VO 2 as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction.

Herein, we report high electrocatalytic activity of monoclinic VO 2 (M1 phase) for the oxygen evolution reaction (OER) for the first time. The single-phase VO 2 (M1) nanoparticles are prepared in the form of uniformly covering the surface of individual carbon fibers constituting a carbon fiber paper (CFP). The VO 2 nanoparticles reveal the metal-insulator phase transition at ca. 65 °C (heating) and 62 °C (cooling) with low thermal hysteresis, indicating a high concentration of structural defect which is considered a grain boundary among VO 2 nanoparticles with some particle coalescence. Consequently, the VO 2 /CFP shows a high electrocatalytic OER activity with the lowest η 10 (350 mV) and Tafel slope (46 mV/dec) values in a 1 M aqueous solution of KOH as compared to those of the vacuum annealed V 2 O 5 and the hydrothermally grown VO 2 (M1), α-V 2 O 5 , and γ'-V 2 O 5 . The catalytically active site is considered V 4+ components and V 4+/5+ redox couples in VO 2 . The oxidation state of V 4+ is revealed to be more favorable to the OER catalysis compared to that of V 5+ in vanadium oxide through comparative studies. Furthermore, the amount of V 5+ component is found to be increased on the surface of VO 2 catalyst during the OER, giving rise to the performance degradation. This work suggests V 4+ and its redox couple as a novel active component for the OER in metal-oxide electrocatalysts.