Introduction

The PEM fiiel cell is promising as the power source for use in mobile and stationary applications primarily because of its high power density, all solid components, and simplicity of operation. For wide acceptability of this power source, its cost has to be competitive with the presently available energy sources. The fuel cell requires continuous humidification during operation as a power source. The humidification unit however, increases fuel cell volume, weight, and therefore decreases its overall power density. Great advantages in terms of further fuel cell simplification can be achieved if the humidification process can be eliminated or minimized. In addition, cost reductions are associated with the ease of manufacturing and operation.

At BCS Technology we have developed a technology of self-humidified operation of PEM fuel cells based on the mass balance of the reactants and products and the ability of membrane electrode assembly (MEA) to retain water necessary for humidification under the cell operating conditions. The reactants enter the fuel cell chambers without carrying any form of water, wheather in liquid or vapor form. Basic principles of self-humidified operation of fuel cells as practiced by BCS Technology, Inc. have been presented previously in literature [1, 2, 3]. Here, we report the operation of larger self-humidified single cells and fuel cell stacks. Fuel cells of areas up to 100 cm2 have been operated. We also show the self-humidified operation of fuel cell stacks of 50 and 100 cm2 electrode areas.

Attempts to introduce self-humidification to the fuel cell led to the introduction of the special membrane that was recast from the solubilized PEM electrolyte in the presence of Si02 or Ti02 by Watanabe et al. [4]. This membrane was modified by deposition of Pt film (approx. 0.1 mg-cm'2) inside a bulk of the membrane via cation exchange followed by reduction. The self-humidification mechanism was based on the ability of such membrane to transport reactants, both hydrogen and oxygen, which then recombined on the embedded Pt film. The water, product of this reaction, directly humidified the membrane. Such mode of operation required very high stio-chiometries of reactants, 5 and 10 for hydrogen and air, respectively. Other works on self-humidified operation of the fuel cell concentrated on conveying water from feed point to the membrane itself by introduction of some kind of transport mechanism. Cisar et al. [5] developed a membrane cast in the presence of special micro-tubing. The embedded micro-tubes provided internal water passages providing water for membrane humidification. Similar approach, also based on direct addition of water to the membrane was developed by Watanabe et al. [6],

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