(Digital Presentation) Novel Organic Materials for Non-Aqueous Redox Flow Batteries: Implementation of Triarylamine and Phenazine Core Structures
Authors: Romadina Elena , Stevenson Keith J 
Published: July 2022

The rapid growth of the role of renewable energy sources dictates new requirements for the efficiency, stability and scales of electrochemical energy storage devices for stationary applications [1]. Among the storage systems, redox flow batteries (RFBs) are regarded as a promising technology, since their advantages of excellent scalability, low cost, easy fabrication and operation, long lifetime, and safety. Today inorganic RFBs are penetrating the market, however, low specific capacity in conjunction with low electrochemical stability window of aqueous electrolytes (≈1.5 V) and safety issues, hinders their wide-scale commercialization. [2]. Replacing the inorganic materials with environment-friendly organic redox-active molecules may solve the capacity problems and safety issues. Moreover, the application of non-aqueous electrolytes provides a wide electrochemical stability window (e.g., up to 5 V for acetonitrile) enabling high-voltage batteries with increased energy density [3].
Within the framework of the current project, we implemented a comprehensive study for a large group of novel highly soluble organic materials based on aromatic amines with general formulas of NPh3RnBrm (M1-M4) and N2Ph5RnBrm (M5-M7) where R=-(OCH2CH2)2-OCH3 (Fig. 1a). All the compounds demonstrated high solubility in MeCN (from >2.2 M up to complete miscibility), which can potentially enable outstanding specific capacities of organic RFBs approaching 134 Ah L-1 [4]. Compounds demonstrated one or two quasi-reversible electron transition processes with redox potential up to 0.6 V vs. Ag/AgNO3 electrode, which makes them perspective for the investigation in the RFBs as catholyte materials.
For the RFB investigation butylviologen perchlorate (-0.75V vs. Ag/AgNO3, ~1.15 V battery voltage) was chosen as the redox pair (Fig. 1b, d). On the first step, the selection of the most appropriate electrolyte was performed: it was shown that the usage of electrolytes that contained lithium cations (Li+) and hexafluorophosphate anions (PF6
-) leads to fast decreasing of all the parameters of the RFBs, whereas the usage of the tetrabutylammonium tetrafluoroborate (TBABF4) and NaClO4 produces the stable characteristics (Fig. 1e). Final RFB tests proved that the most promising systems are capable to exhibit 65% of maximum capacities and more than 95% coulombic efficiency after 50 cycles [4] (Fig. 1f).
In the next step, we focused on the creation of low-voltage anolyte material: thus, we synthesized and investigated novel phenazine derivative with oligomeric ethylene glycol ether substituents as promising anolyte material for non-aqueous organic RFBs (Fig. 1c) [5]. The designed compound undergoes a reversible and stable reduction at -1.72 V vs. Ag/AgNO3 and demonstrates excellent (>2.5 M) solubility in MeCN. A non-aqueous organic redox flow battery assembled using novel phenazine derivative as anolyte and substituted triarylamine derivative as a catholyte exhibited high specific capacity (~93% from the theoretical value), >95% coulombic efficiency, 65% utilization of active materials and good charge-discharge cycling stability (Fig. 1g).
To summarize, triarylamine-based and phenazine-based materials establish themselves attractive for future research: obtained redox potentials, high solubility, fast diffusion and kinetics opens promising future directions for their usage as organic cathodic and anodic materials for non-aqueous RFBs.

References

[1] Panwar N., Kaushik, S., Kothari S. Renewable and Sustainable Energy Reviews
2011, 15 (3), 1513-1524.
[2] Placke T., Heckmann A., Schmuch, R., Meister P., Beltrop K., Winter, M. Joule
2018, 2 (12), 2528-2550.
[3] Elgrishi N., Rountree K., McCarthy B., Rountree E., Eisenhart T., Dempsey J. J. Chem. Educ.
2018, 95 (2), 197-206
[4] (a) Romadina E., Volodin I., Stevenson K., Troshin P. JMC-A,
2021, 9, 8303-8307; (b) Romadina E., Troshin P., Stevenson K., Patent RU 2 752 762 C1 “Highly soluble triphenylamine-based catholyte and electrochemical current source based on it”
[5] Romadina E., Komarov D., Stevenson K., Troshin P. ChemComm, 2021, 57, (24), 2986-2989

Acknowledgements

Romadina Elena acknowledges the support provided by Haldor Topsøe A/S Scholarship 2021.

Figure 1