Ternary Pd-Based Nanocrystals Supported On Nitrogen-Doped Reduced Graphene Oxide As Highly Active Electrocatalysts

In the past few decades,Pt catalysts have been considered to be the best catalysts;however,typical problems facing Pt,including unsatisfactory long-term durability due to dissolution,methanol crossover and CO poisoning,are hindrances to mass application and make it a nontrivial issue to develop some novel cathode electrocatalysts.Palladium catalysts,a substitute for Pt catalysts,were found to have competitive intrinsic electrocatalytic performances in ORR compared with Pt-based catalysts.Furthermore,incorporating other transition metals with Pd demonstrates the great improvement of catalytic performances and the reduction of Pd loading owing to their remarkable synergistic catalytic effects.In this work,graphene was adopted as supporting materials,the electronic structure of graphene can be modified by doping atoms of other elements,for example,nitrogen doping not only increases the conductivity of graphene but also affects the spin density around the carbon atom,which generates more activated regions on the surface.the physical and electrochemical properties of Pd can be modified by combining it with other transition metals,resulting in multi-functional catalysts.The addition of other metals changes the electronic properties of Pd and promotes the formation of hydroxide at a low potential,which reduces the Pd level.Finally,Pd-based ternary alloy / nitrogendoped graphene nano-catalysts were obtained.The results are as follows:(1)The nitrogen-doped graphene(NG)was prepared by solid-phase ball milling heat treatment.Graphene and melamine ratio on the structure and properties were studied.The structure of nitrogen-doped graphene was characterized by XRD,XPS,Raman spectroscopy and infrared spectroscopy.The results showed that a large amount of oxygen-containing functional groups in the graphene oxide were removed during the thermal reduction process.The Raman spectra showed the peaks of D and G.And the 2D peaks show that nitrogen-doped graphene was thin layer structure.XPS indicates that nitrogen in graphene mainly exists in three states: pyrrolic-N,pyridinic-N and graphitic-N.When the ratio of oxidized graphite and melamine is 2:1 and the temperature is 800 ? has the best ORR performance.The initial potential is-0.19 V,the half-wave potential is-0.32 V,and the limiting current density is 1.80 m A cm-2.(2)The ternary alloy of Pd Cu Ni was synthesized by chemical reduction method,and it was dispersed on the surface of nitrogen-doped graphene.The results were further confirmed by SEM,XRD,Raman spectra and XPS spectroscopy.In this experiment,CV and RDE show that Pd Cu Ni/NG has high catalytic activity.The results show that the half-wave potential of Pd Cu Ni/NG is-0.210 V and the limit current density was 3.88 m A cm-2 at 1600 rpm.The catalytic performance was better than 20% of commercial Pd/C(-0.235V),and the limit current density is 3.50 m A cm-2,which is follow the 4-electron reaction process.The Pd Cu Ni/NG,20% Pd black electrode material is circulated for 5000 cycles and then test the linear scanning.The half-wave potential of Pd Cu Ni/NG nanocatalyst did not change,and 20% Pd black was weakly shifted.The current density is reduced by 0.1 m A cm-2 while the 20% Pd black is reduced by 0.3 m A/cm2.Finally,Pd Cu Ni/NG nanocatalyst was used in lithium air battery.The discharge capacity of Pd Cu Ni/NG was 6976 m Ah/g,which was higher than 20% commercial Pd/C(3279 m Ah/g).The value of charge and discharge potential of Pd Cu Ni/NG was about 0.95 V,which was lower than 20% commercial Pd/C(1.35 V).It is proved that the redox performance of Pd Cu Ni/NG is the best and the stability is strongest.(3)Pd/NG,Pd Sn/NG,Pd Co/NG and Pd Sn Co/NG nanocatalysts were prepared by solvothermal method.And the nanocatalysts were characterized by scanning electron microscope(SEM),EDS,transmission electron microscopy(TEM),X-ray diffraction(XRD),and XPS.The oxygen reduction performance of the Pd-based composite nanocatalyst and the performance of the lithium-air battery were tested.half-wave potential of Pd Sn Co / NG was-1.90 V,limiting current density was 3.75 m A cm-2 at 1600 rpm,which was better than 20% commercial Pd/C(-0.28 V,3.5 m A cm-2).The first discharge capacity of Pd Sn Co / NG was 6750 m Ah/g,which was higher than that of Pd/NG(3952 m Ah/g),commercial Pd/C(3279 m Ah/g)and NG(2087 m Ah/g).The value of charge and discharge potential difference of Pd Sn Co/NG electrode material was about 0.8 V,which was lower than Pd/NG,commercial Pd/C,and NG.This is due to the reduction of electrochemical polarization of Pd Sn Co / NG nanocatalyst,which increases its oxygen reduction performance,so the lithium air battery has high capacity and good stability.(4)The Pd Ni Sn/NG was prepared by chemical reduction method.The experiment was tested by linear scanning curve and cyclic voltammetry to judge the performance of oxygen reduction.By contrast,The half-wave potential of Pd Ni Sn/NG was-0.210 V,which was better than 20% commercial Pd/C(-0.235 V),NG(-0.320 V),and the limiting current density at 1600 rpm was 3.50 m A/cm2,which was better than 20% commercial Pd/C.The Pd Ni Sn/NG electrode material exhibited the highest specific capacity of 8630 m Ah/g,which was higher than commercial Pd/C(3279 m Ah/g),Pd Sn Co/NG(6750 m Ah/g)and Pd Cu Ni/NG(6976 m Ah/g).Limiting capacity to test its cycle stability,Pd Ni Sn/NG electrode material was recycled over 63 cycles,which was better than Pd Sn Co / NG(60 cycles),Pd Cu Ni / NG(55 cycles),20% commercial Pd/C(28 cycles)and NG(20 cycles).It is further proved that Pd Ni Sn/NG nanocatalyst can greatly improve the catalytic performance and reduce the electrochemical polarization.