Direct deposit of catalyst on the membrane of direct feed...

Details Direct deposit of catalyst on the membrane of direct feed... Direct deposit of catalyst on the membrane of direct feed...

1998-02-10

2001-04-24

Bell, Bruce F. (Department: 1741)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S047000, C429S006000, C429S006000

Reexamination Certificate

active

06221523

ABSTRACT:

FIELD
This disclosure generally relates to organic fuel cells and in particular liquid feed organic fuel cells and the manufacturing thereof.
BACKGROUND
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells use renewable fuels such as methanol; typical products from the electrochemical reactions are mostly carbon dioxide and water. Fuel cells can be an attractive alternative to the combustion of fossil fuels.
In the past, fuel cells used reformers to convert methanol into hydrogen gas for use by the fuel cells. Direct oxidation fuel cells offer considerable weight and volume advantage over the indirect reformer fuel cells. However, initial direct oxidation models used a strong acid electrolyte which caused corrosion, degradation of catalyst, and other problems that compromise efficiency. Problems associated with such conventional direct liquid-feed cells are well recognized in the art.
Jet Propulsion Laboratory (JPL) developed an improved direct liquid-feed cell using solid-state membrane electrolyte. The JPL fuel cell eliminates the use of liquid acidic and alkaline electrolyte and therefore solves many problems in the conventional fuel cells. The subject matter of this improvement is described in U.S. Pat. No. 5,599,638, U.S. patent application Ser. No. 08/569,452 filed Dec. 8, 1995, now U.S. Pat. No. 5,773,162 and U.S. patent application Ser. No. 08/827,319, filed Mar. 26, 1997, now U.S. Pat. No. 5,945,231 the disclosures of which are herewith incorporated by reference to the extent necessary for proper understanding.
FIG. 1
illustrates a typical structure
100
of a JPL fuel cell with an anode
120
, a solid polymer proton-conducting cation-exchange electrolyte membrane
110
, and a cathode
130
enclosed in housing
102
. An anode
120
is formed on a first surface of the membrane
110
with a first catalyst for electro-oxidation and a cathode
130
is formed on a second surface thereof opposing the first surface with a second catalyst for electro-reduction. The anode
120
, membrane
110
, and the cathode
130
are hot press bonded together to form a composite multi-layer structure called the membrane electrode assembly (MEA). An electrical load
140
is connected to the anode
120
and cathode
130
for electrical power output.
The membrane
110
divides the fuel cell
100
into a first section
122
on the side of the anode
120
and a second section
132
on the side of the cathode
130
. A feeding port
124
in the first section
122
is connected to a fuel feed system
126
. A gas outlet
127
is deployed in the first section
122
to release gas therein and a liquid outlet
128
leads to a fuel re-circulation system
129
to recycle the fuel back to the fuel feed system
126
. In the second section
132
of the cell
100
, an air or oxygen supply
136
(e.g., an air compressor) supplies oxygen to the cathode
130
through a gas feed port
134
. Water and used air/oxygen are expelled from the cell through an output port
138
.
During operation, a mixture of an organic fuel (e.g., methanol) and water is fed into the first section
122
of the cell
100
while oxygen gas is fed into the second section
132
. Electrochemical reactions happen simultaneously at both the anode
120
and cathode
130
, thus generating electrical power. For example, when methanol is used as the fuel, the electro-oxidation of methanol at the anode
120
can be represented by:
Anode: CH
3
OH+H
2
O→CO
2
+6H
+
+6e
−
and the electro-reduction of oxygen at the cathode
130
can be represented by:
Cathode: O
2
+4H
+
+4e
−
→2H
2
O
Thus, the protons generated at the anode
120
traverse the membrane
110
to the cathode
130
and are consumed by the reduction reaction therein while the electrons generated at the anode
120
migrate to the cathode
130
through the electrical load
140
. This generates an electrical current from the cathode
130
to the anode
120
. The overall cell reaction is:
Cell: CH
3
OH+1.5O
2
→CO
2
+2H
2
O
The energy generated by JPL's direct feed fuel cell and the advantages of using a solid electrolyte membrane fostered further research. Efforts are targeted toward improving manufacturing efficiency while achieving better performance at reduced cost.
Prior art for preparing methanol fuel cell's membrane electrode assembly, as disclosed in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452, involves the formation of catalyst layers on a porous carbon substrate which is then mounted on either side of a solid electrolyte membrane. Although considerable energy output has been achieved at high catalyst loading levels, there may be significant performance limitations associated with this process.
In some resulting catalyst layers, at least fifty percent of the catalyst gets impregnated deep in the pores of the carbon substrate. Hence, the impregnated catalyst are inaccessible for electrochemical reaction and are essentially wasted. Some prior art methods of preparing membrane electrode assemblies for direct methanol fuel cells employ excessive catalyst. Improved techniques of catalyst application may help reduce the amount of catalyst necessary for attaining the desired performance levels. Reduction of the use of expensive catalyst and more efficient catalyst utilization are improvements that may propel this technology toward commercialization.
Another obstacle to desired performance levels is inadequate catalyst/membrane interface. A large area of electrochemically active interface between the carbon-paper coated catalyst layer and the membrane is usually desired for attaining maximum energy output by a particular fuel cell. There are some prior art methods that rely on heat and pressure for membrane electrode assembly fabrication. Since the catalyst layer is kept relatively dry after application of the catalyst onto the carbon paper substrate, the interface formed between the catalyst layer and the membrane is usually not optimum. Improved methods to enhance the area of electrochemical contact at the catalyst layer/membrane interface are desired for attaining high performance levels.
SUMMARY
The inventors disclose a direct feed fuel cell that can be manufactured efficiently while producing better performance at a reduced cost. This direct feed fuel cell features improved catalyst utilization and enhanced catalyst layer/membrane interface.
A process of catalyst application is presented. Instead of coating catalyst layers onto a support substrate, the catalyst mixture is applied directly onto the membrane. This method involves pre-treatment of the membrane in swelling agents, direct application of catalyst mixture onto the pre-treated membrane, and subsequent slow evaporation of the catalyst coating. Direct application of catalyst onto the membrane reduces catalyst waste due to impregnation of the catalyst into the support substrate.
The direct coating process also improves interfacial contact. Softening the membrane by pre-swelling and the proximity of the uniformly deposited catalyst layer to the membrane enhance the interfacial contact area formed between the electrode and the membrane.
This method of directly applying catalyst layers on the membrane offers very high catalyst utilization and improved catalyst/membrane interface. Laboratory tests reveal that at low catalyst loading levels, e.g. 2-3 mg/cm
2
, a fifty percent increase in power density can be achieved using this method. Such results demonstrate significant improvements in fuel cell performance by depositing the catalyst directly on the membrane.
In an effort to bring this innovation closer to efficient mass production, the present inventors further developed a direct spray deposition process. The spray deposition process produces uniformly thin catalyst layers and has the flexibility of producing well-defined multiple thin layers of different composition. The catalyst ink is adjusted to a sprayable composition a

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