Recent developments in electrochemical energy storage have sparked renewed interest in aqueous electrolytes. The advent of unconventional systems and the development of nanostructures and/or hybrid materials necessitate careful analysis of charging processes. This issue has been addressed using sodium manganese oxide (Na4Mn9O18, NMO) as an electrode material operating in an aqueous environment. The charging process was examined using cyclic voltammetry over a wide range of scan rates (ν), and impedance measurements were analyzed using an equivalent circuit to model the electrode response.

Cyclic voltammetry reveals that at low scan rates (ν ≤ 0.1 mVs–1), the charging process is equilibrium-controlled, while at higher scan rates (ν ≥ 0.2 mVs–1), it transitions to diffusion-controlled with overlaid capacitive charging. At very high scan rates (ν > 2 mVs–1), a mixed ohmic control transport is observed. Impedance analysis discriminates the variable charge storage character, highlighting the dominance of low-frequency faradaic insertion and increasing contributions from high-frequency pseudocapacitive and double-layer charging. We verified that the frequency deconvolution of charge mechanisms from this analysis aligns with CV analysis. For further insights, particularly regarding the aforementioned analysis and as a significant aspect of NMO behavior in general, we evaluated the chemical diffusion coefficient of Na ion as a function of potential.