Due to its long cycle life, safety and reliability, and independent power and capacity, all vanadium flow batteries are currently one of the most promising large-scale energy storage technologies. However, its negative electrode reaction rate is slow, resulting in significant energy loss, so increasing the negative electrode reaction rate plays a crucial role in improving battery performance. Although a significant amount of research has been devoted to developing catalysts to accelerate the negative electrode reaction rate, the basic reaction kinetics of negative electrode V2+/V3+have not been thoroughly studied, and the mechanism by which catalysts enhance the negative electrode reaction kinetics needs to be further explored.



[Job Description]

Recently, Professor Liu Suqin and Associate Professor Wang Jue from Central South University analyzed the reaction overpotential and Tafel slope in the negative electrode oxidation and reduction reactions of all vanadium flow batteries, revealing the rate control steps of the negative electrode reaction. On this basis, it is proposed to use electron deficient active sites to accelerate this rate control step, and to use TiB2 rich in electron deficient active sites as a model catalyst to specifically improve the reaction rate of the negative electrode rate control step. At the same time, the catalytic reaction mechanism was deeply explored by calculating the reaction order during the reaction process. The related achievements were published in the internationally renowned journal Advanced Functional Materials under the title "Electron Effective Sites for Improving V2+/V3+Redox Kinetics in Vanadium Redox Flow Batteries". The first author of the paper is Huang Rongjiao, a doctoral student from the School of Chemistry and Chemical Engineering at Central South University, and the corresponding authors are Professor Liu Suqin and Associate Professor Wang Jue from the School of Chemistry and Chemical Engineering at Central South University. This study has received funding from the National Natural Science Foundation of China and the Hunan Provincial Science and Technology Program.



[Core Content]

Firstly, through cyclic voltammetry and Tafel testing, it was revealed that the electrochemical oxidation process of V2+in the negative electrode of all vanadium flow batteries has a higher Tafel slope and polarization potential compared to the reduction process of V3+, and the rate control steps of the negative electrode reaction were determined. Then, a one-step calcination method was used to prepare TiB2 catalysts rich in electron deficient sites. Meanwhile, in order to further regulate the electronic structure of TiB2, electron rich heterogeneous N atoms doped TiB2 were selected to investigate the effect of electron deficient sites on reaction activity. As shown in Figure 1, TiB2 rich in electron deficient sites has a smaller Tafel slope and higher exchange current density compared to the blank electrode. The Tafel slope decreased from 200 mV/dec to 80 mV/dec, a decrease of 60%, greatly reducing the difficulty of V2+electrochemical oxidation. Compared with undoped TiB2, the Tafel slope of TiB2-N increases significantly at 100% SOC compared to 0% SOC. Experimental and theoretical calculations have shown that the electron deficient sites in TiB2 accelerate the electrochemical oxidation of V2+to V3+.



Figure 1. The effect of TiB2 rich in electron deficient sites on the kinetics of negative electrode oxidation and reduction reactions.



Subsequently, the reaction mechanism of TiB2 catalyzed V2+/V3+reaction was further explored through the reaction order, as shown in Figure 2. The reaction orders for V2+, V3+, H+, and SO42- are 1, 1, 0, and 0, respectively, and V2+/V3+represent the inner electron transfer reaction process. Raman, UV vis, and electrochemical tests indicate that the forms of V2+and V3+in sulfuric acid aqueous solution are [V (H2O) 6] 2+and [V (H2O) 6] 3+, respectively. Based on the above analysis, the mechanism of the V2+electrochemical oxidation process catalyzed by TiB2 rich in electron deficient sites is shown in Figure 3. Specifically, at the interface between the electrode and electrolyte, [V (H2O) 6] 2+adsorbs onto the electron deficient sites of TiB2 by losing a water molecule ligand, while TiB2, which is rich in electron deficient sites, acts as an electron collector, accelerating the electrochemical oxidation of V2+to V3+. Finally, the adsorbed intermediate exchanges ligands with water molecules in the solution, and [V (H2O) 6] 3+is desorbed from the active site.



Figure 2. Calculation of reaction order for TiB2 catalyzed V2+/V3+reaction process.



Figure 3. Schematic diagram of the mechanism of the V2+electrochemical oxidation reaction catalyzed by TiB2 rich in electron deficient sites.



Finally, the TiB2 rich in electron deficient sites was assembled into a VRFB single cell, and the battery performance is shown in Figure 4. According to the average charging and discharging voltages at different current densities (Figure 4b), it can be seen that the resistance of the discharge process of the blank battery is greater than that of the charging process, indicating that the discharge process of the battery requires a higher overpotential than the charging process. Assembling TiB2 rich in electron deficient sites into VRFB batteries resulted in a significant decrease in resistance during discharge compared to charging, indicating that TiB2 rich in electron deficient sites can effectively accelerate the electrochemical oxidation process of V2+. In addition, in the charge discharge voltage curve (Figure 4c), TiB2 rich in electron deficient sites can significantly reduce the overpotential during battery discharge, indicating that TiB2 rich in electron deficient sites can effectively reduce battery polarization and improve battery cycling efficiency.



Figure 4. Performance testing of TiB2 catalyst battery rich in electron deficient sites.



Summary

In summary, this work reveals that the fundamental reason for the slow reaction rate of the negative electrode in all vanadium flow batteries is the electrochemical oxidation reaction of V2+, and the use of catalysts rich in electron deficient sites can accelerate this reaction. For example, the model catalyst TiB2, which is rich in electron deficient sites, has excellent electrocatalytic activity and battery cycling performance, and electrochemical kinetics tests have revealed the mechanism of the V2+electrochemical oxidation reaction process catalyzed by TiB2, which is rich in electron deficient sites. This study not only elucidates that increasing the rate of V2+electrochemical oxidation is an effective way to improve the cycling efficiency of all vanadium flow batteries, but also provides new ideas for constructing new highly active electrocatalysts for all vanadium flow batteries.



Rongjiao Huang, Suqin Liu, Zhen He, Weiwei Zhu, Guanying Ye, Yuke Su, Weiwen Deng, Jue Wang, Electron-Deficient Sites for Improving V2+/V3+ Redox Kinetics in Vanadium Redox Flow Batteries, Advanced Functional Materials, 2022, https://doi.org/10.1002/adfm.202111661



The recent representative work of Liu Suqin/Wang Jue's research group on energy storage (such as all vanadium flow batteries and potassium ion batteries) is as follows:

1. Non precise transition metal based electrocatalysts for advanced redox flow batteries: Rational design and perspectives, Journal of Power Sources, 2021, DOI:10.1016/j.jpowsour.2021.230640.



2. Nature of Bismuth and Antimony based Phosphate Nanobundles/Graphene for Superior Potassium ion Batteries, Chemical Engineering Journal, 2022, DOI:10.1016/j.cej.2022.134746



3 Nature of Novel 2D van der Waals Heterostructures for Superior Potassium Ion Batteries, Advanced Energy Materials, 2020, DOI:10.1002/aenm.202000884



4. In Situ Alloying Strategy for Exceptional Potassium Ion Batteries, ACS Nano, 2019, DOI:10.1021/acsnano.9b00634



5. Nature of Bimetallic Oxide Sb2MoO6/rGO Anode for High Performance Potassium On Batteries, Advanced Science, 2019, DOI:10.1002/advs.201900904