A proton conductor electrolyte based on molten CsH₅(PO₄)₂ for intermediate-temperature fuel cells

Molten carbonate fuel cells have been commercialized as a mature technology. Due to the liquid electrolyte in molten carbonate fuel cells, gas seal and low contact resistance are easier to achieve than in other fuel cells. Herein, we report an investigation of the viability of a molten oxoacid salt as a novel type of fuel cell electrolyte. In comparison with molten carbonate electrolytes for MCFCs that operate at 500-700 °C, for which a ceramic support matrix is required, the molten proton conductor electrolyte has a lower working temperature range of 150-250 °C. The present study has shown that an electrolyte membrane, in which molten CsH₅(PO₄)₂ is held in a matrix made of PBI polymer and SiO₂ powder, has excellent thermal stability, good mechanical properties, and high proton conductivity. In addition, a molten proton conductor fuel cell equipped with such an electrolyte membrane operating at 200 °C showed an open-circuit voltage of 1.08 V, and a stable output voltage during continuous measurement for 150 h at a constant output current density of 100 mA cm^-2.