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 q_diff 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 q_CVA in a DMFC based on these membranes are carried out by cyclic voltammetry (CVA). It is found that the values of q_CVA are on average by 15% lower than the corresponding values of q_diff calculated for each membrane based on its individual parameters (area, thickness, permeability coefficient of methanol). It is proposed to explain the observed ratio q_CVA < q_diff 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 q_CVA can noticeably differ from calculated q_diff 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.