Nanoscale conductive niobium oxides made through low temperature phase transformation for electrocatalyst support

Abstract

We report an effective approach to synthesize nanoscale Nb₂O₅ coated on carbon nanotubes (CNTs) and transform it at low temperatures to the conductive form of NbO₂. The latter, when used as a Pt electrocatalyst support, shows significant enhancement in catalyst activity and durability in the oxygen reduction reaction (ORR). Direct phase transformation of Nb₂O₅ to NbO₂ often requires temperatures above 1000 °C. Here we show that this can be achieved at a much lower temperature (e.g. 700 °C) if the niobium oxide is first activated with carbon. Low temperature processing allows retaining nanostructures of the oxide without sintering, keeping its high surface areas needed for being a catalyst support. We further show that Pt supported on the conductive oxides on CNTs has two times higher mass activity for the ORR than on bare CNTs. The electrochemical stability of Pt was also outstanding, with only ca. 5% loss in electrochemical surface areas and insignificant reduction in half-wave potential in ORR after 5000 potential cycles.