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C: Energy Conversion and Storage; Energy and Charge Transport 2
Black Anatase TiO Nanotubes With Tunable Orientation for High Performance Supercapacitors Xin Liu, Patricia Almeida Carvalho, Marit Norderhaug Getz, Truls Norby, and Athanasios Chatzitakis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05070 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019
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Black Anatase TiO2 Nanotubes with Tunable Orientation for High Performance Supercapacitors Xin Liu,1 Patricia Carvalho,2 Marit Norderhaug Getz,1 Truls Norby,1 Athanasios Chatzitakis 1* 1
Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo,
FERMiO, Gaustadalléen 21, NO-0349 Oslo, Norway. 2
Materials Physics, SINTEF Industry, Forskningsveien 1, NO-0373 Oslo, Norway.
*Corresponding
author (
[email protected])
Abstract Black TiO2 is a versatile material that finds many applications in the field of photocatalysis, but also in energy storage devices, such as supercapacitors. In this work, a new method is developed for the synthesis of black TiO2 in the form of nanotubes. Amorphous TiO2 nanotubes (TNTs) are annealed and reduced in the presence of CaH2 and by a simple manipulation, we are able to obtain control over the preferred crystal orientation. The reduced TNTs show high power and energy densities, as well as unprecedented stability under intensive charge/discharge cycling. The oriented reduced TNTs show metallic-like behavior with improved conductivity when compared to the polycrystalline analogue. This work contributes to the development of highly robust and efficient oxides for electrode material in supercapacitors.
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Introduction Supercapacitors bridge the energy and power gap among the traditional dielectric capacitors, batteries and fuel cells.1-2 They have several advantages as electrochemical energy storage devices, such as high power density, fast charge/discharge rates and long life cycle.3-4 The ideal electrode material for supercapacitors should possess high electronic conductivity, high surface area and good wettability. Based on the charge storage mechanism and the electrode material, supercapacitors can be classified into two groups: Electrical double layer capacitors (EDLCs) and pseudocapacitors.5-7 EDLCs are usually based on carbon-active materials, which have high electrical conductivity, large surface area and good stability, but suffer from relatively low capacitance with values ranging from approximately 0.4 to 6 mF cm-2.8 In contrast, pseudocapacitors are usually based on transition metal oxides. They can achieve higher capacitance compared to EDLCs, but they suffer from poor electrical conductivity and gradual loss of the capacitance due to irreversible electrochemical faradaic reactions.9 Therefore, increasing the electrical conductivity and surface of these materials are common strategies to improve their pseudocapacitive performance. Black titania, often referred to as reduced TiO2, has recently gained great attention mainly due to the band gap narrowing (~1.54 eV) and increased optical absorption onset (~1.0 eV).10-11 The black color has been assigned to absorption of light by excitation of effectively neutral oxygen vacancies (vxO), which can be seen as effectively positive v•• O and associated electron polarons
Ti/Ti (Ti3+).12-13 During the hydrogenation process, a significant amount of protons may be absorbed and present in substitutional hydroxide ion defects (OH•O) and they have been suggested to promote fast surface redox reactions and thus increase the pseudocapacitance of the material.14-15 Nanostructured TiO2 in the form of 1D or quasi-1D structures, such as nanorods and nanotubes (TNTs), have high surface area and offer orthogonality for the electrolyte access
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and charge carrier separation and diffusion.16-18 In addition, TiO2 is inexpensive, earth abundant, non-toxic and chemically stable, and hence a suitable candidate for supercapacitor electrodes. Several studies have shown that the preferential [001] texture of the anatase crystals is highly desirable as it improves the charge transport, and therefore increases the pseudocapacitive performance of the material.19-21 It was proposed that the [001] facets may attract the photogenerated holes, improving the oxidation reactions on the surface of the photocatalysts.22 The control over the preferred crystallization growth has been reported in several studies, such as by zinc incorporation in the amorphous nanotubes,23 by adjusting the amount of water and NH4F in the electrolyte, i.e. the –OH groups and F content in the amorphous phase,24 and by NaF treatment before annealing.22 Apparently, the conditions present during the beginning of the crystallization process play a crucial role for the preferred crystal growth. High concentration of OH groups at the outer wall region of the amorphous tubes results in the formation of randomly oriented nanotube arrays and hinders the preferential growth towards the [001] orientation.25 A few groups have already reported the successful implementation of reduced TiO2 nanotubes as supercapacitors. Lu et al.,9 show that hydrogenation of TiO2 nanotube arrays improves the specific capacitance of the material by as much as 40 times compared to their air-treated nanotube arrays. Their largest specific capacitance reported was 3.24 mF cm-2 at a scan rate of 100 mV s-1, and the material showed a remarkable cycling stability with only a 3.1 % reduction of the specific capacitance after 10 000 cycles. Wu et al. have reported a simple method to fabricate black titania nanotubes by using electrochemical reduction.26 They found that the specific capacitance of the films is more than 20 times than that of regular oxidized titania nanotube electrodes, but the capacitance and internal conductivity was significantly degraded after only 2500 cycles. Zhang et al. have achieved a specific capacitance of approx. 20 mF cm-2 at a current density of 1 mA cm-2 with 17.5 μm long black TiO2 nanotubes, with a capacitance retention of 88.2% after 5000 cycles. Moreover, their best performing nanotubes showed an IR drop as high 3 ACS Paragon Plus Environment
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as 46 mV.27 Kim et al. prepared 10 μm long TNTs after 5 h anodization in an ethylene glycolbased electrolyte. The amorphous TNTs were first thermally treated at 200 °C, resulting in red TNTs, which were then electrochemically self-doped and further annealed in N2 atmosphere at 450 °C for 1 h. The resulting TNTs show an areal capacitance of 15.6 mF cm-2 at 100 mV s-1.28 In this contribution, we present a simple, one-step reduction procedure, which allows the control over the crystal structure of the resulting black TiO2 nanotubes, as well as their capacity and conductivity. The nanotube arrays are grown through a well-established anodization process in organic electrolytes, followed by a chemical reduction in CaH2 at elevated temperatures.13, 29 By systematically modifying the annealing conditions, we are able to gradually tune the preferred crystallographic orientation of the TNTs, which has an impact on the electrical conductivity and double layer capacitance of the nanotubes. Electrochemical impedance spectroscopy (EIS) is used to measure the conductivity of the different nanotubes, and a correlation between conductivity, capacitance and preferred structural orientation is discussed.
Experimental Preparation of the black TiO2 nanotube electrodes A two-step electrochemical anodization process was used in order to grow highly organized TiO2 nanotubes. Anodization was performed in a two-electrode configuration with Ti (0.25 mm thick, 99.7% purity) foil as the anode electrode and a Pt sheet (4 cm2) as the cathode. The electrodes were kept at a distance of 2 cm. The Ti foil (1 cm2) was cleaned in an ultrasonic bath for 30 min in isopropanol followed by 30 min in acetone and finally rinsed with water and dried in air. The anodization was performed in an ethylene glycol electrolyte containing 0.25% wt NH4F and 2% wt H2O. The first step was performed at 60 V supplied by a programmable DC power supply (Keithley Instruments Model 2200-72-1DC) for 2 h at room temperature without stirring. Then, the anode was rinsed with water and left to dry vertically in air. The nanotubes film peeled off
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spontaneously upon drying in air and a fresh surface was revealed. The second anodization step was conducted under identical experimental conditions for 45 min. The electrodes were then left in the anodization solution for 1 h in order to improve the adhesion of the film.30 The amorphous TNTs (TNTsAmor) were chemically reduced to form the black titania electrodes in the presence of CaH2. Black TNTs were prepared by placing the amorphous nanotubes in a quartz ampoule under vacuum (
