Uncovering Structure-Activity Relationships in Manganese-Oxide-Based Heterogeneous Catalysts for Efficient Water Oxidation

Artificial photosynthesis by harvesting solar light into chemical energy could solve the problems of energy conversion and storage in a sustainable way. In nature, CO2 and H2O are transformed into carbohydrates by photosynthesis to store the solar energy in chemical bonds and water is oxidized to O-2 in the oxygen-evolving center (OEC) of photosystem II (PS II). The OEC contains CaMn4O5 cluster in which the metals are interconnected through oxido bridges. Inspired by biological systems, manganese-oxide-based catalysts have been synthesized and explored for water oxidation. Structural, functional modeling, and design of the materials have prevailed over the years to achieve an effective and stable catalyst system for water oxidation. Structural flexibility with e(g)(1) configuration of Mn-III, mixed valency in manganese, and higher surface area are the main requirements to attain higher efficiency. This Minireview discusses the most recent progress in heterogeneous manganese-oxide-based catalysts for efficient chemical, photochemical, and electrochemical water oxidation as well as the structural requirements for the catalyst to perform actively.