Recent advancements in membrane-free redox flow batteries

Membrane-free redox flow batteries (RFBs) are promising energy-storage technologies that present an innovative solution to address the critical need for sustainable and efficient energy systems. This review provides a detailed examination of membrane-free RFBs, focusing on recent technological advances and design optimization. Moreover, it highlights the growing importance of membrane-free designs for achieving higher efficiency and scalability in energy-storage systems. These designs offer significant improvements in terms of electrolyte concentration, Coulombic efficiency, and flow management, underscoring the potential of these systems for advanced energy-storage solutions. We explore the utilization of immiscible electrolyte solvents and the engineering of laminar flow dynamics to achieve efficient electrolyte separation without traditional ion-exchange membranes. The article discusses metal-free and metal-phase aqueous/nonaqueous and nonaqueous/nonaqueous immiscible solvent-based RFBs; laminar flow-based RFBs; single-phase co-laminar flow batteries; liquid/solid membrane-free RFBs; and triphasic membrane-free RFBs, highlighting their unique design features and operational benefits, as well as their potential and challenges in energy-storage applications. Key parameters such as the coulombic efficiency, self-discharge, flow dynamics, and impedance are analyzed to provide a comprehensive understanding of the performance metrics critical for the development of next-generation membrane-free RFBs. We provide valuable references for developing membrane-free RFBs and highlight their significance, technological advancements, and implications for future energy-storage applications. In the context of global energy transitions, the research and development of membrane-free batteries will provide crucial technical support for achieving sustainable energy development.