Applications for CO2-Activated Carbon Monoliths: II. EDLC Electrodes

The use of high‐density carbon monoliths (CM) for preparing supercapacitor electrodes is analyzed. The starting CMs, produced by ATMI Co, were treated as follows: (1) under a N2 flow at 1073 K to modify the carbon surface chemistry and (2) activated with CO2 at the same temperature, using different activation times, to increase their porosity. Electrochemical measurements were performed on disks of 1–2 mm thickness which are suitable for direct use in practical devices. Two‐ and three‐electrode cells were used with 2 M H2SO4 solution as electrolyte. The contents of surface oxygen groups were measured by temperature‐programmed desorption (TPD) and X‐ray photoelectron spectroscopy (XPS). The porosity of the starting monolith is increased by physical activation with CO2, the BET surface area increasing from 957 to 1684 m2/g. Upon heat treatments, both, the high density (1.2 g/cm3), as well as the high amount of surface oxygen groups (2411 μmol CO/g) of the starting monolith are reduced; however, the densities of the treated monoliths remain higher than values reported for other porous carbon monoliths. The performance of the CMs as supercapacitor electrodes show as follows: (1) high specific and exceptionally high volumetric capacitances (up to 292 F/g and 342 F/cm3, respectively) due to their appropriate structure, porosity, and density, (2) a long and stable cyclability, (3) a decrease of power density with disk thickness, and (4) a decrease of pseudocapacitance with activation time.