Strong Metal-Support Interaction Of Pt-Based Alloys And Nitrogen-Doped Carbon Matrix To Boost Oxygen Reduction Reaction Performance For Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells（PEMFCs）are environmentally friendly electrochemical devices that convert chemical energy directly into electrical energy.Due to zero carbon emission,high power density,high energy conversion efficiency and low operation temperature,PEMFCs are considered to be one of the most promising energy conversion devices.However,the sluggish oxygen reduction reaction（ORR）kinetics in the cathode of PEMFCs,high performance catalysts are required to accelerate the process of ORR.Currently,the cathodic catalyst of MEA is still dominated by commercial Pt/C.Due to the low reserves,high price,and the poor stability of Pt/C,which seriously limited the large-scale commercial application and development of PEMFCs.This thesis focuses on the problems and challenges of ORR catalysts.Based on strong metal-support interaction,alloying/ordering of transition metals with Pt,and the high graphitization of carbon carriers,which significantly improve the electrochemical performance of the electrocatalysts.The structure and electrochemical performance of the carbon supported Pt-based alloy catalysts were systematically characterized,and the prepared electrocatalysts were further applied in PEMFCs.The details are as follows:1.The copper species immobilized on NC（Cu NC）were firstly prepared by a molten salt-assisted pyrolysis process.Subsequently,the Cu NC and Pt（acac）₂ were dispersed into and reduced by oleylamine.Afterward,the obtained product was annealed at 700°C to produce the Pt Cu nanoalloys rooted in nitrogen-doped carbon nanosheets（Pt Cu NC-700）as highly efficient ORR catalysts.Experimental and theoretical calculations indicated that the N atoms of the nitrogen-doped carbon carrier and the Cu of Pt Cu nanoalloys modulate the d-band center of Pt,synergistically optimize the adsorption free energies between oxygen intermediates and active sites.Therefore,decreases the energy barrier of the rate-determining step and significantly enhances the intrinsic ORR activity of Pt Cu NC-700.Meanwhile,X-ray absorption fine structure spectrum showed the Pt-N bonds in Pt Cu NC-700,which further enhances the stability of the catalyst.As a result,Pt Cu NC-700 exhibited high mass activity(0.45 A mg _Pt^-1)and specific activity(1.05 m A cm^-2),which were 3.7 and4.2 times higher than those of Pt/C,respectively.After 30000 cycles in the accelerated durability test,half-wave potential of Pt Cu NC-700 drops by only 17 m V,which is less than that of Pt/C.Furthermore,Pt Cu NC-700 was employed as the cathode catalyst in H₂/air PEMFCs,which shows peak power density of 929.7 m W cm^-2.Even after 30000 cycles,the Pt Cu NC-700-based device still exhibits power density of820.8 m W cm^-2,which is also superior to that of Pt/C.2.Highly graphitized nitrogen-doped carbon nanoflowers（GCo NC）with Co-N_xsites were prepared as carbon carriers for Pt-based alloy catalysts by an Mn O₂-assisted pyrolysis strategy.Afterwards,ordered Pt Co intermetallic compounds on GCo NC（O-Pt Co@GCo NC）were prepared as efficient and stable ORR electrocatalysts using wet-impregnation and high temperature thermal reduction methods.The obtained GCo NC with hierarchical porous structure and high specific surface area is beneficial to uniform dispersion of nanoparticles and mass transfer.According to the Raman spectra,X-ray diffraction（XRD）patterns and X-ray photoelectron spectroscopy（XPS）results,the GCo NC shown highly graphitized structure.Hence,it was applied as carbon carrier for carbon-supported Pt-based catalysts,which can effectively improve the corrosion resistance of the catalyst,thus enhancing the stability of electrocatalyst.Meanwhile,the ordered Pt Co intermetallic compounds further improve the catalytic activity and stability of the electrocatalyst compared to the disordered Pt Co alloy.The electrochemical tests demonstrated that O-Pt Co@GCo NC possess excellent catalytic activity and stability.Specifically,it shows a half-wave potential of 0.915 V.After 70,000 cycles between 0.6 and 1.0 V,the mass activity and specific activity decreased by 26.5%and 14.6%,respectively.After 10,000 cycles between 1.0 and 1.5 V,the mass activity and specific activity decreased by 10.8%and 13.0%,respectively.