Chemical stability and reactivity of electrochemically grown manganese oxide films

Abstract

The fundamental scientific challenge in fully harnessing the potential of manganese oxide compounds lies in understanding the interplay between microstructure, surface properties, and electrochemical reactivity. In this study, manganese oxides were synthesized via two distinct methods: electrodeposition trough anodic oxidation of Mn2+ in solution and direct anodic oxidation of metallic Mn. Employing electrochemical quartz crystal microbalance measurements under flow conditions, the deposited compounds were successfully identified as function of the applied anodic potentials, revealing a transition from Mn3O4 to Mn2O3. Furthermore, the MnO → Mn(OH)2 conversion was determined to be a critical step in the degradation process of manganese oxides. For the anodic oxidation process, the mere alkaline "pH effect" (solid product formation predicted from Pourbaix Diagram) was demonstrated to be insufficient to form a protective oxide/hydroxide layer on top of metallic Mn. Indeed, the addition of phosphate proved necessary for achieving this protective passive layer formation, particularly in pH 12.0 Na3PO4 electrolyte solution. Potentiostatic growth in the presence of phosphates demonstrated the progressive stabilization of the protective layer consisting of a mix of manganese oxide, hydroxide, and phosphate, as confirmed by Raman and X-ray photoemission spectroscopy. To provide a comprehensive framework for our findings, a revised Pourbaix Diagram for the Mn-H2O-PO43− system is proposed and discussed.