Non-noble Metal Catalysts For Electrochemical CO₂ Conversion Based On Surface Control Strategy

The widespread use of fossil fuels has led to a dramatic increase in carbon dioxide(CO2)emissions,resulting in severe environmental pollution and global climate problems.Electrochemical CO2 reduction reaction(CO2RR)has unique operating conditions(e.g.,room temperature and ambient pressure).The different reduction products can be obtained by adjusting the reaction conditions(e.g.,applied potential,electrode material or electrolyte,etc.).Electrochemical CO2RR can convert CO2 into high value-added chemicals and fuels by utilizing renewable energy sources(e.g.,solar and wind energy),which leads to a true carbon recycling.In recent years,great progress has been made in the exploration of electrochemical CO2RR system,but there are still many problems to be solved,such as the high overpotential in the CO2RR process;low selectivity of reduction products and competition for hydrogen evolution reaction(HER).Therefore,it is of great scientific significance and industrial value to design and prepare highly active CO2RR electrocatalysts to improve the conversion efficiency and high selectivity of the target products.To address the above issues,we have designed and prepared several non-precious metal-based catalysts based on the previous research results.The strategies of coordination environment modulation of metal single atoms and metal-organic complexes,surface molecular modification and control of crystal structure were adopted to enhance the CO2 reduction activity and product selectivity of electrocatalytic materials.The catalytic mechanism of CO2 reduction by these electrocatalysts and the key factors affecting the catalyst performance were also investigated.This work will provide new ideas for the development of efficient CO2RR non-precious metal-based catalysts.The main research of this thesis is as follows:1.Mn single-atom(SAC)catalysts(Mn-NO/CNs)co-ligated with both N and O atoms immobilized on carbon nanosheets were successfully prepared by a one-step high-temperature calcination method.The Mn-NO/CNs catalysts exhibited good catalytic performance for electrochemical reduction of CO2 to CO products.The Faraday efficiency of CO(FEco)was as high as 96.0%at an applied potential of-0.460 V(vs.RHE)in a 0.5 M KHCO3 electrolyte.In flow cell measurements(1.0 M KOH flow solution),MnNO/CNs achieved a current density of 28 mA cm-2 for CO production at a very low potential of-0.425 V(vs.RHE).The FEco remained above 80%after 70 h of electrolysis,indicating the good stability of the Mn-NO/CNs catalyst.Based on X-ray absorption spectroscopy(XAS)results and density function theory(DFT)calculations,it is suggested that the Mn-N2O2 site was the catalytic active center.The Mn-N2O2 sites could facilitate CO2 adsorption and significantly lower the free energy barrier of the intermediate,thus favoring the formation of the key intermediate*COOH.The coordination environment of Mn metal atom with N and O double heteroatoms is the main reason for the enhanced CO2 reduction performance.2.A metal-organic framework(MOF,Zn(atzc)-Cl)were successfully prepared by a hydrothermal method using the 3-amino-1,2,4-triazole-5-carboxylic acid(H2atzc)as a ligand.The electrochemical CO2RR performance of as-prepared Zn(atzc)-Cl was investigated.The results showed that the fraction current density of the CO in an alkaline flow cell was up to 80 mA cm-2.The volume ratio of the reduced product syngas(V(CO)/V(H2)=0.4-4.0)can be adjusted over a wide potential range(-1.476 to-0.766 V vs.RHE)by controlling the applied potential during the electrochemical CO2RR process.The Zn(atzc)Cl catalyst yielded a total product FE of about 100%and a maximum FEco of 80%for CO production at an applied potential of-1.37 V(vs.RHE).The catalytic active site of the electrochemical CO2 reduction reaction was investigated by comparing another Zn-MOF(ZIF-8)with 2-methylimidazole as ligand.Based on experimental and DFT calculations,the catalytic activities of metal Zn catalysts with different coordination numbers were compared in the absence of one ligand defect.We speculate that the coordination environment Zn(atzc)-Cl with a coordination number of 3 and the defective sites may be an advantage for the higher electrochemical CO2 reduction activity of this catalyst.3.Among the various electrochemical CO2 reduction products,formic acid(HCOOH)shows great promise for industrialization due to its high market value.In this work,Cu-MOF(Cu-BTC)materials with octahedral structure were prepared.The effects of three typical surfactants(Hexadecyl trimethyl ammonium Bromide,CTAB;sodium dodecyl benzenesulfonate,SDBS;Polyethylene glycogen,PEG)modified Cu-BTC catalysts on the selectivity of electrochemical CO2 reduction products were investigated.The results showed that CTAB-modified Cu-BTC(Cu-BTC/CTAB)significantly inhibited HER and enhanced the selectivity of HCOOH products during the electrocatalytic process.The Faraday efficiency of Cu-BTC/CTAB electrocatalytic formation of HCOOH was up to 95.5%at an applied potential of-0.876 V(vs.RHE).Electrochemical tests in the flow cell showed that the electrochemical CO2 reduction by Cu-BTC/CTAB could achieve a current density of 100 mA cm-2 with 92.0%FEHCOOH.The morphology and chemical state of Cu-BTC/CTAB after the electrochemical CO2 reduction reaction were characterized and tested.It was found that the Cu-BTC/CTAB reconfigured,and the original octahedral structure disappeared in the CO2RR process.Many 100-200 nm nanoclusters were formed,which consist of Cu and Cu2O.surface and selective conversion to HCOOH.The synergistic interaction between Cu and Cu2O nanoclusters and CTAB molecules in Cu-BTC/CTAB-R enables better adsorption of CO2 on the catalyst surface,which facilitates the selectively catalytic reduction of CO2 to HCOOH.4.Copper-based materials are currently an important class of materials that can be electrochemically converted CO2 to C2/C2+products.Truncated-octahedral Cu2O microcrystals were prepared on carbon paper electrodes by in situ electrochemical deposition.The morphology and the integrity of the exposed crystal surface(111)were successfully regulated by controlling the deposition potential,deposition time and deposition temperature.The truncated-octahedral Cu2O microcrystals exhibited high electrocatalytic CO2 reduction activity for the formation of C2H4.The Faraday efficiency of the C2H4 product FE(C2H4)was up to 42.0%at an applied potential of-1.376 V(vs.RHE)in 0.1 M KHCO3 electrolyte.During 10 hours of continuous electrolysis,the FE(C2H4)remained stable at about 40%.The truncated-octahedral Cu2O microcrystals with fully exposed crystal faces(111)can effectively promote C-C coupling and may be the main active site for catalytic conversion of CO2 to C2H4.