Elucidating the performance-limiting electrode for all-vanadium redox flow batteries through in-depth physical and electrochemical analyses

For the development of all-vanadium redox flow batteries (VRFB) with high power ratings, it is very crucial to identify the reasons that limit the kinetics of vanadium couples at the electrodes. This work, using two different electrode materials (a polyacrylonitrile-based carbon felt and a rayon-based graphite felt), elucidates the redox reactions at the anode as performance-limiting ones through various sophisticated physical and electrochemical analyses as well as the VRFB testings with/without homogeneous catalyst in the electrolytes and different electrode combinations. The electrode material with the properties of higher hydrophilicity arising from abundant surface oxygen functionalities and electrical conductivity exhibit higher VRFB performance, particularly due to improved reaction kinetics at the anode. Moreover, the incorporation of catalyst, especially for the V³⁺/V²⁺ couple, results in a significant improvement of VRFB performance by accelerating the kinetics of V³⁺/V²⁺ redox reactions as well as mitigating the harmful effects of hydrogen evolution reactions and temperature-related effects.