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

Abstract

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.