Ion and Water Transport Characteristics in Membranes for Polymer Electrolyte Fuel Cells Containing H + and Ca2 + Cations

Abstract

The effect of contamination by ions in proton conductive membranes for polymer electrolyte fuel cells was investigated systematically. Ion and water transport characteristics of Nafion membranes, which were equilibrated with 0.02 to of mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that has much higher affinity than to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site , decreased about 19% from 21 for H‐form membrane to 17 for Ca‐form membrane. The water transference coefficient was obtained from streaming potential measurements of Nafion 115 membranes of various cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the content in the membrane increased, especially when the equivalent fraction of in the cationic exchange sites x _HM became less than 0.5. Ionic transference numbers for in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by became less than 0.5. Membrane conductivity changed linearly with composition in the membrane. In strong contrast to the interaction mode between and cations during ionic conduction, which appeared almost independent, a certain extent of interference was observed among water molecules as they were carried along by cations in the membrane. It was predicted that if ions enter the fuel‐cell membrane, they cause serious effects to membrane drying and result in deterioration of fuel‐cell performance. The advantage of this methodology in the study of transport characteristics of fuel‐cell membranes is stressed due to ease and accuracy of measurements.