Preparation Of Tri-metallic Oxide On N-doped Carbon Material Catalysts And Their Electrocatalytic Performance For Oxygen Reduction Reaction

The direct methanol fuel cell（DMFC）is a versatile energy converter with high performance and environmental friendliness.The performance of the fuel cell depends mainly on the cathode ORR performance.The key point of improving fuel cells’performance is to improve and optimize the performance of cathode catalysts,which can increase ORR speed.At present,the best active cathode catalyst is Pt/C,but its high cost and poor methanol resistance directly hinder the commercialization of DMFC.Transition metals and their oxides have always been considered as the materials with the greatest potential to replace precious metal Pt-based catalysts for cathodic oxygen reduction catalysis of fuel cells due to their inherent activity and sufficient stability.Transition metal oxides supported on nitrogen-doped carbon composites have good stability,methanol resistance,and low cost.Research on ORR catalysts has become a hot topic in this field.The first is to prepare the tri-metal oxide/nitrogen-doped carbon material catalyst nano-M₁M₂M₃O₄/N-C-T by using the method of pre-hydrothermal and high temperature calcination（M₁,M₂,M₃ are different transition metals Zn,Ni,Cu,Mn,Co and Fe）,and a series of different catalysts with different N sources,different carbon carriers,different calcination temperatures,different metals etc were prepared,whose composition and structure were characterized by XRD,SEM,etc.The results are as follows:（1）The metal oxide particles are uniformly dispersed on the graphene,and the particle size is basically below 50 nm,which indicates that the oxide particle size of the nano-CuCoFeO₄/N-rGO composite catalyst reaches nanometer size,which is also a catalyst.（2）Nitrogen doping has great influence on the electrocatalytic activity of the catalyst.When metal and nitrogen are simultaneously doped into graphene,the interacted and prepared catalyst material can play good catalytic effects;with multi-wall carbon Carbon nanotubes（MWCNT）,carbon black（BP2000）and reduced graphene oxide（rGO）are the catalysts of carbon carriers.The graphene as carbon carrier has the best electrocatalytic performance.（3）The calcination temperature also has great influence on the electrocatalytic activity of the prepared catalyst.At 700°C,the catalyst nano-CuCo FeO₄/N-rGO-700 had the best ORR activity,and the number of electrons transferred is closest to the cleaning process of 4 electrons to generate H₂O.（4）ORR catalytic activity of trimetal oxide catalysts is superior to those of single metal and bimetallic catalysts,and there is a synergistic effect between metals,which can enhance the ORR performance;the properties of the metal itself also affect the electrocatalytic performance of the catalysts.Among different nano-M₁M₂M₃O₄/N-rGO catalysts with different transition metals,the nano-CuCoFeO₄/N-rGO catalysts containing Cu,Co and Fe have the best catalytic activity for ORR,and the catalyst nano-CuCoFeO₄/N-rGO is the only one with a starting potential and mass activity is larger than the commercial Pt/C catalyst.（5）The optimum catalyst nano-CuCoFeO₄/N-rGO is compared with the commercial Pt/C catalyst for methanol resistance and stability.Both the stability and methanol resistance of the catalyst nano-CuCoFeO₄/N-rGO are superior to commercial Pt/C catalyst.The second part mainly investigates the effect of particle size on the electrocatalytic performance of transition metal oxide doped graphene catalytic materials:（1）During the preparation of the tri-metal oxide catalyst nano-CuCoFeO₄/N-rGO,it was found that whether or not ethylenediamine was added had a significant effect on the particle size and electrocatalytic performance of the catalyst.The particle size of the metal oxide particles in Cu CoFeO₄/N-rGO is small,and its electrocatalytic activity is also better.（2）A simple monometallic oxide catalyst Fe₃O₄/N-rGO was prepared by hydrothermal method,and a series of catalysts with different particle sizes were obtained by changing the hydrothermal temperature.Then the electrocatalytic performance of the catalyst was tested.In the particle size range from 23 nm to56.6 nm obtained in this chapter,all the starting potential,half-wave potential,and mass activity of the catalyst Fe₃O₄/N-rGO show an increasing trend with decreasing particle size.（3）In order to further analyze the relationship between the electrocatalytic activity of the trimetal oxide catalyst and the particle size,attempts were made to prepare nano-CuCoFeO₄/N-rGO catalysts with different particle sizes by changing the hydrothermal temperature.The effect of hydrothermal temperature on the particle size of the prepared catalyst is not significant,but in the limited particle size range,the electrocatalytic activity of the tri-metal oxide catalyst nano-CuCo FeO₄/N-rGO increased with the particle size of prepared catalysts reduced.