Birnessite-type manganese oxide materials applied as supercapacitor electrodes are widely investigated, but they have not been studied as a new type of energy storage system-flow electrochemical capacitor based on flowable slurry electrodes. In the present work, the electrochemical performances of the slurry based on birnessite-type manganese oxide nanosheets were investigated. As compared to the manganese oxide slurry prepared through ball-milling, the slurry obtained through ultrasonic oscillation showed higher specific capacitances, which was attributed to higher electronic conductivity of the latter slurry. The birnessite-type manganese oxide slurry was further assembled with activated carbon slurry to obtain asymmetric flow capacitor. This asymmetric device exhibited high energy densities, such as an energy density of 9.4 W h kg(-1) at a power of 94.5 W kg(-1), which makes them attractive for the application as a flow electrochemical capacitor.