Modulating Water Activity in Electrochemical Systems: Key Factors and Strategies

ABSTRACT Water serves multiple critical roles as a polar solvent and a clean and abundant source for protons and hydroxyl anions in various electrochemical systems. In this context, its activity, governed not only by thermodynamics but also by inter‐ and intra‐molecular dynamics, significantly affects the performance of various electrochemical systems for energy storage and conversion. This review explores how water activity can be modulated through parameters such as pH, electrolyte salts, additives, and temperature to tailor the local water environment and interfacial electrochemical reactions. We examine their influences on various electrochemical systems for CO 2 conversion, ammonia synthesis, water electrolysis, and aqueous batteries. By offering an expanded perspective and comprehensive understanding of water activity across various electrochemical systems, this review provides both mechanistic insights and rational design principles for next‐generation technologies for sustainable energy and chemical processes.