Synthesis Of Co Modified Nitrogen-doped Graphitized-nano-diamond Catalyst For Oxygen Reduction

As an innovate energy conversion device,fuel cell is of–high conversion ratio and clean by products.However,its cathode oxygen reduction reaction（ORR）is quite slow,which affects its energy conversion efficiency.Therefore,developing efficient and durable catalysts has become a key to accelerate the ORR process of fuel cells.Commercially available Pt/C catalysts own very high catalytic activity,but their large cost and poor durability brought about by amorphous C carriers limit their further application.Nano–Diamond（ND）is a carbon material of high stability and is expected to become a durable catalyst support material.In this thesis,Graphitized–Nano–Diamond（GND）was initially prepared to obtain the carbon support material through heat treatment:The graphite shell at the surface of GND is used to provide necessary conditions for the subsequent modification treatments,and the core diamond structure in GND provides structural stability for the catalyst.Then urea and cobalt are used as surface modification agent for GND,to prepare a series of various structures of nitrogen–doped GND compounded with cobalt and cobalt oxide catalysts in various structures.The effects of their chemical composition and microstructure on the electrochemistry performance of prepared catalysts are investigated.Main research results are as follows:（1）Porous cobalt oxide microsphere growing at the surface of on Nitrogen–doped Graphitized–Nano–Diamond（Co O_x/N–GND）was prepared using hydrothermal and subsequent heat treatment process.Porous cobalt oxide of high specific surface area could expose more surface Co²⁺as active sites than bulk one does.In addition,the doping of nitrogen plays a key role in the improvement of catalytic activity.The prepared catalyst shows satisfactory performance,the half–wave potential and initial potential reached 0.82 V and 0.89 V（vs.RHE）respectively,which are close to the performance of Pt/C（0.85 V,0.95 V）;The average number of transferred electrons at0.2–0.6 V is 3.65,and the hydrogen peroxide yield is 18%.Meanwhile,the catalyst also has excellent stability,for instance,its half–wave potential only shows a left shift of 28 m V after operation of 5000 cycles,which is much lower than the left shift of the Pt/C catalyst（60 m V）.（2）The Nitrogen–doped Graphitized–Nano–Diamond with uniformly loaded Co O on the surface（Co O/N–GND）is prepared using a one–step pyrolysis method.Various cobalt addition,urea addition and pyrolysis temperature all affect the phase composition and catalytic activity of the catalyst.Catalyst containing more Co O phase is obtained,at the surface of which more Co²⁺as active sites are exposed under the conditions of 30%Co addition,10 times urea addition,and 700℃heat treatment temperature.The half–wave potential and initial potential of this catalyst are 0.82 V and 0.89 V;Besides,due to the loading of Co O at the surface is more uniform,the number of transferred electrons and the peroxide yield of this catalyst at 0.2–0.6 V reach 3.75 and 13%on average,which are better than the samples prepared using hydrothermal method;Its half–wave potential only shows a left shift of 15 m V after operation of 5000 cycles,and the attenuation degree is also smaller than that of the hydrothermal prepared catalyst.（3）Well–dispersed GND is obtained through acid treatment and ultrasonic cell pulverization.Then the Nitrogen–doped Graphitized–Nano–Diamonds with uniformly loaded Co on the surface（Co/N–GND）is obtained using microwave ethylene glycol reduction and pyrolysis nitrogen–doped treatment.The results of electron microscope observation show that the cobalt nanoparticles in size of about 20 nm locates on the surface of GND uniformly as active sites.Moreover,nitrogen–doped components also act as active sites.The half–wave potential and initial potential of the catalyst reached0.84 V and 0.92 V,respectively;While the number of transferred electrons and peroxide yield at 0.2–0.6 V reached 3.84 and 8%on average;Its half–wave potential only a left shifts 12 m V after operation a cycle of 5000 cycles,demonstrating the most excellent catalytic activity and stability in this thesis.