Strong Interaction Between Redox Mediators and Defect-Rich Carbons Enabling Simultaneously Boosted Voltage Windows and Capacitance for Aqueous Supercapacitors

Energy density, the Achilles' heel of aqueous supercapacitors, is simultaneously determined by the voltage window and specific capacitance of the carbon materials, but the strategy of synchronously boosting them has rarely been reported. Herein, we demonstrate that the rational utilization of the interaction between redox mediators (RMs) and carbon electrode materials, especially those with rich intrinsic defects, contributes to extended potential windows and more stored charges concurrently. Using 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4OH-TEMPO) and intrinsic defect-rich carbons as the RMs and electrode materials, respectively, the potential window and capacitance are increased by 67% and sixfold in a neutral electrolyte. Moreover, this strategy could also be applied to alkaline and acid electrolytes. The first-principle calculation and experimental results demonstrate that the strong interaction between 4OH-TEMPO and defect-rich carbons plays a key role as preferential adsorbed RMs may largely prohibit the contact of free water molecules with the electrode materials to terminate the water splitting at elevated potentials. For the RMs offering weaker interaction with the electrode materials, the water splitting still proceeds with a thus sole increase of the stored charges. The results discovered in this work could provide an alternative solution to address the low energy density of aqueous supercapacitors.