Insights on the Activity of Graphene - supported Tantalate Oxides as Non Noble Metal Catalyst for the Electroreduction of Oxygen

Oxygen reduction reaction (ORR) is a very important process in life and in various energy related applications. In the past decades, the metal oxides of groups 4 and 5 have attracted much interest due to their low cost, abundance, environmental compatibility and excellent stability especially in acid medium. However, these transition metal oxides usually exhibit limited ORR activity and the exact nature of the active sites has not yet been elucidated. We will present our recent studies on Ta-based catalysts aiming to understand the origin of the catalytic activity for the ORR on these materials, and to develop new strategies to obtain high density of active sites. Na2Ta8O21 supported on graphene catalysts are obtained by hydrothermal synthesis using NaOH as precipitation agent followed by pyrolysis at 900°C in Ar atmosphere. This type of catalysts shows higher catalytic activity for ORR than pristine graphene, and its activity can be tuned and further improved through the proper control of the synthesis parameters. Instead, Ta2O5 oxide on nitrogen-doped graphene is obtained when NH4OHis used as precipitating agent. Comparison of the polarization curves recorded for catalysts subjected to similar synthesis conditions evidences: i) the higher electrocatalytic activity of Na2Ta8O21 compared to Ta2O5, and ii) that in the latter case the activity is mainly due to the nitrogen doping of the graphene substrate. Correlations between synthesis conditions, structure and activity will be discussed in detail.