Preparation And Electrocatalytic Performance Of Iron Group/Nitrogen Group/Carbon Based Catalysts For Oxygen Electrode

Lithium-air battery has many advantages such as excellent energy density,environment friendliness and resource conservation,which has become one of ideal energy storage technologies.Air electrode or oxygen electrode is main reaction place of lithium-air battery,where the complex oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?process occur.Due to sluggish reaction kinetic performance,the catalyst is needed to promote the efficiency of the battery.Conventional noble metal based catalysts?Pt,RuO₂ and IrO₂?have the disadvantages of high price,resource shortage and single catalytic performance,which restrict the commercialization of lithium-air batteries.In response to this problem,this study designed and developed ORR/OER bifunctional iron group metal–nitrogen group nonmetal-carbon?M-NM-C?based catalysts and studied their electrocatalytic performances.In this paper,cobalt and nitrogen co-doped carbon materials?Co-N-C?catalysts with bulk,planar and fiber morphologies were prepared by methods of solid phase,recrystallization and electrospinning,respectively.Cobalt and nitrogen co-doped carbon nanofibers?Co-N-CNFs?catalyst possesses three-dimensional network structure,large specific surface area(175.1 m²g^-1),many defect structures?I_D/I_G=1.17?,and exhibits good ORR/OER catalytic performance,ORR onset potential,OER onset potential and ORR/OER potential gap(?E=E^-2_{10 m A cm}-E_{-1 m A}1 m A cm^-2)are 0.874,1.667 and 0.916 V,respectively.Therefore,the effective control of the micromorphology and specific surface area can help to improve the distribution and utilization of catalytic active sites and further improve the catalytic performance.Effect of Co/N molar ratio on the catalytic performance of Co-N-CNFs catalysts was further studied.It is shown that the doping of appropriate amounts of nitrogen element can improve the catalytic performance under the retention of the three-dimensional network structure.Non-metal elements are as important parts of catalytic active sites.Different electronegativity and ionic radius between non-metal elements lead to different sites for oxygen adsorption-desorption and catalytic active sites,and thus affect the catalytic prformances.Therefore,cobalt and phosphorus co-doped carbon nanofibers?Co-P-CNFs?catalyst was prepared by changing the type of non-metal elements.Compared with Co-N-CNFs catalyst,Co-N-CNFs catalyst keeps the high ORR onset potential,and Co-P-CNFs catalyst has low OER onset potential?1.659V?,suggesting that the types of non-metal elements have an effect on the catalytic activities of the catalysts.Cobalt,nitrogen and phosphorus tri-doped carbon nanofibers?Co-N-P-CNFs?catalyst was further prepared by co-doping method of non-metal elements.Compared with the Co-N-CNFs and Co-P-CNFs catalysts,Co-N-P-CNFs catalyst has relatively uniform nanofiber morphology,large specific surface area(214.0 m²g^-1),and many defect structures?I_D/I_G=1.21?,resulting in the high ORR onset potential?0.876 V?,low OER onset potential?1.652 V?and small ORR/OER potential gap??E=0.884 V?.It can be concluded that the changing the type of non-metallic elements helps to improve ORR/OER catalytic activity,but the effect is not significant.Metal elements in the M-NM-C catalyst can not only form M-N_x active sites with pyridine nitrogen,but also can be embedded into the carbon skeleton to form additional catalytic activity sites,which also determine the catalytic performances of the catalysts.Therefore,iron and nitrogen co-doped carbon nanofibers?Fe-N-CNFs?and nickel and nitrogen co-doped carbon nanofibers?Ni-N-CNFs?catalyst were respectively prepared by changing the type of metal elements.Compared with Co-N-CNFs catalyst,due to the different intrinsic properties of iron,nickel and cobalt,the corresponding catalysts also exhibit different forms of metal elements,micromorphologies,specific surface areas and ORR/OER catalytic activities,in which Co-N-CNFs catalyst possesses high ORR onset potential,and Ni-N-CNFs catalyst possesses high OER onset potential?1.584 V?,indicating that the type of metal elements also affects the catalytic performance of the catalyst.Iron,nickel and nitrogen tri-doped carbon nanofibers?Fe/Ni-N-CNFs?catalyst was further prepared by co-doping of double metals.Compared with Fe-N-CNFs and Ni-N-CNFs catalysts,the specific surface area(252.3 m²g^-1)and defect structure of carbon skeleton?I_D/I_G=1.50?of Fe/Ni-N-CNFs(especially Fe_0.5Ni_0.5-N-CNFs)catalysts were significantly improved,and Fe_0.5Ni_0.5-N-CNFs catalyst exhibits high ORR onset potential?0.903 V?,low OER onset potential?1.528 V?and low ORR/OER potential gap??E=0.796 V?,indicating that the synergistic effect of Fe/Ni alloy in Fe/Ni-N-CNFs catalyst effectively improved the catalytic performance.In order to further enhance the ORR/OER catalytic performance of the catalyst,the composite catalyst on the basis of Fe/Ni-N-CNFs catalyst with another bifunctional catalyst perovskite oxide La_1-x-x Sr_x Co_1-y-y Fe_yO_3-??LSCF?was prepared.By optimizing the morphology and element composition,La_0.6Sr_0.4Co_0.8Fe_0.2O_3-??LSCF-6482?catalyst has network structure composed of nanorods and the best elementcomposition,therebyexhibitinghighcatalyticactivity.Fe/Ni-N-CNFs/LSCF composite catalyst was further prepared by method of blending and sintering.When the mass ratio is 1:1,the composite catalyst showed a higher ORR onset potential?0.914 V?,a lower OER onset potential?1.512 V?and a lower ORR/OER potential gap??E=0.659 V?,indicating the synergistic effect between these two catalysts can further enhance the catalytic performance of the composite catalyst.