Preparation And Electrochemical Performance Of Pd-based Nano Alloys Supported On Nitrogen And Sulfur Co-doped Honeycomb Carbon For Lithium Air Battery

Promote the use of green and efficient new energy era in the world,Li-O2 batteries are attracting more and more interest due to their high theoretical energy density and open cell configuration that uses oxygen as a reactant.However,because of high charge/discharge overpotential,low rate capability,and poor cyclic stability,the application of these systems have been seriously restricted.The research shows that the slow rate of oxygen reduction reaction(ORR)and the high reaction potential on the cathode of the Li-O2 batteries are the factors that directly restrict the performance and efficiency of the battery.Researchers suggested that porous catalytic cathode is the key to improve the cyclic performance of the Li-O2 batteries.The commercially available electrocatalyst is Pt/C,however,the high cost and low stability hinder its practical applications.Therefore,it is of great significance to design and develop the non platinum catalyst with high efficiency and catalytic activity.The research shows that Pd and Pt have similar electronic structure and electrochemical performance.And Pd exhibits high oxygen reduction catalytic activity under alkaline conditions.In addition,Pd has a low cost advantageand.So,Pd is a noble metal that can replace Pt.However,the ORR activity of Pd/C is at least five times lower than that of Pt/C.The author thinks that two ways can be used to reduce the Pd and improve the efficiency of the catalyst.First,the formation of Pd?based alloys is an effective way to change the electronic structure of Pd and produce synergistic effects.Second,heteroatom?doping of carbon supports is another approach to improve the activity.The multifaceted synergistic effects not only reduce the amount of precious metal Pd,but also show higher catalytic activity and stability on the the same amount.Based on these factors,we have mainly carried out the following work.(1)In this work,the acrylonitrile oligomer(ANT)with low molecular weight was prepared through radical telomerization of acrylonitrile by using acrylonitrile as monomer,azodiisobutyronitrile as initiator,and 3?mercapto propionic acid as chain transfer agent,andthe effects of the atomic ratio of different acrylonitrile monomers and chain transfer agents on the content of nitrogen and sulfur and the electrochemical properties were studied.The results show that acrylonitrile and 3?mercapto propionic acid with a mole ratio of 1:0.8.After pre-oxidation and carbonization,it is found that the product has the highest degree of cross-linking cyclization at this time by comparing the transformation of-CN into-CN-using the FTIR.The elemental analyzer proved that the relative content of nitrogen and sulfur atoms were 3.23% and 4.35% respectively,and showed the best oxygen reduction catalytic activity.Their initial potential and limiting current were 0.84 V and-1.0 m A cm-2 respectively.(2)The nitrogen nd sulfur co-doped honeycomb-ordered carbon(NS/C)is prepared by using silica as template prepared via classical sol-gel combined with homogeneous precipitation,and functionalized fluidic acrylonitrile telomer(ANT)as C,N and S sources.Firstly,ANT was adsorbed into the Si O2 spheres by a wet impregnation method.Secondly,the sample was successively pre-oxidized at 180 °C for 4 h and 240 °C for 4 h,and then carbonized at 800 °C for 4 h in nitrogen atmosphere.Finally,the NS/C was obtained by etching the Si O2 template with HF solution.The composition and morphology of NS/C were characterized by SEM,BET and XPS.When the size of Si O2 is 150-200 nm,the concentration of HF is 20%,and the stirring time is 24 h,the NS/C are ordered,which have large number of homogeneous pores with rich specific surface area(880 m2 g-1),and pore volume(1.15 cm3 g-1).XPS shows that ANT exists pyrrole-N,pyridine-N,graphitized-N,sulfur carbon and sulfur oxygen.(3)High crystallinity Pd Ni nanoscale alloys with an average size of 3 nm were synthesized by chemical reduction of sodium borohydride and loaded on the surface of NS/C.The composition,structure,and morphology of Pd Ni-NS/C were characterized by using SEM,TEM,XRD,BET,XPS and other instruments.After loading Pd Ni alloys,the structure of the NS/C remained intact.When used as an electrocatalyst for oxygen reduction reaction(ORR)in alkaline solution,the catalytic activity and stability of commercial Pd/C(20 wt %),NS/C,Pd?NS/C,Pd Ni?NS/C are studied.It is found that Pd Ni?NS/C exhibits superior performance both in catalytic activity and stability.The average electron transfer number ofPd Ni?NS/C is about 3.72,which mainly carried out in 4 electron channels.The initial potential is 0.99 V,and the limiting current density is 3.4 m A cm-2.The chronoamperometry(i?t)tests show that the current retention of Pd Ni?NS/C is 91%,while for the commercial Pd/C is only 40% after 10000 s.Finally,the Pd Ni-NS/C was applied to lithium air battery.Compared with other samples,it also showed a lower charge and discharge overpotential(0.8V),a higher specific capacity(5650 m Ah g-1)and a cycle life of 70 cycles(charging to 5.0 V).(4)The dispersed Pd Cu W nanoparticles are supported on the walls of the NS/C to form composite catalyst material(Pd Cu W-NS/C)by a glycol reduction method.Experiments show that the the Pd Cu W alloys particles with the average particle size is about 5.25 nm are small and uniformly dispersed on the honeycomb carbon surface.Measurement and characterization of oxygen reduction(ORR)and lithium air batteries of Pd-based materials.The results showed that Pd Cu W-NS/C sample reaction pathway is close to 4 electron transfer under alkaline conditions.The half wave potential of Pd Cu W-NS/C is0.87 V,and the limiting current density is 2.8 m A cm-2,while the other samples were NS/C(0.80 V,1.3 m A cm-2),Pd/ C(0.82 V,1.8 m A cm-2),Pd-NS/C(0.83 V,2.5 m A cm-2),Pd W-NS/C(0.84 V,2.3 m A cm-2),and Pd Cu-NS/C(0.86 V,2.4 m A cm-2).Finally,the Pd Cu W-NS/C as a part of the cathode materials for lithium air battery possess a specific capacity of 5725 m Ah g-1(deep charge and discharge)at 300 m A g-1.After 5 cycles of deep cycling,the specific capacity of the battery is still 2714 m Ah g-1.In first cycle,the charge and discharge overpotential is only 0.7 V under the condition of constant capacity of 500 m A h g-1,and constant current density of 300 m A g-1.In addition,the cyclic retention of more than 70 cycles(charging to 5 V).In summary,Pd Cu W-NS/C can effectively promote the oxygen reduction reaction,and greatly improve the electrode phenomenon in the lithium air battery,and improve the cycle stability of the battery.