Thesis presented November 27, 2020

Abstract:
Production of solar fuels in water splitting dye-sensitized photoelectrochemical cells is a highly promising approach in order to fulfill future energy demands and to face the environmental problems arising from the combustion of fossil fuels. Our group previously constructed NiO-based dye-sensitized photocathodes for H₂ evolution, based on the first covalent organic dye-catalyst assemblies. Here, we report the synthesis and characterization of two novel dyads, both relying on a ruthenium tris-diimine photosensitizer covalently linked via copper-catalyzed azide-alkyne cycloaddition (CuAAC) to two different H₂-evolving cobalt catalysts. The H₂-evolving photoelectrochemical activities of the resulting photocathodes were assessed under standardized conditions. This allowed us to establish that i) the dyads based on a Ru photosensitizer outperform the previous ones relying on push-pull organic dyes; ii) replacing the cobalt diimine-dioxime catalyst previously employed by the group by a more robust cobalt tetraazamacrocyclic catalyst was crucial to improve the activity, with the highest TON so far reported for hydrogen-evolving dye-sensitized photocathodes. Transient absorption spectroelectrochemical measurements revealed that the performances are limited by the low efficiency of the electron transfer from the reduced dye to the catalytic unit. In addition, long-term stability is strongly affected by the desorption of the dyads from the surface of the electrode during the course of the photoelectrochemical tests. Finally, promising preliminary results were also obtained for photoelectrochemical CO₂ reduction to CO in aqueous medium, thus opening interesting perspectives in the field.

Keywords:
hydrogen, artificial photosynthesis, dye-sensitized photocathodes