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Redox properties of birnessite from a defect perspective

Significance We propose, and preliminarily confirm with experiments, a theoretical model to understand various structure–performance dependences of layered-structure birnessite as an oxygen evolution reaction (OER) catalyst. Besides the well-accepted importance of Mn(III), we emphasize the critical importance of a nonuniform distribution of Mn(III) to OER catalytic activity. Such a distribution contributes to the reduction of the overpotential by building an internal potential step. We further propose the small polaron as a common concept to link the fields of oxygen evolution catalysis and Li-ion batteries, suggesting a promising candidate space for oxygen evolution reaction catalysts. Birnessite, a layered-structure MnO2, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure–performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.