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

Abstract

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.