Theoretical investigation of fullerene nanocage capacity for hydrogen storage

Fullerenes are nanocage compounds that can be used for hydrogen storage. Hydrogen is believed to be a potential alternative energy source, as the energy produced is clean. One of the most important issues in hydrogen–filled fullerene molecules is the determination of the number of hydrogen molecules that can be encapsulated inside the fullerene cage. In this study, the maximum number of hydrogen molecules that can be encapsulated inside C50, C60, C70 and C78 fullerenes was investigated by means of theoretical methods. Various density functional theory (DFT) functionals, together with Hartree–Fock (HF) and post Hartree–Fock methods were used in the computation for this study. Taking into consideration the basis set superposition error (BSSE) correction, it was found that second order Møller-Plesset perturbation theory (MP2) and dispersion corrected semiempirical hybrid density functional theory with perturbative second–order correlation (B2PLYPD), in conjunction with the triple zeta Pople–style 6-311G(d,p) basis set, provide the most reliable results in predicting the stability of nH2@Ck complexes. On the basis of complexation energy calculations, it was confirmed that encapsulation of numerous hydrogen molecules inside Ck (k = 50, 60, 70 and 78) fullerenes is unrealistic. In agreement with results of experimental works, only one hydrogen molecule can be accommodated inside C50 and C60, two inside C70 and three inside C78. Geometrical considerations of encapsulation of H2 molecule(s), host–guest interaction forces, strain energies, dispersion energies, maximum expansion of the fullerene cages that can be reached before breaking some of the C–C bonds and the bond dissociation energies (BDEs) of the cages are all in line with the calculated complexation energies