Preparation And Performance Evaluation Of Pt/MNC Composite Catalyst

At present,the world’s major developed and developing countries are working together to reduce carbon emissions and protect the earth’s ecology.As the next generation of environment-friendly energy,hydrogen energy has been highly valued by all countries,and the hydrogen energy industry has shown great vitality.Hydrogen production by electrolytic water splitting and proton exchange membrane fuel cell are important links of Hydrogen energy economy.However,the hydrogen evolution reaction（HER）in Hydrogen production and oxygen reduction reaction（ORR）in fuel cell have slow kinetic process,and both of them rely heavily on Pt-based catalyst to achieve high energy conversion efficiency.However,the precious metal Pt is expensive and its reserves are scarce,which brings a significant impact on the further development of hydrogen energy industry.Therefore,it is urgent to develop highly efficient electrocatalysts based on non-precious metal and ultra-low precious metal loads.Studies have shown that all metal atoms on a single atomic catalyst can be used as catalytic sites,thereby increasing atomic availability to 100 percent.At the same time,the intrinsic catalytic activity of precious metals can be further regulated by changing the carrier type and coordination environment,so it is considered as the ultimate method to reduce the Pt load.Therefore,Fe/Conitrogen-doped non-noble metal（Fe/CoNC）obtained from the pyrolysis of transition metal-doped Zeolite Imidazole framework-8（ZIF-8）was used as carrier in this paper.After a simple electrochemical reduction method,the Pt precursors loaded on the carrier were reduced.A series of single-atom catalysts with ultra-low Pt loads were obtained,which greatly improved the utilization efficiency of precious metals.The obtained catalysts exhibited excellent performance in the electrocatalytic reactions of ORR and HER.The main work contents are as follows:（1）In order to obtain a uniform catalytic site,good activity,containing transition metal nitrogen binding site of the nanocarbon materials,different synthesis pathways,processes and conditions were explored.Fe/Co-ZIF crystals with regular polyhedral skeleton structure were obtained by hydrothermal method.After heat treatment,Fe/CoNC catalysts were obtained.After heat treatment,the support morphology remained the same and the abundant pore structure was maintained.Transmission electron microscopy（TEM）showed that there were no metal aggregates in the sample,and X-ray diffraction（XRD）did not find the characteristic peak of metal.It is clear that the transition metal Fe/Coexists in the form of a single atom in the carbon skeleton structure.The catalyst 1.5Fe-NC exhibited excellent ORR activity and the half-wave potential reached 0.818 V vs.RHE,which proved that there were a large number of Fe-N_xcoordination structures in the material.Using 1.5Fe-NC as the support to adsorb chloroplatinic acid,then using a simple cyclic voltammetry method to electrochemical reduction of Pt to get Pt/FeNC catalyst.At the same time,by changing the dosage of Pt and electrochemical reduction parameters can be highly controlled to change the Pt load,different types of catalysts from noble metal monatomic catalyst to nanoparticles can be obtained.When the Pt loading was 5.1wt%,the ORR activity of the catalyst was the best,the half-wave potential was 0.913V vs.RHE,and the mass activity（MA）of the catalyst was 299 m A/mg _Pt,which was1.5 times that of commercial Pt/C.On the other hand,when the Pt load was adjusted to a low level by this method,the catalyst also had a good HER activity.When the Pt load was 0.45 wt%,the HER overpotential was as low as 11.4 mV,and the mass activity was 4.5 A/mg _Pt,which was about 10 times that of commercial Pt/C.（2）The research shows that the atomic sites of Co-N_xhave better HER activity.By the same method,the Co₁NC with abundant pore structure and transition metal nitrogen sites can be obtained,and the Pt₁/Co₁NC catalyst can be prepared after electrochemical reduction by using it as the carrier to adsorb the ultra-low capacity Pt precursor of chloroplatinate.The catalyst exhibited excellent HER activity.When the Pt load was 0.4 wt%,the overpotential was only 4.15 mV and the mass activity was32.4 A/mg _Pt,which was 50 times that of commercial Pt/C with 20 wt%.By spherical aberration-corrected electron microscope can clearly distinguish the catalyst Pt and Coare scattered in the form of a single atom,X ray photoelectron spectroscopy shows that the valence of Pt in sample Pt₁/Co₁NC was 2 and no 0 valence existed.But there is zero valence in the comparison sample.The X-ray absorption spectra test results also confirm the above conclusion,Pt and Coexist in the form of single-atom.Raman and XPS spectra showed that nitrogen/carbon defects were the key to the formation of Pt monatomic active sites.Further theoretical simulation showed that atomically dispersed Pt-C₃,Pt-C₄,and Pt-N₁C₃sites on the graphite base had low intermediate free energy barrier,which revealed the source of ultra-high hydrogen evolution activity.In electric catalytic performance tests prove greatly that it reduces the utilization rate of precious metal catalyst.Therefore,compared with traditional commercial nanoparticle catalysts,the synthesized single-atom catalyst greatly improves the atomic utilization,thus improving the electrocatalytic activity and greatly reducing the cost of the catalyst.In this paper,a highly controllable general electrochemical reduction method was developed to prepare Pt single-atom/Pt nanoclusters to Pt nanoparticles loaded on transition metal nitrogen-doped carbon materials.Electrocatalysts with excellent ORR and HER activities were prepared.The product is expected to reduce the cost and promote commercialization in the practical application of hydrogen energy production and application fields such as fuel cell and electrolysis of water to produce hydrogen.