A Biomimetic Redox Flow Battery Based on a Flavin Mononucleotide

Biological redox reactions inspire us to design sustainable and environmentally safe energy storage systems. Flavins have been demonstrated as solid-state electroactive materials for Li-metal batteries. [1,2] The concentration of flavin in those composite electrodes was no more than 50 wt%, however, due to the high content of conductive carbon necessary to complement the low electronic conductivity of flavin. In nature, flavins are often found dissolved in water or fat, such as in biological systems. As they are commonly more useful in this state, so may it be in batteries as well. Interestingly, flavins have yet to be reported as an active material for any redox flow batteries (RFBs). Here we explore the use of riboflavin-5’-phosphate sodium salt (FMN-Na) for RFBs. A diffusion coefficient and a kinetic rate constant of FMN in a strongly alkaline aqueous solution are determined and shown to be comparable to those of vanadium ions used for industrially utilized RFBs. Nicotinamide (known as vitamin B3) is used as a hydrotropic agent to enhance the water-solubility of FMN-Na, and therefore, its energy density in a RFB by as much as 15 times. A RFB using strongly basic FMN-based anolyte and ferrocyanide/ferricyanide-based catholyte shows highly stable cycling performance over the course of 200 cycles. The oxidized and reduced FMN can be stabilized by a variety of resonance structures, leading to the stable cycling performance. This work shows that biological redox centers are shown to be promising materials for environmentally friendly energy storage systems and warrant future exploration in this field. References [1] Hong, J. et al. Biologically inspired pteridine redox centres for rechargeable batteries. Nat. Commun 5, 9, (2014). [2] Lee, M. et al. Redox Cofactor from Biological Energy Transduction as Molecularly Tunable Energy-Storage Compound. Angew. Chem. Int. Ed. 52, 8322-8328, (2013).