Mechanism of Dioxovanadium Ion Reduction on Oxygen-Enriched Carbon Surface

Abstract

The reduction of dioxovanadium(5+) ion (VO 2 + ) was fundamentally investigated using glassy carbon rotating disk electrodes (GC RDEs) as a model of the carbon electrodes of vanadium redox flow batteries (VRFBs). The enrichment of oxygen-containing functional groups on the GC surface was performed by the electrochemical oxidation and checked by X-ray photoelectron spectroscopy (XPS). The XPS results also suggested the coordination of the functional groups to the oxo/dioxovanadium(4+/5+) ions (VO 2+ /VO 2 + ) after immersion into the VO 2+ /VO 2 + -dissolved solution. The enrichment enhanced the VO 2 + reduction and changed the Tafel slope from −0.161 V decade −1 to −0.087 V decade −1 , indicating the reduction mechanism alteration. The Tafel slope was explained by a multi-step reaction mechanism, in which the reduction of the quinone-like functional group generated the hydroxylic functional group that served as the reaction site and coordinated to the oxo/dioxovanadium(4+/5+) species.Electrical energy storage (EES) has been recognized as one of the valuable approaches for leveling intermittent power generation by wind turbines and photovoltaic cells, which would enable the realization of clean and highly efficient energy systems. Redox flow batteries (RFBs) have attracted much interest as a promising EES system due to their suitability for large-scale energy storage and capability to withstand fluctuating power supplies. 1,2Among the various types of RFBs, the VRFBs have various advantages, such as a relatively higher safety and lower risk of crosscontamination by crossover between the electrode compartments upon long-term cycling, and are considered to be in the most advanced stage of research and development. 3-5 Carbon materials are used in the VRFB electrodes and the discharge reactions at the positive and negative electrodes are as follows:The slow kinetics of the VO 2 + reduction is a serious problem in VRFB technology, which has to be overcome to improve the efficiency of the VRFB. 6 Gattrell et al. proposed a mechanism for the positive electrode reactions by considering multi-step chemical and electrochemical reactions of the dissolved vanadium species. 6 In contrast, the potential involvement of the carbon electrode surface in the reaction mechanism, the functional groups on the surface, in particular, has been pointed out and the reaction model shown in Fig. 1 is proposed. 7-9

Vanadium‐Ion Redox Reactions in a Three‐Dimensional Network of Reduced Graphite Oxide

“…Due to the low possibility of physical adsorption by a weak Van der Waals interaction between ions and the carbon surface, it is reasonable to assume that the vanadium‐ion species were coordinated by the oxygen functional groups on the carbon surface and retained due to chemical adsorption . This result was in agreement with that of a previous study that demonstrated the adsorption of vanadium‐ion species from an acidic vanadium solution that contained VO 2+ (5 m m ) and VO 2 + (5 m m ) onto a glassy carbon surface through hydroxylic surface functional groups generated by the reduction of the quinone‐like groups …”

“…[38] This result was in agreement with that of aprevious study that demonstrated the adsorption of vanadium-ion speciesf rom an acidic vanadium solution that contained VO 2 + (5 mm)a nd VO 2 + (5 mm)o nto ag lassy carbon surfacet hrough hydroxylic surfacef unctional groups generated by the reduction of the quinone-like groups. [39]…”

Electrocatalysis at Electrodes for Vanadium Redox Flow Batteries

“…Maruyama et al have studied the electrochemical reduction of the dioxovanadium(5+) ion at a glassy carbon electrode electrochemically enriched by oxidative treatment with oxygen-containing functional groups [219]. Inspection of obtained electrochemical data suggests the pivotal role of OH-groups in the reduction mechanism.…”

Vanadium (V) reduction reaction on modified glassy carbon electrodes – Role of oxygen functionalities and microstructure

“…It has been proposed by several authors that oxygen functional groups play a key role in facilitating the electron transfer process for V(V) reduction at carbon electrode surfaces [8,18,35]. More recent studies have negated the catalytic role of oxygen groups towards V(V) reduction, and have suggested that other properties such as surface roughness and carbon microstructure play a role in improving electrode kinetics [11,13,36].…”