homogeneous phase occur via an "Interaction of 2 M-O units" (I2M) might still be able to carry out the catalytic water oxidation reaction at the surface of an electrode, but will need to proceed through higher energy pathways that can lead to catalyst degradation. 6 Further, given the intrinsic high energy demands for the water oxidation catalysis, it is essential that the anchoring groups that act as an interface between the catalysts and surface are oxidatively resistant.Here on, we report new hybrid materials consisting of molecular WOCs anchored onto Multi-Walled Carbon Nanotubes (MWCNTs) via π-stacking interactions. 9 The resulting materials are extremely stable and allow the anchoring of a large amount of catalyst giving Turnover Numbers (TNs) over a million without apparent deactivation.In a recent publication, 10 we have reported the synthesis of complex {Ru II (tda)(py)2}, 1a, (for a drawing of tda 2-see Scheme 1) and have shown that in its high oxidation states (IV) acts as a precursor for the formation of {Ru V (O)(tda)(py)2} + . The latter is the most powerful molecular water oxidation catalyst described to date achieving Turnover Frequencies (TOF) in the range of 50.000 s -1 . In addition, we showed that the rate determining step for the water oxidation reaction is the O-O bond formation, which in this case occurs via WNA, as evidenced by kinetics and further supported by DFT calculations.Scheme 1. Drawing of the ligands discussed in the present work (top) and complex labelling strategy (bottom).[a]

Both global warming and limited fossil resources make the transition from fossil to solar fuels an urgent matter. In this regard, the splitting of water activated by sunlight is a sustainable and carbon‐free new energy conversion scheme able to produce efficient technological devices. The availability of appropriate catalysts is essential for the proper kinetics of the two key processes involved, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). During the last decade, ruthenium nanoparticle derivatives have emerged as true potential substitutes for the state‐of‐the‐art platinum and iridium oxide species for the HER and OER, respectively. Thus, after a summary of the most common methods for catalyst benchmarking, this review covers the most significant developments of ruthenium‐based nanoparticles used as catalysts for the water‐splitting process. Furthermore, the key factors that govern the catalytic performance of these nanocatalysts are discussed in view of future research directions.