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Hydrogel-Schutzschilde für die Immobilisierung und Stabilisierung von Katalysatoren für die H2-Oxidation und CO2-Reduktion

Fachliche Zuordnung Analytische Chemie
Förderung Förderung von 2015 bis 2019
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 283974326
 
Erstellungsjahr 2019

Zusammenfassung der Projektergebnisse

The ANR-DFG Project SHIELDS lifts major barriers on the path toward implementing cheap catalysts based on earth-abundant elements for sustainable energy conversion. New protection matrices were designed to protect highly active but sensitive catalysts even in the harsh operating conditions of fuel cells without compromising on catalyst utilization. Redox active films based on viologen modified polymers were previously proposed as protection matrices. The electrons generated from the H2 oxidation at the catalyst can be transported through the film to react with O2 which is thus removed at the outer layer before it can reach and deactivate the catalyst in the inner part of the film. Despite its effectiveness for dramatically enhancing catalyst longevity, this protection concept is not useful in practice because the use of thick films (more than 100 µm) limits the current output due to inefficient H2 diffusion and only 0.3% of the catalyst contributes to fuel cell performances. The ANR-DFG project SHIELDS demonstrates that highly O2-sensitive catalysts such as hydrogenases immobilized in films with thicknesses of just a few micrometers can be protected effectively from oxidative damage. Surprisingly, this protection is even more robust than in thick films, all the while achieving effective H2 conversion current at a fraction of the catalyst loading: As much as 50% of the catalyst is now contributing to the catalytic current and the catalytic current is completely immune to O2. Several advances made these achievements possible. The poorly defined building blocks used for matrix assembly were replaced by well-defined redox dendrimers. The crosslinking of the dendrimer units in presence of the hydrogenase resulted in the assembly of very homogeneous films with high control on the thickness even in the low micrometer range, which was not accessible previously. This was a prerequisite to make and then test films as thin as 3 µm. Moreover, these films are significantly more efficient for transporting electrons than the previously used polymeric systems. According to theoretical modeling, the life time of the catalyst is enhanced by higher electron conductivity because it defines how quickly electrons are transported through the film and thus how far from the catalyst O2 can be blocked. The most surprising finding from the project SHIELDS is that the thickness of the film has an extremely strong impact on longevity of the catalyst. In a film of 3 µm the catalysts survive just 10 min in the presence of O2. If the film thickness is doubled to 6 µm, the life time is increased to up to one year and a further increase to a value of 8 µm leads to a theoretical life time of 22000 years. An additional surprising finding, is that the film not only blocks the deactivating molecule (O2) but is also able to reactivate the inhibited catalyst by donating electrons produced from neighboring, still active, catalysts. These synergistic effects make it possible to protect infinitely the catalyst in films as thin as 3 µm and therefore make even fragile catalyst potentially compatible with their use in H2/O2 fuel cells. This new conception of the protection matrix will urge people in the field of energy conversion to rethink catalyst use and design: indeed, as far as achieving robustness is concerned, the traditional paradigm in catalyst optimization has been making the catalyst intrinsically more stable (less sensitive to O2). The results from the AND-DFG project SHIELDS will refocus the attention on the properties of the protective matrix and the enhancement of the catalyst reactivation kinetics, allowing the possibility to apply catalysts that would otherwise rapidly deactivate under operating conditions. Following the breakthroughs from the project SHIELDS, the road towards the realization of widespread application and implementation of bioinspired catalysts in real-world applications will be opened, including other energy converting schemes like solar fuel generation from CO2 reduction.

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