Identifying and preventing degradation in flavin mononucleotide-based redox flow batteries via NMR and EPR spectroscopy.
While aqueous organic redox flow batteries (RFBs) represent potential solutions to large-scale grid storage, their electrolytes suffer from short lifetimes due to rapid degradation. We show how an understanding of these degradation processes can be used to dramatically improve performance, as illustrated here via a detailed study of the redox-active biomolecule, flavin mononucleotide (FMN), a molecule readily derived from vitamin B2. Via in-situ nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) we identify FMN hydrolysis products and show that these give rise to the additional plateau seen during charging of an FMN-cyanoferrate battery. The redox reactions of the hydrolysis product are not reversible, but we demonstrate that capacity is still retained even after substantial hydrolysis, albeit with reduced voltaic efficiency, FMN acting as a redox mediator. Critically, we demonstrate that degradation is mitigated and battery efficiency is substantially improved by lowering the pH to 11. Furthermore, the addition of cheap electrolyte salts to tune the pH results in a dramatic increase in solubility (above 1 M), this systematic improvement of the flavin-based system bringing RFBs one step closer to commercial viability.
Acknowledgements: D.H. acknowledges support from the Sheldrick Scholarship in Chemistry, Jesus College Cambridge, and the EPSRC iCASE PhD Fees Only Studentship. C.P.G. acknowledges support from the European Research Council (ERC) 835073 BATNMR. D.H., R.J., E.W.Z., N.L.F., and C.P.G. acknowledge support from Shell; R.J. and N.L.F. acknowledge support from the EPSRC and Shell via I-Case studentships EP/R511870/1 and EP/V519662/1, respectively. R.B.J. thanks the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 101034413 for funding. E.W.Z. and C.P.G. acknowledge support from Centre of Advanced Materials for Integrated Energy Systems (CAM-IES), via EPSRC grant number EP/P007767/1. We thank D.S. Wright, A.C. Forse, M. De Volder, and E.J. Latchem from University of Cambridge and P.A.A. Klusener from Shell for many fruitful discussions.
Funder: University of Cambridge | Jesus College, University of Cambridge; doi: https://doi.org/10.13039/501100000644
Engineering and Physical Sciences Research Council (EP/P007767/1)
Engineering and Physical Sciences Research Council (EP/R511870/1)