Catalyst Design And Reaction Mechanism Research For The Electrochemical CO₂ Reduction

The consumption of fossil energy has caused a sharp rise in carbon dioxide（CO₂）in the atmosphere,which poses a great threat to the earth’s ecological environment and the sustainable development of human society.Using electrochemical method to reduce CO₂（ECO₂R）can not only decrease the concentration of CO₂,but also obtain high value-added chemicals（such as HCOOH,C₂H₄,C₂H₅OH,etc.）,which is an effective strategy to solve the energy crisis and achieve“carbon neutrality”.Although some progress has been made in the ECO₂R field during recent years,there are still some unresolved issues,such as low selectivity and activity,poor stability,and under-debated catalytic mechanism,which restrict the rapid development of ECO₂R.The performance optimization and formation mechanism of C₁（HCOOH）and C₂（C₂H₄,C₂H₅OH）on Bi-and Cu-based catalysts were studied.Several in-situ characterization techniques and theoretical calculations were used to investigate the catalyst reaction mechanism,evolution process,and CO₂ transformation pathway,to achieve high selectivity,high catalytic activity,and stability of the target product,and also provide theoretical insights for guiding the design of high-performance catalysts.The research details and conclusions are listed as follows:（1）The low current density(<100 mA cm^-2)and the poor stability（<10 h）for HCOOH generation on Bi catalysts hinder the application of ECO₂R.According to the bimetal synergistic strategy,Bi In alloy catalysts were synthesized by ion exchange and electrochemical reduction using metal-organic framework materials（MOFs）as precursors.In-situ Fourier transforms infrared and density functional theory（DFT）calculations prove that the synergistic effect of the dual-metal sites in Bi In alloy is the original reason for the efficient production of HCOOH,and the optimized catalysts achieve high selectivity（92.5%）and high current density(300 mA cm^-2)for HCOOH via optimizing the adsorption of*OCHO intermediates through the Bi-In dual-metal sites.Moreover,the Bi In alloy NPs also achieve superior stability over 25 h with less than a 10%selectivity drop at the current density of 120 mA cm^-2 in a membrane electrode assembly system.（2）The current density(<100 mA cm^-2)and the selectivity（<40%）are low for C₂H₄ generation by Cu-based catalyst,and the oxidation valence state effect is not clear.According to the interface design strategy,Cu⁰@Cu⁺/Cu⁰ core-shell structure was synthesized by calcination of Cu-MOF at high temperature and then electroreduction.The Cu⁺/Cu⁰ interface in the core-shell structure with a mixed oxidation valence state can stabilize*CO intermediate,promote C-C coupling,and then facilitate the production of C₂H₄.Compared with pristine Cu-MOF,the selectivity of C₂H₄ produced by the Cu⁰@Cu⁺/Cu⁰ core-shell structure increases from 15%to 51%,and the current density is 320 mA cm^-2.（3）Cu is the only metallic catalyst that enables the generation of C₂₊products.However,the reaction pathway of CO₂ conversion toward C₂₊is still unclear,and the key step,C-C coupling,is controversial.To solve this problem,the C-C coupling pathway during the formation of C₂₊products was monitored in real-time by using in-situ electrochemical spectroscopies and DFT calculations.The results show that on a Cu surface with low coordination sites,the energy barrier of*CO-COH coupling is lower than that of the*CO-CO coupling,thus the former path is optimal.This study provides the first experimental evidence of*CO-COH coupling and confirms that low coordination is a favorable factor for*CO-COH coupling.These conclusions shed light on the rational design of catalysts to improve the selectivity of C₂₊products.（4）Cu-based compounds undergo structural evolution and surface reconstruction during the electrocatalysis,resulting in the formation of the final active Cu catalyst with high catalytic performance,but the mechanism of structural evolution is still unclear.To solve this problem,in-situ X-ray absorption spectroscopy and in-situ Raman spectroscopy,combined with DFT calculations,were used to observe the evolution process of P-doped Cu-based compounds to Cu（100）facets in real-time.The results show that phosphate ligands can reduce the surface energy of Cu（100）facets formation,promote the co-adsorption of*CO and OH^-in the electrochemical reduction process,and then promote the formation of Cu（100）facets.The selectivity of the Cu（100）catalyst for C₂₊products reached～81%at 350 mA cm^-2.