Flow-electrode capacitive mixing (F-CapMix) and flow-electrode capacitive deionization (FCDI) systems are promising for energy generation and desalination devices, respectively, because constant energy generation and desalination are possible without an intermittent step by employing continuously flowing electrodes. Activated carbon (AC) has been widely used as a flow electrode material for F-CapMix and FCDI due to its good phase stability in an aqueous medium and large surface area capable of ion adsorption. However, the relationship between the physicochemical properties of carbonaceous materials and device performances has not been systematically studied. In this study, we explore activated carbon, natural graphite, synthetic graphite, and expanded graphite with similar particle sizes to a flow electrode material to understand dominant parameters in terms of material properties for the high-performance F-CapMix system and comparison with the FCDI system. The physicochemical properties of carbonaceous materials, their rheological behaviors in an aqueous medium, and corresponding electrochemical properties are systematically studied. Among them, the flow electrode prepared with expanded graphite shows the highest power density of 0.48 W/m2 in F-CapMix due to its solidlike rheological behavior as well as high electrical conductivity and a relatively low FCDI performance of 2.7%. Our findings suggest that the internal resistance of the flow electrode, which is closely related to rheological behavior and powder electrical conductivity, is a dominant factor for the high power density in F-CapMix. In addition, the high specific surface area of carbon material and rheological behavior of the flow electrode are dominant factors for the high salt removal efficiency of the FCDI system.