The research achievements of the project REDOX SHIELDS are the results from a combination of theoretical and experimental investigations.
Extensive modeling activities in cooperation with the group of Christophe Léger at the CNRS Marseille led us to a complete understanding of the protection of catalysts under oxidative conditions (H2 oxidation) even in thin films (<10 µm). We demonstrated that the lifetime of the catalysts scales exponentially with the film thickness. This is important because it makes it possible to theoretically increase the lifetime of protection from just 10 min in a 3 µm thick film, to 1 year in a 6 µm film, and even to 22000 years in a 8 µm film! This remarkable protection enables to finely tune the film properties for a best compromise in terms of current output, stability and catalyst utilization which are the primary requirements for technological applicability of such catalysts in energy conversion. This breakthrough now enables to reconsider catalysts that were previously discarded for the integration into devices.
The protection from O2 was also further extended to protection from other deactivating molecules such as H2O2 which is often a side product from O2 reduction. We have combined the ability of a redox polymer to reduce O2 to H2O2 with the catalytic activity of an electrolyte containing iodide for the dismutation of H2O2 to H2O. We demonstrated experimentally that this approach is extremely valuable for protecting highly fragile catalysts such as the hydrogenase for as long as a week under constant exposure to O2 which would normally destroy the catalyst within seconds. This exceptional performance consolidates the possibility for using highly O2 sensitive catalyst in technological device for energy conversion.
In the final part of the project, we designed the redox active films for protection of highly active but fragile catalysts for reductive processes such as H2 production. Our unique approach to enable protection is based on bidirectional catalysis so that the produced H2 can be reused by the hydrogenase for protecting itself from the inactivating O2 molecules.
The first step was to enable bidirectional catalysis of hydrogenase when embedded in redox-active films which has never been achieved previously. This was made possible by redesigning the electron mediators within the polymer so that their reduction potential matches the one of the H+/H2 couple. The resulting catalytic current were very high for both H2 oxidation and H2 reduction (in the mA/cm2 range).
We then demonstrated the successful protection of the hydrogenase even in the absence of an applied potential which is highly relevant for conversion of electricity from intermittent sources such as wind or solar. We elucidated the precise mechanism for protection and showed how the bias for bidirectionality needs to be adjusted to maintain a sufficient fraction of the electrons within the film to provide a reductive force to eliminate O2.
Based on this breakthrough we constructed an electrolyzer for H2 evolution based on an FeFe-hydrogenase, which is among the most active but also the most fragile catalyst for generating H2. We succeeded in protecting such a fragile catalyst in the harsh conditions of an electrolyzer operating at high current densities (10 mA/cm2) and high energy efficiencies (>80%). This validates the general applicability of the protection concept to any other artificial or natural catalysts and thus open up their use in devices for clean energy conversion.
