Tuning Surface/near-surface Structure Of Platinum-based Nanocatalysts And Their Performance Of Electrocatalytic Reactions In Fuel Cell

Platinum is the most active and durable catalyst in proton exchange membrane fuel cells(PEMFCs).But the scarcity and high price of platinum hinder the commercial development of PEMFCs.Improving the intrinsic activity and stability of platinum is conducive to reducing the amount of platinum and promoting the commercialization of proton exchange membrane fuel cells.Based on the results of metal single crystal model catalysts,it is recognized that the activity and stability of catalysts depend on their surface/near-surface structure.As such,how to optimize the surface/near-surface structure of catalysts has become the hotspot in electrocatalysis.In the synthesis of catalysts,a large amount of surfactant is usually introduced as the structure-directing agent and stabilizer to produce highly active facets.However,the removal of surfactants is rather difficult,and will cause changes in the surface/near-suriace structure of the catalyst,whieh is not eonducive to the acquisition of the relationship of surface structure and activity.Therefore,it is a challenge to optimize the surface/near-surface structure of the catalyst through clean synthesis.In the long-term research,our laboratory focus on the electrochemistry and electrocatalysis of metal monocrystals with clear atomic arrangement surface structure.This paper is devoted to explore the methods of tailoring surface/near-surface structure of Pt-based nanocrystals catalysts with high activity and high stability,especially without surfactants.The main results are as follows:1.Monodisperse Pt concave cubes with uniform size were successfully prepared by periodic adsorption and desorption of hydrogen and oxygen.The Pt concave cube was characterized by TEM and SEM,and the crystal index of its surface was between(210)and(310).The results show that Pt concave cube not only has high electrocatalytic activity,i.e.,5.6 and 6.4 times of electrocatalytic activity relative to commercial Pt/C towards ethanol and formic acid,respectively,but also has high stability.The experimental results show that hydrogen adsorption is the key factor for the formation of concave structure.The DFT results show that on clean Pt surfaces,Pt atoms are preferentially deposited at platform sites,while on hydrogen-saturated adsorbed surfaces,Pt atoms are preferentially deposited at edge sites.2.PtCu alloy nano-octahedron of 10 nm was synthesized by adding I-by solvent heat method.Due to easy removel of I-and highly active(111)facets,the obtained PtCu nanocrystal possess very high ORR activity.At 0.9 V,the specific activity and mass activity of PtCu/C are 4.25 m cm-2 and 1.20 mA gPt-1,respectively,21.3 and 8.6 times higher than that of Pt/C,respectively.After 10,000 cycles of accelerated test,the loss of mass activity is 32%.3.The ORR activity of PtCu/C decreased with the doping of trace Au,but the stability was significantly improved.The specific activity and mass activity of PtCuAu0.0005/C was 3.88 mA cm-2 and 1.00 mA gPt-1,significantly lower than those of PtCu/C.After 10,000 cycles of accelerated experiments,the mass activity of PtCuAu0.0005/C was 0.92 mA gPt-1,with a loss of only 8%.After 20,000 cycles of accelerated experiments,only 17%of the activity was lost.It is suggested that the main role of trace Au is to inhibit the dissolution and migration of low-coordination Pt atoms,retain the highly active(111)site,maintain the octahedral morphology,and sustain the high activity and stability of the catalyst.4.The ORR activity and stability can be improved by introducing phosphorus into the near surface of Pt nanopartides via a simple wet chemical method.Spherical aberration electron microscopy revealed that the surface of Pt was severely distorted after phosphorus doping.By comparing the results of XPS and ICP-AES,it is shown that phosphorus is enriched in near-surface.Angular resolution XPS indicates that the phosphorus partly penetrated into the Pt particle.After phosphorus doping,the area activity and mass activity of PNs-Pt/C were increased by 9 times and 7.1 times respectively.PNS-Pt/C also performed significantly better than commercial Pt/C on fuel cells.DFT theoretical calculation reveals that phosphorus in the subsurface layer can induce surface Pt atoms to sink and form concave sites on the surface.These concave sites have weak adsorption on OH and are the active sites of ORR.In the thesis,we systematically studied the methods of regulating the surface/near-surface structure of Pt based nanocatalysts,and evaluate their activity and stability in fuel cell electrocatalytic reactions.These results highlight the important role of the surface/near-surface structure of naocatalysts on their catalytic performance,which provide a guidance for the design and preparation of fuel cell nano-catalysts with high activity and stability.