High capacity reversible hydrogen storage in zirconium doped 2D-covalent triazine frameworks: Density Functional Theory investigations

Abstract

Employing the state-of-the art Density Functional Theory (DFT) Simulations, we have investigated hydrogen storage capability in zirconium doped novel 2D heterostructures, Covalent Triazine Frameworks (CTFs), specifically CTF-1, rich in nitrogen functionalities. Zirconium atom is strongly bonded to the triazine framework with a -3.61 eV binding energy, and each Zr atom was found to adsorb 7 H2 molecules reversibly with binding energy −0.38 eV per H2 on an average giving a gravimetric storage capacity of 7.1% which accomplishes the US D.o.E. targets for suitable hydrogen storage substrates. The system stability at ambient and higher temperatures as verified using ab initio Molecular Dynamics simulations as well as existence of sufficient diffusion energy barrier preventing metal-metal clustering certifies the practical viability of the system as a high capacity H2 storage device. The mechanism of interaction of Zr on 2D CTF-1 and H2 molecules on Zr+CTF-1 have been analyzed by partial density of states, charge density distribution plot and Bader Charge Analysis. Charge transfer from Zr 4d orbital to 2p orbital of triazine ring was observed, whereas bonding of H2 is through Kubas interaction which involves the charge donation from the filled σ orbitals of hydrogen molecules to the vacant metal d orbitals, and the subsequent back donation of charge from the occupied metal d orbitals to the vacant σ∗ orbital of hydrogen molecules. As the system is stable, can hold high H2 wt% (7.1%) with suitable desorption temperature (442 K at ambient), we propose that Zr doped 2D CTF-1 can act as a potential H2 storage device.