Accumulation of CO and other fuel impurities in the anode recirculation loop of a fuel cell: A zero dimensional transient model

The use of impure hydrogen in polymer electrolyte fuel cells could potentially improve the economics of their implementation. Here, a zero-dimensional transient model is formulated to simulate the operational effects of fuel impurities such as CH4 and CO within an anode recirculation loop with a continuous small bleed. Inert impurities are observed to accumulate until a steady state concentration is reached where entry and exit rates are balanced, for instance resulting in a 20-fold increase in concentration with a 5% bleed rate. With CO, electro-oxidation of adsorbed CO also contributes to CO removal in addition to that lost by bleed. The overall energy conversion efficiency is found to be limited to similar to 60% of the pure hydrogen value without further mitigation for CO impurity levels of 10-80 ppm. If pulsed oxidation is used as a mitigation method, the efficiency can be improved to similar to 90% of the pure hydrogen value. When air bleed is used as a mitigation method, accumulation of nitrogen limits the efficiency to about 80% of pure hydrogen. While oxygen cross-over from the cathode can prevent CO accumulation when the fuel contains low levels of CO (similar to 1 ppm), it has minimal effect at higher CO concentrations (>10 ppm).