As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH) 2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS x has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

Practical-efficiency catalysis of the water oxidation process (2 H 2 O!O 2 + 4 e À + 4 H + ) is the highly sought element of emerging artificial photosynthetic energy-conversion technology. [1,2] While oxygen evolution in naturally occurring photosynthesis, which supports nearly all existing life forms, relies on a sophisticated Mn-based complex, [1a, 3] the majority of artificial molecular water-oxidation catalysts (WOCs) are Ru-based complexes with relatively simple polypyridyl ligands. [4, 5] Recently emerged evidence in favor of alternative O 2 -evolving mechanisms for Ru-catalyzed water oxidation, [5] such as solvent water nucleophilic attack (WNA) and direct O À O coupling via interaction of two M-O units (I2M), [5a, 6-8] demonstrated dramatic mechanistic consequences of different ligand designs, which are yet to be fully rationalized. Understanding of intricate ligand-dependent preferences for one mechanism over the other is necessary for further progress to be made. [5, 7] Unfortunately, vast structural differences between Ru-bound ligands of WOCs which operate by the WNA or I2M mechanism hampers determinations of ligand influence on catalytic pathways.

This perspective article reports the most significant advances in the field of water oxidation-from molecular water oxidation catalysts (WOCs) to photoelectrochemical cells. Different series of catalysts that can be applied in visible light-driven water oxidation catalysis are discussed in details and several key aspects of their catalytic mechanisms are introduced. In order to construct a water oxidation electrode from molecular catalysts, proper immobilization methods have to be employed. Herein, we present one section about how to attach catalysts onto an electrode/material surface. Finally, the state of the art photoelectrochemical cells that achieve visible light-driven water splitting are described.

An approximately planar tetradentate polypyridine ligand, 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline (ppq), has been prepared by two sequential Friedländer condensations. The ligand readily accommodates Co(II) bearing two axial chlorides, and the resulting complex is reasonably soluble in water. In DMF the complex shows three well-behaved redox waves in the window of 0 to -1.4 V (vs SHE). However in pH 7 buffer the third wave is obscured by a catalytic current at -0.95 V, indicating hydrogen production that appears to involve a proton-coupled electron-transfer event. The complex [Co(ppq)Cl2] (6) in pH 4 aqueous solution, together with [Ru(bpy)3]Cl2 and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λmax = 469 nm) produces hydrogen with an initial TOF = 586 h(-1).

Here splits the sun: A dinuclear ruthenium complex has been synthesized and employed to catalyze the homogeneous water oxidation (see picture; purple Ru, green Cl, blue N, red O). An exceptionally high turnover number was observed both for chemical (CeIV as the oxidant) and light‐driven ([Ru(bpy)3]2+‐type photosensitizers) water splitting.