Hydrogen diffusion and storage in substoichiometric TiC

Abstract

In this paper we investigate the nature of hydrogen diffusion in substoichiometric TiC using simulations. Specifically, we examine how well connected the carbon structural vacancies are in TiCx using percolation theory and how many structural carbon vacancies are connected to the surface of TiCx. This provides direct insight into the number of sites available for hydrogen storage and the interconnected nature of the carbon vacancy network. We also compute the binding energies and migration energies of hydrogen in TiCx using density functional theory. This is conducted over a range of carbon concentrations from nearly stoichiometric TiC to Ti2C and includes the differences in vacancy ordering. We find that the migration energy of hydrogen in TiCx generally decreases with carbon loss and is associated with the tetrahedral interstices near them but that the binding energy of hydrogen to TiCx is independent of carbon concentration. Furthermore, we show that this migration path can occur through the tetrahedral interstices, which is influenced by local carbon concentration. These results also demonstrate the vacancy ordered R3¯m Ti2C structure does not possess any more superior ability to store or transport hydrogen than the Fd3¯m Ti2C structure.