Quantitatively Unraveling the Redox Shuttle of Spontaneous Oxidation/Electroreduction of CuO_x on Silver Nanowires Using in Situ X-ray Absorption Spectroscopy

Abstract

Oxide-derived copper catalysts have been shown to enhance CO2 reduction reaction (CO2RR) activity with high selectivity toward hydrocarbon products. However, the chemical state of oxide-derived copper during the CO2RR has remained elusive and is lacking in situ observations. Herein, a two-step process was developed to synthesize Ag nanowires coated with various thicknesses of a CuOx layer for the CO2RR. By employing in situ X-ray absorption spectroscopy, a strong correlation between the chemical state under reaction conditions and the CO2RR product profile can be revealed to validate another competing reaction (i.e., the spontaneous oxidation of Cu(0) in aqueous electrolyte) that significantly governs the chemical state of active centers of Cu. In situ Raman spectroscopy reveals the existence of reoxidation behavior under cathodic potential, and the quantification analysis of reoxidized behavior is revealed to indicate that the reoxidation rate is independent of surface morphology and strongly proportional to the electrochemically surface area. The steady oxidation state of Cu in an in situ condition is the paramount key and dominates the products’ profile of the CO2RR rather than other factors (e.g., crystal facets, atomic arrangements, morphology, elements) that have been investigated in numerous reports.

Operando time-resolved X-ray absorption spectroscopy reveals the chemical nature enabling highly selective CO2 reduction

“…For the DFT + U calculations, we chose the cutoff energy at 500 eV with 8 × 8 × 8 and 4 × 4 × 1 k-point mesh for bulk and surfaces models, respectively. On-site Columbic correction (U = 5.2 eV) was used to prevent delocalization of Cu d-orbitals, that has been demonstrated for modeling the previous Cu 2 O system 63,25 . The bottom two atomic layers were kept frozen with the remaining layers fully relaxed for these Cu 2 O slabs.…”

“…To provide an appropriate solvent effect of CO dimerization reaction, we constructed an explicit water layers (64 water molecules) to represent the hydrogen bond network of the aqueous environment. Water molecules were described by SPC-3f(α) models 67 using LAMMPS molecular dynamics simulator 25 . The liquid-solid interfaces were brought to equilibrium with 2 ns pre-equilibrium simulations 68 at 350 K while the solid geometry (with adsorbate) remained frozen during the molecular dynamics simulations, being represented by atomic charges (from Bader charges of DFT).…”

“…On the other hand, a strong peak overlapping that of Cu 2 O at 8980 eV indicated the presence of Cu(I) species in the CuO x catalyst. Notably, a slight difference between the normalized absorbance at 8981 eV of CuO x and that of Cu 2 O might be attributed to a couple of reasons, such as a dilution effect from metallic Cu species and the catalysts' nanoscale 25 . Wavelet transform was employed to clarify the coordination environment near by the Cu atom and demonstrate the atomic dispersion of Cu in the CuO x sample.…”

“…For example, the employment of pulses voltage has been revealed to enhance the selectivity toward the C 2 products on polycrystalline Cu 21,23,24 . Recently, we have justified the existence of another competing reaction (i.e., the spontaneous oxidation of Cu(0) in aqueous electrolyte) that significantly governs the chemical state of active centers of Cu, establishing a strong correlation between the chemical state under reaction conditions and the CO 2 RR product profile 25 . A similar effect has been also revealed in the case of Fe single atom catalyst, in which the reduction activity toward CO is evidently dominated by the chemical state of metal centers 26 .…”

“…In the cases of XAS and EELS, both of them would require a time-scale in minutes at least for spectrum acquisition. Notably, the chemical state of metal centers could reach a steady state in a few minutes under the eCO 2 RR; as revealed by in situ Raman, the CuO x precursor would achieve a steady-state condition due to the co-occurrence of electrochemical reduction and of spontaneous oxidation caused by the trace oxidant in the electrolyte 25 . Consequently, a time-resolved identification of the characteristics in chemical state of metal centers should be highly desirable, especially for a time-scale of few seconds for revealing the real dominated factor in CO 2 reduction electrocatalysis.…”