Preparation Of Carbon Fibers Embedded With Nickel-Cobalt-Based Catalyst And Its Application In Zinc-Air Battery

The imminent environment pollution problem and energy crisis have promoted the development of efficient and environmental energy conversion and storage tech-nologies.The rechargeable Zn-air battery(ZAB)has been regarded as next-genera-tion sustainable energy conversion device due to its high theoretical specific energy density,excellent safety,and environmental friendliness.However,the development of ZAB has always been restricted by the slow reaction kinetics and high reaction barriers of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).Therefore,the development of high-performance and low-cost non-noble metal bi-functional catalysts for OER/ORR has become the key issue to the development of ZAB.Among non-noble metal catalysts,Ni and Co have become a research hotspot due to their excellent catalytic activity,rich natural reserves,diverse electronic va-lence states,and adjustable electronic structure.Furthermore,the nanofibers prepared by electrospinning have the advantages of hierarchical pore structure,metal particles embedded in-situ in carbon fibers structure,and large-scale preparation.Therefore,in this thesis,Ni-Co composite materials were used as the research object,electro-spinning was used as the materials preparation method,and a variety of Ni-Co bimetal composite materials were prepared through morphology design and component con-trol.Finally,all materials were used as OER/ORR bifunctional catalysts for ZAB.The carbon fibers(CF)in-situ embedded with Ni Co alloy/oxide catalyst(Ni Co/CF-800)was prepared by the morphology design and composition control through electrospinning method.Firstly,the bulk,flake and fibrous morphology cat-alysts were prepared by adjusting the viscosity and conductivity of electrospinning solution.Among them,the fibrous morphology catalyst exhibited more non-graphi-tized carbon structure(I_D/I_G=0.892)and the larger electrochemically active specific surface area(932.75 cm² cm^-2).Furthermore,the nanofiber network with three-di-mensional hierarchical pore structure greatly improved the ion diffusion and elec-tronic conductivity of catalyst.Then,by adjusting the phase composition of nanopar-ticles and increasing the crystallization degree of carbon fibers and Ni Co alloys,the intrinsic activity and electron conduction rate of catalyst were significantly improved.The prepared Ni Co/CF-800 catalyst showed excellent OER/ORR bifunctional cata-lytic activity(?E=0.73 V).The assembled ZAB exhibited a discharge specific capac-ity of 802.6 m Ah g _Zn^-1 at 10 m A cm^-2,which is 97.9%of the theoretical value,and a discharge energy density of 926.8 Wh kg _Zn^-1,which is 85.3%of the theoretical value.At a large discharge rate of 30 m A cm^-2,the ZAB can still maintain the high specific capacity(783.0 mAh g _Zn^-1)and high energy density(887.5 Wh kg _Zn^-1),indicating its excellent high-rate discharge performance.In order to solve above-mentioned problem that Ni Co alloys/oxide were prone to agglomeration during high-temperature carbonization process,coordination mech-anism was employed during electrospinning process.The dispersibility of metal par-ticles in carbon fibers was improved by using the coordination effect of double bond sulfur and amino groups in thioacetamide(TAA)with metal ions,thus increasing the reaction interface.Meanwhile,the introduction of TAA also realized the in-situ co-doping of nitrogen and sulfur in carbon fibers,which will induce the spin redistribu-tion of sp² carbon lattice charge,thus enhancing the reaction intermediate adsorption capacity of carbon support and accelerating the catalytic reaction kinetics.The pre-pared NSCFs/Ni-Co-Ni Co₂O catalyst showed many structural advantages,such as the highly dispersed Ni Co alloy particles(average particle size less than 10 nm),hierar-chical pore structure,high specific surface area(199.38 m² g^-1),and rich carbon defect structure(I_D/I_G=1.171).Compared to commercial Ir O₂ and Pt/C catalysts,NSCFs/Ni-Co-Ni Co₂O exhibited the higher OER/ORR bifunctional catalytic activity(?E=0.69V).The assembled ZAB exhibited a peak power density(171.24 m W cm^-2)at a high discharge current density(284.5 m A cm^-2),indicating its excellent high-rate discharge performance;the ZAB performance showed no obvious degradation after charge-dis-charge cycle test for 380 h at 10 m A cm^-2,indicating its excellent long-term stabilityIn order to solve the problem that it is difficult to prepare carbon fibers embed-ded with metal oxides by electrospinning,we employed a method that synergizes the strong coordination of disulfide bonds in thiourea and the oxidation mechanism of dioxygen bonds in acetamide to prepare N/S co-doped carbon fibers in-situ embedded with Ni Co oxide(NSCFs/Ni Co₂O).X-ray diffraction and X-ray photoelectron spec-troscopy confirmed that the phase structure of metal nanoparticles in NSCFs/Ni Co2O is Ni Co oxide.The NSCFs/Ni Co₂O exhibited the rich carbon defective structure(I_D/I_G=1.281),a large specific surface area(141.65 m²g^-1),a large electrochemically active specific surface area(2692.5 cm² cm^-2)and a high ratio of pyridine-type nitro-gen to pyrrole-type nitrogen.The NSCFs/Ni Co₂O showed a high half-wave potential(0.847 V),a high limiting diffusion current density(5.11 m A cm^-2)and a low Tafel slope value(50.41 m V dec^-1)in ORR catalysis.Meanwhile,it showed a low E_{10 m A cm}-2(1.526 V)and a low Tafel slope value(59.44 m V dec^-1)in OER catalysis.The as-sembled ZAB exhibited excellent high-rate discharge performance(a peak power density of 177.52 m W cm^-2 at a high discharge current density of 278.7 m A cm^-2)and charge-discharge cycle stability(the charge-discharge voltage gap Increase by 17 mV after 1500 h).