The electrochemical flow capacitor (EFC) is a new technology for grid energy storage that is based on the fundamental principles of supercapacitors. The EFC benefits from the advantages of both supercapacitors and flow batteries in that it is capable of rapid charging/discharging, has a long cycle lifetime, and enables energy storage and power to be decoupled and optimized for the desired application. The unique aspect of the EFC is that it utilizes a flowable carbon-electrolyte suspension (slurry) for capacitive energy storage. Similar to traditional supercapacitor electrodes, this aqueous slurry is limited in terms of energy density, when compared to batteries. To address this limitation, in this study a pseudocapacitive additive has been explored to increase capacitance. A carbon-electrolyte slurry prepared with p-phenylenediamine (PPD), a redox mediator, shows an increased capacitance on the order of 86% when compared with KOH electrolytes, and a 130% increase when compared to previously reported neutral electrolyte based slurries. The redox-mediated slurry also appears to benefit from a decrease in ohmic resistance with increasing concentrations of PPD, most likely a result of an increase in the ionic diffusion coefficient. Among the tested slurries, a concentration of 0.139M of PPD in 2M KOH electrolyte yields the largest capacitance and rate handling performance in both cyclic voltammetry and galvanostatic cycling experiments. The improved performance is attributed to the addition of quick faradaic reactions at the electrolyte-electrode interface as PPD undergoes a two-proton/two-electron reduction and oxidation reaction during cycling.