Preparation Of Multi-metal Nanocrystals By Seeding Method And Study Of Structure-activity Relationship In Fuel Cell

Throughout the development process of energy from firewood to coal to oil,the transformation of energy is closely related to national development.As a major carbon based energy consumer,China still faces a daunting task in achieving the"carbon peak"goal in the next few years.Driven by the"dual carbon"policy,the historical trend of low carbon/zero carbon emission new energy has arrived.Fuel cells focus on carbon reduction and decarbonization,which are effective measures to accelerate energy transformation,such as hydrogen/hydrazine hydrate fuel cells with zero carbon emissions,and low-carbon green alcohol/formic acid fuel cells.Numerous studies have shown that Pt has an appropriate adsorption strength in the volcanic map and is the preferred material for the anode and cathode of fuel cells.However,the shortcomings of high cost,low activity,susceptibility to poisoning,and poor stability greatly limit the further large-scale use of fuel cells.Developing low platinum or non-platinum catalysts with high activity has always been a research focus for researchers.In this thesis,a variety of low platinum/non-platinum electrocatalysts with novel structures were synthesized by seed crystallization method,and their electrocatalytic properties and structure-activity relationships in fuel cells were investigated.The specific research contents are as follows:1.Using copper rich octahedral Pt Cu as the crystal seed,a novel Pt Cu subnanoclusters/Pt/Pt Cu heterogeneous bimetallic nanocrystals electrocatalysts(labeled Pt Cu_1.60)with unique structure was synthesized by epitaxial growth.The surface of Pt Cu_1.60 subnanoclusters is rich in low coordination atoms,exposing more active sites,which is beneficial for improving their activity.The existence of a core-shell structure inhibits the electrochemical dealloying effect of Cu,which is conducive to enhance stability.In acidic media,the mass activity of Pt Cu_1.60 oxygen reduction reaction（ORR）is 1.42 A mg _Pt^-1,8.9 times that of commercial Pt/C.Remarkably,Pt Cu_1.60/C electrocatalyst has achieved 140,000 durability cycles without decreased activity,and it is the catalyst with longest cycles in acidic media.The surface structure of Pt Cu_1.60/C remains stable due to epitaxial growth mode,and the mode effectively prevents surface cluster aggregation and loss of near surface active centers.Structural characterization and theoretical calculations confirm that Pt-rich Pt Cu clusters are beneficial to ORR activity and thermodynamic stability.In practical proton exchange membrane fuel cell（PEMFC）applications,Pt Cu_1.60/C also exhibits high power density and durability without attenuation of current density for up to 100 h（air）/50 h（O₂）.2.Developed a general strategy for visible light induced template synthesis for the first time,using Bi₂Te₃ nanosheets with a thickness of 4.5 nm as crystal seeds,overcoming the differences in physical and chemical properties of different metals and crystal growth limitations,and successfully synthesized a series of ultrathin and large sized multicomponent non-Pt alloy nanosheets,such as Pd Bi Te,Au Bi Te,and Pd Au Bi Te.The compressive strain,lower d-band centers,and the introduction of highly dispersed Au in Pd Au Bi Te nanosheets give Pd Au Bi Te nanosheets high mass activity（MA）,high resistance to carbon monoxide（CO）poisoning,high resistance to methanol interference,and excellent durability in cathodic ORR.In oxygen and air atmospheres,the peak power densities of Pd Au Bi Te nanosheets in methanol fuel cell devices can reach 235.7 m W cm^-2 and 173.5 m W cm^-2,respectively,which are the highest values reported in the literature.The voltage attenuation of Pd Au Bi Te alloy nanosheets and Pt/C is 15.0%and 55.1%during durability test in oxygen.In air,the voltage attenuation of Pd Au Bi Te alloy nanosheets can be negligible.3.Based on the progress of previous work,we optimized our experiments and successfully synthesized ultrathin polycrystalline Pt Ag Bi Te nanosheets assembled from sub-5 nm nanoparticles using a green and simple visible light induction method.It has been found that Pt Ag Bi Te has a unique structure rich in defects such as interfaces,low coordination edge atoms,and twin boundaries,resulting in special electrocatalytic activity.It is a multifunctional anode catalyst and has good catalytic effects on small molecules such as hydrazine hydrate,alcohols,and formic acid.Theoretical calculations show that during the oxidation of hydrazine hydrate,*N₂H₃ is deeply activated and the energy barrier is lowered,which is beneficial to the removal of hydrogen and improves the catalytic activity.In situ infrared spectroscopy confirmed that Pt Ag Bi Te did not produce CO in the oxidation of alcohols and formic acid,which was beneficial for performance enhancement.The fuel cell performance of Pt Ag Bi Te as anode for fuel cell devices was further explored.The peak power density of the direct hydrazine hydrate/methanol/ethanol/ethylene glycol/glycerol/mixed alcohol/formic acid fuel cell reached 532.9/207.3/132.8/129.8/196.9/133.4/240.3 m W cm^-2,while the commercial Pt/C was only 394.7/124.2/42.19/84.5/105.7/83.35/91.57 m W cm^-2.4.Based on the successful exploration of the visible light induced seeding method for the above system,a series of dumbbell-shaped nanocrystals have been prepared by further use dumbbell-shaped Bi Te nanocrystals with regular morphology as seed crystals.The electronic synergistic effect of Pd and Pt,as well as the combined effect of rich defect sites on the surface,endow dumbbell-shaped Pd Pt Bi Te nanocrystals with high mass activity in C1-C3 alcohol oxidation.Among them,dumbbell shaped Pd Pt Bi Te-1 nanocrystals reach 18.32 A mg^-1（methanol）,25.39 A mg^-1（ethanol）,23.18A mg^-1（ethylene glycol）,and 16.78 A mg^-1（glycerol）respectively,which are8.19/13.77（methanol）,10.57/16.31（ethanol）,5.18/10.73（ethylene glycol）,and5.24/7.95 times higher than commercial Pt/C and Pd/C.High mass activity improves the utilization rate of Pt and reduces the use cost,making dumbbell-shaped Pd Pt Bi Te nanocrystals have the potential for direct application to fuel cells.In direct ethanol fuel cells,the peak power density reaches 142.2（oxygen）/55.55（air）m W cm^-2,while commercial Pt/C is only 66.07（oxygen）/39.09（air）m W cm^-2.