Development of In Situ Formed Metal Pyrophosphates (MP₂O₇, Where M = Sn, Ti, and Zr)/PA/PBI Based Composite Membranes for Fuel Cells

Development of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) at elevated temperatures is important for the enhancement of tolerance toward CO impurities and for the development of non-precious metal catalysts. The key challenge in such HT-PEMFCs is the high temperature polymer electrolyte membranes. Herein, the development of in situ formed metal pyrophosphates (MP₂O₇, where M = Sn, Ti, and Zr) in phosphoric acid doped polybenzimidazole (PA/PBI) composite membranes for HT-PEMFCs is reported. The formation mechanism of MP₂O₇, and characteristics of MP₂O₇/PA/PBI composite membranes are studied in detail. In contrast to the rapid decay in performance of pristine PA/PBI membrane cells, the in situ formed MP₂O₇/PA/PBI composite membranes show significantly higher proton conductivity, improved performance, and stability at elevated temperatures of 200–250 °C. The best results are obtained on the in situ formed SnP₂O₇/PA/PBI composite membrane cells, exhibiting a high peak power density of 476 mW cm⁻² and proton conductivity of 51.3 mS cm⁻¹ at 250 °C. The excellent durability of SnP₂O₇/PA/PBI composite membrane is due to the uniform distribution of in situ formed SnP₂O₇ nanoparticles in PBI membranes and the formation of a gel-like region, thin and irregular amorphous layer on the SnP₂O₇ with the high acid retention ability. This effectively alleviates the PA leaching at elevated temperatures of the new HT-PEMFCs.