Effect of a Proton Conducting Filler on the Physico-Chemical Properties of SPEEK-Based Membranes

Composite membranes based on sulphonated polyetherether ketone (SPEEK) having a 60% degree of sulphonation (DS = 0.6) and containing 23 and 50 wt.‐% hydrated tin oxide (SnO2·nH2O) were prepared and characterised. The lower water uptake (WU) and the higher conductivity values recorded for the composite membranes with respect to pure SPEEK reference suggested the involvement of SnO2·nH2O in the proton conduction mechanism. Pulsed‐field‐gradient spin‐echo (PFGSE) NMR was employed to obtain a direct measurement of water self‐diffusion coefficient in the membranes. Differences were observed between the unfilled SPEEK and the composites, including departures from the normal correlation between water diffusivity and proton conductivity in the case of composites. To better understand the SnO2·nH2O effect on the proton transport properties of the SPEEK‐based membrane, we employed an analytical model that predicts the membrane conductivity as a function of its hydration level and porous structure. The comparison of the model results with the experimental proton conductivity values demonstrated that the tin oxide phase provides additional paths between the water clusters for proton transport, resulting in reduced tortuosity and enhanced proton conductivity. Moreover, the composite showed reduced methanol crossover with respect to the unfilled membrane.