Catalysis of Vanadium Ion Redox Reactions on Carbonaceous Material with Metal–N4 Sites

Abstract The low energy efficiency caused by the slow reaction rates of vanadium ion redox reactions is the main drawback in vanadium redox flow batteries (VRFB), which is a promising large‐scale electrical energy storage system capable of leveling fluctuating power generation by wind turbines and photovoltaic cells. The high catalytic activity was found at a carbonaceous thin film with a metal ion and four nitrogen atoms (metal–N4 site) combined in a carbon matrix in the square‐planar configuration and exposed to the surface. Only a specified metal caused the activity as the central metal according to the redox reactions. Regarding the dioxovanadium ion (VO2+) reduction, Fe was the active metal, and the VO2+ reduction behavior indicated that the reduction proceeded through the inner sphere mechanism involving the hydroxylic group attached to the central Fe in contrast to the outer sphere mechanism for the other metals and conventional carbon electrodes.