Flow batteries have very high specific energy but low specific power and slow response while mediator supercapacitors give high specific power and fast response but relatively low specific energy. A hybrid flow-battery/supercapacitor is fabricated and operated under different conditions in order to provide both high specific energy and power. The porous electrodes of the consisted of catalysts for redox reactions (Ni, Pt, Ru, etc.), mediators (NaI/I₂, VOSO₄, K₄Fe(CN)₆, etc.), and polymer electrolytes as binders. An ion exchange membrane was used as the separator. An array of complex iron cyanide compound were synthesized and used as the redox materials in slurry reservoirs. This investigation focuses on the effects of flow rate, temperature, pressure, and mediator concentration on the performance of the energy storage device. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were conducted under a wide range of flow rate and mediator concentration. The redox potential of the slurry was monitored using an electrochemical redox sensor. The electrochemical kinetics of the mediators and redox materials were evaluated using linear polarization methods. The results indicate that at a given scan rate for CV or current for GCD, both flow rate and mediator concentration greatly impact the charge/discharge behaviour, the total energy, and efficiency. There is a critical flow rate at which a capacitor-like behaviour transits to battery-like behaviour. A simple numerical model is created to analyse the experimental data and provide an explanation of the observations.