The effects of vacuum pyrolysis conditions on wood biochar monoliths for electrochemical capacitor electrodes

Vacuum pyrolysis is mostly employed to extract liquid biofuels from biomass in industry, but the potential application of its by-product (biochar) is seldom studied. The wood biochar monoliths were prepared by pyrolyzing cotton rose stem under vacuum conditions and a common nitrogen atmosphere, respectively. The effects of vacuum conditions on the physicochemical properties of biochar monoliths were investigated by various techniques, such as Raman, FTIR, XPS, TG, BET, CO2 adsorption and SEM methods. The results showed that the vacuum pyrolysis biochar, in comparison with nitrogen atmospheric pyrolysis biochar, features a higher degree of graphitization, a richer micro-porosity (57.2% vs. 33.9%), and a more reactive surface. The comparison of electrochemical performance suggested that the electrode prepared by vacuum pyrolysis overtakes that by nitrogen atmosphere pyrolysis with a specific capacitance of 161 F g−1 versus 133 F g−1 and an ease ion delivery capacity. Moreover, 125% of initial capacitance was retained over 10000 cycles at 50 mA cm−2. The vacuum conditions are more favorable for the preparation of biochar as well as the biochar derived activated carbon materials for application as electrodes.