Electrosorption on activated biochar: effect of thermo-chemical activation treatment on the electric double layer capacitance

Biochar, a by-product of woody biomass pyrolysis, is investigated as a renewable and low-cost carbon-based electrode material for electric double layer (EDL) applications. To increase the surface area and porosity of the biochar chemical (7 M KOH) and thermal (at 675 and 1,000 °C, respectively) activation treatments are applied. The thermo-chemically activated biochar samples are investigated by a combination of physico-chemical surface characterization and electrochemical methods to reveal the relationship between the activation process variables, the resulting porous carbon structural features and EDL capacitance. For electrochemical testing, the activated biochar is sprayed onto Ni mesh current collectors with or without Nafion^® as binder. Based on cyclic voltammetry experiments in 0.1 M NaCl–0.1 M NaOH a maximum EDL capacitance of 167 F g^?1 is obtained for the activated biochar electrode prepared at 675 °C. The latter capacitance is about 50 times higher than the EDL capacitance of a Vulcan XC-72 electrode prepared and tested under identical conditions. The activated biochar electrodes show also promising galvanostatic charge/discharge behavior and electrical conductivities up to 0.058 S cm^?1 indicating suitability for EDL-type applications.