Design Of Carbon-based Nanomaterials And Study Of Their Electrocatalytic Properties

With the rapid development of human civilization and the advancement of science and technology,traditional fossil energy-based traditional energy sources have gradually become insufficient to meet the needs of human production and life.Human beings have increasing demands for new energy and new materials.Traditional fossil energy is non-renewable resources,and the environmental pollution it caused has become a major problem that restricts human development and threatens the survival of human.It is imperative to vigorously develop new energy utilization and storage methods which are clean,environmentally friendly,efficient,convenient and sustainable.Fuel cells and metal-air batteries are a class of clean and environmentally friendly new energy batteries with high energy conversion efficiency.The oxygen reduction reaction(ORR)and the hydroxide reaction reaction(HOR),which are the cores of the operation of these two types of battery,have a huge demand for platinum-based precious metal catalysts,resulting in the high cost of such batteries,limiting their commercial application.As one of the most widely distributed elements on earth,carbon exists in various forms.Carbon-based materials are widely used in various fields due to their own chemical and physical properties,and new carbon-based materials have been continuously developed and utilized.The excellent performance of carbon-based nanomaterials in energy conversion and storage has made it a hot spot in global research.In this paper,a series of carbon-based nanomaterials are designed and synthesized for the catalytic reactions of fuel cells and metal-air batteries,and their catalytic mechanisms are studied in depth.The carbon-based materials developed include:(1)Non-doped carbon materials(CDs-1000)based on carbon nanodots(CDs);(2)N,B co-doped graphene materials(NBC-1000);(3)N self-doped porous carbon material(C111-900)obtained from spirulina by pyrolysis;(4)Composite catalyst(Pt W₆O₂₄-CDs-4)composed of carbon dots(CDs)and polyoxometallates(POMs).Works in this paper can be mainly divided into the following parts:1.First,a simple method of electrochemically ablating graphite rods was used to prepare carbon nanodots(CDs)with functional groups,then,a non-doped carbon material(CDs-1000)was obtained by using CDs as the only reactant and annealing at high temperature under the protection of inert gas.This non-doped carbon material has similar catalytic behavior to Pt when p H?10,exhibiting excellent oxygen reduction catalytic activity.And it also has stronger stability and resistance to methanol poisoning than Pt.In order to explore the catalytic mechanism of this catalyst,a variety of characterization methods such as Cs-corrected TEM and synchrotron radiation X-ray absorption spectroscopy(XAS)were used to study and analyze the interface structure and fine electronic structure of this catalyst.And the law of catalytic performance with p H was studied,then it is determined that the C-O-K group generated in an alkaline environment is the real active site.Structural models(CD-K,CD-Na,Pt-111)were constructed and DFT calculations were performed to compare and analyze the oxygen adsorption energy,the distance of O-O bond opened,and the charge on each O atom at these active sites.The results show that the CD-K and CD-Na structures have catalytic activity similar to that of Pt(111).The HOPG treated by O plasma produced catalytic activity for ORR,and the obvious presence of potassium atoms was observed on the STM image.2.Graphene was used as the base material,2,3-diaminophenazine(C₁₂H₁₀N₄)and boric acid(H₃BO₃)were the sources of N and B,respectively.N and B co-doped graphene nanosheet(NBC-1000)was synthesized by a simple two-step method.The material exhibits oxygen reduction performance close to that of 20%Pt/C electrocatalyst under alkaline conditions,while exhibiting excellent stability and excellent resistance to methanol poisoning.The N,B co-doped carbon materials synthesized in this work show higher catalytic activity than other reported catalysts of the same type.The relationship between the pyrolysis temperature and the ratio of types of bonding for the N element was studied by XPS spectra.We studied the difference of electron transfer number and the proportion of H₂O₂ during the reaction between diatomic doping and single doping.Those results indicated that the synergistic effect of diatomic doping plays an important role in the improvement of catalytic activity for ORR.3.We used spirulina as the biomass precursor,mixed with Si O₂ and Zn(NO₃)₂as templates.And then a biomass-derived carbon material catalyst(C111-900)with a multi-stage pore structure and a large BET area(1446 m² g^-1)was synthesized at a high temperature of 900?.The rich protein of spirulina makes this carbon material have a large and uniform doping of N atoms,and the final doping amount of nitrogen reached about 8.24%,which is beneficial to the generation of catalytic active sites.During the electrochemical test,C111-900 exhibits excellent oxygen reduction catalytic activity,with a onset potential of 0.95 V and a half-wave potential of 0.85 V.The catalytic reaction is a 4-e^-process,which is close to that of commercial 20%Pt/C,Moreover,the cyclic stability of C111-900 is much better than that of 20%Pt/C.In addition,the C111-900 catalyst was used as cathode material in zinc-air battery,and the battery shown higher open circuit potential(1.49 V),power density(138.5 m W cm^-2),specific capacity(766.4 m Ah g_Zn^-1)and energy density(958 Wh Kg_Zn^-1)than that of 20%Pt/C.In addition,it has better stability than 20%Pt/C during high current discharge testing.Our work proves that with the use of dual templates,cheap and available biomass materials can be converted into carbon-based electrocatalysts with high efficiency oxygen reduction activity.The simple synthesis method combined with the wide source of raw materials could provide the possibility for the large-scale production catalysts with efficient ORR activity.4.A HOR catalyst(Pt W₆O₂₄-CDs-4)with low platinum content was synthesized by combining CDs with platinum-containing polyoxometalates.In acidic environment,this low-platinum(2.18%)electrocatalyst exhibits HOR properties far exceeding 20%Pt/C,and its mass activity is about 9.5 times than that of commercial Pt/C.In addition,the catalyst has excellent resistance to CO poisoning and excellent stability.In this work,it was the first time that polyoxometalates were used as HOR catalysts in acidic environments,finding a new way for the development of HOR catalysts.It is interesting to note that the catalytic activity was enhanced by the CDs introduced.On the one hand,the proton adsorption capacity of carbon dots could accelerate the stripping speed of the reaction products,which could greatly promote the catalytic kinetic process of HOR.On the other hand,CDs have a significant enhancement effect on the resistance of materials to CO poisoning.In addition,carbon dots can form abundant hydrogen bonds with POMs,which could greatly enhance the structure stability of this material in solution.