Process for the manufacture of a solid oxide membrane electrode assembly

Abstract

A process for the preparation of a membrane electrode assembly comprising providing, in the following layer order, (I) a green supporting electrode layer comprising a composite of a mixed metal oxide and Ni oxide; (IV) a green mixed metal oxide membrane layer; and (V) a green second electrode layer comprising a composite of a mixed metal oxide and Ni oxide; and sintering all three layers simultaneously.

Claims

1. A process for the preparation of a membrane electrode assembly, the process comprising: providing, in the following layer order, a green supporting electrode layer comprising precursors to a first composite of a mixed metal oxide and Ni oxide; a green membrane layer comprising precursors to a mixed metal oxide; and a green second electrode layer comprising precursors to a second composite of a mixed metal oxide and Ni oxide; and sintering the green supporting electrode layer, the green membrane layer, and the green second electrode layer simultaneously; wherein the ratio of Ni oxide to mixed metal oxide in the first composite and the second composite after sintering is 0.2 to 0.8 on a volumetric basis.

2. The process as claimed in claim 1, wherein the providing step further comprises: coating the green membrane layer onto the green supporting electrode layer; and coating the green second electrode layer onto said green membrane layer.

3. The process as claimed in claim 1, wherein the providing step further comprises: preparing the green supporting electrode layer; coating the green membrane layer onto said green supporting electrode layer; and coating the green second electrode layer onto said green membrane layer.

4. The process as claimed in claim 1, wherein the green supporting electrode layer, the green membrane layer, the green second electrode layer, or a combination thereof further comprises an organic additive before sintering.

5. The process as claimed in claim 1, wherein the green supporting electrode is formed by extrusion.

6. The process as claimed in claim 1, wherein, after sintering, the mixed metal oxide of the supporting electrode material, the membrane layer, the mixed metal oxide of the second electrode material, or a combination thereof independently comprise a material of formula (I)
AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y  (I) wherein A is Ba, Sr, Ca, or a mixture thereof; the sum of a, b, and c equals 1; b is from 0 to 0.45; c is from 0.1 to 0.5; Acc is Y, Yb, Pr, Eu, Sc, In, or a mixture thereof; and y is a number such that formula (I) is uncharged.

7. The process as claimed in claim 6, wherein A is Ba.

8. The process as claimed in claim 6, wherein Acc is Y.

9. The process as claimed in claim 6, wherein the material of formula (I) is BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y.

10. The process as claimed in claim 1, wherein, after sintering, the mixed metal oxide in the supporting electrode layer is the same as the mixed metal oxide in the second electrode layer.

11. The process as claimed in claim 1, wherein the sintering is performed at a sintering temperature of between 1200 and 1800° C.

12. The process as claimed in claim 1, wherein after sintering, the process further comprises a reducing step.

13. A membrane electrode assembly obtained by the process as claimed in claim 1.

14. A membrane electrode assembly comprising, in the following layer order: a supporting electrode material comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y, wherein the ratio of Ni to AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y is 0.2 to 0.8 on a volumetric basis; a membrane layer material comprising AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y; a second electrode material comprising a Ni composite of formula Ni-AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y, wherein the ratio of Ni to AZr.sub.aCe.sub.bAcc.sub.cO.sub.3-y is 0.2 to 0.8 on a volumetric basis; wherein, for each material independently, A is Ba, Sr, Ca, or a mixture thereof; the sum of a, b, and c equals 1; b is from 0 to 0.45; c is from 0.1 to 0.5; Acc is Y, Yb, Pr, Eu, Sc, In, or a mixture thereof; and y is a number such that the material is uncharged.

15. The membrane electrode assembly as claimed in claim 14, wherein: the supporting electrode material comprises a Ni composite of formula Ni—BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y; the membrane layer material comprises BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y; and the second electrode material comprises a Ni composite of formula Ni—BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y.

16. The process as claimed in claim 1, wherein, after sintering, the membrane layer comprises a ceramic mixed metal oxide.

17. The process as claimed in claim 6, wherein, after sintering, the ratio of Ni to the material of formula (I) in the supporting electrode material and/or the second electrode material is from 0.2 to 0.8 on a volumetric basis.

18. The process as claimed in claim 6, wherein 2.75≤y≤2.95.

19. The process as claimed in claim 12, wherein the reducing step comprises applying hydrogen or diluted hydrogen gas at a temperature between 500 and 1200° C.

20. The membrane electrode assembly as claimed in claim 14, wherein 2.75≤y≤2.95.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a general schematic of a membrane electrode assembly (MEA) showing an anode at which reactions occur that generate electrons, a cathode at which reactions occur that consume electrons, and an electrolyte which conducts ions but is insulating towards electrons. In this case, the overall reaction is the oxidation of hydrogen to generate water.

(2) FIG. 2 is a micrograph of a fracture surface showing the structure resulting from the use of a single co-sintering step to produce the MEA architecture. Note that the electronically active component of the electrodes (dark grey phase) has undergone a further reduction step.

(3) A electrode support structure is illustrated in FIG. 3. The first layer comprises the electrode support. The second layer comprises the membrane. The third layer comprises the second electrode. In a further embodiment, a catalyst layer can be adhered to the surface of the MEA or lie freely on top (not shown).

(4) A tubular membrane-support structure/design is illustrated in FIG. 4. Two general designs are possible. One with a catalyst layer on the surface of electrode layer 3 and one with the catalyst layer on the inside of the tube. An arrangement with the catalyst layer on the surface of the tube is advantageous if the dehydrogenation reaction is slow. If the reduction reaction of O.sub.2 and/or the water formation reaction and/or the diffusion of water/O.sub.2 to the membrane are the slowest the arrangement with the catalyst layer inside the tube will be advantageous. FIG. 4 shows the support layer (1) with membrane layer (2) and outer electrode layer 3.

(5) An embodiment for a planar reactor design is illustrated in FIG. 5. Modules of catalyst-membrane-support assemblies are stacked horizontally arranged so that the support faces a support of a second assembly, and the catalyst faces a catalyst of a third assembly and so on. This stacking form channels for the reactant gas and the purge gas respectively. Each assembly is sealed at the end with suitable sealing material, such as a glass which is non-catalytically active towards coke formation.

(6) The embodiment shown has a counter-current gas flow. This configuration has a similar hydrogen pressure gradient ΔP in the two end segments. The first segment is located at the inlet of the reactant gas. The hydrogen concentration will be highest at this point, while the oxygen content in the purge gas will be the lowest. In the other end, of the air inlet, the hydrogen pressure will be at the lowest point, while the oxygen pressure will be at the highest. The pressure gradient in the two ends will be approximately equal, which is also true for the part between the two ends. This will ensure a homogeneous dehydrogenation along the membrane, which furthermore will stabilize the conversion towards carbon formation. In this way a constant thickness of the membrane can be used throughout the reactor.

(7) FIG. 6 shows the membrane reactor in action. The membrane separates a first zone from a second zone. The first zone contains methane and steam. Upon dehydrogenation of the methane, hydrogen in the form of protons passes through the membrane to the second zone. Current is applied across the membrane to encourage transport.

(8) FIG. 7 presents a flow chart of the current MEA production process.

(9) FIG. 8 shows the effect of the present invention to reduce the number of steps required to produce a complete MEA.

Example 1

(10) This example used a water-based system for the slip casting of a closed-one-end tube as the supporting electrode. A ‘green’ ceramic tube was produced by casting a ceramic-laden slip into a plaster mold. The ceramic material contained 60% NiO by weight and 40% by weight of a mixture of BaSO.sub.4, CeO.sub.2, ZrO.sub.2, and Y.sub.2O.sub.3 in molar ratios to yield BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y on decomposition and reaction.

(11) The reactants, including NiO, are mixed as powders to form a powder mix. The powder mix was milled for 24 hours to improve homogeneity. A slurry was then made from the milled powder by adding, relative to the weight of the powder, 30 to 60 wt % water, 0 to 1% ammonium polyacrylate dispersant, and 1 to 5% acrylic emulsion as a binder. The resulting slurry was cast into the plaster mould and dried. A tube was produced of 13.5 mm diameter and 0.75 mm wall thickness. This tube is called the green substrate and acts as the first electrode after densification.

(12) After removal of the green substrate from the plaster mold, a coating of ceramic laden slurry was applied to the outside of the tube by means of a spray coating process. This slurry consists of 40 to 60 wt % materials to form the electrolyte in an organic spray vehicle. The ceramic precursors were a mixture of BaSO.sub.4, CeO.sub.2, ZrO.sub.2, and Y.sub.2O.sub.3 as a heterogeneous dispersion in molar ratios to yield BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y on decomposition and reaction. The spray vehicle comprises a methyl methacrylate resin dissolved in a mixture of organic solvents and a dispersant. The resulting tube, consisting of a green thick-walled tube of electrode material with a coating of green membrane material, was allowed to dry overnight. The membrane layer had an approximate thickness of 25 to 30 μm.

(13) The tube with was dip coated by submerging into a container bearing a coating slurry. A mixture comprising 94 wt % electrode powders, 2 to 3% methylcellulose binder, up to 2% starch, up to 2% plasticizer, up to 2% dispersant was prepared. This was then diluted with approx. 50 wt % water. The electrode powder mixture was the same as that used in example 1, i.e. the ceramic material contained 60% NiO by weight and 40% by weight of a mixture of BaSO.sub.4, CeO.sub.2, ZrO.sub.2, and Y.sub.2O.sub.3 in molar ratios to yield BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y on decomposition and reaction.

(14) The tube was then retracted from the dip coat slurry to produce a layer of green second electrode material. The thickness of the green second electrode material was controlled by the solid fraction of the slurry, the time the tube remains in the slurry, and the rate at which the tube was extracted from the slurry. The tube was then allowed to dry.

(15) The length of the second electrode was controlled by a combination of masking and/or adjusting the depth of submersion of the tube in the dip coating procedure.

(16) The resulting ‘green’ tube comprised a green electrode support with green electrolyte coating and outer green electrode coating. This tube was then sintered to produce a concentric MEA in an electrode-supported configuration consisting of an electrode support of 750 μm nominal thickness, ionic membrane of 25 μm nominal thickness, and second electrode of 35 μm. The sintering process used a tube hang fired in a muffle furnace using tubular kiln furniture as a support. The furnace temperature was raised slowly (c. 1°/min) to 250-350° C. for debinding and sintered at high temperature (at least 1550° C.) to produce a dense membrane electrode assembly.

Example 2

(17) This example used a water-based system for the extrusion of the supporting electrode. First, a mixture of the supporting electrode powders was homogenized in a high shear mixer. The materials contained 60% NiO by weight and 40 wt % by weight of a mixture of BaSO.sub.4, CeO.sub.2, ZrO.sub.2, and Y.sub.2O.sub.3 in molar ratios to yield BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y on decomposition and reaction. To this was added an aqueous binder system such that a paste was made containing 75 to 85 wt % electrode powders/support oxide, 2 to 3 wt % methylcellulose binder, up to 2% starch, up to 2% plasticizer, up to 2% dispersant and 9 to 12% water.

(18) The paste was then extruded through an annulus using a 45 tonne hydraulic ram extruder to produce a green tube of 13.5 mm outer diameter and 0.8 mm wall thickness. The tubes were cut to an arbitrary length, typically 35 to 40 cm.

(19) These tubes were then coated using a computer controlled ultrasonic spray coater with a slurry consisting of 40 to 60 wt % ceramic materials to form the electrolyte in an organic spray vehicle. The ceramic precursors were a mixture of BaSO.sub.4, CeO.sub.2, ZrO.sub.2, and Y.sub.2O.sub.3 in as a heterogeneous dispersion in molar ratios to yield BaCe.sub.0.2Zr.sub.0.7Y.sub.0.1O.sub.3-y on decomposition and reaction.

(20) The spray vehicle contained a methyl methacrylate resin dissolved a mixture of organic solvents and a dispersant. The resulting tube, consisting of a thick-walled tube of electrode material with a coating of membrane material, was allowed to dry overnight.

(21) A dip coating slurry was made containing a mixture of a). 50 wt % of a blend of 75 to 95 wt % electrode powders, 2 to 3 wt % methylcellulose binder, up to 2% starch, up to 2% plasticizer, up to 2% dispersant and b). 50 wt % water. The tube was then submerged into a container bearing the dip coating slurry. The tube was then retracted from the dip coat slurry to produce a layer of green material, the thickness being controlled by the solid fraction of the slurry and the extraction rate. The tube was then allowed to dry.

(22) The tube was then hang fired in a muffle furnace using tubular kiln furniture as a support. The furnace temperature was raised slowly (c. 1°/min) to 250-350° C. for debinding and sintered at high temperature to produce a dense membrane electrode assembly. An example of the microstructure that results (after reduction of the electronically active component of the electrodes) is given in FIG. 2.