Proton transport in the short side chain perfluorosulfonic ionomer membranes

The influence of ion exchange capacity (IEC) on the water sorption properties of high IEC, short side chain (SSC) perfluorosulfonic acid (PFSA) ionomer membranes, and the relationships between water content, proton conductivity, proton mobility, water permeation, and oxygen diffusion were investigated. Transport properties of SSC ionomer membranes were compared with a series of long side chain (LSC) PFSA membranes. At 25 °C, fully-hydrated SSC ionomer membranes were characterized as possessing higher water contents, moderate λ values, high analytical acid concentrations, and moderate conductivity than LSC PFSA membranes; but lower than anticipated effective proton mobility. Complementary measurements of water permeability and oxygen diffusion also revealed lower than expected values given their much higher water contents than LSC analogues. Potential benefits afforded by reducing the side chain length of PFSA ionomer membranes, such as increased crystallinity, higher IEC, and high hydrated acid concentration were diminished by a less-developed, disorganized hydrophilic percolation network, which provides a motivation for future improvements of transport properties for this class of material.