There is great enthusiasm for the development of high power density fuel cell systems for defense and civilian applications. Taking into consideration the main causes for efficiency losses--activation, mass transport and ohmic overpotentials--the only fuel cell systems capable of achieving high power densities are the ones with alkaline and solid polymer electrolyte. High power densities (0.8 W/cm2 at 0. Even higher power densitiesHistorically, the first type of fuel cell system to find a major application (auxiliary power source in the Gemini Space Vehicles) is the one with solid polymer electrolyte as the electrolyte. These fuel cell systems were developed by the General Electric Company. Since the sixties, great strides have been made in increasing the power density from 50 mW/cm2 to about 2-3 W/cmz. The latter performance was achieved at Ballard Technologies Corporation which used practically the same technology as General Electric CompanylUnited Technologies Corporation--Hamilton Standard. The platinum loading in these fuel cells is 4 mg/cm2 on each electrode.Research and Development at Los Alamos National Laboratory and in our laboratory has led to the attainment of high power densities (zl watt/cm2> in solid polymer electrolyte fuel cells with ten times lower platinum loading in the electrodes (i .e., 0.4 mg/cm2>. used to attain these goals are as follows:The necessary criteria and the methods (1) Extension of the three dimensional reaction zone by the impregnation of a proton conductor (i.e., the ion-exchange membrane) into the electrode structure. Hot pressing of the proton-exchange membrane and electrodes at a temperature above the glass transition temperature and at a pressure of 50 atm. Adequate humidification of the reactant gases by passing these gases through humidification chambers set at temperatures of 10°C for H2 and 5OC for oxygen or air higher than the cell temperature. Enhancement of the electrode kinetics of the hydrogen oxidation and the oxygen reduction reactions and particularly of the mass transport rates of the reactant gases to the electrode by operation at elevated temperatures and pressures, say 95OC and 5 atm. Localization of the platinum near the front surface of the electrode to reduce the thickness of the active layer and provide a higher concentration of platinum sites on the front surface to reduce mass transport and ohmic overpotentials within the porous electrode and at the electrodelelectrolyte interface.( 2 )
Polymer electrolyte membrane based direct methanol fuel cells (PEM-DMFC) have a complicated system design, and their fuel efficiency is limited by methanol cross-over through the proton exchange membrane. In addition, the methanol oxidation kinetics as well as oxygen reduction kinetics are quite slow in the acid electrolyte system. In contrast to acid DMFCs, it is known that electro-oxidation (and oxygen electro-reduction) reactions are improved in basic media. Alkaline DMFCs, unlike their acid counterparts, do not require MEAs, presently the most costly and complex stack component. It is however unknown if alkaline DMFC could be used as PEM alternatives for portable power applications. The technical challenge then, is to develop AFC stacks that can be used to fabricate compact low-power (20 W) systems that have sufficient resilience to carbonate build-up inherent in alkaline cells. In addition to carbon dioxide from ambient air, the methanol fuel is an additional source of electrolyte carbonation (CO2 generated in situ). For portable power applications, one way to circumvent this issue is to replenish the spent alkaline electrolyte on a regular maintenance schedule. This paper reports the results of some preliminary work carried out to investigate the viability of alkaline DMFC for portable power application.
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