Method of fabricating a fuel cell electrode

Electrocatalyst and hydrophobic polymer particles are combined to form an aqueous suspension which is then applied to a conductive substrate to form an electrode. The colloidal chemistry of the suspension of electrocatalyst particles and hydrophobic polymer particles is controlled prior to the application of the suspension to the substrate such as by the incorporation of a sol of a polyvalent metal oxide or solution of a salt of a polyvalent metal into the suspension.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the construction of electrodes for 
use in an electrochemical device. 
2. Description of the Prior Art 
The advantages of lightweight electrodes for use in fuel cells have been 
recognized. These electrodes essentially comprise an admixture of 
electrocatalyst and hydrophobic binder deposited on a substrate material 
such as a porous carbon or metal support, wire mesh or grid. The 
electrodes are extremely thin, having low internal electrical resistance 
and furthermore, take up only limited space permitting the construction of 
highly compact cells having a high energy to volume and energy to weight 
ratio. One difficulty with these electrodes, however, is obtaining a 
controlled distribution of electrocatalyst particles with the hydrophobic 
polymer particles throughout the electrode structure. The performance of 
polytetrafluoroethylene (PTFE) bonded platinum black and platinum 
supported on carbon fuel cell electrodes is limited by the size of the 
effective catalyst clusters interspersed in the porous gas diffusion 
electrode structure. The effective size of the catalyst clusters (i.e., 
"agglomerates") between the gas diffusion channels maintained by the 
dispersed PTFE is usually large; therefore, utilization of the catalyst in 
the interior of the electrolyte filled catalyst agglomerate requires the 
molecules of gaseous reactant to travel a long diffusion path. 
Conventional methods for dispersing the electrocatalyst particles, such as 
fluid energy milling, ball milling, ultrasonic agitation, and the addition 
of non-ionic surfactants, have been generally unsuccessful in sufficiently 
reducing the effective catalyst agglomerate size, so that the catalyst 
agglomerate in prior art fuel cell electrode structures ranges between 
1.times.10.sup.-6 and 20.times.10.sup.-6 m. This is considered too large. 
SUMMARY OF THE INVENTION 
It is proposed to overcome the deficiencies of the prior art methods of 
constructing lightweight electrocatalyst/hydrophobic polymer electrodes by 
carefully controlling the colloid chemistry of an aqueous suspension of 
electrocatalyst particles and hydrophobic polymer particles prior to the 
application of the suspension to a conductive substrate, by introducing 
into the suspension a sol of a polyvalent metal oxide or salt solution of 
a polyvalent metal to adjust the surface charges and thence the 
interactions between the electrocatalyst particles themselves and between 
the electrocatalyst particles and the hydrophobic polymer. More 
particularly, according to the present invention a separate aqueous 
suspension of electrocatalyst particles and a separate aqueous suspension 
of hydrophobic polymer particles are mixed to form a combined suspension, 
which also includes a sol of a polyvalent metal oxide or salt solution of 
a polyvalent metal. The sol or salt solution is introduced into the 
combined suspension either as part of the separate aqueous suspension of 
electrocatalyst particles or after the separate suspensions have been 
combined. To work properly the valency of the metal of the sol or salt 
solution must be three or greater. The charged metal oxide particles if 
the sol is used or the metal cations from the salt solution adsorb on the 
high surface area catalyst particles and prevent massive agglomeration of 
the electrocatalyst. Electrodes fabricated using this catalyst/polymer 
suspension have an improved electrochemical performance and improved 
reliability of construction, which means a higher yield of acceptable 
electrodes. 
It is not clear exactly what occurs in the fabrication of the electrodes as 
a result of modifying the colloid interactions of the component particles 
prior to deposition. It is theorized, however, that by modifying the 
colloid interactions--i.e., by adjusting the surface charges of the 
electrocatalyst particles and hydrophobic polymer particles--the zeta 
potentials of the suspended particles are altered, thereby altering 
interaction between them. The modification of colloid interaction can be 
accomplished with most polyvalent metals, such as Th.sup.4+, Zr.sup.4+, 
Al.sup.3+, Fe.sup.3+, Ce.sup.3+, or the like. The valency must be at least 
3+. Valencies of 2+ do not seem to retard the agglomeration sufficiently. 
Cationic organic surfactants may also accomplish the same results, and, 
although not preferred, are contemplated as being within the scope of the 
present invention. One possible organic surfactant is Hyamine 1622 from 
Rohm and Haas, Philadelphia, Pennsylvania, which is a di-isobutylphenoxy 
ethoxy ethyl dimethyl benzyl ammonium chloride. 
The foregoing and other objects, features, and advantages of the present 
invention will become more apparent in light of the following detailed 
description of preferred embodiments thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In carrying out a preferred embodiment of the invention, proper amounts of 
polymer and electrocatalyst are conveniently formed into separate aqueous 
suspensions using colloidal hydrophobic polymer particles and 
electrocatalyst particles. In one construction the electrocatalyst 
comprises an electrocatalytic metal on carbon particles. The weight of the 
metal catalyst may be anywhere from 1 to 20% of the total electrocatalyst 
weight but is preferably 5 to 10% of the electrocatalyst weight. The 
electrocatalyst/polymer admisture will contain from about 70-40 weight 
percent electrocatalyst and from about 30-60 weight percent polymer. 
Preferably the electrocatalyst will comprise 45-55 weight percent of the 
admixture and the polymer will comprise 55-45 weight percent. The 
foregoing percentages are based upon experience with a platinum supported 
on carbon electrocatalyst and PTFE polymer; however, the suggested ratios 
of electrocatalyst to polymer would also apply if unsupported platinum 
black were used as the electrocatalyst. 
A polyvalent metal is added to the electrocatalyst suspension in the form 
of either a sol of a polyvalent metal oxide or a solution of a salt of a 
polyvalent metal. The concentration of the metal oxide in the sol and the 
concentration of the metal cation in the salt solution is preferably on 
the order of 10.sup.-2 to 10.sup.-7 M. A salt solution is preferred. The 
suspension of electrocatalyst with the polyvalent metal is them combined 
with the PTFE suspension and stirred. It is desirable, although not always 
required, to adjust the pH of the electrocatalyst suspension after adding 
the polyvalent metal to produce maximum dispersion of the electrocatalyst. 
This procedure simply gives another element of control. The proper pH is 
determined by the nature of the electrocatalyst and polyvalent metal to be 
added. For example, if the electrocatalyst is platinum black or platinum 
supported on carbon, the polymer PTFE, and the polyvalent metal Thorium, 
the pH of the electrocatalyst suspension should be adjusted to between 9.0 
and 10.0; if the electrocatalyst is platinum supported on carbon, the 
polymer PTFE, and the polyvalent metal Iron, the pH of the electrocatalyst 
suspension should be adjusted to between 2.5 and 5.0; if the 
electrocatalyst is platinum supported on carbon, the polymer PTFE, and the 
polyvalent metal Aluminum, the pH of the electrocatalyst suspension should 
be adjusted to between 3.5 and 6.5. In the case of the salt solution it is 
possible that the polyvalent metal in solution is hydrated or solvated and 
that this species (oxy-hydrate or hydroxide) controls the colloid 
chemistry; it may be that the optimum pH for any particular combination is 
the pH at which an aqueous solution permits the formation of the hydrated 
metal ion; this pH can readily be determined by trial and error by a 
person having ordinary skill in the art. 
After the two suspensions are combined and stirred, a catalyst/polymer 
composite layer is formed by applying the suspended solids to a suitable 
conductive substrate such as a porous metal or carbon substrate, or a wire 
grid or mesh by any of various techniques including filtration, spraying, 
or by forming a paste or the like and applying the paste to the substrate 
with a flat knife-like surface, doctor blade, or similar means. As a 
result of the process of this invention, the maximum effective catalyst 
agglomerate size is usually about 1.0.times.10.sup.-6 m. Preferably, the 
electrode is heated in air or oxygen to a temperature sufficient to remove 
any organic surfactant which may be in the suspensions and to bond the 
electrocatalyst and polymer particles to each other and to the substrate. 
Since the bonding temperature of the polymer is always sufficiently high 
to volatilize or decompose the surfactant, this can be accomplished in a 
single step. The bonding temperature of polytetrafluoroethylene is, for 
example, about 320.degree. C. The electrocatalyst/polymer composite 
preferably will be applied to the support at a catalyst loading of from 
about 0.05 mg metal per cm.sup.2 up to about 10 mg metal per cm.sup.2. 
Normally, as low an electrocatalyst loading as possible for any particular 
application is desired due to the expense of the catalyst. It is possible 
to use catalyst loadings outside of the above range up to as high as about 
35 mg metal per cm.sup.2 of electrode surface and higher, but normally 
this is not necessary or desirable. In accordance with the present 
invention, the amount of electrocatalyst utilized can be lowered due to 
the uniform distribution of electrocatalyst and as a result of the 
improved utilization characteristics of the electrode. 
Some substrates particularly useful herein are metal screens, expanded 
metal, porous sinters of carbon or metal, metal felt, or mesh. It is 
essential that the structure be electrically conductive and able to 
withstand the corrosive environment of a fuel cell. Suitable metal 
supports are from about 0.5 to 1.0 mm thick having a high porosity, i.e., 
from 35 to 90%, and preferably are composed of nickel, copper, iron, 
titanium, tantalum, silver, gold, and alloys thereof, primarily from the 
standpoint of the exceptional resistance of these metals to the corrosive 
environment in the fuel cell. 
The polymer which is to be utilized in accordance with the present 
invention must be relatively hydrophobic. Exemplary polymers include 
polytetrafluoroethylene, polyfluoroethylenepropylene, 
polytrifluorochloroethylene, polyvinylfluoride, 
perfluoroalkoxypolyethylene, polyvinylidene fluoride, 
polytrifluoroethylene, and co-polymers thereof. However, because of its 
exceptional hydrophobicity as well as its resistance to heat and the 
corrosive environment of the electrolyte, polytetrafluoroethylene is 
presently preferred. 
The electrochemically active metal which is to be applied to the metal 
support as a suspension with the hydrophobic polymer can be any of various 
metals which will favorably influence an electrochemical reaction. Such 
metals include nickel, cobalt, iron, gold, copper, silver, palladium, 
platinum, ruthenium, osmium, and iridium, alloys and oxides thereof. Due 
to their exceptional properties insofar as favorably influencing an 
electrochemical reaction, the Group VIII and Ib metals of Mendeleyev's 
periodic table are preferred. The most desirable metal is platinum. 
The electrodes prepared by the process of the present invention can be 
employed in various types of fuel cells including those using alkaline and 
acid electrolytes. Alkaline electrolytes are preferably the alkaline metal 
hydroxides but can include the alkaline earth hydroxides as well as the 
alkaline earth carbonates. Preferred alkaline electrolytes are potassium, 
sodium, rubidium, and cesium hydroxides. The strong mineral acids such as 
phosphoric acid, sulfuric acid, and hydrochloric acid and organic acids 
such as trifluoromethanesulfonic acids or polymers thereof are the 
preferred acid electrolytes. Preferably the electrodes are employed in 
acid or alkaline electrolyte fuel cells wherein the electrolyte may be 
trapped or contained in a hydrophilic matrix; however, they could also be 
used in cells operating with a free electrolyte. Such cells are normally 
operated from ambient to about 220.degree. C. using oxygen or air as the 
oxidant, and hydrogen or hydrocarbons as the fuel. 
The following specific examples are illustrative of this invention: 
EXAMPLE I 
A catalyst comprising 5 weight percent platinum supported on graphitized 
Vulcan.RTM. XC-72.RTM. (a furnace black from Cabot Corporation, Boston, 
Mass. is suspended by ultrasonic vibration in a quantity of six times 
distilled (6D) water (K .congruent. 1.times.10.sup.-6) sufficient to give 
a catalyst concentration of 1.times.10.sup.-4 g/ml. To this suspension is 
added Fe.sup.3+ ions from an Fe.sub.2 (SO.sub.4).sub.4 solution which is 
0.01 M in Fe.sup.3+, such that the ratio, by weight, of Fe.sup.3+ to 
catalyst is 6.6.times.10.sup.-2. The pH of the suspension is raised to 3.4 
by adding 0.1 N NaOH dropwise. The resulting suspension is stirred 
vigorously to aid in agglomerating the catalyst and is then filtered 
through 0.45.mu. Millipore.RTM. filter paper. The filtered material is 
washed with 6D water to remove any excess salts and is then resuspended 
(while still damp) by ultrasonic agitation for five minutes in the same 
volume of 6D water as used in the first suspension of the catalyst. The 
step of removing the excess salts may not be required when the electrode 
is to be used in a phosphoric acid cell since the salts will be dissolved 
naturally by the acid. After removing from the agitator, TFE-42 (a PTFE 
aqueous suspension from Dupont Corporation, Delaware, Maryland) is added 
dropwise, while stirring, so as to give a one to one ratio, by weight, of 
catalyst to PTFE. The catalyst/PTFE suspension is filtered onto 0.45.mu. 
Millipore filter paper. Using this filtered catalyst/PTFE mixture an 
electrode is prepared as follows: A mixture of 80 weight percent TFE-42 
and 20 weight percent graphitized Valcan XC-72.RTM. is suspended in water 
and filtered on top of this catalyst/PTFE mixture to form a porous 
conductive substrate. While still damp and on the filter paper, the filter 
cake is pressed at 300 psi into a 50 mesh gold plated tantalum screen 
using absorbent paper to absorb the water squeezed out of the damp filter 
cake. The layer is dried in air at about 50.degree.-70.degree. C., and 
then sintered at 335.degree. C. to form the finished electrode. 
The performance of two electrodes (designated A and B) made in accordance 
with the procedure of Example I is shown in Table 1. These electrodes had 
a catalyst loading of 0.5 mg Pt/cm.sup.2 and were run in half cell tests 
using pure oxygen and 96 weight percent H.sub.3 PO.sub.4 as the 
electrolyte. The operating temperature of the cells was 160.degree. C. 
TABLE 1 
______________________________________ 
ELECTRODE PERFORMANCE 
Electrode Potential vs. 
H.sub.2 Reference Electrode 
Electrode in the Same Electrolyte 
Current Density (volts) 
(ma/cm.sup.2) A B 
______________________________________ 
1000 .44 .57 
500 .56 .62 
300 .61 .65 
100 .69 .71 
50 .73 .74 
______________________________________ 
EXAMPLE II 
Electrodes having good performance characteristics were also made by the 
procedure of Example I except the catalyst used consisted of 5 weight 
percent platinum supported on non-graphitized Vulcan XC-72.RTM.. 
EXAMPLE III 
Electrodes were made using the same procedure as Example I except that 
instead of adding Fe.sup.3+ ions from an iron salt solution, Th.sup.3+ 
ions were added from a solution of Th(NO.sub.3).sub.4 solution which was 
0.01 M in Th.sup.4+, such that the ratio of Th.sup.4+ to catalyst, by 
weight was 0.22; the resulting suspension was raised to a pH value of 7.5 
(instead of 3.4) by adding 0.1 N NaOH dropwise. 
Table 2 gives performance data of electrodes C and D made according to the 
procedure of this example under the same test conditions used with the 
electrodes of Table 1. Electrode C had a catalyst loading of 0.05 mg 
Pt/cm.sup.2 and electrode D had a platinum loading of 0.1 mg Pt/cm.sup.2. 
TABLE 2 
______________________________________ 
ELECTRODE PERFORMANCE 
Electrode Potential vs. 
H.sub.2 Reference Electrode 
Electrode in the Same Electrolyte 
Current Density (volts) 
(ma/cm.sup.2) C D 
______________________________________ 
1000 .53 .49 
500 .60 .59 
300 .66 .65 
100 .71 .71 
50 .74 .75 
______________________________________ 
In the foregoing examples the sol or salt solution was added to an aqueous 
suspension of electrocatalyst particles before combining the 
electrocatalyst suspension with the polymer suspension. An alternate and 
equally useful embodiment is described in the following Example IV wherein 
the polyvalent metal is added after the suspensions of electrocatalyst and 
polymer are combined. 
EXAMPLE IV 
A catalyst consisting of 15 weight percent platinum on non-graphitized 
Valcan XC-72.RTM. is added to a beaker containing an aqueous suspension of 
0.4 mg/ml TFE-42 such that the total weight of the PTFE equals the weight 
of catalyst. The catalyst and PTFE are then suspended by ultrasonic 
agitation. The suspension is heated to about 50.degree.-70.degree. C., and 
then Al.sup.3+ ions, in the form of an Al.sub.2 (SO.sub.4).sub.3 solution, 
is added dropwise until 10 mg Al.sup.3+ has been added for every 6 mg 
electrocatalyst. This suspension is heated for about two hours, making 
sure to maintain the volume by adding water occasionally. The suspension 
is then cooled and resuspended by ultrasonic agitation. The resuspension 
is then filtered onto a 0.45.mu. Millipore filter paper as was the 
resuspension of Example I. From this point on the procedure for forming 
finished electrodes is the same as the procedure of Example I. Electrodes 
designated E and F were made by this procedure. Their performance 
characteristics are shown in Table 3. 
An electrode designated G was also made by this procedure except that the 
step of adding Al.sup.3+ ions was eliminated for the purpose of 
comparison. The performance data for electrode G is also shown in Table 3. 
All electrodes of Table 3 had a platinum loading of 0.05 mg Pt/cm.sup.2 
and were tested under the same conditions as the electrodes of Tables 1 
and 2. 
TABLE 3 
______________________________________ 
ELECTRODE PERFORMANCE 
Electrode Potential vs. 
H.sub.2 Reference Electrode 
Electrode in the Same Electrolyte 
Current Density 
(volts) 
(ma/cm.sup.2) E F G 
______________________________________ 
1000 .62 .62 .56 
500 .66 .66 .62 
300 .68 .69 .65 
100 .73 .72 .70 
50 .76 .75 .74 
______________________________________ 
In yet another embodiment, exemplified by the following Examples V and VI, 
the sol or salt solution is used as the aqueous suspending medium for the 
electrocatalyst particles which are added thereto as a powder to form the 
electrocatalyst suspension. The electrocatalyst suspension is then mixed 
with a hydrophobic polymer suspension in a manner similar to Example I. 
EXAMPLE V 
A thorium oxide (ThO.sub.2) sol is prepared by peptizing ThO.sub.2 powder 
having a surface area of 80-90 m.sup.2 /g in dilute aqueous HNO.sub.3 at a 
pH value of from 3 to 3.5 for one hour at 80.degree. C. The concentration 
of ThO.sub.2 in the sol should be between 0.05-5.0 mg/ml. Twenty-five mg 
of a platinum black having a surface area of from 20-40 m.sup.2 /g, is 
mixed with 4.6 ml of the ThO.sub.2 sol to form a suspension. This 
suspension is blended with the 45 ml dilute HNO.sub.3 at a pH value of 
from 3-4 and is ultrasonically dispersed; it is then mixed with a TFE-42 
suspension having a ratio of PTFE to platinum, by weight, of 0.1. This 
suspension is formed into an electrode by applying it to a conductive 
carbon substrate. The electrode layer is dried at 50.degree.-70.degree. C. 
and is then heated at 350.degree. C. for ten minutes. 
The performance of an electrode H made in accordance with the procedure of 
this example and having a platinum loading of 0.5 mg Pt/cm.sup.2 was 
tested under the same test conditions used with the electrodes of Table 1. 
Performance data for this electrode is given in Table 4. 
TABLE 4 
______________________________________ 
ELECTRODE PERFORMANCE 
Electrode Potential vs. 
H.sub.2 Reference Electrode 
Electrode in the Same Electrolyte 
Current Density ( volts) 
(ma/cm.sup.2) H 
______________________________________ 
200 .74 
100 .77 
50 .81 
20 .86 
10 .89 
______________________________________ 
EXAMPLE VI 
Electrodes similar to those of Example V may also be prepared using 
ZrO.sub.2 powder to form a zirconium oxide sol which is then formed into 
an electrode by the same procedure as Example V. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail 
thereof may be made therein without departing from the spirit and the 
scope of the invention.