Process optimization and electrochemical investigation of nanostructured carbon materials for mixed assembly supercapacitor applications

Abstract

In this study, the effect of the inclusion of a hydrothermal pre-treatment procedure and varying the mass ratios of the raw material and the activating agents on the electrochemical performance of laboratory-synthesized activated carbon (AC) nanostructures is reported. Symmetric cells and mixed assembly cells were fabricated and tested in this work. The AC materials were obtained using an environmentally friendly, mild-alkaline potassium based activating agents (AAs). Hibiscus biomass waste and polypyrrole materials were adopted as the raw materials for source of carbon in the optimization process using these AAs to synthesize porous electrochemically active materials. The incorporation of the hydrothermal (HT) pre-treatment procedure for the hibiscus biomass waste material decreased the optimal carbonization time to 1 hour (sample was noted as (HTAC-1) and increased the specific surface area (SSA) of the material. The effect of the mass ratio of the AA and raw material was analysed for the polypyrrole (PPY) raw material and the SSA as well as the pore volume were found to increase with an increase in the AA ratio to a maximum threshold of 6:1 (sample was noted as AC-PPY-6). A mixed assembly (MA) device fabricated from the HTAC-1 and the AC-PPY-6 samples demonstrated an extended operating potential window of 1.70 V. A good stability was displayed by the MA device after a stability test of 10 000 constant galvanostatic charge-discharge cycles. The device had a capacitance retention of 82% with a corresponding coulombic efficiency of 99.6% respectively. Most importantly, the performance of the device improved after an 80 hour voltage holding ageing test, the capacitance of the device increased as time progressed, from 94.4 F·g-1 to 219.5 F·g-1. The stability metrics displayed by the mixed assembly device was relatively better as compared to the AC-PPY-6 and HTAC-1 symmetric devices. Although the capacitance value was lower compared to the AC-PPY-6 symmetric device, the MA device showed better energy storage capability after voltage holding (floating) tests.