The conversion of CO₂ into useful compounds via electrocatalytic conversion* is one of the strategies being considered to limit CO₂ emissions and develop a circular carbon economy. For this purpose, modified electrodes based on molecular catalysts are excellent candidates because they allow for control not only of the catalyst's structure but also of its surface environment, thereby enabling the development of selective and efficient cathodes. These electrodes are indeed capable of producing carbon monoxide (CO, an essential synthon* in the chemical industry for Fischer-Tropsch* or hydroformylation* processes) very efficiently thanks to the integration of cobalt macrocycles* onto carbon surfaces. 

In previous work, the modification of the CoN₄H catalyst (a cobalt-based tetraazamacrocyclic complex) with a pyrene anchoring group* enabled researchers at CEA-Irig/LCBM/SolHyCat to develop an active and selective molecular cathode for CO production in an aqueous medium. In this new study, the researchers leveraged the non-covalent nature of the interaction between the catalyst and the surface to control and decreased the surface concentration of catalytic sites in order to measure their intrinsic activity. Under these conditions, catalytic rates on the order of 5 s^–1 were observed, and post operando measurements allowed for the characterization of ligand degradation processes responsible for the electrode's loss of efficiency over time. The authors were thus able to postulate the existence of at least two populations of catalytic sites at the electrode: a first population whose redox response can be observed and whose degradation over time can be measured; and a second population, whose redox responses remain “silent" while catalytic activity continues to be measurable, at very low surface concentrations. The hypothesis that site orientations vary depending on surface concentration could explain these differences in electrochemical responses and have implications for the modulation of site activity as well as for the degradation processes of these sites.​ 

 

This work has made it possible to characterize the intrinsic activity of catalytic sites grafted onto the surface, while improving our understanding of the deactivation mechanisms of the electrode during operation. In particular, this study highlights the importance of detailed characterization of molecular assemblies at the surface in order to better understand the mechanisms at play.

The ability to control the orientation of active sites and their surface environment opens up new avenues for research, particularly to modulate the selectivity and stability of electrode sites, with the aim of integrating these cathodes into functional electrolysers designed for CO₂ utilization.

​Tutelles UMR : Université Grenoble Alpes (UGA), CNRS, CEA.

Fundings : ANR (Labex ARCANE, CBH-EUR-GS, ANR-17-EURE-0003, ANR-21-CE50-0004 and ANR-22-PESP-0010: Project “POWERCO2" within the PEPR project SPLEEN)​. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal number CH-6609.