CO Product Selectivity For Electrochemical CO₂ Reduction On Zn-based Materials

The technology of electrocatalytic reduction can convert CO2 molecule into the industrial chemicals with high-added value,thus being an effective method to alleviate the massive emission of greenhouse gas and the shortage of fossil energy.Among the reduction products,as a component of industrial syngas,CO gas is the main target product due to its great commercial application value.In the electrochemical CO2 reduction system for CO generation,the CO2 molecule will receive coupling transfer of electron and proton for two times.Firstly,the CO2 molecule is reduced to a CO2·-radical by one-electron transfer;secondly,the CO2·-radical which adsorb on the electrode surface will obtain proton from the electrolyte to generate COOH*intermediate adsorbates;subsequently,COOH*intermediate adsorbates acquire proton and electron to generate CO*adsorbates;finally,CO*adsorbates desorb from the electrode to form CO products.Due to the activation process of linear inert CO2 molecule requiring a high energetic barrier,the one-electron reduction process that generates CO2·? radicals and the proton transfer process that generates COOH*intermediate adsorbates are usually the rate-determining steps in the reaction system,which are the key factors to affect the efficiency of CO generationAffordable metal Zn electrodes with abundant crustal content are beneficial for the adsorption of COOH*intermediates and the desorption of CO*adsorbates,thus being an ideal electrocatalyst for the industrial production.However,a secondary reaction of hydrogen evolution will occur at the surface of Zn electrode due to the CO2 electrochemical reduction takes place in an aqueous solution,therefore leading to a decrease of CO selectivity.Moreover,the bulk Zn electrode also has the disadvantage of slow kinetics during the CO2 reduction process,as a result,building effective active sites in the Zn-based catalyst is a promising strategy to change its catalytic properties and improve the CO selectivity.In this paper,from the aspects of inhibiting the hydrogen evolution,facilitating the activation of CO2 molecule and the adsorption of CO2·-radical and adjusting the proton concentration,the enhancement of CO product selectivity was achieved by building surface adlayer and ionic active site on the Zn-based materials.The main research contents and conclusions are as followsUsing Cl-ion to construct a Zn-Cl adlayer on the surface of Zn electrode and exploring the mechanism of hydrogen evolution inhibition to improve the selectivity of CO product.Adsorption is the essential prerequisite to build an electron-transfer channel between the reactant and catalyst,moreover,the method of surface modification is an effective strategy to adjust the adsorption characteristic of catalyst In this paper,a bulk Zn catalyst was chosen as the working electrode.The weakly solvated Cl-ion in KCl solution could interact with the Zn electrode through a covalent bond to form a Zn-Cl adlayer for the blocking of proton adsorption,thus inhibiting the hydrogen evolution and promoting the CO faradaic efficiency of bulk Zn electrode up to 48%.Besides,the existence of Cl-ion accelerated the electron transfer to the vacant orbits of CO2 molecule and decreased the energy barrier for CO2·-radical formation to improve CO selectivityConstructing Zn2+/Zn+redox active site in low-crystalline porous ZnGa2O4 material for the efficient activation of CO2 molecule to improve CO selectivity.When adsorbed on the electrode surface,the hybridization between CO2 molecule and catalyst will lead to the formation of bonding orbital and vacant anti-bonding orbital The existence of ionic Zn2+/Zn+redox active site will facilitate the rate of electron injection into the vacant antibonding orbital of CO2 molecule.The porous structure of ZnGa2O4 material was conducive to the adsorption of CO2 molecule and the low crystallinity was beneficial for the formation of abundant Zn2+/Zn+active sites.The strong interaction between Zn2+/Zn+site and CO2 molecule not only promoted the generation of CO2·-radical but also effectively accelerated the proton transfer process for the formation of COOH*intermediate.In this mechanism,the CO faradaic efficiency of porous ZnGa2O4 material could reached 96%.Adjusting the HCO3-ion concentration of electrolyte to verify the proton source of CO2 reduction reaction and regulating the selectivity of CO product.During the reaction of CO2 reduction,the COOH*intermediate is formed by the interaction between CO2·-radical and proton,besides,the stable adsorption of COOH*on the electrode surface will make a significant influence on the CO selectivity.In this paper,the adjustment of HCO3-ion concentration in the ZnGa2O4 system was performed to make sure the role of proton source and its advantage for the formation of COOH*intermediate.Meanwhile,the negatively charged HCO3-ion could easily combine with the positively charged Zn2+/Zn+active site on the catalyst surface and simultaneously dissociated out of CO2 molecule to increase the reactant concentration near the electrode surface.In this mechanism,the interaction between Zn2+/Zn+active site,CO2 molecule and HCO3-ion lead to the realization of CO selectivity regulation.