Supercapacitors (SCs) can reach a high specific energy (25-50 Wh/kg) by doping mediators in electrolyte contained in porous electrode structure. Mediators in electrolyte can be considered as redox materials to provide pseudocapacitance and to increase the conductivity of electrolyte. However, the charge/discharge behaviors of the mediators with respect to current and voltage are difficult to be resolved because of the complicated structure and charge/discharge kinetics of the electrode materials. In this work, a 3D microscopic model is established to investigate the roles of different components, especially mediators in charge/discharge processes. The microscopic model (Figure 1) is solved using COMSOL Multiphysics. Activated carbon, carbon fibers, mediators and polymer electrolyte are explicitly represented in this model. The simulation describes the process after applying a voltage on the mediator SC. During charging, electric current flows into the mediators at positive electrode and flows out from the mediators at negative electrode. The charge rate at electrical double layers is much faster than the charge rate for the mediator domains due to different charge storage mechanisms. The mediators can only store the charges at the certain potential range. With the increase of the mediator capacitance, the time to reach a stable state increases. The mediators can also affect the stable potential of the electrolyte. As such, the model can be used to design the composition of the mediator SCs and predict the effect of mediators on the performance of the mediator SCs. In addition, the information regarding the stable electrolyte potential vs. mediator capacitance can be used to choose the electrolyte with specific potential windows to avoid electrolyte degradation at electrode/electrolyte interface.