Electrochemical performance of electrodes produced from carbon aerogels of lignocellulosic wastes: effects of chemical activation using KOH and ZnCl₂

Renewable and sustainable biomass materials have begun to be used in many new areas. In this context, lignocellulosic wastes such as hazelnut shells (HS), sunflower seed shells (SSS), and rice hulls (RH) were activated chemically using a base (KOH) or a salt (ZnCl₂) followed by physical activation at 800–900 °C. Then, carbon aerogels were produced from these pre-activated biomasses and used to prepare symmetrical supercapacitor (SS) electrodes. Electrochemical tests of SSs were performed using the potentiostat/galvanostat where the reference electrode and counter electrode for the three-electrode system were an Ag/AgCl electrode and a Pt wire, respectively, and the electrolyte was 0.5 M H₂SO₄ /PVA gelled. Although the specific surface area of the aerogel produced from ZnCl₂-activated SSS was only 137 m²/g, it provided a specific capacitance of 275.11 F/g at a scan rate of 5 mV/s and a long cycle test comprising 10,000 cycles, while the aerogel produced from ZnCl₂-activated HS with specific surface area of 1154 m²/g showed lower electrochemical potential. Besides, the electrochemical performance of KOH-activated HS outperformed the KOH-activated SSS. It is concluded that to prepare aerogels that have high specific capacity, biomass with high holocellulose- and extractive substances- content (like SSS) should be activated by ZnCl₂, while biomass with high lignin content (like HS) should be activated by KOH. It is also concluded that only the specific surface area can not predict the specific capacitance and other factors such as having a hierarchical pore structure and having a suitable surface chemistry may be more decisive.