Advances in Functional Hydrogel Electrolytes for Flexible Supercapacitors: Adhesive, Self‐Healing, Structurally Resilient and Anti‐Freezing Systems

ABSTRACT Hydrogels possess distinctive characteristics such as softness, high wettability, excellent ionic conductivity, and enhanced device safety compared to conventional liquid electrolytes. These features make them particularly promising for flexible supercapacitors, where frequent mechanical deformation, including twisting, bending, and stretching are inevitable. However, traditional hydrogels face several limitations in long‐term applications. They often exhibit poor water retention, leading to dehydration and degradation of electrochemical performance over time. Additionally, inadequate interfacial adhesion results in delamination or detachment at electrode‐electrolyte interfaces. Under large mechanical strain, conventional hydrogels tend to suffer partial loss of charge storage capability. Their performance also deteriorates significantly under extreme environmental conditions, restricting practical applicability. To permanently replace liquid electrolytes in flexible supercapacitors, hydrogels must be engineered to possess advanced functionalities. This can be achieved by manipulating polymer network architectures, tuning crosslinking types and densities, modifying chemical structures, or incorporating functional moieties and transformative nanostructures. This review article highlights the essential functionalities required for hydrogel‐based electrolytes, discusses current challenges, and outlines strategic pathways toward the development of next‐generation hydrogel systems for high‐performance flexible supercapacitor applications.