Surpassing water-splitting potential in aqueous redox flow batteries: insights from kinetics and thermodynamics

Aqueous redox flow batteries (AQRFBs) employing non-flammable electrolytes are recognized for their inherent safety and eco-friendliness, making them promising candidates for large-scale energy storage systems. Furthermore, the unique architecture of this battery technology enables autonomous decoupling of power and energy, resulting in higher capacity and enhanced cost-effectiveness compared to other battery technologies. Nonetheless, the limited electrochemical stability of water leads to water electrolysis during the electrochemical process, triggering undesired parasitic reactions, namely, the hydrogen evolution reaction, and ion-cross-over. These reactions significantly affect the electrochemical performance of the system, giving rise to several challenges, including low Coulombic efficiency and a short cycle life, hindering the advancement of AQRFBs. To overcome these obstacles and achieve high-potential AQRFBs, it becomes essential to incorporate a reaction-inhibitor to encounter water electrolysis during battery operation. This perspective review focuses on addressing and mitigating the thermodynamic limitations through improved strategies, proposing effective approaches to suppress aforementioned side reactions.