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Title: Toward Improved Catholyte Materials for Redox Flow Batteries: What Controls Chemical Stability of Persistent Radical Cations?

We report catholyte materials are used to store positive charge in energized fluids circulating through redox flow batteries (RFBs) for electric grid and vehicle applications. Energy-rich radical cations (RCs) are being considered for use as catholyte materials, but to be practically relevant, these RCs (that are typically unstable, reactive species) need to have long lifetimes in liquid electrolytes under the ambient conditions. Only few families of such energetic RCs possess stabilities that are suitable for their use in RFBs; currently, the derivatives of 1,4- dialkoxybenzene look the most promising. In this study, we examine factors that define the chemical and electrochemical stabilities for RCs in this family. To this end, we engineered rigid bis-annulated molecules that by design avoid the two main degradation pathways for such RCs, viz. their deprotonation and radical addition. The decay of the resulting RCs are due to the single remaining reaction: O-dealkylation. We establish the mechanism for this reaction and examine factors controlling its rate. In particular, we demonstrate that this reaction is initiated by the nucleophile attack of the counter anion on the RC partner. The reaction proceeds through the formation of the aroxyl radicals whose secondary reactions yield the corresponding quinones. The O-dealkylationmore » accelerates considerably when the corresponding quinone has poor solubility in the electrolyte, and the rate depends strongly on the solvent polarity. Finally, our mechanistic insights suggest new ways of improving the RC catholytes through molecular engineering and electrolyte optimization.« less
 [1] ; ORCiD logo [1] ; ORCiD logo [2] ;  [3] ;  [3] ; ORCiD logo [2] ;  [3] ;  [1]
  1. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research and Chemical Sciences and Engineering Division
  2. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research and Materials Science Division
  3. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research; Univ. of Michigan, Ann Arbor, MI (United States). Department of Chemical Engineering
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Volume: 121; Journal Issue: 42; Journal ID: ISSN 1932-7447
American Chemical Society
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
25 ENERGY STORAGE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; chemical kinetics; electron spin resonance; energy storage; radical ions; radical reactions; redox flow battery
OSTI Identifier: