First-principles study of hydrogen storage in non-stoichiometric TiCx

In this work, the first principles calculation has been performed to study the hydrogen storage in non-stoichiometric TiCx. It is found that hydrogen absorption in stoichiometric TiC is energetically unfavorable, while it is favorable in non-stoichiometric TiCx. This indicates that the existence of carbon vacancies is essential for hydrogenation storage in TiCx. At the same time, multiple hydrogen occupancy of the vacancy has been confirmed and it is calculated that as many as four hydrogen atoms can be trapped by a carbon vacancy. These absorbed hydrogen atoms tend to uniformly distribute around the vacancy. However, it is also found that the diffusion of hydrogen atoms in TiCx is difficult, especially in TiCx with high x.

► The absorption of hydrogen in non-stoichiometric TiCx is thermally favorable.
► As many as four hydrogen atoms can be trapped by a carbon vacancy.
► The diffusion of hydrogen in TiCx is difficult, especially in TiCx with high x.