Controlled Synthesis Of Pt Based Intermetallic Compounds Catalysts And Their Catalytic Performance Towards Oxygen Reduction Reaction

Proton exchange membrane fuel cells(PEMFCs)are widely regarded as one of the most promising clean energy technologies due to their high energy conversion efficiency,ease of operation and zero/low emissions.Unfortunately,the current electrocatalysts suffer from high platinum loading and insufficient stability,which limits the commercialization of fuel cell technologies.It is,therefore,highly desirable to develop new,highly-efficient and long-life electrocatalysts.Intermetallic compounds(IMCs),featuring atomically ordered structures,have attracted increasing research attention over the last decade,because of their exceptional electronic and structural properties.In contrast to disordered Pt-M alloys(solid solution),ordered Pt-M intermetallic compounds have well-defined stoichiometries and provide much better control the local geometry of metal atoms,resulting in uniform distribution of active sites on the same surface plane.More importantly,the stronger lattice strain of Pt on intermetallic surfaces endows Pt-M intermetallic catalysts with better activity and structural/chemical stability.To promote metal atom arrangement and form atomically ordered intermetallic phases,high temperature annealing is generally a necessary process,which often results in serious agglomeration and poor size distribution.To address this issue,effective synthetic strategies should be explored.In addition,it is of great significance to study new and more effective strategies to further improve the catalytic performances of Pt-based intermetallic catalysts such as composition tuning.Pt-M intermetallic could preserve its pristine crystal structure when metal M was partially substituted by another suitable metal.Such composition flexibility can enable the optimization in their catalytic performance.Based on this,this thesis focus on the studies of size control and composition tuning of Pt-based intermetallic catalyst.The main contents and achievements are as follows:(1)A controllable synthesis strategy using MOF derived mesoporous carbon as both confined template and carrier combined with freeze drying technique was proposed to synthesize IMCs for the first time.Porous carbon materials with mesoporous structure were prepared by controlled pyrolysis using ZIF-8 as precursor.Then,ZIF-8-derived mesoporous carbon(DMC)was impregnated with a solution containing Pt and Co precursors.After freeze-drying and heat treatment in a reducing atmosphere,the Pt Co binary intermetallic catalyst with porous carbon as the support was prepared.XRD and TEM analysis showed that the nanoparticles developed by this strategy were intermetallic phase with an ultra-small size of 3 nm,which was much smaller than that of the intermetallic compounds prepared using carbon black as support(6 nm)and other reported methods(>5 nm).In addition,when changing the type of MOF derived carbon and the element of intermetallic,the synthesized ordered particles remained precisely controlled at 3 nm,which proved the universality and effectiveness of the synthesis method for particle size control,and provides an important solution and example for the synthesis of intermetallic with small particle size.(2)XRD and HAADF-STEM characterization results showed that the nanoparticles of Pt-Co two-component intermetallic catalyst prepared by the method described in(1)had a typical ordered structure of Pt₃Co intermetallic phase.The Pt₃Co/DMC-F catalyst had superior activity and stability/durability.The ORR activity reached 0.83A mg^-1Pt,which was5.2 times that of the commercial Pt/C catalyst and 4.6 times that of the carbon black supported Pt₃Co intermetallic catalyst.After 5000 ORR cycles,the activity attenuation of Pt₃Co/DMC-F was only 13.3%,while that of Pt/C catalyst is 40.6%.Furthermore,we have evaluated Pt₃Co/DMC-F catalyst on H₂/air PEM single cell.It was found that the performance of the membrane electrode assembly(MEA)with a Pt₃Co/DMC-F cathode(1300 m A cm^-2 at 0.6 V)was about 40%higher compared to the MEA with a commercial Pt/C cathode.After 20000cycles of accelerated stress test(AST),the MEA performance attenuation of Pt₃Co/DMC-F was only 36%of that of commercial Pt/C,indicating that ordered Pt₃Co/DMC-F catalyst had excellent durability far superior to commercial Pt/C.HRTEM characterization of Pt₃Co/DMC-F and commercial Pt/C before and after AST cycles indicated that the morphology and particle size of Pt₃Co/DMC-F remained almost unchanged after AST while Pt/C catalyst showed significant agglomeration and growth.These results emphasize that the porous carbon carrier can effectively inhibit the migration,agglomeration and abscission of the active particles during the electrochemical reaction process.(3)We have firstly synthesized an ordered Pt-Co-Ti core-shell electrocatalyst possesed a 3 nm Pt₃Co_0.6Ti_0.4 intermetallic as core and a 2-3 atomic layers of Pt as shell,by partial substitution of Co by Ti in the ordered Pt₃Co.The results showed that the partial replacement of Co by Ti can significantly improve the durability of the catalyst.By optimizing the composition,we found that the Pt₃Co_0.6Ti_0.4/DMC catalyst with 40%Co replaced by Ti has the best performance.The ORR mass activity of Pt₃Co_0.6Ti_0.4/DMC reached 7.8 times that of the commercial Pt/C catalyst and 1.8 times that of the binary Pt₃Co/DMC catalyst.Furthermore,Pt₃Co_0.6Ti_0.4/DMC presented excellent performance on H₂/air PEM single cell measurement.It was found that the current density of the MEA with a Pt₃Co_0.6Ti_0.4/DMC cathode reached 1350 m A cm^-2 at 0.6 V,which was 1.5 times that of MEA with a commercial Pt/C cathode.After 30000 AST cycles,the ECSA attenuation of MEA with a Pt₃Co_0.6Ti_0.4/DMC cathode was only 28%of that of MEA with a commercial Pt/C cathode.The further characterization and DFT calculation revealed that the introduction of a small amount of Ti could precise regulate the surface electronic structure and the position of the d-band center of the catalyst,and modify the oxygen binding energy to the optimal value.(4)Based on the above research,we further studied the Pt-Co-Y ternary catalyst prepared by substituting a small amount of Co by Y in Pt₃Co intermetallic.It was found that partial substitution of Y could effectively improve the performance of the catalyst,in which the Pt₃Co_0.8Y_0.2/DMC catalyst with 20%Co replaced by Y showed the best performance.The mass activity of Pt₃Co_0.8Y_0.2/DMC catalyst(ORR activity,1.98 A mg^-1 Pt at 0.9 V)was 10.4times that of commercial Pt/C catalyst and 2.4 times that of the Pt₃Co/DMC.After 20000ORR cycles,the mass activity attenuation is only half of that of commercial Pt/C.XPS analysis showed that the addition of Y could reduce the binding energy of Pt 4f by 0.2 e V relative to Pt₃Co,which improved the electronic structure of Pt and enhanced the catalytic performance.