Pegylated Viologen Derivatives to Improve Performance of Aqueous Organic Redox Flow Battery

Renewable energy sources like wind and solar power are great alternatives for a clean way to produce electricity but have the inconvenience to fluctuate 1 . To address this issue and promote the implementation of renewables sources, scientists are developing new ways to store energy while production is at a maximum for a subsequent release when there is demand. Redox flow batteries (RFBs) are the most appropriate solution and are increasingly gaining attention for stationary, large scale electrochemical energy storage 2,3 . One variation of RFBs are aqueous organic redox flow batteries (AORFBs), where water-soluble organic molecules are use as the redox couple. Viologen type molecules are particularly well-suited as a negative potential species to develop cheaper and better AORFBs with great scalability, reversibility and long-term stability 4 . In this study, viologen derivatives were synthesized with various N-functional groups to explore the impact of the substituent on their solubility, viscosity and redox potential. We showed that the use of short chains of poly(ethylene glycol) improved the solubility of the viologens derivatives which could lead to batteries with higher capacity. The symmetric viologen (with two identical substituents) provided a great solubility in water and the formal potential was unaffected by the chemical nature of the substituents. We demonstrated that the 1,1’-(bisethylene glycol)-4,4’-bipyridium (PEG2-V-PEG2) showed the most promising properties with a high solubility (2.3 M in 1M KCl) and low viscosity (3.01 mPa*s at 1M). As such, this derivative was selected for further evaluation in an asymmetrical labs-scale RFB prototype cell versus bis(trimethylamoniumpropyl)ferrocene (BTMAP-Fc). The compound was studied at various concentrations to assess the maximum energy density that can be reached and the effect of concentration on cycling life by achieving a comparison to the symmetrical 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridium (SPr-V-SPr) at 0.5 M. Figure 1: Redox flow battery scheme using Viologen derivative as the negolyte and BTMAP-Fc as the posolyte Reference Shi, X., Qian, Y. and Yang, S. Fluctuation analysis of a complementary wind–solar energy system and integration for large scale hydrogen production. ACS Sustainable Chemistry & Engineering, 8(18), pp.7097-7110. (2020) 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. Journal of Power Sources, 481, 1-23. (2021) Poullikkas, A., A comparative overview of large-scale battery systems for electricity storage. Renewable and Sustainable energy reviews, 27, pp.778-788. (2013) Gentil, S., Reynard, D. and Girault, H.H. Aqueous organic and redox-mediated redox flow batteries: a review. Current Opinion in Electrochemistry, 21, pp.7-13. (2020) Figure 1