Air activation of charcoal monoliths for capacitive energy storage

Charcoal monoliths derived from waste wood were activated with air for the application of electrochemical capacitor electrodes and an insight was given into the activation mechanism. The mild air activation is effective and pollution-free compared to the common chemical activation using KOH etc. for the preparation of crack-free carbon monoliths. The activation process was controlled by altering the activation temperature and time, and their effects on the nanostructure of charcoal monoliths were studied. As the activation temperature or time increased, air eroded the defective surface of charcoal layer-by-layer, with the oxygen atoms being introduced by chemisorption and oxidation reactions and removed by dehydration and decomposition reactions. Meanwhile, micro-pores were produced. The electrode activated at 300 °C for 1 h, with a specific surface area of 567 m² g^-1 and a high micro-porosity of 86%, exhibited a specific capacitance of 203 F g^-1 and 35.5 F cm^-3. Moreover, it presented a higher total capacitance of 3.6 F cm^-2 than most reported pellet electrodes. These findings give a reasonable picture of the air activation process and are instructive to prepare activated carbon monoliths under an oxidizing environment.