A review of transport properties of electrolytes in redox flow batteries

Redox flow battery is a competitive grid-level energy storage technique that is especially suitable for large-scale and long-duration energy storage. In redox flow batteries, the energy is stored in the electrolyte electrochemically, which circulates between the reservoir and the electrode, driven by the pump. Therefore, the electrolyte is one of the most important components in redox flow batteries and its physicochemical properties greatly determine the battery performance. Here, the transport properties of various types of electrolytes in redox flow batteries are reviewed, including viscosity, diffusion coefficient, and conductivity. This paper outlines the measuring methods and principles for these fundamental transport properties, provides typical values of viscosity, diffusion coefficient, and conductivity for different types of electrolytes, and examines the impact of those properties on the mass and charge transport as well as the overall battery performance in redox flow batteries. Insightful perspectives are proposed to bridge the electrolyte transport properties to technological relevance for better understanding and optimizing redox flow batteries.