Understanding Redox Reaction Mechanisms of a Flavin Mononucleotide By in Situ NMR and EPR Techniques
Authors: Hey Dominic , Zhao Evan Wenbo Wenbo , Jethwa Rajesh , Grey Clare P. 
Published: July 2022

There are many types of energy storage technologies available at present, such as batteries and supercapacitors.1,2 Common battery chemistries include lead3 and lithium-ion4; however, scaling them up to the grid level is challenging.2 Recently, redox flow batteries (RFBs) have attracted much attention, as they can overcome the intermittent nature of renewable energy sources. We have previously developed a new approach to study RFBs by using in-situ nuclear magnetic resonance (NMR) and in-situ electron paramagnetic resonance (EPR), allowing the real time tracking of the redox state of various species in a system. As a result of this work, the electrochemical processes can be better understood. Investigating and understanding the electrolyte properties is a major goal in flow battery research and this will help to develop further RFB systems.
Here, we report the analysis of the redox mechanism of a flavin mononucleotide-based anolyte (FMN) in a RFB system with the in-situ NMR and EPR setups. It represents one of the safest families of organic molecules for flow battery applications as it is a molecule derived from riboflavin (vitamin B2) and commonly used as an orange-red food additive.5–7 Nevertheless, the redox properties of flavins in strongly alkaline solution have yet to be thoroughly studied. In particular, the mechanism of the redox reaction for this system is not well understood.
To understand the chemistry of FMN in strongly alkaline solution, first the NMR properties were studied. It was revealed that the chemical structure of FMN changed over time, i.e., additional signals were observed to grow in, and, additionally, the existence of a paramagnetic species was confirmed. The paramagnetic species formed on dissolution and was responsible for significant line broadening in the NMR spectrum. To examine how these changes affect the redox properties of FMN, a RFB was cycled over multiple cycles. Here we observed a second charge plateau developed as we continued to cycle the battery that was not mirrored during discharge. The in-situ EPR and NMR analysis reveal that the second plateau was likely caused by a side product derived from FMN. Therefore, possible degradation mechanisms of flavins, such as hydrolysis, need to be considered. Herein we propose that the observed changes over time are caused by the hydrolysis of FMN and provide an explanation for the different redox properties.
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