Thursday, November 10, 2022
Huali WU defended his thesis
“Molecular doping of copper-based catalysts for the electrocatalytic conversion of CO2 to multi-carbon products”
front of the jury composed of:
– Raffaella BUONSANTI, Professor, Swiss Federal Institute of Technology – Lausanne – Rapporteur
– Cicero GIANCARLO, Associate Professor, Politecnico di Torino – Rapporteur
– Marc ROBERT, Professor, Paris Cité University, CNRS – Examiner
– Sara CAVALIERE, Professor, University of Montpellier – Examiner
– Philippe MIELE, Professor, University of Montpellier – Thesis supervisor
– Damien VOIRY, CNRS Research Fellow, University of Montpellier – Co-thesis supervisor
Abstract:
The rapid increase of CO2 concentration due to fossil energy consumption poses a great threat to the ecological environment of the planet and the sustainable development of human society. To reduce the concentration of CO2 while achieving carbon recycling, the electrochemical reduction of CO2 is considered a promising approach and has attracted worldwide attention in recent decades. So far, copper, which is one of the few transition metals, can effectively catalyze the electrolysis of CO2 into multi-carbon products such as ethylene, ethanol, acetate, propanol, which have higher market values and are more energy concentrated. Therefore, intensive efforts have been devoted to improving the selectivity of the reaction towards the production of C2+ molecules, including alloying, surface doping, ligand modification and interface engineering. It has been reported that partially oxidized copper (Cuδ+, 0<δ<1) sites on the surface of copper catalysts can facilitate the conversion of CO2 to multi-carbons by decreasing the energy barrier associated with the CO dimerization and the formation of *OCCOH intermediate via efficient charge transfer between the surface step sites and the reaction intermediates. Nevertheless, the instability of Cuδ+ species, especially the high cathodic potentials during the electro-synthesis of multi-carbons, made the study of the role of Cuδ+ tedious, and it may eventually lead to a rapid loss of the performance. Therefore, the control of the oxidation state of Cu and the presence of Cu+ species on the surface of the electrodes has recently been a central focus in CO2RR notably via controlled
oxidation, pulse polarization, or molecular doping. In this thesis, we sought to fine-tune the behavior of the active sites of copper-based catalysts surfaces through molecular engineering. We firstly modified the surface of the bimetallic silver-copper catalyst with aromatic heterocycles such as thiadiazole and triazole derivatives to increase the conversion of CO2 into hydrocarbon molecules. We identified that the electron withdrawing nature of functional groups orients the reaction pathway towards the production of C2+ species (ethanol and ethylene) and enhances the reaction rate on the surface of the catalyst. As a result, we achieve a high Faradaic efficiency (FE) for the C2+ formation of ≈ 80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm-2 for C2+ using functionalized Ag-Cu electrodes. We anticipate that our strategy can further be extended to improve the selectivity of the reaction towards the production of specific multi-carbons molecules. Therefore, based on this proof of concept experiments, we then explored a library of aryl diazonium salts with different electron-withdrawing ability to functionalize copper. By combining density functional theory (DFT) calculations with operando Raman and X-ray absorption spectroscopy (XAS), we identified the role of the surface oxidation state of Cuδ+ with 0<δ<1 on the selectivity and the formation rate of C2H4. As a result, we obtained a FE and a specific current density for C2H4 as large as 83±2% and 212 mA cm-2, respectively on partially oxidized Cu0.27+. This corresponds to an energy efficiency of 26.9% and an electrical power consumption (EPC) of 61.4 kWh N-1m-3. When coupled with an Ag-based membrane electrode assembly (MEA) cell to generate CO from CO2 in a cascade flow process, an energy efficiency of 40 % with a FEC2H4 of 86± 2% was achieved, corresponding to a record low EPC of 25.6 kWh N-1m-3. Overall this thesis provide a route towards practical developments for the CO2-to-C2H4 conversion reaction using valence engineering of the Cu sites.
