High-capacity reversible hydrogen storage properties of metal-decorated nitrogenated holey graphenes

Bilayer Heterostructure of Boron Nitride and Graphene for Hydrogen Storage: A First-Principles Study

“…Hydrogen molecules binding capability on the h-BN/Gr is tested first by calculating the binding energy of H 2 molecules using the following equation ,,, E normalB normalE = E normalT normalo normalt normala normall − ( E h − B N / G r + n E H 2 ) A single hydrogen molecule prefers a parallel orientation at the hexagonal site of the BN-ring with a binding energy of −1.37(−1.48) eV calculated by employing the DFT-D2(D3) Grimme’s dispersion method. Grimme’s semi-empirical D2 and D3 dispersion correction are proven to be better choice for weakly interacting hydrogen-bonded systems. , The single hydrogen molecule binding energy on h-BN/Gr is higher in comparison to many available results in the literature and also compared to its pristine counterparts. ,,,, We further tested the binding energy of a single hydrogen molecule on two more bilayer structures with AA type (where hexagon of graphene is symmetric with hexagon of boron nitride) and AB-Boron type (where boron atom is at the center of graphene hexagon) [refer to Figure S8 in Supporting Information] stacking orientation of h-BN/Gr. The single hydrogen molecule adsorption energies on these AA-type and AB-type configuration are −0.94(−0.97) and −1.01(−1.03) eV, respectively.…”

“…The average adsorption energy [ E ads (eV/H 2 )] are computed using the following equation: ,,, E normala normald normals = E normalT normalo normalt normala normall − ( E h − B N / G r + n E H 2 ) n where E Total is the total energy of the complex (h-BN/Gr+H2), E h–BN/Gr is the total energy of the h-BN/Gr heterostructure, n is the number of H 2 molecules, and E H 2 refer to the total energy of H 2 molecule. The hydrogen molecules are gradually increased on the hexagonal site of the cavity up to the maximum available adsorption sites, keeping in mind the average adsorption energy lying in the desirable range of −0.15 – −0.60 eV/H 2 . ,, In the case of 4H 2 molecules, adsorption energy per H 2 is −0.47 eV.…”

“…We observe that with the increase in the number of H 2 molecules inside the cavity, there is a gradual decrease in the average adsorption energy. The decrease in average adsorption energy can be a result of the steric repulsion between the increased number of hydrogen molecules. ,, Once the cavity is completely filled, the second layer of hydrogen molecules is introduced at the cavity region. The ILD increases beyond 6.5 Å, and the average adsorption energy becomes positive.…”

“…The result of the Mulliken charge analysis is listed in Table . Along with the enhanced orbital interactions, increased charge transfer can also be the reason for an improved adsorption energy per H 2 .The desorption temperature for 4–24H 2 molecules from the h-BN/Gr heterostructure are calculated using the Van’t Hoff equation ,,, T normalD ( K ) = false| E normala normald normals false| K normalB true( normalΔ S R − ln nobreak0em.25em⁡ P true) − 1 whereas E ads is the average adsorption energy, R = 8.3145 JK –1 Mol –1 is the gas constant, K B = 1.38 × 10 –23 JK –1 is the Boltzmann constant, Δ S represents change in the H 2 entropy from gas to liquid phase, and P is the equilibrium pressure taken to be 1 atm, respectively. − The desorption temperature T D calculated using DFT-D2/D3 is summarized in Table . Additionally, cut-plane plots of electron density difference (EDD) for the 4-12-24H 2 hydrogen-adsorbed h-BN/Gr system are performed to further analyze the interaction between the hydrogen molecules and the h-BN/Gr heterostructure and are presented in Figure (a–c), while rest of the plots are provided in the Supporting Information [refer to Figure S11 (a–d)].…”

“…The mode of hydrogen storage on solid-state material is classified by physisorption and chemisorption processes. The former process involves hydrogen storage through weak Van der Waals and electrostatic interactions [in the case of many two-dimensional (2D) materials], − whereas the latter process involves a chemical bonding mechanism (in the case of metal hydrides). − Promising hydrogen storage materials in general should have (i) high hydrogen storage capacity and (ii) good operability conditions, that is, storage and desorption at moderate temperature and pressure. As per the United States Department of Energy (USDOE), for on-board storage applications, the hydrogen storage materials must fulfill the gravimetric density and binding energy criteria of 5.5–9.5 wt % and −0.15 to −0.60 eV/H 2 , respectively. − …”