Zr doped C24 fullerene as efficient hydrogen storage material: insights from DFT simulations

In this article, we report the hydrogen storage capacity of zirconium (Zr) decorated C-24 fullerene using state-of-the-art density functional theory simulations. Our study shows that zirconium, like most other transition metals, tends to bind strongly on the C-C bridge of C-24 fullerene with a maximum binding energy of -3.64 eV. Each Zr atom decorated over C-24 fullerene can adsorb a maximum of 7H(2) molecules with an average adsorption energy of -0.51 eV/H-2, leading to a gravimetric density of 7.9 wt%, which is higher than the prescribed target of 6.5 wt% set by United States-Department of Energy. There is a charge transfer from Zr to C atoms in C-24 fullerene, which is the primary cause of the binding of Zr with C-24 fullerene. H-2 molecules are adsorbed over Zr sorption sites via Kubas-type interactions, which include charge donation from the filled s orbitals of hydrogen to the vacant 4d orbital of Zr and subsequent back charge donation to unfilled s* orbital of hydrogen from the filled 4d orbital of Zr. The structural stability of the Zr + C-24 system at a high temperature of 500 K is verified using ab-initio molecular dynamics calculations. The high diffusion energy barrier of Zr (2.33 eV) inhibits clustering between the Zr atoms decorated on the C-24 fullerene and ensures the system's practical feasibility as a high-capacity H-2 adsorbing system. Therefore, our computational studies confirm that Zr decorated C-24 fullerene is stable and can be regarded as a potential candidate for H-2 storage systems with optimum adsorption energy range.