Study Of Flexible Asymmetric Supercapacitors Based On Anodic TiO₂ Nanotube Arrays

In addition to the advantages of rapidly charge-discharge ability and long-term cycle life of traditional supercapacitors,the flexible solid-state supercapacitors also have excellent flexibility.And they conform to the development trend of miniaturization and wearability of modern electronic products.In this paper,titanium dioxide(TiO2)nanotube arrays were prepared by anodic oxidation on a titanium foil with a thickness of about 15 ?m.After a series of modification treatments,they were used as a flexible anode for supercapacitors.Meanwhile,polyaniline(PANI)was electrochemically polymerized in TiO2 nanotube arrays in situ to prepare PANI/TiO2 nanotube composite cathode.Finally,the anode and cathode were assembled into a "sandwich" type flexible asymmetric solid-state supercapacitor with polyving akohol(PVA)/H2SO4 gel as solid electrolytes,and its performance was studied.The water vapor treatment and annealing process were adopted to further improve the electrochemical performance of anodized TiO2 nanotube arrays.The results show that TiO2 nanotube arrays reacted with saturated steam(formed by 1.0 mL deionized water)at 180 ? for 20 min,showing the best electrochemical performance.The final specific capacitance of the modified TiO2 nanotube arrays electrode was as high as 11.38 mF cm-2 after annealing at 450?in Ar atmosphere,and it was more than 10 times compared with the pristine TiO2 nanotube arrays(with a specific capacitance less than 1.0 mF cm-2).After adjusting the electrochemical polymerization parameters such as the electrochemical reduction technology and deposition cycle number,it was found that the optimum polymerization process of PANI is:First,TiO2 nanotube arrays were reduced at a scan rate of 500 mV s-1 for 1 cycle,then used dynamic potential scaning for 5 cycles at 25 mV s-1 in the polymerization solution.Consequently,the composite materials with high filling rate of PANI into TiO2 nanotube can be obtained.It showed that the specific capacitance of the PANI/TiO2 nanotube composite electrode was as high as 33.91 mF cm-2,and a cycling stability with 71%of initial capacitance was retained after 2000 charge-discharge cycles.Finally,the working electrolyte and potential window of the flexible asymmetric solid-state supercapacitors were studied.Thus,a flexible asymmetric supercapacitor with TiO2 nanotube arrays as anode and PANI/TiO2 nanotube composite as cathode was fabricated.Under the operating voltage of 1.0 V,the measured capacitance of the device was 5.13 mF cm-2 at a discharge current density of 0.1 mA cm-2.And the capacitance remains unchanged under different bending states.Furthermore,the capacitance retention of the flexible asymmetric supercapacitor was 83%after 2000 cycles at a scan rate of 100 mV s-1 showing a good electrochemical stability.