CA1120098A

CA1120098A - Carbonate fuel cell anodes

Info

Publication number: CA1120098A
Application number: CA000339077A
Authority: CA
Inventors: Rafael A. Donado; Leonard G. Marianowski; Hansraj C. Maru
Original assignee: GTI Energy
Current assignee: GTI Energy
Priority date: 1978-11-20
Filing date: 1979-11-02
Publication date: 1982-03-16
Also published as: DE2945565C2; GB2039131B; JPS5574065A; GB2039131A; DE2945565A1; US4247604A; JPS6010422B2; NL7907877A

Abstract

CARBONATE FUEL CELL ANODES
Abstract of the Disclosure A molten alkali metal carbonates fuel cell porous anode with a stabilizing agent to maintain porosity and surface area during fuel cell operation. A molten alkali carbonates fuel cell having the above stabilized anode and a method for production of such porous anodes.

Description

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This invention relates to molten carbonate fuel cells and particularly to anodes in contact with alkali metal carbonates electrolytes over long periods of high temperature operation. The invention more specifically relate~ to carbonate fuel cell anodes principally of nickel or cobal~ or mixtures ~hereof having added minor amounts of a stabilizing agent of chromium9 aluminum or ~irconium in oxide or alkali metal salt forms and mixtures thereof under cell operating conditionsO It is preferred to use chromium. Additionally9 i~ is preferable to use alumina as an effective dispersing agent in blending of ~he powders in the manufacture of the anode~
of this invention.
Molten carbonate fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, ~ -an electrolyte tile making contact with both the electrodea a~d a cell housing to ,~ ~ ' ', " - '.

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physically retain the cell components.
Under fuel cell operating conditions, in the range o about 500 to about 700~C., the entire electrolyte tile, the carbonate and the inert support material, forms a paste and thus the electrolyte diaphragms ~ this type are known as paste electrolytes.
The electrolyte is in direct contact with the electrodes where the three phase reactions (gas-electrolyte-electrode) take place. Hydrogen is consumed in the anode -~
area producing water, carbon dioxide and electrons. The electrons flow to the cathode through an external circuit producing the desired current flow. At the anode there must be ready entry for the reactant gas, ready exit for the chemical reaction products and ready exit for the product electrons. To maintain a high level of stable performance, both electrolyte and electrode design and propertles must be optimized and stabilized at the gas-electrolyte-electrode interface.
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LZ(~098 Improved molten carbonate- ~uel cell electrolytes are described in UO SO Patent 4,079,171 and contain sbout 50 to 70 weight percent carbonates in a high surface area inert support portionO With the use of electrolytes such as described in the patent providing high power densities over long periods of time 9 the problem of loss of cell performance with time due to anode instability was recognized by the applicants. - -Porous anodes of cobalt or nickel have been previously used in molten . - ..
carbonate fuel.cellsO Porous anodes of this type can be produced by a variety of powder metallurgical techniques to form a green compact with voids bet~een the - : -particles forming interconnected pore channels throughout the compact. The _ ~ -green compact is ~hen sintered by heating ' at temperatures of greater than about 70 percent of the melting point temperature.
This produces anodes having pore channels ~hroughout the structureO The loss of fuel cell power ou~put of mol~en carbonate ~ .

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~lZ0098 fuel cells after as short as a few hundred hours of use has been noted with porous cobalt and nickel anodes. The molten carbonate fuel cell power output los8 appears to be related to the surface area loss of the porous anodeO Thus9 ~'stabilityl' as referred ~o in this disclosure and claims relates to maintenance of fuel cell power output and anode surface area.
Various methods have been attempted to increase the porosi~y of electrode materials. One method has been to incorporate in the electrode material an alkali soluble material such as aluminum, silicon or boron which is dissoived out of the primary electrode material as taught by U. S. Patent NosO 3,359,099 and 3,4149438.
However, such Raney-type electrodes produced according to ~hese pa~ents~ while they may have greater initial porosi~y have the same long term instability under molten carbonate fuel cell operation as normally produced porous nickel or cobalt anodes.

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attempted to produce high surface area electrodes for molten carbonate fuel cells such as electrodes having metal fiber wick~ -as described in U. SO Patent 3p826,686~
It is known from ~eachings of general powder metallurgy to incorporate critical amounts of specific si7ed inert dispersoid particles in the base metal to produce porous sintered metal materials suitable for uses such as fluid flow distributors and filters as taught by U. S.
Patent 3,397,968. This patent teaches that sintered articles produced with the inert dispersoid particles ; :
are dimensionally stable with respect to overall shapes and sizes. Belgian Patent 849,639 ~eaches use of conductive dispersoid particles of chromium, molybdenum, tungsten and mixtures thereof to produce thermally stable ~ -sintered porous metal structures for use as high tempera~
ture heating elements, conductive metallic grids, batteries ~-and conductive elements for electrostatic precipitationsO ::
However, the teachings of these patents do not relate ~o fuel celi anode use and, particularly, they do not relate to surface area stability under carbonate fuel cell operating conditions~ For example, combination of nickel with a disp~rsed phase of magnesium . oxide or calclum oxide taught to produce . .
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overall dimensional s~ability by both ehe 3,397~968 patent 2nd by the article "Sintering of Metal Powder Compacts Containing Ceramic Oxidesl', M. H. Tikkanen9 B. 0. Rosell and O. Wiberg, Powder Metallurgy9 No. 109 pg. 49-609 (1962) does not result in suitable porous anode material providing surface area stability under carbonate fuel cell operating conditions to provide relatively constant power output of the fuel cell over periods of time in the order of several thousand hoursO
It is an object of this invention to provide a molten alkali metal carbonates fuel cell having a stable anode providing high power densities necessary for practical fuel cell operation over long periods o~ time. ~ :
It is another object of this invention to provide improved moIten alkali metal ..
carbonates fuel cell porous anodes providing : -stability over long periods of ~ime under fuel cell operating conditions.
It is yet another object of this :
invention to provide a method of production of porous anodes principally of nickel or ,, ' . ~ ;~
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~12~3098 cobalt or mixtures thereof which are stable over long periods of operation under molten -carbonates fuel cell operating conditions.
Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings showing preferred embodiments wherein:
Fig. 1 is a graph showing comparisons of surface areas of preferred embodiments of fuel cell anodes of this invention with prior ar~ fuel cell anodes a~ ~emperatures of molten carbonate fuel cell operation;
Fig. 2 is a graph showing compari-son of molten carbonate fuel cell power -~
densities using anodes of preferred embodiments of this invention compared with prior art fuel cell anodes;
Fig. 3 is a photomicrograph of ~
90% nickel-10% chromium anode according to one embodiment of this invention prior to fuel cell conditions;
-Fig. 4 is a photomicrograph of a 90% nickel-10% chromium anode according to . .
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.. :. , , ' ` i ' : ' ~L12(~098 the embodiment shown in Fig. 3 after 50 hours in a furnace in carbonates electrolyte environmene a~ fuel cell operational temperature of 650Co;
Figo 5 is a photomicrograph of an anode according ~o the embodiment shown in Fig. 3 after 500 hours in a furnace in carbonates electrolyte environment at fuel cell operational temperature of 650C~;
Fig. 6 is a photomicrograph of an anode according to the embodiment shown in Fig. 3 after 3000 hours in a urnace in carbonates electrolyte environment at uel cell operational temperature of 650C.; and Fig. 7 is a graph showing mean pore size of a 90% nickel ~ 10% chromium anode according to one embodiment of this invention during fuel cell operation.
Molten alkali metal carbonates fuel cell porous anodes according to ~his invention comprise about 0.5 ~o about 20 weight percen~, based upon the metal9 of a stabilizing agen~
selected from the group consisting of chromium9 zirconium and aluminum in metal9 oxide .. , . . : .

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- ~L120098 or alkali metal salt forms and mixtures thereof, the remainder of the anode being substantially a metal selected from the group consisting of nickel, cobalt and mixtures thereof. It is especially preferred that the stabilizing agent be present i~
about 1 to about lO weight percent9 based upon the metal. The porous anodes of this invention are particularly suited for use in molten carbonate fuel cells of the ~ype as described in U. S. Patent 49073,171. Such fuel cells have a binary or ternary electrolyte system of lithium and sodium or potassium carbonates and are suitable for use in ~
conjunction with this invention. In these ~ -fuel cells under operating conditions9 the alkali metal salts are predominately lithium chromite LiCrO2; lithium aluminate LiAlO2; and lithium zirconate Li2ZrO3.
The molten alkali metal carbonates ~-fuel cell of ~his invention is the type having an anode and a cathode with their respective current collectors9 an electrolyte ~ile making contact with said anode and cathode, and a .
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, ;, : ::., ~ZV~)98 cell housing to physically retain the cell components, the electrolyte tile comprising alkali metal carbonates and an inert support material which upon cell operation at temperatures of about 500 to about 700Co forms a paste in direc~ contact with a porous anodeO The improved fuel cell of this invention has a porous anode comprising about 0.5 to about 20 weight percent9 based upon the metal, of a stabilizing agent selected from the group consisting of chromium, zirconium and aluminum in metal, oxide ~or alkali metal salt forms, and mixtures thereof with the remainder being substantially a metal selected from the group consisting of nickelg cobal~ and mixtures thereof.
Under fuel cell operating conditions, the anode stabilizing agents of this invention will form oxides and in the alkali -metal carbonates environment will form alkali metal saltsO The lithium salt is preferentially formed. We have found that even though oxidation and alkali metal salt formation , reactions take place 9 the s~abilizing agents .

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0~38 of this invention do not migrate from the anode. After long term cell operation~ we have found over 90 weight percent of the stabiliæing agents remaining in the anodeO
We have added the stabilizing agents to the anode in both the metal and the oxide formsO
To reduce undesired active electrolyte carbonate loss during ~uel cell operation, it is preferred to add the stabilizing agents in the lithium salt form or to react the stabilizing agents in the anode to form their lithium salt prior to cell -operation.
Fig. 1 shows changes in anode surface area as a function of time in ~-accelerated furnace tests at 750C. except for the one furnace test indicated at 650C., in a fuel cell feed gas containing hydrogen, ;~
carbon dioxide, and water vaporO The results of one anode of 90 weigh~ percent nickel ~ -10 weight percent chromium according to this invention and operated in a molten carbonate ;
fuel cell at 650C. is also shown in Fig. 1.
I~ is seen from Fig. 1 that the accelerated , ~ ~
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furnace tests do have validity with respect to surface area changes of ~he anode material as related to actual anode use in molten csrbonates fuel cells. We have found ln the fuel cell environment that the surface area change is somewhat retarded in time. Fig. 1 shows the surface area of conventional porous nickel and cobalt anodes by dashed linesO
It is seen that in the conventional anodes without the stabilizing additives of this invention the surface area decreased in the order of a factor of 3 to 40 Both cobalt and nickel porous anodes containing chromium and zirconium stabilizing additives of this invention show very marked decrease in surface area change over the time interval shown.
Figo 2 shows fuel cell power outputs versus time during cell operation at 650~C.
with lithium carbonate/potassium carbona~e binary eutectic electrolytes having a sub -stantially lithium aluminate inert carrier structure and a nickel oxide ca~hodeO The cells were comparable except for the anodes9 ,A .
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the solid lines representing anod~s of this invention and the dashed line a conventional cobalt porous anode. Figo 2 shows the power output versus time with the various anodesO
The curve for porous cobalt anodes without a stabilizing agent is the average of the operation of two cells; the curves representing the alumina and zirconia additives are each the average of two cells; and the curve representing the chromium additive is the i average of three cells. The stabilized ' fuel cell power output obtained by use of anodes of this invention is clearly shown by Fig~ 20 Figs. 3 through 6 are photomicrographs showing the structural changes of an anode according to this invention having 10 weight percent chromium stabilizing agent and the remainder nickel. Figs~ 3 through 6 are the same magnification and ~he size indicator beneath Fig~ 3 applies to all Figs. 306.
Fig. 3 shows the anode structure before subjecting the anode to heat and an electrolyte envi-onment. FigsO 4, 5 and 6 sh the same , , ~ IGT-768 o14~ -~

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anode material after 50 hours, 500 hours and 3000 hours9 respectively9 maintenance in a furnace st 650C. in an electrolyte environment of H2-C02-H20 fuel gas~
Figs. 3-6 show the lack of agglomeration and particle growth and illustrate the sequence of changes also observed in actual cell tests where only initial and final structures can be observed. The sequence shows the development of porosity or - j -cavities in the nickel grains and confirm ~-that fuel cell power output as shown in -Fig. 2 correlates with surface area as shown in Fig. 1. Figs. 3-6 show that the ~ -stabilized anodes of this invention maintain ¦
a high surface area under fuel cell -operating conditions.
It is also desired to obtain optimum porosity and pore size o the anode structure for long term high and stabilized power output of the fuel cell. We have found mean pore diamet~rs of about 2 microns -to about 20 microns to be suitableO Tests have , -sh~wn that the mean pore size of a nickel-10 , .

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11;~0~)98 percent chromium stabilized anode according to this invention was reduced by about 33%
after about 15,000 hours of cell operation.
Fig. 7 shows results of these tests and the stability of mean pore diameter of an anode according to this invention during long term fuel cell operation.
We have found that about one~half of the total decay in cell power outpu~ ln both 3 sq. centimeter cells and 100 sq.
centimeter cells can be attributed to increases in ohmic losses which may be due to carbonate losses from ~he electrolyte caused by the lithium carbonate in the electrolyte reacting with the metal stabilizing agent in the anode to form lithium metal salt. Therefore, it is a preferred embodiment of this invention to react metal stabilizing agent with lithium carbonate prior to introduction into the anode material or prior to incorporation of the anode into a fuel cell.

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~lZ~0~8 The porous anodes of this invention may be prepared by mixing abou~ O.S to about 20 weight percent9 based upon the metal, of a stabilizing agent powder selected from the group consisting of chromium, zirconium and aluminum in metal9 oxide or alkali metal salt form and mixtures thereof with the remainder being a metal powder having a siæe of about 0.1 to 200 microns selected from the group consisting of nickel, cobalt and mixtures thereof; pressing the mixture into a green compac~ with voids between the particles forming interconnected pore channels :
throughout the compact, the pore channels sized by the pressing ~o produce mean pore size of about 2 to abou~ 20 microns in the sintered anode; and sin~ering ~he green compact at temperatures of greater than about 70 percent of the melting point of the metal powder. We have found ~ha~
addition of about 0O5 ~o abou~ 5 weight percent alumina powder to the powders provides dispersion of the stabilizing agent throughout the metal ,:

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powder, resu~ting in a more uniform anode structureO The alumina powder should have a low bulk density and a high surface area.
Degussa "C~' alumina is appropriate.
While in the foregoing specificatio~ ..
this invention has been described in relation to certain preferred embodiments thereof9 and many details have been set forth for purpose of illustration9 it will be apparen~ ~o those skilled in the art ~hat the invention is susceptible to additional embodiments and that cer~ain of the details described herein can be varied considerably ~ithout departing from the basic principles ~f ~he nveA~Io~.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A molten alkali metal carbonates fuel cell porous anode having mean pore diameters of about 2 to 20 microns and surface area about 0.09 to 0.16 square meters per gram having improved surface area stability under carbonate fuel cell operating conditions at about 500° to /00 C. comprising about 0.5 to about 20 weight percent, based upon the metal, of a surface area stabilizing agent selected from the group consisting of chromium, zirconium and aluminum in metal, oxide or alkali metal salt forms and mixtures thereof and the remainder being substantially a metal selected from the group consisting of nickel, cobalt and mixtures thereof.

2. The fuel cell anode of claim 1 wherein said stabilizing agent is present in about 1 to about 10 weight percent.

3. The fuel cell anode of claim 2 wherein said stabilizing agent is chromium.

4. The fuel cell anode of claim 3 wherein the chromium is substantially in the form of lithium chromite.

5. The fuel cell anode of claim 2 wherein said stabilizing agent is zirconium.

6. The fuel cell anode of claim 5 wherein the zirconium is substantially in the form of lithium zirconate.

7. The fuel cell anode of claim 2 wherein said stabilizing agent is aluminum.

8. The fuel cell anode of claim 7 wherein the aluminum is substantially in the form of lithium aluminate.

9. The fuel cell anode of claim 1 additionally having about 0.5 to about 5 weight percent alumina dispersing agent which is converted to alkali metal aluminate upon operation of the fuel cell.

10. The fuel cell anode of claim 9 wherein said stabilizing agent is present in about 1 to about 10 weight percent.

11. The fuel cell anode of claim 10 wherein said stabilizing agent is chromium.

12. The fuel cell anode of claim 11 wherein the chromium is sub-stantially in the form of lithium chromite.

13. The fuel cell anode of claim 10 wherein said stabilizing agent is zirconium.

14. The fuel cell anode of claim 13 wherein the zirconium is sub-stantially in the form of lithium zirconate.

15. The fuel cell anode of claim 1 wherein the surface area is maintained at about 0.09 to 0.15 square meters per gram after at least 1000 hours operation in a molten alkali metal carbonates fuel cell at about 650° to about 750 C°.

16. In a molten alkali metal carbonates fuel cell of the type having an anode and a cathode with their respective current collectors, an electrolyte tile making contact with said anode and cathode, and a cell housing to physically retain the cell components, said electrolyte tile comprising alkali metal carbonates and an inert support material which upon cell operation at temperatures of about 500°
to about 700°C. forms a paste in direct contact with a porous anode, the improve-ment of said porous anode having mean pore diameters of about 2 to 20 microns and surface area about 0.09 to 0.16 square meters per gram having improved surface area stability under carbonate fuel cell operating conditions comprising about 0.5 to about 20 weight percent, based upon the metal, of a surface area stabilizing agent selected from the group consisting of chromium, zirconium and aluminum in oxide or alkali metal salt forms and mixtures thereof under cell operating conditions and the remainder being substantially a metal selected from the group consisting of nickel, cobalt and mixtures thereof.

17. The molten alkali metal carbonates fuel cell of claim 16 wherein said alkali metal carbonates are selected from the group consisting of binary lithium and sodium carbonates and lithium and potassium carbonates and ternary lithium, sodium and potassium carbonates.

18. The molten alkali metal carbonates fuel cell of claim 17 wherein said stabilizing agent is present in about 1 to about 10 weight percent.

19. The molten alkali metal carbonates fuel cell of claim 18 wherein said stabilizing agent is chromium.

20. The molten alkali metal carbonates fuel cell of claim 19 wherein the chromium is substantially in the form of lithium chromite.

21. The molten alkali metal carbonates fuel cell of claim 18 wherein said stabilizing agent is zirconium.

22. The molten alkali metal carbonates fuel cell of claim 21 wherein the zirconium is substantially in the form of lithium zirconate.

23. The molten alkali metal carbonates fuel cell of claim 18 wherein said stabilizing agent is aluminum.

24. The molten alkali metal carbonates fuel cell of claim 23 wherein the aluminum is substantially in the form of lithium aluminate.

25. The molten alkali metal carbonates fuel cell of claim 1.7 additionally having about 0.5 to about 5 weight percent alumina dispersing agent which is con-verted to alkali metal aluminate upon operation of the fuel cell.

26. The molten alkali metal carbonates fuel cell of claim 25 wherein said stabilizing agent is present in about 1 to about 10 weight percent.

27. The molten alkali metal carbonates fuel cell of claim 26 wherein said stabilizing agent is chromium.

28. The molten alkali metal carbonates fuel cell of claim 27 wherein the chromium is in the form of lithium chromite.

29. The molten alkali metal carbonates fuel cell of claim 26 wherein said stabilizing agent is zirconium.

30. The molten alkali metal carbonates fuel cell of claim 29 wherein the zirconium is in the form of lithium zirconate.

31. The molten alkali metal carbonates fuel cell of claim 17 wherein over 90 weight percent of said stabilizing agent remains in said anode during long term cell operation.

32. The molten alkali metal carbonates fuel cell of claim 37 wherein said anode maintains a substantially constant surface area during long term cell operation.

33. The molten alkali metal carbonates fuel cell of claim 32 wherein the surface area is maintained at about 0.09 to 0.15 square meters per gram after at least 1000 hours at about 650° to about 750°C.

34. A method for preparation of porous anodes having a mean pore size of about 2 to about 20 microns for molten alkali metal carbonates, fuel cell comprising: mixing about 0.5 to about 20 weight percent, based upon the metal, of a stabilizing agent powder selected from the group consisting of chromium, zirconium and aluminum in metal, oxide or alkali metal salt form and mixtures thereof with the remainder being a metal powder selected from the group con-sisting of nickel, cobalt and mixtures thereof; pressing the mixture into a green compact with voids between the particles forming interconnected pore channels throughout the compact, said pore channels sized to produce mean pore size of about 2 to about 20 microns in the anode; and sintering the green compact at temperatures of greater than about 70 percent of the melting point of the metal powder.

35. The method of claim 34 wherein about 0.5 to about 5 weight percent alumina is added to the mixture providing dispersion of said stabilizing agent throughout said metal powder.