T. Okada, N. Nakamura, M. Yuasa, and I. Sekine
[J. Electrochem. Soc., Vol.144, No.8, pp.2744-2750, 1997]

The effect of contamination by Ca²⁺ 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 0.03 kmol m^-3 of HCl/CaCl₂ mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis showed that Ca²⁺ has much higher affinity than H⁺ to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site H₂O/SO₃, 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 H⁺/Ca²⁺ cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca²⁺ content in the membrane increased, especially when the equivalent fraction of H⁺ in the cationic exchange sites x_HM became less than 0.5. Ionic transference numbers for H⁺ in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H⁺ became less than 0.5. Membrane conductivity changed linearly with H⁺ composition in the membrane. In strong contrast to the interaction mode between H⁺ and Ca²⁺ 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 Ca²⁺ 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.