Methods of making anodes for high temperature fuel cells

Methods of fabricating anodes for high temperature fuel cell in which an alloy powder with a major phase of a base metal and a minor phase of a stabilizing, alloying metal is preformed into the shape wanted in the anode. This green structure is sintered under conditions which produce a metallic, essentially oxygen-free structure. The sintered structure is selectively oxidized in situ in a fuel cell under conditions which promote internal oxidation and a consequent increase in the stability of the anode under operating conditions where the anode is exposed to high temperatures for long periods of time.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to novel, improved methods of fabricating 
anodes for high temperature fuel cells. 
BACKGROUND OF THE INVENTION 
Fuel cells were invented in 1839 by Sir William Grove. A fuel cell is an 
electrochemical device which directly combines a fuel and an oxidant such 
as hydrogen and oxygen to produce electricity and water. It has an anode 
and a cathode separated by an electrolyte. Hydrogen is oxidized to 
hydrated protons on the anode with an accompanying release of electrons. 
At the anode, oxygen reacts with protons to form water, consuming 
electrons in the process. Electrons flow from the anode to the cathode 
through an external load, and the circuit is completed by ionic current 
transport through the electrolyte. 
Fuel cells do not pollute the environment. They operate quietly, and high 
temperature fuel cells have a potential efficiency of ca. 80 percent. 
Virtually any natural or synthetic fuel from which hydrogen can be 
extracted--by steam reforming, for example--can be employed. 
As suggested above, the anode is a major component of a high temperature 
fuel cell, and a successful fuel cell of that character which will remain 
stable when heated at high temperatures for long periods of time--for 
example, at the 500-750.degree. C. operating temperatures and in the 
corrosive environment of molten carbonate fuel cells. 
Anodes for high temperature fuel cells are typically based on nickel 
although other metals have been proposed. These are principally cobalt 
(which is of limited practicality because of its posture as a strategic 
metal) and copper (see U.S. Pat. No. 4,752,500 issued Jun. 21, 1988 to 
Donado for PROCESS FOR PRODUCING STABILIZED MOLTEN CARBONATE FUEL CELL 
POROUS ANODES). 
The base metals suitable for high temperature fuel cell anodes undergo 
sintering at the temperatures to which the anodes of high temperature fuel 
cells are subjected. As a result, the anode, which is required to be 
porous, consolidates with use; and the pores close, causing countless 
problems. This consolidation occurs because of the reducing atmosphere, 
compressive load, and temperatures utilized in molten carbonate and other 
high temperature fuel cell processes. Furthermore, the stack of anodes in 
the cell bundle gets thinner and thinner with use. This results in pore 
closure which causes gaps in the stack and with resulting lapses in 
electronic and ionic communication through the stack and a consequent 
reduction in operating efficiency. 
Other potential microstructural instability mechanisms in a porous anode 
under cell operating conditions are: 
Compressive Creep--Particles are drawn closer together as the metal yields 
under load. 
Particle Rearrangement--Large pores collapse as the particles rearrange via 
grain boundary sliding under compressive load. 
Additives have been employed in an effort to stabilize nickel-based anodes 
intended for high temperature fuel cells. One commonly suggested 
stabilizing additive for nickel in fuel cell anodes is chromium. 
Currently state-of-the-art are Ni-Cr anodes fabricated by: blending Ni and 
Cr powders, forming the mixture into a sheet form, and sintering the sheet 
into a coherent, porous, green structure. The green structure is installed 
directly in the fuel cell in which the anode is to be used; the fuel cell 
is brought to operating temperature; and fuel and oxidant are introduced 
into the cell. The Cr constituent of the anode thereupon oxidizes and 
forms Cr.sub.2 O.sub.3 and LiCrO.sub.2 on the surface of the nickel base 
metal. 
External oxides have only a limited ability to inhibit anode creep. 
Consequently creep of unacceptable magnitude is a continuing problem, even 
in state-of-the-art Ni-Cr anodes. 
Nickel-aluminum fell cell anodes in which the aluminum is employed to 
stabilize the nickel base metal have also been proposed by a number of 
investigators, but their efforts to date have met with only limited 
success. 
In any event, it has been found that internal oxidation of the alloying 
element (Cr or Al in nickel) is necessary to obtain a creep resistant 
anode. Internal oxidation of a fuel cell anode is a prolonged process 
because it involves solid state diffusion of oxygen in nickel at a 
controlled temperature and atmosphere. For example, in one investigation 
where the anode composition was Ni+Cr, the internal oxidation step was 
carried by heating the anode structure at 600.degree.-800.degree. C. for 
24 hours in an atmosphere containing 80-100 volumes of steam to one volume 
hydrogen. 
Also, to obtain a structure with the requisite porosity, Ni-Al fuel cell 
anodes are generally prepared by sintering a mixture of nickel and 
aluminum powders. However, nickel and aluminum powder mixtures do not 
readily form alloys when heated because Al oxidizes in normal reducing 
atmospheres employed in sintering processes. Therefore, it is difficult a 
to fabricate Ni-Al anode from Ni and Al powders. Moreover, if one starts 
with a pre-alloyed Ni-Al powder, sintering of the powder into a coherent 
structure is also difficult in normal reducing atmospheres because the gas 
usually contains a residual oxygen that readily oxidizes the Al phase, and 
the oxide product inhibits powder sintering. The problem described in the 
preceding paragraph has resulted in the development of a multi-step 
process designed to produce a usable Ni-Al fuel cell anode. In that 
process, Ni-Al powder is oxidized under controlled conditions such that 
the surfaces of the resulting particles are NiO powder and the interiors 
consists of aluminum oxide and metallic nickel. By then heat treating the 
oxidized powder in a reducing atmosphere, the NiO is reduced; and the 
particles are sintered together. 
This process is undesirably complex. And it is expensive and 
time-consuming. For example, in an investigation where the anode 
composition was Ni+Al, the oxidation step was performed in an 
oxygen-containing atmosphere at 700.degree.-1000.degree. C. for 1-10 hours 
to oxidize both the Ni and Al. Afterwards, the Ni phase had to be reduced 
back to metal in a hydrogen-containing atmosphere at 
600.degree.-1000.degree. C. for 0.5-2.0 hours. 
SUMMARY OF THE INVENTION 
It has now been discovered that the foregoing and other drawbacks of 
previously proposed methods for fabricating high temperature fuel cell 
anodes can be overcome to an acceptable extent by a novel process in which 
a powder alloy of a base metal and a stabilizing metal is preformed into 
the wanted anode shape and in which the resulting green structure is 
sintered in an environment which discourages oxidation of the alloying or 
stabilizing phase. This produces a metallic structure which, among others, 
has the advantage that it is not brittle like pre-oxidized anodes 
typically are. 
The unoxidized structure is installed directly in the high temperature fuel 
cell in which it is to be employed and selectively oxidized in a manner 
which results in internal oxidation--that is, oxidation of the alloying or 
stabilizing phase and dispersion of the oxide particles within the 
particles of the base metal and at grain boundaries. The internally 
dispersed oxide particles effectively inhibit creep deformation of the 
anode by impeding the movement of those dislocations which cause creep 
deformation. They also effectively inhibit degradative particle growth by 
sintering of the base metal phase of the alloy from which the anode is 
fabricated. The tiny oxide particles at the grain boundaries restrict the 
unwanted particle rearrangement that would otherwise result from grain 
boundary sliding. 
Internal oxidation parameters that are taken into account in treating the 
metallic, green anode structure are: 
alloy content of the starting powder; 
partial pressure of the oxygen in the oxidizing, fuel cell environment; 
oxidation temperature; 
alloying phase solubility in the base metal; 
diffusion coefficients of the alloying phase and oxygen in the base metal; 
free energy of formation of the oxide phase; and 
microstructure of starting alloy powder. 
These parameters are not independent. Consequently, it may be most 
expeditious to empirically develop an appropriate or optimum set of 
parameters for a given application of the principles of the invention. 
From the foregoing, it will be apparent to the reader that one important 
and primary object of the present invention resides in the provision of 
novel, improved anodes for high temperature fuel cells. 
A related, also important and primary object of the invention resides in 
the provision of methods for fabricating anodes as characterized in the 
preceding object. 
Other important objects of the invention, as well as additional advantages 
and features, will be apparent to the reader from the foregoing and the 
appended claims and as the ensuing detailed description and discussion 
proceeds in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
As discussed above, the starting material for the novel, herein disclosed 
method of fabricating a high temperature fuel cell anode is an alloy 
powder containing a major proportion of a base metal and a minor 
proportion of a stabilizing or alloying metal. 
Suitable base metals, taking affordability into account, are: nickel, 
copper and cobalt. As a practical matter, cobalt may be an inferior choice 
because of its position as a strategic metal; and the creep of copper at 
the operating temperature of a high temperature fuel cell such as a 
MCFC--typically 650.degree. C.--poses a formidable problem. Therefore, 
nickel will probably most often be the preferred metal. 
Characteristics taken into account in selecting an alloy for stabilizing 
the microstructure of the base metal at the high, prolonged temperatures 
experienced by fuel cell anodes are their solubility in the base metal and 
their oxide stability, mechanical properties, availability, cost, ease of 
alloying with the base metal, and--in the case of a MCFC--their stability 
in carbonate melts. Suitable alloying metals, based on the foregoing 
criteria, are: aluminum, chromium, titanium, iron, silicon, beryllium, 
yttrium, and thorium with aluminum, titanium, and chromium being 
preferred. 
At least in the case of nickel-based anodes, aluminum may typically prove 
to be the stabilizing element of choice. This metal is available at 
relatively low cost, and techniques for making Ni-Al alloy powders and for 
fabricating components from such alloy powders are well understood. 
Aluminum may be employed in the alloy in amounts ranging from 0.1 to five 
percent based on the total weight of the alloy in those applications of 
the invention employing nickel as the base alloy. Appropriate amounts of 
the stabilizing element for alloys based on other metals can range up to 
12 percent. 
An anode forming, nickel plus 3 percent aluminum powder is shown in FIG. 9. 
The particles were smooth, approximately spherical, and in a desirable 10 
to 40 micron size range. The powder was easily sintered. 
An internally oxidized anode made from this powder had an acceptable degree 
of porosity and a good creep resistance (&lt;1% creep in 140 hours at 100 psi 
and 650.degree. C.). The creep curve of the structure is shown in FIG. 7 
and identified by reference character 10. 
The anode of the present invention with the creep resistance shown by curve 
10 was significantly superior in this respect to a state-of-the-art Ni-Cr 
anode operated under the same conditions as can readily be seen by a 
comparison of curve 10 with the creep resistance curve 12 for the 
state-of-the-art anode. 
The preforming of the selected alloy powder into a green anode structure of 
the required shape is straightforward. One preferred technique is tape 
casting. In this process, the alloy powder is suspended in a liquid 
vehicle containing a solvent which can be an aqueous or organic liquid or 
a mixture of both aqueous and organic liquids; a plastic binder such as a 
methyl cellulose, polyvinyl alcohol, or an acrylic; and other additives 
such as plasticizers, dispersants, anti-foaming agents, etc. This produces 
a slurry which is cast into a thin sheet by tape casting on a flat 
substrate using a doctor blade. The tape is dried, leaving powder material 
held together by the plastic binder. 
Alternatively, alloy powder can be placed in a flat graphite mold and 
compacted by the application of pressure. 
In both the graphite mold and tape casting processes, very little pressure 
(if any) will typically be required to form the powder into the wanted 
shape. However, pressures of up to 2000 psi can be applied to the 
mold-contained powder or the cast tape as appropriate to keep the porosity 
of the anode structure at a maximum of 65 percent. 
Irrespective of whether the molding or tape casting approach is employed, 
the goal is a green structure with a thickness of 10-50 mils, preferably 
one with a thickness in the 20-40 mil range. 
As was pointed out above, Ni and Al powder mixtures do not form alloys 
readily because Al oxidizes in normal reducing atmospheres. Therefore, it 
is difficult to fabricate a Ni-Al anode from Ni and Al powders. If one 
starts with a pre-alloyed Ni-Al powder, sintering into a coherent 
structure is also difficult in normal reducing atmospheres because the gas 
usually has residual oxygen that readily oxidizes the Al phase; and the 
oxide product inhibits powder sintering. 
In anode fabricating processes employing the principles of the present 
invention, these problems are eliminated by sintering the green structure 
or preform made from the Ni-Al alloy powder in one heat treatment step at 
1000.degree. C. to 1200.degree. C. in a controlled environment which is 
either pure dry H2, a dry inert gas (He or Ar), or a vacuum (10.sup.-4 
torr or lower). These sintering atmospheres inhibit the oxidation of the 
alloying phase. The resulting anode structure is consequently metallic and 
not brittle as typical of pre-oxidized anodes. This is a decided advantage 
as it facilitates the handling of the preform. 
It has unexpectedly been discovered that a sintered anode structure 
fabricated as just described can be selectively oxidized in a manner which 
favors the wanted internal oxidation of the anode forming alloy by 
installing the anode directly in a fuel cell and effecting the selective 
oxidation of the anode in situ in the fuel cell. This is a significant 
advance in the art. The selective in situ oxidation of the sintered and 
directly assembled anode eliminates the long and expensive ex situ 
oxidation step heretofore employed to oxidize the alloying phase and 
thereby improve the creep resistance of state-of-the-art and other 
previously proposed high temperature fuel cell anodes. 
In one representative application of the present invention, a tapelike 
anode 3.59 cm long by 1.26 cm wide by 29.3.+-.2 mils thick over 
seven-eighths of its area and weighing 7.19 gm was fabricated by pressing 
a eutectic Ni-5Al powder resembling in appearance and particle size the 
Ni-Al powder shown in FIG. 9. 
The green structure was sintered by: placing it in a furnace, heating the 
furnace to a temperature of 1000.degree. C. over a period of five hours, 
holding the specimen at 1000.degree. C. for two hours, and then allowing 
it to cool in the furnace to ambient temperature (the cooling step is not 
critical and may range from less than 30 minutes to as long as 10 hours 
depending on the characteristics of the particular furnace or oven being 
used). 
Throughout this single step heat treatment, an inert atmosphere--i.e., one 
not favorable to oxidation of the Al stabilizing element--was maintained 
in the furnace. Specifically, hydrogen gas passed through Drierite to 
remove moisture and a filter or oxygen getter made of steel wool and 
placed in front of the anode structure was employed to remove residual 
oxygen from the gas. 
The sintered structure was 3.54 cm long by 1.24 cm wide by ca. 28.88 mils 
thick (shrinkage of 1.42%.times.1.37%.times.1.5%). It weighed 7.14 gms 
(weight loss of 0.68%) and had a porosity of 57.06% (prior to oxidation of 
the Al alloying element). 
As shown in FIGS. 1 and 2, each grain of the sintered specimen had a smooth 
cross-section typical of a single-phase alloy composition. 
Referring still to the drawings, FIG. 8 depicts a test cell 20 in which 
centered anode structures embodying the principles of the present 
invention can be selectively oxidized, and then evaluated. 
Fuel cell 20 has an elongated, cylindrical casing 2 in which the anode 
(identified by reference character 24) is in a sandwich that also contains 
a tile 26 filled with a molten carbonate electrolyte, a cathode 28, and 
current collectors 30 and 32. A fuel and an oxidant are introduced into 
fuel cell 20 by way of axially aligned tubes 34 and 36 concentrically 
disposed in housing 22 on opposite sides of the anode-tile-cathode 
sandwich. 
Effluent fuel is exhausted from fuel cell 20 through the annular passage 38 
between fuel inlet tube 34 and fuel cell housing 22. Effluent oxidant is 
similarly discharged from the fuel cell through an annular passage 40 
between housing 22 and oxidant inlet tube 36. 
For purposes of evaluation, the effluent fuel and oxidant sampling tubes 42 
and 44 are installed in the effluent fuel and oxidant exhaust passages 38 
and 40, respectively; and reference electrodes 46 and 48 are supported 
from fuel cell housing 20 in an electrode holder 50. 
Test cell has a 3 cm.sup.2 active area, a 70-mil hot pressed tile with a 
molten carbonate electrolyte, a 15-mil in-situ oxidized NiO cathode, a 30 
mil thick anode constructed in accord with the principles of the present 
invention, a perforated nickel anode current collector, and a perforated 
stainless steel current collector. The fuel was 60% H.sub.2/ 40% CO.sub.2 
humidified to 20 percent, and the oxidant was 30% air/70% CO.sub.2 
humidified at room temperature. The utilization was kept at 15% for the 
oxidant and 6.7% for the fuel for a 200 mA/cm.sup.2 current density. 
The sintered anode structure was directly assembled in 3 cm.sup.2 fuel cell 
20 after sintering but prior to any oxidation of its stabilizing element. 
The cell test was then operated for 2000 hours, and the cell performance 
was rated as very good. 
By comparing the micrographs of the anode cross-section before and after 
the foregoing operation of the fuel cell (FIGS. 1 and 3 vs. FIGS. 2 and 
4), it can be seen that internal oxidation of the Al phase took place in 
the fuel cell. In the pre-test micrographs (FIGS. 1 and 3), each grain has 
a smooth cross-section typical of a single phase alloy composition. In the 
post-test sample (FIGS. 2 and 4), the grain cross-section appears porous. 
This is a result of Al oxidation within the grains of the anode alloy. 
Curves 14 and 16 in FIG. 10 compare the preand post-test pore-size 
distributions of an anode like that just described but with a 63% porosity 
and a mean pore size of 6.4 microns and fabricated from a Ni-3% Al powder 
alloy. The anode was internally oxidized in situ in fuel cell 20. The pore 
size distributions were similar. Thus, the FIG. 1 curves show that the 
pore structure of the oxide dispersion strengthened anode remained stable 
over the extended duration operation of the fuel cell. 
It was pointed out above that a number of parameters affect the internal 
oxidation rate of a sintered, metallic anode structure internally oxidized 
in situ in a fuel cell in accord with the principles of the present 
invention. Of particular importance as far as the in situ oxidation step 
is concerned are temperature and the partial pressure of the oxygen in the 
anode environment. 
The effect of oxygen partial pressure pO.sub.2 (indicated by pH.sub.2 
O/pH.sub.2) on the internal oxidation rate of the sintered Ni+3% Al anode 
structure is shown by curves 16 and 17 in FIG. 5. A high pO.sub.2 is 
desired as this produces effective internal oxidation. 
The effect of temperature on the oxidation rate of the Ni+3% Al anode 
structure is shown by curves 18 and 19 in FIG. 6. Although it is faster to 
oxidize at higher temperatures, one must ensure that external oxides are 
not formed excessively. 
The invention may be embodied in many forms without departing from the 
spirit or essential characteristics of the invention. The embodiments 
disclosed herein are therefore to be considered in all respects as 
illustrative and not restrictive, the scope of the invention being 
indicated by the appended claims rather than by the foregoing description. 
All changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.