High power, long cycle life capacitive carbon from Hibiscus cannabinus, a Agri-bio-waste with simultaneous value addition in water treatment application

Abstract

Herein we report a simple scalable process to convert Hibiscus cannabinus (HBC), a nontoxic, ubiquitous agricultural waste into multi-functional carbon material for energy and environmental application, in which supercapacitor of high specific capacitance, high-power density, and long cyclic stability was prepared, and the end product was tested for the removal of organic pollutants with high adsorption capacity and multiple reusability. Through specific surface area (SSA), CV, GCD and kinetics of charge storage analysis of the carbon materials prepared at different pyrolysis temperature (700, 800 and 900 °C). We demonstrate that, although increased pyrolysis temperature results in increased surface area, the specific capacitance is largely affected by the removal of hydrophilic and pseudocapacitive functional groups at high pyrolysis temperature. The carbon materials prepared at 800 °C provides the optimum combination of SSA, residual functional groups facilitating electrode wetting and pseudo-capacitance contribution from ion diffusion and surface redox reaction and exhibited high specific capacitance of 241.2 F/g at 0.5 A/g (three electrodes set up). Importantly, the HBC-800//HBC-800 symmetric supercapacitor could be charged/discharged in less than a second at current exceeding 26 A/g, leading to high power density of 30 kW/kg, and could be reversibly cycled with only 1.6% capacitance loss over 30,000 cycles (at 10 A/g). Furthermore, when used as contaminated water treatment media the HBC prepared at 700 °C showed excellent malachite green (MG) adsorption capacity of 397.76 mg/g with 84 % retention of capacity over 10 cycles.