Energy and environment are two principle themes in the 21st century, the research and development of large-scale energy storage technology and devices is becoming increasingly important. Electrochemical energy storage (EES) has attracted extensive attention due to its high energy density and no special requirement for geographical conditions. Electrochemical flow capacitors (EFC) is an important representative of EES due to they have both advantages of high energy of the flow batteries and the high power density of supercapacitors. Semi-solid flow electrodes are the core part of this type of devices. Nevertheless, their microstructure is relatively complex, usually involving cross-scientific issues such as rheology, interface chemistry, and electrochemistry, and so on, which is the focus of current research. This paper reviewed the study progress of the design of EFC’s electrode microstructure and device, characterization and testing methods, and further discussed the key scientific issues and important research directions for future research.