Preparation Of NiFe/Carbon Composite Catalysts And Their Applications In Zn-air Batteries

Secondary Zn-air battery,which has the advantages of high theoretical energy density and abundant resources,is a promising new generation of energy conversion and storage technology.At present,the slow kinetics of the oxygen reduction/evolution reaction（ORR/OER）on gas electrodes results in large charge/discharge polarization and low cycle efficiency of batteries,thus seriously restricting the large-scale development of Zn-air batteries.Developing highly active bifunctional catalysts/electrodes,can significantly enhance the reaction rate and improve the battery performance.Transition metals and carbon-based materials are abundant in reserves,and their structures and compositions are easily to regulate,but there are problems such as low intrinsic activity and limited active sites.In this paper,the 4e^-ORR and 2e^-ORR catalytic mechanism of N-doped carbon fibers was studied by regulating the pore structure and composition.Then,efficient and stable bifunctional Ni Fe/carbon composite catalysts were constructed by surface activation and defect/vacancy regulation,and its application characteristics in Zn-air batteries were investigated.The main research contents and results are summarized as follows:（1）The excellent 4e^-ORR activity and high 2e^-ORR selectivity of N-doped porous carbon fiber（NPCF）were achieved through the regulation of pore structure and composition,respectively.NPCF-1 with high porosity and NPCF-2 with medium porosity were prepared by electrospinning and high-temperature pyrolysis with ZIF-8 as the pore-forming agent.NPCF-1 produces a large number of pores and defects during the high-temperature pyrolysis and exhibits high graphitization,thus forming a high proportion of graphitic N structure.NPCF-2 has moderate porosity,uniform pore distribution,and high content of pyrrole N.Theoretical calculation confirms that the adsorption capacity of pyrrole N for OOH*intermediate is moderate,close to the ideal OOH*free energy of 2e^-ORR.Graphite N spontaneously promotes the formation of OOH*,O*and OH*intermediates,preferring 4e^-ORR catalytic pathway.Therefore,NPCF-1 exhibits efficient 4e^-ORR catalytic activity,and the NPCF-1 based Zn-air battery has a peak power density of 209.8m W cm^-2.NPCF-2 possesses excellent 2e^-ORR selectivity（82%）in acidic solution and achieves a high H₂O₂ yield of 1390.2 mg L^-1 h^-1.（2）Aiming at the problems of insufficient active sites in N-doped carbon fiber-coated Ni Fe alloy catalysts,an NH₃ surface/interface activation strategy was proposed.Ni Fe nanoalloy embedded in N-doped porous carbon nanotubes(Ni_0.6Fe_0.4@NPCF-NH₃)was prepared by electrospinning,high-temperature pyrolysis and NH₃ activation.The abundant micro/mesoporous pores on the carbon tubes were constructed by the NH₃ activation,which effectively increased the doping content of heteroatom N and the specific surface area of catalysts,not only increasing and exposing a large number of ORR/OER reactive sites but also promoting the effective contact between the electrolyte/gas and active sites.Therefore,the Ni_0.6Fe_0.4@NPCF-NH₃ possesses excellent ORR/OER catalytic activity and stability.Ni_0.6Fe_0.4@NPCF-NH₃ based Zn-air batteries exhibit a peak power density of 256 m W cm^-2 and are stable for 1600 charge-discharge cycles.（3）A CN vacancy(V_CN)regulation strategy was designed to enhance the intrinsic activity of the Ni Fe-PBA and N-doped carbon fiber（NCF）flexible composite electrodes.The V_CNwas in-situ formed on Ni Fe-PBA grown on NCF by N₂ plasma technology to prepare a flexible self-supporting electrode（N₂-Ni Fe-PBA/NCF/CC-60）.Theoretical calculation confirms that V_CN can promote the formation of OH*intermediates,reduce the theoretical overpotential of OER,and suppress the escape of Fe elements during the catalytic process,ensuring excellent stability.In addition,the flexible integration of Ni Fe-PBA and NCF rooted in carbon cloth（CC）not only provides efficient and sufficient ORR/OER active sites,but also avoids the side effects of binders.Therefore,the N₂-Ni Fe-PBA/NCF/CC-60exhibits efficient and stable ORR/OER catalytic performance.The N₂-Ni Fe-PBA/NCF/CC-60 based Zn-air battery can steadily charge/discharge for 2000 cycles,and flexible Zn-air batteries show good bendability and battery performance.（4）Fe-NC catalysts have high atomic utilization,but suffer from insufficient active sites and poor OER performance.To this end,highly dispersed binary Ni/Fe-NC bifunctional sites were constructed.A flexible self-supporting gas electrode with the in-situ coupling of Ni/Fe-NC nanoparticles and N-doped carbon fiber network was fabricated by in-situ growth and high-temperature pyrolysis.The highly dispersed binary Ni/Fe-NC ensures excellent ORR intrinsic activity while being an efficient OER active site.Theoretical calculation confirms that the Ni/Fe-NC sites can accelerate the formation of OOH*intermediates and reduce the theoretical overpotential of OER.The in-situ coupling of Ni/Fe-NC and NCF/CC carrier provides abundant and stable active sites,and ensures fast charge transfer and gas transport.In addition,there is a synergistic catalytic effect between Ni/Fe-NC and NCF/CC carrier,thus realizing the efficient operation of ORR/OER.The Ni/Fe-NC/NCF/CC-based Zn-air batteries can steadily charge/discharge for 2150 cycles.The corresponding flexible Zn-air batteries exhibit excellent bendability and a peak power density of 105.0 m W cm^-2.