Biomimetic molecular water splitting catalysts for hydrogen generation

During the last few decades, the scientific community has been striving hard to develop new and alternative sources for renewable energy and fuel. Hydrogen or carbon free fuels obtained from catalytic water splitting using sunlight offer an attractive solution for a cleaner and greener future. In this pursuit, to establish effective molecular catalytic systems for efficient water oxidation is considered to be a bottleneck, hampering the design, implementation and exploitation of electrochemical and photo-electrochemical modular devices for light driven energy conversion into hydrogen-based storable fuels. From metal oxides to composite materials, noble metal complexes to transition metal organometallics, multinuclear to mono-site catalysts, various water oxidation complexes (WOCs) have been investigated both in a homogeneous environment and on surfaces in photo- or electrochemical conditions. However, a truly biomimetic catalytic system that matches the performance of photosystem-II for efficient water splitting, operating with four consecutive proton coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be achieved. We here present an overview of biomimetic molecular complexes that have been investigated recently for water oxidation catalysis in homogeneous solution using chemical oxidants, or as heterogeneous species for catalytic electrochemical systems.

► Molecular catalysts offer a route to use water as raw material for solar fuel.
► Photosystem-II provides design principles for artificial water splitting systems.
► An overview of biomimetic molecular complexes for oxygen evolution is presented.
► High energy states in the cycle can be avoided by proton coupled electron transfer.
► A perspective on how available systems can be further improved is described.