Implementation of nature´s power into DAFC

The objective of this project is to generate electrical energy from simple alcohols. Thereby the concept of fuel cells is used, in which the chemical energy from hydrogen is converted into electrical energy. Given the small size of a hydrogen molecule, storage is often problematic, as it diffuses even through compact storage containers and is lost in the process. In addition, expensive precious metals such as platinum or palladium are usually required as catalysts to perform this kind of transformation. Instead of inserting hydrogen directly into a fuel cell, alcohols can alternatively be used as fuel. Alcohols are excellent chemical and sustainable hydrogen sources, especially methanol which is the most economical one that has no impact on the food chain. Hydrogenase enzymes are nature`s catalysts and the most efficient biological systems for the mutual conversion of alcohols and hydrogen. Enzymes are macromolecules where the transformation of chemical substances takes place at active sites, which are defined areas on the surface on those macromolecules. Therefore, this project is working on synthesizing organometallic compounds that have the properties of the active sites of such hydrogenase enzymes. These organometallic complexes consist of two metal atoms stabilized by an organic backbone. Thereby, it will be investigated how different modified organic backbones influence the properties of the metal atoms. These well-defined artificial hydrogenase compounds are embedded onto suitable support materials and their activity in converting alcohols to hydrogen and subsequently to water and electric current is investigated. This is an innovative way to utilize the chemical properties of hydrogenase enzymes in synthesized organometallic complexes and use them as electrocatalysts for direct alcohol fuel cells. Here, every single metal atom would be involved in the conversion of alcohols into electrical energy, thus significantly reducing the amount of (noble) metals required. Furthermore, the rational design of such well-defined molecular catalysts (=artificial hydrogenase compounds) brings better selectivity and reactivity control. Likewise, fundamental reaction steps of such catalytic processes are investigated to develop a better understanding of them. In summary, this project aims to develop a fundamentally new approach to sustainable energy generation using so-called metal-organic fuel cells. These play a fundamental role on the progress towards a climate-neutral energy economy, particularly in the field of mobility.

Enzymes are natural catalysts and the most efficient biological systems in which the transformation of chemical substances takes place at so-called active sites (small defined areas). Artificial active sites in the form of organometallic compounds, which have similar properties, are of great interest. These organometallic complexes often consist of two metal atoms stabilized by an organic backbone. Utilizing the chemical properties of hydrogenase enzymes in synthesized organometallic complexes and using them as electrocatalysts for direct alcohol fuel cells is of crucial importance. Here, every single metal atom would be involved in the transformation of alcohols to electrical energy, which would significantly reduce the amount of (noble) metals. In this project, I synthesized several binuclear as well as some mononuclear Ru and Cu complexes. These could be fully characterized and investigated for their reactivity. The dinuclear Ru-olefin complex is non-symmetrically bridged by a 1,4-bis(5H-dibenzo[a,d]cyclohepten-5- yl)-1,4-diazabuta-1,3-diene (trop2dad) ligand. Its structure is best described as a folded ruthenadiazacyclopentadienide that undergoes 5 coordination at the second ruthenium center. This central five-membered ring is formed from a ruthenium and a trop2dad2-enediamido ligand. This dinuclear ruthenium complex is an active catalyst for the dehydrogenation of ammonia-borane to selectively elongated polycondensed borazine rings, predominantly BN hexabenzocoronene (Chem. Commun., 2024, 60, 885-888). We also discovered that a previously known binuclear Rh/Rh complex can be used as a homogeneous catalyst for the selective conversion of ammonia-borane to borazine (Dalton Trans., 2024, 53, 14212-14218). Remarkable bi-nuclear copper olefin complexes were synthesized in this project. These exhibit an unusual bridging coordination of the phosphane ligand between the two copper centers. The bond distance between the two copper atoms is remarkably small at 2.43 Å. The described complexes showed catalytic activity for the azide-alkyne cycloaddition.