EPR Studies Research Articles

Aqueous organic redox flow batteries (RFBs) are promising energy storage systems especially for grid-level storage, where low-costs, and scalability are essential.1 However, for deep market penetration of organic flow battery systems, when compared to other storage technologies, their relatively low energy densities and short lifetimes due to degradation of the electrolytes need to be addressed.2,3 Additionally, most organic systems suffer from high toxicity, e.g., viologen systems, and/or relatively high costs due to complex synthetic methods, despite being comprised of earth-abundant elements.4-6 Therefore, a system for grid-scale storage with a long lifetime, minimal degradation, low toxicity, high energy density, and low cost is urgently needed.To achieve this goal, advanced characterization methods to study RFBs are needed. They will improve the understanding and enhance the performance, ultimately extending the battery life of both organic- and inorganic-based chemistry. Ex situ characterization can be challenging, due to the high reactivity, sensitivity to sample preparation and short lifetimes of some of the oxidized and/or reduced redox-active molecules and ions within the electrolyte.2 We present amongst other things how the biomolecule flavin mononucleotide (FMN), when cycled under the right conditions, is a commercially viable anolyte in an RFB: it overcomes the limitations described above as it is commercially available, chemically stable and non-toxic and it exhibits excellent capacity retention when used in a RFB. Specifically, we show how in-situ NMR and EPR techniques can be coupled to first understand degradation. The techniques allowed us to detect the reaction intermediates directly, identify the degradation products, and explain the changes in the electrochemistry that occurred in the RBF on cycling. Critically, we were then able to use this insight to mitigate the degradation of this redox-active species. This work clearly demonstrates how a combined in-situ NMR and EPR study can be used to understand and then mitigate degradation in flow batteries. Due to the simplicity of the setup, which consists essentially of a lab-scale RFB, a flow NMR and a flow EPR sampling tube, the technique can be widely applied. We stress that the methods are general and can be applied to study a wide range of RFB chemistries, including vanadium-based batteries, and our work should inspire further research to explore and develop further in-situ characterisation of flow batteries. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M. D. & Schubert, U. S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials. Angew. Chemie - Int. Ed. 56, 686–711 (2017).Zhao, E. W. et al. In situ NMR Metrology Reveals Reaction Mechanisms In Redox Flow Batteries. Nature 579, 224–228 (2020).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Hu, B., DeBruler, C., Rhodes, Z. & Liu, T. L. Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) Toward Sustainable And Safe Energy Storage. J. Am. Chem. Soc. 139, 1207–1214 (2017).Jin, S. et al. Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species. Adv. Energy Mater. 10, 1–10 (2020).Lin, K. et al. A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy 1, 1–8 (2016).

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. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M. D. & Schubert, U. S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials. Angew. Chemie - Int. Ed. 56, 686–711 (2017).Lin, K. et al. Alkaline Quinone Flow Battery. Science (80-. ). 349, 1529–1532 (2015).Zhao, E. W. et al. In situ NMR Metrology Reveals Reaction Mechanisms In Redox Flow Batteries. Nature 579, 224–228 (2020).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Hu, B., DeBruler, C., Rhodes, Z. & Liu, T. L. Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) Toward Sustainable And Safe Energy Storage. J. Am. Chem. Soc. 139, 1207–1214 (2017).Jin, S. et al. Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species. Adv. Energy Mater. 10, 1–10 (2020).Lin, K. et al. A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy 1, 1–8 (2016).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat. Energy 3, 508–514 (2018).Wei, X. et al. Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Lett. 2, 2187–2204 (2017).Orita, A., Verde, M. G., Sakai, M. & Meng, Y. S. A Biomimetic Redox Flow Battery Based On Flavin Mononucleotide. Nat. Commun. 7, 1–8 (2016).Nambafu, G. S. et al. An organic bifunctional redox active material for symmetric aqueous redox flow battery. Nano Energy 89, 106422 (2021).Turck, D. et al. Dietary Reference Values for riboflavin. EFSA J. 15, (2017).

EPR Studies Research Articles

Articles published on EPR Studies

Tuning the electrochemical properties of ZnO through chromium doping and its suitability for supercapacitor application

DFT and EPR study of spin-assisted processes in poly(3-alkylthiophene) oligomers and their composites

Surface Modification of p‐type ZnO Nanorods by Nitrogen Doped SiO2 Dots as an Efficient Solar Photocatalyst for Degradation of Ciprofloxacin in Water

Influence of Cu doping on the functionality of spray coated SnS2 thin films and its photocatalytic degradation of dyes and antibacterial activity

Selective C-H Activation of Molecular Nanodiamonds via Photoredox Catalysis.

Photochemical Synthesis of 1,4‐Dicarbonyl Containing Quinoxalin‐2(1H)‐Ones through Consecutive Photoredox Catalysis

An EPR study of the cognitive processes underlying the impact of self-relevant information on emotional word processing

Mn2+ doped SrSn(PO4)2 nanopowder for new novel LED material

Synthesis, crystal structure and magnetic properties of two 1D coordination polymers containing Co2+ and Cu2+ ions and 1,3,5-tris(4-carboxylphenyl)benzene

Na₂ – Eosin Y – Photocatalyzed Cross Dehydrogenative C−S Coupling of Arylthiols with Indole Derivatives

EPR and optical study of Mn2+ doped BaCO3 for potential yellow laser application

Development and application of nanodiscs for EPR studies of membrane-bound influenza A M2 protein

Evaluation of the anionic effect on the formation of biologically active {CuII-phenx; x = 1, 2, 3} fragments - Synthetic and structural variations, antimycobacterial and antiblastoma effects

Experimental and DFT Study of Monensinate and Salinomycinate Complexes Containing {Fe3(µ3-O)}7+ Core.

Structural Flexibility of Tau in Its Interaction with Microtubules as Viewed by Site-Directed Spin Labeling EPR Spectroscopy.

Structural evolution of versatile Bi2O3-B2O3-Cr2O3: ZrO2 glass ceramics monitored by Raman and EPR spectroscopy

EPR and optical study of erbium-doped CeO2 and CeO2 / CeF3 nanoparticles

Vanadium-Containing Keggin-Type Polyoxometalates, [VM12O40]3- and [VVM11O40]4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites.

In-situ Studies of Aqueous Organic Redox Flow Batteries

Preventing Degradation in a Flavin Mononucleotide-Based Redox Flow Battery: An NMR and EPR Study

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