During the last decades, significant efforts have been made to directly convert CO₂ as a potential feedstock into hydrocarbons as fuels and/or basic chemicals for industrial applications. The electrochemical CO₂ reduction reaction (CO₂RR) is a promising alternative for a large-scale production of hydrocarbons. However, there are still some challenges including poor product selectivity and highly complex multiple-step reaction mechanisms.[1] In order to convert CO₂ on a Cu electrode, high overpotentials up to 1.0 V are required, which make the energy efficiency still rather poor and lead to competition with the H₂ evolution.[2] Additionally, the morphology of the Cu materials i.e. structure and ‘chemical state’ (metallic vs oxidized, high vs low coordinated) strongly influence the performance of the CO₂RR. Recently, we have shown the critical potential of oxide-metal transition processes for Cu oxide foam annealed at 300°C probed by operando XAS, XRD and Raman spectroscopy.[3,4] All three operando techniques showed an entire reduction of Cu oxide to metallic before the production of hydrocarbons starts. [3,4]

In this work, we have investigated the kinetics of both electrochemical oxide-metal reduction and CO₂RR on nanoporous Cu foams as catalyst precursor annealed at four different temperatures (100°C, 200°C, 300°C, 450°C) in air using operando Quick X-ray Adsorption Spectroscopy (Quick-XAS). The Quick-XAS measurements were carried out in a custom-made spectro-electrochemical flow cell using 0.5 M KHCO₃ as the electrolyte, while the XAS-Spectra were measured in transmission mode. The Quick-XANES data was analyzed by linear combination fit (LCF) and principle component analysis (PCA) to monitor the potential dependent changes of the chemical state and coordination number of the Cu species. Based on the Cu K-edge XANES and EXAFS data, we show that the annealing temperature strongly influences the chemical state of the Cu species. More precisely, the population of the Cu(II) species within the as prepared foams increases with increasing annealing temperature. Starting from the different ratios of Cu(0):Cu(I): Cu(II), the oxide-metal transition processes are shifted in the cathodic direction by applying potential steps of 100 mV. With an increase in annealing temperature, this oxide-metal transition is more rapid and occurs at lower cathodic overpotentials, but still before the production of hydrocarbons begins. In contrast, the potential jump experiments of several hundreds of mV lead to different kinetics of the oxide-metal reduction of Cu species. These transition processes and the resulting structure of porous Cu foams have a huge impact on the product distribution for CO₂RR. Altogether, our results provide deeper insights into the oxide-metal transition processes to form the catalytically active Cu species for hydrocarbon formation during CO₂RR.

Reference:

[1] S. Nitopi et al., Chemical Reviews, 2019, 119, 7610. [2] C. W. Li et al., Journal of the American Chemical Society, 2012, 134, 7231. [3] A. Dutta et al., Chimia, 2021, 75, 733. [4] A. Dutta et al., Journal of Catalysis, 2020, 389, 592.