Composite nafion/zirconium phosphate fuel cell membranes: Operation at elevated temperature and reduced relative humidity.

摘要

High temperature polymer electrolyte fuel cells are being developed because of expected improvements in the operating tolerance for carbon morioxide (CO) in the hydrogen fuel stream. However, increases in fuel cell operating temperature typically lead to reductions in membrane water content due to evaporation, and the associated increase in membrane resistance decreases power output and thermal efficiency. Modifications to traditional perfluorinated sulfonic acid membranes (such as Dupont Nafion®) can improve the performance of these membranes at higher temperature and reduced relative humidity. The addition of inorganic additives like zirconium hydrogen phosphate (Zr(HPO₄)₂) modifies specific membrane properties relevant for operation under these conditions.; Fuel cell testing of the composite Nafion/zirconium phosphate membranes in both hydrogen and methanol fuel cells demonstrates significantly improved performance over unmodified membranes at high temperature (130–150°C) and dehydrating conditions. To understand the reasons for these membrane improvements in more detail, specific physical and chemical membrane characteristics were studied. The ionic cluster structure of modified membranes and changes upon swelling in water was investigated using small angle x-ray scattering (SAXS). A barometric sorption technique and AC impedance spectroscopy were used to measure equilibrium water uptake and conductivity over a range of relative humidities and temperatures. Finally, water transport measurements and a water flux model were used to investigate the effects of changes to diffusion and evaporative resistances on membrane water content.; When compared to unmodified membranes, Nafion/zirconium phosphate membranes exhibit an increase in water uptake but a decrease in extent of membrane reorganization with water uptake. This change relates to the reduction in membrane chemical potential due to the hydrophilic zirconium phosphate and greater stability of the composite membrane to thermal treatments. Despite these improvements, the proton conductivity and diffusive transport are reduced, due to lower water and proton mobility in the ionic clusters. To explain the discrepancy between the reduced proton conductivity and the improvement in fuel cell performance, a simple water flux model is proposed, which indicates that reducing evaporative flux with respect to the diffusive flux can increase steady state water content and proton conductivity.