Core-shell tin pyrophosphate-based composite membrane for fuel cell with durability enhancement at elevated temperatures

Abstract

Tin pyrophosphate has been employed as a proton conductor in polymer electrolyte membranes (PEMs) at temperatures over 200 °C. However, neither the aggregation of the SnP2O7 nanoparticles nor the proton conduction mechanism of the material has been clearly clarified in PEM. The present work reports a polybenzimidazole (PBI)/SnP2O7 composite membrane with homogeneous distribution of the inorganic phase. The membrane is successfully fabricated by in-situ transformation of SnO2 nanoparticles to core-shell SnP2O7 nanoparticles in the PBI matrix during fuel cell operation. Further in-depth analysis shows that the core-shell SnP2O7 contains a crystalline SnP2O7 core, an amorphous SnP2O7 intermediate layer and a gel-like outer layer mainly containing H4P2O7. In addition, the crystal growth of the SnP2O7 nucleus consumes ions from the amorphous SnP2O7 layer, where the reacted ions are compensated by the diffusion of Sn4+ and P2O74− ions from the gel-like outer layer. Also, the fuel cell with the composite membrane shows superior proton conductivity and durability compared to the pristine phosphoric acid-doped PBI membrane at 240 °C for over 50 h. That is due to the presence of H4P2O7 in the outer layer, which contributes to high and stable proton conductivity of the core-shell SnP2O7 at 220–260 °C.