Molecular catalytic assemblies for electrodriven water splitting.

Clean energy carriers obtained from renewable, earth-abundant materials and by using the virtually unlimited supply of sunlight have potential to serve as future sustainable power sources. A quest for new materials for oxygen evolution from catalytic water oxidation and carbon dioxide reduction, which aim to build up solar-to-fuel conversion devices that use water as raw material, has been developing during the last two decades. Most of the research in the field of materials science and chemistry has been focused on the development of inorganic materials and molecular complexes for water oxidation, in particular bioinspired catalytic systems. Recently, various molecular water-oxidation complexes with mono- or multinuclear catalytic sites have been tested for solution-phase dioxygen generation. Catalyst immobilization and functionalization on an electrode surface is required for electrocatalytic or photoelectrochemical water oxidation devices; however, there are only a few examples in which a molecular catalyst has been placed on a transparent conducting surface in an electro- or photoelectrochemical system. Herein, a brief overview of surface-immobilized molecular assemblies for electrochemical water oxidation is presented, and an analysis of recent progress in catalyst design and performance is provided, including systems integration of modules for future stand-alone solar-to-fuel conversion devices. A view on the thermodynamics features of various intermediates and the mechanism of O–O bond formation in single-site complexes and binuclear water oxidation catalysts is also presented.