Supercapacitors beyond energy storage: Multi-functional devices for sensing, actuation, and smart systems

Supercapacitors (SCs) are evolving from passive high-power energy storage units into active, multifunctional elements that simultaneously store energy, sense, actuate, harvest, and communicate. This review critically examines how electrodes, electrolytes, separators, and current collectors can be engineered to couple charge storage with secondary functions such as mechanical, chemical, and thermal sensing, electrochemical actuation, electrochromism, self-healing, and self-charging. Rather than cataloguing demonstrations, we compare material families (carbonaceous materials, conducting polymers, transition metal oxides, MXenes, MOFs) and device architectures (flexible, stretchable, micro-, fiber/yarn and structural SCs) using common figures of merit: energy/power density, sensitivity, response time, durability, safety, and integration complexity. Particular attention is given to trade-offs between capacitance and transduction sensitivity, energy density and mechanical robustness, and multifunctionality and long-term stability under coupled electro-chemo-mechanical loading. We highlight cross-cutting design strategies such as hierarchical porosity, interfacial/spacing engineering, healable solid and gel electrolytes, and 3D or textile-integrated formats, and assess their practicality for wearable systems, soft robotics, e-skin, smart windows, and IoT nodes. Finally, we identify key gaps, including limited energy density, inadequate standards for benchmarking multifunctional performance, and immature system-level integration, and outline research directions towards manufacturable, safe, and truly smart SC-based power–sensing–actuation platforms.