Valorization of Macadamia Nut Shell Waste into Activated Carbons for Electrochemical Supercapacitor Electrodes

Supercapacitors are promising energy storage devices owing to their high power density and excellent cycling stability. The conversion of biomass waste into activated carbon with hierarchical pore structures has emerged as a sustainable strategy for developing high-performance supercapacitor electrodes. Herein, activated carbons were synthesized from macadamia nut shell (MNS) waste via a two-step process comprising carbonization at 600 °C and subsequent chemical activation with potassium hydroxide (KOH) at 800 °C, using weight ratios of carbonized MNS to KOH of 1:1, 1:2, and 1:3. The resulting activated carbons exhibited an amorphous structure with a predominant microporosity. KOH activation substantially increased the specific surface area from 325 to 1362 m² g^-1 through gas evolution, etching, and potassium intercalation, thereby promoting the predominant micropore formation along with partial mesopore development. Increasing the KOH ratio induced greater structural disorder, widened micropores, increased the mesopore fraction, enriched C-O functionalities, and suppressed CO and O-CO groups. Electrochemical measurements in a 6 M KOH electrolyte using a three-electrode system revealed that the sample activated at a 1:3 ratio delivered a maximum specific capacitance of 170 F g^-1 at 1 A g^-1 and exhibited predominantly electric double-layer capacitor (EDLC) behavior with a minor pseudocapacitive contribution, as confirmed by power-law, Trasatti, and Dunn analyses. This performance is attributed to its large specific surface area, partially developed mesoporosity, and enhanced surface wettability. A symmetric coin-cell supercapacitor assembled using this material delivered an energy density of 5.4 W h kg^-1 and a power density of 2500 W kg^-1, while maintaining excellent cycling stability over 10,000 charge-discharge cycles at 3 A g^-1. These results reveal the potential valorization of MNS-derived activated carbons as sustainable and efficient electrode materials for high-performance supercapacitors.