Discussion of factors affecting the electric double-layer capacitor characteristics of carbon powders derived from HyperCoals

Carbon powder derived from hypercoal (HPC) is obtained by treating carbon precursors prepared by precipitation with a stabilization and carbonization process. Its carbon powder was labelled SB- or B- based on the type of HPC raw material (sub-bituminous or a bituminous) and the carbonization temperature. The specific surface area of the carbon powder (SB-900) obtained by heat-treating a carbon precursor manufactured from sub-bituminous coal (mainly used as boiler fuel) at 900 °C was found to be 737 m²/g. In contrast, the specific surface area of carbon powder (B-900) obtained by heat-treating a carbon precursor manufactured from bituminous coal (primaly used as a coke precursor) at 900 °C was found to be 400 m²/g. SB-900 showed a specific surface area of approximately 1.8 times greater than that of B-900. Furthermore, it was confirmed that the structure primarily consist of microporous pores. When SB-900 and B-900 were applied as electrode materials in electric double-layer capacitors (EDLCs), electrodes made from SB-900 were found to exhibit higher capacitance than those made from B-900 or commercially available activated carbon (YP-50F). Examining the factors contributing to this, based on the chemical composition of the starting materials and the amount of oxygen-containing functional groups as determined by temperature-programmed desorption (TPD) measurements, revealed that factors other than oxygen-containing functional groups significantly affected SB-900 and B-900. A detailed examination of the pore characteristics using Ar adsorption measurements showed that the pore size distribution of the HPC-derived carbon powder exhibited a peak at 0.46 nm when analyzed using the Horvath-Kawazoe method. The differential pore volume increased in the vicinity of this peak. Since the pores were similar in size to the ions in the sulfuric acid electrolyte used to evaluate the EDLC characteristics, it was inferred that the powder would demonstrate higher capacitance than commercially available activated carbon and be suitable for use in EDLCs with an aqueous electrolyte. Additionally, increased adsorption was observed at 0.6 nm or less for SB-900 compared to B-900. It is suggested that SB-900 contains a higher proportion of ultramicropores (0.7 nm or less) within its micropores than B-900.