Redox flow battery technologies are leading candidates for stationary energy storage, which provide a potentially cost-effective approach that would be beneficial for renewable energy integration, balancing the mismatch between supply and demand, as well as improving the overall reliability and efficiency of the grid. Until very recently, most research in this area has focused on aqueous electrolyte systems, with the choice of electroactive materials mostly focused on transition-metal redox species, such as the all-vanadium and Fe/Cr redox flow batteries. This choice limits aqueous systems to low energy density because of the low solubility of metal salts in water. Some of the high-cost transition-metal elements further limit the potential of cost reduction of the flow battery system.

Materials selection and chemistry development are therefore of critical important to expand the chemical space for the flow battery technology. Through a molecular ligand design approach, we demonstrate that the structure of organic and inorganic redox active species can be tuned to capture the most attractive properties, such as favorable potential, high solubility, and low cost, etc. In the Zn-I redox chemistry, alcohol (ethanol(EtOH)) can be used as a stabilizing agent to mitigate the triiodide dissociation and subsequent precipitation reaction through direct coordination of EtOH with the zinc cation (Figure 1).¹ A new total organic aqueous redox flow battery (OARFB) has also been developed using low-cost and sustainable methyl viologen (MV, anolyte) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO, catholyte), and benign NaCl supporting electrolyte (Figure 2).²The electrochemical properties of the redox active materials and their performance in flow cell will be presented.

Figure 1 a. Triiodide complexed zinc cation and b EtOH-complexed zinc cation formed in the catholyte during the charging process. c. Efficiencies of the cell with (2.5 M ZnI₂ + 10 vol% EtOH) and Nafion 115 as membranes tested under the current density of 10 mA cm^-2at different temperatures.

Figure 2. Cyclic voltammograms of MV (blue trace) and 4-HO-TEMPO (red trace); conditions: 4.0 mM analyte in 0.5 M NaCl electrolyte; scan rate: 50 mV/s; glassy carbon working electrode; glassy carbon counter electrode; Ag/AgCl reference electrode.

Acknowledgements

The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery and Energy Reliability (OE) (under Contract No. 57558). Pacific Northwest National Laboratory is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

  

References

(1)        Li, B.; Nie, Z.; Vijayakumar, M.; Li, G.; Liu, J.; Sprenkle, V.; Wang, W. Nat Commun 2015, 6, 6303.

(2)        Liu, T.; Wei, X.; Nie, Z.; Sprenkle, V.; Wang, W. Advanced Energy Materials 2015, n/a.