Halide Anion-Induced Reconstruction of Copper Catalysts During Acidic CO₂ Electroreduction

The electrochemical CO₂ reduction reaction (CO₂RR), which converts CO₂ - a major contributor to climate change - into economically valuable energy sources or chemical feedstocks through electrolysis, is advancing rapidly. The products from CO₂RR heavily depend on the catalyst material, and copper is a notably promising material for converting CO₂ into multi-carbon products with high energy density such as ethylene, ethanol, and propanol. Consequently, to effectively utilize copper catalysts in CO₂RR, significant attention has been dedicated to researching their properties and developing appropriate synthesis methods. Simultaneously, achieving high carbon conversion efficiency is essential for the practical application of this technology. Recent studies have demonstrated that this high efficiency can be achieved by using acidic electrolytes, which regulate the local reaction environment to induce carbon confinement, rather than relatively mild alkaline or neutral electrolytes. However, these corrosive media degrade the stability of copper catalysts by accelerating the dissolution kinetics, leading to more extensive reconstruction than that caused by other electrolytes. Thus, to effectively utilize acidic electrolytes with high carbon conversion efficiency, a precise understanding of the copper catalyst reconstruction mechanism during acidic CO₂RR is necessary.
In acidic electrolytes, the abundance of hydrogen ions promotes a kinetically favorable hydrogen evolution reaction (HER). This competing HER in CO₂RR can be suppressed by adjusting the electric field of the double layer using alkali cations (e.g., Li⁺, Na⁺, K⁺). Highly soluble alkali metal halide compounds such as KCl, KBr, and KI are widely used to introduce a large amount of alkali cations into the electrolyte. However, the influence of the coexisting halide anion on the reconstruction mechanism remains unclear. In this study, we investigated the reconstruction of copper catalysts during acidic CO₂RR, focusing on the accelerated dissolution of the copper catalyst by the acidic environment and the interactions between dissolved copper cations and halide anions. It appears that the halide anion interacts with Cu²⁺ to form a [Cu-halide]⁺ complex. Typically, the halide anion is repelled from the catalyst surface under reductive potential, making redeposition difficult. However, by forming positively charged complexes with dissolved copper cations, it is presumed that halide anions can be redeposited onto the catalyst surface despite the reductive potential. In other words, this complexation-driven reconstruction transforms copper into a halide-doped copper catalyst. Furthermore, the type of halide anion (e.g., Cl^-, Br^-, I^-) influences the structural reconstruction behavior of the copper, which affects CO₂RR performance. In this work, we explored microstructural and phase transformations using TEM, XPS, in-situ XAS, and other methods to elucidate the reconstruction through dissolution and redeposition mechanisms occurring during acidic CO₂RR in the presence of halide anions. We also evaluated the influence of the type of halide anion on CO₂RR selectivity. It is expected that this research will contribute to enhancing understanding of the acidic CO₂RR system and improving its performance.