Preparation And Oxygen Reduction Catalytic Performance Of Cobalt Oxide And Its Carbon-nitrogen Composites

Magnesium air battery has many advantages,such as high energy density(3910Wh/kg),high theoretical voltage(3.09 V),large theoretical specific capacity(2205m Ah/g),low cost,light weight,environment friendly and abundant magnesium storage,showing great application potential.At the cathode,the oxygen reduction catalyst provides the key catalysis,which is responsible for accelerating the reaction of the anode material with oxygen in the electrolyte.Therefore,the performance of the cathode oxygen reduction catalyst directly affects the performance of the whole electrochemical system(energy efficiency,rate,life and cost,etc.),which plays a crucial role in the final performance of the magnesium air battery.Therefore,looking for high performance,high efficiency,durable,low cost,stable oxygen reduction(ORR)catalyst is the key to the development of metal air battery.In this paper,Co₃O₄ and its carbon and nitrogen composites with different morphologies were prepared by one-step hydrothermal method and two-step hydrothermal method,as well as high temperature pyrolysis method.The microstructure and crystal structure of the experimental products were characterized,and the electrochemical properties of the composites were tested by electrochemical workstation and rotating disc electrode.The main research contents and results are as follows:(1)Three kinds of Co₃O₄ with different micromorphology(rod-like,spherical and flake-like)were prepared by one-step hydrothermal method with cobalt acetate as the cobalt source by adding different alkali sources,solvents and active agents.The mechanism of morphology formation is discussed and analyzed.The specific surface area test and electrochemical performance test results showed that the three different morphologies of Co₃O₄ had certain catalytic activity for oxygen reduction,and the rod-like Co₃O₄ with the largest specific surface area had the highest activity.The initial potential and half-wave potential of Co₃O₄ were 0.78V and 0.62V,respectively,which still had a certain gap with commercial Pt/c.However,it paved the way for the subsequent modification and preparation of composite materials.(2)Carbon and nitrogen composites(N-C)were prepared by two-step hydrothermal method,and then nano-sized Co₃O₄-CeO₂/N-C composites were prepared with different metal oxides.By contrast,Co₃O₄/N-C and CeO₂/N-C composites were also prepared at the same time.Among them,Co₃O₄-CeO₂/N-C showed the best catalytic activity,and its initial potential and half-wave potential were 0.88 V and 0.72 V,respectively,which were close to the 0.91 V and 0.74 V of commercial Pt/c.And after5,000 cycles,the half-wave potential has shifted to the left by only 25 mV.(3)Co-MOF-900Ar precursor was prepared by hydrothermal method using dimethyl imidazole as organic ligand and adding metal salt.Then,Co-MOF-900Ar material was pyrolyzed into rhomboid dodecahedron at 900?by high temperature pyrolysis in Ar atmosphere.For comparison,At the same time,the pyrolysis products in air atmosphere and at 700?were also prepared.Among them,the experimental products pyrolysis in 900?Ar atmosphere have the best catalytic activity,and their initial potential and half-wave potential reach 0.86 V and 0.71 V,which is close to commercial Pt/c catalyst.In addition,after 5000 cycles,the half-wave potential of its only shifted to the left by 22 mV,while after 5000 cycles,the half-wave potential of commercial Pt/c catalyst shifted to the left by 30 mV,indicating that it showed better cycle stability.