Effect of pH and activation on macroporous carbon derived from cocoa-pods for high performance aqueous supercapacitor application

Abstract

Activated carbon (AC) electrodes for supercapacitors (SCs) are typically prepared by high-temperature carbonization processes and involves multiple stages of activation. Herein, we report a facile, hydrothermal carbonization of biomass cocoa-pods via ZnCl2 activation during pyrolysis that results in macro-porous carbon (AC1). ZnCl2 serves as a template and interacts with the cocoa-pods promoting efficient carbonization reaction resulting in large specific surface area (SSA = 1061.6 m2g-1) macro-pores, while the AC2 (KOH activation) results in SSA of 819.79 m2g-1. FESEM micrographs reveal well-defined large-size pores of ∼3.2 μm diameter that facilitate easy access of the electrolyte ions into the pores thereby contributing to the efficient charge-storing ability suitable for SCs performance. The porous AC1 exhibits high specific capacitance and excellent rate capability over a wide pH range from 2.5 to 13.5. The highest specific capacitance (320 Fg-1) was obtained in 6 M KOH (pH = 13.5) at 1 Ag-1 due to the high ionic conductivity and high mobility of OH− anions in KOH electrolyte. The symmetrical SC with AC1 as the electrode delivers a high energy density of 126 Whkg−1 at a power density of 11,520 Wkg-1 with a wide operating voltage of 1.7 V in 1 M Li2SO4 (pH = 5.6) electrolyte. AC1 delivered higher power density than AC2 (KOH-activation) and 100% capacitance retention over 24,000 cycles. The work demonstrated here outlines a new strategy for optimizing the efficient performance of biomass derived carbon electrodes in aqueous electrolytes of varying pH.