Mass Transport, Electron Transfer and Coupling of Multi-Electron Electrolyte in Aqueous Flow Battery for High Energy and Power Densities

Description

Flow battery (FB) is a promising technology that meets the requirement of renewable energy storage, which is critical for integrating sustainable electricity in the grid. However, the energy density limitation restricts wide deployment of current FB, which commonly useactive material involving mainly single-electron transfer reaction. While new types of FB have been studied to increase the energy density, the power density decreased due to high electrolyte viscosity and/or sluggish reaction kinetics. Discovery of an aqueous electrolyte with multi-electron transfer reaction is thus favorable for both high energy and power densities due to multiple charge capacity at the same concentration. Based on our preliminary study, Bi(III)/Bi redox reaction involves three-electron transfer and its equilibrium potential is within the stable potential window of water, avoiding parasitic reactions. Moreover, aqueous bismuth electrolyte with relatively high concentration wasobtained with good thermal stability, ensuring high energy density and electrolyte stability. Coupling with cerium as positive electrolyte (posolyte), bismuth-based FB achieved competitive energy and power densities to state-of-art FB. To explore the potential of multielectron electrolyte for cutting-edge FB, a fundamental probing study on bismuth redox reaction is pivotal. Therefore, we propose to investigate the Bi(III)/Bi redox couple for the first time for aqueous FB to fill the knowledge gap on bismuth electrodeposition anddissolution, assess the impact of electrolyte composition and electrode material on the mass transport, electron transfer and electrodeposition uniformity, which is essential for opening up a new research direction for FB and is the key objective of the project.To couple with the high specific capacity of bismuth, we adopted multi-redox posolyte of cerium and vanadium in our preliminary work, where the influence of cerium on vanadium reaction kinetics was observed. However, the absence of knowledge on reaction kinetics in multi-redox electrolyte hinders the explanation of such influence. Thus, we propose to investigate the interaction of multiple redox couples in mixed electrolyte to reveal the quantitative impact on diffusion coefficients and standard rate constants, which will form thefoundation for exploiting complex redox reactions.Further, to form theoretical elucidation of the interplay between mass transport, electron transfer and electrodeposition morphology, a 2-D simulation model of bismuth-based FB will be developed, illustrating the electric field, flow field and species distribution at the cross-section as well as the shape and thickness of the electrodeposition layer, which is essential for understanding the processes involved, but unachievable through experiment alone. Combining such detailed information obtained by simulation with diffusion coefficient and standard rate constant obtained by experiment, the impact of electrolyte compositions, reaction rates and flow rates of both sides on power density and cycling efficiency can be precisely analyzed. Moreover, simulating the discharge potential-current relationship in full-cell configuration can efficiently optimize the coupling of two sides. The approach taken here can be extended to other potential redox couples with multi-electron transfer as well as multi-redox electrolytes, forming the foundation for cell design and optimization of operation conditions. The success of this project will represent a methodological breakthrough for research on FB with electrodeposition.