No abstract

A simplified model for water management in a polymer electrolyte membrane ͑PEM͒ fuel cell operating under prescribed current with iso-potential plates is presented. The consumption of gases in the flow field channels, coupled to the electric potential and water content in the polymer membrane, is modeled in a two-dimensional slice from inlet to outlet and through the membrane. Both co-and counter-flowing air and fuel streams are considered, with attention paid to sensitivity of along-the-channel current density to inlet humidities, gas stream composition, and fuel and oxygen stoichiometries. The parameters describing the nonequilibrium kinetics of the membrane/catalyst interface are found to be fundamental to accurate fuel cell modeling. A new parameter which models nonequilibrium membrane water uptake rates is introduced. Four parameters, the exchange current, a membrane water transfer coefficient, an effective oxygen diffusivity, and an average membrane resistance, are fit to a subset of data and then held constant in subsequent runs which compare polarization curves, current density and membrane hydration distributions, water transfer, and stoichiometric sensitivity to the balance of the experimental data.

For widespread exploitation of proton exchange membrane fuel cells (PEMFCs) the cost of the stack must be reduced, and the performance per unit volume increased. Significant cost reduction has been achieved by the development of a high-volume, low cost, electrode manufacturing process and from reductions in the electrode precious metal catalyst loadings. The performance of membrane electrode assemblies (MEAs) employing printed cathodes (0.6 mg Pt/cm 2 ) and anodes (0.25 mg Pt/cm 2 , 0.12 mg Ru/cm 2 ) in Ballard Mark V single-cell and advanced-stack hardware are at least comparable to current stack MEAs comprising high loading unsupported platinum black electrodes containing 4.0 mg Pt/cm 2 . Optimum cell performance has provided high power densities of 0.42 W/cm 2 at 0.7 V Furthermore, under motive and utility test conditions, the low-cost electrodes show minimal loss in performance after over 3000 h of stack operation and, in short and full sized stacks, the cell-to-cell reproducibility is excellent, highlighting the high consistency of product available from the electrode manufacturing process. Incorporation of the low cost electrodes in commercial PEMFC stacks is anticipated in the near future. J.St.-P. wishes to thank the Natural Sciences and Engineering Research Council of Canada for an Industrial Research Fellowship. The authors wish to acknowledge the contributions made by Shanna Knights, Ross Bailey, and other employees of Ballard Power Systems and Malcolm Gascoyne, Jan Denton, and other employees of Johnson Matthey, who have contributed to the development of low cost electrode technology.