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

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 SnP 2 O 7 varies vigorously due to different fabrication methods, molar ratios of phosphorous to tin and sintering temperature. In this work, a core-shell SnP 2 O 7 (c-SnP 2 O 7) has been fabricated via a conventional acid method between SnO 2 nanoparticles and various phosphoric acids including H 3 PO 4 , H 4 P 2 O 7 , (HPO 3) x and P 2 O 5 under temperature as low as 200 °C. The transformation of SnO 2 to c-SnP 2 O 7 in these phosphoric acids follows dissolving and reprecipitation mechanism where Sn(HPO 4) 2 is the intermediate species. In addition, the dominated factor for the transformation is the presence of P 2 O 7 4− due to the condensation or hydration of various phosphoric acids. The as-synthesized c-SnP 2 O 7 contains crystalline SnP 2 O 7 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-SnP 2 O 7 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 SnP 2 O 7 is a promising proton conductor to be used in elevated temperature fuel cells. [ABSTRACT FROM AUTHOR]