The energy density of carbon-based electrochemical capacitors can been improved by grafting electroactive quinone-based molecules.1 The improvement in energy density of the quinone-grafted carbon electrode is attributed to both the redox capacitance of the electroactive molecules and the electrical double layer capacitance of activated carbon. The redox reaction of the quinone-grafted carbon electrodes at high scan rate typically shows quasi-reversible behavior, suggesting rather sluggish charge transfer. In this study, to understand the redox behavior of electroactive molecules in various environments, we investigated the electrochemical properties of activated carbon slurry electrode with electroactive molecules dissolved in the electrolyte (Scheme 1). The behavior of the electroactive molecules was investigated in terms of the pore size, the surface functional group, and specific surface area of activated carbon. The electrochemical measurements were conducted using a three-electrode cell. A glassy carbon electrode (6 mm in diameter) was used as the working electrode. 4,5-dihydroxy-1,3-benzenedisulfonic acid (BQDS) was used as the electroactive molecule. Activated carbon (AC, average pore diameter = 0.8 nm) was dispersed in n mM BQDS + 0.5 M H2SO4 electrolyte (n = 1, 3, 10, 100 mM). Cyclic voltammograms show that the ΔE of BQDS dissolved in H2SO4 (1 mM BQDS + H2SO4 | GC) was 345.5 mV at a scan rate of 20 mV s- 1 (Fig. 1a). By adding activated carbon into the electrolyte (1 mM BQDS + AC-dispersed H2SO4 | GC), the peak to peak potential difference ΔE of AC dispersed 1 mM BQDS + 0.5 M H2SO4 | GC was only 3.4 mV at a scan rate of 20 mV s- 1, which is a signature of a fast surface redox reaction (Fig. 1b). The reversibility of the redox reaction was clearly observed when the BQDS concentration was 1-10 mM, while a quasi-reversible reaction was observed when the BQDS concentration was 100 mM (Fig. 1c). The results suggest that the fast surface redox behavior of BQDS is observed only when the BQDS concentration in the electrolyte is low, i.e. below 100 mM BQDS. AC with various pore sizes was also studied. Fast surface redox behavior was obtained when AC rich with the micropores (pore size < 1 nm) were used even without grafting treatment. The result suggests that the BQDS is trapped in micropores via interaction in confined pores and is responsible for the fast redox reaction.2 , 3 G. Pognon, T. Brousse and D. Bélanger, Carbon , 49, 1340 (2011).Y. Hanzawa, T. Suzuki and K. Kaneko, Langmuir, 10, 2857 (1994).K. Kaneko, R. F. Cracknell and D. Nicholson, Langmuir, 10, 4606 (1994). Figure 1