Recent Advances in Catalysts Based on Molecular Cubanes for Visible Light-Driven Water Oxidation

Increasing climate change and environmental pollution caused by the excessive use of fossil fuels have prompted intensive research into clean and efficient renewable sources as a substitute for traditional fossil fuels. A very promising approach is to mimic the water splitting process that occurs in plants during photosynthesis, in order to convert solar energy into chemical energy. A successful water splitting reaction, which comprises two half reactions (water oxidation and the reduction of protons), can generate H-2 and O-2 from water. Hydrogen is a promising renewable energy carrier because of its clean combustion and high calorific value. Light-driven water splitting is considered to be a feasible way to transform water and solar energy into hydrogen energy. However, water oxidation is considered to be the bottleneck process of water splitting because it advances in a thermodynamically uphill manner with the involvement of 4e(-) and 4H(+). Inspired by the nature of Mn4CaO5 in photosystem II (PS II), the comprehensive understanding of its key features for use in active molecular water oxidation catalysts (WOCs) remains challenging. Extensive effort has been devoted to researching and manufacturing the structure and biomimicking the catalytic activity of Mn4CaO5 clusters that contain the Mn3CaO4 cubane structure, for the construction of low-cost and robust WOCs. WOCs can be divided into heterogeneous and homogeneous catalysts. Although heterogeneous WOCs are convenient for recycling and are easily prepared on a large scale, homogeneous WOCs, especially complexes based on organic ligands or polyoxometalates (POMs), have more advantages owing to their catalytic efficiency, structural modifications, and mechanistic understanding. Thus, recently, some molecules with an M4O4 (M = transition metals, mainly Mn, Co, Ni, and Cu) cubic structure have been reportedly used as photocatalytic WOCs. In this review, we present an overview of the most important and recent advances based on M4O4 cubic WOCs that contain first-row transition metal cubanes for visible light-driven water oxidation. Our main focus is on the structure of cubane catalysts, including metal complexes, POMs, and a system containing BiVO4 or polymeric carbon nitride (PCN) as a photosensitizer, and cubic complexes as WOCs. Results have shown that the activity and stability of the catalyst can be tuned by the ligand stability, metal center, coordination environment, and other factors. This review will be helpful for designing new cubane catalysts for photocatalytic water oxidation that are highly efficient and stable.