Eco-friendly supercapacitor for temperature sensing and energy storage in dual-function operation

Hydrogel-based supercapacitors are emerging as sustainable platforms for multifunctional energy devices, yet their use as temperature sensors is still limited by transport-related instabilities. In this work, an eco-friendly hydrogel supercapacitor is investigated as a dual-function device for simultaneous energy storage and temperature sensing. It is shown that conventional charge–discharge readout becomes unreliable above the hydrogel gelation transition (~303 K), where leakage currents induce non-monotonic capacitance and increased energy dissipation. To overcome this intrinsic limitation, a pulsed-current sensing strategy is introduced, extracting temperature from the equivalent series resistance (ESR). The ESR exhibits a smooth and monotonic temperature dependence across 278–333 K, enabling fast and leakage-insensitive measurements. The method provides 50 ms response times and ultra-low energy consumption (<9 μJ per measurement), largely independent of sensing current and device geometry. A comprehensive electrochemical analysis combining impedance spectroscopy, galvanostatic measurements, and transient characterization reveals two distinct transport regimes governed by gelation and leakage–capacitance interplay. The results identify ESR as a robust temperature observable and define the physical limits of capacitance-based sensing in hydrogel systems. These findings establish a general readout strategy for sustainable supercapacitors operating in dual-function mode and highlight their potential for sensing in low-power IoT applications.