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

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 KNi1–xH2x(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.