Hemp-derived activated carbons for supercapacitors

Activated carbons are prepared from raw hemp stem (hurd and bast) via hydrothermal processing and chemical activation. Both hemp bast and hemp hurd are converted into activated carbons of low-dimensional structures under certain experimental conditions. The experimental results show that hemp hurd is a better precursor than hemp bast for the preparation of activated carbons. The activated carbons are used to construct the electrodes of supercapacitor cells. The electrochemical performance of the activated carbons used in the supercapacitor cells is dependent on hydrothermal processing conditions and the mass ratio of KOH/biochar in the chemical activation. Excellent electrochemical performance metrics are achieved, including a specific capacitance of 160 F/g, and a high energy density of 19.8 Wh/kg at a power density of 21 kW/kg. Both the specific capacitance and capacitance retention increase with the increase of surface area and mesopore fraction. A simple relationship between the specific area capacitance and the fraction of micropores is proposed, via the rule of mixtures, and is supported by the experimental results. This relationship reveals the effect of the distribution of pore sizes on the specific area capacitance of electrochemical double layer capacitors.