Neutral pH Viologen-TEMPO Based Aqueous Organic Redox Flow Battery—The Critical Role of Electrolyte Formulation on Stability and Efficiency

The intermittent availably and power of renewable energy sources has become an issue, efficient energy storage systems must be introduced to overcome this obstacle.<br/>Redox-flow batteries stand out as relevant candidates for large-scale stationary energy storage due to their many advantages, including their unique capability to decouple energy storage and power generation.¹<br/>In recent years, there has been a transition from redox inorganic materials to the development of water-soluble organic redox active materials owing to their low cost, high sustainability, and their structural diversity and tailorability.²<br/>Redox flow batteries based on aqueous organic electrolyte have emerged as promising competitors to traditional redox flow batteries. Acidic, alkaline, and neutral AORFBs using different organic molecules/polymers as redox-active materials have been explored lately.<br/>Despite efforts devoted to the development of new active materials, search for stable high redox potential molecules for catholyte remains a challenge, mainly limited to ferrocyanide and TEMPO derivatives which render low cell voltages^{3, 4} or present uncertain stability.^{5, 6}<br/>As a solution to above mentioned problem, we have contributed to the development of neutral pH Aqueous Organic Redox Flow Batteries employing a novel anolyte-catholyte combination leading to cell voltages &gt;1.2 V and outstanding stability and efficiency. A case of study for Viologen-TEMPO based electrolyte including synthesis, characterization and battery testing is provided.<br/>Considering that the supporting electrolyte and the ion exchange membrane are key components in RFBs and have significant impact on the energy efficiency and power density we perform a study to understand how these factors affect the battery performance.<br/>Several electrolyte formulations were evaluated attending to parameters such as the concentration of active materials and conductive salts. The influence of the cation on the transport phenomena, as well as the role of the anion on the stability of the solution will be discussed. Cell resistance and water transport can be modulated by wise electrolyte formulation. Moreover, different commercial CEM membranes were evaluated aiming at neutral pH systems with high RTE (&gt;80% EE at 60 mA/cm²) and high power densities.<br/>Finally, with an optimized combination of membrane and supporting electrolyte, cycling test was conducted to reveal the long cycling stability with negligible capacity decay (0,001%/day) over 1000 cycles.<br/><br/>[1] Sánchez-Díez, E.; Ventosa, E.; Guarnieri, M.; Trovò, A.; Flox, C.; Marcilla, R.; Soavi, F.; Mazur, P.; Aranzabe, E.; Ferret, R., Redox flow batteries: Status and perspective towards sustainable stationary energy storage. <i>Journal of Power Sources </i><b>2021,</b> <i>481</i>, 228804.<br/>[2] Li, Z.; Lu, Y.-C., Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies. <i>Adv. Mater. </i><b>2020,</b> <i>32</i> (47), 2002132.<br/>[3] Kwabi, D. G.; Ji, Y.; Aziz, M. J., Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review. <i>Chem. Rev. </i><b>2020,</b> <i>120</i> (14), 6467-6489.<br/>[4] Luo, J.; Hu, B.; Debruler, C.; Bi, Y.; Zhao, Y.; Yuan, B.; Hu, M.; Wu, W.; Liu, T. L., Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries. <i>Joule </i><b>2019,</b> <i>3</i> (1), 149-163.<br/>[5] Winsberg, J.; Stolze, C.; Schwenke, A.; Muench, S.; Hager, M. D.; Schubert, U. S., Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High-Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery. <i>ACS Energy Letters </i><b>2017,</b> <i>2</i> (2), 411-416.<br/>[6] Fan, H.; Wu, W.; Ravivarma, M.; Li, H.; Hu, B.; Lei, J.; Feng, Y.; Sun, X.; Song, J.; Liu, T. L., Mitigating Ring-Opening to Develop Stable TEMPO Catholytes for pH-Neutral All-organic Redox Flow Batteries. <i>Adv. Funct. Mater. </i><b>2022,</b> <i>32</i> (33), 2203032.