Thesis presented February 10, 2023

Abstract:
Hydrogenase enzymes were first discovered in the 1930s. The first report of {Fe₂µ-S₂)(CO)₆} based catalyst was reported in 1928 without any information of [FeFe] hydrogenases. The first crystal structure of [FeFe] hydrogenases was reported in 1990s, which triggered the development of bioinspired catalysis based on the {Fe₂(µ-S₂)(CO)₆} core due to its structural similarities with the [2Fe]H cofactor.19,91 Yet, it remains a highly active research field with continued efforts to find the optimum balance between activity, durability, sustainability and cost of the catalysts. This always motivates me to look for a better catalytic system, and also resulted in this thesis. In the eSCALED project, our objective was to develop a noble metal free bioinspired device for H₂ production or CO₂ reduction using solar electrolysis. I was responsible for developing cathode materials for H₂ production. Considering that objective and my personal motivation, my overall doctoral journey is summarised in the following manner.
In Paper I, we aimed straight towards an immobilisation strategy for a bioinspired {Fe₂(µ-S₂)(CO)₆} based active sites, as this constitutes a key step for molecular based electrode materials for device applications. The anchoring method we settled for was π-π interactions, utilizing pyrene. We showed an improvement in loading capacity on the electrode as compared to previous reports, and an appreciable electronic communication between electrodes and catalysts. Finally, and importantly, the anchoring group appeared highly stable during catalysis. Still, the activity remained substandard under aqueous condition due to poor interaction with substrate (H⁺ from buffer) and degradation of the catalyst/active site under catalytic conditions.
In Paper II, we aimed at encapsulating the [FeFe] site inside a water-soluble polymeric scaffold, to provide a basic outer coordination sphere along with an anchoring group in a single platform. The strategy met our expectations with regards to improved activity, but decreased the loading of active sites 4-5-fold, compared to the diiron complex deprived of polymer scaffold described in Paper I. Unfortunately, the polymeric scaffold could not protect the active site from degradation. In addition, we performed a life cycle assessment study to further highlight the importance of the metallopolymer strategy from a sustainability perspective.
In Paper III, we redesigned our metallopolymer by replacing the active site with what was expected to be a more robust diiron active site. In parallel, the new design could allow to tune and increase the fraction of active sites in the polymer chain. However, we found that the active site still remains the weak link in this assembly. Hence further improvement is still required.
On a more positive note, it is noteworthy to mention that we demonstrated through Papers II and III, that the metallopolymer approach is a highly promising for their employment in an energy conversion context. Clearly this could be extended to various energy conversion applications in future.
Finally, in Paper IV, we explored a biohybrid system by preparing and characterising semisynthetic hydrogenases. The detailed analysis provides some valuable insight towards the design of bioinspired catalysts for H⁺/H₂ conversion. For example, replacing one CN ligand with a CO ligand in [2Fe]H resulted in a semisynthetic hydrogenase with very different properties i.e. activity, sensitivity towards inhibitors etc., depending on the host protein scaffold.
In closing, I think the insights that we learned throughout this thesis work motivates taking inspiration from Nature in the continued search for an ideal catalyst. The metallopolymer approach is promising. Here, the design of active and robust catalytic sites is crucial. Finally, evaluating environmental footprint of the product is essential from a sustainability perspective.

Keywords:
bioinspired, biohybrid, electrode, hydrogen

On-line thesis. ​