We first study the static, fully optimized structures with single proton (H + ) or protium (H atom) confined in the interstitial space of layered crystals of h-BN, MoS 2 , and graphite. Details of the simulations are shown in Methods section and SI (Figures ). A proton located in the interstitial space of layered h-BN binds to a N atom, forming a N-H bond with a bond length of 1.05 Å, and locally slightly decreases interlayer separation. On the contrary, a H atom forms a B-H bond of 1.32 Å length and slightly increased interlayer separation. Details are shown in Figure and Table in SI. Due to the additional fourth bond, which forms upon binding H + or H, both N and B atoms transform from sp 2 to sp 3 hybridization and thus pyramidalize to approach a tetrahedral configuration. In MoS 2 , proton (or H atom) connects to a S atom, forming a S-H bond of 1.37 (1.43) Å, and also slightly decreases (increases) locally the interlayer distance. For metallic graphite, proton immediately neutralizes once it enters into the interlayer space, and binds to a C atom, forming a C-H bond of 1.12 Å, and the interlayer distance increases due to the pyramidalization of the binding carbon atom, similar to the case of h-BN. Considering the experimental setup of Hu et al., we estimate the total transfer energies of H + or H from the hydrogenated Pd side electrodes and the layered crystals. The transfer reactions