ION AND WATER TRANSPORT CHARACTERISTICS OF NAFION MEMBRANES AS ELECTROLYTES

Transport characteristics of Nafion membranes, that have been publishe d earlier, are re-evaluated. It is found that the specific conductivit y of the membranes is not only determined by the mobility of the ions, but largely also by the interaction of ions with water and with micro scopic membrane channel structures. Similarly, the water transference coefficient, defined as the number of moles of water transported per F araday through the membrane, is governed by two effects: an electrosta tic effect between ion and water dipoles, and an effect due to the siz e of the cation. Contributions to electro-osmotic water transfer are w ater of hydration to cations and hydrodynamically pushed water molecul es. The size of the ion compared to the channel diameter, has a major impact on the electric conductivity, but also on water transport. It i s shown that hydrophilic cations can promote an enlarged hydrophilic d omain in the membrane, that is accompanied by a lower membrane resista nce. Criteria for designing high performance ion conducting membranes are given based on this basis. (C) 1998 Elsevier Science Ltd. All righ ts reserved.