The Study On The Electrocatalysis Of Improved Carbon Materials For Oxygen Reduction Reaction

Fuel cell is an ideal energy conversion device for converting chemical energy from renewable sources directly into electrical energy through a chemical reaction with oxygen in a sustainable,high-efficient and environmentally friendly way.However,the high cost and short lifespan of traditional cathodic oxygen reduction catalyst severely hindered the commercial application of this electrochemical device.Thus,dedicated to develops an alternative Pt-based catalyst in oxygen reduction reaction with comparable catalytic activities,prolonged durability and a satisfactory cost is particularly important for the large-scale implementation of the advanced energy conversion device.Recently,great efforts have been made to explore non-noble metal catalysts,which has been taken for one of the most promising alternative to Pt-based catalyst.Among the non-noble metal catalysts,the carbon-based metal-free electrocatalysts,carbon-based non-precious metal-free electrocatalysts,as well as the contribution of intrinsic carbon defects have hence spurred on an extensive search in academic.However,the development of new catalysts has taken the traditional trial and error method,which is time-consuming and labor-intensive,and the fundamental issue of the origin of oxygen reduction activity is far from being clarified,the two greatly limited the further development of the oxygen reduction catalyst.Focus on carbon materials,this work is dedicated to the development of carbon-based non-precious metal catalysts that are different from traditional methods.To pursue the precise and controllable synthesis of materials and exploring the structure of active sites in materials,attempts to reveal the nature of active sites at the micro level.This work firstly develops a novel method to design a special g-C₃N₄@GO gel structure,which is a nanohybrid and can be named van der Waals heterostructures.In the structure of g-C₃N₄@GO,a composed of g-C₃N₄ nanolayers and graphene assembled alternatively in a layer-by-layer fashion,they are held together by interlayer?-?interaction,van der Waals forces and hydrogen bonding of oxygen-containing groups.And a specific structure of N-doped graphene nanosheets with high level of pyridinic-N?56 at.%?and edge-rich defect structure can be obtained when pyrolysis of the nanohybrid at high temperature.The prepared N-doped graphene nanosheets exhibited a great electrocatalysis for oxygen reduction reaction interms of the activity,durability,methanol tolerance,and the reaction kinetics.And the excellent electrocatalytic performance stems from the effective N-doped sites that the nitrogen atom is successfully doped at the defective edges of graphene,and the annealing temperature can play significant role of the doping pattern and location of N.The research provides a new insight into the enhancement of electrocatalysis for ORR based on nonmetal carbons by using the novel N-doping method.Then,simultaneous N-doped and S-doped in carbon materials have also been studied,and obtained a N and S dual-doped functionalized acetylene black catalyst.In this experiment,acetylene black was firstly exfoliated and functionalized through a simple single-pot method then be assembled to a novel porous carbon material by utilizing sulfur as both template and sulfur source.And after a pyrolysis process in the presence of melamine as N doping agent,the sulfur template was decomposed and yielded the N,S codoped porous carbon material with additional porosity and enhanced specific surface area.The defect-rich structures of the obtained acetylene black in the oxidation process play a critical role in providing more appropriate sites for heteroatoms doping.The synergistic effects between the defect-projecting and heteroatoms-doping,and the synergistic effects between the N atoms and S atoms boost the activity of electrocatalysts in terms of the enhanced oxygen reduction efficiency and optimized kinetic process,both are better than that of commercial Pt/C.Further,a specific pure carbon microsphere without dopants has been explored and the effect of intrinsic defects in carbon materials on electrocatalytic performance has been discussed.The carbon microspheres were prepared by the iodine-catalyzed carbonization of ethanol at low temperatures by solvothermal synthesis.The carbon microspheres have a lot of surficial broken fringes,edges and the holes through the shell,the unique morphology endows the carbon microspheres with an enlarged specific surface area and pore volume for active sites loading and a smooth mass transfer inside the catalyst layer.The excellent performance of oxygen reduction reaction revealed a great contribution of the intrinsic carbon defects in the carbon materials.Finally,we have explored the effect of possible residual metals on electrocatalytic performance which origin from the preparation of catalyst.It has been found that the metal component contained in the raw material of the catalyst could not be completely removed by acid etching,which can be detected by X-ray fluorescence spectroscopy and inductively coupled plasma emission spectroscopy.Typically,the residual Mn impurities in the carbon-based catalyst prepared by the Hummers method can play a key role in the electrocatalyst properties of the material,even a trace amount of Mn in catalyst can significantly affect the catalytic activity of oxygen reduction reaction,and the activity of the catalyst varies with the content of Mn.Additionally,a pyrolysis-induced graphitic-carbon-layer-encapsulated MnO₂ structure was presented in the corresponding reduced acetylene black oxide products,which improves the stability of nanocarbon catalysts and catalyzes graphitic structure in the resulting carbon structure,yielding a superior activity of oxygen reduction reaction as well as an extraordinary long-term durability for catalyst.