ALD-Engineered CuO Overlayers Transform ZnO Nanorods for Selective Production of CO in Electrochemical CO₂ Reduction

The electrochemical CO₂ reduction reaction (CO₂RR) holds tremendous promise as a strategy for lowering atmospheric CO₂ levels and creating new clean energy sources. The conversion of CO₂RR to CO, in particular, has garnered significant scientific interest due to its industrial feasibility. Within this context, the CuZn-based electrocatalyst presents an attractive alternative to conventional CO-selective electrocatalysts, which are often costly and scarce. Nevertheless, the wide-range utilization of CuZn electrocatalysts requires a more comprehensive understanding of their performance and characteristics. In this study, we synthesized ZnO nanorods through electrodeposition and subsequently coated them with CuO overlayers prepared by atomic layer deposition (ALD). CuO significantly enhanced CO selectivity, and 88% CO selectivity at a relatively low potential of -0.8 V was obtained on an optimized CuO overlayer thickness (CuO-250/ZnO). The addition of CuO on ZnO was found to dramatically increase the electrochemical surface area (ESCA), lower the charge-transfer resistance (R_ct), and introduce new active sites in the ε-CuZn₄ phase. Furthermore, electrochemical Raman spectroscopy results showed that the CuO-250/ALD electrode developed a ZnO layer on the surface during the CO₂RR, while the bare ZnO electrode showed no evidence of ZnO during the reaction. These results suggest that the addition of CuO by ALD played a crucial role in stabilizing ZnO on the surface. The initial amount of CuO was shown to further affect the redeposition of the ZnO layer and hence affect the final composition of the surface. We attribute the improvement in CO selectivity to the introduction of both ε-CuZn₄ and ZnO that developed during the CO₂RR. Overall, our study provides new insights into the dynamic behavior and surface composition of CuZn electrocatalysts during CO₂RR.