How to make an efficient and robust molecular catalyst for water oxidation

Abstract

Energy has been a central subject for human development from Homo erectus to date. The massive use of fossil fuels during the last 50 years has generated a large CO concentration in the atmosphere that has led to the so-called global warming. It is very urgent to come up with C-neutral energy schemes to be able to preserve Planet Earth for future generations to come and still preserve our modern societies' life style. One of the potential solutions is water splitting with sunlight (hν-WS) that is also associated with "artificial photosynthesis", since its working mode consists of light capture followed by water oxidation and proton reduction processes. The hydrogen fuel generated in this way is named as "solar fuel". For this set of reactions, the catalytic oxidation of water to dioxygen is one of the crucial processes that need to be understood and mastered in order to build up potential devices based on hν-WS. This tutorial describes the different important aspects that need to be considered to come up with efficient and oxidatively robust molecular water oxidation catalysts (Mol-WOCs). It is based on our own previous work and completed with essential contributions from other active groups in the field. We mainly aim at describing how the ligands can influence the properties of the Mol-WOCs and showing a few key examples that overall provide a complete view of today's understanding in this field.

Rational Design Rules for Molecular Water Oxidation Catalysts based on Scaling Relationships

“…These examples show that the ROC mechanism can indeed lead to powerful catalysts for water oxidation that are active at low overpotential. To design new transition‐metal catalysts that operate through this mechanism, two prerequisites need to be fulfilled: 1) the metallo‐oxo species should have sufficient spin density at the oxygen atom (Figure , intermediate C) and 2) two metal‐oxo species should be able to react to form the oxygen–oxygen bond . To allow this final step, the complexes should at least have sufficient space around the oxygen radical to allow this reaction, and one may also facilitate this step by preorganizing the two metal sites.…”

Ruthenium Nanoparticles for Catalytic Water Splitting

“…Today, most mechanistic knowledge available on the OER comes from research performed for the last 40 years on ruthenium molecular complexes . Even cycling on the millisecond timescale, ruthenium complexes have proven to be highly active electrocatalysts for the OER, showing remarkable stability if properly designed . However, the reaction rates of their heterogeneous counterparts, namely, RuO 2 , are typically several orders of magnitude lower, even if NP materials are employed; turnover frequency (TOF) values are below 1 s −1 (see Section 3), which highly contrasts with TOFs in the 10 000 s −1 range achieved for the best molecular Ru OER catalysts .…”

Synthesis, Structure, and Redox Properties of a trans-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States

“…8 Particularly interesting is the application of Ru complexes for the catalytic oxidation of water to dioxygen, a reaction of interest within the context of new renewable energy conversion schemes. 9,10,11,12 For redox catalysis, one of the unavoidable requirements to display significant redox activity is the presence of a Ru-OH2 group in the complex. 13,14,15,16 This allows to reach reactive high oxidation states via proton-coupled electron transfer at a relatively low energy.…”