Negative electrodes in vanadium redox flow batteries: recent advances and outlook

• The impacts of slow kinetics, hydrogen evolution reaction, and mass transport limitations are discussed • Specific desired properties of electrode materials are analyzed • The alignment of the band structure, work function, and Fermi level is essential • Balanced controlled doping and defect introduction are also essential • The multi-scale composite idea is a promising route for creating an ideal electrode Vanadium redox flow batteries are among the most promising technologies for large-scale energy storage, offering high safety, scalability, and stable performance through an aqueous electrolyte system. These attributes enable efficient integration of intermittent renewable sources such as wind and solar power. However, widespread deployment is constrained by limitations at the negative electrode, where the V 3+ /V 2+ redox couple exhibits sluggish kinetics compared to the V 5+ /V 4+ reaction at the positive electrode. Graphite felt, the most commonly used electrode material, satisfies several desirable properties: high conductivity, chemical stability, and large surface area; but suffers poor electrochemical activity toward V 3+ /V 2+ conversion. This mismatch in reaction rates, combined with parasitic hydrogen evolution within the operating potential window and polarization losses, significantly reduces overall battery efficiency. This review critically examines strategies to overcome these challenges, including the role of functional groups on the electrode surface, electrocatalyst incorporation, and alternative electrode materials. The discussion begins with an analysis of fundamental limitations, side reactions, and degradation mechanisms, followed by criteria for electrode selection and the role of surface functional groups in enhancing kinetics. Approaches for electrocatalyst design and their impact on reaction rates are highlighted, and future research directions are proposed to accelerate the development of such batteries. By addressing the bottleneck at the negative electrode, these advancements aim to improve energy efficiency and durability, paving the way for broader adoption of such redox flow battery technology in grid-scale applications.