Tailoring crystalline Mn5O8 nanostructures: Kinetic control and stabilization for enhanced electrocatalytic O2 evolution

Abstract

The stabilization of manganese oxide nanostructures with tunable oxidation states and morphologies is crucial for regulating catalytic properties. In this study, we aim to stabilize the metastable monoclinic Mn5O8 phase and control the morphology of the Mn5O8 nanostructures using a hydrothermal assisted calcination process in the presence of different precipitating agents. Raman experiments confirmed that the interaction of aqueous Mn2+ salts with different precipitating agents such as NaOH, ammonia, and oleylamine, resulted in the formation of Mn5O8. On the other hand, the precipitation of Mn2+ salt using diammonium oxalate monohydrate as a precipitating agent led to complete oxidation forming Mn2O3. Detailed phase and microstructural analyses were conducted using PXRD, XPS, and FESEM. The analyses revealed the formation of the monoclinic Mn5O8 phase with various morphologies, including hexagonal nanoplates, nanorods, and random nanoplates, when using NaOH, ammonia, and oleylamine as precipitating agents. The synthesized Mn5O8 phase was deposited onto an iron sheet as a substrate. Furthermore, we demonstrated the effect of the structure, morphology, and size of the Mn5O8 material on O2 evolution activity. The catalytic performance of irregularly shaped Mn5O8 nanoplates showed significantly lower η10 (270 mV) and η50 (350 mV) compared to Mn5O8 hexagonal nanoplates, Mn5O8 nanorods, and Mn2O3 nanostructure. Additionally, an insight into the catalytic performance was investigated in detail by evaluating electrochemical active sites, impedance spectroscopy, and hydrophilicity/hydrophobicity of the Mn5O8 material.