Recent advances in porous electrodes for vanadium redox flow batteries in grid-scale energy storage systems: A mass transfer perspective

Energy storage systems that serve as reservoirs for the power management of existing power grids and renewable power generation facilities have become increasingly important. Vanadium redox flow battery (VRFB) technology provides a balanced solution for large-capacity energy storage within power management strategies. More than 30 years have passed since the discovery of vanadium among redox couples for water-based flow cells. A major challenge in VRFB applications is the optimization of mass transfer effects in micro-scale porous electrodes characterized by close interactions between flow and electrochemical reactions. This paper reviews current research on mass transfer phenomena in the porous electrodes of flow cells using redox couples. Instead of comparing the details of experiments or numerical models, we focused on the following keywords to characterize mass transfer: modifications (thermal or catalytic) to porous electrodes; a flow channel located outside or inside the electrode; architecture of the porous electrode; and nanofluidic electrolytes, or slurry electrodes, with nanoparticle dispersion. This review paper introduces the flow characteristics of electrodes as diffusion and convection-dependent mass transfer phenomena to provide insights into the development of porous electrodes for studies on the experimental aspects or numerical simulations of flow cells.