Studies On Magnetic Field Effect Of Electrochemical CO₂ Reduction Based On In Situ Spectroscopy And Ultramicroelectrode

Electrochemical CO₂ reduction technology can convert CO₂ into high-value chemicals,alleviating environmental and resource crises.At present,the above technology has some problems,such as low reaction rate and poor selectivity.To solve the above problems,the development of traditional optimization strategy has entered the bottleneck stage.In recent years,the use of magnetic field to promote electrochemical reactions has attracted wide attention.However,the effect and mechanism of the magnetic field on the electrochemical reduction of CO₂ are still unclear,and new methods need to be developed to study the above mechanisms at the molecular level.In situ spectroscopy（such as in situ Raman spectroscopy,in situ ultraviolet spectroscopy,in situ infrared spectroscopy,etc.）combines spectroscopy with electrochemical experiments.By monitoring the intensity or frequency changes after the interaction between light and electrode surface substances,information at the molecular level can be obtained directly,so as to obtain the fingerprint information of intermediates on the electrode surface during the reaction process and assist in interpreting the reaction mechanism.The above technique can realize the real-time on-line monitoring of electrode surface microstructure.Based on this,the magnetic field effect and mechanism of HCOO^-and CO systems generated by electrochemical reduction of CO₂ were studied in this paper by means of in-situ spectroscopy and ultramicroelectrode experiments.The specific research contents are as follows:（1）Study on the magnetic field effect of electrochemical reduction of CO₂ by Bi nanosheets based on in-situ spectroscopy and ultramicroelectrode:A two-step electrodeposition method was used to prepare ultra-thin Bi nanosheets,which realized the efficient electrochemical reduction of CO₂ to generate HCOO^-.The reaction current was significantly increased under the magnetic field,and the yield of HCOO^-was increased by50%.Under the magnetic field,the absorption peak intensity corresponding to CO₂^*-anion radical at 205 nm in the in-situ UV spectrum increased,indicating that the magnetic field increased the local CO₂ concentration on the electrode surface through the magnetohydrodynamics effect,thus improving the reaction efficiency.In situ Raman spectroscopy verified the above mechanism,and the signal peak intensity corresponding to CO₂^*-anion radical at 175 cm^-1 was significantly increased under the magnetic field.When a magnetic field of 0.3 T was applied,the signal peak corresponding to HCO₃^-at 1007 cm^-1 in the in-situ Raman spectra was significantly increased,which further indicated that the magnetic field increased the HCO₃^-concentration on the surface of Bi nanosheet electrode through magnetohydrodynamic effect,and promoted the mass transfer of CO₂.At the same time,the intensity of the signal peak corresponding to another key intermediate OCHO^*radical at 2890 cm^-1 in the spectrum was significantly increased under the magnetic field,indicating that the magnetic field could significantly promote the generation of OCHO^*intermediate,which could be desorbed directly on the electrode surface to form formate,and thus the yield of formate was significantly increased under the magnetic field.Ultramicroelectrode experiments further showed that the catalyst performance is improved by the combined effect of magnetohydrodynamics and magnetic field-induced free radical spin state change.（2）Study on the magnetic field effect of electrochemical reduction of CO₂ by Au nanoparticles based on in-situ spectroscopy and ultramicroelectrode:By applying magnetic field,the electrochemical reduction performance of CO₂ to CO on Au nanoparticles was improved.Under the action of 0.9 T magnetic field,the yield of CO can be increased by28.5%.In situ UV spectra showed that the magnetic field increased the signal peak intensity corresponding to CO₂^*-anion radical at 205 nm,which verified the mechanism of magnetohydrodynamic effect.The signal peak corresponding to HCO₃^-at 1007 cm^-1 in the in-situ Raman spectra under magnetic field was significantly increased,which further indicated that the magnetic field increased the HCO₃^-concentration on the surface of Au electrode through magnetohydrodynamic effect,and promoted the mass transfer of CO₂.According to in situ infrared spectroscopy,the HCO₃^-signal peak corresponding to 1243cm^-1 was enhanced under the magnetic field,while the signal peak intensity of^*COOH intermediate corresponding to 1369 cm^-1 and^*CO intermediate corresponding to 2340 cm^-1remained unchanged.The results showed that the magnetic field only increased the HCO₃^-transient concentration on the electrode surface through the single effect of magnetohydrodynamic effect,but had no significant effect on the^*COOH intermediate and^*CO intermediate.Ultramicroelectrode test showed that the rate-determining step of the reaction of CO production by electrochemical reduction of CO₂ on Au electrode does not involve the process of free radical binding to form radical pair,thus there is no magnetic field-induced effect on spin state of radical pair.For electrochemical reduction of CO₂ to CO on Au electrode,the enhancement effect of magnetic field is mainly attributed to the magnetohydrodynamics effect.