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

Progresses on the understanding of the charge storage mechanism of birnessite-type manganese dioxide (MnOx) in aqueous electrolytes has been essentially studied by conventional electroanalytic techniques. However, understanding the pseudocapacitive response of MnOx 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 MnOx in Na2SO4 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 SO42- 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 MnOx electrode and its electrochemical response was successfully obtained by employing this unique combination of in situ localized techniques complemented with conventional voltammetry.