Preparation Of Carbon Materials Supported Palladium Nanocatalysts And Their Catalytic Performance For Oxygen Reduction Reaction

In recent years,fuel cells have received extensive attention due to their high efficiency and environmental friendliness under the background of"energy saving and carbon reduction".The cathodic reaction,oxygen reduction reaction（ORR）,is one of the factors hindering the widespread application of fuel cells due to its inherently slow reaction kinetics.Palladium-based catalysts has good resistance to CO and methanol poisoning,but its electronic structure is not conducive to the efficient electron transfer in the ORR process.The controllable preparation of nano-palladium structure can be realized by selecting appropriate supports and preparation methods.It is necessary to design the supported nano-palladium catalysts and to promote the synergistic mechanism between Pd and supports for improving the ORR activity and stability.Aiming at the above problems,palladium nanocatalysts supported on carbon materials were designed by integrating the advantages of different types of carbon materials and Pd nanoparticles with different morphologies and structures.Reduced graphene oxide（r GO）and zeolite imidazolate framework（ZIF）derived porous carbon materials were used as supports,respectively and were modified with heteroatoms.The controllable preparation of Pd nanoparticles were realized by using different capping agents,reducing agents and assisted methods.r GO-supported coral-like 2D Pd nanosheets and Pd concave nanocubes catalysts,and ZIF-derived carbon-supported Pd nanoparticle catalysts were prepared.The catalysts were characterized by XRD,TEM,XPS and Raman et al,and the catalytic activity and stability were investigated toward ORR in alkaline medium.The relationship between the structure,morphology,composition and catalytic performance of the catalysts was explored.A layered template between cetyltrimethylammonium bromide（CTAB）and r GO was designed and constructed by using r GO as the carrier.Coral-like porous Pd nanosheets（Pd/r GO-u）were obtained under the sonication condition,while square Pd nanosheets（Pd/r GO-c）were produced via traditional chemical reduction using Na₂Pd Cl₄ as palladium source and ascorbic acid（AA）as reducing agent.The interaction between GO and CTAB made the alkyl chain assembles to a 2D lamella micelles which limit the growth of Pd atoms resulting in the formation of 2D nanosheets.The energy generated by the acoustic cavitation of ultrasound promotes the creation of coral-like structures.Pd/r GO-u exhibits higher catalytic activity than Pd/r GO-c in ORR via a similar four-electron reaction mechanism with commercial Pt/C with an onset potential of 0.96 V,a half-wave potential of 0.80 V（vs.RHE）,and a limiting diffusion current density of 3.60 m A cm^-2.Palladium concave nanocubes（Pd CNs）with high-index facets were synthesized through a facile none-seeded routine by using CTAB as capping agent and AA as reducing agent.The as-prepared Pd CNs have interstices inside the Pd CNs with an average particle size of 20.5 nm.Pd nanocubes first grew into larger size with numerous interstices inside the Pd CNs,and then the inner interstices were filled along with oxidative etching on the surface.The optimal molar ratio R of CTAB to Na₂Pd Cl₄ranges from 2.0 to 2.4,the reaction time is 1.5 h,and the reaction temperature is 60°C.Pd CNs/r GO was prepared by loading Pd CNs on r GO.The synergistic effect of the two materials achieves high ORR catalytic activity and stability following a similar four-electron reaction mechanism to Pt/C with an onset potential of 0.99 V,a half-wave potential of 0.88 V（vs RHE）,and a limiting diffusion current density of 3.68 m A cm^-2.Fe Co-ZIF was prepared by in-situ introduction of Fe species in the synthesis of ZIF-67,and Fe Co alloy-modified N-doped porous carbon（Fe Co NC）was prepared by pyrolyzing Fe Co-ZIF as a carrier for supported nano-palladium catalyst（Pd/Fe Co NC）.Fe Co alloy particles are uniformly dispersed and encapsulated in graphene layers with an average particle size of 45.7 nm.A good electon mobility can attribute to the synergistic effect of Fe Co alloy and nitrogen-doped carbon.Meanwhile,pyridine nitrogen and Fe/Co-N provided more active sites for the molecular activation of O₂ on the surface,which achieved a higher limiting diffusion current than that of Pt/C in ORR.The ORR catalytic activity of Pd/Fe Co NC is much higher than that of Pd/C,Pd/Co NC and Pt/C with an onset potential of 1.04 V,a half-wave potential of 0.88 V,and a limiting diffusion current density of 5.03 m A cm^-2.The N species in Fe Co NC can play the role of anchoring Pd,which improves the dispersion of Pd nanoparticles and prevents the agglomeration of metal nanoparticles in corrosive media.After aging test for 30000 s,Pd/Fe Co NC catalyst still maintains a current of 86%of initial,which exhibits much higher catalytic stability than commercial Pt/C and Pd/Co NC in ORR.By density functional theory（DFT）calculations,a 2e^-pathway on Fe Co NC catalyst surface is more likely to occur than the 4e^-pathway,while Pd species benefits the 4e^-process on the surface of Pd/Fe Co NC catalyst.The overpotentials of Fe Co NCs and Pd/Fe Co NCs in ORR are 0.67 V and 0.21 V,respectively.The introducing of Pd significantly reduced the ORR overpotentials of Fe Co NC,which is in good agreement with the experimental results,verifying the synergistic effect between the Pd species and the Fe Co NC support.The experimental results show that Pd nanoparticles can be anchored to the carbon materials to realize high dispersion and catalytic stability.The catalytic performance of palladium nanoparticles can be effectively improved by the synergistic effect between Pd and carbon materials.This work provides a new idea for the preparation of two-dimensional metal composites,and offers a reference for the preparation of ORR catalysts with high efficiency.