Carbon Nanocomposites Derived From Metal-organic Frameworks:Synthesis And Application In Electrocatalysis

As a new renewable energy,hydrogen is considered to be one of the best energy carriers in the 21st century due to its high energy density,easy storage,no pollution,raw materials and products are water.Water electrolysis is environment-friendly and sustainable method to produce hydrogen.Platinum-based catalysts are currently the most advanced electrocatalysts for hydrogen evolution reaction.However,due to the scarcity of resources,high price and poor stability,the large-scale application of Pt is limited.Therefore,it is urgent to develop catalysts with excellent HER catalytic performance to reduce the overpotential of hydrogen evolution reaction and improve the stability.As a kind of energy conversion device,fuel cell has the characteristics of high energy conversion efficiency,environment-friendly,wide fuel source,etc.,which has great development and utilization value.For fuel cells,the mechanism of cathode oxygen reduction reaction(ORR)is complex and the kinetic process is slow.Therefore,it is necessary to develop catalysts with high catalytic activity and high stability to improve the performance of the fuel cells.In this thesis,carbon nanomaterials derived from metal organic frameworks(MOFs)are used as precursors.Various electrocatalysts were prepared by different ways such as nano-engineering,alloying,and doping,etc..The morphologies,structures,compositions and electrocatalytic performances of the target materials were investigated.The main points of this thesis are presented as follows:(1)Using ZnO nanorods as templates,a layer of ZIF-8 were grown on the surface of ZnO nanorods by self-assembly reaction(ZIF-8@ZnO).Nitrogen doped porous carbon nanotubes(NCNTs)were obtained by carbonization ZIF-8@ZnO at low temperature and further etching with HCl.N,P,F-doped porous carbon nanotubes(N,P,F-CNTs)were prepared by calcination of NCNTs mixed with ammonium hexafluorophosphate at high temperature.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and Specific surface area and aperture analyzer(BET)were used to investigate the morphology and structure of the materials.The synthesized N,P,F-CNTs have a length of about 4?m,wall thickness of 20 nm,inner diameter of 120 nm,a large specific surface area(1013.9 m²g^-1)and abundant pore structure.When N,P,F-CNTs were used as ORR electrocatalyst,a good catalytic activity,stability and resistance to methanol are obtained in 0.1M KOH solution.The onset potential and half-wave potential of N,P,F-CNTs were 0.94 V and 0.834 V,respectively.The electron transfer number of ORR was close to 4.The ORR performance of N,P,F-CNTs was better than commercial Pt/C.(2)Fe,Co-ZIF-8 was obtained by adding cobalt nitrate and iron acetylacetone in the synthesis process of ZIF-8.Then Fe,Co co-doped nitrogenous carbon polyhedron(Fe,Co-NC),which has rich Fe-N-C and Co-N-C catalytic structure,was obtained by high temperature carbonization Fe,Co-ZIF-8 and further etching with dilute acid.The morphology,structure and composition of the materials were characterized by SEM,TEM,XRD,XPS and BET.Fe,Co-NC catalyst has regular dodecahedral shape,uniform particle size,large specific surface area(618.12 m²g^-1)and abundant pore structure.When used as a ORR electrocatalyst in 0.1 M KOH solution,Fe,Co-NC shows a excellent oxygen reduction performance.The onset potential,half-wave potential,limit current density and number of electron transfers of Fe,Co-NC are 0.97V,0.82 V,4.05 m A cm^-2and close to 4,respectively.At the same time,Fe,Co-NC also shows good stability.The ORR performances of Fe,Co-NC is better than the commercial Pt/C.(3)Nitrogen doped porous carbon polyhedron(NPC)was prepared by carbonization ZIF-8 under Ar atmosphere at 1000 ^oC.Then,porous carbon polyhedron inset with Rh nanoparticles(Rh@NPC)was synthesized by microwave-assisted glycol reduction method.The morphology and structure of the materials were analyzed by SEM,TEM,XRD,XPS,BET and ICP-OES.The catalytic properties of Rh@NPC for HER and ORR were investigated by cyclic voltammetry(CV),linear sweep voltammetry(LSV),electrochemical impedance spectroscopy(EIS)and I-t technologies.With the appropriate amount of Rh(9.84 wt.%),Rh@NPC exhibited excellent HER catalytic activity.To reach the current dentisy of 10 m A cm^-2,Rh@NPC only needs the overpotential of 20 m V,23 m V and 45 m V in alkaline,acid and neutral media,respectively.Rh@NPC also shows excellent ORR performance in alkaline medium.Its half-wave potential is 0.84 V,which is better than commercial Pt/C catalyst.In addition,due to the ultra-fine size of Rh nanoparticles and the carbon coating effect,Rh@NPC catalyst exhibited excellent long-term stability during HER and ORR.(4)Nitrogen doped porous carbon polyhedron(NPC)was prepared by carbonization ZIF-8 under high temperature condition.Then,a series of nitrogenous porous carbon polyhedrons inset with Ru Pd alloy nanoparticles(Ru_xPd_1-x@NPC)were prepared by microwave-assisted glycol reduction method.The morphology and structure of the material were analyzed by SEM,TEM,XRD,XPS,BET and ICP-OES.The HER catalytic performances of the catalyst were investigated by CV,LSV,EIS and I-t curves.The results show that the prepared catalyst has excellent HER catalytic activity due to the synergistic effect of composition and structure.Ru_0.7Pd_0.3@NPC showed the best HER activity in alkaline solution,with the?₁₀of 20 m V;Ru_0.3Pd_0.7@NPC showed the best HER activity in acidic solution,with the?₁₀of 35m V.In addition,the catalysts exhibited good electrocatalytic mass activity and long-term stability in both alkaline and acidic solutions,which were far superior to commercial Pt/C catalysts.(5)Using the nitrogen-doped porous carbon polyhedron(NPC)obtained from high-temperature carbonization of ZIF-8 as the carrier,the N,P co-doped porous carbon polyhedron loaded with ultrafine Ru P₂nanoparticles(Ru P₂@NPC)was prepared by microwave-assisted reduction and low-temperature phosphating treatment.SEM,TEM,XRD,XPS,BET and ICP-OES were used to analyze the microscopic morphology and structure of the material.The results show that Ru P₂@NPC has a regular and uniform dodecahedron structure and a very low amount of precious metal Ru(5.62 wt.%).Moreover,the size of the encapsulated Ru P₂nanoparticles is ultrafine.When using Ru P₂@NPC as a HER electrocatalyst,it shows greatly improved HER properties after the phosphating process.To reach the current dentisy of 10 m A cm^-2in alkaline medium,Ru P₂@NPC only needs the overpotential of 19 m V,which is better than commercial Pt/C catalyst(?₁₀=30 m V).At the same time,the catalyst also showed excellent quality activity and long-term stability.In addition,Ru P₂@NPC catalyst also showed good electrocatalytic activity(?₁₀=44 m V)and long-term stability in acidic solution.