Light-Mediated Electrochemical Synthesis of Manganese Oxide Enhances Its Stability for Water Oxidation

Abstract

New methods are needed to increase the activity and stability of earth-abundant catalysts for electrochemical water splitting to produce hydrogen fuel. Electrodeposition has been previously used to synthesize manganese oxide films with a high degree of disorder and a mixture of oxidation states for Mn, which has led to electrocatalysts with high activity but low stability for the oxygen evolution reaction (OER) at high current densities. In this study, we show that multipotential electrodeposition of manganese oxide under illumination produces nanostructured films with significantly higher stability for the OER compared to films grown under otherwise identical conditions in the dark. Manganese oxide films grown by multipotential deposition under illumination sustain a current density of 10 mA/cm 2 at 2.2 V versus reversible hydrogen electrode for 18 h (pH 13). Illumination does not enhance the activity or stability of manganese oxide films grown using a constant potential, and films grown by multipotential deposition in the dark undergo a complete loss of activity within 1 h of electrolysis. Electrochemical and structural characterization indicate that photoexcitation of the films during growth reduces Mn ions and changes the content and structure of intercalated potassium ions and water molecules in between the disordered layers of birnessite-like sheets of MnO x , which stabilizes the nanostructured film during electrocatalysis. These results demonstrate that combining multiple external stimuli (i.e., light and an external potential) can induce structural changes not attainable by either stimulus alone to make earth-abundant catalysts more active and stable for important chemical transformations such as water oxidation.

Characterizing Biogenic MnOx Produced by Pseudomonas putida MnB1 and Its Catalytic Activity towards Water Oxidation

“…Furthermore, manganese oxides have been of interest for catalytic water oxidation (Equation ( 1)) and its broader connections to the oxygen evolving complex (OEC) in Photosystem II [17,[21][22][23][24][25][26][27][28][29][30][31][32][33][34]. The OEC is the only natural system capable of water oxidation, occurring as part of the photosynthetic process [17,35].…”

Electronic Structure Modification of MnO2 Nanosheet Arrays with Enhanced Water Oxidation Activity and Stability by Nitrogen Plasma

“…However, many experimental and theoretical studies on MnO 2 electrocatalysts have demonstrated that Mn 3+ is the most unstable oxidized state of Mn, suffering from electrochemical instability. − Under OER oxidation conditions, Mn 3+ undergoes charge disproportionation into Mn 4+ and Mn 2+ . Therefore, it is essential to stabilize Mn 3+ to enhance the electrocatalytic performance of MnO 2 .…”