Proton Hopping-Induced Fast Proton Transfer through Carbon Nanotubes Revealed by Molecular Dynamics Simulations

Carbon nanotubes (CNTs) have emerged as promising materials for fast proton transport owing to their high mechanical and thermal stability. However, the detailed mechanisms of proton transport in CNTs under an electric field and the influence of the CNT diameter remain unclear. In this work, the electric field-driven proton transfer through CNTs with varying diameters was evaluated using molecular dynamics simulations, along with that in a bulk water system for comparison. Wide CNTs exhibited high proton diffusivity with a relatively low proton flux, like in the bulk, whereas narrow CNTs showed low proton diffusivity with a higher proton flux. Furthermore, narrow CNTs exhibited enhanced directional proton hopping frequency along the CNT axis. These findings reveal that the CNT diameter critically affects the mode and efficiency of proton conduction: narrow CNTs promote hopping-mediated efficient transport, whereas wide CNTs induce bulk-like mass transfer. These insights advance the design of CNT-based proton-conducting membranes for next-generation fuel cells.