Strategic factors to design the next generation of molecular water oxidation catalysts: Lesson learned from ruthenium complexes

Abstract Energy conversion through sustainable means is essential to counter global warming and an urgent solution through a multidisciplinary approach is required. The temperature change stems from the emission of greenhouse gases largely contributed by burning fossil fuels; thus, its replacement with a clean, cheap, and sustainable energy source is paramount. Water is one of the integral sources and is potentially attractive to produce hydrogen (H2) gas to substitute fossil fuels. In this respect, molecular ruthenium complexes are by far the best molecular water oxidation catalysts (WOCs) to produce proton (H+) as a source for H2 gas production and molecular oxygen (O2) as a clean by-product. Here, we have made an insightful chronology of the ruthenium-based water oxidation catalysts (RWOCs) and quantify and classified the activation and deactivation pathways in chemical and electrochemical water oxidation reactions (WORs). In this insightful chronology, RWOCs are classified into three main groups, including Ru(N)6, Ru(N)5(O), and Ru(N)4(O)2 which the last group is the most robust and powerful WOCs. However, still, there is ∼200 mV overpotential for the Ru(N)4(O)2 group of complexes which is one of the current challenges in WOR. Based on the experimental data collected over the last four decades and extracted from these three categories, we suggest a new family of RWOCs that can be the workhorse of RWOCs with very low overpotential. Moreover, the lesson learned from RWOCs has been applied to redesign metal-based WOCs. Such insights provide unparalleled and practical information to produce the WOCs with various metals with high activity and durability that can be employed to cap global warming.