| Therefore,people have gradually turned to the research of non-precious metal-based electrocatalysts.In a series of non-noble metal electrocatalysts developed,carbon nanotubes have a unique hexagonal subsidiary structure,exhibiting an abnormal specific surface area and electrochemical performance,and providing more active sites.On the other hand,nitrogen doping on carbon nanotubes?N-CNTs?can improve the local charge density of carbon nanotubes,thereby accelerating electron transport performance and reducing resistivity.At the same time,the high affinity of the non-precious metals Ni/Co/Fe and N can be beneficial to activate the outer graphite layer of the carbon nanotubes,increase the existence of porous defects,accelerate the interface electron transfer between the catalyst surface and the reaction intermediate,thereby improving the ORR activity.Therefore,the development of Ni/Co/Fe-N-CNTs catalysts has excellent prospects for oxygen reduction reactions.In addition,the current research on zinc-air batteries mainly uses concentrated alkaline electrolytes,but due to the spontaneous dissolution of anode zinc in strong alkaline medium and corrosion under concentrated alkaline conditions,the conductivity and battery of ionic electrolytes are slowed down.Lifetime,while neutral electrolytes can be used to solve corrosion problems caused by alkaline electrolytes and to avoid spontaneous discharge phenomena.In this thesis,Ni/Co/Fe was used as the metal catalyst center,dicyandiamide was used as the nitrogen source,and glucose/sucrose was used as the main carbon source to prepare Ni/Co/Fe-N-CNTs catalyst.The electrocatalysis of the catalyst for oxygen reduction reaction was studied.The neutral zinc-air battery was activated and assembled,and its discharge performance was tested.?1?A mixture of nickel/cobalt salt,glucose and dicyandiamide was directly pyrolyzed at600?and 800?in a nitrogen atmosphere to synthesize a nitrogen-doped hollow tubular composite.The morphology and composition of each catalyst were analyzed by SEM,TEM,EDS and XRD.Electron microscopy results show that when the metal is contained,the material has a distinct hollow tubular morphology structure,and the material obtained without metal addition has a sheet shape.In addition,the presence of nickel is more favorable than cobalt to form long,dense nanotubes,and when nickel and cobalt are present simultaneously,the tubular morphology is more uniform and has a diameter of about 25 nm.The electrocatalytic activity of catalysts for ORR was tested in acid and alkaline electrolytes.The results show that the initial potential of oxygen reduction of Ni2?-n?Co3?-n?/N-CNT catalyst is closer to Pt/C than other catalysts,at 1600 rpm,the ultimate diffusion current density of Ni2?-n?Co3?-n?/N-CNT in acidic medium reaches 7.6 mA·cm-2,which is significantly larger than Pt/C and several other synthetic catalysts.In addition,the Ni2?-n?Co3?-n?/N-CNT catalyst also exhibited good catalytic stability.?2?A mixture of dicyandiamide,glucose,and nickel nitrate/cobalt nitrate salt was pyrolyzed at a high temperature to obtain Ni/C-N,Co/C-N,and Ni-Co/C-N catalysts.The oxygen reduction polarization curves of catalysts under pure oxygen and air conditions were tested,and it was proved that Ni-Co/C-N has relatively better catalytic oxygen reduction performance.In addition,the discharge performance of the assembled neutral zinc-air battery was tested in a 0.5 mol·L-1 KNO3 electrolyte.Similarly,the Ni-Co/C-N electrode has the best discharge performance as the cathode,and its open circuit voltage is 1.11V,which is closer to Pt/C?1.12V?,and the maximum power density is 65 mW·cm-2,which is much higher than Pt/C and several other types of electrodes.The constant current discharge test chart shows that when the discharge current density is maintained at 50 mA·cm-2,100 mA·cm-2 and 150mA·cm-2,the discharge duration of the Ni-Co/C-N battery is 202,93 and 11 hours,respectively.And the platform voltage is higher than Pt/C.Further,the Ni-Co/C-N battery can be repeatedly discharged at a high current density of 150 mA·cm-2 by merely replacing the anode zinc sheet.A series of tests have shown that the Ni-Co/C-N catalyst has high oxygen reduction activity,high stability and repeatability as a neutral zinc-air battery cathode material.?3?Co-C-N and Fe-Co-C-N catalysts were obtained by directly pyrolyzing a mixture of dicyandiamide,glucose and ferric nitrate/cobalt nitrate at 600?and 800?in a nitrogen atmosphere.The physical properties of catalysts were characterized by SEM,TEM,XRD and XPS.The oxygen reduction activity of catalysts was studied by cyclic voltammetry,linear sweep and chronoamperometry.The results show that the catalytic performance of Fe-Co-C-N catalyst on ORR is relatively best.The catalyst was tested as a cathode material assembled zinc-air battery.The results show that the open circuit voltage and maximum power density of the Fe-Co-C-N electrode are comparable to those of the commercial Pt/C electrode.The constant current discharge test at 25 mA·cm-2,50 mA·cm-2 and mA·cm-2 was maintained,and it was confirmed that the Fe-Co-C-N catalyst can be repeatedly discharged for a long time.Therefore,the Fe-Co-C-N catalyst can be applied to a neutral zinc-air battery as a substitute for a noble metal Pt-based catalyst.?4?NiCo@ht-CN catalysts was prepared by directly pyrolyzing a mixture of dicyandiamide,sucrose,nickel acetate and cobalt acetate in a nitrogen atmosphere at 600?and 800?,respectively.The morphology,elemental composition and structural composition of catalysts were studied by different analytical characterization methods.The catalytic activities of the Ni and Co ratios of different metals in oxygen reduction in acidic,basic and neutral solutions were tested.The results show that the initial potential and half-wave potential of Ni2Co3@ht-CN catalyst are relatively highest in acidic,alkaline or neutral solutions,and the ultimate diffusion current density is close to 7 mA·cm-2@2000rpm in acidic and neutral solutions.In addition,in any medium,the number of ORR electron transfer on the Ni2Co3@ht-CN catalyst is between 3.9 and 4.0,indicating that the reaction process is mainly carried out by a 4-electron reaction,and O2 is directly reduced to H2O.?5?The experimentally prepared Ni1Co1@ht-CN,Ni2Co3@ht-CN and Ni3Co2@ht-CN catalysts and zinc sheets were assembled into a neutral zinc-air battery for discharge performance test.The results showed that Ni2Co3@ht-CN electrode has the best discharge performance,its open circuit voltage is about 1.08V,at 130 mA·cm-2,the maximum power density exceeds 50 mW·cm-2.The discharge curves at constant current density(25 mA·cm-2,50 mA·cm-2,100 mA·cm-2,and 150 mA·cm-2)show that the Ni2Co3@ht-CN electrode has a higher voltage platform than the Pt/C electrode.And the constant current discharge time at150 mA·cm-2 may exceed 10 hours.It shows that Ni2Co3@ht-CN catalyst can be used as an excellent cathode material for neutral zinc-air batteries. |
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