Proton exchange membrane fuel cells: water permeation through Nafion(R) membranes

Water permeation through Nafion® membranes and catalyst-coated membranes are measured. Three types of water permeability measurements are conducted in order to systematically study the effect of the phase of water in contact with the membrane: vapour permeation (termed vapour-vapour permeation), pervaporation (termed liquid-vapour permeation) and hydraulic permeation (termed liquid-liquid permeation). Measurements are taken at 70oC. The largest water permeation flux was observed when the membrane was exposed to liquid water on one side and water vapour at the other, i.e., liquid-vapour permeation. Water permeabilities were found to increase: with increasing differential chemical potential developed across the membrane; with progressive hydration of the membrane; and when the membrane is in contact with liquid water. Water permeability measurements obtained ex-situ are correlated to in-situ fuel cell water balance measurements at 70oC. The back permeation (i.e., water transport from cathode to anode), is largely driven by liquid-vapour permeation, and is sufficient to offset the electro-osmotic drag flux (i.e., proton-driven water transport towards cathode). Ex-situ and in-situ water transport measurements were extended to membranes with thicknesses ranging 6 to 201 μm. Under liquid-liquid permeation condition, water permeation fluxes increased with reduction in membrane thickness; under liquid-vapour and vapour-vapour permeation conditions, water permeation fluxes increased with reduction in membrane thickness but changed little for thickness below 56 μm. Estimation of internal and interfacial water transport resistances revealed that interfacial water transport resistance is dominant for thin membranes – explaining why further increases in liquid-vapour and vapour-vapour permeation fluxes are not observed with decreasing membrane thicknesses below 56 μm. Water permeabilities of catalyst-coated membranes and pristine membranes are found to be similar under all three modes of water permeation. The effect of catalyst layer on membrane water permeation is negligible. In summary, the formation of a membrane/liquid interface is found to enhance the permeability of water through Nafion® membranes. In contrast, presence of a membrane/vapour interface diminishes the rate of water permeation. Under fuel cell operating conditions, when the membrane/liquid interface is formed at the cathode, it is found that a sufficient rate of back permeation effectively regulates the water balance within the fuel cell.