The influence, on membrane nanostructure and properties, of water or ethanol as swelling solvent of sulfonated poly(ether ether ketone) (SPEEK) and zirconia-SPEEK hybrid membranes prepared using the sol-gel process has been investigated. Independent of the solvent, small-angle X-ray scattering measurements reveal the existence of a two-level hierarchical structure in SPEEK of greater sulfonation level, consisting of solvent-swelled spatially correlated primary SO 3 H-rich ionic clusters of around 15 Å in size, forming larger secondary aggregates well dispersed in the PEEK matrix. The size of the primary nanodomains and the connectivity between domains are determining parameters for protonic conductivity, solvent swelling ability and permeability of the membranes. For both SPEEK and zirconia-SPEEK membranes containing ethanol, the pronounced affinity of ethanol molecules with SPEEK leads to an increase in the size of ionic clusters and of the number of connecting channels between clusters compared to membranes containing water. This promotes solvent swelling and proton conductivity. The increase in permeability to water induced by incorporation of ethanol in place of water in both polymeric and hybrid membranes is less in the hybrid membranes. This result suggests that the potential use of zirconia-SPEEK hybrid membranes in direct alcohol fuel cells is more promising than that of pure SPEEK, due not only to the less probable alcohol and water penetration in the membranes associated with their lower permeability, but also to the fact that an eventual penetration of alcohol in hybrid membranes should reduce the risk of cathode flooding compared to zirconia-free SPEEK membranes.

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