Unveiling Anion-Dependent Redox Kinetics of the V4+/V5+ Couple in Acidic Electrolytes: A Combined Powder Microelectrode and Intervalence Charge Transfer Study

The redox kinetics of the V 4+ /V 5+ couple govern the power density and stability of all-vanadium redox flow batteries (VRFBs). Using electrolyte additives is the most common strategy to accelerate the kinetics, but their underlying mechanisms remain unclear. Herein, the electrochemical behaviors of the V 4+ /V 5+ couple in sulfuric acid (H 2 SO 4 ) with phosphoric acid (H 3 PO 4 ) and perchloric acid (HClO 4 ) as additives were systematically investigated using powder microelectrode (PME) techniques. Based on the in-depth electrochemical analysis using the PME, the electrochemically active species are identified as dimers, whose electron transfer (ET) kinetics can be accelerated by H 3 PO 4 but inhibited by HClO 4 . Fitting the PME-acquired Tafel plots with the Marcus–Hush–Chidsey (MHC) ET theory demonstrates that H 3 PO 4 reduces the reorganization energy and enhances electronic coupling strength, while HClO 4 shows the opposite effect. Furthermore, H 3 PO 4 addition yields a lower-energy intervalence charge transfer (IVCT) absorption band and a higher electronic coupling matrix element, indicating that it not only reduces the formation energy of the dimers but also enhances V 4+ /V 5+ electron transfer kinetics, which is further corroborated by Raman spectroscopy. This work provides a reliable approach to evaluate ET kinetics and offers theoretical guidance for designing high-performance VRFBs.