Studies Of X-Ray Absorption Spectroscpy On The Structure-Activity Relationships Of Electrocatalytic Surface And Interface

Ensuring energy security is crucial to promoting economic and social development,and improving national living standards.With the increasing consumption of traditional fossil fuels,efficient utilization of renewable energy is one of the key measures to alleviate the energy crisis and protect the ecological environment.Renewable energy is a kind of clean energy relative to non-renewable energy(such as fossil fuels),including solar energy,wind energy,tidal energy and other natural energy that can be automatically renewable.However,due to the limitation of natural conditions,achieving efficient use of renewable energy remains challenging.At present,electrochemical conversion and utilization of renewable energy technology has broad application prospects,including the technical path of hydrogen as the energy carrier,for example,electrocatalytic hydrogen evolution(HER)can convert electrical energy into chemical energy(H₂).Then,the H₂ energy that is stored and transported to where it is needed,is converted to electrical energy via the Fuel Cell technology.In view of this idea,efficient,low-cost and stable electrocatalysts can greatly improve the conversion efficiency,reduce the energy loss in the conversion process,achieve the purpose of reducing the cost of hydrogen energy,and finally realize the"economy"of large-scale renewable energy utilization.Supported metal catalysts are widely used in electrocatalytic reactions.The catalytic activity,selectivity and stability are closely related to catalysts size.For single/dual-atom sites,sub-nanometer clusters and heterostructured catalysts,their catalytic properties are often influenced by different factors,but the similarity is that the catalytic reaction usually takes place at the surface or interface of the catalysts.Therefore,in this paper,with the help of Synchrotron radiation X-ray absorption fine structure(XAFS),the struct-activity relationship at the surface or interface of these catalysts was studied in order to understand the catalytic reaction mechanism and provide ideas for the design of new and efficient catalysts.The specific research contents of this paper are as follows:1.Understanding the structure-activity relationship on the surface of single/dual-atom catalystsIn the electrochemical oxygen reduction(ORR)and HER reactions,N and C ligands modified metal sites and atomically doped MoS₂ materials show excellent catalytic performance,respectively.We cooperated with the experimental research groups on uncovering the coordination environment of atomically dispersd sites:1)Besides the commonly identified single Fe/Co atom sites,we confirmed the single Cr atom was surrounded by 4 N atoms via the combination of wavelet-transformation analysis and EXAFS fitting,which showed excellent ORR performance and robust stability;2)On the basis of Co-based electrocatalysts for ORR,it is found that the optimization of Co content can significantly improve its ORR performance.Among the catalysts with excellent performance,wavelet-transformation analysis and EXAFS fitting were used for the first time to find the existence of abnormal short Co-Co bond at 2.12(?),and DFT simulation finally confirmed that the active structure was Co₂N₅.3)In Pd doped MoS₂catalysts for HER,the EXAFS analysis confirmed that atomic doping of Pd can effectively induce the phase transformation of MoS₂ from 2H to 1T,regulate the electronic structure of the adjacent S site,and significantly activate the intrinsic activity of Pd.On the basis of the further design of the second doping sites Ru,the results showed that the introduction of OH accompanied with Pd sites dramatically improved the reactant concentration on the inner Helmholtz plane.The well-conditioned surface brings with up-to-date the highest HER performance among the MoS₂ based catalysts.2.Exploring the active structures of sub-nanometer Ptclusters for HERPtsub-nanometer clusters(Pt-SNCs)inherit the comprehensive advantages of single atom sites and nanoparticles with both highest atom utilization and abundant active sites,which stimulates its application in many chemical processes.However,since the surface structure of PtSNCs is very sensitive to reaction conditions,it remains challenging to understand the real structure-activity relationship.In this work,PtO_x/CNT SNCs(PtO_x/CNT)are highly dispersed on carbon nanotubes,showing excellent HER performance with extremely low overpotential(18 m V at current density of 10 m A/cm²)and high mass activity(19 A/mg _Pt at overpotential of50 m V).In-situ XAFS experiments showed that:1)PtO_x was unstable and was reduced to PtSNCs;2)Chemisorbed H is inserted into the interface between the metal and CNT support,which weakens the metal-support interaction;3)With the increase of hydrogen coverage,the morphology of PtSNCs changed to 3D-like shape,which led to the performance degradation in the long-term reaction.This work for the first time illustrates the structural evolution of PtSNCs at electrochemical interfaces and provides guidance for the understanding on the working mechanism of relevant catalysts.3.Synergistic effect on the MoC/Mo₂C heterogeneous interfacial structuresThe construction of heterostructured catalysts has become an important strategy for the design of efficient electrocatalysts.However,because the interface structure is difficult to explore,its underlying mechanism remains unclear.To this end,we achieved the regulation of two-phase substances(MoC and Mo₂C)by controlling the amount of metal Co,and prepared electrocatalysts containing simplified heterostructures.In alkaline HER reaction,the catalysts containing MoC/Mo₂C heterostructure showed superior electrocatalytic activity compared with the single component.Based on high-resolution transmission electron microscopy,XAFS LCF(linear combination fitting)analysis and density functional theory calculation,we revealed the synergistic effect between MoC and Mo₂C,which suggested that MoC site is beneficial for the dissociation of water molecules,and the combination of chemisorbed protons is prone to happen at Mo₂C sites.