Hydrogel Electrochemistry as a Structured Interfacial Platform: From Early Concepts to Emerging Applications.

Hydrogels are emerging as structured electrochemical interfaces that do more than replace liquid electrolytes in soft and aqueous systems. Because they combine ionic conductivity with a chemically and structurally tunable polymer network, hydrogels can define where electrochemical reactions occur, how ions partition and move, and how interfacial signals are stabilized or transduced. This minireview discusses recent progress in hydrogel electrochemistry through three connected themes, including spatial control of confined electrochemical reactions, molecular engineering of ion transport in hydrogel networks, and dynamic or architected hydrogel electrochemical interfaces. Across these themes, representative studies show that hydrogels can localize gelation and deposition, regulate ionic transport through fixed charges, coordination environments, and solvation effects, and create responsive or predesigned interfaces that enhance sensing and electrochemical performance. Rather than serving simply as soft electrolyte matrices, hydrogels can be viewed as programmable interfacial media in which reaction space, ionic environment, and signal generation can be engineered together. This view also points to the need for more direct links between hydrogel design and measurable electrochemical behavior, particularly in sensing, iontronic, and biointerfacial systems, as well as systems for energy storage and conversion.