Carbyne Ring Activated Using ZnCl2 for Hydrogen Adsorption: DFT Study

We have studied the feasibility of activated carbyne as a good hydrogen storage material. Density functional theory (DFT) simulations through van der Waals interactions have been applied to investigate calcium sorption on activating carbyne with zinc dichloride (ZnCl2) and also interactions of molecular hydrogen with pristine carbyne and Ca functionalized on an activated carbyne C-12-ring. The obtained results showed that (i) the chemical activation of the C-12-ring with ZnCl2 increases its area by 5.17% with respect to pristine carbyne. (ii) Ca atoms at small concentrations tend to get atomically sparse on carbyne, donating +0.94e and +1.05e to the ring, according to Mulliken population analysis and the electrostatic potential fitting charges, respectively. Furthermore, in the presence of calcium, hydrogen sorption increases by 21.8% in comparison with Ca-decorated pure carbyne. (iii) Seven hydrogen molecules per Ca atom have adsorption energy close to the range of similar to 0.3-0.5 eV per H-2, which is necessary for effective charge/discharge cycles. (iv) Theoretical uptake (7.11 wt %) with a single Ca atom is higher than the U.S. Department of Energy target (5.5 wt %). Therefore, an activated C-12-ring can bind three Ca atoms with its seven H-2 molecules reaching 13.8 wt %. (v) Equilibrium pressure for CaC12-7H(2) and Ca3C12-21H(2) systems (5-15 MPa) by means of adsorption isotherm calculations. The calculated van't Hoff desorption temperatures exceed considerably the boiling point of liquid nitrogen. In addition, we also performed DFT-based molecular dynamics simulations for the C-12, CaC12, CaC12-7H(2), and Ca3C12-21H(2) systems to study thermal stability. Our results confirm the potential of Ca-decorated carbyne for hydrogen storage.