Preparation Of One-Dimensional Cobalt-Based Carbon Fibers By Electrospinning And Its Oxygen Reduction Performance

Fuel cell is regarded as one of the best alternatives to fossil fuels due to its high energy conversion effciency and environmental friendliness.However,the cathodic oxygen reduction reaction?ORR?exhibits slow kinetics,which requires effective electrocatalysts.Pt-based materials are widely used as state-of-the-art ORR electrocatalysts in fuel cells due to their excellent electrocatalytic activity.Nevertheless,their disadvantages of high cost and poor stability significantly limited their large-scale commercial applications.Therefore,developing a kind of highly active,low-cost non-precious metal ORR catalysts has been considered as one of effective strategies to accelerate the commercial application of fuel cell.In this thesis,electrospinning technology were employed to construct 1D carbon nanofibers with controllable structures achieved by adjusting the synthesis conditions.Meanwhile,N and transition metal Co were rationally incorporated into the carbon nanofibers toenhance their catalytic activity.Moreover,the effects of catalysts composition and morphology on the ORR performance were investigated by further physical characterization and electrochemical tests.The details are as follows:?1?N-doped 1D Co-based carbon nanofibers?N-Co@CNFs?was prepared by an electrospinning method and subsequent urea post-treatment method.The electrochemical results show that the N-Co@CNFs exhibited positive ORR peak potential of 0.78 V,satisfactory half-wave potential of 0.77 V,and limiting current density of 4.90 mA/cm² at 0.2 V vs.RHE.The HO₂^-yield at 0.4 V is only 18.94%,corresponding to an electron transfer number of 3.60.The current density still maintained 86.7%of the initial value after 10000 s operation at the half-wave potential.Such excellent electrocatalytic properties of N-Co@CNFs should be ascribed to the effective integration of highly active N-doped sites and Co-based sites on the conductive skeletonof 1D carbon nanofibers via electrospinning technology coupled with urea post-treatment strategy.?2?A metal-organic-framework?MOF?of ZIF-67 with rich N and Co was employed as the precursor and co-eletrospun with polyacrylonitrile?PAN?through electrospinning to prepare highly ZIF-67-dispersed porous carbon nanofiber?ZIF-67@PAN?,the final catalyst of ZIF-67@PAN-800 was obtained after pyrolysis at 800°C.The electrochemical results show that the prepared ZIF-67@PAN-800 exhibited more positive ORR peak potential?0.83 V?,satisfactory half-wave potential?0.82 V?and high limiting current density?4.90 mA/cm² at 0.2 V vs.RHE?.The HO₂-yield was 16.47%?at 0.6 V vs.RHE?,corresponding to an electron transfer number of3.71.The current density was still maintained 89.1%of initial value after 10000 s operation at half-wave potential.The incorporation of ZIF-67 nanoparticles significantly increased the N doping level in the carbon nanofibers?4.47%⁷.77%?and the graphitization degree,thereby improving the catalytic activity,four-electron selectivity and durability of the ZIF-67@PAN-800catalyst.?3?In order to further impove the utilization of N and Co sites and reaction mass transfer dynamics,Zn?acac?₂,PS and PAN were co-electrospun to construct interconnected porous multi-channels carbon nanofibers?MCCNFs?.Furthermore,well-defined Co-core-Co₃O₄-shell nanoparticles were embedded on the multi-channel carbon nanofibers skeleton via solvothemal growth of ZnCo₂O₄ on the precursor nanofibers as the intermediate and subsequent carbothermal reduction and evaporation of Zn.Electrochemical tests indicated that the obtained Co/P-MCCNFs exhibited a high half-wave potential?⁰.82V?,high limiting current density?⁵.08mA/cm²?,low HO₂^-yield?below?10%,corresponding to the electron transfer number of?3.90?as well as satisfactory long-term durability with 88.4%of initial current after 10,000 s operation at half-wave potential.Such electrosping technology combined with ZnCo₂O₄ intermediate method can facilely achieve the synchronous construction of the porous multi-channel structures and highly active Co-core-Co₃O₄-shell nanoparticles,significantly improving the ORR catalytic activity and the four-electron selectivity of the prepared Co/P-MCCNFs catalyst.