Effects of Microwave Treatment on Carbon Electrode for Vanadium Redox Flow Battery

Chemical Doping and O-Functionalization of Carbon-Based Electrode to Improve Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) holds promise for large-scale energy storage applications, despite its lower energy and power densities compared to advanced secondary batteries available today. Carbon materials are considered suitable catalyst electrodes for improving many aspects of the VRFB. However, pristine graphite structures in carbon materials are catalytically inert and require modification to activate their catalytic activity. Among the various strategies developed so far, O-functionalization and chemical doping of carbon materials are considered some of the most promising pathways to regulate their electronic structures. Building on the catalytic mechanisms involved in the VRFB, this concise review discusses recent advancements in the O-functionalization and chemical doping of carbon materials. Furthermore, it explores how these materials can be tailored and highlights future directions for developing more promising VRFBs to guide future research.

Insights into the Modification of Carbonous Felt as an Electrode for Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) has been regarded as one of the best potential stationary electrochemical storage systems for its design flexibility, long cycle life, high efficiency, and high safety; it is usually utilized to resolve the fluctuations and intermittent nature of renewable energy sources. As one of the critical components of VRFBs to provide the reaction sites for redox couples, an ideal electrode should possess excellent chemical and electrochemical stability, conductivity, and a low price, as well as good reaction kinetics, hydrophilicity, and electrochemical activity, in order to satisfy the requirements for high-performance VRFBs. However, the most commonly used electrode material, a carbonous felt electrode, such as graphite felt (GF) or carbon felt (CF), suffers from relatively inferior kinetic reversibility and poor catalytic activity toward the V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples, limiting the operation of VRFBs at low current density. Therefore, modified carbon substrates have been extensively investigated to improve vanadium redox reactions. Here, we give a brief review of recent progress in the modification methods of carbonous felt electrodes, such as surface treatment, the deposition of low-cost metal oxides, the doping of nonmetal elements, and complexation with nanostructured carbon materials. Thus, we give new insights into the relationships between the structure and the electrochemical performance, and provide some perspectives for the future development of VRFBs. Through a comprehensive analysis, it is found that the increase in the surface area and active sites are two decisive factors that enhance the performance of carbonous felt electrodes. Based on the varied structural and electrochemical characterizations, the relationship between the surface nature and electrochemical activity, as well as the mechanism of the modified carbon felt electrodes, is also discussed.

Degradation of Carbon Electrodes in the All-Vanadium Redox Flow Battery

The charge-discharge operation of the vanadium redox flow battery degrades the electrodes over time and results in a performance and efficiency decay. The impact of extended charge-discharge cycling operation on carbon electrodes is investigated using carbon paper as a model electrode. Electrode degradation along with 70 % degradation of charge-discharge capacity was observed after 100 charge-discharge cycles of a single cell vanadium redox flow battery operating at a current density of 80 mA cm^-2 at room temperature (23 °C). Raman mapping of the electrodes shows a decrease in structural defects in the negative electrode, and an increase in defects in the positive electrode, indicating differences in the degradation mechanism at each electrode. Electrochemical investigation reveals an increase in the activation overpotential at both the positive and negative electrodes. However, the negative electrode showed a higher activation overpotential indicating a higher impact of electrode degradation on the negative side. X-ray photoelectron spectroscopy shows around an eightfold increase in surface oxygen functional groups after degradation in both positive and negative electrodes. The composition of oxygen functional groups was also observed to change significantly after degradation from dominantly carbonyl-based to a combination of carbonyl- and carboxyl-based groups. This study provides insight into the electrode degradation mechanism and highlights the differences in the mechanism for the positive and negative electrodes.