Gas-diffussion electrodes for polymeric membrane fuel cell

The present invention describes an improved electrode suitable for application in solid polymer electrolyte fuel cells, comprising a thin, porous, planar, conductive substrate having one side coated with a pre-layer consisting of conductive carbon having a low surface area mixed to a first hydrophobic agent, to which is applied a catalytic layer consisting of platinum supported on conductive carbon with a high surface area, mixed to a second hydrophobic agent. The hydrophobic degree of the applied layers are suitably adjusted to obtain the best exploitation of the catalyst and to improve the water balance of the process.

DESCRIPTION OF THE INVENTION 
Fuel cells are apparatuses wherein reaction energy released by the 
combination of a fuel (e.g. hydrogen or admixtures thereof) with a 
comburent (e.g. pure oxygen, air, chlorine or bromine) is not completely 
transformed into thermal energy, but is converted to electric energy, as 
direct current. In said apparatuses, the fuel is fed to the anode, which 
acquires a negative polarity, and the comburent is fed to the cathode, 
which becomes viceversa positive. The evolution of electric energy in the 
most common systems of fuel cells, namely those cells which are fed with 
hydrogen and oxygen or with mixtures thereof, is quite interesting for the 
high efficiencies of the utilized fuel and for the very low, nearly 
negligible, negative effect on the environment (absence of harmful 
emissions and noise). 
A schematic classification of fuel cells is typically based on the kind of 
electrolytes used to separate the anodic and cathodic compartments, and, 
as a consequence, on the range of temperatures they may be operated at. 
This kind of classification is directly reflected by the use that may be 
devised for said types of fuel cells. 
In particular, fuel cells operating at high temperatures, i.e. above 
200.degree. C., are by now becoming an alternative electric energy source 
in large-size plants also for the interesting co-generation possibilities 
ensured by the high thermal level. On the contrary, in the field of 
low-temperature cells (25-200.degree. C.), an increasing interest is 
focused on solid polymer electrolyte fuel cells, the negative and positive 
compartments of which are respectively fed with hydrogen (pure, or in 
admixture, produced by the catalytic conversion of a precursor thereof) 
and with pure oxygen, preferably with air. 
Among the various advantages offered by these systems, particular attention 
is to be given to the extremely fast start-up, the nearly instantaneous 
ability to follow the required power variations, the high electric 
efficiency in a very wide field of supplied energy. For all these reasons, 
the very favorite application field of solid polymer electrolyte fuel 
cells is the small-size domestic supply of electric energy, small 
continuity power units, high efficiency energy-recoversion from hydrogen 
formed as a by-product in chemical and electrochemical plants, electric 
transport. 
The typical embodiment of solid polymer electrolyte consists of 
ion-exchange membranes, characterized by a high ionic conductivity. This 
kind of electrolyte had been developed as an alternative to the more 
traditional acidic or alkaline solutions (e.g. orthophosphoric acid or 
potassium hydroxide) to solve the problems connected with the utilization 
of liquid electrolytes which, although typically contained in porous 
matrixes, pose serious limitations due to instantaneous pressure 
unbalances between the two compartment. Furthermore, as said electrolytes 
are quite corrosive, extremely expensive construction materials are 
needed. 
The main drawback initially faced in the field of polymeric ion exchange 
membrane fuel cells was the difficulty of providing for a perfect 
electrical continuity between the membrane area where electric current is 
transported by a positive charge carrier (the H.sup.+ ion) and the two 
electrodic surfaces, from which on electric conductivity is ensured by the 
electron flow. The charge passage between the two carriers takes place on 
the catalyst particles which activate the electrode by means of the anodic 
and cathodic reactions. In the typical case of a cell having the anode fed 
with a mixture containing hydrogen as the fuel and the cathode fed with a 
mixture containing oxygen, the reactions are respectively: 
##STR1## 
To obtain a very effective device, the contact points between the catalyst 
particles and the membrane must be easily reached by the gaseous 
reactants. For this purpose, the electrodes contain a hydrophobic agent, 
(such as polytetrafluoro-ethylene P.T.F.E.) which permits to locally 
segregate the water produced by the cathodic reaction leaving free access 
to the gas. Only the points where the contact between membrane and 
catalyst and concurrently the access of the reactants are ensured are 
efficient reactions sites. 
The first solution found in the prior art to maximize these contact points 
foresaw the use of a high quantity of catalyst on the two sides of the 
membrane (typically 40-50 grams per square meter of membrane on each 
side). Platinum black is the only catalyst capable of ensuring a 
sufficient efficiency for industrial applications. However the cost of 
this material was prohibitive hindering completely the industrial 
development of this technology. For comparison sake it must be noted that 
the closest fuel cell technology, using phosphoric acid embedded in a 
matrix as the electrolyte, uses platinum loads ten times lower. The most 
commonly used electrodes in phosphoric acid fuel cells are activated by a 
catalyst consisting of platinum supported on active carbon particles, 
applied to a substrate made of an electrically conductive thin carbon 
cloth. These electrodes, commercialized by E-TEK, U.S.A. under the 
trademark ELAT.TM., are described in U.S. Pat. No. 4,647,359. ELAT.TM. 
electrodes are specifically intended for use in phosphoric acid fuel 
cells. The carbon cloth acting as the substrate in fact is activated on 
one side with a mixture of catalyst and a hydrophobic agent and on the 
other side with conductive carbon also mixed with a hydrophobic agent in 
order to physically constrain the electrolyte inside the porous supporting 
matrix, as already described. The electrode described in U.S. Pat. No. 
4,647,359 foresees a distribution of the hydrophobic binder completely 
unsuitable for use in polymeric ion exchange membrane fuel cells. 
First of all this configuration foresees a hydrophobic surface opposed to 
the active surface facing the membrane. This is due, as aforesaid, to the 
need of constraining the liquid electrode inside the porous matrix but is 
completely useless in the case of a solid electrolyte as it introduces 
without any need an additional ohmic penalty. Furthermore in 
mass-production, it would be disadvantageous to activate both surfaces as 
this introduces a superfluous complication in an automated fabrication 
process. The method described in U.S. Pat. No. 4,647,359 foresees also 
that the active surface of the electrode comprise a uniform mixture of 
catalyst and hydrophobic binder. This involves the loss of a remarkable 
quantity of catalyst inside the roughness of the substrate. 
U.S. Pat. No. 4,876,115 describes the use of ELAT.TM. electrodes also in 
membrane fuel cells. The invention consists in impregnating the active 
surface of the electrodes with a protonic conductive liquid thus creating 
a tridimensional reaction zone which practically extends the membrane 
phase beyond the more external surface of the electrode, increasing 
thereby the platinum exploitation of one order of magnitude. A subsequent 
stage consisting in heat pressing the electrode on the membrane, as 
described in U.S. Pat. No. 3,134,697, permits to obtain a 
membrane-electrode assembly having the same electrochemical properties as 
the electrodes having a higher platinum content of the prior art. The 
typical noble metal load required for the best performance of electrodes 
in membrane fuel cells is reduced to 5 grams per square meter of active 
surface. Thanks to this invention, the ELAT.TM. electrode found a quick 
application in this field, although it was not originally conceived for 
this aim. 
The combination of these two techniques, which in principle gives to the 
membrane-electrode assembly the desired electrochemical characteristics, 
is however not completely satisfactory from an industrial standpoint. In 
particular, heat pressing of the two electrodes on the solid electrolyte 
is a very expensive procedure due to the problems connected with its 
automation. In fact each membrane-electrode assembly must be subjected to 
heat and pressure for a time sufficient to cause the intimate contact 
among the components, which time is usually in the range of some minutes. 
Further the temperature must necessarily exceed 100.degree. C. with a 
relative humidity close to 100%, otherwise membranes suitable for use in 
any fuel cell presently commercialized or described in the literature 
would undergo an irreversible decay. The high cost of the necessary 
components makes unacceptable to discard defective assemblies which are 
unavoidable in mass-production wherein several parameters (times, 
temperatures, pressures, relative humidity) must be kept within very 
strict tolerance limits. In addition, membranes undergo remarkable 
expansion under the thermal cycle and the variations of the relative 
humidity. Conversely the electrodes are practically dimensionally stable. 
This causes dangerous stresses at the interface involving possible damages 
to the heat-pressed assemblies, which must be kept under strictly 
controlled conditions before assembling in the cell, thus adding to the 
process costs. 
These drawbacks, which substantially hindered industrial success for the 
solid polymer electrolyte fuel cells have been overcome by the assembly 
described in U.S. Pat. No. 5,482,792 which describes assembling of a cell 
wherein the heat-pressing of the membrane-electrodes assembly is carried 
out in situ, after stacking of the single components, thanks to the use of 
a current collector exhibiting residual deformability. This current 
collector provides for a homogeneous distribution of the contact points at 
the same time evenly distributing the pressure exerted by the clamping of 
the cells on both electrodes in a close point-pattern. 
It is the main object of the present invention to improve the prior art 
electrodes comprising a thin, porous conductive substrate and devised for 
the application in liquid electrolyte cells, by modifying the 
characteristics to make them perfectly suitable for application in solid 
polymer electrolyte cells. In particular, the present invention consists 
in activating only one side of said thin, porous conductive substrate with 
a pre-layer comprising a conductive carbon having a low surface area and a 
first hydrophobic agent and subsequently superimposing a catalytic layer 
comprising platinum supported on conductive carbon having a high surface 
area, mixed to a second hydrophobic agent, and adjusting the degree of 
hydrophobicity of the applied layers in order to obtain an optimum 
exploitation of the catalyst. 
For an optimization of the electrochemical characteristics of the 
electrodes for polymer fuel cells the following goals are to be achieved: 
maximum increase of the active contact area between catalyst and proton 
conductor, that is the number of catalytic particles simultaneously in 
contact with the membrane and efficaciously fed by the gaseous reactants; 
best water balance to the membrane-electrode assembly, to completely 
hydrate the electrolyte in order to ensure a perfect electrical 
conductivity without causing an excessive water load in the catalytic 
particles, which would prevent the reactants access. 
It has been surprisingly found that it is extremely advantageous to 
distribute the hydrophobic binder in a decreasing degree between the 
inside and the outside of the cathode, while no similar result is obtained 
at the anode. Different treatments have been consequently applied to the 
cathode and anode and for each one the best formulation has been devised. 
In both cases, a pre-layer of conductive carbon having a low surface area 
mixed to a hydrophobic binder has been first applied to the substrate. 
Said pre-layer is aimed at both giving the required hydrophobic 
characteristics to the electrode, and at substantially filling the 
substrate roughness in order to obtain an extremely even surface. A 
catalytic layer comprising a platinum-based catalyst supported on a carbon 
having a high surface area, mixed to a second hydrophobic agent has been 
then applied to the substrate obtained as previously described. The 
platinum/carbon ratio in the catalyst has been modified in order to expose 
the largest surface of platinum. With catalysts having excessively 
dispersed platinum, in fact, when the quantity of noble metal is applied, 
thicker catalytic layers are obtained which may lead to the risk of having 
a high quantity of platinum hidden in too deep layers, for which no 
contiguity can be attained with the membrane. Catalysts with too 
concentrated platinum, on the contrary, present a too reduced specific 
surface (that is related to the weight of the applied metal). 
In the application in fuel cells fed with non-pure hydrogen, the platinum 
is often deactivated due to poisoning. In these cases significant 
advantages are obtained by the activation of the fuel cell anode with 
catalysts containing platinum as platinum alloy. For example, the 
modifications to the ratio platinum/carbon in terms of weight are also 
extended to the binary platinum-ruthenium alloy. 
The following examples show that: 
the optimum noble metal dispersion on carbon, both in the case of pure 
platinum or alloy thereof, is comprised in the range of 30-40% by weight. 
the optimum P.T.F.E. concentration in the cathodic pre-layer ranges from 50 
to 65% by weight. 
The optimum P.T.F.E. concentration in the anodic pre-layer is comprised 
between 15 and 25% by weight. 
The optimum P.T.F.E. concentration in both anodic and cathodic catalytic 
layer is comprised in the range of 10-20% by weight. Preferably, the anode 
and/or cathode are subjected to an additional thermal treatment above 
300.degree. C.

EXAMPLE 
Some samples of electrodes for use in fuel cells have been prepared 
according to the following procedure: 
--an aqueous dispersion of the pre-layer components was applied to the 
substrate surface and dried at ambient temperature up to obtain a specific 
load of 25 grams of carbon per square meter; 
--an aqueous dispersion of the catalytic layer components was subsequently 
applied to the pre-layer and dried at ambient temperature up to obtaining 
a specific load of 6 grams of noble metals per square meter; 
--the thus activated substrate was thermally treated for 30 minutes at 
350.degree. C. 
--a 5% hydroalcoholic suspension of perfluorinated sulphonated polymer, 
commercialized by Du Pont de Nemours under the trademark Nafion.RTM., was 
applied to the activated substrate by brushing and subsequent drying at 
ambient temperature. The final load was 10 grams/m.sup.2. 
The substrates consisted alternatively of a conductive carbon cloth 0.35 mm 
thick (indicated in Table 1 as TC) or a reticulated nickel material, 
commercially known as "metal foam", completely flattened (indicated in 
Table 1 as SM). 
Shawinigan Acetylene Black carbon and P.T.F.E. as the hydrophobic binder 
were used for the prelayer. 
The same hydrophobic binder in combination with Pt supported on Vulcan 
XC-72 carbon was used for the catalytic layer. 
The samples had the following characteristics: 
TABLE 1 
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% by weight 
of nobel 
P.T.F.E. 
Nobel metal 
metal on 
P.T.F.E. in the in the carbon in 
Substrate 
in the catalytic 
catalytic 
the catalytic 
Sample 
type pre-layer 
layer layer layer 
______________________________________ 
A TC 15% 50% Pt 30% 
B TC 30% 50% Pt 30% 
C TC 40% 50% Pt 30% 
D TC 50% 50% Pt 30% 
E TC 65% 50% Pt 30% 
F SM 50% 50% Pt 30% 
G TC 20% 50% Pt 30% 
H TC 25% 50% Pt 30% 
I SM 60% 50% Pt 30% 
J TC 70% 50% Pt 30% 
K TC 60% 15% Pt 20% 
L TC 60% 15% Pt 40% 
M TC 60% 15% Pt 50% 
N TC 60% 15% Pt 70% 
O TC 15% 30% Pt 30% 
P TC 15% 10% Pt 30% 
Q TC 15% 20% Pt 30% 
R TC 20% 15% Pt 30% 
S TC 60% 25% Pt 30% 
T TC 60% 40% Pt 30% 
U TC 60% 10% Pt 30% 
V SM 15% 15% Pt:Ru 1:1 
30% 
W TC 15% 15% Pt:Ru 1:1 
50% 
X TC 15% 30% Pt:Ru 1:1 
50% 
______________________________________ 
Some samples of ELAT.TM. electrodes have been obtained from E-TEK, Inc., 
U.S.A.. The samples, prepared according to the teaching of U.S. Pat. No. 
4,647,359, had a platinum load of 6 grams per square meter. A layer of 
liquid Nafion.RTM. was applied to the samples according to the same 
procedure used for the samples of Table 1. These additional samples have 
been identified by Y. A fuel cell having an active area of 25 cm.sup.2 
prepared according to the teachings of U.S. Pat. No. 5,482,792, with pure 
hydrogen fed at the anode and air fed to the cathode was alternatively 
equipped with the electrode samples of Table 1 in combination with a 
Nafion 117 membrane. All the tests were carried out at the same operating 
conditions and for a equal periods of 6 hours at 3 kA/m.sup.2, The cell 
voltages were detected at the end of each test. The results are reported 
in Table 2. 
TABLE 2 
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Test N.sup..degree. 
Anode Cathode Cell voltage at 3 kA/m.sup.2 
______________________________________ 
1 Y Y 730 mV 
2 D D 740 mV 
3 F F 740 mV 
4 A D 755 mV 
5 B D 750 mV 
6 C D 745 mV 
7 J D 720 mV 
8 G D 755 mV 
9 H D 755 mV 
10 A B 715 mV 
11 A C 745 mV 
12 A E 760 mV 
13 A I 765 mV 
14 A J 740 mV 
15 O E 770 mV 
16 P E 775 mV 
17 Q E 775 mV 
18 Q S 795 mV 
19 Q T 780 mV 
20 Q U 795 mV 
21 Q K 760 mV 
22 Q L 790 mV 
23 Q M 775 mV 
24 Q N 765 mV 
25 V U 790 mV 
26 W U 790 mV 
27 X U 780 mV 
______________________________________ 
The foregoing description identifies the characterizing features of the 
invention and some applications thereof. Further applications are however 
possible for the described electrode structures and equivalent ones 
without departing from the scope of the present invention and should be 
included within the scope of the following claims.