Revealing Structure-activity Relationship Of Metal Atomically Dispersed Electrocatalysts By In Situ Synchrotron Techniques

To alleviate the increasing energy and environmental problems,various renewable energy storage and conversion devices have attracted a lot of attention from researchers.However,some chemical conversion processes,such as oxygen reduction/oxidation reactions(ORR/OER),which involve multiple complex steps usually require efficient catalysts to lower the energy barrier and thus improve the overall efficiency of the devices.Metal atomically dispersed electrocatalysts with well-defined active site structure and high active metal utilization provide an ideal configuration for studying electrocatalytic mechanisms at the atomic scale.Therefore,the atomically precise electrocatalysts are considered to be the most valuable and promising materials in the field of energy catalysis.At present,in-situ tracking of the dynamic evolution of metal active sites at the catalytic reaction interface is of great significance for understanding the complex oxygen-electrocatalytic reaction mechanism and for the targeted design of advanced electrocatalysts.Under the reaction kinetic potentials,the structure and valence state of the atomically dispersed metal active site are prone to dynamic evolution,accompanied by adsorption and desorption of multiple oxygen-containing intermediates on the electrodes surface.However,the acquisition of dynamic information about the catalytic reactions remains challenging due to the complexity of electrode surfaces in aqueous environments and the limitations of effective detection techniques.In recent years,synchrotron-based spectroscopic detection techniques,with the advantage of high brightness of light sources,have been developing rapidly in the field of energy catalysis.Among them,the synchrotron X-ray absorption fine structure spectroscopy(SR-XAFS)with high penetration in aqueous solutions and high sensitivity to local structural changes,and the synchrotron infrared Fourier transform spectroscopy(SR-FTIR)with molecular fingerprinting of the surface oxygen functional groups offer the possibility to describe the catalytic mechanism of electrocatalysts from the atomic/molecular level.Based on metal atomically dispersed electrocatalysts,this dissertation focuses on the complex and important oxygen electrocatalytic reactions(ORR/OER)in new energy technologies,and uses the unique in situ synchrotron radiation characterization technique to investigate the dynamic evolution of catalyst active centers in the catalytic reaction process and discusses the potential catalytic mechanism of multi-step oxygen electrocatalytic reactions.The specific studies in this dissertation are as follows.1.In situ synchrotron radiation techniques reveal the evolving dynamics of the coordination-unsaturated iron atomic active sitesIn this work,we have synthesized single-atom catalysts with Fe atomically dispersed on nitrogen-doped carbon substrates by hydrothermal method.The catalysts achieved efficient electrocatalytic oxygen reduction(ORR)activity.The dynamic formation process of the active structure(OH-Fe-N2)during the reaction was probed using in-situ XAFS characterization technique.Meanwhile,synchrotron radiationbased in-situ FTIR characterization showed that the coordination-unsaturated active structure(OH-Fe-N2)could promote the production of the key reaction intermediate*OOH and the cleavage of O-O bond,thus enhancing the intrinsic kinetic activity and selectivity of the catalyst.This work reveals the dynamic evolution and activity effect of catalytic active structure during the reaction process,which provides new ideas for the study of catalytic microscopic mechanism.2.In situ spectroscopic study of the structure-activity relationship of single-atom iridium-based catalysts in water oxidation reactionsIt is of great significance to design the active structure with metal atomically dispersed and study the reaction mechanism of oxygen electrocatalysis.In this work,the electrocatalysts with Ir atomic-level dispersed on the surface of three-dimensional carbon paper were designed and prepared by an "electrically driven amine-induced"strategy.The stable hetero-nitrogen-configured Ir sites(hetero-Ir-N4)enable the Ir catalysts high electrochemical acidic water oxidation(OER)activity and stability.By in situ XAFS and SRIR experimental characterization techniques,we have elucidated that an oxygen atom dynamically couples to the Ir site under low potential driving to form a stable "O-hetero-Ir-N4" active structure.This active structure significantly lowers the kinetic barrier of the four-electron reaction and promotes the production of the key intermediate*OOH,resulting in the catalytic OER process with a low overpotential of 216 mV at 10 mA cm-2.This work is important for understanding the catalytic mechanism and actual reaction pathways of oxygen electrocatalytic reaction,and guiding the design of efficient and stable oxygen electrocatalysts.3.Utilization in situ synchrotron radiation techniques to research the regulation of scaling relationship during reactions by dual-atom sites catalystsCompared with single-atom sites catalysts,dual-atom sites catalysts have interaction between adjacent metal sites,which provide the possibility to regulate the catalytic mechanism.In this work,atomically dispersed Pt=N2=Fe dual-atom sites on amino-functionalized carbon nanosheets were designed and synthesized.Electrochemical tests showed that the dual-atom active sites exhibited an intrinsic activity nearly two orders of magnitude higher than that of commercial Pt/C catalysts.In situ XAFS spectroscopy shows that oxygen-containing intermediates can co-adsorb on suitably spaced Pt=N2=Fe sites to form a Pt-O-O-Fe dual-site adsorption mode.In situ SR-FTIR reveals that the catalyst produces*O-O*intermediates during catalytic ORR and promotes the O-O bond cleavage directly.This effectively avoids the conventional end-adsorption mode of*OOH intermediates on single-atom sites that are difficult to be further cleaved.This two-site reaction path effectively regulates the scaling relationship between the adsorption energies of multiple reaction intermediates,which fundamentally accelerates the catalytic kinetics effectively.This work provides meaningful design principles and ideas for the development of highly active catalysts.