Lay summary

The storage of light energy in chemical bonds is by far the most promising and sustainable strategy to solve our worldwide energy and climate problems. Not only is solar energy inexhaustible, its general use would also drastically reduce global CO2 emission. Water is an equally ubiquitous resource so that artificial photosynthesis based on water splitting is the ideal carbon-neutral way out of the fossile fuel dilemma. Water reduction catalysts (WRCs) promote hydrogen production, whereas water oxidation catalysts (WOCs) facilitate the challenging oxidative half reaction of oxygen evolution. Extensive synthetic, mechanistic and optimization work is now required to develop WRC/WOC catalyst systems that produce solar fuels at a competitive price. Cheap, replaceable and non-toxic WRC/WOC couples would offer distinct operational advantages over photoelectrolytic water splitting, such as single step and low cost processes without preliminary generation of electricity.

Whereas an increasing number of studies report on the integration of WRCs and WOCs into heterogeneous water splitting systems, visible light driven homogeneous overall water splitting with WRC/WOC pairs has not been described to date. Bio-inspired homogeneous catalysts are exceptionally promising due to their manifold structural tuning options that can render artificial photosynthesis setups close to the robustness, self-repair options and high performance of the natural photosystems. The Sinergia project thus explores forefronts of overall water splitting research: we integrate preparative, physico-chemical and theoretical expertise into the quest for the first homogeneous WRC/WOC catalytic system. Our goal is a unified concept for water splitting through robust, stable and economically viable homogeneous catalysts. The synthetic part (projects 1 and 2) is thus focused on the development of easily accessible homogeneous WRCs and WOCs based on abundant metals (e.g. Co, Mn, Fe and combinations thereof). Special emphasis will be placed on water-stable WRC complexes and bio-inspired polyoxometalate-based WOCs with Mn cores. Project 3 is concentrated on the investigation of the catalytic WRC/WOC half reactions with advanced in situ time-resolved IR spectroscopy. This will provide highly sought-after insight into the operational mechanisms of these catalysts that is required for their informed design. In parallel, theoretical models of WRCs/WOCs in solution will be developed in project 4 to optimize the synthetic strategy with a grid approach and to correlate the spectroscopic results with calculated water reduction/oxidation pathways.

The most challenging task is the quest for the “missing chemical links” between WRCs and WOCs, namely common photosensitizers and electron shuttles. Cutting-edge in situ studies of the underlying reaction pathways will provide the requested mechanistic pathways to bring the homogeneous water splitting concept to life through the systematic development of recyclable and robust electron transfer agents. Such pioneering investigations would add new directions to Swiss research programs on renewable energy sources, such as the Swiss Hydrogen Association HYDROPOLE, and we aim to place Swiss research on sustainable hydrogen technology at the forefront of current European research activities.