DFT insights into Cu-driven tuning of chemisorption and physisorption in the hydrogen storage by SnC monolayers

Abstract

A Density-Functional-Theory investigation about the effect of decorating a hexagonal SnC monolayer with Cu on the H2 storage was performed. The feasibility for capturing and releasing a H2 molecule was inferred by means of its degrees of chemisorption and the complementary physisorption, respectively determined through the non-dispersion and dispersion contributions to the adsorption energy, and correlated with the formed-bonds strength as measured by the Mayer bond order. The results confirm that for the reference case of C-pure graphene there is only physisorption, whereas it is predicted that in contrast to graphene, the pristine SnC monolayer performs as a material for hydrogen storage due the H2 molecule is equally chemi- and physisorbed. Likewise, Cu-decorated SnC monolayer strongly chemisorbs one H2 molecule without dissociation at the Cu site, substantially enhancing the hydrogen storage functionality. However, Cu also induces a weaker chemisorption than in the pristine SnC at the C sites around it because its unpaired electron inhibits the C's electron acceptor character. The above results indicate chemisorption drives the H2-storage performance, as following described. Null chemisorption causes the binding energy be out below the ideal operating range because in this case adsorption is mediated only through weak long-range dispersion forces. Instead, appearing of some chemisorption degree, <20 %, activates the H2-storage performing since, even leading to binding energies still below the ideal range, there exists Kubas interaction between H2 molecules and d orbitals of either Sn or Cu atoms. Further chemisorption-degree increasing to around 50 % produces a binding energy within the ideal range, whereas at 80 % the Kubas interaction intensifies on the Cu site putting the binding energy in the zone of the best H2-storage performing.