With the increasing demand on energy resources and concerns of climate change, widespread utilization of renewable energy becomes the target for sustainable development. As intermittent resources, energy storage systems are essential for integrating renewable energy into the grid, which have received growing scientific attention. Owing to the advantages of large capacity, simple configuration, flexible design, safety and long cycle life, flow battery (FB) has been extensively studied as a promising technology for large-scale application. However, it is yet to be widely deployed due to energy density limitation.

While numerous approaches have been proposed to increase the concentration of active species and cell voltage for high energy density, the impact of these approaches on the power density of FB was barely studied. Employing highly soluble active materials and suppressing precipitation using ligands have been used to increase the concentration, resulting in high viscosity, which generally increases inner resistance and impairs reaction kinetics, and consequently the power density decreases. Concurrently high energy and power densities ensure feasible designs and diverse applications of FB.

In addition to cell design and inner resistance, rate of the redox reaction is the key factor affecting power density, which is controlled by diffusion and electron transfer processes, i.e. reaction kinetics of the redox couple. As interfacial reactions, all factors about electrode surface and redox species, such as electrode materials, surface functional groups, ligands, ions and temperature, can affect the reaction kinetics. In this work, the influence of temperature and electrode material is studied in V(IV)/V(V) reaction. The diffusion coefficients on both metal and carbon electrodes increase with temperature, and most are in the order of 10^-6 cm² s^-1. The positive effect of temperature on standard rate constants is most significant on the glassy carbon electrode. Comparison between glassy carbon and metal electrodes indicates a promising potential of carbon-based materials. The impact of ligand on cerium reaction kinetics is studied on glassy carbon, where inevitable influence of electrochemical measurements on the electrode surface is revealed. The charge transfer resistance is reduced by ca. 70% through the proposed in situ treatment of glassy carbon.

Further, adopting multi-electron reaction in aqueous FB to improve energy density is explored in this work. Although the multi-electron reaction is potential for increasing the energy density, it was barely used in FB due to the difficulty to discover a suitable redox couple with high solubility and appropriate equilibrium potential. High energy density can be achieved by multi-electron active material at a lower concentration comparing to single-electron, benefiting power density. Fortunately, bismuth reaction involves three electrons transfer within the stable potential window of water and high concentration of 1.5 M is achieved in MSA. Thus, an aqueous bismuth-based FB is successfully developed, demonstrating high energy density of 90 Wh L^-1 and power density of 295 mW cm^-2. Moreover, the impact of ligands on bismuth reaction kinetics is analyzed, providing guidance for further enhancement of bismuth-based FB.