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Sustained potential oscillations are experimentally observed in a proton exchange membrane fuel cell with PtRu as anode catalyst and with H 2 /108 ppm CO as the anode feed when operating under a constant current density mode. These oscillations appear at fuel-cell temperatures below 70°C. A threshold value exists for both the current density and the anode flow rate at a given fuel-cell temperature for their onset. The temperature dependence of the oscillation period shows an apparent activation energy around 60 kJ/mol. The potential oscillations are believed to be due to the coupling of anode electro-oxidation of H 2 and CO on the PtRu catalyst surface, on which OH ad is formed more readily, i.e., at lower overpotentials. A simple kinetic model is provided that can reproduce the observed oscillatory phenomenon both qualitatively and quantitatively.

Pt dissolution and further precipitation within the membrane of proton exchange membrane fuel cells (PEMFCs) was investigated at open-circuit voltage conditions. It was found that the location of Pt precipitation is affected by both normalH2 and normalO2 permeability through the membrane. A simple model is developed that can predict the location of the Pt precipitation band as a function of normalH2 and normalO2 partial pressure, which agrees well with measurements. The implications of Pt deposition on membrane chemical degradation are briefly discussed.

A detailed mathematical analysis is performed to understand the anode potential oscillations observed experimentally in a proton exchange membrane fuel cell ͑PEMFC͒ with H 2 /CO feed ͑Ref. 9͒. Temperature and anode flow rate are found to be key bifurcation parameters. The time dependence of all the key surface species must be accounted for in order for the model to predict the oscillatory behavior, while the time dependence of CO concentration in the anode chamber need not necessarily be considered. The bifurcation diagram of CO electro-oxidation rate constant agrees very well with the effect of temperature on the oscillation pattern. The oscillator model is classified as a hidden negative differential resistance oscillator based on the dynamical response of the anodic current and surface species to a dynamic potential scan. A linear stability analysis indicates that the bifurcation experienced is a supercritical Hopf bifurcation.

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