Monday, November 30, 2009

Asahi Glass Carbon Nanotube Fuel Cell Membrane Solves Interlayer Flooding Problem

Asahi Glass Company, Limited (Chiyoda-ku, JP) developed a fuel cell membrane using carbon nanotubes that exhibits superior performance in both humid and low humidity environments. Solid polymer electrolyte fuel cells are required to be operated under a low humidity environment, primarily in their use for automobiles. The nanocarbon material exhibits high proton conductivity in a low humidity environment for a proton-conductive polymer used in a fuel cell membrane electrode assembly, according to U.S. Patent Application 20090291345. Carbon nanofiber is particularly preferred since it is fine and has excellent electron conductivity. The carbon nanofiber may be a vapor-grown carbon fiber, carbon nanotube single wall, double wall, multiwall or cup-laminated type.

By providing an interlayer composed mainly of carbon fiber between the catalyst layer and the gas diffusion layer, water is readily transferred by a capillary phenomenon from the catalyst layer to the interlayer whereby a flooding problem during the operation of the polymer electrolyte fuel cell may readily be solved say inventors Satoru Hommura, Shinji Kinoshita, Hiroshi Shimoda, Susumu Saito, Seigo Kotera, Tetsuji Shimohira and Hideki Nakagawa.

The fiber diameter of the carbon fiber is preferably from 50 to 200 nm, and the fiber length is preferably from 1 to 50 microns. By using such carbon fiber, it entangles with an electron-conductive material (platinum or a platinum alloy, and a carbon carrier) contained in the catalyst layer at the interface between the interlayer 1-84 and the catalyst layer to form a new electroconductive path in addition to an electroconductive path by a point contact with the electroconductive material, whereby the electron conductivity of the catalyst layer will be improved. Further, such carbon fiber is likely to be entangled to form void spaces at the time of applying a coating fluid containing the carbon fiber, whereby such void spaces will function as a gas channel.

The ratio of the carbon material to the proton-conductive polymer (carbon material/proton-conductive polymer) is preferably from 1/0.1 to 1/5 (mass ratio), more preferably from 1/0.2 to 1/1. Within such a range, the dispersibility of the carbon material, the adhesion between the interlayer and the gas diffusion layer, and the gas diffusing property and water-discharging property of the interlayer will be good.

In actual driving of an automobile having a polymer electrolyte fuel cell mounted, the polymer electrolyte membrane will be exposed to various humidity environments. As a test to simulate such a situation, a cycle test (hereinafter referred to as a moistening/drying cycle test) has been proposed wherein the polymer electrolyte membrane is repeatedly exposed to each of a dried environment with a relative humidity of at most 25% and a moistening environment with a relative humidity of 100%. In such a moistening/drying cycle test, the polymer electrolyte membrane swells in the moistening environment and shrinks in the dried environment and thus undergoes swelling and shrinkage repeatedly along with moistening and drying cycles and thus undergoes a dimensional change especially in a planar direction. Therefore, by the moistening/drying cycle test, it is possible to evaluate the mechanical durability of the membrane/electrode assembly in an environment where moistening and drying are repeated.

In order to improve the proton conductivity of the proton-conductive polymer, the ion exchange capacity may be increased. However, if the ion exchange capacity is increased, the water content of the proton-conductive polymer will increase. If the water content of the proton-conductive polymer to be used for the catalyst layer becomes too high, the proton-conductive polymer will swell and clog void spaces in the catalyst layer, thus leading to a problem of a so-called flooding phenomenon. If the flooding phenomenon takes place, the diffusion of the gas supplied to the catalyst layer decreases, whereby the power generation performance of the polymer electrolyte fuel cell will be substantially deteriorated. Asahai Glass says their membrane solves the flooding problem.

An iRAP study identified 3,870 organizations involved in fuel cells, hydrogen energy and related nanotechnology who earned and spent an estimated $8.4 billion in 2008.

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