Preparation And Performances Of Cobalt/iron-carbon Composites As Cathode Catalysts In Zn-air Batteries

As an electrochemical energy storage and conversion device,the Zn-air battery has the advantages of large capacity,high specific energy,and stable discharge.However,its energy density and power are greatly restricted by the slow kinetics of oxygen reduction reaction(ORR)on the positive electrode.At present,the most effective Zn-air battery cathode catalyst is platinum-based precious metal materials,such as Pt/C and Pt alloys.In addition to being expensive and scarce in resources,these catalysts also have disadvantages such as poor durability,low selectivity,and poor methanol tolerance.Therefore,it is particularly important to design non-noble metal ORR electrocatalysts with high activity,high efficiency and good stability.Among them,transition metals have a variety of compound con Fig.urations and exhibit highly efficient and adjustable electrocatalytic activity.However,transition metal compounds generally have the disadvantage of low conductivity.Combining them with conductive carbon materials can achieve complementary advantages and further enhance ORR electrocatalytic activity,as well as stability.In this thesis,high-performance ORR electrocatalysts were prepared by combining cobalt,iron-based transition metal compounds and heteroatom-doped carbon materials.Zn-air batteries assembled a Zn-air battery for research.The main contents are expressed as follows:(1)Using reduced graphene oxide(rGO)as the carbon matrix,an adsorption-calcination method was used to prepare nitrogen-doped graphene-supported cobalt-based multi-component nanoparticles to obtain excellent ORR electrocatalytic performance.The cobalt ion solution is mixed with the functional-rich rGO dispersion solution,and the rGO with a large number of cobalt ions on the surface is obtained by electrostatic adsorption.Then,an appropriate amount of ammonia is added to obtain the conversion of cobalt ions into cobalt ammonia coordination ions(CO(NH₃)₆²⁺).Finally,composite featured by N doped graphene dispersed with cobalt,cobalt oxide and cobalt nitride multi-component nanoparticles(Co/CoO_x/Co N/NG)were obtained by calcination.Using it as ORR electrocatalyst,the obtained half-wave potential is 0.80 V vs.RHE,and the limiting current density is 4.6 m A cm^-2.The assembled Zn-air battery has a high specific capacity of 843 m Ah g _Zn^-1 and good stability.(2)Ultrafine CoO nanoparticles and Co-N-C nanosheets supported by mesoporous carbon(MC)were prepared by co-adsorption-calcination method using CMK-3 as mesoporous carbon matrix.The process uses Co²⁺as cobalt source,dicyandiamine(DCDA)as nitrogen and carbon source for co-adsorption on functionalized CMK-3,followed by calcination in nitrogen atmosphere.In the process of co-adsorption,Co²⁺is dispersed among and coordinated with DCDA molecules.During the calcination process,part of Co²⁺was embedded in DCDA and converted to Co-N-C lamellar;while the other part was aggregated and converted to CoO nanoparticles.The obtained CoO/Co-N-C/MC has a good electrocatalytic performance for ORR with a half-wave potential of 0.78 V vs.RHE and a limiting current density of 5.01 m A cm^-2.The specific capacity of the assembled Zn-air batteries is 842.5 m Ah g _Zn^-1,and it has good rate performance and stability.(3)Using ferrocene and melamine as raw materials and controlling the relative contents of the two precursors,iron and nitrogen doped carbon micron ball loaded with carbides and oxides(Fe₅C₂/Fe₂O₃/Fe-N-C-3)were synthesized by solid phase pyrolysis method.The composite material has excellent ORR electrocatalytic performance.The composite material has excellent ORR electrocatalytic performance,the half-wave potential is 0.87 V vs.RHE,the limiting current density is 5.34 m A cm^-2,and the specific capacity of the assembled Zn-air batteries reaches 878.6 m Ah g _Zn^-1.This is attributed to the high dispersion of Fe₅C₂/Fe₂O₃particles and Fe and N doped atoms in the matrix,so that the active sites can be fully contacted with the electrolyte.In addition,the Fe,N doped carbon matrix can provide high specific surface area and abundant active sites while ensuring efficient electron transport.