Unraveling the dynamic transformation of azobenzene-driven redox electrolytes for Zn-ion hybrid capacitors†

The incorporation of redox-active molecules into aqueous electrolytes addressed the challenges in the development of energy storage devices by enhancing energy density, expanding voltage windows, and improving cycling stability. Aqueous zinc-ion hybrid capacitors (ZICs), which combine the characteristics of metal-ion batteries and supercapacitors, have emerged to meet high energy-power demands. Herein, we investigate the irreversible and reversible dynamics of azobenzene compounds in the development of redox electrolytes for ZIC applications. The results reveal that Sunset Yellow (SY) additives promote reversible Zn²⁺ stripping/plating at the Zn anode through ion–ligand coordination, while the redox behavior of the –NN– group enhances the interfacial charge transfer. The passivation behavior of redox electrolytes, derived by SY molecules, was investigated using in situ electrochemical atomic force microscopy. Morphology evolution, coupled with nanomechanical tests, shed light on the transformation of SY-containing electrolytes. The redox electrolytes improve the lifespan of Zn//Zn cells from 50 to 1700 cycles at 2 mA cm⁻² and 1 mA h cm⁻² by suppressing parasitic reactions and mitigating by-product formation. The redox-enhanced ZIC delivers an increased capacity of 113.3 mA h g⁻¹ with improved cycling stability, compared to 75.2 mA h g⁻¹ of ZnSO₄ electrolytes without SY molecules at 0.6 mA g⁻¹. A range of azobenzene-based compounds with varying structures were investigated to demonstrate the synergistic modulating effects. The results demonstrate the molecular engineering of redox electrolytes and provide fundamental insights into electrochemical interphase transformation.