Redox-Mediated Processes for Flow Batteries—From Capacity Boosting to Combined Renewable Hydrogen Generation

Redox flow batteries (RFBs) have recently attracted significant attention as a grid-scale energy storage technology due to their ability to decouple energy and power ratings, unprecedented scalability, and cost-effectiveness in long-duration storage [1]. However, the low energy density of RFBs, primarily due to the limited concentration of redox-active species in electrolytes, remains a critical challenge [2-3]. One promising approach to address this limitation is the utilization of solid charge storage materials in the tanks. Similar to a typical flow battery operation, the mediator (i.e., redox-active species), which is oxidized in the flow cell, is pumped to the external tanks. Upon interaction between the mediator and the solid charge storage materials in the tank (i.e., the redox-mediated process), the oxidized mediator is reduced, and the charge is transferred to the solid charge storage material [4]. In this way, the redox-mediated process enables the shuttling of charges between the flow cell and the solid charge storage material, greatly increasing the theoretical capacity of the system [4-5].<br/><br/>In this presentation, an overview of the presenter’s most recent research utilizing redox-mediated processes for addressing the key issues in RFBs will be provided. In the first part, a high energy density redox-mediated flow battery system using highly stable redox active species and Prussian Blue analogue solid storage materials will be introduced. The efforts to understand the interplay between two kinetic processes, the electrochemical reaction in the flow cell and the indirect redox-mediation reaction in the tank, will be discussed. Furthermore, an in-line ultramicroelectrode voltammetry setup is employed to gain fundamental insights into the interactions between solid charge storage materials and mediators, as well as how intercalation cations influence the kinetics of the indirect redox targeting reaction. In the second part of the talk, our research groups’ recent progress on dual-circuit RFBs [6], a different concept utilizing the redox-mediated processes, combining energy storage with indirect water electrolysis for renewable hydrogen generation, will be summarized.<br/><br/><br/><b>References</b><br/><br/>[1] Z. Li, M. S. Pan, L. Su, P-C. Tsai, A. F. Badel, J. M. Valle, S. L. Eiler, K. Xiang, F. R. Brushett, Y-M. Chiang <i>Joule</i>, <b>2017</b>, 1 (2), 306-327.<br/>[2] S. Gentil, D. Reynard, H. H. Girault <i>Current Opinion in Electrochemistry</i>, <b>2020</b>, 21, 7-13.<br/>[3] F. Zhang, M. Gao, S. Huang, H. Zhang, X. Wang, L. Liu, M. Han, Q. Wang <i>Advanced Materials</i>, <b>2022</b>, 34, 2104562.<br/>[4] M. Moghaddam, S. Sepp, C. Wiberg, A. Bertei, A. Rucci. P. Peljo <i>Molecules</i>, <b>2021</b>, 26, 2111.<br/>[5] J. Egitto, T.C. Gokoglan, S.K. Pahari, J.N. Bolibok, S.R. Aravamuthan, F. Liu, X. Jin, P.J. Cappillino, E. Agar <i>Journal of Electrochemical Energy Conversion and Storage</i>, <b>2022</b>, 19, 041005.<br/>[6] S. Chaurasia, S. R. Aravamuthan, C. Sullivan, M. B. Ross, E. Agar <i>ACS Applied Energy Materials, </i><b>2024. </b>