In situ localized studies to unveil the charge storage mechanism of manganese oxide in aqueous media

• Detailing the mechanism of charge storage of birnessite-type MnO x . • Combination of in situ localized techniques with conventional voltammetry analysis. • Local micro-potentiometric analysis of H + and Na + at material/electrolyte vicinity. • Quantifying mass transfer using beam deflection and multiflux convolution analysis. • Close relationship between ionic flux at the vicinity of the MnO x electrode and its electrochemical response. Progresses on the understanding of the charge storage mechanism of birnessite-type manganese dioxide (MnO x ) in aqueous electrolytes has been essentially studied by conventional electroanalytic techniques. However, understanding the pseudocapacitive response of MnO x materials during charge/discharge remains challenging due to the dynamics of electrolyte ions at the electrode interface. In this work, the role of different ions in the storage mechanism of MnO x in Na 2 SO 4 electrolytes was revealed using localized electrochemical techniques for in situ measurement of ions distribution during charge/discharge. SIET (scanning ion-selective electrode technique) was used to record local pH and pNa evolutions. In PBD (probe beam deflection), in situ beam deviation was measured and analyzed using a multiflux convolution treatment, specially designed in this work to quantify the mass transfer flows. PBD results showed individual and equal flux contributions of Na + and SO 4 2− ions, highlighting their charge compensating role in the mechanism. Localized micro-potentiometry revealed that redox reactions were responsible for major changes in pH profile, which was additionally affected by (de)intercalation processes. Thus, a close relationship between ionic flux at the vicinity of the MnO x electrode and its electrochemical response was successfully obtained by employing this unique combination of in situ localized techniques complemented with conventional voltammetry.