Surface/interface Engineering And Synchrotron-based Spectroscopic Studies Of Oxygen Reduction Catalysts

As advanced energy devices,proton exchange membrane fuel cell(PEMFCs)and metal-air batteries can directly convert chemical energy into electricity by electrochemical method,which avoid the limitation of Carnot cycle.Due to the high energy conversion efficiency,large power density,environmentally friendly and safety,they have shown a broad application prospect in the fields of electric vehicle,standby emergency power and military security.However,the air cathode of PEMFCs and metal-air batteries often suffers from the slow kinetic process of oxygen reduction,which needs electrocatalysts to accelerate the reaction.Oxygen reduction reaction(ORR)is a multiphase catalytic reaction,which includes multiple proton coupled electron transfer.A typical ORR process usually not only involves reactant adsorption,intermediate transformation and product desorption on catalyst surface,but also refers to mass transport at gas-solid-liquid three-phase interface and electron transfer.In addition,benefiting from the high brightness,high-resolution,continuous tunable wavelength and excellent beam focusing ability of synchrotron radiation light source,synchrotron radiation X-ray absorption spectroscopy technology plays an irreplaceable role in the study of the fine structure of the catalyst surface and interface.This thesis focuses on the important scientific problems of oxygen reduction catalyst for PEMFCs and metal-air batteries.Based on the key influence factors including active sites,reaction energy barrier,electron transfer and mass transport of ORR catalysts,we have explored various surface/interface regulating strategies to achieve high-efficiency oxygen reduction reaction.Furthermore,by virtue of in-situ and ex-situ synchrotron radiation spectroscopy technology,we have investigated the reaction kinetics at the surface/interface and active sites for ORR catalysts under actual working conditions.The structure-activity relationship between surface/interface structure and catalyst performance has also been studied in depth at atomic and molecular scales,which will lay a solid theoretical and experimental foundation for the application and development of fuel cells and metal-air batteries.The main research contents of this paper are as follows:1.We have developed an interface engineering strategy for the preparation of a series of metal sulfide/graphene hybrid materials,which enhanced the catalytic activity of ORR by optimizing the interfacial charge transfer capability.Taking cobalt sulfide/graphene catalyst as an example,soft X-ray absorption spectra characterization confirmed that there are a large number of Co-N/S-C strong coupling bonds at the interface between cobalt sulfide nanoparticles and graphene,which makes this hybrid exhibit a controllable interface coupling effect.On the one hand,the strong coupling interface can accelerate charge transport process during the ORR process;on the other hand,the presence of the strong-coupled Co-N/S-C bonds can also serve as the active sites to enhance the intrinsic catalytic activity.As expected,the hybrid material showed excellent ORR performance with high half-wave potential and four-electron reaction selectivity.Rechargeable zinc-air battery devices assembled based on this material also exhibited excellent power density and stability.Moreover,this interface engineering strategy also has a certain universality and can be extended to other metal sulfide hybrid materials.This work opens up a new avenue for interface design and optimization of oxygen reduction catalysts.2.We have developed a surface anion modification strategy to prepare novel atomically dispersed Cu-N3S1 coordination species,which can be used as an efficient ORR electrocatalyst for flexible Zn-air battery.Synchrotron radiation absorption spectra confirmed that this new Cu-N3S1 active site showed strong electron interaction with neighboring heteroatoms.Due to its high intrinsic activity,strong electron transfer ability and excellent O2 adsorption energy,the catalytic system showed significantly enhanced ORR catalytic activity.The flexible Zn-air battery base on this material achieved excellent open-circuit voltage(1.41 V),high power density(138.2 mW cm-2)and low charge-discharge polarization.In addition,the constructed Zn-air battery also showed a stable discharge voltage platform at different folding angles under the charge-discharge current density of 2.0 mA cm-2.This surface modification strategy can also be extended to prepare Fe-N3S1 active sites with excellent ORR activity.This work provides theoretical and experimental guidance for the design and optimization of the active site structure for high-performance ORR electrocatalysts.3.We have developed an ammonia surface etching strategy that can directionally convert pyridinic nitrogen of FeNx sites to pyrrolic nitrogen,thus producing high-purity pyrrole-type FeN4 structure.Synchrotron radiation absorption spectroscopy and theoretical calculations confirmed that the prepared pyrrole-type FeN4 sites exhibited high intrinsic catalytic activity,appropriate O2 adsorption energy and complete four-electron reaction selectivity due to its unique atomic and electronic structure.As expected,this high-purity FeN4 catalyst exhibited excellent ORR activity with an ultrahigh active area current density of 6.89 mA m-2 in acid medium.PEMFCs built with this material also achieve high open circuit voltage(1.01 V)and large peak power density(exceeding 700 mW cm-2).This work provides a new idea for the study of structure-activity relationship between FeNx site structure and ORR activity as well as the design of high-performance metal-nitrogen-carbon catalysts.4.We proposed a surface thermal migration strategy that can couple isolated FeN4 sites to form a planar-like Fe2N6 structure with ultra-high surface density on carbon substrate.Through the study of oxygen reduction kinetics,it was found that this Fe2N6 active sites optimized the ORR catalytic pathway of traditional FeNx sites,and showed excellent oxygen reduction activity in the application of PEMFCs system.In-situ synchrotron radiation absorption spectroscopy techniques reveal that,benefiting from the synergistic advantages of optimized adsorption status for oxygen intermediates and high density of active sites,this planar-like Fe2N6 structure showed a unique ORR reaction pathway and strong driving force for O-O bond breaking,thus accelerating the oxygen reduction kinetics and inhibiting the two-electron side reaction.Therefore,compared with the isolated FeN4 site,our developed planar-like Fe2N6 structure greatly improves the catalytic activity and stability of metal-nitrogen-carbon materials.An ultra-high peak power density of 845 mW cm-2 was also obtained for PEMFCs based on this catalyst,which was higher than that of most reported non-noble metal electrocatalysts.This work represents an important breakthrough in the practical application of metal-nitrogen-carbon materials in PEMFC systems.