In Situ Characterization of Atomic-Scale Surface Restructuring of Copper Electrodes Under CO2 Electroreduction Conditions

Abstract

The electrochemical CO2 reduction reaction (CO2RR) by copper-based heterogeneous catalysts has attracted widespread attention due to its potential social, economic, and environmental benefits, as it utilizes electricity from renewable sources to transform CO2 into value-added chemicals, thus closing the carbon cycle[1]. However, because the energy efficiency, selectivity and lifetime of CO2RR on Cu are still far away from those required at industrially conditions, a knowledge-based catalyst design to optimization of the catalytic system is urgently needed. The reactivity and selectivity of CO2RR depends on the catalyst material as well as on surface and interfacial structure/composition and the nanoscale morphology of the catalyst[2]. With the development of in situ and operando characterization techniques, the fundamental understanding of the structure-reactivity/selectivity relationships of catalysts under reaction conditions can be obtained. Here we monitor the CO-induced surface reconstruction of Cu(100) electrodes during CO2RR in 0.1M KHCO3 solution by in situ scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and Raman spectroscopy. In situ STM images during step-wise potential changes show reversible cluster formation. The nanosized clusters start to appear at a potential ≤ -0.2 V vs. RHE and slowly disappear once the potential is changed back to more positive potentials, the clusters slowly disappear. However, the formation of Cu nanocluster leads to an irreversible modification of the surface structure on the molecular level. According to our previous research [3],freshly prepared Cu(100) electrodes are covered by a well-ordered (bi)carbonate adlayer in the double layer regime. After a potential excursion to the range of cluster formation, a disordered adlayer phase with only short-range order is observed in STM images, which can be explained by the presence of Cu adatoms and surface vacancies that remain on the electrode after dissolution of the nanoclustersIn the in situ SXRD studied, crystal truncation rods (CTRs) were recorded at different potentials from the double layer (0 V) to the CO2RR (-1.1 V) regime and then back to 0 V. Quantitative analysis based on a surface adatom/vacancy pairs model exhibits a clear increase in the surface defects density at potentials ≤ -0.2 V and a change in the defects coverage of ≈ 4%. After increasing the potential back to 0 V, the coverage of these defects decreases only slightly, indicating that some Cu adatoms remain on the surface. These results are in good agreement with our STM studies. The vertical expansion of the topmost atomic Cu layer exhibits a distinctly different potential dependence. This surface relaxation is not related to cluster formation, but can be attributed to changes in the type of chemisorbed species on the surface.To identify the adsorbate species on copper surface, in situ surface-enhanced Raman was employed. In the double layer range, bidentate carbonates and (bi)carbonate anions are dominant surface species. Below -0.04 V, the characteristic bands of carbonate become weak or disappear. In the meantime, new bands assigned to adsorbed carboxylate or carboxyl groups emerge, which can be attributed to the first reduction intermediates of CO2[4]. These bands disappear at potential below -0.34 V. Adsorbed CO appears on the surface starting from -0.24 V in the same potential range as the Cu nanocluster. In the reverse potential scan back to more positive values, only weak carbonate bands reappear and formate bands remain constant up to 0.26 V. These observed behavior suggests that some intermediates are formed irreversibly, in accordance with the irreversible changes in the molecular structure of the adlayer.[1] S. Nitopi et. al., Chem. Rev. 2019, 119(12), 7610–7672[2] Y. Y. Birdja at. al, Nature Energy 2019, 4, 732–745[3] R. Amirbeigiarab et. al., Angew. Chemie Int. Ed. 2022, 61 (46), No. e202211360[4] W. Shan et. al., ACS Nano 2020, 14(9), 11363–11372