Photochemically Assisted Formation of Layered Manganese Oxide Nanosheets with High Pseudocapacitance for Potential Environmental Applications

Manganese (Mn) oxides have attracted considerable attention for their affordability, environmental compatibility, and favorable electrochemical properties, making them promising materials for various environmentally relevant electrochemical applications, such as capacitive deionization and ion-selective electrochemical separations. However, conventional Mn oxide synthesis methods typically involve strong oxidants, external heating, extended synthesis times, or expensive equipment, underscoring the need for a greener, more energy-efficient synthesis approach for Mn oxides with reduced environmental footprints. Inspired by the natural Mn oxidation process driven by sunlight-induced reactive radicals, this study presents a greener and photochemically assisted method for synthesizing Mn oxide solids by optimizing aqueous chemistry conditions to accelerate Mn2+ oxidation. During nitrate photolysis in the presence of a pyrophosphate buffer, Mn2+ was rapidly oxidized to form birnessite, eliminating the need for external heating and significantly reducing energy input. The resulting Mn oxides exhibited high pseudocapacitance performance, achieving a capacitance of 172.4 ± 16.6 F/g, surpassing Mn oxides synthesized via redox oxidation (144.1 ± 4.2 F/g) and those commercially purchased (14.5 ± 0.6 F/g). By integrating enhanced pseudocapacitance with sustainable, energy-efficient synthesis, this study introduces a promising strategy for producing Mn oxides with significant potential for environmentally relevant electrochemical applications.