Post-doping Of Carbon Nanomaterials And Their Application In Oxygen Reduction Reaction And Li-S Batteries

Recently,carbon nanomaterials have been used widely in the field of energy conversion and storage due to their exceptional mechanical,electrical and chemical properties.However,the intrinsic inert surface and unable band gap adjustment greatly limit their broadened application.Doping heteroatoms into carbon frameworks can effectively alter their electronic structure and improve the physicochemical properties,which can widen their electrochemical application.Generally,to make the best use of these heteroatoms-doped carbon-based electrode materials,high dopant amount and optimized morphology,structure and composition are always pursued.Nevertheless,how to achieve highly efficient and optimized heteroatoms doping by a facile and controllable approach is still a big challenge for us.In this dissertation,carbon nanotubes and graphene were both used as the doping objects,and the post-doping methods were adopted to dope heteroatoms into carbon nanomaterials.The main works of our study focused on how to achieve highly efficient doping by introducing structural defects,constructing 3D carbon precursor and realizing multiple heteroatoms co-doping,and how to optimize the doping effect by controlling oxidation degree of pristine carbon and imposing post-treatment upon the prepared heteroatoms-doped carbon.Additionally,the electrochemical applications of the as-prepared heteroatom-doped carbon nanomaterials in the fields of oxygen reduction reaction(ORR)and lithium sulfur batteries were also investigated in depth.The specific research content was summarized in the following six points:(1)Pristine CNTs were chemically tailored into O-UCNTs by modified Hummers method,then the N-UCNTs were successfully synthesized by a facile pyrolysis process using the O-UCNTs and melamine as the carbon precursor and N source,respectively.The morphology,structure and composition of the N-UCNTs were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and so forth.The results showed that the N-UCNTs exhibited much larger specific surface areas(215.8 m2/g)and significantly higher N doping content(6.65 at%)when compared with those of N-CNTs.Besides,the ORR performance of N-UCNTs was also investigated by LSV,and the result demonstrated that N-UCNTs showed better ORR performance than N-CNTs,revealing their great potential in the application of fuel cell.(2)The N-UCNTs were firstly prepared by a facile hydrothermal process using O-UCNTs and ammonia as the carbon precursor and N source,respectively,then they were mixed with sulfur in an appropriate proportion by the thermal infusion method to obtain the N-UCNTs/S composite.The morphology,structure and composition of the N-UCNTs/S composite were characterized by SEM,Raman,XPS and so on.The results showed that the N-UCNTs had large specific surface areas and a high N content of 6.02 at%,which endowed them with excellent dispersibility and adsorbability for sulfur species.Besides,the electrochemical performance of the N-UCNTs/S cathode was studied by CV and galvanstatic charge/discharge testing,and the results demonstrated that the N-UCNTs/S cathode exhibited superior cyclability and rate capability to the counterparts.(3)The nitrogen,boron co-doped unzipped carbon nanotubes(NBUCNTs)were firstly synthesized by a facile hydrothermal process using the O-UCNTs,urea and boric acid as the carbon precursor and heteroatom sources,respectively.Then the NBUCNTs were mixed with sulfur in an appropriate proportion by the thermal infusion method to prepare the NBUCNTs/S composite.The morphology,structure and composition of the obtained NBUCNTs/S composite were characterized by SEM,XRD,XPS and so on.The results showed that the NBUCNTs exhibited enlarged specific surface areas,improved conductivity and notably increased doped N and B contents(6.6 at%and 7.0 at%)when compared with the counterparts,indicating superior dispersibility and adsorbability for sulfur species.In addition,the electrochemical performance of the NBUCNTs/S cathode were studied by CV,galvanstatic charge/discharge testing and electrochemical impedance spectroscopy(EIS),and the results demonstrated that the NBUCNTs/S cathode showed the best cycling stabiliy and rate performance.(4)The 3D nitrogen-doped graphene/unzipped carbon nanotubes composite(NG-NUCNTs)were fabricated by a facile pyrolysis method using the 3D graphene oxide/oxidized unzipped carbon nanotubes composite(GO/O-UCNTs)and urea as the carbon precursor and N source,respectively.The morphology,structure and composition of the NG-NUCNTs were characterized by SEM,Raman,X-ray photoelectron spectroscopy(XPS)etc.The results showed that NG-NUCNTs with special 3D framework displayed much higher specific surface areas and N doping content(as high as 8.0 at%)when compared with those of their counterparts.Furthermore,the ORR performance of NG-NUCNTs was also studied by CV and LSV,and the result demonstrated that NG-NUCNTs as a very promising ORR electrocatalyst had the comparative ORR activity with the commercial Pt/C catalyst.(5)Oxidized O-E-CNTs were firstly prepared by refluxing pristine CNTs with concentrated nitric acid,then N-O-E-CNTs were successfully fabricated by a scalable hydrothermal process using the oxidized O-E-CNTs and urea as the carbon precursor and N source,respectively.When the N-O-E-CNTs were mixed with sulfur in an appropriate proportion by the thermal infusion method,the N-O-E-CNTs/S composite could be synthesized.The morphology,structure and composition of the N-O-E-CNTs were characterized by SEM,Raman,XPS and so forth.The results showed that N-O-E-CNTs with special open-ended structure achieved successful N doping while retaining excellent conductivity,and they exhibited improved dispersibility and adsorbability for sulfur species when compared with pristine CNTs.Apart from these,the electrochemical performance of the N-O-E-CNTs/S cathode were studied by CV,galvanstatic charge/discharge testing and EIS,and the results demonstrated that the N-O-E-CNTs/S cathode showed clearly enhanced cycling stability and rate capability.(6)Nitrogen,sulfur co-doped graphene(NSG)were prepared by a one-step hydrothermal process using graphene oxide(GO)and(NH4)2S as the carbon precursor and heteroatoms source,respectively.Then the NSG suffered from post-heat treatment at 700 ? for 1 h under N2 atmosphere to synthesize heat-treated NSG(HNSG).The morphology,structure and composition of the HNSG were characterized by SEM,XRD,XPS and so on.The results showed that HNSG exhibited much more crumpled morphology,better graphitization degree and higher content of pyridinic N as well as graphitic N and-C-S-C-,respectively.Besides,the ORR performance of HNSG was also investigated by CV and LSV,and the result demonstrated that HNSG showed a nearly four-electron ORR pathway,and presented much better ORR performance than the counterparts.