Fundamental studies and perceptions on the spillover mechanism for hydrogen storage.

In this feature article, the atomic-scale understanding of the hydrogen spillover mechanism for hydrogen storage in metal-doped carbon materials and metal-organic frameworks is discussed by critically assessing recent computational and experimental studies. It is argued that the spillover mechanism involves: (a) the generation and desorption of mobile H atoms on the metal nanoparticles (b) the diffusion of H atoms in weakly-bound states on the support (c) the sticking and immobilization of H atoms at preferential locations of the receptor where barriers to sticking are decreased, and, (d) the Eley-Rideal recombination of the adsorbed H atoms with diffusing mobile H atoms to form H(2). The implications and open questions on the mechanism and effectiveness of hydrogen storage by spillover are critically assessed.

Enhanced hydrogen storage by spillover on metal-doped carbon foam: an experimental and computational study.

A lightweight, oxygen-rich carbon foam was prepared and doped with Pd/Hg alloy nanoparticles. The composite revealed high H2 sorption capacity (5 wt%) at room temperature and moderate pressure (2 MPa). The results were explained on the basis of the H2 spillover mechanism using Density Functional Theory.

DFT study of hydrogen storage by spillover on graphite with oxygen surface groups.

DFT modeling was used to understand the role of epoxide (C-O-C) and hydroxyl (C-OH) functional groups on the spillover mechanism for hydrogen storage on graphite oxide and oxygen-modified carbons. A primary spillover model was used, consisting of a Pt(4) cluster, a graphite substrate model, and O and OH functional groups adsorbed on graphite. The spillover mechanism was found to proceed via the migration of dissociated hydrogen atoms from the Pt cluster to epoxide groups adjacent to the cluster (to form OH), followed by H migration by hopping on the adsorbed O atoms. The low energy barriers required for the relevant elementary steps indicate that the spillover process is facile when the carbon substrate is decorated with oxygen functionalities, leading to enhanced hydrogen uptake and faster charge/discharge kinetics. However, a reaction path was also identified, in which surface OH groups can react to form water, which can have adverse consequences for hydrogen storage on oxygenated carbons via spillover.