Fine Structures And Properties Of Iridium Oxide Studied By XAFS

IrO₂-Ta₂O₅/Ti electrode is regarded as the best anode for oxygen evolution reaction,but the actual service life of IrO₂-Ta₂O₅/Ti electrode is generally lower than 3 years,which is far less than the service life of Ruthenium oxide electrode used in chlor-alkali industry.Although there are many researchers to investigate the short service life of IrO₂-Ta₂O₅/Ti electrode,the reason is still unknown due to the lack of the research methods.Therefore,this article relies on the modern computational theory（i.e.Density functional theory,DFT）,combined with advanced testing method including synchrotron radiation X-ray absorption fine structure（XAFS）and in-situ electrochemical technology to study the relationship between fine structure and properties of IrO₂-Ta₂O₅/Ti electrode.This investigation on IrO₂-Ta₂O₅/Ti electrode was divided into pure IrO₂ and pure TaO_x,by this way the essential failure reason for IrO₂-Ta₂O₅/Ti electrode was obtained.Firstly,the main failure mechanism of IrO₂/Ti electrode during cathodic electrolysis was studied.By using electrochemical test,XAFS and DFT,the relationship between fine structure and electrochemical catalyst of IrO₂/Ti electrode was obtained.The test results show IrO₂/Ti electrode exhibited different cathodic reaction mechanism while electrolysis under varies current density.Based on XAFS and DFT analysis results,the Ir-O bond coordination number was observed increasing after cathodic electrolysis under 5 mA·cm^-2,indicating OH-formed by H₂O decomposition were absorbed in oxygen vacancies of IrO₂/Ti electrode.The Ir-O bond coordination number was decreased after cathodic electrolysis under the current density of 100 mA·cm^-2,then was increased in the continuous cathodic electrolysis process,showing that oxygen vacancies were formed owing to the precipitation of oxygen atoms from IrO₂ lattice,and OH-were absorbed on oxygen vacancies,hydrogen atoms were absorbed on the oxygen atoms during cathodic electrolysis process as well.Hydrogen adsorption and the formation oxygen vacancies improve the electrocatalytic performance of the electrode:the polarization resistance of the electrode increased after OH-adsor:ption,and the hydrogen electrochemical catalyst was changed slowly.The fracture of Ir-O coordination bond and the oxygen atom dissolution of IrO₂/Ti electrode under the current density of 100 mA·m-2 will lead to the instability of IrO₂ crystal,then IrO₂ coat was broken and spilled by the sharply increased pressure caused by the large amount of hydrogen gas produced during cathodic electrolysis process,which is the main reason for the failure of IrO₂/Ti electrode.Secondly,the relationship between the fine structure and properties of tantalum oxide prepared by thermal decomposition method was discussed.Research results show the crystal structure of TaO_x powder was transformed from α-Ta₂O₅ toβ-Ta₂O₅ with the temperature increase from 550℃ to 650℃,while the Ta-O coordination number was reduced from 5.63 to 5.36,illustrating that the oxygen vacancies were formed after the crystal structure transformation.Then the relationship between the electrochemical activity and the fine structure of TaO_x/Ti electrode was discussed.With the decreasing of coating layers,the double-layer capacities were increased,and the fine structure of TaO_x/Ti electrode was seriously distorted.Calculated by DFT,it could be found out that the distortion of TaO_x/Ti electrode was caused by the existence of oxygen vacancies,and will improve the electrical conductivity of tantalum oxide.After cathodic electrolysis process,the distortion of TaO_x coordination structure was decreased,but the electrochemical activity of TaO_x/Ti electrode was significantly increased.Coupled with the test results of EDS,XANES and EXAFS,the fine structure transformation of TaO_x was found caused by the absorption of OH-,the semiconductor type and donor concentration of TaO_x/Ti electrode were changed after then.The electrocatalytic activity of TaO_x/Ti electrode was much lower than that of IrO₂/Ti electrode,but the surface morphology were not changed after cathodic electrolysis,this may be the main reason for TaO_x regarded as the stable component in IrO₂-Ta₂O₅/Ti electrode.Thirdly,the fine structure of IrO₂-Ta₂O₅/Ti electrode was discussed.The surface morphology,crystal structure and element distribution of IrO₂-Ta₂O₅/Ti electrode were firstly obtained with FESEM,XRD,EDS and TEM,and the fine structure of IrO₂-Ta₂O₅/Ti was analysed with XAFS and DFT.The research results show tantalum atoms doped into IrO₂ crystal will lead to the appearance of oxygen vacancies.Contradicted to the easily dope of tantalum into IrO₂,Iridium was hardly doped into Ta₂O₅,implies the fine structure of IrO₂-Ta₂O₅/Ti electrode includes IrO₂,amorphous tantalum oxide and Ir-Ta solid solution formed by tantalum dope into Iridium oxide.Finally,the main deterioration mechanism of IrO₂-Ta₂O₅/Ti electrode under the current density of 100mA·cm^-2 during cathodic process was discussed.The research results revealed the mixture of Iridium oxide and tantalum oxide was unstable during cathodic electrolysis process,as Ir-O coordination bond was easily broken,a large amount of oxygen vacancies were formed and lead to abundant hydrogen gas produced during cathodic electrolysis.The electrode coating was broken after then.The breakage of the electrode coating was the main reason for the falling off of IrO₂-Ta₂O₅/Ti electrode.