Preparation Of Metal Porphyrin-based MOFs Composite Electrocatalyst And Its CO₂ Reduction Performance

Reducing the carbon dioxide(CO2)content in the atmosphere has been a worldwide research topic.Electrocatalytic reduction of CO2(CO2RR)has attracted much attention because it can use renewable energy to drive CO2 conversion.However,due to the thermodynamic stability and low electron affinity of the CO2 molecule,its electrochemical conversion requires high energy.It is necessary to select a suitable electrocatalyst.The metal center plays an important role in determining the CO2 reduction product.But it is still a huge challenge to develop a highly selective catalyst for a certain reduction product.So far,M-N4(M=metal)catalysts with clear active centers-porphyrins and phthalocyanine molecular catalysts,as well as extended structures that combine these active centers,such as porphyrin-based metal organic frameworks(MOFs)formed by Zr6/Zr8 clusters combined with porphyrin carboxylic acids,have shown unique advantages in the field of CO2RR.It is a significant topic to study the selectivity of metals in the M-N4 structure by using the advantages of Zr-based MOFs with adjustable pore structure and high water stability.However,its conductivity is still the bottleneck that limits its catalytic activity due to the electrical insulation of the Zr cluster.Therefore,improving the electron transfer capability of MOFs is our primary consideration.In the previous research,we found that the porphyrin-based MOFs—PCN-222(Fe)has high selectivity to CO(91%)in CO2RR,and the activity of other transition metal centers remains to be studied.Therefore,we choose PCN-222 and PCN-222(M)(M=Mn,Co,Ni,Zn)with clear M-N4 coordination as the research objects in this study.We discuss the influence of different metal centers in the same topological structure on the CO2RR performance by combining theory with experiments.To improve the conductivity of MOFs,we combine PCN-222 series materials with highly conductive materials CNT and electron-rich polymetallic oxygen clusters(POMs).The main contents are as follows:1.Theoretical prediction of CO2RR performance of PCN-222(M)(M=Mn,Co,Ni,Zn)A series of PCN-222(M)materials are prepared by the solvothermal method.With the help of IR,UV,PXRD,BET,XPS series characterization,the structure and specific surface area of crystalline materials and the oxidation state of metals in MOFs are determined,respectively.Taking the metal center as the active site and the porphyrin ligand as the calculation model,density functional theory(DFT)is used to calculate the Gibbs free energy change(ΔG)for each catalyst to convert CO2 to CO,and the frontline molecular orbital(the highest occupied orbital(HOMO),lowest unoccupied orbital(LUMO)),and the partial density of states(PDOS)of the reduced catalyst.The results show that the formation process of the intermediate*COOH(*represents the active site)is the rate-determining step(RDS)of the reduction process in all catalysts.In RDS,PCN-222(Mn)and PCN-222(Co)have lower ΔG*COOH 0.87 eV and 0.58 eV,respectively,and may have higher CO2 catalytic activity.PCN-222(Zn)and PCN-222 have similar ΔG*COOH(2.34 eV and 2.35 eV,respectively)in the RDS,and their reduced LUMO orbitals are also similar.In addition,the N1-2p and N2-2p near the Fermi level of PCN-222(Zn)overlap,suggesting that it may have potentially high catalytic activity.The large ΔG*COOH(2.15 eV)of PCN-222(Ni)suggests that its CO2RR may have a greater obstacle.2.Study on CO2RR performance of PCN-222(M)/CNT(M=Mn,Co,Ni,Zn)compositePCN-222/CNT and PCN-222(M)/CNT are synthesized in situ by the solvothermal method and loaded on carbon paper electrodes,respectively.We study the catalytic activity and stability of the composite material against CO2 by linear sweep voltammetry(LSV),cyclic voltammetry(CV)and chronoamperometry,and explore the role of metal centers.The results show that the PCN-222(Mn)/CNT,PCN-222(Co)/CNT and PCN-222(Zn)/CNT composites all exhibit high selectivity to CO(FEco>80%)in the range of-0.60～-0.70 V vs.RHE.Their FEco are 88.46%,89.29%,92.49%at the optimal potential and Tafel slopes are 159.85 mV decade-1,174.93 mV decade-1 and 184.54 mV decade-1.respectively.The catalytic activity of PCN-222/CNT is lower than the above three catalysts.Its FECOmax is 80.7%and the Tafel slope is 336.2 mV decade-1.PCN-222(Ni)/CNT is affected by the hydrogen evolution reaction,and its highest FEco is only 51.97%.Its Tafel slope is as high as 373.22 mV decade-1.Therefore,the metal center has a significant effect on the activity of CO2RR,and M=Mn,Co,Zn all have a promoting effect on it in the same topology.3.Study on CO2RR performance of P2W18Co4@PCN-222 and P2W18Co4@PCN-222(M)(M=Fe,Co)composite[(PW9O34)2Co4(H2O)2]10-(referred to as P2W18Co4)is embedded into the pores of MOFs by a simple impregnation method to prepare P2W18Co4@PCN-222 and P2W18Co4@PCN-222(M)composite catalysts.Then we mix them with carbon black and load them on the carbon paper electrodes,respectively.We use LSV,CV and chronoamperometry to carry out electrocatalytic research.The results show that P2W18Co4 improves the catalytic activity and conductivity of PCN-222.At-0.75 V vs.RHE,the FEco of P2W18Co4@PCN-222 is as high as 72.13%(before doping:32.88%,PCN-222).At the same potential,the current density of P2W18Co4@PCN-222 is more than 6 times that before doping.P2W18Co4 plays the role of an electronic relay in it and promotes the charge jump on PCN-222.However,its CO2RR activity don’t improve significantly for the P2W18Co4@PCN-222(M)system.This may be attributed to the fact that P2W18Co4 and PCN-222(M)compete for electrons and some electrons preferentially undergo(1e-+H+)reduction on P2W18Co4 in the CO2RR process,result in promoting the hydrogen evolution process.