Flow-electrode capacitive mixing (F-CapMix) is a promising energy harvesting system, utilizing the salinity gradient difference between seawater and river water. However, the conventional F-CapMix system has a low power density due to the limited charge storage of porous carbon materials through the electrical double layer. In this study, we first report manganese dioxide-coated activated carbon (MO@AC) as a flow-electrode material for the F-CapMix system. MnO2 nanoparticles could be easily decorated on the surface of activated carbon (AC) by the reduction of KMnO4 in an acidic medium. We systematically study electrochemical properties of AC and MO@AC-based
flow-electrodes as positive and negative electrodes for both symmetric and asymmetric F-CapMix systems. The asymmetric F-CapMix system employing the MO@AC flow-electrode exhibits an outstanding power density of 2.22 W/m2 through the reversible redox reaction with Na+ ions. Our strategies can be extended to other materials that have redox reactions with Na+ and Cl– ions for the F-CapMix system.