Thin TiO2 Overlayers As Catalysts for Oxygen Reduction and Evolution Reactions

We report on the electrochemical activity of thin TiO₂ overlayers on titanium micro-particles toward oxygen reduction and evolution reactions in a CsH₂PO₄ solid acid electrochemical system. The electrocatalysis at the interface of the super-protonic (at 245 °C) CsH₂PO₄ solid acid with the electrodes occurs at active sites which are also accessible to the humidified gases. Pt remains the best catalyst for the O₂ reduction reaction in the cathodes of the solid acid fuel cells. Because of the strong O=O bond, oxygen reduction is a slow reaction even on a Pt surface at 245 °C, requiring high loadings of the precious Pt catalyst.  Solid state TiO₂ catalysts are an attractive alternative. TiO₂ catalysts show a range of activity depending on the relative ratio of the rutile to anatase polymorphs. Rutile phase exhibits a lower overpotential for the ORR. TiO₂ films are grown on Ti metal micro-particles under slightly oxidizing environment at elevated temperature (600-900 °C). The thickness and relative ratio of the rutile to anatase phase is controlled by the growth temperature and holding time. X-ray diffraction analysis of the films reveals that the amount of the oxide increases with increasing the growth temperature. At higher growth temperatures, the oxide layer formed is mostly rutile.  The activity of the TiO₂ films is controlled by both the phase and thickness of the films. Between 700-900 °C, the activity of the TiO₂ overlayers for oxygen reduction reaction increases with increasing the growth temperature. Interestingly, thin oxide overlayers with mostly rutile phase grown at 600 °C show higher activity. We will discuss the origin of the improved activities of thin TiO₂ overlayers.