Performance Of Formic Acid Production By CO₂ Electrochemical Reduction Based On A SnO₂ Nanosheet Cathode

In recent years,a large amount of fossil energy consumption has led to a dramatic increase in global CO₂ concentration,leading to a serious"greenhouse effect".In order to cope with a series of environmental problems such as global warming and sea level rise,researchers are making efforts to achieve emission reduction,conversion and utilization of CO₂ through biomass carbon sequestration,photocatalytic reduction,thermocatalytic reduction and electrochemical reduction.Among them,the electrochemical reduction technology has become the research hotspot because of its advantages of mild reaction condition and high conversion efficiency.It can use the unstable energy（such as wind,solar,tidal,etc.）to realize CO₂ electrochemical reduction to chemical products with high added value（such as formic acid,carbon monoxide,etc.）,and is considered as an effective way of CO₂ emission reduction and energy storage.At present,the main problems faced by electrochemical reduction of CO₂ include low selectivity of products,serious side reactions,low current density and low electrode life,which limit the development and application of electrochemical reduction of CO₂.In this study,in order to improve the performance of electrochemical reduction of CO₂,researches were carried out from three aspects:catalyst and electrode,reactor and reaction system.First of all,the Sn O₂ nanosheets catalyst was prepared by hydrothermal self-growth method on the carbon cloth.It was shown that the modification made the electrode hydrophobic,increased the three-phase reaction interface,and improved the performance of electrochemical reduction of CO₂.Secondly,a membrane-free single-channel flow reactor was designed to compare the performance of CO₂electrochemical reduction with the traditional H-cell reactor.Then,the effects of electrolyte concentration,p H,flow rate and gas flow rate were studied.At the same time,the gas-liquid two-phase flow was visualized and analyzed.Finally,Cu O anode was prepared to replace the conventional Pt anode,and CO₂RR+MOR system for anodizing methanol to produce formic acid was proposed,and its performance was compared with that of the traditional CO₂RR+OER system.The main conclusions of this study are as follows:（1）The Faradaic efficiency and current density of formic acid were 62.46%and26.87 m A·cm^-2,respectively,for the electrochemical reduction of CO₂ prepared by twice hydrothermal process.The Faradaic efficiency of formic acid was 77.2±1.9%at-1.8 V（vs.Ag/Ag Cl）electrolysis potential for the surface modified Sn O₂/CC-HB electrode,which was 14.7%higher than that of unmodified Sn O₂/CC-HL electrode.This is because the hydrophobic surface of the electrode increases the three-phase reaction interface of the electrode,solution and CO₂.After 12 hours of electrolysis test,the Faradaic efficiency of formic acid of Sn O₂/CC-HB electrode can still maintain72.6%,and the electrode still has a certain hydrophobicity.（2）In the membrane-free single-channel flow reactor,the current density and Faradaic efficiency of formic acid were 40.03 m A cm^-2 and 84.23%for electrochemical reduction of CO₂ at-1.8 V（vs.Ag/Ag Cl）.Compared with the traditional H-cell,the current density and Faradaic efficiency were increased by 44.26%and 9.15%,respectively.At the same time,it was found that the optimal electrochemical reduction performance of CO₂ was obtained when the electrolyte flow was 0.5 m L/min and the gas flow was 25 SCCM in 0.5 M KHCO₃ solution saturated with CO₂（p H=7.3）.（3）The Faradaic efficiency of formic acid of methanol oxidation by Cu O/CF electrode prepared by one-step oxidation method reached 72.7%at the potential of 0.4V（vs.Ag/Ag Cl）.After 18 h long test,the morphology of the catalyst showed little change,and the Faradaic efficiency remained 67.2%.The results of electrochemical comparison between the CO₂RR+MOR system and the CO₂RR+OER system at cathode potential-1.8 V（vs.Ag/Ag Cl）showed that the formic acid yield of the CO₂RR+MOR system was 32.7%higher than that of the traditional CO₂RR+OER system.At the same time,the potential between cathode and anode of CO₂RR+MOR system decreased by 32.6%.