Toward efficient electrochemical reduction of CO2 to CO: Decorating ZnO nanorods with CuxO

This thesis investigates CuxO-ZnO based electrocatalyst as a potential cost-effective electrocatalyst for electrochemical reduction of CO2 to CO (CO2RR). CO2RR is a viable solution that can offer a potential source of energy while also reducing the amount of CO2 in the atmosphere. CO2RR to CO presents a promising route due to having the highest economical visibility among other targeted products. In response, we have synthesized and examined two sets of CuxO-ZnO electrocatalysts derived from earth-abundant elements that have high Faradic efficiency (FE) for CO, exceeding 70%. We initially synthesized CuxO/ZnO electrocatalysts with different amounts of CuxO, using the electrodeposition technique. The optimized CuxO/ZnO electrode exhibited elevated selectivity towards CO, achieving a Faradaic efficiency (FE) of 75% at a low overpotential of -0.8 V vs. RHE. This, in itself, marked a clear improvement from the performance of bare ZnO, which only achieved a CO selectivity of 48% under identical conditions. However, an even more significant enhancement in CO selectivity was observed when CuxO was introduced using atomic layer deposition (ALD). Specifically, the CuxO-250/ZnO electrode, prepared with 250 ALD cycles, exhibited a remarkable CO selectivity of 88% at the same potential, demonstrating the superior performance of ALD CuxO/ZnO over both bare ZnO and the electrodeposited CuxO/ZnO electrodes. The addition of CuxO was found to enhance CO selectivity through the introduction of the new active phase ε-CuZn4 and a reduction in charge transfer resistance (Rct). Furthermore, the incorporation of CuxO significantly influenced the surface reconstruction process, resulting in a distinct surface morphology for the CuxO/ZnO electrodes after CO2RR. This led to an increased surface area compared to bare ZnO, suggesting a possible contribution to enhanced CO2RR performance. We also demonstrated that the applied potential substantially impacts the reconstruction process, affecting the Cu/Zn ratio at the surface, which in turn influences CO selectivity. We proved that after CO2RR at -0.8 V, the Cu/Zn atomic ratio was higher than that after -1.2 V of the same electrode, aligning with our findings on CO selectivity and highlighting the crucial role of surface Cu in CO production. We further noted that after CO2RR, the surface composition underwent significant changes, and the highest Cu/Zn ratio was not necessarily associated with the greatest initial amount of Cu. Rather, the maximum Cu/Zn ratio was observed in the CuxO- 250/ZnO electrode, which also exhibited the highest FE of CO. We attribute this variation to the surface reconstruction process and the galvanic effect between Cu and Zn atoms, both of which influence the surface composition and, by extension, electrode selectivity. In summary, this thesis presents a new earth-abundant electrocatalyst that is highly selective toward CO and provides new insights into the dynamic behavior and surface composition of Cu-Zn electrocatalysts during CO2RR. These insights will significantly contribute to future studies aiming to design more effective and efficient Cu-Zn based electrocatalysts.