Hydrogel shields to support and protect catalysts of H2 oxidation and CO2 reduction – shields

Previously, the two groups had shown that a hydrogenase (a biological catalysts of H 2 oxidation and production) becomes self-protected in a film of redox polymer deposited on an electrode. H 2 diffuses in the film from the edge of the film, opposite from the electrode. A fraction of it is oxidized by the enzyme near the electrode and transformed into a current, and another fraction is oxidized at the other end of the film, near the film/solution interface, to produce electrons that reduce O 2 and prevent it from
penetrating further. The idea that the two reaction layers had to be far apart from one another lead to
use very thick films, which had many inconveniences: thick films prevent H 2 from diffusing in,
resulting in small current densities and the vast majority of the enzyme loaded in the film would not
contribute to current.

These problems are now solved thanks to a combination of very important conceptual advances. The very surprising observation that thin films (in the µm range) actually protect the enzyme better while allowing the use of a much larger fraction of the catalyst load was perfectly explained by the
simulations of the reaction and diffusion processes within the film. This theoretical understanding made it possible to determine the smallest thickness below which the film fails to protect the enzyme,
but also to show that one can go below this theoretical limit by modifying the catalyst to increase the
rate at which the catalysts reactivates upon reduction. The researcher could dig into a library of
engineered enzymes to find the hydrogenase variant that illustrates this synergy between film
protection and fast enzyme reactivation, to obtain hydrogenase films thinner than 3 µm that can resist
O 2 for ever. The model explains how the rate of electron transfer within the film determines the
current, the life time of the film and its optimal thickness.

This work shows how subtle phenomena that result from the coupling between reaction and diffusion within the film can modify - actually improve - the catalysts' properties. This could be used with a variety of molecular catalysts, even for reductive reactions such as H 2 production and CO 2 reduction
under aerobic conditions.

These results have been published in 8 high IF articles, and two other papers are being prepared. They have been highlighted by the editor of Nature Chemistry Reviews in 2019.

The use of metalloenzymes or synthetic inorganic complexes as catalysts in fuels cells or photoelectrochemical cells may open key routes in energy production and in industrial synthesis. However, the intrinsic fragility and oxygen sensitivity of these catalysts has been an obstacle. The German partner in this project has recently demonstrated that hydrogenases, the very efficient but very fragile biological catalysts of H2 oxidation, could be protected from O2 damage upon integration into a specifically designed redox hydrogel, which reduces oxygen at the polymer surface and thus provides self-activated protection from oxygen [Plumeré et al, Nature Chemistry, 2014]. Following the publication of this result, the French and German partners have initiated a collaboration, which already proved fruitful [Fourmond et al, J. Am. Chem Soc., 2015], to rationalize the protection mechanism and optimize the design of the catalyst-polymer films. These recently published results have set the stage for the full investigation that is the goal of this international and interdisciplinary ANR/DFG project.

We plan to explore this new concept by examining a variety of configurations (oxidative or reductive catalysis in thick or thin films), using enzymes such as hydrogenases and CO dehydrogenases as models of fragile catalysts. The enzymes, which will be prepared by the French partner, have been selected because they exhibit various properties (reversible or irreversible catalysis, reversible or irreversible inhibition by O2, rates of inactivation and reactivation that can be tuned by protein engineering). Each of these enzymes catalyses, at rates in excess of thousands per seconds, a reaction that is important in the context of energy and environment (oxidation and production of H2, reduction of CO2). A tight collaboration between the two partners is absolutely crucial in this project because understanding how the hydrogel protects the catalysts requires that the kinetic and geometrical properties of the film be determined, and used in realistic mathematical models that take into account the various chemical reactions and diffusion processes occurring in the depth of the film; the models should then be validated by experimental measurements of how the presence of O2 affects the catalytic current, before the knowledge that has been acquired is used to guide the new design of the film (hydrophobicity of the polymer backbone, redox potential of the redox moieties, thickness, load, etc.). The two partners have demonstrated that they have the right expertise to accomplish their parts in the project, and that they can unite their force and knowledge (biochemistry and mathematical modelling in France, polymer design and physical electrochemistry in Germany) by working together.

Our ultimate goal is to fully understand the function of these complex systems where the catalysts is embedded into a protective redox-active hydrogel, by building upon the proof of concept of the protection [Plumeré et al, Nature Chemistry, 2014] and its initial modelling [Fourmond et al, J. Am. Chem Soc., 2015]. We will understand the factors governing current density and protection, and become able to rationally design robust catalytic electrodes for actual applications using metalloenzymes or any other efficient but fragile catalyst. Of course this knowledge will be usable by all researchers in the field.