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 N-2 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 m(2)/g. Upon heat treatments, both, the high density (1.2 g/cm(3)), as well as the high amount of surface oxygen groups (2411 mu mol 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/cm(3), 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.