Highly ordered titania nanotube arrays were synthesised on titanium metal foil through electrochemical anodisation. The annealed samples were characterised through scanning electron microscopy and X-ray diffraction analysis. The electrochemical characterisations of the arrays were done through cyclic voltammetry, galvanostatic charge discharge and electrochemical impedance spectroscopy analyses. The titania nanotube arrays exhibited a specific capacitance of 6.8 mF cm–2 at 5 mV s–1 scan rate, which is very much higher than that reported earlier. Pseudocapacitive metal oxides were deposited on these arrays forming composite supercapacitor electrodes and their supercapacitor properties were compared with same deposited on bare titanium foil substrates. Pseudocapacitive metal oxides deposited on these titania nanotube array substrates exhibited improved supercapacitor performance and stability over the same deposited on titanium foil substrates.
Titania nanotubes have found various applications in solarcells [1, 2, 3], capacitors [4, 5, 6, 7, 8, 9, 10, 11], photocatalysis [12, 13, 14], sensors [15, 16, 17], antibacterial studies [18, 19, 20], paints [21, 22], sun creams [23, 24], etc. In recent years, titania nanotube arrays (TNTA) have played a vital role in the field of electrochemical capacitors as they offer a very high surface area, long cycle life, low manufacturing cost, fast charge–discharge, enhanced reversibility, well designed geometry with an open channel for charge transport and wide potential window . They can act as substrates for the deposition of active materials because they are mechanically robust, chemically stable and can eliminate the use of binder material. The presence of interfacial defects in natural semiconducting titania hinders the Ti4+ to Ti3+ redox reaction limiting the faradaic contribution of capacitance observed in the case of many other metal oxides, exhibiting small amount of electrical double layer capacitance in aqueous electrolytes owing to its nanoporous morphology [26, 27].
Extensive studies are being conducted at TNTA for supercapacitor electrode applications. Salari et al. reported the synthesis of TNTA electrodes, which exhibited a capacitance of 538 μF cm–2 at 100 mV s–1 scan rate . An areal capacitance of 2.4 mF cm–2 was reported for TNTA electrodes at 50 mV s–1 scan rate by Kim et al. . It is the length and crystallinity of the nanotubes that mainly governs the capacitive performance. The regular pore size and hollow tube morphology of TNTA electrodes facilitate easy charge transfer and enhanced interfacial ion movement, resulting in improved capacitive performance of around 4 times that of nanoparticle electrodes. Annealing treatment [30, 31], electrochemical treatment [32, 33, 34], plasma treatment , nitridation , etc. have been utilised by many researchers for enhancing the performance of TNTA electrodes. Salari et al. introduced an optimised heat treatment process of TNTA to develop a non-stoichiometric structure, resulting in an improved capacitance of 1620 μF cm–2 at a scan rate of 1 mV s–1, with high retension over 1000 cycles . Annealing temperature, time and atmosphere plays a key role towards increased capacitance. Annealing in reductive environment facilitates anatase to rutile phase transformation, Ti4+ to Ti3+ reduction and creation of oxygen vacancies. Xie et al. reported an increase in the electrochemical capacitance of TNTA electrodes from 0.26 mF cm–2 to 3.14 mF cm–2 after the nitridation process . Wu et al. subjected TNTA to H2 plasma illumination in plasma enhanced chemical vapour deposition system, which showed a capacitance of 7.22 mF cm–2 at 0.05 mA cm–2 .
Pseudocapacitive materials are capable of delivering very high specific capacitance and energy density compared to electrical double layer capacitive (EDLC) materials. However, they suffer from its low cyclic stability and efforts are being made to improve the stability by hybrid approaches incorporating EDLC materials possessing high cycle life. The structural parameters of the active material overlay are considered as another crucial parameter in the electrode fabrication of pseudocapacitors. Various limitations such as non-uniformity, instability, etc. shown by conventional planar supercapacitor electrode architectures are expected to be overcome by the use of TNTA substrates. TNTA with energy storage behaviour have been found suitable for the in-situ fabrication of various composite electrodes. However, the alkali hydrothermal method results in powdery titania nanotubes having disordered arrangement and a small tube diameter below 10 nm. The high interfacial energy hinders the entry of pseudoactive materials into these nanotube channels and thus the inner tube surfaces become inaccessible for the electroactive materials. An electrochemical anodisation method has been widely utilised for the preparation of highly ordered TNTA having a regular architecture, which can act as substrates for supercapacitor electrodes. Synthesis and properties tuning of TNTA by newly developed electrolytes have been widely explored in the recent years [37, 38, 39, 40]. Their open and wide tubular channels offer a support structure for loading various well designed pseudoactive materials [41, 42, 43]. The tailored morphology provides more easily available spaces for electrochemical reactions resulting in the maximum utilisation of these electroactive materials and offer super highways for charge transport. Thus, highly ordered TNTA on titanium foil as an electrode substrate enhance utilisation of active materials, charge transport and decrease the fading effect of electroactive materials improving the cyclic stability.
Here in this work, we successfully synthesised TNTA through an inexpensive and self-optimised electrochemical anodisation procedure. The synthesised one-dimensional compact nanotube arrays exhibited a specific capacitance of 6.8 mF cm–2 at 5 mV s–1 scan rate, which is very much higher than that reported earlier. These TNTA were used as substrates for the deposition of pseudocapacitive oxides and exhibited enhanced performance with extra high stability compared to bare titanium foil substrates.
2 Experimental procedure
2.1 Materials and methods used
Titanium metal sheet (Alfa Aesar, 99.5 %), ammonium fluoride (SRL, 98%), ethylene glycol (SRL, 99%), ethanol and potassium chloride (SRL, 98%) were purchased and used as received without further purification.
Titanium dioxide nanotube arrays were prepared by an electrochemical anodisation procedure. Figure 1 shows the schematic representation of the anodization setup for the synthesis of ordered TNT Arrays. The anodisation set up included a two-electrode cell with a platinum wire as cathode and pure titanium foil as anode or working electrode. Titanium metal sheets of area 3 cm × 2 cm were cleaned by ultrasonication in a mixture of isopropanol and acetone for 10 min. Then rinsed with deionised water and dried under nitrogen gas flow. 0.3 wt.% ammonium fluoride solution in 100 ml ethylene glycol–water system containing 2 vol.% water was used as the electrolyte and anodisation was carried out at a voltage 100 V for 1 hour with constant stirring. The temperature of the solution was maintained at temperature 12 ± 2°C by using an ice bath. The foil was taken out after anodisation and kept in ethanol for 15 min, dried, annealed in a tube furnace at 400 °C for 1 hour in air atmosphere and labelled as TNTA.
An X-ray diffractometer (Model D8 Advance, Bruker) with Cu-Kα radiation (λ = 1.54178 Å) was used for X-ray diffraction (XRD) analysis and performed at a scanning rate of 0.02° s–1 in the 2θ range from 10°–80°. Scanning electron microscopy (SEM) analysis was done using a Hitachi SU8010 scanning electron microscope.
An electrochemical workstation (Biologic SP 240) with Ag/AgCl (KCl) reference electrode, platinum counter electrode and 1 M KCl electrolyte was used for electrochemical characterisations of the TNTA sample (area 5 cm2). The potential window for cyclic voltammetry (CV) analysis and galvanostatic charge discharge (GCD) analysis was fixed at–0.8 to 0 V and the frequency range from 1 MHz to 100 Hz was chosen for electrochemical impedance spectroscopy (EIS) analysis.
2.3 Application of TNTA as SC substrates
The TNTA were used as substrates for the deposition of pseudocapacitive oxides such as ceria and vanadia. For the deposition of nanoceria, 100 ml 0.1 M Ce(NO3)3 (Merck, 99.5%) aqueous solution was taken in a beaker and an already prepared TNTA sample of area 5 cm2 was immersed in a slanting position for 2 h. After 2 h, the sample was taken out and dried. The deposited cerous ions were converted to ceria by annealing treatment in a tube furnace at 500 °C for 2 θ in air atmosphere. The ceria deposited samples were labelled as TNTA-ceria. For comparison, pure nanoceria was synthesised on pure titanium metal foil by similar procedure and labelled as Ti-Ceria.
An in-expensive hydrothermal treatment followed by annealing was used for the deposition of nanovanadia on TNTA and pure titanium metal foil substrates. 20 ml 0.1 M Ammonium metavanadate (Merck, 99%) solution was taken in a teflon cup and as prepared TNTA sample of area 5 cm2 was immersed into the solution. Then it was placed in an autoclave and was subjected to hydrothermal treatment at 170 °C for 6 θ in a hot air oven. After the reaction time, the sample was washed with distilled water, dried and annealed at 500 °C for 2 θ and marked as TNTA-Vanadia. For comparison, bare nanovanadia was deposited on pure titanium metal foil by the same procedure and labelled as Ti-Vanadia.
The CV analyses of all the four samples were done in order to measure the improved performance of TNTA based electrodes quantitatively.
3 Results and discussion
3.1 Structural and morphological studies
Figure 2a and b shows the surface SEM images of TNTA with a cross-sectional SEM image of TNTA in the inset of Fig. 2a. From SEM images, it is clear that anodisation of the titanium metal through carefully optimised conditions resulted in the formation of self-organised and vertically aligned one dimensional compact ordered tube array morphology with uniform pore diameter. The nanotube has a length of 3.93 ± 0.001 μm and diameter 85 ± 5 nm. Conventional anodisation procedures result in disordered TNTA which shows a negative effect on the practical applications of TNT arrays. Here the synthesised highly ordered TNTA with uniform pore diameters acts as short super highways for charge transfer, making it a potential candidate for practical supercapacitor applications.
Figure 3 shows the XRD pattern of the TNTA. The diffraction peaks at 2θ values 25.4, 37.9, 48.2, 53.8, 55.2, 62.7 and 70.7 clearly matches with the peaks of the anatase form of crystalline titania (JCPDS No. 89 – 4921), corresponding to reflections from the (101), (004), (200), (105), (211), (204) and (220) planes respectively. The other intense peaks are due to the reflections from the planes of titanium metal foil (JCPDS No. 44 – 1294) and well-marked in the plot.
3.2 Electrochemical Studies
CV analysis was carried out for TNTA sample at scan rates of 100 mV s–1, 50mV s–1, 25mV s–1, 10mV s–1 and 5 mV s–1 and the results are given in Fig. 4a. The specific capacitance (Cs) values can be calculated from the cyclic voltammograms by using Eq. (1).
Here, A is the area of the working electrode in cm2, v is the scan rate in V s–1 and (V2 – V1) is the potential window expressed in V. Figure 4b shows the plot of specific capacitance against scan rate. From the plot, it is clear that specific capacitance decreases as scan rate increases. The reduced diffusion of the electrolyte ions at higher scan rates is responsible for the decline in specific capacitance value as scan rate increases. There occurs effective redox reaction between the electrolyte ions and active titania at lower scan rates resulting in high capacitance. From calculations, it is obtained that the synthesised TNTA exhibit a specific capacitance of 6.8 mF cm–2 at 5 mV s–1 scan rate, very much higher than that reported earlier for TNTA. Generally, the traditional redox capacitance observed in case of transitional metal oxides is negligible in case of nanosized titania and exhibits EDLC type mechanism in aqueous electrolytes. In order to understand the contribution proportion of pseudocapacitance to total capacitance, the CV data were further analysed using Trasatti’s method. According to the Eq. (2), there is a linear correlation between the reciprocal of specific capacitance (Cs–1) and square root of scan rate (v1/2).
where, Cs, CT and v represent calculated specific capacitance, total specific capacitance and scan rate respectively. The total specific capacitance equals the sum of pseudocapacitance and electrical double layer capacitance and is calculated as the reciprocal of the y-intercept of the Cs–1 vs. v1/2 plot.
There exists a linear correlation between the calculated specific capacitance (Cs) and the reciprocal of square root of scan rate (v–1/2) also, as described by the Eq. (3).
Where, C, CEDL and v are the calculated specific capacitance, maximum electrical double layer capacitance and scan rate respectively. The maximum electrical double layer capacitance is measured as the y-intercept of C vs. v–1/2 plot. The difference between the total specific capacitance and maximum electrical double layer capacitance gives the maximum pseudocapacitance. The percentage capacitance contribution can be calculated by the Eqs. (4) and (5).
Figure 5 summarises the percentages of electrical double layer capacitance and pseudocapacitance as obtained from the Trasatti method. TNTA exhibited an EDLC contribution of 23% and a very high pseudocapacitance contribution of 77%, which accounts for its improved performance. Here the well-ordered nanotube arrays with very high pore diameter possess enhanced surface area and improve EDLC mechanism of capacitance contribution. The double layer capacitance is proportional to the surface area of active materials. The open uniform one dimensional nanotube arrays accounted for effective redox reaction between the multivalent titanium ions and electrolyte ions resulting in pseudocapacitance contribution. The supercapacitive properties were further confirmed by GCD analysis.
Figure 6 shows the GCD curves of TNTA obtained at current densities 0.5 mA cm–2, 1 mA cm–2 and 5 mA cm–2. GCD curves depict similar charging–discharging times, exhibiting good coulombic efficiency. Equation (6) gives an expression for the calculation of capacitance (Cs) from these charge–discharge curves.
Here I is the discharge current expressed in A, ΔV is the voltage window expressed in V, Δt is the discharge time in s and A is the area of the working electrode in cm2. From GCD calculations, it is obtained that the TNTA exhibit a specific capacitance of 6.2 mF cm–2 at a current density of 0.5 mA cm–2.
The kinetic properties of the TNTA sample were studied through EIS analysis. Figure 7 shows the obtained Nyquist plot in the frequency range 1 MHz to 100 Hz with inset plot showing the high frequency part only. The plot consists of a linear part at low frequencies and a semi-circular part at high frequencies. The charge transfer resistance, Rct to the redox reaction is calculated as the diameter of the semicircular part and the series resistance (RX) is measured as the high frequency intercept on the x-axis. The contact resistance at the solid–liquid interface, internal resistance of active material and electrolyte resistance together makes the series resistance (RX). The measured values of charge transfer resistance, Rct and series resistance, RX for the sample TNTA were 1.76 and 1.34 ohms respectively. The wide open one-dimensional nanotube channels for smooth charge transfer result in low resistance and exhibit enhanced capacitor performance.
Bare titanium foil and synthesised TNTA were used as substrates for the deposition of the pseudocapacitive oxides ceria and vanadia and the electrochemical characterisation of the samples was done using CV analysis. Figure 8a shows the CV curves of ceria deposited on titanium metal foil and TNTA substrates, while Fig. 8b shows the CV curves of vanadia deposited on titanium metal foil and TNTA substrates at scan rate 5 mV s–1.
The specific capacitances of the above four pseudosupercapacitor electrodes were calculated from cyclic voltammograms and are given in the Table 1. The advantages of TNTA substrates over bare titanium foil substrate are evidenced by the higher capacitance values exhibited by super capacitor electrodes using TNTA substrates. From calculations, it was obtained that ceria deposited on TNTA electrode exhibited a specific capacitance of 55.38 mF cm–2, while ceria deposited on Ti metal foil electrode exhibited a specific capacitance of 8.44 mF cm–2 at 5 mV s–1 scan rate. Similarly, vanadia deposited on TNTA electrode also exhibited a superior capacitance performance over vanadia deposited on Ti metal foil electrode with specific capacitances of 152.11 mF cm–2 and 37.46 mF cm–2 respectively at 5 mV s–1 scan rate. Basu et al. reported phase-pure VO2 nanoporous structures on carbon fiber electrodes, which exhibited a capacitance of 33 mF cm–2 at 10 mV s–1 scan rate . Ren et al. reported VO2 nanoparticles on edge oriented graphene foam electrodes with specific capacitance of 119 mF cm–2 at 2 mV s–1 scan rate . Thus, it is clear that the capacitive performance of vanadia on TNTA electrode is superior over others. As discussed in detail in the introduction part, the better adhesion of pseudocapacitive oxides on TNTA substrates imparts higher stability and the open porous morphology of electrodes provides easily available spaces for electrochemical reactions and smooth channels for transport of charge carriers enhancing specific capacitance.
|Sample||Specific Capacitance, Cs (mF cm–2) @ 5 mV s–1 scan rate|
|Ceria on bare titanium foil||8.44|
|Ceria on TNTA||55.38|
|Vanadia on bare titanium foil||37.46|
|Vanadia on TNTA||152.11|
Highly ordered titania nanotube arrays (TNTA) were synthesised through a simple electrochemical anodisation process and morphological and electrochemical studies were done by SEM, XRD, CV, GCD and EIS analyses. As synthesised highly ordered TNTA exhibited a specific capacitance of 6.8 mF cm–2 at 5 mV s–1 scan rate and capacitance is very much higher than that reported earlier owing to the greater contribution of pseudocapacitance towards total capacitance. The TNTA were used as substrates for the deposition of the pseudocapacitive oxides ceria and vanadia and their supercapacitor properties were compared with same deposited on bare titanium foil substrates. The advantages of TNTA substrates over bare titanium foil substrate are evidenced by the higher capacitance values exhibited by super capacitor electrodes using TNTA substrates. TNTA provides easily available spaces for electrochemical reactions and smooth channels for transport of charge carriers enhancing the capacitor performance of pseudocapacitive oxides. The nanotube openings improve the adhesion of pseudocapacitive oxides to titanium foil substrates imparting cyclic stability for supercapacitor electrodes.
Funding statement: KMT and BKV acknowledge the funding from KSCSTE project (Order No. 1562/2016/KSCSTE) UGC start-up grant No.F 30-384/2017 (BSR) and CSIR. We also acknowledge Kannur University for providing research facilities.
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