Odom, Susan A.

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University of KentuckyUKnowledgeChemistry Faculty Publications Chemistry2-21-2018Preventing Crossover in Redox Flow BatteriesThrough Active Material OligomerizationSusan A. OdomUniversity of Kentucky, saodom0@uky.eduRight click to open a feedback form in a new tab to let us know how this document benefits you.Follow this and additional works at: https://uknowledge.uky.edu/chemistry_facpubPart of the Chemistry Commons, and the Power and Energy CommonsThis Article is brought to you for free and open access by the Chemistry at UKnowledge. It has been accepted for inclusion in Chemistry FacultyPublications by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu.Repository CitationOdom, Susan A., "Preventing Crossover in Redox Flow Batteries Through Active Material Oligomerization" (2018). Chemistry FacultyPublications. 128.https://uknowledge.uky.edu/chemistry_facpub/128Preventing Crossover in Redox Flow Batteries Through Active Material OligomerizationNotes/Citation InformationPublished in ACS Central Science, v. 4, issue 2, p. 140-141.Copyright © 2018 American Chemical SocietyThis is an open access article published under an ACS AuthorChoice License, which permits copying andredistribution of the article or any adaptations for non-commercial purposes.Digital Object Identifier (DOI)https://doi.org/10.1021/acscentsci.8b00099This article is available at UKnowledge: https://uknowledge.uky.edu/chemistry_facpub/128Preventing Crossover in Redox Flow Batteries through ActiveMaterial OligomerizationThe ability to store large quantities of energygenerated by renewable, but intermittent, sourceslike solar or wind in systems integrated within thecountry’s electrical grid is critical to the reduction of CO2emissions and slowing the rate of global warming. Amongthe energy storage technologies under consideration, redoxflow batteries are promising candidates. While large-scaleinstallations of vanadium-containing flow batteries have beenutilized in Japan and China, the widespread adoption ofthese batteries for grid storage purposes has been limited dueto economics, safety concerns, and limited bankability. Newelectrolytes and longer-running installations have begun tomitigate concerns and have proven particularly useful inremote locationsespecially island communities and thosein which the grid is less reliableas well as in connection tomunicipal grids in the United States (e.g., WA, MA), manyof which are cofunded by the DOE’s Office of ElectricityDelivery and Energy Reliability.Redox flow batteries consist of a few major components:tanks of charged (or to-be-charged) electrolytes, electronics todo the charging (discharging), membranes to separate theelectrolyte solutions, and pumps to move liquids around.In commercial vanadium-based species, systems installed byJapan-based Sumitomo Electric Industries are still successfullyin operation decades in, but the cost of materials limits overalleconomic viability. Replacing current vanadium-based electro-lytes with those containing organic redox materials is thoughtby many to be a route to obtaining more commercially com-petitive flow batteries. While some groups are focused onorganics in aqueous solutions of various pHs, others haveexplored nonaqueous electrolytes. In the latter case, a majoradvantage stems from the increase in the stability window ofaprotic, nonaqueous electrolytes, which may be about 3 times(possibly 4 times) greater than that of aqueous solutions.The result would beif all other performance metrics areequalsmaller footprint batteries. Given the massive sizesof these batteries, realizing smaller footprints would greatlyassist in their ease in installation in existing communities.To provide an example, the company UniEnergy advertisesthat “a compact footprint of only five 20’ containers” arerequired to deliver 600 kW of power and 2.2 MWh maximumenergy (Figure 1).1The bane of many a flow battery chemist’s existence isthe performance of the membrane that separates the positiveand negative solutions, called the posolyte (or catholyte) andnegolyte (or anolyte). Separating the components of thesesolutions from one another is critical to keeping chargedcomponents (and their neutral versions) in the appropriatetanks. If species crossover occurs, capacity losses of 50% areinevitable. In acidic aqueous flow batteries, membranesalbeitexpensiveexist for keeping species in their respective tanks,but for other pHs and for nonaqueous electrolytes, membranesare still in development. Researchers can choose to wait forbetter membranes, work on developing better membranes,orin the case of Hendriks et al.can tailor the electrolyte fora size exclusion approach. In the latest work from the Sanfordgroup at the University of Michigan, with collaborators Helms,Sigman, and Minteer, a dual approach is reported: thecombination of an oligomeric posolyte (or catholyte) with amembrane composed of polymers exhibiting intrinsic micro-porosity (PIMs).2In their recent publication in ACS Central Science, Hendrikset al. report the synthesis and characterization of tris(dial-kylamino)cyclopropenium oligomers, some of which arerepresented in the chemical drawing below.2 The oligomerswere prepared to evaluate their performance with size exclusionmembranes composed of PIMs. By increasing the molecularweight of the oligomers in tandem with decreasing membranepore size through cross-linking, the authors hoped to minimizemembrane crossover of this active species. A recent publica-tion by coauthor Helms showed that the PIMs limited thecrossover of other oligomers containing redox-active viologen,Published: February 21, 2018Susan OdomDepartment of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United StatesCould the secret to better battery storage lie inlinkage length?First Reactions© 2018 American Chemical Society 140 DOI: 10.1021/acscentsci.8b00099ACS Cent. Sci. 2018, 4, 140−141FIRST REACTIONSThis is an open access article published under an ACS AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.Downloaded via 128.163.8.74 on February 6, 2019 at 20:16:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. pyridinium, and alkoxyarene oligomers.3,4 The stability ofSanford’s tris(dialkylamino)cyclopropenium redox moietyprompted the team to study it as an oligomeric material witha PIM separator (Figure 2).5This systematic study first involved a comparison of olig-omers in which spacer distance was varied, allowing the team tosystematically evaluate the impact of linker length on theelectrochemical potential and stability of these molecules. Theauthors found that a minimum linker length of four carbonswas needed to isolate redox couples from one another. Next,larger units were explored to measure crossover. While pre-vious results evaluating oligomeric active materials with PIM-based membranes showed that increases in oligomer size andcross-linking content limited crossover to undetectable limits,the stability of the redox species was not extensively probed.Here, Sanford and co-workers construct a full flow cell con-taining the oligomeric electrolyte for the positive electrode(posolyte) with a monomeric negolyte (they note that a stableoligomeric negolyte has not been developed). Cycling of thesesolutions showed rapid capacity loss due to negolyte cross-overnot a surprise given the small size of the active speciesbut that the posolyte remained stable and on its original sideof the membrane.In summary, the authors present an exciting study in which amodified membrane and oligomerized active material func-tion in a flow battery without crossover, providing a possiblesolution to a problem that has plagued groups working withnonaqueous electrolytes. I look forward to these authors’ futurework in the identification of a stable oligomeric negolyte withwhich to pair their tris(dialkylamino)cyclopropenium materials.Author InformationE-mail: susan.odom@gmail.com■ REFERENCES(1) http://uetechnologies.com/images/product/UET_UniSystem_Product_Sheet_reduced.pdf or UniEnergy’s website: http://www.uetechnologies.com/.(2) Hendriks, K. H.; Robinson, S. G.; Braten, M. N.; Sevov, C. S.;Helms, B. A.; Sigman, M. S.; Minteer, S. D.; Sanford, M. S. High-Performance Oligomeric Catholytes for Effective MacromolecularSeparation in Nonaqueous Redox Flow Batteries. ACS Cent. Sci. 2018,DOI: 10.1021/acscentsci.7b00544.(3) Doris, S. E.; Ward, A. L.; Baskin, A.; Frischmann, P. D.;Gavvalapalli, N.; Cheńard, E.; Sevov, C. S.; Prendergast, D.; Moore, J.S.; Helms, B. A. Macromolecular design strategies for preventingactive-material crossover in non-aqueous all-organic redox-flowbatteries. Angew. Chem., Int. Ed. 2017, 56, 1595−1599.(4) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.;Msayib, K. J.; Tattershall, C. E. Polymers of intrinsic microporosity(PIMs): Robust, solution-processable, organic nanoporous materials.Chem. Commun. 2004, 230−231.(5) Sevov, C. S.; Samaroo, S. K.; Sanford, M. S. Cyclopropenium saltsas cyclable, high-potential catholytes in nonaqueous media. Adv. EnergyMater. 2017, 7, 1602027.Figure 1. The 1 MW/4MWh Uni System installed at Schweitzer Engineering Laboratories in Pullman, Washington. Credit: http://uetechnologies.com.Figure 2. A representation of the chemical structures of selectoligomers reported by Hendriks et al.2 where the counteranion ishexafluorophosphate.This systematic study firstinvolved a comparison ofoligomers in which spacer dis-tance was varied, allowing theteam to systematically evaluatethe impact of linker length on theelectrochemical potential andstability of these molecules.REFERENCESACS Central Science First ReactionsDOI: 10.1021/acscentsci.8b00099ACS Cent. Sci. 2018, 4, 140−141141FIRST REACTIONS

Preventing Crossover in Redox Flow Batteries Through Active Material Oligomerization

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Preventing Crossover in Redox Flow Batteries Through Active Material Oligomerization

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