Preparation Of Fe₃C@N-Doped Carbon Composites And Study On Electrocatalytic Performances For Oxygen Reduction Reaction And Ethylene Glycol Oxidation

With the continuous development of science and technology,the use of fossil energy such as coal,natural gas and oil is increasing.For decades,human have been constantly looking for relevant sustainable energy sources that can replace fossil fuels.At present,fuel cell is one of the feasible methods to realize effective energy conversion and storage,and it has great potential.Although precious metals such as platinum and ruthenium have high catalytic activity,their large-scale commercial application is restricted due to their high price and poor stability.Therefore,it is significant to study high-efficiency and low-cost non-precious metal catalysts to replace precious metal catalysts.Also,it is very important to provide theoretical and experimental basis for the commercialization of new energy.In this work,the relevant knowledge and reaction mechanism of fuel cells,oxygen reduction and alcohol oxidation,the current research status of electrocatalysts and the application of Fe/C/N in the electrochemical field were introduced.In addition,the main problems and solution methods for the non-precious metals and Fe/C/N electrocatalysts were in the research of catalysts materials.By exploring simple synthesis and preparation methods,Fe/C/N electrocatalysts with tube and sheet morphologies were obtained.Then,by using them as precursors,a series of nanosized composites with larger specific surface area and oxygen defects were obtained with controlling preparation conditions.The two-dimensional ultra-thin Fe/C/N material and Fe/C/N-based composites have excellent electrocatalytic performance and potential applications.The contents of the work are as follows:1.The iron carbide encapsulated nitrogen-doped carbon nanotube and nanosheet（Fe₃C@N-CNTs,Fe₃C@N-CNSs）electrocatalysts were synthesized by a one-step calcination method.They were used in cathode materials for the oxygen reduction reaction（ORR）.Both electrocatalysts had a larger specific surface area,which was benficient to the improvement catalytic activity.The electrochemical measurement results indicated that the half-wave potentials(E_1/2)on the Fe₃C@N-CNTs-and Fe₃C@N-CNSs-modified glassy carbon electrodes（Fe₃C@N-CNTs/GCE,Fe₃C@N-CNSs/GCE）reached 0.845 V and 0.860 V,respectively.Compared with 20wt%commercial Pt/C(E_1/2=0.822 V),the E_1/2on the Fe₃C@N-CNSs/GCE shifted positively.The results implied that both electrocatalysts had good electrocatalytic activity for ORR.The onset potentials(E_onset)on both electrocatalysts were 0.996 V（Fe₃C@N-CNTs）and 1.056 V（Fe₃C@N-CNSs）,which was higher than that on commercial Pt/C（0.992 V）.In addition,the diffusion-limited current density of Fe₃C@N-CNTs and Fe₃C@N-CNSs electrocatalysts was higher than that of commercial Pt/C.Moreover,both electrocatalysts had a good stability,and superior to commercial Pt/C.On the another hand,the stability of the Fe₃C@N-CNTs was better than that of the Fe₃C@N-CNSs electrocatalyst.The reason was that the Fe₃C particles were encapsulated tightly by carbon nanotubes in the Fe₃C@N-CNTs structures.2.The CeO₂/Fe₃C@N-CNSs（CF）composite electrocatalyst with a large number of oxygen vacancies was synthesized by the hydrothermal and calcination methods.The structure and morphology were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The optimal conditions were obtained by controlling of the feed ratio of the reactants and the hydrothermal temperature.It was found that the rod-shaped CeO₂was uniformly bonded to the Fe₃C@N-CNSs of the ultra-thin layer structure.The results implied that the E_1/2and E_onsetof the CF modified GCE catalyst（CF/GCE）were0.923 and 1.088 V,respectively,which was higher that that of commercial Pt/C.electron transfer number and reaction kinetics data.After 12 h,the current density retention rate was 97%on the CF electrocatalyst,which was higher than the pure phase Fe₃C@N-CNSs（81%）and commercial Pt/C（63%）electrocatalydsts.The results indicated that CeO₂rich in oxygen vacancies helps fix more nitrogen sources and can quickly reduce H₂O₂intermediates in ORR.Therefore,the excellent catalytic activity and durability of the catalyst are further improved.3.Pd and FeTa₂O₆ are uniformly loaded on the Fe₃C@N-CNSs,and the three-phase composite electrocatalytsts（Pd/FeTa₂O₆-Fe₃C@N-CNSs,PFFC）were constructed.They exhibited excellent electrocatalytic activity for ethylene glycol oxidation reaction（EGOR）and ORR in an alkaline condition.The results show that the PFFC has the best electrocatalytic activity for EGOR and ORR when usage of Pd（Ac）₂is 5%（mass atio）.In 0.5 M KOH+1 M EG solution,the current density of EG electrooxidation on the Pd/FeTa₂O₆-Fe₃C@N-CNSs modified GCE（PFFC@GCE）was 1.394 A mg^-1,which was 15.4 times that of commercial Pd/Cmodified GCE(Pd/C@GCE,0.090 A mg^-1).In 0.1 M KOH solution,the E_onsetand E_1/2of PFFC@GCE can reached 1.103 V and 1.002 V,which was higher than commercial Pt/C（20 wt%）.In addition,the PFFC@GCE exhibited excellent durability in an alkaline medium.The fixed potential is-0.137 V.The peak current densities of PFFC5 and commercial Pd/C modified GCEs after 6000 s were 0.074 A mg^-1and0.006 A mg^-1,respectively.The PFFC electrocatalyst was operated at a constant voltage（1.002 V vs.RHE）for 12 h,the current retention rate was 97%.