HYDROGEN INSERTION IN SUBSTOICHIOMETRIC TITANIUM CARBIDE TiCx THIN FILM
Résumé
There is nowadays a growing need for the development of cheap and effective energy storage and conversion materials. That is why more and more recent studies do not focus on original chemical compositions, but on the tailoring of structures and shapes of already existing materials. Titanium carbide is ceramic mainly known for its refractarity (melting point ca. 3067 °C), its metallic conductivity (σ ≈ 106 Ω-1 m-1) and its high hardness (HV = 28 GPa) making him is one of the most widely applied hard coating materials. TiC presents a large range of composition from TiC0.55 to TiC0.98. In two recent electrochemical studies [1-2], it has been shown that hydrogen can be inserted in the substoichiometric titanium carbide TiC0.6, but not in TiC0.9, this latter being closer to the stoichiometry. Moreover, in contrast to carbon or nanoporous gold, TiC seems to represent a more viable, stable, cathode for aprotic Li-O2 cells [3].
Objective of this work is to investigate the electrochemical properties of TiC in the form thin films that would be suitable as an electrode. Substoichiometric titanium carbide thin films TiCx were synthesized by reactive magnetron sputtering using titanium target and a variable flow of gaseous benzene. Poisoning of the target was studied by optical emission spectroscopy and the monitoring of the cathodic voltage, pressure and deposition rate. The samples present a dense microstructure composed with columnar grains of 30 nm width. X-Ray Diffraction reveals that the films present a (111) preferred orientation. In complement to XRD, X-Ray Photoelectron Spectroscopy measurements suggest that for a low carbon content, a Ti-rich metallic phase is present while for a high carbon content, nanocrystallites of TiCx could be embedded by an amorphous carbon matrix. The compositions TiC and TiC0.6 were approximately obtained for 30 and 22 sccm respectively. The electrochemical measurements were carried out at room temperature, in a standard three-electrode cell. The solution was a 1 M H2SO4 aqueous solution deaerated by pure Ar bubbling. TiCx thin films were mounted on a 2000 rpm rotating disk as a working electrode. Results show that hydrogen insertion is possible in the thin film TiC but not in TiC0.6. This behaviour is totally different from bulk TiCx synthesized by classic sintering.
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Electrochemical hydrogen storage in TiC0.6, not in TiC0.9
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Electrochemical hydrogen insertion in substoichiometric titanium carbide TiC0.6: Influence of carbon vacancy ordering
(2013) Langmuir, 29 (38), pp. 12036-12042.
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A stable cathode for the aprotic Li-O2 battery
(2013) Nature Materials, 12 (11), pp. 1050-1056