Preparation And Electrochemical Properties Of Transition Metal/Nitrogen Doped Carbon Composites

With the development of sustainable energy economy,fuel cell（FC）and metal-air battery（MAB）have attracted global attention because of their high energy density and the ability to effectively convert and store electric energy.The cathodic reaction of these two devices is essentially oxygen reduction reaction（ORR）,but the widely used platinum-based precious metal catalysts have some key problems,such as high cost,poor stability,poor alcohol resistance and so on,which limit their large-scale application.Therefore,it is necessary to explore and develop ORR catalysts with high activity,low cost and excellent stability.Due to its large specific surface area,adjustable porosity and uniform pore structure,MOFs,as precursors,are designed and synthesized by a simple in-situ calcination method for transition metal/carbon composites,which have good catalytic properties.The synergism effect between different metal components makes them produce different active centers,and the derived carbon materials can also provide better conductivity for the catalyst,so they are considered as potential candidates for ORR catalysts.Based on this,this paper used nickel-based MOFs as precursors to synthesize nickel/nitrogen/carbon composite materials in situ bycarbonization and chemical vapor deposition,and further explored the effect of the synthesis process on the physical and chemical properties and electrochemical properties of the materials,through Introducing binary metals and graphene quantum dots to construct a multi-component composite material of nickel-cobalt alloy/graphene quantum dots/nitrogen-doped carbon nanotubes,in order to improve its ORR catalytic performance in many ways.The main contents are as follows:1.Flake nickel-based MOFs precursors were synthesized from nickel nitrate hexahydrate and pyromellitic tricarboxylic acid.Using nickel as the metal source for the growth of carbon nanotubes by chemical vapor deposition and dicyandiamide as the carbon source for the growth of carbon nanotubes,a nitrogen-doped carbon-supported nickel composite was prepared by one-step carbonization,in which carbon carriers exist in the form of carbon nanotubes and MOF-derived carbon.Its morphology,structure and element morphology were characterized.The effects of metal doping amount of the precursor,the proportion of raw materials,thetemperature and time of carbonization on the morphology and electrochemical properties of the material were investigated.When the molar ratio of metal to ligand in the precursor is 3:1,the mass ratio of Ni₃-MOF to dicyandiamide is 1:10,the carbonization temperature is 700C and the carbonization time is 1h,the prepared catalyst Ni₃-NCNT shows the best catalytic performance of ORR.Its half-wave potential is 0.772V（V vs.RHE）.After adding methanol,i Murt test is carried out,and the current density is 78.76%of the initial value.2.Based on the above work,cobalt nitrate hexahydrate was introduced into the synthesis of precursor Ni₃-MOF to prepare bimetallic MOFs,and their morphology and structure were characterized.Using bimetallic MOFs and dicyandiamide as raw materials,graphene quantum dot dispersion was added in the mixing process,and then carbonized at high temperature to obtain nickel-cobalt alloy/graphene quantum dots/nitrogen-doped carbon nanotubes multicomponent composites.The effect of the ratio of metal in the precursor on the morphology and electrocatalytic performance of the catalyst was investigated.It was concluded that the Ni Co₂-MOF synthesized when the molar ratio of nickel to cobalt in the precursor was 1:2,and the catalyst Ni Co₂-GQD-NCNT was obtained by carbonization,which had excellent ORR catalytic performance.The onset potential of the catalyst is 1.180V（V vs.RHE）;the half-wave potential is 0.803V（V vs.RHE）,which is higher than the half-wave potential of commercial Pt/C 0.797V（V vs.RHE）;after adding methanol,the current density is 90.79%of the initial value,while the commercial Pt/C is only 82.94%.Many results show that the catalyst Ni Co₂-GQD-NCNT has better catalytic performance for oxygen reduction than commercial Pt/C.