Properties Of IrO₂/TNTs/Ti Composite Electrode And Its Application In Trivalent Chromium Plating

Ti-based Iridium oxide electrode is widely used in the fields of electroplating,electric-metallurgy and degradation of organic matter,due to its lower oxygen evolution potential,better electrocatalytic activity and stability in acid solution.Although IrO₂ itself is very stable,the microstructure of IrO₂ coating electrode prepared in traditional thermal decomposition method.exists many “Turtle” cracks What’s more,the solution is easier to penetrate the coating and contact with the substrate,then leading to the dissolution of substrate and the spalling of coating in service.Therefore,the study of Ti-based iridium oxide electrode with highly active and long service life not only has academic value,but also broad application prospect.In this paper,conductive TiO2 nanotube arrays prepared by anodic oxidation and heat treatment is used as an interlayer,which has excellent chemical stability and can prevent the titanium substrate from oxidizing to produce unconductive TiO2 lay in service.On the other hand,the adhesion strength between active material and matrix can be improved due to the orderly nanotube arrays structure.In addition,the TNTs possesses large specific surface area,free of charge transfer path,which can further enhance the electrocatalytic properties of electrode.Different IrO₂/TNTs/Ti electrodes were prepared by thermal decomposition and constant current electrodepsition method,the corresponding electrocatalytic performance and morphology were characterized as well as its application in chrome plating.In chapter 3,IrO₂ coating titanium anode had been fabricated by traditionally thermal decomposition method.Experimental results show that the morphology of IrO₂/TNTs/Ti electrode is regularity without any cracks.And the IrO₂/TNTs/Ti electrode shows much higher activity and longer lifetime than the IrO₂/Ti electrode without a TNTs layer.Calcination temperature and IrO₂ loading amount are the two key factors affecting the electrochemical properties of IrO₂/TNTs/Ti electrode.It’s found that the catalytic activity of oxide electrodes will be weaken at extremely high calcination temperatures as a result of high degree of crystallinity,grain growth,accumulation of active component and the collapse of TNTs.Low calcination temperature（400°C）plays a positive role in the surface active sites and electrocatalytic activity for oxygen evolution while the service life is very short due to the poor cohesion strength and the unstable structure of active component.With the increasing of IrO₂ amount,the film resistance and the charge transfer resistance of oxide electrode decreases gradually and reaches the lowest level at IrO₂ amount of 6 g/m2 after which the redundant IrO₂ accumulates on the surface of TNTs and results in the increase of film resistance and charge transfer resistance.As a consequence,the electrode calcined at 500°C with IrO₂ amount of 6 g/m2 exhibits the higher electrocatalytic activity for oxygen evolution than the other electrodes.In chapter 4,different IrO₂/TNTs/Ti electrodes were prepared by the constant current electrodeposition.Results show that the electrode with deposition charge of 500 mC and heat treatment temperature of 450°C,presents better structure,catalytic active of the ORE and service life.With the decreasing of deposition charge,the amount of active oxide is reduced,resulting in poor performance.And the increasing of deposition charge will lead to form a dense oxide layer on the surface of TNTs interlayer by the accumulation of active oxide.In this case a lot of inner active oxides will be covered and the utilization of active oxide will be reduced.Low heat treatment temperature leads to the low crystallinity of IrO₂ grain and then affects the catalytic activity of electrode.The crystallinity of IrO₂ grain is high under higher heat treatment temperature,in this case the grain grows too fast and becomes thick.The bonding strength between active oxide layer and TNTs interlayer is weak,then the electrode is deactivated due to spalling of active component.The accelerated life of IrO₂/TNTs/Ti electrode,prepared in optimal condition,is only about 4 h,far less than the electrode prepared by thermal decomposition method.The mainly reason is that the active oxide layer of electrode usedelectrodeposition method is easier to falls,especially in the strong gassing environment.In chapter 5,the homemade iridium electrode is adopted for chromium plating.Anodic polarization behavior of electrodes in chromium electrolyte was discussed by linear sweep voltammetry.The cathode current efficiency under different current density for different anode were also studied.Experimental results show that IrO₂/TNTs/Ti electrode prepared by electrodeposited emerges short service life and easy to deactivation,and couldn’t be used in the chromium plating.The as-prepared IrO₂/TNTs/Ti electrode in thermal deposition exhibits an excellent property compared with the graphite electrode in that the peak potential for trivalent chromium oxidation is more positive and the oxygen evolution reaction is facilitated.The cathode current efficiency using this IrO₂/TNTs/Ti reaches 18%.