Preparation Of Sandwich-type Polyoxometalates Derivatives And Their Performance In Electrocatalytic Reduction Of CO₂

Reducing carbon dioxide(CO2)levels in the atmosphere is already a key to solving global environmental problems.The electrocatalytic CO2 reduction(ECR)is particularly competitive due to its controllability and mild reaction conditions.Moreover,it can be combined with renewable energy and storage excess of them in chemical forms with added values,thus promoting carbon neutrality.However,electrocatalytic CO2 reduction faces challenges of large overpotentials,slow electron transfer kinetics,suboptimal selectivity,and low energy efficiency due to the accompanying solvent decomposition reactions and the inherent high thermodynamic stability of the CO2 molecule with low electron affinity energy.At the same time,high overpotential also leads to increased competitive hydrogen evolution activity(HER)and decreased CO selectivity.Therefore,the development of effective electrocatalysts is essential for high selectivity and stability in ECR.Polyoxometalates(POMs)have attracted a lot of attention in the field of electrocatalysis due to their clear structure,tunable composition,and unique redox properties.POMs are known as electron storage sponges.The remarkable ability to store and provide electrons gives POMs the property of reversible multi-electron transfer,which ensures that they can perform reversible multi-electron redox reactions in external electric fields while maintaining structural integrity.Furthermore,the electron transfer behavior of the POMs can be modulated by simply adjusting the constituent elements without altering their structure.These advantages make POMs ideal materials for constructing electrocatalysts with adjustable electron transfer pathways.However,the high solubility of polyoxometalates in water and polar organic solvents limits their catalytic activity.Porphyrin-based molecular catalysts with abundant redox active sites,structural diversity,and high stability are ideal materials for binding to POMs.The porous structure of MOFs with porphyrin ligands as structural units can facilitate CO2 adsorption/activation and shorten the transport distance between CO2 molecules and active sites to promote ECR activity.However,the low electron conductivity,low electron transfer,and low electron donating ability of most porphyrin-based molecular catalysts limit the electron current density and CO2 diffusion.Therefore,compounding POMs and porphyrin-based electrocatalysts is a feasible method to overcome the above limitations,which can make full use of the stability of porphyrin framework structure and the strong electron transfer ability of POMs to promote the generation of high-value reduction products.We assembled sandwich-type polyoxometalates K10[(PW9O34)2M4(H2O)2](M=Mn,Ni,Zn)into the structure of porphyrin-based molecular catalysts PCN-222 and Fetpyp with clear M-N4 coordination,and investigated the effect of the filled polyoxometalates with different metal centers on the performance of porphyrinbased electrocatalysts.At the same time,the mechanism of CO2 reduction and electron transfer process were discussed by using the DFT theory.The main contents are as follows:1.Study on ECR performance of P2W18M4@PCN-222(M=Mn,Ni,Zn)compositePCN-222 was prepared by the solvothermal method,and P2W18M4(M=Mn,Ni,Zn)was encapsulated in the hexagonal pores of PCN-222 with the help of a simple impregnation technique to prepare the P2W18M4@PCN-222(M=Mn,Ni,Zn)composite electrocatalysts.The test results showed that when P2W18M4 were immobilized in the pore channel of PCN222,their ECR behavior exhibited a polyoxoanion-dependent activity.The ECR activity of P2W18Mn4@PCN-222 is significantly improved in the range of-0.50～-0.65V vs.RHE,especially the Faraday efficiency of CO(FEco)is 72.6%at-0.60V vs.RHE.It is more than four times that of PCN-222(FECO=18.1%)and exhibits excellent electrochemical stability over 36 h.The FEco of P2W18Ni4@PCN-222 increased slightly(26.9%),while the ECR activity of P2W18Zn4@PCN-222 decreased significantly(3.2%)due to the hydrogen evolution reaction.2.Theoretical study of ECR performance of P2W18M4@PCN-222(M=Mn,Ni,Zn)compositeDensity flooding theory(DFT)calculations were performed to help understand the mechanism and electron transfer pathways of CO2 reduction.The results show that the formation of the bound*COOH(*represents the active site)intermediate is the ratedetermining step(RDS)of the reduction process in all catalysts.The RDS energy ΔG*COOH of P2W18Mn4@PCN-222 and P2W18Ni4@PCN-222 decreased significantly compared with PCN-222,which were 0.95 eV and 1.70 eV,respectively.On the contrary,the ΔG*COOH of P2W18Zn4@PCN-222 increased to 2.64 eV.Frontier molecular orbital electron distribution results show that electrons transfer from P2W18Mn4/P2W18Ni4 to the active center of the porphyrin ring,promoting the activity of electroreducing CO2.In contrast,P2W18Zn4 may accumulate electrons of PCN-222,thus promoting hydrogen evolution(HER).Therefore,POMs with different metal centers in the same topology have different effects on the ECR activity of PCN-222.3.Study on ECR performance of P2W18M4-Fetpyp(M=Mn,Co)compositeA series of POM-porphyrin composite electrocatalysts P2W18M4-Fetpyp(M=Mn,Co)have been prepared by using electrostatic interaction to fix the POM anion P2W18M4(M=Mn,Co)in the Fetpyp structure.The composite electrocatalysts were mixed with conductive carbon black and loaded on the surface of carbon paper electrodes to study the ECR activity.The structural integrity of the polyoxometalates and free ligand in the composites was determined by IR and UV spectra,while the morphology of the composites produced regular changes,demonstrating the successful compounding of the polyoxometalates with Fetpyp.The study of the catalyst ECR performance revealed that the composites showed a significant increase in activity in the range of-0.50～-0.90 V vs.RHE.In particular,the Faraday efficiency of P2W18Co4-Fetpyp for CO was enhanced from 13.1%to 70.1%and jCO from 0.17 mA cm-2 to 2.63 mA cm-2 at-0.70 V vs.RHE.The FEco(43.7%)and jCO(0.68 mA cm-2)of P2W18Mn4Fetpyp were also both enhanced.Thus,the introduction of polyoxometalates has a significant promotion effect on the ECR activity of porphyrin-based molecules.