Improving selectivity in methanol fuel cell membranes: A study of a polymer-zeolite composite membrane

Direct methanol fuel cells require membranes with the dual properties of high proton conductivity and low methanol crossover. New membranes need improved selectivity: i.e., a higher ratio of proton conductivity to methanol permeability. The approach taken in this research involves a proton conducting polymer membrane loaded with proton conducting, methanol impermeable zeolites. In this scenario, protons travel a direct path through both the polymer and zeolite phases, while methanol has a more tortuous path around the zeolite particles.; The composite membranes consisted of mordenite particles embedded in a PVA matrix. The hydrophilic nature of both materials prevents the formation of non-selective voids at the PVA-mordenite interface. These membranes were tested for both methanol permeability and proton conductivity. Methanol permeability was determined using a diaphragm diffusion cell interfaced with a differential refractometer for tracking concentration change. Proton conductivity was measured in the traverse direction of the membrane using a two-point probe technique. Composite membranes, consisting of 50% mordenite by volume, represent up to a 20-fold improvement in selectivity over Nafion.; The improved behavior is a result of the proper tailoring of diffusion properties for methanol and protons between the polymer and dispersed phase. Predictions using Maxwell's theory for diffusion in composite media are in good agreement with the experimental selectivity values. Thus, the experimentally determined increase in selectivity, correlated with simple membrane theory, demonstrates the feasibility of the composite membrane approach for direct methanol fuel cell membranes.