Plasma‐Assisted Surface Engineering to Stabilize Mn3+ in Electrodeposited Manganese Oxide Films for Water Oxidation

Abstract Manganese oxide is a unique type of transition metal oxide that effectively functions as a catalyst for the oxygen evolution reaction (OER). Here, manganese oxide (MnO x ) polymorphs are synthesized through electrochemical deposition and treated with an atmospheric pressure plasma jet (APPJ). The APPJ surface treatment can generate numerous oxygen vacancies and modify the crystallinity of the MnO x films, which can enhance the long‐term stability of the MnO x films by stabilizing the Mn 3+ content in the highly oxidizing environment. The increase in Mn 3+ content and concentration of oxygen vacancies in the material synergistically increase the adsorption capacity of OH * and the electron‐transferring capacity of MnO x films in the OER process, making them more stable and effective for OER. MnO x films treated with APPJ exhibit significantly higher activity, better stability, and lower Tafel slopes for OER than untreated MnO x films. The MnO x films treated with APPJ can remain stable for up to 92 hours during OER with a current density of 10 mA cm −2 , with an onset overpotential of 310 mV. This strategy, which combines APPJ surface treatment techniques with electrodeposition methods, is innovative in the surface modification of manganese oxides with mixed valences to create OER catalysts with stable Mn 3+ content.