pH and Cation Concentration Effect on the Interfacial Recharging of Mn Oxides: Experiment and Modeling

The influence of electrolyte composition on the kinetics of electrocatalytic reactions was discussed in the seminal works of Frumkin. However, recent findings have reignited interest in the role of cations and pH in electrocatalysis, particularly in alkaline environments. Manganese oxides have attracted much attention as catalysts for both the oxygen reduction (ORR) and oxygen evolution reactions (OER). In addition, some Mn oxides are promising materials for supercapacitors due to their high pseudocapacitance. In this work we combine electrochemical measurements with microkinetic modeling to unveil influence of pH and cation concentration on the interfacial recharging behavior of α-MnOOH, α-Mn₂ O₃ and LaMnO₃ . Titration experiments revealed that the isoelectric point of the manganese oxides is around 5, indicating that they are negatively charged in alkaline environments. Cyclic voltammetry showed that both OH⁻ and Na⁺ concentrations influence the Mn(III)/Mn(IV) redox transition at the oxide-electrolyte interface, with distinct behaviors for α-MnOOH, α-Mn₂O₃ , and LaMnO₃ . It is noteworthy that α-MnOOH exhibits two types of interfacial redox centers, protonated and cationic, which emphasises that the redox behavior is determined by ion-exchange equilibria.