Redox flow batteries show promise for grid-scale energy storage due to their simple manufacturing, decoupled power and energy, and long lifetimes^1,2. However, state-of-the-art flow batteries utilizing aqueous electrolytes and transition metals are too expensive to meet the U.S. Department of Energy’s long range cost targets³ spurring research activities aimed to reducing system cost. Nonaqueous RFBs seek to lower battery costs through increased energy density enabled by high voltage nonaqueous electrolytes with high concentrations of redox materials. Redox active organic compounds are of particular interest due to their tunable molecular structure and potential for low cost synthesis⁴.

Recent work has suggested that using redox active macrostructures, such as redox active oligomers (RAOs)⁵, polymers (RAPs)⁶, and colloids (RACs)⁷, could reduce RFB costs as fluorinated ion-exchange membranes can be replaced by inexpensive size-selective separator. However, the effects of using macrostructures, instead of monomers, on electrode kinetics, species transport, and electrolyte properties are not well understood. Here, we begin to deconvolute these effects by examining the electrochemical and solution properties of a series of RAOs using electrochemical, flow cell, and analytical techniques. This work provides insights on design directions for next-generation active species.

References

(1) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Gostick, J. T.; Liu, Q. Redox Flow Batteries: A Review. J. Appl. Electrochem. 2011, 41 (10), 1137–1164.

(2) Su, L.; Kowalski, J. A.; Carroll, K. J.; Brushett, F. R. Recent Developments and Trends in Redox Flow Batteries. In Rechargeable Batteries; Zhang, Z., Zhang, S. S., Eds.; Green Energy and Technology; Springer International Publishing, 2015; pp 673–712.

(3) Akhil, A. A.; Huff, G.; Currier, A. B.; Kaun, B. C.; Rastler, D. M.; Chen, S. B.; Cotter, A. L.; Bradshaw, D. T.; Gauntlett, W. D. DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA. Ed Albuq. NM Sandia Natl. Lab. 2013.

(4) Kowalski, J. A.; Su, L.; Milshtein, J. D.; Brushett, F. R. Recent Advances in Molecular Engineering of Redox Active Organic Molecules for Nonaqueous Flow Batteries. Curr. Opin. Chem. Eng. 2016, 13, 45–52.

(5) Doris, S. E.; Ward, A. L.; Baskin, A.; Frischmann, P. D.; Gavvalapalli, N.; Chénard, E.; Sevov, C. S.; Prendergast, D.; Moore, J. S.; Helms, B. A. Macromolecular Design Strategies for Preventing Active-Material Crossover in Non-Aqueous All-Organic Redox-Flow Batteries. Angew. Chem. Int. Ed. 2017, 56 (6), 1595–1599.

(6) Nagarjuna, G.; Hui, J.; Cheng, K. J.; Lichtenstein, T.; Shen, M.; Moore, J. S.; Rodríguez-López, J. Impact of Redox-Active Polymer Molecular Weight on the Electrochemical Properties and Transport across Porous Separators in Nonaqueous Solvents. J. Am. Chem. Soc. 2014, 136 (46), 16309–16316.

(7) Montoto, E. C.; Nagarjuna, G.; Hui, J.; Burgess, M.; Sekerak, N. M.; Hernández-Burgos, K.; Wei, T.-S.; Kneer, M.; Grolman, J.; Cheng, K. J.; Lewis, J. A.; Moore, J. S.; Rodríguez-López, J. Redox Active Colloids as Discrete Energy Storage Carriers. J. Am. Chem. Soc. 2016, 138 (40), 13230–13237.