Electrochemical hydrogen insertion in substoichiometric titanium carbide TiC0.6: influence of carbon vacancy ordering.

Substoichiometric titanium carbide of formula TiC0.6 with different degrees of carbon vacancy ordering has been synthesized first by reactive sintering at high temperature (2100 °C) and then by annealing at low temperature (730 °C) for different durations. The effect of annealing on the structure of the carbide and its capacity to electrochemically insert hydrogen has been investigated. The XRD study reveals two phase transitions in the carbide during annealing. First, annealing of disordered TiC0.6 (space group Fm3¯m) for 40 h leads to a trigonal superstructure of space group R3¯m thanks to ordering of carbon vacancies. After the longest annealing time of 120 h, the structure of the carbide becomes cubic anew, but with a space group of Fd3¯m. The electrochemical hydrogen insertion in these different types of TiC0.6, as studied by cyclic voltammetry, strongly depends on the crystalline structure of the carbide. The maximum storage capacity is obtained for 40 h, corresponding to the R3¯m ordered phase. Because the R3¯m structure consists of alternately empty and full (111) carbon atomic planes, we show that the hydrogen insertion and diffusion in the material is rendered possible by the presence of vacant (111) planes. This behavior is very similar to that of a cation-ordered manganese-nickel spinel vis-à-vis lithium ion insertion.