Reduction of VO2+ in electrolysis cell combined with chemical regeneration of oxidized VO2+ electrolyte for operando capacity recovery of vanadium redox flow battery

Gradual capacity fade of vanadium redox flow batteries (VRFB) is one of the greatest challenges for further distribution of this technology, since capacity stability is crucial for industrial energy storages. It is suggested that the main reason for this issue is vanadium ions crossover, however recent studies show that appearance of chemical and electrochemical side reactions leading to increase of the electrolyte average oxidation state (AOS) should be considered as a greater contributor to capacity fade. It should be noted that increase of overall AOS is more challenging to eliminate compared to vanadium ions crossover. To date, a great variety of VRFB capacity fade mitigation methods have been proposed, but very little attention has been paid to development of methods for the battery capacity recovery. Despite several methods for AOS decreasing via partial posolyte reduction have been demonstrated, none of them were implemented for operando capacity recovery due to a number of ongoing issues. In this work, a novel method for operando VRFB capacity is proposed. It consists of partial electroreduction of posolyte in electrolysis cell utilizing chemically regenerative electrolyte. Regeneration of VO2+ by reducing agents is cost-effective, and separation of this process from the battery using electrolysis cell allows proceeding VRFB capacity recovery operando. This study is devoted to investigation of both chemical and electrochemical stages of the proposed method, and its optimization for rebalancing under cycling conditions. It is suggested that integration of the VRFB stack with the proposed rebalancing system allows to demonstrate high battery performance while maintaining nominal capacity indefinitely.