Research progress on mass transfer optimization of filter-press type electrochemical reactors for flow batteries

Filter-press electrochemical reactors have become the dominant reactor configuration for flow batteries owing to their simple structure, high modularity, and low cell voltage. They also show broad application potential in energy storage batteries, electrochemical water treatment, and electrosynthetic chemical processes. Among various flow battery systems, the all-vanadium redox flow battery represents the most commercially advanced and extensively studied technology. However, inadequate mass transfer continues to limit further performance development since it increases concentration polarization, reduces voltage efficiency, speeds up long-term performance degradation, and eventually raises system costs. This review focuses primarily on all-vanadium redox flow batteries and discusses mass-transfer enhancement from the perspectives of both key reactor components and system operation. Recent advances in the mass-transfer optimization of electrodes, flow fields, membranes, and other core components are systematically summarized. The effects of operating parameters, including flow rate, temperature, current density, electrolyte concentration, and tank-to-stack ratio, on mass-transfer behavior are analyzed. Based on this, coordinated optimization solutions including reactor structure and system operation are suggested, and the coupling relationship between component-level mass-transfer characteristics and total battery performance is elucidated. From an engineering perspective, mass-transfer optimization in all-vanadium redox flow batteries should not be limited to component modification alone. Instead, it should also account for stack scale-up effects, tank-stack matching, operating-parameter regulation, and the balance between electrochemical performance and system energy consumption. This review aims to provide theoretical guidance and technical references for the engineering design and performance improvement of filter-press flow battery reactors.