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A Tetrasubstituted Hydroquinone Ether Catholyte for Non-Aqueous Redox Flow Batteries: Bicyclic Substitution Enabling High Stability

Thursday, 5 October 2017: 14:10
Maryland A (Gaylord National Resort and Convention Center)
J. Zhang (Argonne National Laboratory, Joint Center for Energy Storage Research (JCESR)), Z. Yang (Pacific Northwest National Laboratory, Joint Center for Energy Storage Research), I. A. Shkrob (Argonne National Laboratory, Joint Center for Energy Storage Research (JCESR)), R. Assary (Materials Science Division, Argonne National Laboratory, Joint Center for Energy Storage Research (JCESR)), S. O. Tung (University of Michigan, Joint Center for Energy Storage Research (JCESR)), B. Silcox (University of Michigan), W. Duan (Pacific Northwest National Laboratory, Joint Center for Energy Storage Research), B. Hu (Argonne National Laboratory), L. T. Thompson (University of Michigan, Joint Center for Energy Storage Research (JCESR)), X. Wei (Pacific Northwest National Laboratory, Joint Center for Energy Storage Research), C. Liao (JCESR at Argonne National Laboratory), Z. Zhang (Argonne National Laboratory, Joint Center for Energy Storage Research), and L. Zhang (Joint Center for Energy Storage Research (JCESR), Argonne National Laboratory)
The practical application of non-aqueous redox flow batteries (NRFBs) sets stringent requirements on the electrochemical stability of the redox active materials. In the redox pair for NRFBs, the high-potential species is referred to as the catholyte, which is oxidized into the radical cation state in the charging process. Hydroquinone ethers are a key class of catholyte materials, but the intermolecular reaction between the radical cations is a dominating decomposition pathway of the catholyte during functioning. In order to increase the electrochemical stability of the catholyte materials, the introduction of bulky alkyl spacers at 2,5- positions of the arene ring is commonly used as a strategy in the molecular design to suppress this type of undesirable side reaction. However, the unsubstituted 3,6- positions that are favored for the electrochemical reversibility are responsible for the radical cation reaction in the long-time cycling. Tetrasubstitution of the arene core with either primary or secondary alkyl groups often leads to a compromised redox reversibility. This talk will describe a tetrasubstituted hydroquinone ether-based catholyte molecule that retains excellent redox reversibility and illustrates exceptional stability. Unique bicyclic alkyl groups are incorporated to the molecular scaffold, eliminating the bimolecular reaction between the radical cations. A hybrid NRFB using such catholyte has been operated for over 150 charge-discharge cycles with minimal loss of capacity.