The Role of the Copper Oxidation State in the Electrocatalytic Reduction of CO2 into Valuable Hydrocarbons

Juan-Jesús Velasco-Vélez^1,2, Travis Jones², Dunfeng Gao^3,4, Emilia Carbonio^2,10, Rosa Arrigo^5,6, Cheng-Jhih Hsu⁷, Yu-Cheng Huang^7,8, Chung-Li Dong⁷, Jin-Ming Chen⁸, Jyh-Fu Lee⁸, Peter Strasser⁹, Beatriz Roldan Cuenya^3,4, Robert Schlögl^1,2, Axel Knop-Gericke^1,2, Cheng-Hao Chuang^7*

¹Department of Heterogeneous Reactions, Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany
²Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
³Department of Physics, Ruhr-University Bochum, Bochum, Germany
⁴Department of Interface Science, Fritz-Haber-Institute of the Max-Planck Society, Berlin, Germany
⁵Diamond Light Source Ltd., Harwell Science & Innovation Campus, Oxfordshire, UK
⁶School of Environment and Life Sciences, University of Salford, Manchester, UK
⁷Department of Physics, Tamkang University, New Taipei City, Taiwan
⁸National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁹Department of Chemistry, Technical University Berlin, Berlin, Germany
¹⁰Helmholtz-Center Berlin for Materials and Energy, BESSY II, Berlin, Germany

* Presenter:Cheng-Hao Chuang, email:chchuang@mail.tku.edu.tw

Redox-active copper catalysts with accurately prepared oxidation states (Cu⁰, Cu⁺, and Cu²⁺) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO₂, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ micro-reactors, it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e., during fabrication and electrocatalytic reactions). It was found that the surface, subsurface, and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially reduced copper catalysts yields the key dissociative proton-consuming reactive adsorption of CO₂ to produce CO, allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions, are mediated by thermodynamics, but critically, kinetics dominate near the oxide/metal phase transitions.

Keywords: CO2RR, In situ X-ray spectroscopy, Copper carbonate passivation layer, Electrocatalytically active reduced copper oxides