electrode layer of 46:54. After tape casting, the green thickness was in the range of 10 - 20 pm.

[0066] Layer 3: The suspension comprised YSZ with green thickness in the range of 10 - 20 pm.

[0067] Layer 4: The suspension comprised Ceo.9Gdo.1O1.95 (CGO10) with green thickness in the range of 10 - 20 pm.

[0068] The second step comprised assembling of the single tape-cast layers 1 through 4 by lamination.

[0069] In the third step the laminated half-cell was cut into the desired shape by knife punching, resulting in areas of about 15x10 cm2to be sintered.

[0070] In the fourth step the half-cell was sintered for 12 h at approximately 1350°C

[0071] In the fifth step, the sintered samples were laser-cut into areas of 5.3 x 5.3 cm2. The fifth step comprised deposition of an LSCF oxy-electrode by screen-printing onto a 4 x 4 cm2area in the centre of the 5.3 x 5.3 cm2cells.. The green thickness was approximately 30 pm.

[0072] The sixth step was sintering of the cell at about 1050°C for 24 h. The seventh step was heating of the cell in inert atmosphere, followed by reduction of the cell, after sealing, at 800°C for 2 hours initially in a 10:90 vol.% H2:N2mixture, and then in pure H2for a further 2 hours, to obtain a solid oxide cell comprising a porous electrode layer of the doped (Nio.85Feo.i5) :YSZ (46:54) to be tested.

[0073] One such cell was tested in a conventional set-up for 1500 hours under the conditions of 725°C, -1 A / cm2, H2O / H2: 90 / 10 (inlet), 57% H2O conversion, and air to the oxy-electrode. The performance as a function of time is measured via recording the cell voltage over time and by recording electrochemical impedance spectra during the galvanostatic electrolysis test. The cell resistance contributions were deconvoluted using an equivalent circuit model (ECM), such that the individual electrode process resistances could be quantified.

[0074] Compared to an un-doped cell comprising a pure Ni:YSZ 45:55 composite electrode, the degradation of the fuel electrode, as measured by in operando electrochemical impedance under galvanostatic operation, was significantly reduced from 279 mQ cm2to 144 mQ cm2(48% reduction in degradation). This is illustrated in Figure 1, which shows the fuel electrode charge transfer resistance (Rctr) as a function of time during the test, for both an undoped Ni:YSZ cell (tested for 1000 h) and the doped (Nio.85Feo.is) :YSZ cell.

[0075] Conclusion: This reduction in fuel electrode degradation caused by the dopant modification to the conventional Ni:YSZ composite fuel electrode can drastically extend the lifetime of the SOEC by lowering degradation, since degradation of the fuel electrode in particular is considered one of the primary degradation issues for SOECs.

[0076] Example 2: Manufacturing and testing of a solid oxide cell SOC with a (Co-doped Ni) :YSZ 45:55 voL% composite.

[0077] A solid oxide cell comprising a porous electrode layer of the doped

[0078] (Nio.85Coo.i5) :YSZ was manufactured using the same procedure as in example 1, except a precursor solution of Co(NO3)2.6H2O was used for the multiple wet impregnations of NiO to obtain a ratio of Co:Ni of 0.15 :0.85 and and pre-calcination at 950 °C resulting in a (Nio.85Coo.i5)Ox(Ni-TM) precursor powder (Co:Ni~0.18: l). Using this precursor powder in the procedure of example 1, resulted in a solid oxide cell comprising a porous electrode layer of the doped (Nio.85Coo.is) :YSZ (45:55) to be tested.

[0079] One such cell was tested in a conventional set-up for 1000 hours under the conditions of 725°C, -1 A / cm2, H2O / H2: 90 / 10 (inlet), 57% H2O conversion, and air to the oxy-electrode. The performance as a function of time is measured via recording the cell voltage over time and by recording electrochemical impedance spectra during the galvanostatic electrolysis test. The cell resistance contributions were deconvoluted using an equivalent circuit model (ECM), such that the individual electrode process resistances could be quantified.

[0080] Compared to an un-doped cell comprising of a pure Ni:YSZ 45:55 composite electrode, the degradation of the fuel electrode, as measured by in operando electrochemical impedance under galvanostatic operation, was significantly reduced from 279 mQ cm2to 209 mQ cm2(25% reduction in degradation). This is illustrated in Figure 2, which shows the fuel electrode charge transfer resistance (Rctr) as a function of time during the test, for both an undoped Ni:YSZ cell (tested for 1000 h) and the doped (Nio.85Coo.is) :YSZ cell.

[0081] Conclusion: This reduction in fuel electrode degradation caused by the dopant modification to the conventional Ni:YSZ composite fuel electrode can extend the lifetime of the SOEC by lowering degradation, since degradation of the fuel electrode in particular is considered one of the primary degradation issues for SOECs.

[0082] Example 3: Manufacturing and testing of further Co-doped Ni-YSZ electrodes

[0083] SOE cells comprising Co-doped Ni-YSZ electrodes with TM:Ni ratios of 0.03: 1 and 0.05: 1 were also manufactured and tested in the same way as in Example 2. Reductions in degradation of 6% and 9%, respectively, were observed after a 1 kh test period.

[0084] Based on this work, but without being bound by theory, the inventor's hypothesis on cause of reduced degradation due to the TM doping of the fuel electrode is:

[0085] It is hypothesised that the reduction in degradation of the fuel electrode is due to an increased tolerance towards impurities in the testing environment, particularly SiO2, which is known to have a detrimental impact on fuel electrode performance. Doping of Ni with TM elements, such as Co and Fe amongst others, is believed to result in a slightly decreased electrocatalytic activity at the electrochemical triple-phase boundaries, which leads to a decreased likelihood of impurity deposition in these zones. Ni is known to be an excellent catalyst for the steam reduction reaction, and thus replacing Ni with small quantities of any other TM dopant from Groups 3 - 12 is hypothesized to have a similar effect as exemplified with Co and Fe dopants.. A second hypothesis for the reduction in fuel electrode degradation is that doping the Ni phase with specific TM elements (such as Co and Fe) will increase the interface strength between the metal and zirconia phases, thus increasing the fuel electrode overpotential threshold for Ni migration to occur. This may delay, or in some cases prevent, the occurrence of Ni migration in the electrode, which is a crucial degradation mechanism in SOECs operated at high current density. The increase in interface strength is due to a stronger metal-oxygen (M-O) bond of the TM dopant, compared to Ni.

Claims

CLAIMS1. A solid oxide cell (SOC), said SOC comprising - in order - a porous electrode layer as fuel electrode, a solid-oxide electrolyte layer; optionally a barrier layer; and an oxy-electrode layer, wherein said porous electrode layer is a composite comprising dispersed nickel particles and stabilised zirconia particles, and said composite further comprising at least one transition metal (TM) from Groups 3 - 12 as dopant, preferably wherein said transition metal is selected from Co, Cu, Fe, Mn, Mo or W or mixtures thereof, and wherein said transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni).

2. The solid oxide cell (SOC) according to claim 1, being a solid oxide electrolysis cell (SOEC) or a solid oxide fuel cell (SOFC).

3. The solid oxide cell (SOC) according to any one of the preceding claims, wherein said porous electrode layer comprises a nickel volume fraction of the solid phase of >60, >50, >45, >40, >30, or >20 vol%, most preferably >30 vol%.

4. The solid oxide cell (SOC) according to any one of the preceding claims, wherein the volume ratio of doped nickel :stabilised zirconia in the porous electrode layer is between20:80 - 70:30 or 20:80 - 50 :50, preferably 40:60 - 60:

405. The solid oxide cell (SOC) according to any one of the preceding claims, wherein the transition metal (TM) is present in said porous electrode layer in an amount of between 0.0005: 1 - 0.05: 1, 0.001 : 1 - 0.01 : 1, 0.01 : 1 - 0.1 : 1, or 0.05: 1 - 0.25: 1, , expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni).

6. A method for manufacturing a solid oxide cell (SOC) according to any one of claims 1- 5, said method comprising the steps of: providing a powder mixture for a porous electrode layer, said powder mixture comprising : a. nickel precursor powder, b. stabilised zirconia powder andc. dopant precursor, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, in which said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, and wherein - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1 expressed as a molar ratio (TM :Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder, optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form a layer on said substrate, exposing said layer to sintering in oxidising conditions to form an electrode precursor layer, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode of the solid oxide cell.

7. The method according to claim 6, wherein the providing a powder mixture comprises a first prior step of forming a nickel-transition metal (Ni-TM) precursor powder, and wherein - in said Ni-TM precursor powder - the transition metal (TM) is present in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1 expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) of the nickel precursor powder, and a second prior step of mixing the Ni-TM precursor powder with the stabilised zirconia powder to provide the powder mixture.

8. The method according to claim 7, wherein the first prior step of forming a nickel- transition metal (Ni-TM) precursor powder further comprises impreganting the nickel precursor powder with the dopant precursor in an aqueous solution and subsequently performing a pre-calcination of the impregnated nickel precursor powder at 400-1000 degC to provide the powder mixture.

9. The method according to any one of claims 6 to 8, wherein said powder mixture comprises or consists of:19 a. 45-75 weight %, preferably 55-67 weight %, more preferably 59-62 weight % nickel oxide powder, b. 25-55 weight %, preferably 30-45 weight %, more preferably 35-40 weight % yttria-stabilized zirconia (YSZ) and c. dopant precursor, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, wherein said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof wherein - in said powder mixture - the transition metal (TM) is present in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni) of the nickel oxide powder.

10. A method for manufacturing a solid oxide cell (SOC), said method comprising the steps of: providing a powder mixture for a porous electrode layer comprising (a) nickel precursor powder, (b) stabilised zirconia powder, and (c) a dopant precursor solution, wherein said dopant precursor comprises or consists of a transition metal (TM) precursor, in which said transition metal (TM) is a metal selected from Groups 3 - 12, such as Co, Cu, Fe, Mn, Mo or W, or a mixture thereof, Ni oxide optionally, milling said powder mixture, forming a slurry from said powder mixture casting, e.g. tape casting, the slurry on a substrate, to form an electrode precursor layer on said substrate, exposing the electrode precursor layer to thermal treatment in reducing conditions to form an electrode layer on said substrate, impregnating said electrode layer with the dopant precursor solution (c), optionally, exposing the electrode precursor layer to thermal treatment in reducing conditions, such that said transition metal (TM) is present in said porous electrode layer in an amount of between 0.0001 : 1 - 0.25: 1, or 0.0001 : 1 - 0.1 : 1, expressed as a molar ratio (TM:Ni) of the transition metal (TM) to the nickel (Ni), and incorporating the electrode layer on said substrate into a solid oxide cell (SOC), so that the electrode layer is arranged as the fuel electrode of the solid oxide cell.2011. The method according to any one of claims 6-10, wherein said transition metal precursor is the transition metal oxide or the transition metal carbonate or the metal nitrate, preferably the transition metal oxide.

12. A process for the electrolysis of a fuel stream to a product stream in the solid oxide cell (SOC) according to any one of claims 1-5, said SOC comprising - in order - a porous electrode layer as fuel electrode, a solid-oxide electrolyte layer; optionally a barrier layer; and an oxy-electrode layer, when the solid oxide cell (SOC) is a solid oxide electrolysis cell (SOEC), said process comprising the steps of: feeding said fuel stream to the fuel electrode side of said SOEC, - optionally feeding a flush stream to the oxy-electrode layer of said SOEC, and applying a voltage across said SOEC, so as to electrolyse said fuel stream to a product stream.

13. The process according to claim 10, wherein the fuel stream is a water-rich stream and the product stream is a hydrogen-rich stream, or wherein the fuel stream is a CO2-rich stream and the product stream is a CO-rich stream, or wherein the fuel stream is a waterrich and CO2-rich stream and the product stream is a hydrogen-rich and CO-rich stream.

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