Mediating Zn loading on Cu-Zn bimetallic catalyst for electrochemical CO2 reduction into tunable syngas

Abstract

Electrochemical CO₂ reduction (ECR) syngas (CO/H₂) using renewable electricity is considered as a promising approach to realizing carbon neutrality. However, balancing the activities of the CO₂ reduction reaction (CO₂RR) and the hydrogen evolution reaction (HER) to achieve adjustable CO/H₂ ratios remains challenging. In this study, Cu-Zn bimetallic catalysts were synthesized via electrodeposition for ECR to syngas, and the CO/H₂ ratio was tuned over a broad range of 0.2–2.1 by adjusting the deposition time and applied potential. The effect of deposition time on Zn loading and ECR performance was investigated. As deposition time increased, the density and uniform dispersion of Zn sites were enhanced, which improved CO selectivity and increased the CO/H₂ ratio and syngas production rate. Density functional theory (DFT) calculations revealed that the alloying effect between Cu and Zn sites endowed the catalyst with moderate energy barriers for *COOH formation and *CO desorption in CO₂RR, and moderate desorption intensity of *H for HER. Increasing Zn loading raised the energy barrier for *H formation but facilitated *CO desorption, thereby enhancing CO production. CO₂ conversion to *COOH was then identified as the rate-determining step in CO₂RR. Excessive prolongation of the deposition time negatively impacted CO₂RR performance, resulting in decreased CO/H₂ ratio and syngas production rate due to overloading and uneven dispersion of Zn sites, as well as increased charge transfer resistance. The Cu-Zn-675 catalyst, with an optimal deposition time of 675 s, exhibited the highest activity and selectivity for syngas production, achieving a high syngas Faradaic efficiency (FE) of approximately 90 % and a tunable CO/H₂ ratio of 0.6–2.1 over a wide potential range of −0.61 to −1.21 V versus reversible hydrogen electrode (RHE). Cu-Zn-675 also demonstrated excellent long-term operational stability, maintaining a stable CO/H₂ ratio and syngas production rate of 2.0 and 104.2 μmol/h/cm², respectively, for up to 7.5 h at a constant potential of −0.81 V vs. RHE. This work presents a promising strategy for developing robust catalysts for ECR to syngas with tunable CO/H₂ ratios.