Tracking the electronic and structural configurations of water splitting catalysts for Artificial Photosynthesis

Replacing fossil fuels with renewable energy sources is one of the most promising research fields that can provide a solution towards solving the global energy crisis. Although today’s state of the art technology has achieved progress in producing electricity using solar, wind, tidal, and hydroelectric power sources, these intermittent sources will find limited applications without proper energy storage and transport. One way of storing solar energy is to convert it into chemical energy through fuel forming reactions inspired by natural photosynthesis, such as the light induced water splitting into hydrogen and oxygen. The prospect of using molecular hydrogen as a carbon-free fuel has motivated the development of catalysts for photo-induced water oxidation, proton reduction, and their integration in catalyst-photosensitizer systems. However, although active synthetic efforts have been invested in developing efficient water splitting complexes, there is no clear understanding between their stability and performance to their structures and ligand geometries.

 In this context, time-resolved X-ray absorption with X-ray emission spectroscopy are powerful tools for visualizing the “real-time” electronic and geometric changes involved in a photocatalytic system with picosecond-microsecond time resolution. This talk will demonstrate the reaction pathways of several cobalt, nickel and copper-based hydrogen evolving complexes, examined in unprecedented detail with picosecond time resolution. The mechanistic pathways followed by these catalysts with spectroscopic and kinetic characterization of the different intermediates towards the hydrogen evolution pathway and H-H bond formation will be explained. Experimental results combined with theoretical simulations will reveal new aspects about the catalytic intermediates, and step by step time frames used for the hydrogen evolution reaction in purely aqueous conditions. Results shown will enable the rational design of molecular hydrogen-evolving photocatalysts that can perform beyond the current microsecond time scale, and suggest ways in which the ligand structures can be adjusted to facilitate protonation and catalytic efficiency.