Study On Structure Control And Oxygen Reduction Performance Of Platinum Copper Catalyst For Fuel Cell

Proton exchange membrane fuel cell(PEMFC)is a clean and efficient device,which can obtain electricity directly from hydrogen energy through electrochemical reaction.However,due to the short life and high price,PEMFC is still in the early stage of application.Its cathodic oxygen reduction reaction(ORR)is relatively slow,so a large amount of platinum(Pt)must be used to promote kinetics of the reaction.The high price of Pt directly increases the cost of PEMFC.Moreover,the commercialized Pt/C catalysts still cannot meet the requirements in terms of activity and stability.It is necessary to reduce the dosage of Pt while ensuring the catalytic performances before developing fuel cells on a large scale.It is found that the structural regulation of Pt-based nanoparticles including preparation of ordered alloy,core-shell,and nanoframe is an important way to design catalysts,which can not only improve the utilization rate of Pt,but also enhance the activity because of the regulation of Pt electronic property.Based on these questions,we designed and prepared a series of PtCu-based catalysts with multi-dimensional and optimized their catalytic performance through designing micro-nano structure combined with the composition control strategy to optimize the crystal structure and electronic structure.The study provides theoretical and experimental basis for the commercialization of ORR catalysts.And this article mainly includes the following four parts:(1)In order to improve the stability of the catalyst with zero-dimensional structure,the advantages of surface Mo modification,ordered core-shell structure are combined.Firstly,the PtCu alloy was prepared by the polyol reduction method,and surface Mo atoms was doped by thermal decomposition of ammonium molybdate forming MoOx-PtCu/C to adjust the Pt electronic structure,then an ordered core-shell structure catalyst(MoOx-PtCu@Pt/C)was formed by annealing at high temperature and chemical dealloying to further improve the activity and stability of the catalyst.Through a series of physical characterizations,it was found that the alloy degree of MoOx-PtCu@Pt/C increases with the increase of annealing temperature.And the ordered structure was formed when the annealing temperature reach 700℃.Moreover,it was found that Mo doping can fine-tune the crystal structure and electronic structure of PtCu,and the core-shell structure could be controlled by changing the time of chemical dealloying,both of which could optimize the catalytic activity.Combining the advantages of ordered structure,core-shell structure and surface modification,MoOx-PtCu@Pt/C behaved a mass activity of 0.65 A mgpt-1 at 0.9 V.Moreover,the surface-doped Mo could inhibit the dissolution of internal Cu,thereby improving the stability of the catalyst.(2)In order to reduce the metal agglomeration in the catalysis process,the PtCu catalyst with one-dimensional structure was prepared,and its activity was further enhanced by porous structure.As a sacrificial template,Cu nanowires underwent a displacement reaction with chloroplatinic acid under ultrasoundassisted conditions to form a porous and alloy structure(PtCu PNWs).The porous structure could provide efficient channels for electron and mass transfer.Through comprehensive comparison,it was found that ultrasound and precursor ratio played an important role in the formation and regulation of the pore structure.At the same time,the addition of Cu could optimize the catalytic activity through tuning the crystal structure and electronic structure of catalyst.Based on the electrochemical test,the mass activity of PtCu PNWs catalyst reached 0.8 A mgpt-1 at 0.9 V.In addition,the stability test proved that the onedimensional structure and porous structure could significantly improve the stability of PtCu PNWs through strengthening the firmness of the structure.As a result,the activity of the PtCu porous nanowire catalysts retained 91.5%after 10,000 potential cycles of stability test.(3)In order to improve the utilization rate of Pt,a hollow nanoframe structure catalyst with highly exposed active area was prepared.Firstly,PtCu nanoframes(PtCu NF)were prepared by solvothermal method,and then the surface was doped with Mo forming Mo-PtCu NF to improve the stability of the frame structure.Pt segregated to the surface during the doping process to form a PtMo skin.The excess Cu in the frame structure was removed by acid pickling forming A-Mo-PtCu NF/C to make the frame structure more obvious and expose more Pt atoms,which could improve the utilization of Pt.The insertion of Cu changed the crystal structure and electronic structure of Pt,which reduced the oxygen adsorption energy and improved the oxygen reduction performance of catalyst.The Pt electronic property on the surface could be further adjusted via changing the doping amount of Mo.Combining the advantages of highly open frame structure,alloy effect and surface Mo modification,A-Mo-PtCu NF/C exhibited excellent mass activity of 1.3 A mgPt1 at 0.9 V.Moreover,the stability of the catalyst was greatly improved after surface Mo modification,mainly because the external Mo could protect Cu from dissolution and prevent the structure collapse during the catalytic process.(4)So far,a variety of non-precious metal materials have been prepared to replace the current Pt-based catalysts,but the activity of such catalysts under acidic conditions is too low to meet the application requirement.Therefore,a Co,B,and N codoped carbon material(Co,B-MF-900)was synthesized by using melamine formaldehyde gel as a precursor,and was modified with trace amounts of PtCu alloy to prepare ultra-low platinum catalysts with excellent performance under acidic conditions.By investigating the effect of precursor and annealing temperature on the carbon material,it was found that the doping of Co and B could effectively increase the defect amount and produced a variety of active sites of the carbon material,thereby improving the ORR activity.Moreover,the carbon materials annealed at 900℃ exhibited the highest nitrogen content as well as activity.A small amount of PtCu alloy was deposited on the prepared carbon material by the impregnation reduction method forming PtCu/Co,B-MF catalyst.And the characterization found that there was a synergistic coordination effect between the alloy,boron,and nitrogen in the catalyst,which helped to improve the activity of the catalyst.And the catalytic activity of the carbon material was further improved after modified with trace PtCu alloy,and the mass activity of PtCu/Co,B-MF reached 0.52 A mgpt-1 at 0.9 V.The prepared catalyst not only improved the activity but also reduced the cost,which showed a good application prospect.