Overcoming Out-of-Plane Conduction by Changing Nanosheet Morphology

The development of solid electrolytes exhibiting fast proton transport remains a central challenge for next-generation fuel cells. Two-dimensional oxide nanosheets have attracted considerable interest as proton-conducting materials; however, their practical implementation remains limited because interlayer barriers severely hinder out-of-plane ion transport. Here, we report a proton-conducting membrane based on HTaWO₆ nanosheets that overcomes this limitation through a unique nanotube morphology formed by spontaneous rolling of the sheets. The resulting membrane exhibits the out-of-plane proton conductivity of 1.7 mS/cm at 80°C, with an activation energy as low as 0.22 eV. Notably, the anisotropy between in-plane (48 mS/cm) and out-of-plane conductivity (1.7 mS/cm) is reduced to only one order of magnitude, significantly smaller than that of conventional nanosheet assemblies. When implemented as a fuel-cell electrolyte, the membrane delivers a maximum current density of 1.26 A cm^-2 and a power density of 294 mW cm^-2 at 80°C. These results demonstrate that curvature-induced topology in oxide nanosheets represents a powerful strategy to enable efficient cross-plane ion transport, opening new avenues for high-performance solid electrolytes based on two-dimensional materials.