Shifting Target Reaction from Oxygen Reduction to Superoxide Disproportionation by Tuning Isomeric Configuration of Quinone Derivative as Redox Mediator for Lithium-Oxygen Batteries

Abstract

Quinones having a fully conjugated cyclic dione structure have been used as redox mediators in electrochemistry. 2,5-Ditert-butyl-1,4-benzoquinone (DBBQ or DB-p-BQ) as a para-quinone derivative is one of the representative discharge redox mediators for facilitating the oxygen reduction reaction (ORR) kinetics in lithium-oxygen batteries (LOBs). Herein, we presented that the redox activity of DB-p-BQ for electron mediation was possibly used for facilitating superoxide disproportionation reaction (SODR) by tuning the isomeric config-uration of the carbonyl groups of the substituted quinone to change its reduction potentials. First, we expected a molecule having its reduction potential between oxygen/superoxide at 2.75 V versus Li/Li+ and superoxide/peroxide at 3.17 V to play a role of the SODR catalyst by transferring an electron from one superoxide (O-2(-)) to another superoxide to generate dioxygen (O2) and peroxide (O-2(2-)). By changing the isomeric configuration from para (DB-p-BQ) to ortho (DB-o-BQ), the reduction potential of the first electron transfer (Q/Q(-)) of the ditert-butyl benzoquinone shifted positively to the potential range of the SODR catalyst. The electrocatalytic SODR-promoting functionality of DB-o-BQ kept the reactive superoxide concentration below a harmful level to suppress superoxide-triggered side reaction, improving the cycling durability of LOBs, which was not achieved by the para form. The second electron transfer process (Q(-)/Q(2-)) of the DB-o-BQ, even if the same process of the para form was not used for facilitating ORR, played a role of mediating electrons between electrode and oxygen like the Q/Q(-) process of the para form. The ORR-promoting functionality of the ortho form increased the LOB discharge capacity and reduced the ORR overpotential.