Transport properties of ionomer composite membranes for direct methanol fuel cells

The present study investigates the transport properties of composite Nafion®-based membranes for direct methanol fuel cells (DMFC). The proton conductivity, the methanol permeation rate, and the relative water permeation rate are investigated and the influence of the thickness of the membrane on these properties is determined. The recast membranes, containing 4.3% Aerosil A380 (silicon dioxide), comprising five different values of the thickness between 0.05 and 0.42 mm are studied. It is found that the membranes, filled with silica powder exhibit an increased water uptake and also an increased conductance at 90°C compared with Nafion® 112, 115 and 117. The permeation experiments reveal similar values of methanol permeation rates of composites and of the commercial membranes. Both the conductance and the methanol permeation rate show a significant decrease with increasing thickness, so that an optimisation of this parameter regarding favourable properties is difficult. The combined parameter of methanol permeation rate and conductivity suggests improved behaviour of the modified membranes at 65 C.