VO₂ as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction

Herein, we report high electrocatalytic activity of monoclinic VO₂ (M1 phase) for the oxygen evolution reaction (OER) for the first time. The single-phase VO₂ (M1) nanoparticles are prepared in the form of uniformly covering the surface of individual carbon fibers constituting a carbon fiber paper (CFP). The VO₂ 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 VO₂ nanoparticles with some particle coalescence. Consequently, the VO₂/CFP shows a high electrocatalytic OER activity with the lowest η₁₀ (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 V₂O₅ and the hydrothermally grown VO₂ (M1), α-V₂O₅, and γ'-V₂O₅. The catalytically active site is considered V⁴⁺ components and V^4+/5+ redox couples in VO₂. The oxidation state of V⁴⁺ is revealed to be more favorable to the OER catalysis compared to that of V⁵⁺ in vanadium oxide through comparative studies. Furthermore, the amount of V⁵⁺ component is found to be increased on the surface of VO₂ catalyst during the OER, giving rise to the performance degradation. This work suggests V⁴⁺ and its redox couple as a novel active component for the OER in metal-oxide electrocatalysts.