Exploring the feasibility of using spent rooibos (Aspalathus linearis) tea waste–derived activated carbon for supercapacitor applications

The global drive toward sustainable and environmentally friendly energy solutions has increased interest in developing efficient energy storage devices. In this context, using waste materials as precursors for activated carbon (AC) production has garnered substantial attention. This study investigates the feasibility of using spent rooibos (Aspalathus linearis) tea waste (SRTW), a hitherto unexplored precursor, to synthesize AC for supercapacitor applications. The sample generated under optimum conditions (700 ℃, 75 min, and 2 g/g, activation temperature, time, and ratio), designated as 2AC700-75, was subjected to morphological analysis by field emission scanning electron microscope (FESEM), which revealed the porous nature of the prepared AC. Concurrently, Fourier transform infrared spectroscopy (FT-IR) elucidated the presence of carbonaceous functional groups. Cyclic voltammetry (CV) analyses were employed to determine the electric double-layer capacitance (EDLC) in the presence of a 6-M potassium hydroxide (KOH) electrolytic medium. Electrochemical impedance spectroscopy (EIS) generated a Nyquist plot, revealing a charge transfer resistance value (R_ct) of 11.36 Ω. Furthermore, a galvanostatic charge–discharge (GCD) study determined the specific capacitance values of 109.16 and 210 F/g at 0.5 A/g for two- and three-electrode systems, respectively. Remarkably, it exhibited excellent cycling stability, retaining 99.99% of its capacitance and maintaining 100% coulombic efficiency after 5000 cycles at a current density of 2 A g⁻¹. In a symmetric coin supercapacitor cell with a 6 M KOH solution, 2AC700-75 electrodes exhibited exemplary EDLC behavior, featuring an energy density of 15.16 W h kg⁻¹ and a power density of 999.2 W kg⁻¹. The findings illuminate the promising future of waste-to-energy conversion and its critical role in a sustainable and clean energy landscape. Graphical Abstract: (Figure presented.)