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

Abstract

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−x H 2x (PO 3 ) 3 •yH 2 O, 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 (KO 6 ) and (NiO 6 ) chains and PO 4 tetrahedral chains, retained the rigid framework up to 600 °C. Two oxygen sites of water molecules located adjacent to each other along the PO 4 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 N 2 gas flow, a 4% H 2 -96% Ar gas flow, and an O 2 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 nextgeneration fuel cells.