Low-temperature supercapacitors enabled by date-seed-derived activated carbon and NaClO4-based aqueous ternary electrolyte system

• Ternary NaClO 4 –water–PG electrolyte stays liquid to −80 °C with high conductivity. • Date-seed-derived AC provides sustainable, high-performance electrodes. • The interconnected micro–mesoporous structure of the bio-derived AC enhances charge storage and ion transport at low temperatures. • Developed supercapacitors remain stable over 10,000 cycles at −40 °C and 1 A g -1 . • A 2.0 V stability window enables an energy density of 73 W kg -1 at low temperatures. Supercapacitors are crucial at both room and low temperatures (LTs) due to the demand for high-power energy and rapid charge-discharge capabilities. Herein, a new ternary electrolyte system (TES) consisting of sodium perchlorate (NaClO 4 ), water, and a non-toxic co-solvent, propylene glycol, was designed. Additionally, date-seed-derived activated carbon (AC) was utilized for supercapacitor assembly to demonstrate that bio-derived AC can perform as effectively as commercial alternatives at LTs. High-surface-area (2423 m 2 g -1 ), free-standing AC electrodes were incorporated into a symmetric supercapacitor employing the TES. The interconnected micro-mesoporous structure of the AC enhances charge storage and transport, ensuring reliable operation across a range of temperatures; while the 6 m TES remains liquid down to -80 °C and maintains favorable physicochemical properties, with a viscosity of 104.9 mPa·s and a conductivity of 7.18 mS cm -1 at -20 °C. The combined system exhibits excellent electrochemical performance, retaining nearly 100 % Coulombic efficiency and capacitance over 10,000 cycles at 1 A g -1 at -40 °C. At this temperature, stable charge storage is sustained with minimal IR drop, and the electrochemical stability window (ESW) extends up to 2.0 V. This widened ESW allowed us to increase the energy density of supercapacitors up to 73 W kg -1 , which is essential for their performance. The combination of a bio-waste-derived electrode and a non-toxic, water-based electrolyte presents a sustainable and scalable approach to energy storage.