Tunnel-Structured Phosphate Exhibiting High Proton Conductivity and Thermal Stability over a Wide Intermediate...

For the practical application of fuel cells in vehicles, it is a challenge to develop a proton solid electrolyte that coexhibits thermal stability and high proton conductivity at wide intermediate temperatures. Here, we report on the tunnel structured phosphate KNi_1-H₂(PO₃)₃·yH₂O, which exhibits high proton conductivity at room temperature up to 500 °C, with the conductivity value reaching 1.7 × 10^-2 S cm^-1 at 275 °C for x = 0.18. This material, composed of the smallest cations that form the tunnel framework with face-shared (KO₆) and (NiO₆) chains and PO₄ tetrahedral chains, retained the rigid framework up to 600 °C. Two oxygen sites of water molecules located adjacent to each other along the PO₄ tetrahedral chains in the tunnel provided the proton conduction pathway. The sample maintained a conductivity of 5.0 × 10^-3 S cm^-1 for 10 h at 150 °C while changing the measurement atmosphere to a N₂ gas flow, a 4% H₂-96% Ar gas flow, and an O₂ gas flow. The conductivity value at x = 0.18 obtained from the DC measurement was in the order of 10^-6 S cm^-1, close to the instrument's measurement limit. These results demonstrate that tunnel phosphate has potential as a proton solid electrolyte for next-generation fuel cells.