Formation of the Metastable Mn III Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution-Deposition Process Involving Labile Mn II .

Birnessite, the closest naturally occurring analog of the Mn 4 CaO 5 cluster of photosystem II, is an important model compound in the development of bio-inspired electrocatalysts for the water oxidation reaction. The present work reports the formation mechanism of the key Mn III intermediate realized through the study of the effects of several electrolyte anions and cations on the catalytic efficiency of birnessite. In situ spectroelectrochemical measurements show that the activity is controlled by a dynamic dissolution-oxidation process, wherein Mn III is formed through the oxidation of labile uncomplexed Mn II that reversibly shuttles between the birnessite and the electrolyte in a manner similar to the photoactivation in photosystem II. The role of electrolyte cations of different ionic radii and hydration strengths is to control the interlayer spacing, whereas electrolyte anions control the extent of deprotonation of complexed Mn II in the lattice. Both in turn govern the shuttling efficiency of uncomplexed Mn II and its subsequent electro-oxidation to Mn III .