Significant enhancement of proton conductivity in solid acid at the monolayer limit

Proton transport in nanofluidic channels is not only fundamentally important but also essential for energy applications. Although various strategies have been developed to improve the concentration of active protons in the nanochannels, it remains challenging to achieve a proton conductivity higher than that of Nafion, the benchmark for proton conductors. Here, taking H₃Sb₃P₂O₁₄ and HSbP₂O₈ as examples, we show that the interactions between protons and the layer frameworks in layered solid acid H_nM_nZ₂O_3n+5 are substantially reduced at the monolayer limit, which significantly increases the number of active protons and consequently improves the proton conductivities by ∼8 ‒ 66 times depending on the humidity. The membranes assembled by monolayer H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheets exhibit in-plane proton conductivities of ~ 1.02 and 1.18 S cm^-1 at 100% relative humidity and 90 °C, respectively, which are over 5 times higher than the conductivity of Nafion. This work provides a general strategy for facilitating proton transport, which will have broad implications in advancing both nanofluidic research and device applications from energy storage and conversion to neuromorphic computing.