Water oxidation catalysts based on abundant 1st row transition metals

•Efficient water oxidation catalysts based on earth abundant elements are highly sought after.•Many molecular complexes have been applied in homogeneous solution but often decompose under the strongly oxidizing test conditions.•The metal coordination environment is important in development of complexes that are stable during catalysis.•Further mechanistic studies are required to elucidate the actual species or material active in catalysis.•Integration of water oxidation catalysts with photosensitizers for solar driven water oxidation remains a challenge.

Hydrogen production through water splitting is an excellent potential source of clean energy for the future, providing it can be generated from renewable energy sources. An attractive approach is to use the abundance of solar energy that reaches the earth every day to convert water into hydrogen and oxygen (a by-product). Catalysts are a very important component of efforts to design and develop efficient water splitting technologies. Ideally, these catalysts need to operate at low overpotentials and be able to be coupled to systems that harness solar energy to achieve light-driven water splitting. Moreover, for such technologies to have maximum practical utility, the components of water splitting devices need to be made from cheap and abundant materials. Our efforts, and also those of many other research groups, have been directed at the development of catalysts for the energetically demanding and mechanistically complex water oxidation reaction (2H2O ⇌ O2 + 4H+ + 4e−) which can be coupled with catalysts for proton reduction (2H+ + 2e− ⇌ H2) to achieve water splitting. In this review, we focus specifically on water oxidation catalysts incorporating inexpensive first row transition metals, e.g., Fe, Mn, Co and Cu, an area enjoying significant progress in recent years. We firstly review molecular water oxidation catalysts which have been tested in homogeneous solution and when introduced into heterogenous systems. Secondly, we cover the application of cheap and abundant metal oxides, based mainly on manganese, cobalt and nickel oxides, as water oxidation catalysts in electrocatalytic and photoelectrochemical devices.