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Title: Elucidating Factors Controlling Long-Term Stability of Radical Anions for Negative Charge Storage in Nonaqueous Redox Flow Batteries

Radical anions of electrochemically reduced compounds (anolytes) have been suggested for storage of negative charge in nonaqueous redox flow batteries. The lower the redox potential of the anolyte molecule, the higher is the stored energy density. However, the stability of the radical ions frequently suffers as their redox potentials become extreme, and there is a compromise between the energy density and the chemical stability in the active form. In this paper, we scrutinize this trade-off using one such “extreme,” the heterocyclic anolyte 2,1,3-benzothiadiazole, BzNSN, by adjusting the redox potential of BzNSN via installed electron-donating and electron-withdrawing groups. We show that the stability of the radical anion strongly depends on the degree of ion pairing in solution, with the worst being for the contact lithium ion pairs. For BzNSN derivatives, there is a strong correlation between the lifetime of the radical anion and the redox potential. The root cause appears to be the proton transfer from the solvent (acetonitrile) to the radical anion, which is concerted with the dimerization of the solvent molecules. These interactions cause the radical anion to become unstable even though the redox couple falls well within the electrochemical stability window of the solvent. Steering this redox potentialmore » toward the middle of this window using electron-withdrawing groups did not pay off as it opened additional decomposition pathways. Finally, our study, therefore, suggests that there can be natural limitations to the energy density that is realistically achieved using neutral, closed-shell anolyte molecules for charge storage in the redox flow cells.« less
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  1. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research. Chemical Sciences and Engineering Division
  2. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research; Univ. of Illinois, Urbana, IL (United States). Dept. of Chemistry
  3. Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research. Materials Science Division
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Volume: 122; Journal Issue: 15; 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
OSTI Identifier: