ZIF Assisted Synthesis PtCo Based High Performance ORR Catalyst For Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cells（PEMFC）have emerged as a promising new generation of mobile energy storage and conversion devices after lithium batteries.However,the cathode oxygen reduction electrochemical reaction kinetics are relatively slow,and Pt-based oxygen reduction catalysts have long been dominant and require the consumption of rare and expensive Pt precious metals,which affect their service life due to leaching of alloying elements,growth and agglomeration of particles and corrosion of carbon support during long-term service.In order to reduce Pt usage,increase cell power density and improve fuel cell service life,the development of Pt-based catalysts with high Pt exposure area,high intrinsic activity and a composition and structure with high cycling stability is a key factor.By analyzing internal causes of construction of high performance Pt-based catalysts,we have identified a comprehensive set of factors that need to be met:highly dispersive loading of Pt and Co,adequate alloying and ordering,size confinement and the resistance to sintering and agglomeration,pore structure with hierarchical pore size distribution and smooth mass transfer,high graphitized carbon support,good electrical conductivity and high stability,and a relatively simple,controlled and scalable production process.ZIF（imidazolium-based metal organic framework）is a class of materials with a large number of meso-micro pores,embedded with highly dispersed metals and is well suited to the requirement above.We have therefore introduced ZIF materials into the preparation of PtCo-based oxygen reduction catalysts,organically integrating the needs of Pt dispersion,alloying,size confinement,pore creating and graphitization into a relatively simple,controlled and scalable process,and have prepared nano-porous PtCo nanoframes,ultra-small PtCo nanoparticles,small-sized highly ordered intermetallic PtCo Zn nanoparticles and the self-dispersed,one-pot,one-step ordered L10-PtCo,which have good oxygen reduction activity and stability after half-cell and full-cell tests,providing a promising strategy for the development of high-performance oxygen reduction catalysts.The main content are as follows.Chapter 2 reports a strategy for the successful synthesis of novel PtCo nanoframeworks（PtCo NFs）by one-pot mechano-chemical methods,featuring Pt-based 3D nanoframes with Pt-skin surfaces and abundant defect sites,and nitrogen-doped carbon with high specific surface area and hierachical pore structures.The prepared Pt-skin Pt_1.8Co NFs-HP exhibited a mass activity of 1.73A mg_Pt^-1and a specific activity of 1.94 m A cm_Pt^-2 in a 0.1 M HCl O₄ electrolyte at0.9 V（vs RHE.）,which is 9.25 and 5.6 times higher than the standard Pt/C catalyst（TKK Pt/C,47.6 wt%）.In addition,after 30,000 accelerated durability tests（ADTs）,the mass activity（MA）of Pt-skin Pt_1.7Co NFs-HP remained with 0.531A mg_Pt^-1,which is 2.83 folds of the initial value of the commercial Pt/C.This enhancement can be attributed to the high electro-chemical active surface area,the three-dimensional open structure,the step-atom-rich Pt-skin surface and the smooth mass transfer with a hierachical porous carbon structure for smooth mass transfer.In Chapter 3,a method for depositing highly dispersed,self-confined PtCo alloys on Co,N co-doped mesoporous carbon（PCN-MC）is developed by a facile dual-template confinement strategy.Ultrafine PtCo alloys of 2.7 nm with 2-3atomic layers of Pt-skin were obtained by exploiting size confining effect of Zn in the bimetallic ZIF and the Mg（OH）₂ template.By adjusting the Co/Zn feeding ratio in the bimetallic ZIF to 8/7,the degree of alloying and nanoparticle size were optimised and excellent ORR activity was achieved,with a mass activity（MA）of up to 0.956 A mg_Pt^-1 in 0.1 M HCl O₄,which is approximately 7.5 times higher than that of commercial Pt/C.In addition,significant durability was achieved after30,000 cycles between 0.6 and 1.0 V,with an initial MA retention of 81%（relative to RHE.）.These characteristics were also validated by H₂-air fuel cell tests,achieving a combination of mass activity,power density and durability.This strategy provides a viable route for the large-scale synthesis of highly dispersible PtCo-alloy catalysts.In Chapter 4,as intermetallic ordered PtCo is effective for high oxygen reduction reaction（ORR）activity and stability.However,preparing small-sized,highly ordered Pt M alloys is still challenging.Herein,we report a controlled two-stage confinement strategy,in which highly ordered PtCo Zn/NC nanoparticles of 5.3 nm size were prepared in a scalable process.The contradiction between the high ordering degree with the small particle size as well as the atomic migration with the space confinement was well resolved.An outstanding PEMFC performance was achieved for L1₀-PtCo Zn/NC with a high mass activity（MA）of 1.21 A mg_Pt^-1 at 0.9 V_{i R-free},80.1%MA retention after 30k cycles in H₂-O₂ operation,and a high mass-specific power density of 8.24 W mg_Pt^-1in H₂-Air operation with a slight loss of cell voltage@0.8 A cm^-2 of 28 m V after 30k cycles.The high performance can be ascribed to the high Pt area exposure,the enhanced Pt-Co coupling,and the prevented agglomeration in the mesoporous carbon wall.Overall,this strategy may contribute to the commercialization of fuel cells.In Chapter 5,we modified the conventional impregnation method for the preparation of intermetallic ordered PtCo catalysts,as to overcome the precursor segregation issue in the solvent evaporation stage through a Pt,Co organic precursor self-diffusion mechanism.As a result,uniformly distributed ordered L1₀-PtCo alloys of less than 6 nm can be easily obtained in a one-pot,one-step process.Due to the highly dispersion of the alloy,high Co/Pt ratio and the enhanced electronic coordination effect between the uniformly distributed Pt and Co atoms,o-PtCo achieves an enhanced ORR mass activity of 0.54 A mg_Pt^-1 at 0.9V,4.9 times that of commercial Pt/C,as well as an excellent durability after 30 k cycles as demonstrated by ADT testing.Comparative experiments revealed the essence for the introduction of 3-methylpyridine,which aided the diffusion and re-dispersion of the precursor during the low temperature calcination stage and mitigated precursor segregation without residual additional carbon shells.The mechanism of self-dispersion was then further characterised by non-in situ XRD,TG and UV-Vis characterizations:the properties of the coordination effect,the affinity of the carbon support,the low melting point and the suitable volatility characteristics were essential for the dispersion mechanism.