Comprehensive Operando Electrochemical XAS Study on Nanoporous Cu Oxide Foams for CO2 Reduction Reaction

Abstract

To secure our global energy economy and chemical industry in the future, new and improved strategies are needed due to the strong dependence on limited fossil resources and the socio-environmental problems associated with increasing anthropogenic emissions of CO2. In this context, electrochemical CO2 reduction reaction (CO2RR) into hydrocarbons and alcohols is a very promising route to counteract this problem and simultaneously to reduce the concentration of CO2 as a well-known greenhouse gas. Different reaction mechanisms and kinetics for the CO2RR are postulated.[1-4] In particular, the role of Cu oxide during the C2 formation like ethylene and ethanol is poorly understood to date. Further critical issues need to address: (i) high overpotentials, (ii) broad product distribution, (iii) fast degradation by catalyst poisoning and (iv) competition reaction at high cathodic potentials, referred to as hydrogen evolution reaction. Our work is focusing on the fundamental understanding about the role of nano-porous copper oxide foams probed by operando electrochemical X-ray absorption spectroscopy (XAS) technique. The nano-porous Cu oxide foams prepared by H2-assisted electrodeposition and thermal annealing in air exhibit large surface area-to-volume ratios, improved catalytic performance and high C2 selectivity compared to a flat Cu surface. The catalytic properties (activity and selectivity) can be controlled by the thermal annealing process in air, where different kind of Cu oxides (Cu(I) vs. Cu(II)) are generated. By using operando XAS, we studied the stability of the Cu oxides as function of the applied potential during the CO2RR. In addition, potential jumps experiments were performed to investigate the kinetics of the reduction process of Cu oxides prepared by different annealing temperatures. Linear combination fitting and EXAFS analysis provide us information about the potential-controlled changes of oxidation state and local environment of the nanoporous Cu oxide foams. More interestingly, we compare our results with operando Raman spectroscopy and operando X-ray diffraction to highlight the enormous importance for the appropriate design of operando measurements. In addition, we identified the potential range for the catalyst aging via coarsening and suggest strategies to stabilize the structure of the Cu foams during the CO2RR. Based on our operando XAS studies, we obtained a deeper insight to the mechanism and kinetics of the CO2RR on porous Cu foams and clarified the role of Cu oxides for the C2 production.