Preparation Of Non-noble Metal Electrocatalyst Derived From Metal Organic Framework And Performance Of Oxygen Reduction Reaction

The overexploitation and use of non-renewable fossil fuels has caused two major problems:energy shortage and environmental pollution.The development of efficient,green and friendly new sustainable energy has become a major challenge in the 21st Century.New fuel cells such as proton exchange membrane fuel cells have high efficiency,low emissions,and high energy density,and are a sustainable advanced energy technology.In these fuel cells,the kinetic properties of the cathodic oxygen reduction reaction（ORR）are very slow（six or more orders of magnitude slower than the anode reaction）,which limits the performance of the cell and affects its commercial application.Therefore,it is particularly important to develop efficient ORR catalysts.Currently,precious metal platinum and its alloys are considered the best ORR catalysts.However,the limited reserves and expensive price of platinum have seriously affected the commercial application of fuel cells.In addition,platinum-based catalysts perform poorly in terms of methanol resistance and durability.Therefore,the development of low-cost,efficient,and stable oxygen reduction catalysts has become a current research focus.Nitrogen-doped carbon-based transition metal（Co,Fe,etc.）nanocomposites have shown great potential in catalyzing oxygen reduction reactions and are expected to replace Pt/C.Metal organic frameworks（MOFs）is a kind of crystalline porous material which is self-assembled by metal ions and organic ligands.Compared with traditional porous materials,these materials have the advantages of building units and diversity of composition,and can introduce different functional groups and metal centers through reasonable design.The metal elements and organics that make up the metal organic framework compounds can be converted to metal/metal oxides and porous carbon under certain conditions.Due to the uniformity and purity of MOFs,the obtained metal oxides and porous carbon have very uniform structure and composition.Selecting appropriate organic precursors and metal species can obtain porous carbons with different morphologies,while other precursors can be added during the synthesis process to introduce heteroatoms to further improve the properties of porous carbon.In this work,metal organic framework compounds are used as precursors to prepare porous carbon electrocatalysts for non-noble metal composites,and the applications of these electrocatalysts for ORR are studied in detail.They mainly include the following parts:（1）A high temperature annealing method using MOFs as precursors was used to prepare iron carbide（Fe₃C）/N-doped tremella-like porous MOFs nanosheets（NPM）composites.The experimental results show that the obtained electrocatalyst has the characteristics of many active sites,large specific surface area,high degree of graphitization,and good conductivity,and nitrogen doping further improves its oxygen reduction performance.FE-SEM tests show that the electrocatalysts prepared with different amounts of ferrous sulfate heptahydrate pyrolyzed at high temperature have different morphologies.When the added amount is 0.04 g,a special tremella-like morphology for porous carbon complex was obtained.TEM results further confirmed that the NPM-0.04 electrocatalyst has a fluffy tremella-like morphology,and the material contains nanoflakes and spherical nanoparticles.XPS characterization confirms the type of N doping and confirmed the presence of Fe-N₂,which is also one of the ORR active sites.In addition,the presence of defective oxygen in the electrocatalyst can promote the transport of electrons in O₂,and ensure the rapid exchange kinetics of O₂^-/OH^-,thereby improving the electrochemical performance of ORR.The BET results show that the electrocatalyst has a high specific surface area and a rich microporous structure.The fluffy,multilayer,interconnected and porous three-dimensional structure is conducive to electron transport.The ORR performance test was performed on the rotating disk electrode（RDE）and the rotating ring disk electrode（RRDE）in an alkaline system and the NPM-0.04electrocatalyst shows good electrochemical performance.The Koutecky-Levich（K-L）equation analysis shows that the number of electron transfers in the reaction is very close to 4,which is an ideal four-electron path process.Using the chronoamperometry method in a 0.1 M KOH saturated O₂electrolyte for 10000 s,NPM-0.04 retained 97%of its initial current value,and its durability was better than Pt/C.（2）Plasma enhanced atomic layer deposition（PE-ALD）was used to deposit cobalt oxide on MOFs,and annealing was performed at 925℃to generate cobalt-based composite nanomaterials.The experimental results show that the use of MOFs as the substrate can increase the conductivity of the electrocatalyst.The resulting material is derived from carbon nanotubes,and the cobalt nanoparticles are encapsulated on the top of the material.From the analysis of XRD data,the electrocatalyst contains cobalt substance.FE-SEM test proves that carbon nanotubes derived from MOFs provide a large specific surface area and help to enhance the electrocatalytic activity of the catalyst.TEM confirms that there were a large number of nanotubes on the surface of the NPM-200 electrocatalyst and the distribution is uniform,and the nanoparticles are encapsulated at the top.Corresponding EDS images indicate the uniform presence of cobalt,carbon,oxygen,and nitrogen,while the cobalt element exists only at the top of the carbon nanotubes.When used as an ORR electrocatalyst,the surface has good electrochemical performance.NPM-200 has the highest reaction onset potential and half-wave potential.According to analysis of RDE and RRDE data,ORR has an ideal four-electron path.The chronoamperometry method was used to test the stability of the NPM-200 electrocatalyst for 9 h.It retains 97.8%of its initial current value and exhibits better durability than Pt/C.（3）Nanotubes were derived from N-doped porous MOFs nanosheets（NPM）using PE-ALD technology combined with heat treatment and phosphating processes.Results show that on NPM-200 Co/Co P species were produced during the phosphating process,thereby forming a carbon nanotube-coated Co/Co P composite nanostructure on NPM with a large specific surface area.The XRD data confirms that the addition of different amounts of phosphorus sources caused the components in the nanostructure of the electrocatalyst to be regulated accordingly.SEM analysis shows that the amount of different phosphorus sources also had a great effect on the morphology of the products.High-resolution TEM further proves that cobalt phosphide nanoparticles were encapsulated on the top of carbon nanotubes.XPS tested the composition of the surface elements of the electrocatalyst,and shows that Co,C,N and P elements were present in the electrocatalyst.N and P were successfully doped into the defect sites of CNTs by replacing C or O atoms.The rich pore volume and the presence of micropores greatly increase the specific surface area and provide a high density of active sites.When used as an ORR electrocatalyst,NPMCNT-300 shows good electrochemical performance,and its initial potential for ORR is 0.924 V.In addition,the electrocatalyst also exhibits excellent stability.