| Supercapacitors have recently attracted great attention due to promising applications in pulse power technology, pure electric vehicles and hybrid electric vehicles. One-dimensional TiO2nanotube arrays possess unique physicochemical properties, large specific surface areas, direct pathways for charge transport and wide potential windows, thus holding promising capabilities in supercapacitors. However, the pristine (without intentional doping or modification) TiO2nanotube arrays generally suffer from poor capacitive behavior, since TiO2is a wide bandgap semiconductor with a limited conductivity. In this work, anodic TiO2nanotube arrays (ATO) were prepared by two-step anodization of Ti foils, and emphasis was given to the bandgap engineering of ATO nanotube arrays for enhancing their conductivity and electrochemical performance.Firstly, self-organized ATO nanotube films were fabricated by two-step anodization of Ti foils. The corresponding anodization process and electrochemical properties of ATO nanotube films were studied in detail. The results showed that the anodic current response of the as-prepared ATO electrode at more positive potentials was strongly limited. A relatively low specific capacitance of0.97mF/cm2at0.05mA/cm2was obtained from the charge-discharge test.Secondly, the influence of a hydrogen (H2) plasma treatment on morphology and electrochemical properties of ATO nanotube electrodes were investigated. Compared with the pristine ATO, a distinct color evolution of the nanotubes treated by H2plasma illumination (ATO-H) was revealed from white to dark gray. ATO-H nanotubes presented a rough and amorphous layer at the surface of nanotubes with simultaneously incorporated Ti3+and-OH groups, leading to increased specific surface areas and electronic conductivity. The novel ATO-H electrode also exhibited excellent electrochemical properties. The specific capacitance of ATO-H electrode substantially increased-7times, with the value as high as7.22mF/cm2at a current density of0.05mA/cm2in charge-discharge measurements. Moreover, ATO-H electrode also exhibited excellent rate capability (6.37mF/cm2at a current density of2mA/cm2). The CV curves could still keep quasi-rectangular shape as the scan rate was up to1.2V/s. Significantly, cycling performance of ATO-H nanotubes had no degradation after10,000cycles.Finally, a simple, efficient and fast approach to ATO modification for improving electrochemical properties, known as electrochemical hydrogenation doping, was developed. The morphology of the ATO nanotubes treated by the electrochemical hydrogenation doping (ATO-H-2) did not show any significant change in comparison with the pristine ATO. However, ATO-H-2nanotube electrodes also revealed a color evolution from white to gray-blue. The electrochemical doping process of ATO nanotube films was studied. The results demonstrated that doping voltage, interelectrode distances and doping time play a key role in the electrochemical performances of ATO-H-2electrodes. The optimum doping process was:the doping voltage5V, the interelectrode distance2.5cm and the doping time30s. ATO-H-2electrodes prepared under these conditions exhibited a very high average specific capacitance of20.08mF/cm2at a current density of0.05mA/cm2,-20times more than that of the pristine ATO nanotube electrodes. The areal capacitance can obtain9.07mF/cm2at the highest current density of4.00mA/cm2. The supercapacitor device assembled by the ATO-H-2electrodes deliverd a specific capacitance of5.42mF/cm2at the current density of0.05mA/cm2. The specific capacitance of the device decreased considerably after5000cycles. However, the electrochemical performances of the supercapacitor after cycling can be recovered easily by the same doping process. This strategy makes it possible to fabricate a renewable supercapacitor whose performances can be reversibly restored. |
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