Effect of surface oxygen functionalities on capacitance of activated carbon in non-aqueous electrolyte

Electric double-layer capacitors (EDLCs) are composed of two activated carbon (AC) electrodes and an electrolyte/separator, in which the ACs contain numbers of surface oxygen functionalities (SOFs). In this work, the effect of SOFs on the EDLC's capacitance in non-aqueous electrolytes is studied by using a 1.0 m (molality) LiPF6 3:7 (wt.) ethylene carbonate-ethyl methyl carbonate electrolyte and a commercial activated carbon. Results show that the SOFs on one hand contribute to Faradic pseudocapacitance, and on the other hand adversely reduce the EDLC's performances, including the initial reversibility, coulombic efficiency, and capacitance retention. It is found that the AC behaves significantly different in the Li/AC half cells and in the AC/AC full cells and that the SOF's pseudocapacitance increases with widening the EDLC's operating voltage. The latter is attributed to the large-voltage hysteresis of the redox of SOFs. In this paper, the AC's unique behaviors in Li salt electrolyte are presented, and a possible mechanism for the observed behaviors is proposed.