Ti₄- and Ni₄-Doped Defective Graphene Nanoplatelets as Efficient Materials for Hydrogen Storage

Abstract

We report a detailed theoretical investigation of the structural and electronic properties of titanium- and nickel-doped defective graphene nanoplatelets, which are shown to be efficient materials for hydrogen storage. We found that H2 bond cleavage is favored by Ti4-doped defective graphene nanoplatelets because of the strong interaction between the hydrogen 1s and titanium 3d levels that leads to the formation of metal hydrides, while H2 adsorption on Ni4-doped defective graphene favors the formation of Kubas complexes as hydrogen 1s levels only interact with the nickel 4s levels. A comparison between adsorption energies, number of H2 adsorbed molecules, and hydrogen gravimetric content shows that Ti4-doped graphene has a better performance for hydrogen storage with a notably high hydrogen gravimetric content of 3.4 wt %; than Ni4-doped graphene with a 10-fold lower gravimetric content of only 0.30 wt %. This observation can be explained by three factors: Ti is a lighter transition metal, it absorbs a larger amount H2 per metallic atom, and it presents a planar geometry that increases the coverage of the graphene layer and makes possible that all atoms in the cluster participate in the H2 adsorption. Our results support the hypothesis that a controlled introduction of defects in graphene followed by the anchoring of small metallic clusters is a feasible way to enhance the hydrogen gravimetric content of graphene nanoplatelets and to fine-tune hydrogen absorption energies to achieve a reversible operation at ambient temperature and moderates pressures, addressing one of the main challenges of a sustainable hydrogen-based economy.