Study On The Synthesis Of Nitrogen-Doped Cabon Catalysts For Oxygyen Reduction Reaction Through MnO₂ Sacrificed Template Method

Oxygen reduction reaction（ORR）is of utmost importance in fuel cell technologies.Nevertheless,the ORR is kinetically very sluggish and strongly relies on precious and scarce platinum-based catalysts,which serious hampers the large scale production of hydrogen-powered fuel cell vehicles.At this juncture,design and fabrication of low-cost but highly efficient catalysts for ORR has been the key issue for the large-scale application of fuel cells.Carbon-based nonprecious metal catalysts（NPMCs）are one of hopeful candidates to replace Pt-based catalysts.In the past decades,considerable progresses have been achieved in the development of NPMCs,however,the electrochemical performance of them is still far from that of Pt-based catalysts.Currently,the research directions in this field focus on the exploration of advanced technologies for increasing the density of active sites,anddesigning catalysts with reasonable nano/microstructures.Therefore,this thesis has carried out the following two aspects research work:（1）Conventional template methods for the synthesis of 3D nanostructured catalysts often require complex synthetic and removal procedures,which add much production cost and make these techniques economically unfeasible.In this work,we successfully prepared three different nanostructured PANI,namely,PANI nanotube,PANI nanoflower and PANI nanorod,based on“MnO₂ sacrificed template”method.Then subjecting them into high temperature carbonization process could obtained three different nanostructured nitrogen-doped cabon catalysts（NC）.Electrochemical experiments demonstrated that the ORR activity of as-prepared NC catalyst strongly depends on their morphology.The PANI nanotube derived NC（NC-tube）exhibits the best ORR catalytic activty in alkaline media,whose value of the half-wave potential was only 23mV lower than that of commercial JM-Pt/C.However,SEM results showed that the structure of three NC catalysts collapsed at high temperature,which would unfavor for the effective exposure and ultilization of nitrogen-containing active sites.（2）In order to preserve the morphology and structure of pristine precursors during high temperature carbonization process and thus left the exposure and ultilization of nitrogen-containing active sites,we elaborately designed and synthesized a well-defined porous nitrogen-doped carbon nanotube with open-ended channels（O-NCNTs）as ORR catalyst by a twice pseudomorphic transformation of MnO₂ nanotubes.During pyrolysis,the NaCl crystal functions as a fully sealed nanoreactor to facilitate nitrogen incorporation and graphitization,and a structure fixer to ensure the final non-collapsed carbon structure.Benefiting from its fantastic structure,the increased exposure of the active sites is easily achieved since the open-ended channels with the porous surface allow for high utilization of the plentiful active sites on both the outside and the inner surface.The results of electrochemical tests clearly showed that the obtained O-NCNT-SS presented a superior ORR activity in alkaline media and its value of the half-wave potential was 10mV higher than that of commercial JM-Pt/C.Then we also deeply analyzed the influence of temperature and Fe³⁺/PANI ratio on ORR activity,and found that when pyrolysis temperature was 900℃and the mass ratio of FeCl₃ to PANI was 1:1,the obtained product owned the best ORR activity.