Comparative study on pilot-scale production of CuO-loaded activated biochar and hydrochar from oil-palm empty fruit bunches for high-performance symmetric supercapacitor application

•Pilot-scale slow pyrolysis or HTC were used to produce carbon electrodes from EFB.•CuO/ABC electrodes exhibited high specific capacitance of up to 495.4F/g.•CuO/ABC electrodes produced energy density of 22.83 Wh/kg at power density of 1,200 W/kg.•CuO/ABC electrodes had capacitance retention rate of 92.4% after 5,000 cycles.•CuO/AC is a promising electrode which can be mass produced from pilot-scale production. This study reports a comparative study on the integration of double-layer capacitive activated carbon with CuO, which imposes pseudocapacitive activity to the electrode materials, for supercapacitor application in the presence of different electrolytes. The activated carbon was prepared from different pilot-scale char production procedures. The CuO/activated char (CuO/AC) electrode material with an exceptional electrochemical performance was efficiently produced by slow pyrolysis or by hydrothermal carbonization through the chemical activation to produce activated biochar (ABC) or activated hydrochar (AHC). The specific surface area and total pore volume of the produced ABC was 810–1008 m2 g−1 and 0.53–0.79 cm3 g−1, respectively. In a three-electrode cell, the CuO/ABC electrode material exhibited superior specific capacitances of 495 and 274F g−1 in 2 M KOH and 1 M Na2SO4 electrolytes, respectively. The energy density of the symmetric supercapacitor produced in KOH electrolyte was 11.42 Wh kg−1 with power density of 600 W kg−1 and could maintain up to 6.17 Wh kg−1 at 6,000 W kg−1. In Na2SO4 aqueous electrolyte, the cells achieved the maximum energy density of 6.01 Wh kg−1 at 700 W kg−1. In the cyclic stability test, the initial capacitance was retained at 92.7% after 5,000 cycles by using Na2SO4 as the electrolyte, signifying that CuO/ABC is a promising electrode material which can be mass production from pilot-scale to perform the excellent electrochemical energy storage capacity with outstanding long-cycling life.