Anode for a solid oxide fuel cell and composition and method for forming same

Abstract

The invention relates to solid oxide fuel cell anodes, in particular anodes which containing porous particles coated with catalytic nickel. The use of porous particles as a carrier for the nickel catalyst helps to overcome some of the redox stability issues experienced by some systems and improves the internal reforming properties of the system and permits less nickel to be used in SOFC systems.

Claims

1. An anode for a solid oxide fuel cell (SOFC), the anode comprising: a matrix comprising a doped metal oxide; and an electrocatalyst, wherein the electrocatalyst comprises porous particles supported by the matrix, the porous particles having pore sizes greater than 1 nm and containing a steam reformation catalyst material trapped within the pores of the porous particles.

2. An anode according to claim 1, wherein the steam reformation catalyst material is selected from: Fe, Co, Ru, Ni, Rh, Pt, Pd, or a combination thereof; and optionally wherein the steam reformation catalyst material is selected from: Fe, Co, Ni, or a combination thereof.

3. An anode according to claim 1, wherein the steam reformation catalyst material is nickel.

4. An anode according to claim 1, wherein the doped metal oxide is an electrically conducting ceramic material selected from the perovskites lanthanum strontium chromite (LSCr), lanthanum strontium chromite manganite (LSGM), and doped strontium titanates.

5. An anode according to claim 1, wherein the doped metal oxide is an electrically conducting ceramic material.

6. An anode according to claim 1, wherein the doped metal oxide is a rare-earth doped ceria, optionally selected from: gadolinium doped ceria (CGO); samarium doped ceria; or combinations thereof.

7. An anode according to claim 1, wherein the porous particles containing the steam reformation catalyst material are present in the range 5 to 70% wt. based on the weight of the total anode.

8. An anode according to claim 1, wherein the porous particles containing the steam reformation catalyst material are present in the range 10 to 60% wt. based on the weight of the total anode.

9. An anode according to claim 1, wherein the porous particles have pore sizes of less than 1 μm.

10. An anode according to claim 1, wherein the matrix surrounds the porous particles.

11. An anode according to claim 1, wherein the porous particles have pore sizes in the range 1 nm to 500 nm.

12. An anode according to claim 1, wherein the porous particles are mesoporous particles.

13. An anode according to claim 1, wherein the porous particles are dispersed throughout the matrix.

14. An anode according to claim 1, wherein the porous particles comprise praseodymium doped ceria (PDC).

15. An anode according to claim 1, wherein the porous particles comprise a rare-earth doped ceria.

16. An anode according to claim 1, wherein the matrix further comprises in the range 5%-70% wt. of the steam reformation catalyst material, optionally selected from nickel and nickel oxide.

17. An anode according to claim 1, wherein the anode is for a metal supported solid oxide fuel cell, and is provided on a metal support substrate.

18. A composition for making an anode for a solid oxide fuel cell, the composition comprising: a matrix precursor comprising a doped metal oxide; and an electrocatalyst, wherein the electrocatalyst comprises porous particles having pore sizes greater than 1 nm and containing the steam reformation catalyst material trapped within the pores of the porous particles.

19. A method of making an anode for a solid oxide fuel cell, comprising the steps of: i) applying a composition according to claim 18 to a substrate; and ii) sintering the composition.

20. A solid oxide fuel cell comprising the anode according to claim 1, and optionally, wherein the anode is for a metal supported solid oxide fuel cell, and is provided on a metal support substrate.

21. An anode according to claim 1, wherein the porous particles are of higher porosity than the matrix.

22. An anode according to claim 1, wherein the surface of the pores of the porous particles are coated with active metal catalyst nanoparticles.

23. An anode according to claim 1, wherein the steam reformation catalyst material in the porous particles comprises nickel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shoes a schematic diagram of a PDC particles prior to the addition of a catalytic coating.

(2) FIG. 2 shows an SEM micrograph of PDC catalyst support particle at a0 2500× magnification and b) 30000× magnification.

(3) FIG. 3 shows a schematic diagram of the PDC particles following impregnation with catalyst material.

(4) FIG. 4 shows an SEM micrograph of the PDC particles after impregnation at 30000× magnification.

(5) FIG. 5 shows a schematic representation of a conventional SOFC cermet anode for a metal supported SOFC.

(6) FIG. 6 shows a schematic representation of an anode of the invention comprising impregnated PDC particles.

(7) FIG. 7 shows SEM micrographs of cross sections of an SOFC cell with an anode made up of impregnated PDC showing (a) the anode microstructure, (b) the anode-substrate interface and (c) the anode-electrolyte interface.

(8) FIG. 8 shows a schematic representation of an SOFC anode where impregnated PDC particles are incorporated into a conventional cermet structure.

(9) FIG. 9 shows SEM images of a cross section of an SOFC anode where impregnated PDC particles are incorporated into a conventional cermet structure at a) 5000× and b) 15000× magnification.

(10) FIG. 10 shows a schematic representation of impregnated PDC particles incorporated into a matrix of La.sub.0.75Sr.sub.0.25Mn.sub.0.5Cr.sub.0.5O.sub.3.

(11) FIG. 11 shows SEM cross sectional images of PDC particles incorporated into a matrix of La.sub.0.75Sr.sub.0.25Mn.sub.0.5Cr.sub.0.5O.sub.3.

DETAILED DESCRIPTION

(12) The embodiments described herein use 20% praseodymia-doped ceria (PDC, Ce.sub.0.8Pr.sub.0.2O.sub.1.9) supplied commercially by Solvay and which is typically used as a support for water gas shift catalysts in the automotive industry to reduce NO.sub.x emissions. PDC is obtained in the form of porous, approximately spherical particles of approximately 3 μm diameter, and they have a very high internal surface area of 150-200 m.sup.2g.sup.−1; the spherical particles being made up of agglomerates of nanometre-scale crystallites. A schematic representation of the spherical porous PDC particle 1 is shown in FIG. 1 comprising a particle body 3 and a plurality of pores 5.

(13) The PDC particles are impregnated with a suitable metallic catalyst in order to line the pores of the PDC particles with active metal. In all embodiments described here the active metal catalyst is nickel or an alloy thereof. Nickel is known to be catalytically active for the dissociation of molecular hydrogen (which is an important step in the electrochemical oxidation of hydrogen in an SOFC anode), and the steam reforming of methane to release hydrogen and carbon monoxide (another important SOFC anode reaction). A catalyst particle impregnated with metal catalyst nanoparticles is shown schematically in FIG. 3 containing a particle 1 comprising pores 5 and a particle body 3, wherein the surface of the pores are coated with a plurality of catalyst nanoparticles 7. An SEM micrograph of an impregnated particle is shown in FIG. 4 in which the nanoparticles are too small to be seen individually.

(14) The process of incipient wetness impregnation involves the following steps:

(15) (a) Establishing empirically the specific pore volume (expressed as cm.sup.3/g) of the catalyst support, by adding deionised water to a known mass of catalyst support drop-wise until the catalyst starts to appear slightly damp (the point of incipient wetness). This is the volume of water the pores in the catalyst can absorb without leaving excess water outside the catalyst particles.
(b) Making up a solution of the nitrate salts (though other salts, e.g. chlorides, could be used in principle) of the active metal(s) to be impregnated in deionised water. In this case a saturated solution of nickel and optionally cobalt nitrates are prepared, to maximise the amount of metal which could be impregnated onto the catalyst support in a single step.
(c) Adding the solution of metal nitrates drop-wise to a known mass of PDC catalyst support particles whilst continuously mixing the catalyst support until a volume of solution just below that previously determined to be the point of incipient wetness has been added.
(d) Transferring the catalyst support impregnated with nitrate solution to an oven, and drying off the water to leave the catalyst impregnated with anhydrous metal nitrate coating the inside of its pores.
(e) Transferring the dried impregnated catalyst to a suitably ventilated furnace, and calcining it at a temperature (650° C. was used for in all cases here) high enough to cause the metal nitrates to decompose to the equivalent metal oxides, with the emission of nitrogen dioxide.

(16) Subsequent manufacturing processes to incorporate the impregnated PDC particles into a fuel cell system are performed with the impregnated catalyst in its oxide form. On exposure to hydrogen and temperature when the SOFC is first operated, the metal oxides are reduced to their native metal form, in which they provide catalytic activity.

(17) Comparison with a Conventional SOFC Anode

(18) A conventional SOFC anode for a metal-supported application, such as those disclosed in GB1315744.1 and GB1315746.6 is shown schematically in FIG. 5. Here the anode is deposited between the ferritic stainless steel substrate 11 and the gadolinium-doped ceria (CGO) electrolyte 13. The anode is porous to enable gaseous reactants to diffuse through it to/from the anode-electrolyte interface.

(19) The anode consists of a sintered ceramic-metal composite of CGO 15 and metallic phases 17, where the particles making up the anode structure are typically in the size range 0.5-2 μm. Normally, the anode is deposited and sintered as a mixture of CGO and metal oxides, and the metal oxides are reduced to the active metal upon exposure to hydrogen during the first SOFC operation.

(20) Anode Deposition Processes

(21) For all the embodiments described here, the deposition process is essentially as described in GB1315746.6. As a first step, the impregnated PDC particles are formulated into an ink suitable for screen printing comprising: an organic solvent (Texanol); metal oxide(s) dispersed within the solvent system as pigments (nickel and cobalt oxides), typically in the range 50-80 wt % of the total ink mass; a polymer binder (e.g. Polyvinylbutyral (PVB) Eastman Butvar B76 grade); one or more dispersing agents (commercially available dispersants from Byk Chemie were used) to stabilise the metal oxide powders in the ink and minimise the tendency of the components to settle out or agglomerate; and a wetting/defoaming agent (Byk-057 made by Byk Chemie), to aid levelling of the printed layer and destabilise any bubbles trapped in the ink, which can lead to print defects.

(22) Methods for dispersing the metal oxide(s) into the ink include those well known in the art for making inks and paints such as bead milling, use of a high-shear disperser and triple-roll milling, either singly or in combination. The resulting ink is screen printed onto a metal substrate to form a deposited anode layer which is then passed through an oven to evaporate off the solvent to form a dried printed layer.

(23) The dried printed layer is passed through a furnace at a temperature high enough to burn off the binders and dispersant polymers in the ink. Isostatic or uniaxial pressing of the burnt-out layer is performed to increase its green density. The pressed layer is then placed in a furnace and fired it at a temperature of up to 1050° C. in air to cause the particles of metal oxide powder to sinter together to form a porous ceramic structure. Optionally the electrolyte layer may be printed over the anode in the process disclosed in PCT/GB2016/050256 and GB1502035.7, wherein the burnout, pressing and sintering steps combined.

(24) Anode Consisting Entirely of Impregnated PDC Particles.

(25) One embodiment of the invention is shown schematically in FIG. 6, comprising an electrolyte 13, an anode made up of a sintered impregnated PDC particles 19 and the ferritic stainless steel substrate 11. The invention is shown in SEM micrographs of a cross-sectioned cell in FIG. 7. This has the advantage of very good REDOX stability and internal steam reforming activity relative to a conventional SOFC anode cermet because, without being bound by theory, it does not rely on the metallic phase for its mechanical stability.

(26) The disadvantage of this embodiment is that the electronic conductivity of the anode layer is relatively low as there is no contiguous metallic phase to carry electronic current from the anode-electrolyte interface to the substrate. This results in a relatively high ohmic voltage loss in this part of the cell. To avoid this issue whilst maintaining the advantage of the invention, two other embodiments have also been developed.

(27) Accordingly, active metal-impregnated PDC particles are typically incorporated into conventional cermet anode structures, partially replacing the metallic phase. In this instance the presence of some contiguous metallic phase results in a greatly enhanced electronic conductivity, at the expense of some REDOX stability and catalytic activity for internal steam reforming. However both of these properties are still enhanced by comparison with a conventional cermet anode.

(28) This structure is shown schematically in FIG. 8, showing the electrolyte 13, the steel substrate 11, and the anode comprising impregnated PDC particles 19, CGO particles 21 and metallic particles 23. An anode of this type is shown as an SEM cross section in FIG. 9.

(29) A standard cermet anode contains 42 wt % CGO, and 58 wt % a 90:10 mixture of NiO and CuO upon initial manufacture. It has been shown that reducing the NiO/CuO content in these structures results in enhanced mechanical and REDOX stability, at the expense of electrochemical performance and internal steam reforming activity. It has been demonstrated that a desirable combination of high electrochemical performance, high REDOX stability and high internal reforming activity may be achieved by maintaining the 42 wt % CGO, but partially replacing the NiO/CuO content with impregnated PDC. The anode shown in FIG. 9 has the composition CGO 42 wt %, impregnated PDC 33 wt % and NiO/CuO 25 wt %. This results in an anode only containing around 28 wt % metal which has electrochemical performance comparable with a conventional cermet anode with 58 wt % metal. This reduction in metal content enhances the mechanical and REDOX stability of the anode.

(30) Incorporation of Impregnated PDC Particles into a Matrix of Electronically Conductive Ceramic

(31) The active-metal impregnated PDC particles are typically incorporated into a matrix of a suitable electronically conductive ceramic. The porous conductive ceramic matrix provides the mechanical structure of the anode and provides a current collection path from the anode-electrolyte interface to the substrate. This is shown schematically in FIG. 10, showing the electrolyte 13, the impregnated PDC particles 19, the conductive ceramic 25 and the steel substrate 11. FIG. 11 shows an SEM cross section of PDC particles incorporated into a conductive ceramic matrix.

(32) The choice of suitable electronically conductive ceramic is limited to materials which are stable and electronically conductive in a reducing atmosphere at SOFC operational temperatures. Suitable materials include the perovskites La.sub.0.75Sr.sub.0.25CrO.sub.3 (lanthanum strontium chromite, LSCr) and La.sub.0.75Sr.sub.0.25Mn.sub.0.5Cr.sub.0.5O.sub.3 (Lanthanum strontium chromium manganite, LSCrM). Of these two materials LSCrM is favoured due to its greater sinterability relative to LSCr. It will be noted that the relative ratios of lanthanum and strontium on the A-site of the perovskite, and chromium and manganese on the B-site may be varied significantly. Other suitable materials include the doped strontium titanates. This has been found to be advantageous of demonstrating very high REDOX stability as the mechanical structure of the anode is made of fully REDOX stable ceramic.

(33) Table 1 shows a summary of testing data for the above described systems, and a comparison with standard anode cermets. The PDC was impregnated with 8 wt % nickel and 2 wt % cobalt, the cobalt being added to enhance sintering of the layer. SOFC power at 570° C. and 0.75V/cell is measured in 56% H.sub.2/44% N.sub.2 fuel. It can be seen that the power output of Embodiment 2 is comparable with the standard anode, with somewhat lower power in the case of Embodiment 1 due to higher ohmic resistance in the anode as described previously.

(34) The overall and internal methane conversions are a measure of the catalytic activity of the anode for internal methane steam reforming. This is measured with a stack temperature of 610° C., with the stack being operated on partially steam reformed methane with a thermodynamic equilibrium temperature of 540° C., with a stack fuel utilisation of 65%. The overall methane conversion is the methane conversion between the reformer feed and the stack fuel outlet. The internal methane conversion is the percentage of the methane in the stack fuel feed converted within the stack. The reformate equilibrium of 540° C. means that 55% of the methane fed to the system is converted externally, with the remainder converted within the stack. The fuel feed composition for these measurements are shown in Table 2. The methane conversion is calculated based on a measurement of the fuel gas composition leaving the stack using an infra-red gas analyser.

(35) TABLE-US-00001 TABLE 1 Summary of performance testing metrics Overall Internal CGO/ PDC/ NiO/CuO/ Mean cell power methane methane Variant wt % wt % wt % at 570° C., 0.75 V/W REDOX stability conversion/% conversion/% Embodiment 1 0 100 0 17.45 >180 cycles 93 84 Embodiment 2 42 33 25 20.54 >200 cycles 93 84 0.3% overall performance change Standard anode 44 0 56 21.45 >200 cycles 91 80 cermet 1% overall performance change

(36) TABLE-US-00002 TABLE 2 Fuel gas composition for internal methane conversion measurements. Gas Mole % in stack gas feed Hydrogen 42.3 Steam 35.6 Carbon monoxide 2.7 Carbon dioxide 8.6 Methane 10.9

(37) It can be seen from Table 1 that embodiment 2 in particular offers advantages over the standard anode in terms of reduced performance loss through REDOX cycling and enhanced internal methane reforming.

(38) Although features described herein may be referred to as “comprising” part of the invention, it is also envisaged that the invention may “consist” or “consist essentially” of one or more of said features. Further, all numerical ranges are not to be interpreted literally but as being modified by the term “about” to encompass those values deviating in a literal but non-technically material manner.