Perspective on electrocatalysts and performance hindrances at the negative electrode in vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are widely used in energy storage systems due to their large storage capacity and stable performance. As one of the critical components of VRFBs to provide the reaction sites for redox couples, an ideal electrode should possess excellent conductivity, electrochemical and chemical stability, good reaction kinetics, and a low price. Due to their favorable properties, carbon-based materials such as graphite felt (GF) and carbon paper (CP) are widely used as VRFB electrodes. However, these electrodes suffer from poor electrochemical activity towards VO²⁺/VO₂⁺ and V²⁺/V³⁺ redox couples, caused by sluggish kinetics and high polarization, limiting the operation of VRFB at high current density. Specifically, the negative electrode is performance-limiting due to the V²⁺/V³⁺ reaction overlapping with the potential range of the hydrogen evolution reaction (HER), further hindering performance. Researchers have developed different strategies to improve the performance of VRFB electrodes towards the V²⁺/V³⁺ reaction. Here, the leading causes of capacity losses in VRFB towards the V²⁺/V³⁺ reaction, including the undesirable side reactions, such as the HER and degradation of carbon materials, are briefly reviewed. The electrochemical kinetics, the mechanism, and the role of surface functional groups on carbon electrodes are highlighted. The general phenomena, mechanisms, and methods of HER inhibition in the negative electrode are also reviewed. Furthermore, approaches to improve the performance in the negative half-cell of VRFB are outlined. Finally, the ongoing challenges to improve the performance of electrode materials towards the V²⁺/V³⁺ reaction are identified, and future research directions are proposed.