Design And Synchrotron Radiation Study Of Binary Synergistic Transition Metal Electrocatalytic Materials

Energy crisis and environmental pollution are the two most severe and challenging basic problems.It is urgent to develop green and renewable energy.The development of the energy systems such as fuel cells and water electrolysis devices which rely on electrochemical technology,is crucial to boosting the renewable energy industry.Among them,the design and synthesis of electrocatalysts with high activity and low cost is the core problem of realizing commercialization in new energy systems.Binary synergistic transition metal electrocatalytic materials have the characteristics of unique two-site tunability and electron co-coupling effect.It provides the possibility to accelerate the slow kinetics during hydrogen evolution and oxygen reduction.Nonetheless,how to optimize the properties of binary synergistic electrocatalytic materials and reveal the synergistic mechanism of bimetallic sites is a great challenge that has been widely concerned.To achieve this goal,it is important to deeply understand the dynamic evolution of binary synergistic transition metal electrocatalysts.The synchrotron radiation X-ray absorption fine structure（SR-XAFS）spectroscopy have an element specificity and high local structure sensitivity,and the Fourier transform infrared（SR-FTIR）spectroscopy is highly sensitive to surface species.which are powerful tools for characterizing binary synergistic electrocatalysts.Operando synchrotron radiation technology can monitor the dynamic structural changes of metal sites and the evolution of reaction intermediates under working conditions.By fully utilizing the techniques,we can clarify the relationship between the composition,structure and catalytic activity of the electrocatalysts.The obtained results will guide the design of high-performance electrocatalysts.In this dissertation,binary synergistic electrocatalytic materials were successfully prepared by strategies of solid-liquid reaction and pyrolysis treatment.Based on the binary synergistic materials,the electronic structure and coordination environment of electrocatalysts can be tailored,which effectively solve the low activity and insufficient stability of catalysts for electrochemical reduction reaction in alkaline and acidic electrolytes.Finally,the electrocatalytic performance was improved significantly.The SR-XAFS and SR-FTIR spectroscopy can track the dynamic structural evolution of electrochemical interface active sites and the evolution of reactive intermediates on the electrode surface during electrocatalysis process,which can reveal the structure-activity relationship for catalysts.It can provide guidance and reference for the development of efficient and stable electrochemical reduction catalysts.The specific research contents of this paper are as follows:1.Operando XAFS technique reveals the dynamic process of atomically dispersed Ru sites in alkaline HERAccurately manipulating the electronic structure of metal active sites under working conditions is central to developing efficient and stable electrocatalysts in industrial water-alkali electrolyzers.However,the lack of an intuitive means to capture the evolution of metal sites during the reaction state inhibits the manipulation of its electronic structure.Here,atomically dispersed Ru single-sites on cobalt nanoparticles confined onto macromicroporous frameworks（M-Co NPs@Ru SAs/NC）with tunable electron coupling effect for efficient catalysis of alkaline hydrogen evolution reaction（HER）are constructed.Using operando X-ray absorption and infrared spectroscopies,a dynamic Co-Ru bond shrinkage with strong electron coupling effect under working conditions is identified,which significantly promotes the adsorption of water molecules and then accelerates its dissociation to form the key H*over Ru sites for high HER activity.The well designed M-Co NPs@Ru SAs/NC delivers efficient HER performance with a small overpotential of 34 mV at 10 mAcm^-2 and a high turnover frequency of 4284 H₂ h^-1 at-0.05 V vs.RHE,40 times higher than that of the benchmark Pt/C.This work provides a new point of view to manipulate the electronic structure of the metal active sites for highly effective electrocatalysis processes.2.Utilization operando spectroscopy techniques to research the reaction mechanism of symbiotic Fe catalyst for high-selectivity oxygen reductionAccurately regulating the oxo-hydroxy adsorption capacity is crucial to control the catalytic efficiency and selectivity of the oxygen reduction reaction（ORR）.Here,we report a symbiotic Fe catalyst with atomically dispersed Fe sites coupled onto the Fe nanocrystal（SBT-Fe-NC@SA）that realizes an excellent ORR activity and selectivity.By using in situ synchrotron radiation infrared and X-ray absorption spectroscopies.we directly identify that a self-evolution Fe site is adsorbed on the oxygen-bridge to form an active Fe-O-Fe-N2 structure with enhanced dinuclear Fe-Fe interaction under working conditions,which obviously enables a moderate oxo-hydroxy adsorption capacity for rapid breakage of O-O bond.As a result.the optimal SBT-Fe-NC＠SA exhibits>95%efficiency of four-electron ORR pathway to generate H2O/OH-.The SBT-Fe-NC@SA delivers an excellent ORR activity with a half-wave potential of 0.93 V,an ultra-high mass activity of～2490 A g_metal^-1 and an extremely large kinetic current density of 142.8 mAcm^-2 at 0.8 V vs.RHE,34 times those of Pt/C catalyst.These findings provide a unique perspective for accurately regulating intermediates adsorption capacity of metal sites toward superior activity.3.Synchrotron radiation spectroscopy techniques reveal structure-activity relationship of metallic Ni₃N and NiO nanosheet toward ORRThe development of efficient and stable transition-metal-based electrocatalysts for the oxygen reduction reaction（ORR）in fuel cells is highly desirable,yet remains a great challenge.Here,we report a novel Ni₃N quantum dot（QD）/NiO heterostructure material,fabricated by immobilization of metallic Ni₃N QDs onto the surface of NiO nanosheet,as a highly active and durable electrocatalyst for efficient oxygen reduction performance.The electrochemical characterizations and theoretical calculations reveal that a strong interface coupling effect in Ni₃N QDs/NiO heterostructure is realized by the interfacial hetero Ni species,synergistically accelerating the dissociation of adsorbed water molecules and reductive kinetics of adsorbed oxygen molecules during the ORR process.Hence,the developed Ni₃N QDs/NiO heterostructure yields prominent oxygen reduction performance with a small half-wave potential（E1/2）of 0.76 V and high kinetic current density（Jk）of 15.4 mAcm^-2 at 0.7 V vs.RHE,much superior to those of NiO nanosheet(0.65 V v;0.81 mAcm^-2)and comparable to those of commercial Pt/C catalyst(0.80 V;13 mAcm^-2).In addition,the catalyst shows long-term catalytic stability with robust methanol tolerance,serving as a promising noble-metal-free ORR catalyst for fuel cells.