NMR studies of proton transport in fuel cell membranes at sub-freezing conditions

Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including Nafion®-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin–lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafion®-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.