  Recent experimental reports on in-plane proton conduction in reduced graphene
oxide (rGO) films open a new way for the design of proton exchange membrane
essential in fuel cells and chemical filters. At high humidity condition, water
molecules attached on the rGO sheet are expected to play a critical role, but
theoretical works for such phenomena have been scarcely found in the
literature. In this study, we investigate the proton migration on
water-adsorbed monolayer and bilayer rGO sheets using first-principles
calculations in order to reveal the mechanism. We devise a series of models for
the water-adsorbed rGO films as systematically varying the reduction degree and
water content, and optimize their atomic structures in reasonable agreement
with the experiment, using a density functional that accounts for van der Waals
correction. Upon suggesting two different transport mechanisms, epoxy-mediated
and water-mediated hoppings, we determine the kinetic activation barriers for
these in-plane proton transports on the rGO sheets. Our calculations indicate
that the water-mediated transport is more likely to occur due to its much lower
activation energy than the epoxy-mediated one and reveal new prospects for
developing efficient solid proton conductors.
