Characterization of gas crossover and its implications in PEM fuel cells

With pure hydrogen as the fuel, PEM fuel cell operation at or near 100% fuel utilization is desirable to achieve a high stack efficiency and zero emissions. However, typical membranes used in PEM fuel cells allow a finite amount of permeation rates or crossover of hydrogen, oxygen, and nitrogen across the membrane. The hydrogen and oxygen that permeate through the membrane are consumed with the generation of heat and water but without the generating of useful work, leading to a fuel inefficiency. Nitrogen crossover, on the other hand, from the cathode side to the anode side accumulates at the exit of the anode flow fields, lowering the hydrogen concentration and resulting in local fuel starvation. In this study, an in-situ electrochemical technique has been applied to determine the magnitude of the hydrogen crossover over a range of relevant fuel cell operating temperatures and pressures. Permeability coefficients thus obtained are compared to values reported in the literature. A mathematical model is developed to predict the extent of nitrogen accumulation along the anode flow fields, and fuel recycle as a mitigation method is simulated by improving hydrogen distribution. The model results were validated by comparison with experimental results. (c) 2006 American institute of Chemical Engineers.