Overcoming Out‐of‐Plane Conduction by Changing Nanosheet Morphology

ABSTRACT 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 HTaWO 6 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.