Mechanism Study Of The Interfacial Properties Of TiO₂ Thin-film Porous Electrodes In Supercapacitors

The development of supercapacitors having enhanced energy is of technological importance.Since capacitance scales not only with the specific area of the electrode but also with the dielectric constant of the interface,this study focused on the role of the dielectric constant in enhancing capacitance.TiO₂ porous thin-films were deposited on Ti coupons and Si wafers by sol-gel dip-coating techniques,which were employed in the study of electrochemistry and optics,respectively.Interfacial potentials,refractive indices and capacitance were evaluated on the TiO₂ porous thin-film electrodes under the influence of solution chemistry including protons?pH?and specifically adsorbing ions?phosphate?.The influence of solution chemistry on surface potential and space charge potential was studied through zeta potential measurements and open circuit potential?OCP?studies.Both Zeta potential and OCP varies as a function of pH and phosphate adsorption.The plot of Zeta potential of TiO₂ versus pH exhibits an isoelectric point,pH_iep,which is close to 5.7.With the adsorption of phosphate,the pH_iep shifts to lower pH values.The slopes of the OCP versus pH curve were close to but lower than the Nernstian.The shift of OCP-OCP_pztc?OCP evaluated at the Point of Zero Total Charge?with phosphate adsorption implies that the space charge?charge developed inside the solid oxide material itself?potential,varies with phosphate concentration.Using ellipsometry under controlled humidity through a designed humidity controlling system,the refractive index of water inside pores?pore water?was measured on TiO₂ thin-films coated Si wafers.The inhomogeneity of TiO₂ thin-film generated by dip-coating,and changes in crystal structure with different firing temperature were shown to be important variables in this system.Surprisingly,the refractive index of pore water decreases with increasing ionic strength,which is the opposite case of bulk water as measured by the ellipsometer.Furthermore,surface potential also influences the refractive index of pore water.The higher the surface potential,the higher the refractive index,the lowest refractive index occurring close to the pH_iep.The presence of the specifically adsorbed phosphate ions on the pore walls increases the refractive index.Electrochemical charge-discharge testing and cyclic voltammetry measurements were performed using Ti O₂ thin-films coated on platinized Ti coupons to obtain the capacitance of these TiO₂ thin-film systems.The electrochemical capacitance increases with the deposition of TiO₂ thin-films on the platinized Ti electrode that directly relates to its Zeta potential with the lowest values in capacitance occurring near the pH_iep,in the absence of phosphate.Capacitance can be improved with phosphate adsorption over the pH range of 5 to 9 for aqueous based systems.Capacitance also depends on the voltage range referenced to the OCP and the voltage range itself.The highest value of capacitance?encompassing both Pseudo and EDL capacitance?was 155F/g and obtained over a voltage range of 1V at a scan rate of 5mV/s for a pH of 6.Asymmetric capacitive deionization?ACDI?system was proposed based on the study of surface potential of porous metal oxide,using it as coating on the carbon fibers which changes the physical-chemical properties of the carbon.SiO₂-?-Al₂O₃ ACDI system was built due to their surface potential,which will favor the electro-adsorption of ions and will avoid ions of the opposite charge to adsorb during the regeneration step.SiO₂ was replaced with phosphate adsorbed TiO₂ to further enhance the deionization rate.It should be noted that refractive indices that obtained from measurements using the ellipsometer only qualitatively relate to capacitance since they were acquired at much higher frequency than those normally used to measure capacitance.However,the combined value of these experiments should help add insight into the nature of structured water and fundamental electrochemistry inside biased nanoporous oxide thin-films.These TiO₂ porous thin-films were believed to be used as electrodes or coatings for electrodes due to their high surface area,good wettability of the pore walls and controllable Zeta potential.This work will lead to more investigations regarding these TiO₂ and other oxide films such they could provide higher energy storage and in the water world–a higher efficiency of capacitive deionization.