Supercapacitors Under Nonequilibrium Conditions: Mechanisms and Design Principles

Supercapacitors have attracted considerable interest as high-power energy storage devices for grid frequency regulation, backup power systems, and fast-response applications, which are increasingly operated under conditions far from equilibrium, including floating states, high-frequency cycling, rapid charge and discharge, wide temperature windows, and high-voltage regimes. Under these nonequilibrium conditions, performance and stability are governed by coupled kinetic and thermodynamic processes, including ion transport limitations, interfacial polarization, electrolyte decomposition, and structural stress evolution. These processes span multiple length and time scales, from ion desolvation and surface reactions through pore-level transport mismatch to macroscopic degradation. This review presents a unified perspective on the mechanisms that govern supercapacitor behavior under nonequilibrium conditions. Representative operating scenarios are analyzed to identify three common degradation origins, namely, transport mismatch, interfacial instability and thermo-mechanical coupling. Building on this mechanistic understanding, rational material-design principles are derived on ion transport regulation, interfacial chemistry regulation, electrolyte optimization, and structural adaptability. Finally, future research directions are discussed for bridging the gap between laboratory evaluation and real-world operation and for guiding the development of robust supercapacitors for dynamic energy storage applications.