Oxidation State and Surface Reconstruction of Cu under CO₂ Reduction Conditions from In Situ X-ray Characterization

Area of Science:

• Electrochemistry
• Materials Science
• Surface Science

Background:

• Copper-based catalysts are crucial for the electrochemical carbon dioxide reduction reaction (CO2RR), enabling the conversion of CO2 into valuable multi-carbon products using renewable energy.
• The precise chemical and structural state of copper catalyst surfaces during CO2RR is not well understood, hindering catalyst optimization.

Purpose of the Study:

• To investigate the structural evolution of polycrystalline copper electrode surfaces under *in situ* conditions during the CO2RR.
• To elucidate the dynamic changes in the near-surface region of copper catalysts relevant to CO2RR performance.

Main Methods:

• Utilized a combination of *in situ* grazing incidence X-ray absorption spectroscopy (GIXAS) and grazing incidence X-ray diffraction (GIXRD) to probe the catalyst surface.
• Applied electrochemical techniques to control and monitor the reaction conditions and applied potentials.

Main Results:

• The surface oxide layer on copper electrodes is completely reduced to metallic copper before the CO2RR onset potential.
• The copper catalyst maintains its metallic state throughout the potential range relevant to CO2RR.
• Preferential surface reconstruction towards (100) facets occurs in the presence of CO2, with increased reconstruction at more negative potentials.
• The observed surface reconstruction is persistent even after returning to more positive potentials.

Conclusions:

• The surface of copper electrocatalysts is highly dynamic during the CO2RR, undergoing significant structural changes.
• Understanding these *in situ* surface dynamics is critical for designing and optimizing copper-based catalysts for efficient CO2RR.
• The study emphasizes the necessity of *in situ* characterization techniques for accurate insights into electrocatalytic processes.