Fast In-Plane Proton Transport in H–Nb ₃ O ₈ Nanosheets

Proton transport in solid oxides is well established at elevated temperatures; however, achieving high proton conductivity below 100 °C remains challenging because rigid oxide lattices rarely form continuous proton-transfer networks. Here we demonstrate that layered H–Nb 3 O 8 nanosheets enable fast in-plane proton transport without conventional defect engineering. A highly oriented H–Nb 3 O 8 nanosheet membrane exhibits an in-plane proton conductivity of 0.14 S cm –1 at 55 °C. Under humidified conditions, the oriented nanosheet membrane incorporates water molecules into the interlayer space, forming hydrated nanochannels that provide continuous hydrogen-bonded networks for proton transport. Temperature-dependent XRD measurements indicate that this hydrated interlayer structure changes with increasing temperature, and this structural change appears to correlate with the temperature dependence of proton conductivity. In addition to the hydrated interlayer nanochannels, the Nb 3 O 8 framework contains quasi-one-dimensional corrugated oxygen rows formed by corner-sharing NbO 6 octahedra. Bond-valence-based energy calculations identify this oxygen arrangement as a favorable proton migration pathway with a low diffusion barrier of 0.082 eV, and density functional theory calculations further support the presence of low-energy proton migration paths within the Nb–O framework. The intrinsic in-plane proton transport is directly utilized at the device level: a H 2 /O 2 fuel cell employing the H–Nb 3 O 8 membrane as an in-plane electrolyte delivers a short-circuit current density of 820 mA cm –2 at 55 °C. These results suggest that fast proton transport in layered oxide nanosheet membranes is enabled by hydrated interlayer nanochannels, with framework-associated oxygen arrangements providing additional low-energy pathways that may assist proton migration.