The semi-solid flow cell system, consisting of an electrochemical charge-discharge cell connected to an external circulatory system of pumps and reservoirs, has been proposed as the grid-scale energy storage. Among the flow cells, electrochemical flow capacitor (EFC) stores electrical charge in an electrical double layer at the carbon-electrolyte interface in a flowable carbon-electrolyte suspension (slurry), so the EFC exhibits higher power density than that of semi-solid flow battery [V. Presser, et al., Adv. Energy Mater, 2, 895 (2012).]. However, compared to the battery system, EFC has the limited energy density only derived from electrical double layer capacitance. Our previous study revealed that the impregnation of redox-active organic compounds into the nanoporous carbon enable to enhance the high energy density (>20 Wh kg-1) of aqueous electrochemical capacitor with maintaining the high power density (>5 A g-1) and long cycle lifetime (more than 10000 cycles) [T. Tomai, S. Mitani, D. Komatsu, Y. Kawaguchi, I. Honma., Sci. Rep., 4, 3591, (2014).]. In this study, to address the limitation of the energy density of EFC, we employed the nanoporous carbon beads holding quinonic/hydroquinonic compounds to carbon-electrolyte suspension (quinone/carbon complex slurry) for both flowable electrodes of the EFC. The flowable quinonic/hydroquinonic slurry electrode was prepared by dispersing the organic compound-carbon composite in 0.5 M H2SO4 and 0.05 M HCl aqueous solution. We employed tetrachlorohydroquinone (TCHQ) as cathode active material and dichloroanthraquinone (DCAQ) as anode active material in positive and negative slurry electrodes, respectively. The weight ratio of the quinonic/hydroquinonic compound to the nanoporous carbon bead was fixed to be 3:7. The weight ratio of the organic compound-carbon composite to 0.5 M H2SO4 and 0.05 M HCl aqueous solution was fixed to be 13:87. Charge-discharge measurement at static operation, the EFC using the quinone/carbon complex slurry shows the high energy density (19.4 Wh/kg) in the applied potential range between 0-1 V. This value is about two times larger than that of the EFC using the unmodified carbon slurry. Thus, by using the redox reactions of quinonic/hydroquinonic compounds in positive and negative slurry electrodes, we achieved the enhancement of the energy density of EFC. Notably, the EFC using TCHQ/carbon complex slurry and DCHQ/carbon complex slurry for both electrodes investigated in this study exhibits 1.6 times higher energy density than the EFC using only hydroquinone/carbon complex slurry [M. Boota, K. B. Hatzel, E. C. Kumbur, Y. Gogotsi., ChemSusChem 2015, 8, 835.]. Moreover, charge-discharge measurement at intermittent flow operation, we demonstrated the prepared slurry electrodes were applicable to semi-solid flow system.