Various technologies have been developed for the safe and efficient storage of hydrogen. Hydrogen storage in its solid form is an attractive option to overcome challenges such as storage and cost. Specifically, hydrogen storage in carbon-based structures is a good solution. To date, numerous theoretical studies have explored hydrogen storage in different carbon structures. Consequently, in this review, density functional theory (DFT) studies on hydrogen storage in graphene-based structures are examined in detail. Different modifications of graphene structures to improve their hydrogen storage properties are comprehensively reviewed. To date, various modified graphene structures, such as decorated graphene, doped graphene, graphene with vacancies, graphene with vacancies-doping, as well as decorated-doped graphene, have been explored to modify the reactivity of pristine graphene. Most of these modified graphene structures are good candidates for hydrogen storage. The DFT-based theoretical studies analyzed in this review should motivate experimental groups to experimentally validate the theoretical predictions as many modified graphene systems are shown to be good candidates for hydrogen storage.

The study of novel materials for H₂ storage is essential to consolidate the hydrogen as a clean energy source. In this sense, the H₂ adsorption on Pd_4-nNi_n (n = 0–3) clusters embedded on pyridinic-type N-doped graphene (PNG) was investigated using density functional theory calculations. First, the properties of Pd_4-nNi_n (n = 0–3) clusters embedded on PNG were analyzed in detail. Then, the H₂ adsorption on these composites was computed. The E_int between the Pd_4-nNi_n (n = 0–3) clusters and the PNG was greater than that computed in the literature for Pd-based systems embedded on pristine graphene. Consequently, it was deduced that PNG can more significantly stabilize the Pd_4-nNi_n (n = 0–3) clusters. The analyzed composites exhibited a HOMO–LUMO gap less than 1 eV, indicating good reactivity. Based on the E_ads of H₂ on Pd_4-nNi_n (n = 0–3) clusters embedded on PNG, it was observed that the analyzed systems meet the standards set by the DOE. Therefore, these composites can be viable alternatives for hydrogen storage.

A density functional theory (DFT) investigation was conducted to study the O₂ and CO₂ adsorption on very small Pd_3−nNi_n (n = 0–2) clusters supported on N-doped graphene quantum dots (N-GQDs). The study was carried out in two stages. First, the interaction between Pd_3−nNi_n (n = 0–2) clusters and N-GQDs was analyzed. Subsequently, the adsorption behavior of O₂ and CO₂ molecules on the supported clusters was examined. The calculated interaction energies (E_int) of Pd_3−nNi_n (n = 0–2) clusters on N-GQDs were found to be higher than those on pristine graphene, indicating enhanced cluster stability on N-GQDs. Furthermore, the adsorption energies (E_ads) of the O₂ molecule on the Pd₃ and Pd₂Ni clusters deposited on N-GQDs were similar. Meanwhile, the PdNi₂ cluster deposited on N-GQDs exhibited the highest E_ads (−1.740). The E_ads of CO₂ on Pd_3−nNi_n (n = 0–2) clusters embedded in N-GQDs were observed to be close to or exceed 1 eV. Upon adsorption of O₂ and CO₂ on the Pd_3−nNi_n (n = 0–2) clusters supported on N-GQDs, an elongation of the O–O and C–O bond lengths was observed, respectively. This structural change may facilitate the dissociation of these molecules on the supported clusters.