040

Figure 2: Effect of Nitrogen Dilution on PEM Stack Performance. Polarization curves (constant stoichiometry) for five hydrogen/nitrogen mixtures.

independent. At lower stoichiometry levels, individual cells tend to show unstable performance. The data control system is programmed to abort tests when individual cell voltages reach unac-ceptably low values, thus preserving stack integrity.

Conclusions:

Demonstrated voltage losses with nitrogen and/or carbon dioxide dilution are larger than predicted by the Nernst equation. Operating contemporary stacks with dilute hydrogen mixtures involves voltage penalties, especially at lower stoichiometry values. The exact explanations for these losses remain under investigation and may involve moisture content, formation of surface contaminants, and electrode flooding. Indeed, some performance loss may even result from pore flooding, leading to mass transport limitations within the cathode electrode; however most of the losses are probably within the anode structure. Most likely hydrogen solubility in the Nafion™ solution (encapsulating the Pt catalyst particles) is displaced by nitrogen and carbon dioxide.(4) These data tend to suggest that earlier performance loss noted with C02 may be this result, contrasted to a reverse shift generation of CO.(l) These results point to the need for both improved electrode structures for these fuel processing product mixtures and improved techniques for handling two-phase flow within these stacks. With improved stack designs however, robust and efficient performance should prove technically feasible.

Acknowledgment:

These studies are supported by the Office of Transportation Technology, the United States Department of Energy.

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