Activated carbon/ZnO composites prepared using hydrochars as intermediate and their electrochemical performance in supercapacitor

• Hydrochars as intermediate to prepare activated carbon/ZnO composites.
• Activated carbon/ZnO showed excellent electrochemical performance in supercapacitors.
• Activated carbon with large surface area can be obtained by removing ZnO.

We report a new methodology to prepare activated carbon and activated carbons/ZnO composites from walnut shell-derived hydrothermal carbons (hydrochars), which were prepared under hydrothermal condition in presence of ZnCl2. For this method, activated carbon/ZnO composites were prepared via heat treatment of hydrochars under inert environment and activated carbons were prepared by removing the ZnO in activated carbon/ZnO composites. The chemical structure of walnut shell, hydrochars, activated carbon/ZnO and activated carbon was investigated by Fourier transform infrared spectroscopy, Raman, X-ray powder diffraction, thermogravimetric analysis and N2 adsorption/desorption measurements. It is found ZnCl2 plays multiple roles, i.e., helping to remove the oxygen-containing groups during hydrothermal stage, improving the surface area of activated carbon and acting as the precursor of ZnO in heat-treatment stage. The specific surface areas up to 818.9 and 1072.7 m2 g−1 have been achieved for activated carbon/ZnO composites and activated carbon, respectively. The activated carbon/ZnO as electrode materials for supercapacitors showed that specific capacitance of up to 117.4 F g−1 at a current density of 0.5 A g−1 in KOH aqueous solution can be achieved and keeps stable in 1000 cycles.