Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment

Here, we have developed porous nanostructured Zn electrocatalysts for CO2 reduction reaction (CO2RR), fabricated by reducing electrodeposited ZnO (RE-Zn) to activate the CO2RR electrocatalytic performance. We discovered that the electrochemical activation environment using CO2-bubbled electrolyte during reducing ZnO in a pretreatment step is important for highly selective CO production over H-2 production, while using Ar gas bubbling instead can lead to less CO product of the Zn-based catalyst in CO2RR later. The RE-Zn activated in CO2-bubbled electrolyte condition achieves a Faradaic efficiency of CO production (FECO) of 78.5%, which is about 10% higher than that of RE-Zn activated in Ar-bubbled electrolyte. The partial current density of CO product had more 10-fold increase with RE-Zn electrodes than that of bulk Zn foil at -0.95 V vs RHE in KHCO3. In addition, a very high FECO of 95.3% can be reached using the CO2-pretreated catalyst in KCl electrolyte. The higher amount of oxidized zinc states has been found in the high performing Zn electrode surface by high-resolution X-ray photoelectron spectroscopy studies, which suggest that oxidized zinc states induce the active sites for electrochemical CO2RR. Additionally, in pre- and post-CO2RR performance tests, the carbon deposition is also significantly suppressed on RE-Zn surfaces having a higher ratio of oxidized Zn state.