Methanol permeability and performance of Nafion-zirconium phosphate composite membranes in active and passive direct methanol fuel cells

Methanol crossover through polymer electrolyte membranes represents one of the major problems to be solved in order to improve direct methanol fuel cell (DMFC) performance. With this aim, Nafion/zirconium phosphate (ZrP) composite membranes, with ZrP loading in the range 1-6 wt%, were prepared by casting from mixtures of gels of exfoliated ZrP and Nafion 1100 dispersions in dimethylformamide. These membranes were characterised by methanol permeability, swelling and proton conductivity measurements, as well as by tests in active and passive DMFCs in the temperature range 30-80 °C. Increase in filler loading results in a decrease in both methanol permeability and proton conductivity. As a consequence of the reduced conductivity the power density of active DMFCs decreases with increasing ZrP loading (from 46 to 32 mW cm⁻² at 80 °C). However, due to the lower methanol permeability, the room temperature Faraday efficiency of passive DMFCs, with 20 mA cm⁻² discharge current, nearly doubles when Nafion 1100 is replaced by the composite membrane containing 4 wt% ZrP.