Method of making gas diffusion layers for electrochemical cells

Details Method of making gas diffusion layers for electrochemical cells Method of making gas diffusion layers for electrochemical cells

2001-12-19

2002-10-15

Cameron, Erma (Department: 1762)

Coating processes

Heat decomposition of applied coating or base material

Coating decomposed to form carbide or coating carbonized

C427S358000, C427S359000, C427S365000

Reexamination Certificate

active

06465041

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a three-step method of making a gas diffusion layer for an electrochemical cell by combining carbon particles and surfactant in a typically aqueous vehicle, typically by high shear mixing, adding fluoropolymer by low shear mixing, and applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 6,127,059 describes a method of coating a gas diffusion layer for use in an electrochemical cell with a composition made by simultaneously mixing fluororesin and carbon black in water.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) combining a vehicle with carbon particles and one or more surfactants to make a preliminary composition, typically by high shear mixing; b) adding one or more highly fluorinated polymers to said preliminary composition by low shear mixing to make a coating composition; and c) applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
What has not been described in the art, and is provided by the present invention, is a three-step method of mixing and coating a carbon/fluororesin composition which provides the high shear treatment of the carbon particles but avoids high shear treatment of the fluororesin.
In this application:
“vehicle” means a fluid which carries the particulate in a dispersion, which typically includes water or an alcohol;
“highly fluorinated” means containing fluorine in an amount of 40 wt % or more, but typically 50 wt % or more, and more typically 60 wt % or more;
“high shear mixing” means a mixing process wherein the fluid to be mixed encounters zones of shear having a shear rate greater than 200 sec
−1
, and more typically greater than 1,000 sec
−1
, typified by mixing with a high speed disk disperser or Cowles blade at sufficient rpms;
“ultra high shear mixing” means a mixing process wherein the fluid to be mixed encounters zones of shear having a shear rate greater than 10,000 sec
−1
, and more typically greater than 20,000 sec
−1
, typified by bead milling or sand milling at sufficient rpms;
“low shear mixing” means a mixing process wherein the fluid to be mixed does not substantially encounter zones of shear having a shear rate greater than 200 sec
−1
, more typically not greater than 100 sec
−1
, more typically not greater than 50 sec
−1
, and more typically not greater than 10 sec
−1
, typified by paddle mixing, hand stirring, or low-rpm mixing with a high speed disk disperser;
“low shear coating” means a coating process wherein the fluid to be coated does not substantially encounter zones of shear having a shear rate greater than 2000 sec
−1
, more typically not greater than 1000 sec
−1
, more typically not greater than 500 sec
−1
, and more typically not greater than 100 sec
−1
, typified by three-roll coating;
“carbon bleed-through” refers to the presence of carbon particles on an uncoated side of an electrically conductive porous substrate which have migrated through the substrate from a coated side, typically in an amount sufficient to be visible to the naked eye or more; and
“substituted” means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide a method of making a coating composition for coating a gas diffusion layer for use in an electrochemical cell which provides both highly dispersed carbon and highly dispersed fluororesin.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) combining a vehicle with carbon particles and one or more surfactants to make a preliminary composition, typically by high shear mixing; b) adding one or more highly fluorinated polymers to said preliminary composition by low shear mixing to make a coating composition; and c) applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
Fuel cells are electrochemical cells which produce usable electricity by the catalyzed combination of a fuel such as hydrogen and an oxidant such as oxygen. Typical fuel cells contain layers known as gas diffusion layers or diffuser/current collector layers adjacent to catalytically reactive sites. These layers must be electrically conductive yet must be able to allow the passage of reactant and product fluids. Typical gas diffusion layers are coated with a layer of carbon particles and fluoropolymers on the surface adjacent to the catalyst. This invention concerns improvements in coating the carbon/fluoropolymer layer. Specifically, the method according to the present invention provides for high shear treatment of the carbon particles, resulting in increased wetting-out and dispersion, but avoids high shear treatment of the fluororesin which can cause agglomeration.
The coating composition may employ any suitable aqueous vehicle. The vehicle comprises water and may additionally comprise alcohols, and more typically comprises only water or alcohols. Most typically the vehicle comprises water alone.
The coating composition may comprise any suitable surfactant or dispersant, including amine oxide surfactants described in co-pending patent application Ser. No. 10/028,173, filed on even date herewith and incorporated herein by reference. Suitable amine oxides may belong to formula II: R
3
N→O, where each R is independently selected from alkyl groups containing 1-20 carbons, which optionally include ether and alcohol groups, and which may be additionally substituted. Typical amine oxide surfactants according to the cited disclosure are alkyl dimethylamine oxides according to formula (I):
wherein n is 9 to 19 or more typically 11 to 15. Most typically, n is 11 or 13. The amine oxide according to formula (I) is optionally substituted. Suitable amine oxide surfactants may include those available under the trade names Genaminox®, Admox®, Ammonyx®, and Ninox®.
Other suitable surfactants may include alcohol alkoxylates such as Triton™ X100.
The coating composition typically comprises 0.1-15% surfactant by weight, more typically 0.1-10% by weight, and most typically 1-5% by weight.
Any suitable carbon particles may be used. It will be understood that the term “carbon particles” as used herein can refer to primary particles, typically having a average size of 1-100 nm, primary aggregates of primary particles, typically having an average size of 0.01-1 microns, secondary aggregates of primary aggregates, typically having an average size of 0.1-10 microns, and agglomerates of aggregates, typically having an average size of greater than 10 micron. Most typically, the term “carbon particles” refers to primary particles or primary aggregates. Typically a carbon black is used, such as Vulcan XC-72 (Cabot Corp., Special Blacks Division, Billerica, Mass.), Shawinigan Black, grade C55, (Chevron Phillips Chemical Company, LP, Acetylene Black Unit, Baytown, Tex.) or Ketjenblack EC300J (Akzo Nobel Chemicals Inc., Chicago, Ill.). The aqueous coating composition typically comprises 1-50% carbon particles by weight, more typically 1-20% by weight, and most typically 5-15% by weight. Typically, the aqueous coating composition comprises lower weight percent content of carbon particles where smaller particles are used.
The carbon particles are typically suspended in the vehicle by high shear mixing to form a preliminary composition. High shear mixing advantageously provides improved wetting-out of carbon particles with the vehicle as well as improved dispersion and de-agglomeration. In addition, the preliminary composition may be degassed or defoam

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