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The electrochemical CO2 reduction reaction (CO2RR) by copper-based heterogeneous catalysts has attracted widespread, as it utilizes electricity from renewable sources to transform CO2 into value-added chemicals. However, because the efficiency and selectivity of CO2RR on Cu are still far away from those required at industrial conditions, a knowledge-based catalyst design to optimization of the catalytic system is urgently needed. 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. However, the formation of Cu nanocluster leads to an irreversible modification of the surface structure on the molecular level, which can be explained by the presence of Cu adatoms and surface vacancies that remain on the electrode after dissolution of the nanoclusters.
In the in situ SXRD study, crystal truncation rods (CTRs) were recorded at different potentials. 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. And 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. 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.