Ion and water transport characteristics in membranes for polymer electrolyte fuel cells containing H{sup +} and Ca{sup 2+} cations
- National Inst. of Materials and Chemical Research, Tsukuba, Ibaraki (Japan)
- Science Univ. of Tokyo, Noda, Chiba (Japan)
The effect of contamination by Ca{sup 2+} 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{sup 3} of HCl/CaCl{sub 2} mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that Ca{sup 2+} has much higher affinity than H{sup +} to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site H{sub 2}O/SO{sub 3}{sup {minus}}, 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{sup +}/Ca{sup 2+} cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca{sup 2+} content in the membrane increased, especially when the equivalent fraction of H{sup +} in the cationic exchange sites x{sub HM} became less than 0.5. Ionic transference numbers for H{sup +} in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H{sup +} became less than 0.5. Membrane conductivity changed linearly with H{sup +} composition in the membrane. In strong contrast to the interaction mode between H{sup +} and Ca{sup 2+} 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 Ca{sup 2+} 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.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 536468
- Journal Information:
- Journal of the Electrochemical Society, Vol. 144, Issue 8; Other Information: PBD: Aug 1997
- Country of Publication:
- United States
- Language:
- English
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