Testing of Polymer Film–Sulfonated Polystyrene Proton-Exchange Composite Membranes in a Direct Methanol Fuel Cell at 60°C. Methanol Crossover

The coefficients of diffusion permeability of methanol through the synthesized polymer film–sulfonated polystyrene composite membranes and a Nafion-115 membrane are measured. For several composite membranes with significantly different transport properties, the values of the diffusion flux of methanol qdiff through these membranes under the conditions of a direct methanol fuel cell (DMFC) at 60°C and a concentration of the feed solution of 1–2 M are calculated. Direct measurements of the crossover current and methanol crossover qCVA in a DMFC based on these membranes are carried out by cyclic voltammetry (CVA). It is found that the values of qCVA are on average by 15% lower than the corresponding values of qdiff calculated for each membrane based on its individual parameters (area, thickness, permeability coefficient of methanol). It is proposed to explain the observed ratio qCVA < qdiff by the experimentally uncontrolled and, probably, incomplete oxidation of methanol at the cathode. It can be concluded based on the obtained data that the experimental values of the crossover qCVA can noticeably differ from calculated qdiff and real values of methanol crossover in a DMFC without monitoring the degree of oxidation of methanol at the DMFC cathode. A comparative study of the current–voltage characteristics of DMFCs based on the synthesized composite membranes with significantly different transport properties and a Nafion-115 membrane is carried out. It is found that, at 60°C and a concentration of the feed solution of 1 M, the value of methanol crossover has practically no effect on the current–voltage characteristics of the DMFCs.