| Fuel cell is an efficient and clean energy conversion device that can directly convert chemical energy into electrical energy.However,the process of cathodic oxygen reduction reaction（ORR）is kinetically slow and very complicated.Therefore,a suitable catalyst is needed to improve the reaction rate and efficiency.The current catalyst used to catalyze the ORR reaction is mainly the noble metal Pt,which displays the best electrochemical performance in the catalytic oxygen reduction reaction.However,the high cost and rare resources of the precious metal platinum limits the further development and commercial application infuel cell technology.Therefore,the development of non-precious metal electrocatalysts with excellent catalytic performance and low price is the main research direction.A large amount of studies show that heteroatom-doped modification of carbon materials can optimize its morphology,composition,microstructureand surface physicochemical properties.It is therefore important to preparethe electrocatalysts with high electrocatalytic activity and high stability.In addition,transition metal-doped carbon-based electrocatalysts also have excellent electrochemical catalytic activity and high stability.The purpose of this thesis is to study ORR carbon-based catalysts with low cost and excellent catalytic performance,mainly including the following research contents:（1）Using dicyandiamide（DCD）as the nitrogen source and sucrose,β-cyclodextrin and chitosan as different carbon sources,N-doped graphene structure carbon nanosheet samples CN-nanosh（suc）,CN-nanosh（cyc）,and CN-nanosh（ch）were prepared by simple pyrolysis.The morphology and composition of each catalyst were analyzed by SEM,TEM,AFM,Raman,BET,XRD and XPS.Electron microscopy results show that the prepared sample resembles a nanoflake structure of tulle.The surface layer will be wrinkled a few degrees,and the edges are easy to curl and stack.This special wrinkle structure may contribute to a maximum specific surface area of 432.87 m2·g-1.The sample shows excellent ORR electroactivity in alkaline media,and its performance is close to Pt/C.The maximum power density of an alkaline Zn-air battery using CN-nanosh（suc）as a cathode catalyst is 201.33m W·cm-2.Its discharge time can reach more than 50 h at a constant current density of 100m A·cm-2until the anode zinc foil is completely consumed.It shows that CN-nanosh（suc）as the cathode catalyst shows good catalytic stability.（2）Co/Ni-doped nanocomposite catalysts（CoNi/C-N/CNT）were prepared by pyrolysis of a mixture of cobalt nickel salts,carbon nanotubes,dicyandiamide（DCD）and ethylenediamine.The prepared CoNi/C-N/CNT nanocatalyst consists of Co/Ni nanoparticles,hollow tubular C-N nanocomposites and the pristine CNTs.In the synthesized catalyst,the limiting diffusion current density of Co3Ni2/C-N/CNT45in 0.5 mol·L-1KNO3solution was 6.2m A·cm-2.A neutral Zn-air battery was constructed by using the Co3Ni2/C-N/CNT45nanocomposite as the catalyst of air electrode and metal zinc plate as the anode.In 0.5mol·L-1KNO3solution,the open circuit voltage of the neutral Zn-air battery is 1.16 V,and the maximum power density is 50.1 m W·cm-2.In addition,it can continuously discharge for 88 h and 28 h at a constant current density of 100 and 150 m A·cm-2,respectively.A series of tests showed that Co3Ni2/C-N/CNT45as a cathode catalyst for neutral Zn-air batteries has higher oxygen reduction activity and stability.（3）Dicyandiamide（DCD）,cobalt acetate,nickel acetate,ethylenediamine and different organic carbon sources such as soluble starch,β-cyclodextrin and carbon black are used as raw materials.After mixing,CoNi-loaded C-N tubular nanocomposites were synthesized by pyrolysis at 600℃and 800℃successively in a nitrogen atmosphere.The morphology and composition of each catalyst were analyzed by SEM,TEM,XPS,EDS and XRD.The prepared nanocomposite material has a hollow tubular structure,and the Co/Ni nanoparticles are evenly distributed on the C-N nanotubes.Among the prepared catalysts,the catalyst Co3Ni2/CN（st）showed more excellent performance than the Pt/C catalyst in alkaline medium.Using Co3Ni2/CN（st）as the cathode electrocatalyst,an alkaline Zn-air battery was assembled.In the 6 mol·L-1KOH electrolyte,the open circuit voltage reached 1.51 V and the maximum power density reached 331 m W·cm-2.Under different discharge current densities,Co3Ni2/CN（st）batteries exhibit a more stable and higher plateau voltage than Pt/C batteries.The battery can be continuously discharged for 80 h and 51 h at a current density of 50 and100 m A·cm-2,respectively.The results show that Co3Ni2/CN（st）catalyst has potential application prospect as an excellent alkaline Zn-air battery cathode material.（4）A neutral Zn-air battery was assembled by using the prepared Co3Ni2/C-N（st）as a cathode catalyst of air electrodes and zinc flakes as the anode,and the discharge performance of the battery was tested.The results show that in 4 mol·L-1NH4Cl+1 mol·L-1KCl solution,the maximum power density of the Co3Ni2/C-N（st）battery reaches 97.2 m W·cm-2at 196m A·cm-2,the open circuit voltage is about 1.31 V,The discharge curve at different constant current density(10 m A·cm-2,20 m A·cm-2,50 m A·cm-2and 100 m A·cm-2)shows that the voltage plateau of Co3Ni2/C-N（st）electrode is higher than that of Pt/C electrode,and the battery can discharge as long as 45 h at constant current 100 m A·cm-2.It shows that the Co3Ni2/C-N（st）catalyst can be used as an excellent cathode material for neutral Zn-air batteries.（5）Based on Co3Ni2/C-N（st）catalyst,low Pt loading catalyst Pt/CoNi@CN was also prepared,and its electroactivity for oxygen reduction reaction in neutral,acid and alkaline solution was studied.The results show that Pt/CoNi@CN catalyst exhibits poor oxygen reduction activity in the neutral solution while it has good oxygen reduction activity in acidic and alkaline solutions.It can be used as a high-efficiency cathode catalyst for fuel cells and metal-air batteries. |
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