Optimal electrolyte pH for efficient quinone-based aqueous redox flow battery and solar cell integration

Abstract

Quinone-based redox flow batteries (RFBs) have emerged as promising sustainable alternatives to conventional vanadium systems, offering lower costs, high abundance and compatibility with large-scale aqueous energy storage systems. Their tuneable redox potential with pH can also enable direct integration with photovoltaic (PV) devices for solar charging. However, the design of efficient quinone electrolytes requires a detailed understanding of pH-dependent proton-coupled electron transfer, which drives fast or sluggish reaction kinetics. Here, we apply a pH buffering method to a 2,7-AQDS anolyte to tune the cell voltage, enabling efficient operation of a proof-of-concept solar redox flow battery (SRFB) with maximised solar-to-output electricity efficiency (SOEE). We also investigate how pH affects the charging reaction pathway and the voltage efficiency (VE), establishing that a buffered 2,7-AQDS anolyte achieves optimal performance at pH 8–10. These findings provide key insights into device integration and the pH dependence of VE in this quinone electrolyte. Importantly, this work establishes practical guidelines for pH optimisation in the design of next-generation quinone-based aqueous RFB and SRFB technologies.