Cs₃H₂[BOB(PO₄)₃] - the Best-Performing 1D Borophosphate Intermediate Temperature Proton Electrolyte

The presented family of one-dimensional (1D) polyanionic borophosphates is discussed in the context of a growing class of intermediate temperature electrolytes. The borophosphates are noteworthy for exhibiting high thermal stability under a highly reducing H₂ atmosphere. Here, we report the electrolyte characteristics observed in rubidium borophosphate (Rb₃H₂[BOB-(PO₄)₃]) and cesium borophosphate (Cs₃H₂[BOB-(PO₄)₃]) (a new compound) as newly identified members of the proton-conducting 1D borophosphate polyelectrolyte family. Ab initio molecular dynamics simulations of the compounds suggest extremely high H⁺ mobility correlated with gyrational mobility of the borophosphate chains, which is similar in character to that of previously reported borosulfate proton electrolytes. Particularly noteworthy is the proton conductivity of the Cs₃H₂[BOB-(PO₄)₃] variant, which has the highest conductivity of any of the borophosphates so far, found to be on the order of 10 ^-5 S·cm ^-1 (250 °C and 0.2 atm of water). Cs₃H₂[BOB-(PO₄)₃] was found to be the best-performing 1D borophosphate electrolyte due to the combination of the highest observed proton conductivity, the greatest thermal stability, and attractive mechanical properties. This represents an important advance for intermediate temperature proton conductors and provides a viable path to improve electrolytes for intermediate temperature hydrogen fuel cells.