Anomalous Proton Blocking Property of Pore‐Free Graphene Oxide Membrane

Abstract Graphene oxide (GO) has been attracting intensive attention as a flexible barrier film, however, provides no barrier for proton transfer due to its out‐of‐plane proton conductivity (10 ⁻⁶ S cm ⁻¹ ) based on nanoscale defects with oxygen functional groups. In this study, it is reported that a pore‐free GO (Pf‐GO) membrane with controlled oxygen functional groups exhibits unexpected proton blocking behavior (10 ⁻¹¹ S cm ⁻¹ ). Proton permeation tests conducted in aqueous solution demonstrate that proton permeation is below the detection limit, and lithium metal foils coated with the Pf‐GO show higher chemical stability to water than those coated with previously reported GO. The Pf‐GO has periodic honeycomb atomic structure, which is found to impart the Pf‐GO membrane with novel performance characteristics distinct from those of conventional GO.