Beyond Redox Additives: Accelerating Charge Transfer in Aqueous Energy Storage with Supporting Electrolytes

Achieving fast, reversible interfacial charge transfer remains a critical bottleneck hindering the synergistic realization of high energy and power density in aqueous electrochemical energy storage systems. Redox-active electrolytes (RAEs) offer enhanced energy capacitance but often suffer from kinetic limitations and poor reversibility due to unconstrained reactant diffusion. Here, we demonstrate that engineering the supporting electrolyte by introducing an inert salt (Na₂SO₄) into an [Fe(CN)₆]^3-/4- RAE effectively overcomes these challenges. The resultant elevated ionic strength fundamentally reshapes the electrode-electrolyte interface by compressing the diffusion layer, restructuring the electric double layer (EDL), accelerating ion-exchange dynamics, and significantly suppressing redox product drift away from the interface. Operando multipotential step measurements (MPSM) provide direct quantitative evidence for these kinetic enhancements, revealing a 2.5-fold increase in initial Faradaic current, a substantial ∼60% reduction in detrimental drift currents, and a 3-fold acceleration in equilibration time at the redox potential (reduced from >120 s to <40 s) upon addition of the supporting salt. Consequently, the optimized system exhibits markedly improved performance, including a 78% increase in areal capacitance, excellent rate capability retaining 92% capacitance at 100 mA cm^-2, and robust cycling stability with near-unity Coulombic efficiency over 10,000 cycles. These results establish supporting electrolyte engineering, validated by mechanistic insights from MPSM, as a powerful, cost-effective, and broadly applicable strategy to mitigate kinetic limitations and enhance the performance of aqueous RAEs for supercapacitors, hybrid devices, and flow batteries.