Oxygen-vacancy-enriched MgO/carbon composite as a highly efficient electrocatalyst for phenazine/dihydrophenazine redox reaction of aqueous phenazine redox flow battery

Abstract

Phenazine derivatives are attractive anolyte redox-active species in aqueous redox flow batteries (ARFBs). Unfortunately, they typically suffer from sluggish reaction kinetics, resulting in poor rate capability and inferior energy efficiency, especially under high current densities. Herein, we report a composite of MgO and Ketjen black carbon (MgO/KB) that can serve as a low-cost ARFB electrocatalyst to significantly improve the redox reaction kinetics of phenazine derivatives. The MgO/KB composite catalyst exhibits excellent catalytic performance, with an area specific resistance (ASR) of 1.54 and 1.11 Ω cm2 for the charge and discharge processes, respectively, as well as improved energy efficiency, capacity utilization and durability in practical ARFB applications. Detailed research confirms that the oxygen vacancies within the MgO promote electron transfer and enhance catalytic activity for the electrochemical reaction of phenazine derivatives. Using density functional theory (DFT) calculations, we unveil that the underlying catalytic enhancement mechanism stems from a low free energy barrier of the rate-determining step (RDS) on oxygen-vacancy-enriched MgO for the oxidation reaction of dihydro-phenazine derivative. These findings will guide the design of highly active electrocatalysts for organic ARFBs.