Systematic study of back pressure and anode stoichiometry effects on spatial PEMFC performance distribution

摘要

A segmented cell system was applied to investigate the effects of the anode and cathode back pressure and hydrogen stoichiometry on fuel cell performance in terms of overpotential distributions along the flow field. The segmented cell system was designed with closed loop Hall sensors and a data acquisition system allowing simultaneous spatial electrochemical impedance spectra (EIS) measurements. It was determined that an increase in back pressure for the tested serpentine flow field design results in an improvement of the cell performance and uneven improvement of individual segments' performance. In general, the performance and the overpotentials become more uniform downstream with an increase in the back pressure due to a decrease in activation and mass transfer losses. Spatial EIS data for the PEMFC operated at different back pressures support the overpotential analysis. Hydrogen stoichiometry variations do not affect the performance of the cell or the individual segments at low current density because there is no significant hydrogen concentration gradient in the flow field. However, at high current densities a reduction in hydrogen stoichiometry produces a slight decrease in performance for inlet segments while outlet segments showed a noticeable performance loss. The decrease in performance is attributed to an increase in mass transfer losses due to nitrogen diffusion from the cathode to the anode. This effect becomes more pronounced for the outlet segments due to a downstream nitrogen accumulation. Under high current density conditions, the cell is locally fuel starved even with a high fuel stoichiometry creating conditions leading to cell degradation by carbon corrosion. More importantly, this local degradation is masked by the overall cell performance which remains largely unaffected.