Study On Electrichemical Reduction Of CO₂ Over Electrodeposited Zn And ZIF-8 Supported Ag Catalysts

Since the industrial revolution,with the burning of fossil fuels,the content of CO₂ in the atmosphere has been increasing year by year,which has led to a series of environmental problems such as greenhouse effect and energy depletion.Electrochemical reduction of CO₂ has become the most promising technology for the reuse of excessive CO₂ emissions because it can convert CO₂ into chemical fuels and chemicals with higher added value by using electric energy under mild reaction conditions.In this paper,two series of fibrous porous Zn and Ag/ZIF-8 catalysts were prepared,and they were applied in the electroreduction of CO₂ reaction.The electroreduction performance of the catalyst materials was studied,and the relationship between the structure,morphology,chemical state of the element surface,electrochemical surface area and the electroreduction performance of the catalyst was explored.Firstly,Zn foil was placed in four kinds of different gas（N₂,CO₂,H₂,CO）saturated Zn（NO₃）₂ solution at-1.85 V vs SCE,and treated by electrodeposition method for 300 s to prepare Zn catalysts with different morphologies.It was found that the Zn catalyst（Zn-CO₂）prepared in the CO₂-saturated Zn（NO₃）₂ electrolyte showed a fibrous porous structure.Compared with other catalysts,the fibrous porous Zn catalyst has the highest electrochemical active surface area,which is up to19.295 m F/cm²,which is 62 times that of Zn foil.The electrochemical reduction performance of CO₂ showed that the fibrous porous Zn catalyst not only had the highest current density,but also the faradaic efficiency of CO could reach 73%at-1.2 V vs RHE,which greatly inhibited the hydrogen evolution reaction and realized the efficient conversion of CO₂ to CO.Furthermore,the Zn-CO₂ catalyst showed outstanding stability under-1.2 V vs RHE.The current density and faradaic efficiency of the product CO remained unchanged for 6 h.Through the analysis of the reaction mechanism,it is considered that the first step of electron transfer is the rate control step of the electroreduction of CO₂ over Zn catalyst.In addition,ZIF-8 supported Ag catalysts（Ag/ZIF-8）were prepared by impregnation method,and the effects of Ag loading and impregnation time on the structure and electrochemical performance of the catalysts were investigated.The results of scanning electron microscopy（SEM）and transmission electron microscopy（TEM）showed that the ZIF-8 support showed a hexahedral morphology,and the hexahedral particles gradually agglomerated into lumps with the increase of Ag loading.X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）analysis confirmed that Ag mainly exists in elemental form in Ag/ZIF-8 catalyst.The results of electrochemical reduction of CO₂ showed that when the catalyst was impregnation for 8 h and the Ag loading was 20%,the faradaic efficiency of CO could reach 64.31%under the voltage of-1.35 V vs RHE.The stability test was conducted under the voltage of-1.25 V vs RHE,and it was found that,The 20%Ag/ZIF-8 catalyst can achieve the current density and the faradaic efficiency of CO basically unchanged within 4 h,which reflects the good stability of the 20%Ag/ZIF-8 catalyst.