Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal or alloy coating on...
Reexamination Certificate
1998-12-14
2001-07-10
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal or alloy coating on...
C205S105000, C205S102000, C205S103000, C205S257000, C205S082000, C205S083000, C205S084000
Reexamination Certificate
active
06258239
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrode manufacture. In particular, the invention relates to an electrodeposition process used in the manufacture of electrodes for solid polymer fuel cells, or other electrochemical energy converters.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The electrode layers typically comprise porous, electrically conductive sheet material as a substrate, and an electrocatalyst disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
The electrocatalyst may be applied to the electrode substrate, or to the membrane electrolyte, using a variety of well-documented techniques. Typical fuel cell electrocatalysts are expensive. It is therefore important to use the electrocatalyst material as efficiently as possible. This includes increasing utilization of the electrocatalyst in the fuel cell electrodes.
In fuel cell operation, effective electrocatalyst sites are accessible to the reactant in the fuel cell, are electrically connected to the fuel cell current collectors, and are tonically connected to the fuel cell electrolyte. Electrocatalyst sites which are ionically isolated from the electrolyte are not productively utilized if ions generated by the fuel cell reactions at the electrocatalyst sites do not have a means for being ionically transported to the electrolyte.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance.
U.S. Pat. No. 5,084,144 discloses an electrodeposition method for the preparation of fuel cell gas diffusion electrodes. A layer of proton-conducting polymer is impregnated into one surface of a carbon-containing electrode substrate. Electrocatalyst is then deposited on the surface of substrate using a DC or pulse current electrodeposition technique. Thus, together with a counterelectrode, the gas diffusion electrode is immersed in a bath containing primarily cations (M
+
, M
++
, M
+++
) of the metals of groups VIII or Ib of the periodic table, and a direct current is applied. The current may be constant or several current pulses with a relatively long pulse duration (duration of approximately 6-120 seconds) may be used. As a result, a thin layer of electrocatalyst is deposited only where it is ionically connected to the proton-conducting polymer coating, which in turn will be in contact with the electrolyte in the MEA.
Other electrodeposition processes have been used in the preparation of fuel cell electrodes. For example, in U.S. Pat. No. 5,599,638 and PCT/International Publication No. WO96/12317 (Application No. PCT/US94/11911), a controlled potential is used for the electrodeposition step. A constant voltage is applied for 5-10 minutes continuously, without current or voltage pulsing. This potentiostatic control permits some control of the electrochemical processes which occur, but the growth of the electrocatalyst clusters is difficult to control using the described technique. The substrate is not impregnated with any ion-conducting polymer prior to electrodeposition. Instead, the electrode is impregnated, preferably with a Nafion® or another proton-conductive polymer solution after electrodeposition of the electrocatalyst. Another article (Journal of Power Sources, 1998, 75, 230-235) discloses use of DC or pulsed current electrodeposition of electrocatalyst, again without pre-impregnation of the substrate with an ion-conducting polymer.
In certain of the electrodeposition processes described above, the electrode potentials are uncontrolled, and tend to vary at the surface of the electrodes according to the applied current and over time. Depending on the potential, different electrochemical processes and reactions will occur, as they are dependent on the overpotential. For example, some processes will not occur until the overpotential reaches a particular level. Where the potentials are controlled, a voltage is conventionally applied continuously resulting in relatively uncontrolled growth of electrocatalyst clusters during the electrodeposition process.
By controlling the potential and applying a pulsed voltage profile (varying voltage with time) between the electrodes (which is suited to the particular substrate and electrolyte) it is possible to selectively control the processes which will occur during the electrodeposition of an electrocatalyst, and to control the physical deposition of the electrocatalyst.
SUMMARY OF THE INVENTION
In a process for the manufacture of an electrode for a PEM fuel cell or an electrochemical energy converter, an ion-exchange polymer is applied to one face of an electrode substrate. This may be accomplished, for example, by impregnation or coating with a solution of an ion-exchange polymer, or preferably by laminating or bonding the substrate to a pre-formed membrane layer or film. An electrocatalyst is then applied to the substrate by electrochemical deposition, preferably from a solution containing one or more complexes or salts of the electrocatalyst, or alternatively from a melt containing one or more complexes, salts or metals. The electrochemical deposition occurs by application of a voltage between a pair of electrodes (a working electrode which is the electrode under preparation, and a counterelectrode). The voltage between the two electrodes is controlled by controlling the potential of the working electrode. A pulsed voltage profile is applied between the two electrodes during the electrodeposition process. The preferred potentials and voltage profiles are dependent on the nature of the substrate and the solution in which it is immersed.
The electrode substrate material is a porous electrically conductive sheet material, which is typically carbonaceous. Preferably it is flexible, and suitable for processing in a reel-to-reel type process. For example, it may be a paper, a fabric-like tissue, a woven or non-woven material, a felt or cloth or a composite material containing a particulate carbon-filler. Generally the thickness is in the range of about 50-300 &mgr;m.
Using the described method, electrodes which have an extremely low electrocatalyst loading (as low as a few &mgr;g/cm
2
), but which exhibit substantially the same performance as conventional electrodes (with higher loadings), have been prepared. This allows for a substantial reduction in the cost of the electrodes. A further contribution to cost reduction and simplification of the electrode manufacturing method is that the electrocatalyst does not need to be synthesized before being incorporated into the fuel cell electrode.
Through the use of a flexible carbonaceous substrate and a flexible polymer membrane, a durable, single-electrode/membrane assembly may be prepared in a continuous manufacturing process, which is suitable for large-scale production.
Particularly advantageous is the use of H
2
PtCl
6
as an electrocatalyst precursor. The electrocatalyst is preferably electrodeposited from a solution of this platinum complex, which is readily available at reasonable prices.
REFERENCES:
patent: 3767538 (1973-10-01), Politycki
patent: 3847659 (1974-11-01), Sobajima
patent: 4789437 (1988-12-01), Sing
patent: 4876115 (1989-10-01), Raistrick
patent: 5084144 (1992-01-01), Reddy
patent: 5486998 (1996-01-01), Corso
patent: 5527445 (1996-06-01), Palumbo
patent: 5635039 (1997-06-01), Cisar
patent: 5660706 (1997-08-01), Zhao
patent: 5869201 (1999-02-01), Marchetti
patent:
Stäb Gabriele D.
Urban Peter
Ballard Power Systems Inc.
Gorgos Kathryn
Maisano J.
McAndrews Held & Malloy Ltd.
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