Fuel cells and process for the production of the anode

A fuel cell with a solid oxide (SOFC) or a molten carbonate salt electrolyte (MCFC). The anode of the fuel cell, according to the invention, is a CERMET anode which consists of a solid mixture of tungsten carbide and an ion-conducting oxide, such as yttrium-fully stabilized zirconium oxide (YSZ). The fuel cell anode is prepared by a process whereby in order to form the solid mixture of which the anode is formed, a mixture of anion-conducting oxide powder and WC/WC(O), WO.sub.3 or precursor thereof is prepared, and this mixture is converted into a YSZ-WC mixture in a CO/CO.sub.2 atmosphere in a temperature range of 600.degree.-1000.degree. C.

BACKGROUND OF THE INVENTION 
The invention relates to a fuel cell with a solid oxide (SOFC)--or a molten 
carbonate salt electrolyte (MCFC), (SOFC=solid oxide fuel cell; 
MCFC=molten carbonate fuel cell). It further refers to a process for the 
production of an anode for the fuel cell. 
Fuel cells are the subject of study because of their high energy conversion 
efficiency. They operate by converting chemical energy of fuels directly 
to electrical energy without the need for a thermal energy conversion 
step. This efficiency (not subject to the Carnot cycle limitations) 
coupled with (i) advantageous operating characteristics even at partial 
load, (ii) the possibility of co-generation of energy and (iii) production 
of low pollutants, makes the fuel cell a potentially important energy 
conversion device. 
The present invention relates more specifically to a fuel cell of the type 
which is a ceramic fuel cell. As is usual with fuel cells, the device 
produces electricity by electrochemically combining fuel and oxidant gases 
across a separator. Ceramic fuel cells use an ionic conducting oxide and 
usually use an oxygen-ion conductor or a proton conductor as the 
electrolyte. The ceramic construction allows operation at high 
temperatures (greater than 600.degree. C.). 
A high-temperature fuel cell with a solid [oxide] electrolyte (SOFC) 
requires high reliability of all operating parts over an adequate time 
span during operation. In order to achieve this, at the present time 
systems of different sizes are being tested worldwide. In general, 
so-called classical materials are used with various modifications. 
These materials include: 
Zirconium oxide fully stabilized with yttrium (8 mol. %) (YSZ) as the 
electrolyte; 
Perovskite (LaMnO.sub.3) doped with strontium for lanthanum, and with 
cobalt for manganum, as the air electrode (cathode), 
CERMET of a mixture of metallic nickel and YSZ, as the fuel electrode 
(anode), 
A metal (Cr-base alloy) or ceramic (doped perovskite of lanthanum chromite) 
plate as an interconnector (bipole plate) to connect together cells into a 
stack. 
One of the serious problems of this system lies in the fuel cell anode. Due 
to the thermodynamic instability of the interface between metal and 
ceramic, the nickel metal of the anode tends toward restructuring with a 
resultant loss of electrochemical activity and a breakdown of the 
electrical conductivity of the electrode. This effect is particularly 
great in the case of contamination of nickel with impurities, such as, 
e.g., sulfur or sulfur compounds present in gaseous fuels obtained by the 
gasification of various carbon products. 
In addition, the nickel metal exhibits too high a catalytic activity for 
the internal reforming of a methane-water vapor mixture, whereby the 
electrode can be poisoned by the deposition of carbon at the three-phase 
boundary. Proposals to reduce the high catalytic effect in reforming by a 
partial or complete replacement of Ni by Pd or Co are currently being 
tested. However, the problems of the instability of the interface in 
CERMET cannot be resolved with these systems. 
It is therefore an object of the invention to provide a fuel cell, 
including an anode, which avoids the problems of the art. 
BRIEF DESCRIPTION OF THE INVENTION 
The invention provides a fuel cell with a solid oxide (SOFC) or a molten 
carbonate salt electrolyte (MCFC). The anode of the fuel cell, according 
to the invention, is a solid mixture of tungsten carbide and an 
ion-conducting oxide, such as yttrium-fully stabilized zirconium oxide 
(YSZ). 
The invention also concerns a process for the production of an anode of the 
solid mixture of tungsten carbide and an ion-conducting oxide, such as 
yttrium-fully stabilized zirconium oxide (YSZ). The inventive process 
requires that to form the solid mixture, a mixture of YSZ powder and 
WO.sub.3 is prepared, and this mixture is converted into a YSZ-WC mixture 
in a CO/CO.sub.2 atmosphere at a temperature range of 
600.degree.-1000.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
The solution to these problems according to the invention comprises a 
ceramic fuel cell of the type generally described above and known in the 
art using a solid oxide or molten carbonate salt electrolyte. However, the 
fuel cell has an anode according to the present invention, which consists 
of a solid mixture of tungsten carbide and an ion-conducting oxide, such 
as yttrium-fully stabilized zirconium oxide (YSZ). Therefore, as compared 
with the prior art CERMET anode, the anode of the present invention has 
the metallic nickel of the prior art replaced with tungsten carbide. This 
anode thereby avoids all previously observed disadvantages of a CERMET 
anode. 
The solid mixture of the anode appropriately consists of about 30-80 vol. 
%, preferably 30-70 vol. % and more preferably 40-60 vol. % tungsten 
carbide. The ion-conducting oxide is preferably YSZ. 
Referring to the figure, an improved ceramic fuel cell according to the 
present invention comprises a sandwich structure having a cathode 10, and 
an anode 12 separated by the solid electrolyte 11. 
The figure shows a stack of cells separated by a bipolar plate 13. As a 
practical matter stacking the cells provides a greater output with 
convenience of construction. It is possible to connect the cells in other 
ways although this is most practical to allow the fuel flow shown by arrow 
A over the anode 12 side of the cell and the air flow (oxidizer) shown by 
arrow B over the cathode 10 side of the cell. 
The anode 12 of the present invention replaces the known CERMET anodes in 
ceramic fuel cells. Usual cathodes such as described above and SOFC and 
MCFC electrolytes also described above, can be used. 
In the inventive cell, YSZ electrolyte can be used. Another electrolyte, 
SrCe(Yb)O.sub.3 electrolyte for example (wherein 3 to 5% of the Ce is 
replaced with Yb), can be used in place of the YSZ electrolyte. 
When forming a cell with a solid electrolyte, it is preferred but not 
necessary that the solid electrolyte forms the substrate on which the 
anode and cathode are formed. 
Forming the Anode 
In the process of the invention for the production of the anode a solid 
mixture is formed which is a mixture of YSZ powder and WO.sub.3, or 
another tungsten compound, which other tungsten is a precursor for 
WO.sub.3 which, when subjected to thermal decomposition, forms WO.sub.3. 
This mixture is sintered adhesively with the electrolyte to form a layer 
on the electrolyte and finally converted to a YSZ-WC mixture in a 
CO/CO.sub.2 atmosphere in the temperature range of 
600.degree.-1000.degree. C. Examples of the other tungsten compounds that 
convert to WO.sub.3 and therefore can be used in the mixture include 
tungstic acid and ammonium tungstate. It is also possible to start with a 
mixture of WC and YSZ, and proceed by these steps including heating in the 
CO/CO.sub.2 atmosphere as a preferable step to ensure maximum activity as 
an anode. 
More specifically, to form the anode, WO.sub.3, a powder or a precursor 
powder mixture for WO.sub.3 as noted above, is mixed with YSZ powder and 
applied to the solid electrolyte (e.g. YSZ electrolyte films). It is also 
suitable to start with WC (usually containing some oxygen WC(O)) to mix 
with the YSZ powder. The mixture can be conveniently applied to the 
electrolyte by forming a screen printing paste with a screen printing 
binder (e.g. cellulose), which is then coated in a layer onto the 
electrolyte. However, the film thickness applied on the electrolyte to 
form the anode is normally in the range of 20-50 .mu.m. 
The resulting composite is calcined at about 700.degree. C. in air and 
carburized in a mixture of CO/CO.sub.2 at about 770.degree. C. to form the 
anode as part of a half-cell. 
When WC powder is used, the carburization step is optional as the WC powder 
is already formed; however, it is highly desirable as the carburization 
step ensures that the high activity for the anode is maintained by 
converting any other W compound present into WC. 
The WC/WC(O) powder for use in forming the anode can be preferably formed 
by spray drying an (NH.sub.4).sub.2 WO.sub.4 (solution) or H.sub.2 
WO.sub.4 (gel) to form a powder containing WO.sub.3. The powder is 
calcined at approximately 700.degree. C. and then reduced to WO.sub.2 in 
hydrogen (4% vol. H.sub.2 in argon) at approximately 400.degree. C. The 
WO.sub.2 powder is then carburized to WC/WC(O) powder in a 9:1 mixture of 
CO:CO.sub.2 at approximately 770.degree. C. 
The resulting powder can then be mixed with YSZ powder and applied to the 
solid electrolyte as described above. 
In the procedure wherein a precursor of the WO.sub.3 is used, the precursor 
(e.g. (NH.sub.4).sub.4 WO.sub.4 (powder) or H.sub.2 WO.sub.4 (powder)) is 
formed into a screen printing paste by mixing with a suitable binder, and 
mixing with YSZ powder, this is then screen printed or otherwise applied 
to the solid electrolyte, and the resulting "sandwich" or composite 
structure is calcined and carburized whereby the anode (as part of a 
half-cell) is formed as discussed in detail above. 
Examples 
For testing, tungsten carbide mixed with YSZ in various amounts was screen 
printed onto the surface of the electrolyte as described above, on the 
anode side of an SOFC. Mixtures containing 30, 40, 50, 60 and 70 vol. % 
tungsten carbide in YSZ were formed into anodes for testing. 
Electrochemical tests to determine the polarization voltage of this half 
cell show that this system has a behavior similar to that of a platinum 
electrode. Also no large variations in electrical conductibility were 
measured among the samples. 
Investigations to determine the long-term stability of this system have 
resulted in the conclusion that after storage of the solid mixture in an 
H.sub.2 atmosphere for more than 1000 hours at 1100.degree. C., optical 
metallography and scanning electron microscopic observation and 
measurement of the electrochemical activity determined that no change 
occurred in the structure, i.e., there is no agglomeration of one or the 
other phase. A somewhat shorter test over 350 hours with such a mixture at 
1000.degree. C. in a H.sub.2 S-containing hydrogen atmosphere also led to 
the conclusion that there was no determinable change in structure. 
The spray drying method described above and which involves calcining and 
carbonization of precursors of the powder mixture for forming the anode, 
to convert them into the tungsten carbide solid mixture, has been found to 
lead to a favorable powder morphology and is therefore preferred, even 
though it necessarily involves the decomposition of the precursor 
compounds such as ammonium tungstate or gel of tungstic acid. 
The above is by way of embodiments of the invention but is not considered 
limitative of the scope of the invention which is defined by the following 
claims.