Electrosynthesis of Biomimetic Manganese–Calcium Oxides for Water Oxidation Catalysis—Atomic Structure and Functionality

Water-oxidizing calcium–manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-MnIV oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of MnIII ions at catalytic potentials is identified as a prerequisite for catalytic activity.