Preventing Degradation in a Flavin Mononucleotide-Based Redox Flow Battery: An NMR and EPR Study
Authors: Hey Dominic , Jethwa Rajesh B , Farag Nadia L , Rinkel Bernardine L. D. , Zhao Evan Wenbo Wenbo , Grey Clare P. 
Published: February 2024

Redox flow batteries (RFBs) are a promising energy storage system for grid-level storage, where low-cost and scalability are essential1. To date, many different organic molecules including quinones2–4, viologens5,6, phenazines7,8, and alloxazines7,9 have been investigated as potentially-cheap RFB active molecules. Although a few molecules have shown a good performance in alkaline solution (pH 14)7,8, most organic molecules considered for RFBs generally experience degradation, reducing cell lifetime1. In 2016 Orita et al.10 reported a RFB comprising flavin mononucleotide (FMN3−) at pH 14 as the anolyte against a potassium hexacyanoferrate K4[FeII(CN)6] catholyte. The cell showed a remarkable capacity retention of 99% over the course of 100 cycles. Despite the encouraging capacity retention, an additional FMN reduction plateau appeared during charge, which was assigned to a dimerization process. 10 The process was not seen on discharge leading considerable cell hysteresis. Consequently, the resulting capacity retention and energy efficiency were not good enough for grid-scale storage systems, where even longer long-life times with minimal degradation and high coulombic efficiency are required. More recently, Nambafu et al. attached a 2,2,6,6- tetramethylpiperidinyl-N-oxyl (TEMPO) radical to FMN to form a bifunctional redox active material, which showed improved stability at neutral pH.11 However, significant capacity loss was seen within 100 cycles which was largely ascribed to degradation of the TEMPO functionality.
FMN is a commercially available, non-toxic biomolecule, readily derived from vitamin B2, motivating its further study in an RFB. Flavins generally act as a cofactor in many enzymes that catalyze a wide variety of biological reaction and contain one of the most versatile in vivo redox centers 10. In nature, flavins are often found dissolved in water, fat, or blood, such as in biological systems 10. The molecules are also used in the food industry as an orange-red food color additive, utilized in Europe as E101a 12; the sodium salt is commonly known as E106 and is found in foods for babies and young children as well as jelly, milk products, and sweet products 12.
Here we demonstrate a powerful strategy to study the degradation of FMN3− by coupling in-situ NMR and EPR techniques 4. We explain how degradation, which we show involves the hydrolysis of FMN3− rather than a dimerization process, leads to the additional charging plateau. We investigate the electrochemical behavior of hydrolyzed FMN3− with in-situ NMR and demonstrate that FMN3− acts as a redox mediator, helping to reduce the hydrolyzed product, explaining the lack of an additional plateau on discharge yet good cycling behavior, despite degradation and poor reversibility of hydrolyzed FMN3− redox reactions. Lastly, we provide a strategy to avoid the hydrolysis by lowering the pH. The battery performance is improved significantly and the FMN solubility is increased dramatically, by addition of a simple salt.

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