Abstract
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 V4+/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.
Highlights
Renewable energy sources, such as wind and solar power, are providing an increasing share of the energy supply [1,2]
We demonstrate a vacuum annealing approach for the direct integration of VO2 nanoparticles onto conductive carbon finer paper (CFP), making the integration uniform and highly adhesive at the centimeter scale
The VO2 nanoparticles have been prepared through the vacuum annealing of the
Summary
Renewable energy sources, such as wind and solar power, are providing an increasing share of the energy supply [1,2]. The scarcity and high costs of those materials restrict their large-scale application [9] In this regard, the earth-abundant transition-metal oxides based on Mn, Fe, Co, and Ni have been extensively investigated as alternative OER catalysts. Other descriptor approaches of M-OH bond strength and (∆GO * − ∆GHO *) were suggested to find superior catalysts [16,17] Application of such descriptors is limited to experimental work and finding novel catalysts based on other transition metals such as Nb, Ta, and V [9,18]. Al2 O3 ) is widely known as effective catalysts for selective oxidation of sulfur dioxide, naphthalene, o-xylene, alkyl pyridine, chlorinated hydrocarbon, and mercury in the chemical, petroleum, and environmental industries [32] Despite such investigations, OER activities of binary VO2 and V2 O5 are rarely known. This result can be a cornerstone for understanding the V4+ -related active sites for OER and for the design of novel VO2 -based OER catalysts
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