Studies On TiO₂-based Supercapacitor

Supercapacitor is a new type of electrochemical energy storage device between traditional electrostatic capacitors and rechargeable batteries,and it has high power density,high energy density and excellent cycle performance.It has been widely used in many fields,such as rail transportation,hybrid vehicles,solar energy,wind power,high-power electronics and military equipment.Based on different charge-storage mechanism,supercapacitor can be classified to electric double-layer capacitors and pseudo capacitors.The energy storage of pseudo capacitance depends on the potential deposition,chemical adsorption,mosaic smaller radius cations like H+,Li+,Na+,or turning electrical energy of an external source into chemical substances via electro-oxidation readuction.TiO2 is a simple binary oxide with anatase,rutile and brookite three crystal structure and the band gap is 3.0-3.2eV.Since the transition metal Ti has many valence states,TiO2 can also be used as a lithium ion battery cathode and pseudo capacitance electrode material.However,conductivity and electrochemical activity of TiO2 are poor,and it is necessary to improve its conductivity during research and development of TiO2 capacitor electrode material.The main contents are nanofibers of titanium dioxide prepared by electro-spinning and studies on electrical performance of titanium dioxide nanowires as supercapacitor electrode material.Firstly,we studied impact of factors on electrospinning process and preparation of titanium dioxide fibers.Results show that injection speed,spinning voltage and receiver distance is the key determinant of fiber diameter size distribution and morphology.When injection speed,spinning voltage and receiver distance are 0.4 mL/h,11 kV and 11 cm,TiO2 shows the best capacitive performance.Secondly,we study on equivalent series resistance(ESR)of TiO2 nanofibers capacitor.The results show that:(1)performance of titanium oxide capacitor is poor;(2)internal resistance changes as loaded voltage on capacitor,and it becomes a minimum value when voltage is ±0.2V;(3)loaded voltage affects redox reaction rate,thus affecting the transport speed of electrons and reflecting changes of the resistance finally;(4)it is poor in cycle performance,because of lacking in electrolyte while electrical cycle.In addition,we can enhance the conductivity and capacitance of TiO2 by electrical or chenmical treatment,and chemical treatment methods can improve specific capacitance to 9.65 F/g.Both chemical and electrochemical two methods can cause discoloration of titanium dioxide reaction,and ratio of Ti-OH in the sample electrical treated increases by XPS analysis.With three-electrode method,discoloration reaction occurs on sample loaded with negative cycle voltage.We believe that both two approaches take hydrogen into titanium dioxide,resulting out reduction reaction,discoloration phenomena and improvement in capacitive properties.We believe that both two approaches take hydrogen into the titanium dioxide,resulting out reduction reaction,discoloration phenomena and improvement in capacitive properties.Since carbon material owns good electrical conductivity and it can be incorporated into titanium dioxide to reduce internal resistance and improve capacitive properties.Therefore,we have studied on carbon-based supercapacitor and TiO2/C composite supercapacitor systematically.By complex impedance spectroscopy method,we studied the influence of internal and external factors,including mechanical pressure,temperature,electrolyte concentration and loaded voltage on the performance parameters,especially on the ESR and specific capacitance for carbon supercapacitor.The results indicated that the ESR is mainly contributed from the interface between electrolyte and electrode,and it is not from ohmic resistance of wire,electrolyte,collector and carbon material.In addition,we found that the ESR can be significantly lowered by mechanical pressures from 11 ohm at 0 N to 1 ohm at 800 N by decreasing 90%,and in consequence,the specific capacitance of supercapacitor can be improved dramatically from 19.34 F/g to 44.87F/g by increasing 128%.Basing on the model of interface with a diffusion layer,we put forward a reasonable explanation for the variation of ESR and specific capacitance with the four factors of pressure,temperature,electrolyte concentration and loaded voltage.Finally,it is about modification of titanium dioxide capacitors by incorporation of activated carbon.Overall,it got some improvement by incorporation of activated carbon.With the increase in activated carbon,specific capacity and energy density of the capacitor are enhanced.When the ratio of AC and TiO2 is 1:2,it has optimal performance(specific capacitance is 24.2 F/g and power density is 3.32 Wh/kg).And the internal resistance of the same voltage is reduced,but since the voltage continues to affect reduction reaction,the ESR of capacitor remains larger than ESR of carbon supercapacitor.In a word,it is a good way to improve the performance of titanium dioxide capacitor by incorporation of activated carbon.