Ca Functionalization of the Polyaramid System for Reversible Hydrogen Storage: A DFT Study

In this article, two-dimensional polyaramid (2dpa) is explored as a hydrogen storage medium using density functional theory simulations. Since the pristine 2dpa has less adsorption energy for H 2 molecules, the medium is functionalized with the alkali earth metal, calcium, for adsorbing hydrogen. The Ca atom adsorbs on 2dpa with a binding energy of −2.01 eV due to the charge transfer between the 4s orbital of Ca and C 2p orbitals of 2dpa. Each Ca atom, placed above O on 2dpa, holds five H 2 molecules. The interactions between the 4s and 3d Ca orbitals and 1s of the H atom result in an average hydrogen adsorption energy of −0.29 eV/H 2, which meets the US Department of Energy (DoE-US) standards (0.2–0.7 eV/H 2 ). The desorption temperature for the system is 213.48 K at 1 atm pressure, and the gravimetric weight % is 6.9 with three Ca atoms on one side of 2dpa. The stability of the system is verified through ab initio molecular dynamics (AIMD) and phonopy calculations. The results show that the 2dpa + Ca system can be an excellent reversible hydrogen storage medium.