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Anthonin Moinel

Developing novel push-pull organic dyes and dye-catalyst assemblies for hydrogen production in dye-sensitized NiO photocathodes

Published on 22 November 2022
Thesis presented November 22, 2022

Dye-sensitized photo-electrochemical cells are of great interest since they allow the direct conversion of sunlight into solar fuels such as hydrogen (H2). For the past decade, several examples of dye-sensitized photocathodes for the photo-electrocatalytic production of hydrogen using covalently linked dye-catalyst assemblies, also called molecular dyads, were reported. In these examples, the push-pull structures of the fully organic dyes are quite simple and easy to synthesize, but exhibit optoelectronic properties that can be improved. For this purpose, molecular engineering of the structures of the dyes is essential in order to optimize their optoelectronic properties and thus, to improve their performances. To do so, the dyes usually developed for dye-sensitized solar cells prove to be an appropriate source of inspiration. In this manuscript, we exploited the knowledge acquired in this field for the design and the synthesis of new push-pull organic dyes. A bathochromic and hyperchromic shift of their absorption spectrum compared to the one of the dyes from the first generation was observed, corresponding to a better photon absorption in the visible region, especially for the dye AM1. This improvement is due to the introduction of the electro-withdrawing group benzothiadiazole and the π-conjugated spacers indacenothiophene (AM dyes) and thiophene (pRK dyes) in their structure. NiO-based photocathodes were sensitized with these dyes and then tested, in p-type dye-sensitized solar cells first, then for photo-electrocatalytic hydrogen production. In the first study, the dyes pRK1 and AM1 stood out with power conversion efficiencies within the average of the state of the art. For the second study, in which the catalyst was in solution, pRK1 showed, again, good performances. These first results are promising knowing that additional optimizations on those devices are still possible. Finally, the syntheses of molecular dyads were performed using two approaches. The first procedure is a cupper-catalyzed azide-alkyne cycloaddition that was used by the group for the already-reported examples. Thus, the molecular dyad AM1-Co, active for hydrogen production, was synthesized. This dyad, thanks to its enhanced absorption in the visible range, showed improved performances compared to the previous systems using the same hydrogen evolution catalyst. During these studies, the lithium doping of the NiO-based photocathodes was proven to boost the photovoltaic and photo-electrochemical performances of these devices. The second dye-catalyst coupling approach used was a pallado-catalyzed coupling reaction. Hence, the molecular dyads AM3-Cat1 and pRK3-Cat1 were synthesized. These dyads have the particularity, unlike any example from the literature, to be fully conjugated. Unfortunately, these dyads were poorly or even non-active for hydrogen production. The specificity of the structures likely implies a different electronic behavior than the one classically observed and thus, raises new questions about mechanism followed by these systems for the production of hydrogen.

Push-pull dye, molecular engineering, dye-sensitized photocathode, hydrogen production, solar fuel

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