Contribution of surface oxygen groups to the measured capacitance of porous carbon supercapacitors

Abstract

Porous carbon is used as the electrode material in electrochemical double-layer capacitors (EDLCs) due to its high surface area and electrical conductivity. However, one of the challenges associated with porous carbon is the presence of electrochemically active surface oxygen groups, which undergo rapid charge transfer reactions in alkaline or acidic electrolytes. In this study, oxygen groups have been removed from porous carbon samples in order to quantify their contribution to the double-layer capacitance. The oxygen-free porous carbon samples have similar pore size distributions to the original sample; however, the specific surface area (SSA) is observed to increase slightly with heat treatment. The original porous carbon sample, containing oxygen, has an SSA-normalized capacitance of 12.2 ± 0.2 μF cm−2, while the oxygen-free samples have SSA-normalized capacitances of 8.8 ± 1.0 μF cm−2, regardless of total SSA. Based on these results, the relative contributions of the carboxyl and phenol oxygen groups are determined, which are the two electrochemically active oxygen groups in alkaline solutions. From this work, it is shown that the carboxyl groups and phenol groups contribute approximately 1.10 μF cm−2at.%−1. These results demonstrate the importance of oxygen groups for accurate characterization of the performance of novel porous carbons for EDLCs.