The Thermal-Oxidation Behavior of Pristine and Doped Magneli Phase Titanium Oxides

摘要

Magneli phase titanium oxides (Ti_nO_(2n-1), 4 < n < 10) are promising alternatives to carbon-based materials in aqueous electrochemical technologies, but their passivation during operation remains an important challenge to address. To elucidate the mechanism for their oxidation and to investigate the influence of doping on their oxidation stability, the thermal-oxidation behavior in air of Ti_4O_7 doped with V, Cr, Fe, and Ga was investigated by thermogravimetric analysis. Both thermal and electrochemical oxidation of Ti_4O_7 form the electrically-insulating TiO_2 in sufficiently oxidizing conditions. V- and Fe-doping improved the thermal stability of Ti_4O_7 as evidenced by higher onset temperatures in their thermograms. Three-dimensional diffusion reaction models adequately describe the solid-state kinetics of thermal oxidation of Ti_4O_7 in air as demonstrated by linear model-fitting. Doping shows a mixed influence on these kinetics reducing both the Arrhenius pre-exponential factor and the activation energy. Cr-doping significantly shortens the lifetime of Ti_4O_7 in ambient conditions as determined by kinetic predictions.