Preparation And Performance Research Of Pt-Based Ternary Catalyst For Fuel Cell

Proton exchange membrane fuel cell(PEMFC)is an effective energy transformation application that can convert hydrogen into electricity through electrochemical reactions with multiple advantages.However,the kinetics of oxygen reduction reaction(ORR),a cathodic reaction of PEMFC,is relatively slow,which requires the Pt-based catalysts to accelerate.Due to the scarcity and high cost of platinum(Pt),it is difficult to commercialize fuel cells.And the current commercial Pt/C catalysts are facing the situation of low performance,which cannot fully meet the needs of PEMFC vehicles.Moreover,the scale effect is not obvious for Pt-based catalyst because the cost mainly come from the expensive materials.Only by continuously deepening the understanding of the ORR mechanism,designing and controlling the catalyst structure at the molecular and atomic level,and improving the catalytic performance of the ORR catalyst,can the usage of platinum be reduced.For solving the problems of insufficient electrochemical performance,metal dissolution,unstable structure and high cost of fuel cell catalysts,after the analysis of extensive literature and the guidance of electrocatalytic theory,we prepared four ternary Pt-based catalysts through core doping,sub-surface doping,homogeneous doping and surface doping based on the binary catalyst,and studied the modification and performance control of the doped elements on the catalyst.Moreover,by controlling the morphology and structure of the catalyst,the parameters such as the crystal structure,electronic properties and stress distribution of the catalyst is optimized to improve the utilization of Pt,optimize the oxygen adsorption energy and the corrosion resistance of the catalyst.Through the research,we hope to prepare the low-cost,high-activity,and long-life ternary Pt-based catalysts,and improve the development of ORR catalyst through basic research.The research includes four kinds of ternary catalysts,as follows:1.To solve the high cost of PEMFC catalyst and improve the utilization of Pt atoms,the CuFe@Pt/C ternary catalyst with low-priced metal alloy as the core and Pt atoms as the dense shell was prepared by seed-mediated growth method.The relationship between the core metal and the Pt shell was studied by doping Fe atoms into the core of Cu@Pt and changing its composition ratio.From the characterization,it can be found that the doping of Fe atoms into Cu crystal can shrink the lattice and change the electronic properties of Pt shell.These can tune the adsorption capacity of the catalyst to the reaction intermediate through the strain effect and the ligand effect,thereby improving the ORR activity.Furthermore,electrochemical tests show that the mass activity of Cu0.75Fe0.25@Pt/C reachs 0.91 A mgpt-1@0.9V vs.RHE,which is about 5 times than that of commercial Pt/C catalysts.For the durability,the protective effect of the Pt shell and the alloying of the core metal alleviate the dissolution of transition metals and improve the electrochemical stability of the catalyst,which show a good application prospect.2.In order to avoid uneven deposition during the seed-mediated growth,displacement reaction was used to prepare a sub-surface Au-doped Co@PtAu/C ternary core-shell structure catalyst.The ratio of precursor addition determines the degree of lattice shrinkage and electronic properties of the Pt shell which is realated to the activity.The sub-surface Au atoms will not cover the active sites on the surface,so that the stability is improved without the loss of ORR activity.combined with the comprehensive advantages of activity and stability,the Co2@PtAu0.05/C catalyst the most practical value among the prepared catalysts.The mass activity reaches 0.55 A mgpt-1@0.9V,which is about 3 times than that of commercial Pt/C catalysts.and the half-wave potential only decrease 9 mV after 10000 cycles of stability test.This strategy reduces the usage of precious metals,which has an advantage of cost.3.In addition to the above-mentioned doping research on core-shell structure catalysts,we also studied the homogeneous doping of alloy catalysts.To alleviate the agglomeration of alloy nanoparticles during the ordering process at high temperature,small-sized monodisperse PtCoAu ternary ordered alloy catalyst was prepared by using PtAu@CoO as precursor and introducing the vacancies to lower the ordering temperature.The surface segregation of Au and the reduction of CoO both increase the number of vacancies in the crystal,which can reduce the barrier of atom migration and promote the ordering process to reduce the annealing temperature.Co and Au atoms coordinately tune the ligand property and crystal structure of Pt,which enhance the intrinsic activity for ORR.From the Electrochemical tests,it can be found that the mass activity and specific activity of the L10-PtCoAu0.1/C catalyst reach 1.08 A mgPt1@0.9Vand 1.38 mA cmPt-2@0.9V,which are 6 times and 5 times higher than commercial Pt/C catalysts,respectively.And the strategies such as alloying effect,ordered structure and Au doping effectively alleviate the dissolution of Co and improve the stability of the catalyst.4.The activity of alloy catalysts can be further improved by tuning their morphology and crystal facet.But the metals,especially transition metals present on the surface,are easy to dissolve,resulting in relatively poor stability.In this study,PtCo nano wires with abundant humps on the surface were obtained by the oleylamine method,and then the surface of the nanowires was doped in situ with Au atoms by the replacement reaction and annealing process.The mass activity of as-prepared PtCoAu1.5 nanowire catalyst(Au/Pt atomic ratio is 1.5%)reachs 1.94 A mgpt-1@0.9V,about 11 times than that of commercial Pt/C catalyst,mainly due to the the comprehensive advantages of nanowire structure,high-index crystal face,and alloy effect.The surface Co atoms are replaced by trace Au atoms through doping process,which can keep the the nanowire from dissolved and structural damaged,improving the electrochemical stability of the catalyst.In short,through the core doping,sub-surface doping,homogeneous doping and surface doping,as well as the control of the morphology and structure for the Pt-based binary catalyst,a series of highly active and stable fuel cell ternary low-Pt catalysts have been synthesized by weakening the oxygen affinity and reducing the metal leaching.The regulation mechanism of the doping effect on the structure and performance of the catalyst has been revealed to develope the basic research of fuel cell catalysts.