Strain-controlled Oxygen Reduction Performance Of Ptni-based Nanocatalysts

With the increasingly severe problems of the global environmental pollution and energy crisis,the development of green,safe and efficient energy storage and conversion technology is imminent.Integrating the advantages of high specific power,clean,pollution-free and fast start-up at low temperatures,proton exchange membrane fuel cells（PEMFCs）has broad application prospects.As the core of commercial PEMFCs,Pt-based catalysts still face many problems,such as shortage of Pt resources,high price and poor stability.Therefore,the development of oxygen reduction reaction（ORR）catalysts with low Pt loading and high intrinsic performance has always been a research hotspot.At present,numerous studies have been mainly tuning the d-band center of Pt through various effects,optimizing the adsorption/desorption free energy of oxygen-containing species to promote the ORR rate.In the process of actual synthesis,the crystal structure,size and morphology of catalysts are always changing.Understanding and coordinating the interaction mechanism between the electronic field and microstructure will help guide the synthesis of high-performance ORR catalysts.In this thesis,PtNi-based oxygen reduction catalyst is taken as the research object.And WO_x-PtNi NWs,CS-Pt_xNi_yW_z NR,CS-Pt_xNi_yW_z NF,CS-Pt_xNi_yW_zNP,and N,P-PtNi W NP are prepared,which are carried out by surface modification,strain regulation and anionic doping,respectively.The relationship between interface and internal strain,electron and microstructure,and activity/stability of PtNi-based catalysts are studied by means of various characterization methods.The main contents of this work are as follows:（1）Surface modification of PtNi alloy nanowires by introducing WO_x with unsaturated coordination.On the one hand,the overflow and synergistic effect between WO_x and Pt can effectively regulate the free energy of adsorption/desorption of reaction intermediates on Pt surface and,accordingly,promote the reaction rate.On the other hand,WO_x coordinated with Pt and Ni on the surface can effectively stabilize and avoid the migration/agglomeration of unstable metals during the reaction process,hence improving the stability of the catalyst.The study also found that the activity of the catalysts showed a volcano trend with the content of tungsten oxide.And the highest mass activity（MA）of WO_x-（0.25）-PtNi NWs/C is up to 0.85 A mg_Pt^-1,which is 6 times that of TKK Pt/C.After 30k cycles of accelerated durability tests（ADTs）,its activity decreased only 23.89%.Further controlling the content of WO_x on the surface,the facets of Pt could be directionally exposed to achieve the ORR-performance regulation of PtNi alloy NWs.（2）Through lattice mismatch and strain induction,Pt_xNi_yW_z ternary alloy nanorods（NR）,nanoflowers（NF）and nanoparticles（NP）with core-shell structure were synthesized.Their nanostructure and morphology,and the strain effect between the inner and outer shell layer can be adjusted microscopically.The ORR activity and stability of CS-Pt_xNi_yW_z-NR,NF,and NP show different regularities.The CS-Pt_xNi_yW_z-NP has the highest MA with 1.32 A mg_Pt^-1,and CS-Pt_xNi_yW_z-NF shows the best stability with 25.35%MA loss after 30k cycles of ADTs.The relationship between the electronic structure,microstructure and ORR performance of catalyst is illustrated.（3）By introducing heteroatoms during the calcination process to synergize with the metal and carbon support at the same time,the dissolution/shedding of the ex-situ growth catalysts during the catalytic process can be relieved,and the catalytic performance can be greatly improved.Among them,the d-band center of Pt could be regulated by doping N atoms,which produces compressive strain on the surrounding Pt thus improving the activity of catalyst.The MA of N-PtNi W reaches 2.38 A mg_Pt^-1,which is 17 times as high as the commercial Pt/C.Doped P combines with Pt and Ni to form Pt P₂and Ni P₂,which can alleviate the dissolution and agglomeration of unstable metals to a certain extent during the catalytic process,thereby improving the stability of the catalyst.P-PtNi W can still retain 70%activity after 50k cycles of ADTs.Combining the advantages/disadvantages of N-and P-doping,the ORR activity and stability of the catalyst could be well balanced over controlling the incorporation of N and P.