Proton-shuttling nanosheet membranes enable high-power-density protonic fuel cells.

High-temperature operation enhances the efficiency and design simplicity of electrochemical devices, but conventional polymer membranes lose proton conductivity rapidly due to dehydration. Atomically thin nanosheets can selectively transport thermal protons through nanoscale corrugations and quantum tunneling, making them promising for high-temperature proton-conducting membranes. However, stacked nanosheet assemblies often suffer from poor proton transport between layers. We built nanosheet-based membranes by bridging individual nanosheets using nanoconfined phosphoric acid. This architecture enables low-tortuosity, synergistic proton transport via both through-nanosheet conduction and hydrogen bond-mediated hopping along confined acid layers, resulting in ultrafast, stable proton conduction under anhydrous high-temperature conditions. A polyethylenimine-functionalized graphene/boron nitride bilayer membrane achieves a proton conductivity of 166 millisiemens per centimeter and delivers a power density of 1011 milliwatts per square centimeter in hydrogen fuel cells at 250°C, outperforming most previously reported anhydrous proton-conducting membranes. Furthermore, it exhibits superior methanol tolerance, achieving 502 milliwatts per square centimeter on concentrated methanol. This work offers a versatile platform for next-generation high-temperature proton-conducting membranes.