Chemical and Morphological Changes of Chloride-Affected Cu Catalysts during CO ₂ Reduction Studied by In Situ Scanning Transmission Soft X-ray Microscopy

Copper, the most efficient catalyst for multicarbon (C2+) product generation in electrochemical CO2 reduction (CO2R), is highly susceptible to restructuring in the presence of halides. Chloride ions enhance the performance of Cu catalysts, although a detailed understanding of the morphological and chemical evolution induced by chloride remains lacking. In this work, in situ scanning transmission soft X-ray microscopy (STXM) was used to identify the morphological and chemical changes occurring in chloride-affected Cu catalysts during catalyst synthesis by electrodeposition through to electrochemical CO2 reduction conditions. The initially electrodeposited tetrahedral particles were mainly cuprous chloride (CuCl). When a CO2 saturated KHCO3 electrolyte was introduced, the particles were converted to particles with a metallic Cu core and a shell consisting of a mixture of CuCl and cuprous oxide (Cu2O) under the application of a small negative current. As increasingly negative potentials were applied under CO2R conditions, both Cu2O and CuCl progressively reduced to metallic Cu. Simultaneously, Cu-based particles on the electrode surface partially dissolved and redeposited at the edge of the electrode, along with agglomeration and reconstruction. Our results indicate that chloride does not enhance CO2R performance by stabilizing Cu+, CuCl, or oxidized species during reaction. Instead, chloride drives rapid dissolution, migration, redeposition, and agglomeration, leading to a reconstructed catalyst with highly active nanostructures characterized by increased surface roughness, abundant grain boundaries, and reduced crystallite size. This work provides in situ visualization of these changes and detailed insight into the role of chloride, including the distribution and impact of chloride on the morphological and chemical changes occurring in Cu catalysts during CO2R.