Correlation of Capacitance with the Pore Structure for Nanoporous Glassy Carbon Electrodes

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Abstract
The correlation between the pore structure of monolithic nanoporous glassy carbons (NPGCs) and their double-layer capacitance in KOH-aqueous electrolyte was investigated. The results obtained show that, in general, capacitance increases with surface area. However, the capacitance for NPGCs with high conductivity strongly depends on pore structure. At a low current rate, the double-layer capacitance (DLC) comes mostly from the contribution of micropores with the properties of a metal conductor, while the contribution of external pores with the properties of a semiconductor is very limited. As current densities increase, micropores contribute less to the DLC due to the existence of too narrow micropores, while external pores contribute more to the DLC due to a decrease in the effect of semiconductor behaviors. In addition, it is determined that with the charge-discharge rate increasing, minimum pore sizes that contribute to the DLC increase but are still in the microporous range. It has also been observed that for samples with mesopore or wider micropore size distribution, resistance is low and capacitance becomes insensitive to the charge-discharge rate. Therefore, carbon materials with a wider micropore size distribution have higher surface areas and larger capacity than those with a mesoporosity, being more suitable electrodes for electrochemical capacitor applications.