Understanding The Impact Of Anion Defect On Reactivity Of Transition-metal Based Electrocatalysts

In the context of the energy crisis and the carbon peaking and carbon neutrality goals,it is urgent to develop the sustainable new energy to increase the proportion of sustainable new energy in the energy consumption structure and to reduce the dependence on fossil fuels.Hydrogen energy has broad prospects in the field of new energy development due to its characteristics of high mass energy density,clean emission,and wide sources.Water splitting by electrolysis is an ideal way to obtain hydrogen energy.However,the traditional commercial catalysts for water splitting are still dominated by noble metal-based catalysts,such as Pt,Ru O₂,Ir O₂,etc.,and their high price restricts the development of water splitting.In recent years,transition metal-based catalysts have attracted extensive attention due to their high activity,low cost,and easy preparation,which are considered as the potential substitutes of noble metal-based catalysts.The metal sites in transition metal-based catalysts are generally considered the active sites for surface reactions.The reactivity of transition metal-based catalysts can be improved by tuning the metal sites properties（such as metal valence,metal electronegativity,etc.）.In recent years,it has been found that the anionic sites in transition metal-based catalysts can also directly participate in the reaction as active sites.Anion characteristics,such as anion defect concentration and anion redox activity,have a significant impact on the reactivity of transition metal-based catalysts.However,the mechanism of anion regulating the reaction activity of transition metal-based catalysts is still unclear,and the method of anion regulation is not clear.In this work,ion irradiation is used to introduce pre-created defects in electrocatalysts,and the cation doping effect is used to tune the formation of anion defects in electrocatalysts during electrochemical processes.The evolution of anion defects during electrochemical processes and the mechanism of their effect on the catalyst reactivity are systematically investigated by the combination of in-situ characterizations,advanced spectroscopic techniques,isotope labelled experiments,and detailed theoretical calculations.The main research contents and results are as follows:1)We introduced pre-created S-vacancy with different density in single-layer MoS₂by ion irradiation,and revealed the influence mechanism of S-vacancy on the electronic structure and electrochemical activity for hydrogen evolution reaction.We found that the defects introduced by ion irradiation can significantly affect the properties of the single-layer MoS₂,leading to considerable changes in its photoluminescence characteristics and electrocatalytic behavior.The content of defect in monolayer MoS₂increase with the irradiation fluence.The presence of defects promoted O₂adsorption on defect sites in single-layer MoS₂,which results in a depletion of the n-type carriers（electrons）and an enrichment of the p-type carriers（holes）.As a result of the changes in carrier density,a transition from negatively charged excitons（X^-）through neutral excitons（X⁰）to positively charged excitons（X⁺）was observed in the PL spectra.Such interaction of defects with O₂molecules and their effects on the properties of single-layer MoS₂.was further supported by the in-situ PL and Raman measurements in alternate O₂and Ar ambient.Furthermore,the HER activity of single-layer MoS₂was found to be significantly enhanced with increasing defect density.The Tafel slope decreased from 90.8 m V/dec to 50.3 m V/dec after introducing defects in single-layer MoS₂.Our results showed that the defects could modify not only the intrinsic characteristic structure of MoS₂but also the interaction with the environment,leading to significant changes in the photoluminescence characteristics and electrocatalysis performance.2)We tuned the activity of lattice oxygen in Ni Fe（oxy）hydroxide by Modoping,and revealed the impacts of lattice oxygen activity on the reaction activity and kinetics of oxygen evolution reaction（OER）.We synthesized ultrathin Ni Fe（oxy）hydroxide with Modoping by combining metal ion adsorption method and sacrificial templated method,and investigated the effect of Modoping on lattice oxygen activity in（oxy）hydroxide.Synchrotron-based soft X-ray absorption spectroscopy and in-situ Raman results demonstrated that the MoNi Fe（oxy）hydroxide was with higher local density of states around the oxygen ligands,a higher metal oxidation state,and a delayed cationic electrochemical redox process in comparison with the Ni Fe（oxy）hydroxide.Consistently,density functional theory calculations suggested that MoNi Fe（oxy）hydroxide exhibited higher lattice oxygen activity,which was understood by the weakened metal-oxygen bond,upshifted O 2p center relative to Fermi level,enlarged Mott-Hubbard splits in metallic d orbitals,and lower oxygen vacancy formation energy.In-situ differential electrochemical mass spectrometry measurements and in-situ Raman results based on ¹⁸O-labelling experiments suggested that both Ni Fe and MoNi Fe（oxy）hydroxide follow the lattice oxygen mechanism and MoNi Fe（oxy）hydroxide exhibits a higher lattice oxygen exchange rate.The results of deuterium isotope experiments show that MoNi Fe（oxy）hydroxide is more dependent on the proton activity of the electrolyte,indicating that the potential determining step includes decoupling proton-electron transfer process.Such activation of lattice oxygen shifted the potential determining step from oxygen vacancy formation for the Ni Fe（oxy）hydroxide to the*OOH deprotonation for the MoNi Fe（oxy）hydroxide,resulting in strongly enhanced intrinsic OER activity.As a consequence,the obtained MoNi Fe（oxy）hydroxide displays outstanding OER activity,with an overpotential of242 m V at the current density of 10 m A/cm²,which is lower than that of Ni Fe（oxy）hydroxide（306 m V）.In addition,MoNi Fe（oxy）hydroxide shows an ultrahigh mass activity of 1910A/g_metalat the overpotential of 300 m V,which is 60 times higher than that of the Ni Fe（oxy）hydroxide.The electrolytic cell with MoS₂/Ni Fe LDH|MoNi Fe coupled electrodes exhibited superb activity and stability for the overall water splitting,which required a low voltage of 1.728 V to achieve a current density of 100 m A/cm².3)We tuned the（de）intercalation processes of proton and oxygen anion in Ni hydroxide by Co doping,and revealed the correlation between the proton/oxygen anion characteristics and the activity and selectivity of glycerol oxidation reaction（GOR）.We prepared Co-doped Ni hydroxides by electrodeposition and ion exchange methods,and employed both in-situ characterizations and advanced spectroscopy technologies to reveal how proton and oxygen anion（de）intercalation in hydroxide impact the elementary reaction step of GOR.We found that the ease of proton deintercalation from hydroxide lattice critically impacted the dehydrogenation step in GOR,while the oxygen anion deintercalation process determined the intermediates desorption step and final reaction selectivity.Oxygen vacancies formed in Ni Co hydroxide during GOR increased d-band filling of Co sites,promoting the subsequent 2^ndC-C bond cleavage,which is the potential determining step for formic acid production.Owing to the facilitated proton and oxygen（de）intercalation process,Ni Co hydroxide exhibited outstanding GOR activity,delivering a current density of 100 m A/cm²at 1.35 V with a formate selectivity of 94.3%and a Faraday efficiency close to 100%.In addition,Ni Co hydroxide also exhibited outstanding stability for GOR,with negligible changes in the potential during the chronopotentiometry measurement for 90 h at a constant current density of 100 m A/cm².Coupling GOR with hydrogen production,Ni Co hydroxide required only1.33 V（1.58 V）to reach a current density of 10 m A/cm²（100 m A/cm²）,and exhibited excellent stability with only 27.5 m V increasement in voltage after 110 h electrolyzing at the current density of 100 m A/cm².The above results reveal that the anion characteristics play an important role in the reaction activity of transition-metal based electrocatalysts,and the findings provide new ideas and theoretical guidance for the rational design of high-performance transition metal-based electrocatalysts.