Enhancement of Proton Conductivity in Perfluorosulfonic Acid Membranes with the Addition of Hydrophilic Fillers.

The present work demonstrates an improvement in proton conductivity at 80 °C as a result of the addition of a hydrophilic filler (carboxymethyl cellulose nanofibers, CMCNFs) to Nafion composite membranes. The composite membranes (CMCNF/Nafion) prepared by die coating are translucent and flexible, without pores or cracks. The proton conductivity of the CMCNF/Nafion membrane reaches 0.165 S cm-1 at 80 °C and 80% RH. The water retention of the membranes, based on DSC measurements, indicates that the amount of nonfreezing water (water that does not freeze down to -55 °C) is independent of the addition of CMCNF, but the amount of capillary water (water that melts between -55 and 0 °C) increases with CMCNF addition. We considered that the presence of capillary water at the interface between CMCNFs and Nafion would be one of the essential factors that improve the proton conduction in the electrolyte. The small-angle X-ray scattering (SAXS) profiles prove that the size of the hydrophobic domain water clusters in Nafion becomes smaller with the addition of CMCNF, while the water content remains the same. The microstructure of the CMCNF/Nafion composite membrane and the behavior of the water content (capillary water) would be one possible contribution to show the improvement of proton conductivity.