Phase Separation and Development of Proton Transport Pathways in Metal Oxide Nanoparticle/Nafion Composite Membranes during Water Uptake

Nafion has been extensively used as a proton exchange membrane in the promising energy source of polymer electrolyte fuel cells. However, the phase morphology in Nafion, which basically governs its unique proton transport capability, is very sensitive to the ambient environment. In this work, free volumes as well as phase separations in pristine and hybrid Nafion membranes (Nafion-TiO2 and Nafion-SiO2) during water uptake were studied by the positron annihilation lifetime spectroscopy measurement. Because of the ortho-positronium (o-Ps) annihilation in ionic-water clusters and Nafion skeleton regions, it was attempted to resolve two individual o-Ps lifetime components from the positron annihilation lifetime spectra. Results showed that the development of phases in Nafion membranes as a function of relative humidity was well-correlated with a bimodal o-Ps lifetime distribution. Accordingly, the obviously enhanced proton conductivities in Nafion-TiO2 and Nafion-SiO2 hybrid membranes beyond 6 wt % water content were attributed to the formation of additional ionic-water cluster phases around the hydrophilic SiO2/TiO2 additives in the hybrid Nafion membranes, which reduced the tortuosity of the overall proton transport pathways.