Operando XAS Studies: Electro-Reduction of CO2 on Nanoporous Cu Foams

Abstract

New and improved strategies to secure our global energy economy and chemical industry in the future are needed due to the strong dependence on limited fossil resources and the socio-environmental problems associated with increasing anthropogenic emissions of CO2. To counteract this problem and simultaneously to reduce the concentration of the CO2 as well-known greenhouse gas, a very promising route is the electrochemical CO2 reduction reaction (CO2RR) into hydrocarbons and alcohols. 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 are addressed: (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 mechanism and kinetics for the CO2RR on nano-porous copper (np-Cu) foams probed by XAS technique. The np-Cu foams prepared by dealloying of Cu-based alloys or by electrodeposition 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 pore size and curvature of the ligaments of the np-Cu. By using operando X-ray absorption spectroscopy (XAS), we studied the various structures and oxidation states of the Cu species in np-Cu foams as function of the applied potential, kind of electrolyte and pH value during the CO2RR. Linear combination fitting and multi-edge EXAFS fit analysis provide us information about the potential-controlled changes of oxidation state and local environment of Cu. In addition, we identified the potential range for the catalyst aging via coarsening and suggest strategies to stabilize the ligament structure and size 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. K. P. Kuhl , E. R. Cave, D. N. Abram, and T. F. Jaramillo, Energy Environ. Sci., 5 (2012) p. 7050.A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos, and P. Broekmann, ACS Catal., 6 (2016) p. 2804.H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B. R. Cuenya, Nature Communications, 7 (2016) p. 12123.R. Reske, M. Duca, M. Oezaslan, K. J. P. Schouten, M. T. M. Koper, and P. Strasser, J. Phys. Chem. Lett., 4 (2013) p. 2410.