Interplay between halides in the electrolyte and the chemical states of Cu in Cu-based electrodes determines the selectivity of the C2 product

Abstract

Although still poorly understood, electrolytes can play a key role in obtaining the desired product from the electrochemical reduction of CO₂. To address this issue, it is of great significance to better understand the relationship between the selectivity and the activity of the reaction and the electrode structure and the electrolyte. Herein, the influence of electrolytes, especially those that contain halide ions, on the CO₂ reduction performance was studied using Cu-based electrodes with various surface components, including Cu⁰, Cu⁺ and a mixed state of Cu⁰, Cu⁺ and Cu²⁺. The results show that Cl⁻ and Br⁻ improved the CH₄ selectivity for all tested Cu-based electrodes, while the effect of I⁻ was very sensitive to the surface state of Cu. On the one hand, I⁻ could corrode Cu₂O and form CuI, which could stabilize Cu⁺, resulting in a higher faradaic efficiency for C₂H₄. On the other hand, I⁻ had a negative effect on C₂H₄ selectivity in the presence of Cu⁰ and Cu²⁺. When Cu⁺ was present on the electrode surface, the ratio of C₂H₄/CH₄ was significantly changed from 19.3 (in KHCO₃ electrolyte) to 372.1 (in KHCO₃ + KI electrolyte). Our findings improve the understanding of the cooperation between the catalyst and halide electrolytes to produce the desired product and, in particular, the C₂ product.