(40g) Investigation of Redox Active Oligomers for Nonaqueous Flow Batteries

Redox flow batteries are promising electrochemical technologies for grid-scale energy storage due to their many favorable inherent attributes including a decoupled power and energy rating, long operational lifetimes, and simple manufacturing^1,2. However, current start-of-the-art flow batteries, which utilize acidic aqueous electrolytes and transition metal compounds, are too expensive to meet the U.S. Department of Energy’s long range cost targets³, spurring research into next-generation chemistries that can reduce battery system costs through lower materials costs and improved performance. Recently, RFBs leveraging nonaqueous electrolytes and organic active species have gained traction². Transitioning to nonaqueous electrolytes enables wider electrochemical windows and access to a broader palette of candidate active materials. Redox active organics materials are of particular interest as their molecular structure can be tuned to impart desired electrochemical and physical properties.⁴ Moreover, many candidate active materials may be produced at low costs from abundant feedstocks. Thus, taken together, this approach enables lower system costs through increased energy density.

Recent work has suggested that the use of redox active macroarchitectures, such as redox active oligomers (RAOs)⁵, polymers (RAPs)⁶, and colloids (RACs)⁷, could significantly reduce RFB costs by enabling inexpensive size-selective separators to be used in place of fluorinated ion-exchange membranes. However, shifting from small molecules (monomers) to macrostructures has potentially significant, yet poorly understood effects on electrode kinetics, species transport, and electrolyte resistance; all of which can impact the technical performance and economic viability of the battery system. To begin to deconvolute these effects, we examine the electrochemical and solution properties of a series of electrolytes based on different sized RAOs using a suite of electrochemical, flow cell, and analytical techniques. Insights gained from these studies inform design decisions for next-generation active species and operating envelopes for RFBs based on these materials.

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(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.

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(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.