Beyond Conventional Methods for Evaluating Charge Transfer Kinetics in Aqueous Zinc-Ion Batteries: Insights from Ultramicroelectrode Voltammetry and Marcus–Hush Theory

Understanding intrinsic charge transfer kinetics is essential for aqueous zinc-ion batteries (AZIBs). In this work, we employ fast-scan cyclic voltammetry with ultramicroelectrodes (UMEs) to eliminate mass transfer limitations and accurately extract the exchange current density (j0) and reorganization energy (λ) in representative zinc electrolytes: Zn(ClO4)2, ZnSO4, Zn(TfO)2, and ZnCl2. While the Butler–Volmer model accurately describes kinetics at low overpotentials, it fails to capture the nonlinear Tafel behavior at higher overpotentials. In contrast, the Marcus–Hush model successfully accounts for these deviations while also providing physically meaningful kinetic parameters across a wider potential range. Additionally, electrolytes with larger anions and higher viscosities exhibit lower values for j0 and higher values for λ, indicating that solvation structure and ion–solvent interactions contribute to interfacial kinetics. These findings highlight the limitations of the Butler–Volmer model and demonstrate that Marcus–Hush theory offers a more rigorous and accurate approach for evaluating charge transfer in AZIBs.