Uncovering the Electrochemical Origin of Alkalization in Viologen‐Based Aqueous Flow Batteries

Abstract

Abstract Alkalization of viologen‐based anolytes during charge–discharge cycling poses a formidable obstacle to the practical implementation of aqueous organic redox flow batteries (AORFBs) by promoting molecular degradation and accelerating capacity decay. This effect is most severe under 2‐electron transfer conditions, which hinder the full exploitation of viologen's redox potential and thereby limit the attainable energy density. To uncover the origin of this phenomenon, we developed a multimodal in situ pH‐gas chromatography‐AORFB characterization platform that reveals a two‐stage alkalization mechanism. In Stage I, hydrogen evolution reactions dominate at low reduction potentials, driving a rapid and irreversible pH rise; In Stage II, quasi‐reversible interconversion between quaternary ammonium and pyridinic nitrogen sites engenders sustained pH oscillations. Guided by these insights, we employed a 3 M KCl supporting electrolyte in viologen anolyte, enabling the AORFB with 2 M electron transfer to deliver a practical energy density of 66.9 Wh and retain 99.25%/day capacity retention rate over 200 cycles. This study not only advances the understanding of the alkalization mechanism in viologen‐based anolytes but also establishes a broadly applicable framework for the design of durable electrolytes for AORFBs.