Water sorption and diffusion in a short-side-chain perfluorosulfonic acid ionomer membrane for PEMFCS: effect of temperature and pre-treatment

The sorption and diffusion properties of water vapor in a new, short-side-chain perfluorosulfonic acid ionomer (PFSI) membrane have been investigated at different temperatures in the range of 35÷120°C. Experiments show that pretreatment of the membranes under vacuum at low temperatures leaves in the polymer a residual amount of water, which decreases linearly with increasing evacuation temperature. The actual water content in the membrane, instead, is only slightly affected by temperature at fixed activity. For dry membranes, the sorption follows a dual mode behavior, with a downward curvature versus the activity axis. The water diffusivity increases with temperature and when plotted against water concentration shows a maximum. The higher value of the diffusion coefficient is reached at a water content which corresponds to a minimum of the solubility coefficient; such result suggests that different mechanisms take place during sorption of water molecules in PFSI, above and below a critical concentration value, which affect both the solubility and the diffusivity behavior.

The water vapor sorption and diffusion properties into a new, short side chain PFSI membrane have been determined up to 120°C. Drying the membrane at low temperatures leaves residual amounts of water in the matrix that increase linearly with decreasing drying temperature. The solubility isotherms of the dry membranes show a DSM behavior. The actual water solubility in the membrane increases very weakly with temperature. Both diffusivity and solubility data follow two different mechanisms, below and above a concentration value close to the hydration of all SO3H groups by water molecules. Below this value, the solubility increases rapidly with activity and the diffusivity increases very rapidly with concentration; above this value, the solubility increases less strongly with activity and the average diffusivity decreases with water concentration.