Investigation Of Oxygen Reduction Reaction Catalytic Performance Of Chemically Ordered Platinum-based Catalysts For Fuel Cells

Due to its many advantages such as zero emission and low operating temperature,proton exchange membrane fuel cell(PEMFC)is expected to become a new type of energy conversion device that solves the problems of rapid depletion of fossil energy and environmental pollution caused by petrochemical energy depletion.However,the oxygen reduction reaction(ORR)kinetics on the cathode of fuel cell is slow and requires a suitable catalyst to increase its reaction rate.At present,the practical application of PEMFC still needs a catalyst with a high noble metal platinum(Pt)loading to ensure sufficient output power of the fuel cells.However,the scarce and expensive of Pt resources for current used catalyst greatly limit the widespread use of fuel cells.In addition,the insufficient stability of current commercial Pt/C catalyst in the operation of the fuel cells affects the output efficiency and lifetime of the fuel cells.Therefore,it is an urgent task to promote the commercialization of fuel cells by designing ORR electrocatalytic materials widi low Pt content,high activity,and high stability.Considering the key scientific problems of catalytic materials existing in the commercialization of PEMFC technology,this thesis research work aims to develop high-performance and low-platinum catalysts and to reveal their catalytic mechanisms for oxygen reduction on the basis of electrochemistry,catalytic chemistry,nanomaterials,and physical chemistry theories.In this work,we designed and synthesized a series of Pt-based catalysts with chemically ordered structure and developed a batch controllable preparation process for such catalyst to provide theoretical basis and technical support for the large-scale production of high-performance and low-platinum catalysts for fuel cells.The research work of this paper includes the following four aspects.1.In order to control the metal particle sizes of the chemically ordered structure catalysts,we synthesized precursors of carbon-supported platinum-iron alloy catalysts(D-Pt3Fe/C and D-PtFe/C)(chemically disordered)with different metal ratios via microwave assisted reduction.Then,the chemically disordered alloy structure was subsequently transformed to chemically ordered(O-Pt3Fe/C and O-PtFe/C)by optimizing the high temperature calcination conditions in an inert gas environment.The physical structure characterizations showed that the alloy nanopartieles(NPs)of-Pt3Fe/C and O-PtFe/C were small in size(4Y6 nm)with uniform distribution on the carbon support;the lattice parameters of O-Pt3Fe/C and O-PtFe/C are smaller than that of JM Pt/C.XPS electronic structure characterization showed that the binding energy peaks in O-Pt3Fe/C and O-PtFe/C were negatively shifted compared to those of pure Pt due to the formation of high degree of alloying chemically ordered Pt-Fe alloy formed between Pt and Fe atoms.The electrochemical performance tests showed that the catalytic ORR activities of O-Pt3Fe/C and O-PtFe/C were higher than that of commercial JM Pt/C catalyst and the catalytic activity of O-PtFe/C was higher than that of O-Pt3Fe/C.The increased catalytic ORR activity for these catalysts is ascribed to their chemically ordered structure and their altered electronic properties.In addition,the calcination temperature has a significant effect on the ORR catalytic performance of the catalyst,and 700? is the preferred temperature for preparing the highly active chemically ordered Pt-Fe catalyst.2.In order to reduce Pt usage and improve the surface stability of the active component of Pt-based catalysts,carbon supported Pt3Co(Pt3Co/C)and PtCo(PtCo/C)NPs were synthesized via organic colloid method and then controllably calcined for the formation of chemically structure with Pt-rich surface.By adjusting the programmed calcination temperature and carrying out electrochemical performance tests,we found that the catalyst calcined at 700? has the best catalytic ORR activity.The catalytic ORR mass activities of Pt3Co/C-700 and PtCo/C-700 are 0.4978 and 0.4092 A/mgpt,respectively,which are significantly higher than those of commercial JM Pt/C.The area of hydrogen adsorption peak in JM Pt/C CV curves decreases gradually with the increase of potential cycle scans.However,the CV curves of the two kinds of Pt3Co/C-700 and PtCo/C-700 catalysts at 1000,3000 were over-lapped,indicating surface structure stability of these two chemically orderly catalysts.We conclude that the surface of the metal particles of these two chemically ordered catalysts are stable Pt layer and interact strongly with the carbon support to avoid the loss of catalytic active sites.Quantitative analysis showed that the loss of catalytic ORR activity of Pt3Co/C-700(8.60%)and PtCo/C-700(38.65%)is significantly less than that of JM Pt/C(55.22%),indicating enhanced catalytic ORR stability.3.In order to control the surface microstructure of chemically ordered structure Pt-based catalysts and to explore their ORR catalytic mechanism,we synthesized carbon supported chemically ordered PtFe NPs(PtFe/C)via improved microwave assisted reduction and subsequent programmed calcination;the surface of PtFe/C was doped with transition metals Au and Cr to obtain two kinds of doped catalysts of Au-PtFe/C and Cr-PtFe/C,respectively.Structural characterization using EDS associated with high-angle annular dark-field scanning transmission electron microscopy showed that Au and Cr were successfully doped to the surface of chemically ordered PtFe NPs.The XPS electronic structure characterization showed that the binding energies of Au-PtFe/C and Cr-PtFe/C were negatively shifted after the surface doping compared to PtFe/C,indicating weakened interaction between these catalysts and the oxygen-containing intermediate species,which was favorable for the release of Pt active sites during the catalytic reaction so as to improve its catalytic ability.Electrochemical tests showed that the electrochemical active area(ECSA)of Au-PtFe/C and Cr-PtFe/C was higher than that of PtFe/C;the ORR catalytic activity of the synthesized catalysts was much higher than that of commercial JM Pt/C(0.158 A/mgPt)and increased in the following order of PtFe/C(0.314 A/mgPt)<Au-PtFe/C(0.414 A/mgPt)<Cr-PtFe/C(487 A/mgPt).Among them,the catalytic ORR mass activity of Cr-PtFe/C was higher than the technical index(0.44 A/mgPt)proposed by the US Department of Energy(2020)for fuel cell catalysts,showing excellent catalytic ability.In addition,Au-PtFe/C and Cr-PtFe/C exhibit higher catalytic stability than JM Pt/C in accelerated cycle scan test.The reasons for the improvement of the catalytic properties of Au-PtFe/C and Cr-PtFe/C can be attributed to the increase of the catalytic active sites and the adjustment of the surface electronic structure.4.In order to further promote the practical application of high-performance chemically ordered catalyst for fuel cell,we propose a rapid synthesis program of chemically ordered Au-Pt-Fe high efficiency and low platinum catalyst by combining efficient synthesis of catalyst NPs of microwave assisted reduction process and programmed calclination process;10.0 g of chemically ordered high efficiency and Iow-Pt Au-PtFe/-H catalyst with stable structure and performance was controllably prepared in batch preparation.The electrochemical tests showed that the prepared Au-PtFe/-H reached an ORR catalytic mass activity of 0.66 A/mgPt,and displayed superior catalytic stability than commercial JM Pt/C in stability test of 5000 potential scans.The preparation period of chemically ordered Au-PtFe/-H catalyst synthesis strategy is short and the process is simple and robust,and the obtained catalyst showed excellent catalytic ORR performance.The single-cell performance test found that the maximum output power of single-cell using Au-PtFe/-H(496 mW/cm2)as the catalyst is higher than that of single-cell using JM Pt/C(425 mW/cm2)as the catalyst,showing a certain application potential.This research provides certain theoretical and technical reference value for large-scale production of high-effieiency low-Pt loading catalytic materials.