Phosphate induced formation of core-shell tin pyrophosphate via an acid method for proton conduction at elevated temperatures

Abstract

Tin pyrophosphate is a feasible solid-state proton conductor as electrolyte and ionomers for applications in fuel cells and sensors, etc. Nevertheless, the morphology and proton conductivity of the SnP2O7 varies vigorously due to different fabrication methods, molar ratios of phosphorous to tin and sintering temperature. In this work, a core-shell SnP2O7 (c-SnP2O7) has been fabricated via a conventional acid method between SnO2 nanoparticles and various phosphoric acids including H3PO4, H4P2O7, (HPO3)x and P2O5 under temperature as low as 200 °C. The transformation of SnO2 to c-SnP2O7 in these phosphoric acids follows dissolving and reprecipitation mechanism where Sn(HPO4)2 is the intermediate species. In addition, the dominated factor for the transformation is the presence of P2O74− due to the condensation or hydration of various phosphoric acids. The as-synthesized c-SnP2O7 contains crystalline SnP2O7 inner core, an amorphous phosphate intermediate layer and an outer-layer with gel-like phosphorous-rich species. Furthermore, the phosphorous-rich gel layer contributes to outstanding proton conductivity of c-SnP2O7 up to 8.0 × 10−2 S cm−1 at 260 °C and excellent durability of 115 h at 240 °C and anhydrous conditions. Overall, the core-shell SnP2O7 is a promising proton conductor to be used in elevated temperature fuel cells.