Modification methods of carbon electrodes for vanadium redox flow batteries: Standardized testing and characterization protocols

Vanadium redox flow batteries (VRFBs) are among the most promising technologies for large-scale stationary energy storage because of their scalability, long cycle life, operational safety, flexible design and environmentally friendly operation. However, the sluggish redox kinetics and long-term instability of conventional carbon-based electrodes remain major challenges that limit their commercial deployment. Although many electrode modification strategies have been reported, direct comparison between studies is often difficult due to variations in experimental conditions, operating parameters and electrochemical evaluation methods. In many cases, modified electrodes show excellent catalytic activity in half-cell or three-electrode measurements, but these improvements do not always translate into stable long-term performance in practical full-cell systems. This limitation highlights the urgent need for standardized testing and characterization protocols in VRFB research. Recent advances in electrode engineering, including thermal and microwave treatment, chemical and electrochemical modification, defect engineering, plasma processing, heteroatom doping, incorporation of carbon nanomaterials or metal-based catalysts and ionic liquid-derived carbon materials, have demonstrated strong potential to improve catalytic activity, wettability, conductivity and cycling stability. In addition, advanced in-situ and operando techniques, such as electrochemical impedance spectroscopy (EIS), UV-Vis spectroscopy, Raman spectroscopy, X-ray computed tomography and synchrotron-based characterization, provide deeper understanding of electrode behavior under realistic flow battery operating conditions. This review critically summarizes recent progress in carbon electrode modification strategies, structural and electrochemical characterization methods, operando diagnostic approaches and full-cell evaluation protocols for VRFBs. A novel three-tier standardized reporting framework is proposed to improve reproducibility and enable meaningful comparison across different studies. Emphasis is placed on reporting essential operational parameters, including electrolyte composition, flow rate, current density, compression ratio and state-of-charge range. Establishing standardized testing protocols and advanced operando evaluation methods will support the rational design of durable and high-performance electrodes, thereby accelerating the practical deployment of VRFBs for large-scale renewable energy integration.