Synchrotron Radiation Study On Structural Evolution Of Transition-Metal-Based Electrocatalysts

A clean,low-carbon,safe and efficient modern energy system highly depends on remarkable catalytic processes.Recently,due to its many advantages,electrocatalysis has received extensive attention in vigorously exploring new renewable energy and further improving the clean and efficient utilization of fossil energy.In fact,during the actual catalytic processes,electrocatalysts may undergo an intricate structural dynamic evolution process regulated by their intrinsic properties and external field factors.However,limited by traditional ex-situ characterization techniques,it is hard to obtain true working state of electrocatalysts,which poses a great challenge to the precise preparation of remarkable catalysts.Therefore,in-depth understanding of the structural dynamic evolution of electrocatalysts and then rationally regulating it to obtain desired electrocatalytic performance(activity,selectivity,etc.)have become a hot spot in the field of electrocatalysis.Based on the synchrotron radiation light source,aiming at key scientific problems of structural dynamic evolution process of catalysts,this thesis takes electrocatalytic hydrogen evolution reaction(HER)/oxygen evolution reaction(OER)of transitionmetal-based electrocatalysts as the research object,by using operando synchrotron radiation X-ray absorption fine structure(XAFS),synchrotron radiation infrared spectroscopy(SR-IR)and synchrotron radiation photoelectron spectroscopy(SRPES)to capture the actual electrocatalysis process.At the same time,combined with theoretical calculations,we revealed the nature and influencing factors of the electrocatalyst structural dynamic evolution,clarified the structure-performance relationship of corresponding electrocatalysts,guided the preparation of high-efficiency electrocatalysts,and established the foundation for developing SRMS to visualize the whole catalytic reaction processes.The main research contents of this thesis are as follows:1.XAFS identifing true active sites of Co,O dual-doped Mo-based electrocatalystCombining Co,O heteroatom dual-doped strategy and phase regulation strategy to activate in-plane structure of MoS2-based catalysts,and the prepared Co-O-1TMoS2/SWCNT catalyst exhibited outstanding electrocatalytic HER performance,which could reach a current density of 10 mA/cm2 at an overpotential of 113 mV and a high TOF value of 6.65 H2 s-1 at an overpotential of 200 mV.XAFS combined with theoretical calculations indicated that the constructed in-plane Co-O-Mo active structure accelerated the rate-determining step of water splitting process,thereby significantly promoting the efficiency of electrocatalytic HER under alkaline conditions.This work provided a basis for revealing the structure-performance relationship of MoS2-based electrocatalysts.2.Operando XAFS tracking the "chameleon-like" structural dynamic evolution process of Co-based electrocatalystOperando XAFS,SRPES,SR-IR and electron microscopy confirmed that the prepared Co9S8-SWCNT catalyst experienced a chameleon-like structural dynamic evolution during the actual electrocatalytic OER process.That is,it transformed into Scontaining CoOOH under alkaline conditions,rather than complete transformation into CoOOH.However,under neutral conditions,it self-optimized into oxidized cobalt sulfide(O-CoS-SWCNT).This work provided insight into the nature of the structural dynamic evolution behavior of OER electrocatalysts.3.Comprehensively using synchrotron radiation based multi-techniques to track structural dynamic evolution process of anions in Ni-based electrocatalystBased on synchrotron radiation differential X-ray absorption fine structure(ΔXAFS)spectrum,it was experimentally confirmed that the anion(SOxy-)undergoes a structural self-optimization process to form a new Ni-S bonds during the actual electrocatalytic OER process.Furthermore,using SRPES depth detection and in situ SR-IR to probe the anions self-optimization(ASO)behaviors,it was discovered that the ASO process in the s-Ni(OH)2 catalyst started around 1.4 V(vs.RHE)and occured mainly in the catalyst bulk phase rather than surface.Subsequently.theoretical calculations indicated that the s-Ni(OH)2 catalyst formed a thermodynamically favorable structure after the ASO process,which could optimize the thermodynamic and kinetic processes of electrocatalytic OER and thus endow it with excellent catalytic performance.This work provided an experimental basis for subsequent anion-related studies in electrocatalytic systems.4.Synchrotron radiation techniques guiding the development of Fe-based electrocatalyst for hydrogen productionBased on the analytical results of synchrotron radiation technique,the carbon nanotubes with Fe impurity were directly phosphatized,which was further electrochemically activated under different conditions to regulate their structural dynamic evolution process,and finally three remarkable electrocatalysts(O-FePxSWCNT,P-Fe2O3-SWCNT and P-FeOOH-SWCNT)were obtained.In particular,under neutral conditions,P-Fe2O3-SWCNT only needed a low overpotential of 146.7 mV to reach 10 mA/cm2,as there was no significant performance degradation after continuous HER for more than one week at a large current density as high as 125 mA/cm2.Operando SR-IR and theoretical calculations indicated that the P atom in the P-Fe2O3-SWCNT catalyst not only accelerated the initial step of the electrocatalytic water splitting process,but also directly acted as an active sites to participate in the H adsorption/desorption process.This work not only prepared electrocatalysts with practical application prospects,but also provided an experimental basis for the subsequent rational regulation of the structural dynamic evolution process of electrocatalysts.