Application Of Fe-N-C Catalysts In Fuel Cells And Investigation Of The Decay Mechanism

Proton Exchange Membrane Fuel Cell（PEMFC）is a kind of power generation device that can convert the chemical energy of fuel into electricity directly without combustion reaction,which has the advantages of high efficiency of energy conversion,low noise and environmental friendliness,and has a broad application prospect in the field of new energy vehicles,portable devices and other backup power sources.However,due to the slow kinetic process of oxygen reduction reaction（ORR）at the cathode of proton exchange membrane fuel cells,a large amount of Pt-based precious metal catalysts are currently used to promote the process of oxygen reduction screening at the cathode.The commercial development of fuel cells has been considerably limited.In order to reduce the cost of catalysts and promote the commercialization of PEMFC,it is very important to develop low-cost,high-activity and high-durability non-precious metal catalysts.In recent years,significant breakthroughs in catalytic activity have been achieved with Fe and N co-doped carbon-based catalysts,but such catalysts face serious problems of poor durability when applied directly to the cathodes of PEMFCs,making them hindered in practical references.Secondly,it is extremely important to explore and dissect the decay mechanism of carbon-based catalysts in PEMFC and to find effective strategies to improve their stability/durability for theoretical as well as practical applications.In this thesis,the decay phenomenon and decay mechanism of Fe-N-C catalysts in PEMFC single-cell environment are investigated,and based on this,strategies and methods to enhance the activity and stability of Fe-N-C catalysts in PEMFC are explored.The main research contents and results of this thesis are as follows:（1）We co-carbonized the Fe-doped organometallic framework UIO-66 with g-C₃N₄.g-C₃N₄ acts as a nitriding agent to provide a nitrogen and carbon source in the catalyst to form Fe-N_X sites and carbon nanotubes;The increase in Fe-N_X active sites on the one hand and the improved mass transport rate due to the conductivity and the layered porosity of the carbon skeleton on the other hand provide indispensable prerequisites for increasing the maximum power density of H₂-O₂ PEMFC.After optimization,the catalyst has a half-wave potential of0.86 V（vs.RHE）and 0.76 V（vs.RHE）in 0.1 M KOH and 0.1 M HCl O₄,respectively.More importantly,a high power density of 1150 m Wcm^-2 was generated in the H₂-O₂ PEMFC,and its battery performance can recover to close to 78%of its initial performance after 24 hours of discharge,which was illustrated by a series of characterizations that the dissolution of Fe in the catalyst was the main reason for the poor stability of the Fe-N/C catalyst by causing the destruction of the active center,and The encapsulation of the active center in the N-doped carbon shell effectively prevents the leaching of Fe from the active center.In addition,we found that water flooding of the Fe-N-C catalytic layer,H₂O₂/reactive radical（ROS）attack and carbon corrosion are also important reasons for the rapid performance degradation of Fe-N-C catalysts in PEMFC.（2）Based on the fact that the dissolution of Fe is the main reason for the poor stability of Fe-N-C catalysts by causing the destruction of the active center and carbon corrosion,we used a scheme to prepare Fe-S-NC/Fe₃C graphene nanosheets in situ using iron acetylacetonate as the Fe source and thiourea and melamine as the S and C and N sources,respectively,to encapsulate the active center metal stably in the N and S doped graphene carbon shells.In this work,the Fe-S-NC/Fe₃C 2D graphene nanosheets constructed in situ by optimizing the synthesis conditions are characterized by high specific surface area,abundant mesopores,high graphitization and stable encapsulation of abundant Fe₃C NPs within the N and S-doped carbon shells.Benefiting from the synergistic effect of the above features,the resulting catalysts showed excellent activity and stability during ORR,with half-wave potentials of 0.87V（vs.RHE）and 0.768 V（vs.RHE）in 0.1 M KOH and 0.1 M HCl O₄,respectively,and still maintained 91.31%and 74.07%after 12 h durability test,and more importantly,the catalysts showed excellent activity and stability in More importantly,the catalyst produced high power densities of 720 m W cm^-2 and 201 m W cm^-2 in H₂-O₂ PEMFC and alkaline Zn-Air,respectively,and the current density only decayed by 28.8%after 24h in H₂-O₂ PEMFC after endurance test,and the final performance returns to the initial 90%;no significant performance decay was observed in alkaline Zn-Air after up to 100 hours of charge/discharge.