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

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 ${L_{\mathrm{anolyte}}}^{-1}$ 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.