Abstract
Composite membranes composed of modified S or Si–zirconium phosphate (ZrP), porous polytetrafluoethylene (PTFE) and, glycerol (GLY) were synthesized in this work. ZrP was precipitated via the in-situ reaction of zirconium oxychloride (ZrOCl2) with phosphoric acid (H3PO4). Silicic acid and sulphuric acid were introduced as additives to phosphoric acid with variable Si/P or S/P mass ratios in the acid solution/suspension. The modified membranes were investigated as electrolytes for direct hydrocarbon fuel cells operating at temperatures ∼200 °C. The present work shows that adding a small amount of silicic acid to phosphoric acid enhanced the proton conductivity, i.e. by having a Si/P mass ratio of (0.01) in the acid solution/suspension, the Si–ZrP/PTFE/GLY membrane conductivity was 0.073 S cm−1. This value approached the conductivity of Nafion (0.1 S cm−1). The results also showed that adding sulphuric acid to phosphoric acid led to a significant decrease in the membrane's proton conductivity. SEM analysis results showed a porous structure of the Si–ZrP modified membranes. This porous structure combined with the high proton conductivity reported, would make them good candidates for catalyst layer supports in fuel cell applications.
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