Nanofiber composite membranes with low equivalent weight perfluorosulfonic acid polymers

Two low equivalent weight perfluorosulfonic acid (PFSA) polymers (825 EW and 733 EW) were successfully electrospun into nanofibers by adding as little as 0.3 wt% of high molecular weight poly(ethylene oxide) as a carrier polymer. The electrospun fiber morphology transitioned from cylindrical filaments to flat ribbons as the total concentration of PFSA + carrier in solution increased from 5 wt% to 30 wt%. PFSA nanofiber mats were transformed into defect-free dense membranes using a four-step procedure: (i) annealing the PFSA polymer during which time intersecting fibers were welded to one another at cross points (ii) mechanically compacting the mats to increase the volume fraction of nanofibers to ∼75%, (iii) imbibing an inert polymer, Norland Optical Adhesive (NOA) 63, into the mats (to fill entirely the void space between nanofibers) and then crosslinking the NOA with UV light, and (iv) removing the poly(ethylene oxide) carrier polymer by boiling the membrane in 1.0 M H2SO4 and then in deionized water. The resulting membranes exhibited higher proton conductivities than that of commercial Nafion 212 membrane (0.16 S/cm at 80 °C and 80% relative humidity and 0.048 S/cm at 80 °C and 50% relative humidity for a membrane with 733 EW nanofibers), with low water swelling (liquid water swelling of 18% for membrane with high conductivity). The proton conductivity of both EW nanofiber composite membranes increased linearly with the PFSA nanofiber volume fraction, whereas gravimetric water swelling was less than expected, based on the volume fraction of ionomer. There was a significantly improvement in the mechanical properties of the nanofiber composite membranes, as compared to recast homogeneous PFSA films.