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

Abstract

Abstract 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‐Mn IV oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn III ions at catalytic potentials is identified as a prerequisite for catalytic activity.