Spent tea leaves to multifunctional environmental materials: Boric-acid-activated carbons for acetamiprid removal and energy storage, and a green Co catalyst for NaBH₄ hydrolysis

Spent tea leaves were valorized through an integrated and sustainable approach to develop multifunctional materials for hydrogen production, environmental remediation, and energy storage. Two types of activated carbons were synthesized from spent tea leaves via direct carbonization and hydrothermal pretreatment, followed by boric acid activation, a rarely explored and environmentally benign activating agent. In parallel, a cobalt-based catalyst (Co@HME) was synthesized using hydrothermal tea extract as a green support and evaluated for hydrogen generation via NaBH₄ hydrolysis. Comprehensive characterization revealed that hydrothermal pretreatment significantly enhanced surface functionality, porosity, and metal dispersion. The Co@HME catalyst exhibited high catalytic activity with an activation energy of 32.53 kJ mol⁻¹ and demonstrated reasonable reusability over multiple cycles. The hydrochar-derived activated carbon (Stl-Hc-AC) showed superior acetamiprid adsorption performance compared to directly prepared activated carbon, achieving high adsorption capacities under acidic conditions and following pseudo–second-order kinetics. Additionally, Stl-Hc-AC exhibited excellent electrochemical performance as a supercapacitor electrode, delivering a specific capacitance of 342 F g⁻¹, 92.05% capacitance retention after 5000 cycles, and favorable energy–power characteristics. This work demonstrates a zero-waste strategy for the comprehensive utilization of spent tea leaves, offering a sustainable pathway toward clean hydrogen production, efficient pollutant removal, and high-performance energy storage.