Hybrid Systems for Water Oxidation Catalysis

The current energy system based on fossil fuels has led to unprecedented CO2 concentration in the atmosphere, causing irreversible effects on the ecosystem. Artificial photosynthesis aims to substitute fossil fuels by developing photoabsorbing materials that drive the oxidation of water and subsequent reduction of protons or CO2 into new energy vectors. One of the remaining challenges in this field is to find photoanodes that surpass the thermodynamic and kinetic requirements of the water oxidation reaction. The present thesis aims to develop molecular (photo)anodes through the study of new organic-inorganic hybrid systems.

In the first part of the thesis, a thiophene based polymer is investigated as a rugged scaffold suitable to anchor molecular water oxidation catalysts on graphitic materials. The resulting molecular anode ranks among the best, with current densities in the range of 100 mA·cm-2 at 1.45 V (vs. NHE pH 7). In the second part of this thesis, two systems for light-driven water oxidation are studied. Firstly, two organic dyes of the BODIPY family combined with the recently reported Ru-tPa catalyst precursor (tPa4- = [2,2′:6′,2”-terpyridine]-6,6”-diphosphonate) proved photoactivate towards oxygen evolution in a fully homogeneous system without a sacrificial agent, surpassing the low stability of other well-studied dyes. Later, a photoanode consisting of a macrocyclic Cu-complex and an organic dye of the perylendiimide family, all based on earth abundant materials, is studied. Charge transfer between the different components as well as significant photocurrents are proven by photoelectrochemistry, NMR, UV-Vis and XAS spectroscopy.

Finally, two hybrid colloidal photocatalysts for water oxidation are reported. On one hand, the first organic semiconductive polymer with a covalently attached molecular catalyst is presented, displaying turnover numbers of 54 with just 0.7% Ru loading. On the other hand, a modular and versatile hybrid photocatalyst, based on a molecular complex and nanocrystals of TiO2 on carbon nanotubes is developed. The system is capable of completing a total of 229 turnover numbers, which is the highest value reported for this type of device.

In summary, this doctoral thesis presents a range of new and disruptive molecular (photo)anodes an photocatalysts, rationally designed with the aim of implementing them on artificial photosynthetic (photo)electrochemical devices.

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