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Growth of polyaniline on TiO2 tetragonal prism arrays as electrode materials for supercapacitor
Electrochimica Acta 300 (2019) 373e379
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Growth of polyaniline on TiO2 tetragonal prism arrays as electrode materials for supercapacitor Shaoyun Chen a, Ben Liu a, Xingying Zhang a, Fang Chen a, Hong Shi a, Chenglong Hu a, *, Jian Chen b a
Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan, 430056, China Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou, 510275, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2018 Received in revised form 8 January 2019 Accepted 19 January 2019 Available online 23 January 2019
In this study, the method of hydrothermal synthesis is introduced to prepare the titanium dioxide (i.e.,TiO2) tetragonal prism array on the conducting plane of fluorine-doped tin oxide (i.e., FTO), and then the polyaniline (i.e., PANI) is coated on the surface of TiO2 array by normal chemical oxidation to form the PANI/TiO2 shell/core nanoarray. The array architecture of PANI/TiO2 composite is further confirmed by SEM, XPS, XRD, UVevis and Raman spectroscopy. The regular array structure of PANI/TiO2 can reduce the resistance of ionic diffuse and charge transfer via optimizing the ionic diffusion to obtain a lower impedance and higher specific capacitance. Compared with the individual component (PANI or TiO2), the PANI/TiO2/FTO electrode possesses a higher specific capacitance as the supercapacitor electrode material. The maximum specific capacitances of 633 F/g at the rate of 10 mV/s and 781 F/g at the current density of 1 A/g are obtained in the PANI/TiO2/FTO electrode and it is superior or close to individual PANI or PANIbased materials. In addition, the TiO2 array can undertake some mechanical deformation in the redox process without destroying the electrode material to enhance the cyclic stability of PANI/TiO2/FTO electrode. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value which is higher than that of the PANI/FTO electrode (65%) after 2000 cycles. Therefore, such array architecture composite can be promisingly used as the electrode material for electrochemical capacitive energy storage. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Titanium dioxide Polyaniline Array architecture Supercapacitors
1. Introduction Electrochemical capacitors (also called supercapacitors) as one type of energy storage device are gaining much attention because of their fast charge-discharge property, long life cycle and high power density [1e3]. To date, the developments of supercapacitors can be summarized: (i) Design of novel carbon structures for carbon-based electrodes to increase the specific capacitance (i.e., SC) [4e11]; (ii) Synthesis of transition-metal oxides [12e15] and conducting polymers [16e21] with structures or new concepts for pseudocapacitive materials to develop pseudocapacitors; (iii) Development of new electrolytes to enlarge the cell voltage [22e27]. Among the various electroactive materials, polyaniline (i.e., PANI) has been received as pseudocapacitive material due to
* Corresponding author. E-mail address: [email protected] (C. Hu). https://doi.org/10.1016/j.electacta.2019.01.110 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
its multiple redox states, high conductivity and largest theoretical pseudocapacitance [17,28e31]. For example, the intrinsic PANI with nanobelts [32], cabbage [33], nanorods arrays [34], nanowire arrays [35] has been exploited to act as promising electrode materials for electrochemical capacitors. Moreover, the PANI-based nanocomposites, such as PANI/graphene [36e38], PANI/carbon nanofiber [39,40], PANI/carbon nanotube [41,42], PANI/graphite nanosheet [43] and PANI/transition-metal oxide [44] have been widely designed and optimized for application in supercapacitors. Recently, TiO2 shows great potential in supercapacitors research due to its large specific surface area and charge transport property. For instance, (i) The TiO2 nanotube arrays can be hydrogenated to form HeTiO2, which combines with MnO2 nanoparticles to achieve high specific capacitance (912 F/g) [45]; (ii) The self-doped TiO2 nanotube arrays can be fabricated by a cathodic polarization treatment on the pristine TiO2 to improve the conductivity and capacitive property [46,47]; (iii) The graphene/TiO2 composite has
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been fabricated by in-situ preparation or microwave-assisted technique to improve electrochemical capacitive performance [48,49]. However, the semiconductor nature of TiO2 often leads to poor conductivity and low electrochemical activity, which impedes its applications in supercapacitors. Previous study showed that TiO2 can act as an electron passageway to form electron acceptor/donor pairs when it combines with conducting polymers. Therefore, TiO2 may provide electrical channel for electrons and increase the electrons or ions transport rate during the charge-discharge process. Xie et al. showed reported that the SC value was as high as 732 F/g at 1 A/g when PANI nanowire array encapsulated in TiO2 nanotubes [50]. Su et al. showed that a high specific capacitance of 1020 F/g at 2 mV/s was obtained in the PANI/TiO2/GO nanocomposite [51]. Fu et al. reported that the porous PANI nanoparticles were coated onto the surface of TiO2 nanorods array to achieve improved charge transfer ability compared with individual PANI film [52]. Based on above discussion and our previous studies [29,33,53,54], in this paper, the TiO2 tetragonal prism array is firstly prepared by the hydrothermal method on fluorine-doped tin oxide (i.e., FTO). A hydrothermal route for the preparation of TiO2 tetragonal prism array directly onto FTO electrode represents a simple method for the synthesis of high quality samples with excellent electrical contact to a substrate, which is significant for further electrochemical performance testing. Successively, the aniline monomers adsorb onto the surface of TiO2 tetragonal prism array to polymerize long chain macromolecule to form PANI/TiO2 shell/core structure. It is well known that the mechanical property of PANI and electronic conductivity of TiO2 are poor. However, the good electronic conductivity of PANI and mechanical property of TiO2 can be well combined to select as high performance and stable energy materials when PANI fenced onto the surface of TiO2 tetragonal prism array to form shell/core structure. The PANI/TiO2 shell/core structure is characterized by scanning electron microscope (i.e., SEM), X-ray photoelectron spectroscopy (i.e., XPS), UVevisible spectroscopy and Raman spectroscopy. The electrochemical performances of the PANI/TiO2/FTO electrode are carried out by the electrochemical workstation. The as-prepared electroactive PANI/TiO2 possesses a loose structure for electrolyte access, which enhances the specific capacitance of the electrodes. The specific capacitance of PANI/TiO2/FTO electrode reaches as high as 781 F/g at a current density of 1 A/g. The experiment provides the evidence for potential application of PANI/TiO2 in electrochemical capacitors. 2. Experimental 2.1. Chemicals Ammonium sulfate ((NH4)2S2O8, APS), aniline (i.e., ANI) and hydrochloric acid (HCl wt%: 37%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetrabutyl titanate was obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). FTO (<15 U/sq) was obtained from Shenzhen south China xiangcheng technology Co., Ltd. (Shenzhen, China). 2.2. Fabrication of TiO2 tetragonal prism array electrode (i.e., TiO2/ FTO) The FTO was firstly cleaned in acetone, 2-propanol and ethanol (volume rations of 1:1:1) mixed solvent by sonication, subsequently rinsed with redistilled water, and then dried in N2. 24 mL of redistilled water and 24 mL of hydrochloric acid (wt%: 37%) were mixed by magnetic stirring under ambient condition for 10 min, and then 0.8 g tetrabutyl titanate was added into the mixed
solution. The mixture solution was moved into a Teflon-lined stainless steel autoclave (100 mL volume) after stirring for 10 min. The FTO substrate was placed at an angle (45 ) against the wall of the Teflon-liner. It notes that the conducting plane of FTO must face down within a sealed autoclave. The autoclave was sealed and placed in an oven to react for 5 h at 150 C under autogenetic pressure. The obtained product was washed 4e5 times with redistilled water, and then dried in N2. 2.3. Growth of PANI on TiO2 tetragonal prism array (i.e., PANI/TiO2/ FTO) The PANI was grown onto the TiO2 tetragonal prism array by chemical oxidation. Briefly, 10 mL hydrochloric acid (1.0 M) combines with 0.16 mol ANI monomer to form solution A. 10 mL hydrochloric acid (1.0 M) combines with 0.16 mol APS to form solution B. Subsequently, the solution A and B were mixed by dramatically vibration for 30 s, and then put it into 50 mL beaker. Finally, the TiO2/FTO electrode was vertically putted into the beaker to react for 2 h at room temperature. PANI/TiO2/FTO with various morphologies were synthesized by adjusting the molar masses of ANI monomer (CANI ¼ 0.04, 0.08, 0.16, 0.32 and 0.64 mol) under the same conditions (molar ratio of ANI to APS was a constant). In contrast, the PANI coated onto bare FTO electrode was also prepared under the same conditions (i.e., PANI/FTO). 2.4. Electrochemical measurement The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed by CHI 660E electrochemical workstation (CHI Instruments, Shanghai Chenhua Inc., and China). A saturated calomel electrode (SCE), a platinum sheet and as-prepared FTO electrode acts reference electrode, counter electrode and working electrode, respectively, to form a three-electrode configuration. The electrochemical performances of as-prepared TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes were measured in 1 M H2SO4 aqueous solution. 2.5. Characterization The morphologies of as-prepared samples were characterized by scanning electron microscopy (SEM, Hitachi, SU8000). The elemental analysis of as-prepared samples was obtained by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, KALPHAþ). The Raman spectrum was measured at 532 nm excitation wavelength (inVia Laser, Renishaw). The X-ray diffraction was carried out by the Panalytical X'pert powder diffractometer. The water contact angle of as-prepared samples was recorded by surface Tensiometer (K12, Germany Krüss). The transmittance spectra of as-prepared samples were carried out by a PerkinElmer Lambda 25 spectrophotometer (Perkin Elmer, Waltham, MA). 3. Results and discussion Fig. 1a shows the formation process of TiO2 array and PANI/TiO2 array. The FTO conducting glass with conducting plane is chosen as electrode for in-situ growth of nanoarrays. The thetetrabutyl titanate is widely selected as a precursor solution to hydrolyze at high temperature under hydrothermal conditions for the growth of TiO2 nanoarray, as shown in Fig. 1b. The TiO2 tetragonal prism array directly grows onto the conducting plane of FTO electrode with excellent electrical contact to a substrate. Subsequently, the ANI monomers are adsorbed onto the surface of the TiO2 array to form a small granular structure in the initial chemical synthesis stage, and
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Fig. 1. (a) The formation process of TiO2 tetragonal prism array growing onto the FTO and PANI coated onto the TiO2 array. (b) The formation of TiO2 array by hydrolysis. (c) The optical images of TiO2/FTO and PANI/TiO2/FTO.
then the granular structure grows further to polymerize long macromolecule chain to form the PANI shell fenced onto the surface of the TiO2 nanoarray, as shown in Fig. 1a. The optical images of TiO2 array and PANI/TiO2 array can be found in Fig. 1c. The uniform and white TiO2 array film is obtained by hydrothermal method, while the prepared PANI/TiO2 shell/core array is intensive green and formed a uniform film on TiO2 array substrate. Fig. 2 shows top view and tilted 45 SEM images of the TiO2 and PANI/TiO2 shell/core array. Fig. 2a and b shows that the uniform TiO2 array covers onto the entire surface of the FTO electrode. The surface shape of TiO2 array is a square that indicates pure TiO2 array is tetragonal prism with tetragonal crystal. Also, the TiO2 tetragonal prisms are nearly perpendicular to the conducting plane of FTO with diameters of 40e60 nm and lengths of 450e500 nm (Fig. 2c). Fig. 2d and e shows that the PANI is uniformly grown onto the surface of TiO2 tetragonal prisms, and PANI/TiO2 shell/core array with cylindrical shape and rounded top facets is obtained. Interestingly, the lengths of PANI/TiO2 shell/core array are still keeping the same as TiO2 tetragonal prisms (Fig. 2f and the inset of Fig. 2f). Moreover, the morphology and the alignment of TiO2 tetragonal prisms are preserved after coating. It means the TiO2 tetragonal prisms as the core can exhibit good durability. The XRD patterns of the TiO2 tetragonal prism array and PANI/ TiO2 shell/core array films can be found in Fig. 3. The marked\is assigned to the diffraction peaks of FTO glass, while the markedArepresents the diffraction peaks of TiO2, which reveals that the TiO2 tetragonal prism has been indexed as the tetragonal rutile structure [55,56]. It means that the TiO2 tetragonal prism as one kind of crystalline metal oxides exhibits good cyclic stability. In addition, the diffraction peaks of PANI are not observed due to the amorphous nature of PANI. Previous study showed that the
Fig. 2. Morphologies of TiO2 and PANI/TiO2 array: (a) and (b) the top view SEM images of TiO2 tetragonal prism array at low and high magnification. (c) The tilted 45 SEM image of TiO2 tetragonal prism array. (d) and (e) the top view and SEM images of PANI/ TiO2 array at low and high magnification. (f) The tilted 45 SEM image of PANI/TiO2 shell/core array. The inset is the cross-sectional SEM image of PANI/TiO2 array. The PANI/TiO2 is synthesized at 0.16 mol of ANI.
Fig. 3. XRD patterns of as-prepared TiO2 tetragonal prism array and PANI/TiO2 shell/ core array (prepared at 0.16 mol of ANI).
electrochemical stability of PANI is poor compared with the crystalline metal oxide, but it exhibits good pseudocapacitive performance. Therefore, the TiO2 tetragonal prism array incorporating with amorphous PANI to form PANI/TiO2 shell/core array film is a feasible way to enhance the cyclic stability and electrochemical performance. To compare with the properties of all as-prepared electrodes, the PANI grows directly onto the conducting plane of FTO substrate to form PANI/FTO film under the same condition. The SEM images of TiO2/FTO, PANI/FTO and TiO2/PANI/FTO can be found in Fig. 4a. The result shows that the interconnected network structure is obtained when PANI is directly coated on the FTO substrate. Specially, the thickness of PANI coated onto the surface of FTO is controlled 450e500 nm, keeping the same length of TiO2/PANI array. Fig. 4b shows XPS spectra of TiO2/FTO. The survey spectra of the TiO2/FTO contain the Si2p, C1s, Ca2p, Ti2p, Sn3d and O1s peaks, as shown in Fig. 4a. The Si, Ca and Sn element exists in TiO2/FTO which is originated from FTO glass. The Ti and O element come from the TiO2. The inset of Fig. 4a shows that the Ti2p3/2 and Ti2p1/2 spinorbital splitting photoelectrons for TiO2/FTO are appeared at binding energies of 464.51 and 458.58 eV, respectively [57]. The separation value of Ti2p1/2 and Ti2p3/2 signals is about 5.93 eV in good agreement with previous study [58]. The XPS survey scan of
Fig. 4. (a) The SEM images of TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes (prepared at 0.16 mol of ANI). The XPS wide-scan spectra of (b) TiO2/FTO (the inset is Ti2p core-level spectra of TiO2), (c) PANI/FTO (the inset is N1s core-level spectra of PANI) and (d) PANI/TiO2/FTO electrodes (the inset is N1s core-level spectra of PANI and Ti2p core-level spectra of TiO2, respectively).
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PANI/FTO sample shows the presence of oxygen (O1s: 532.35 eV), nitrogen (N1s: 399.92 eV), carbon (C1s: 284.81 eV) and silicon (Si2p: 103.22 eV), as shown in Fig. 4c. The Si element existing in PANI/FTO is also originated from FTO glass. For the PANI/TiO2/FTO, the XPS survey scan is the same as the PANI/FTO and the Ti2p peak can not be found in the spectra. It means that the PANI is entirely coated on the TiO2 tetragonal prism array. Moreover, the N1s spinorbital splitting photoelectron for PANI/FTO and PANI/TiO2/FTO are appeared at binding energies of 399.22 and 399.57 eV, respectively, indicating each of as-prepared PANI has the same structure at the molecular level. The Raman spectra of the TiO2/FTO, PANI/FTO and PANI/TiO2/ FTO can be found in Fig. 5a. The bands at 240, 444, and 610 cm1 can be indexed to pure rutile TiO2, according with previous studies [59]. The Raman spectra of PANI growing directly onto the conducting plane of FTO substrate clearly show no obvious changes in both peak position and shape compared with the Raman spectra of PANI/TiO2/FTO. It further confirms that each of as-prepared PANI has the same structure at the molecular level, in accordance with the results of XPS spectra. The band at 1595 cm1 is assigned to CC stretching vibration in the quinonoid ring. The bands at 1500 and 1162 cm1 are associated with CN stretching vibration and CH bending vibration in quinonoid units, respectively. The bands at 811 and 420 cm1 are related to the C H deformation. The band at 520 cm1 is assigned to benzene deformation in aromatic rings [29,33,55]. The transmittance of bare FTO, TiO2/FTO, PANI/FTO and PANI/ TiO2/FTO electrodes is measured ranging from 200 to 800 nm in the visible region, as shown in Fig. 5b. The transmittance of FTO (82%) decreases after TiO2 tetragonal prism array grows onto the surface of FTO (73%), and the transmittance of PANI/TiO2/FTO film (26%) is lower than that of PANI/FTO film (56%) in the green gap spectral range (450e650 nm) because the TiO2 tetragonal prism array presents in the as-prepared film. Additionally, the water contact angle of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes is about 92 , 94.5 , 73.5 and 73.0 , respectively (Fig. 5c). It can be found that the hydrophilicity of as-prepared electrodes can be enhanced when the PANI is coated on the bare FTO or TiO2/FTO. It can be helpful to form the intimate contact between aqueous electrolyte and PANI to accelerate the exchange rate of Hþ ions during charge-discharge process.
Fig. 5. (a) The Raman spectra of as-prepared TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes. (b) UVevis spectra of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes. (c) The water contact angle of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/ FTO electrodes. The PANI/TiO2 is synthesized at 0.16 mol of ANI.
Fig. 6. (a) CV curves and (b) GCD curves of the TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes (prepared at 0.16 mol of ANI).
To compare with the electrochemical properties of TiO2/FTO, PANI/FTO and PANI/TiO2/FTO (prepared at 0.16 mol of ANI) as a supercapacitor electrode, the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) are characterized in Fig. 6. The CV curves show that there is nearly no peaks for TiO2/FTO and two pairs of redox peaks for PANI/FTO and PANI/TiO2/FTO, which is ascribed to the leucoemeraldineemeraldine and the emeraldinepernigraniline redox transitions of PANI (as shown in Fig. 6a). Therefore, the capacitance of PANI/TiO2/FTO electrode mainly comes from Faradaic reactions of PANI at the electrode/electrolyte surface [60]. In addition, the GDC curves of the PANI/FTO and PANI/ TiO2/FTO at 1 A/g in Fig. 6b show that a capacitive behavior with almost symmetric charge-discharge curves can be observed which is a typical of pseudocapacitive contribution [61]. Moreover, the CV loop and discharge time of PANI/TiO2/FTO electrode are bigger than those of PANI/FTO electrode, which means larger SC values of the materials when the as-prepared electrodes are tested under the same condition [62]. Therefore, PANI/TiO2/FTO electrode exhibits outstanding electrochemical property compared with PANI/FTO electrode. It indicates that the TiO2 tetragonal prism array can effectively increase the capacitance of PANI. Fig. 7a shows the CV response of PANI/TiO2/FTO electrode (prepared at 0.16 M of ANI) at different scan rates from 10 to 200 mV/s, and the potential window is carried out from 0.8 to 0.4 V. The relationship between the voltammetric currents and scan rates can be regarded as nearly directly proportional, indicating an ideal capacitive behavior (black line in Fig. 7b) [63]. The curve of SC value plots of the scan rate is also demonstrated in Fig. 7b (blue line). The SC value of PANI/TiO2/FTO electrode calculated from CV curve is about 633 F/g at scan rate of 10 mV/s [62], and its value decreases to 201 F/g when the scan rate increases from 10 to 200 mVs1. It is attributed to the fact that ions difficultly diffuse within the inner of electrode material, leading the inner active sites of PANI/TiO2/FTO cannot complete the redox transitions at higher scan rates because the electrode material can be considered full utilization to get high specific capacitance at the slowest scan rate. The SC value of 633 F/g of PANI/TiO2/FTO electrode reported here is
Fig. 7. (a) CV curves of the PANI/TiO2/FTO electrode (prepared at 0.16 mol of ANI) at a scan rate from 10 to 200 mV/s (b)The relationship between the peak current densities and scan rates (black line) and the SC value of PANI/TiO2/FTO electrode at different scan rates (blue line).
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Fig. 8. GCD curves of the PANI/TiO2/FTO electrode at a current density from 1 to 5 A/g. The PANI/TiO2/FTO is synthesized at 0.16 mol of ANI.
much bigger than those of PANI nanofibers (122 F/g) [64], PANI porous nanowires (700 F/g) [65], PANI/Ag composite(512 F/g) [66], PANI arrays/Graphene oxide sheets (555 F/g) [60] and PANI arrays/ Reduced graphene oxide (590 F/g) [67]. The GCD curves of the PANI/TiO2/FTO electrode at different current density are performed to better understand the capacitive behavior. Typical GCD curves of the PANI/TiO2/FTO electrode at different current density are shown in Fig. 8. It can be seen that all of the curves are not symmetric or not ideal straight lines, revealing the involvement of a faradaic reaction process. The result indicates the PANI/TiO2/FTO electrode is feasible for the development of supercapacitors. In addition, the “IR drop” in potential can be easily observed which is caused by internal resistance of PANI/TiO2 composite. The SC value calculated from the charging-discharging curve is about 781 F/g at current density of 1A/g. The inset of Fig. 8 shows the relationship between the specific capacitance and current density. The SC value of PANI/TiO2/FTO electrode decreases with the increase of the current density. It suggests that the PANI/ TiO2/FTO electrode has a good electrochemical reversibility and a large SC value of 781 F/g at a current density of 1A/g, according with the result obtained from CV curve. The effects of ANI concentration on morphologies of PANI/TiO2 array are also investigated in Fig. 9aee. It can be seen that the microstructures of the PANI coated on TiO2 array are significantly influenced by the concentration of ANI. At low concentration of aniline (ANI: 0.04 and 0.08 mol), few PANI is coated on the TiO2 array and the diameter of PANI/TiO2 array is small (Fig. 9a and b). The diameter of PANI/TiO2 array increases with the increase of aniline concentration (ANI: 0.16 mol) to form perfect nanoarrays with diameters of 40e60 nm and lengths of 450e500 nm (Fig. 9c). However, the polymerized PANI inclines to merge together to form irregular nanoarrays when the further increase of concentration of ANI (Fig. 9d). Furthermore, the redundant PANI is obviously produced on the surface of TiO2 array in the synthetic process (Fig. 9e). The transmittance (Fig. 9f) and the water contact angle (Fig. 9g) of PANI/TiO2/FTO film gradually decease with the increase of the concentration of ANI, revealing that the thickness and hydrophilicity of PANI/TiO2 array film increases along with the concentration of ANI, in agreement with the results of SEM. Based on above discussion, the composite material of PANI/TiO2 array may show excellent performance as supercapacitor electrode materials. Therefore, the influence of ANI concentration on the specific capacitance is investigated by cyclic voltammetry (10 mV/ s). The similar shape of all of the CV curves can be found in Fig. 10,
Fig. 9. (a)~(e) PANI/TiO2/FTO with various morphologies are prepared by adjusting the molar masses of ANI monomer. (f) UVevis spectra of as-prepared PANI/TiO2/FTO samples. (g) The water contact angle of as-prepared PANI/TiO2/FTO samples (ANI: 0.04, 0.08, 0.16, 0.32 and 0.64 mol).
Fig. 10. CV curves of as-prepared PANI/TiO2/FTO electrode (ANI: 0.04, 0.08, 0.16, 0.32, and 0.64 mol). The inset is the relationship between SC values and the concentration of ANI.
and the SC value of PANI/TiO2/FTO electrode increases with the concentration of ANI from 0.04 to 0.64 mol, with a higher SC value of 633 F/g at 0.16 mol ANI (the inset in Fig. 10). It is caused by that the coated PANI on the TiO2 arrays grow denser under the higher concentration of ANI monomer. However, the SC value PANI/TiO2/ FTO electrode decreases to 538 F/g when the concentration of ANI further increased to 0.64 mol. Fig. 9d and e shows that the random and the redundant PANI is also produced besides aligned PANI/TiO2 array at the concentration of 0.032 and 0.64 mol. These redundant PANI may act as barrier to reduce the specific capacitance property of the PANI/TiO2/FTO electrode. It is well known that the conducting polymer as the electrode material for supercapacitor can be decomposed during repeated charge-discharge processes because its volume will swell when the ions from electrolyte enter into polymers during charging and shrink when the ions go back to electrolyte during discharging. As a result, the conducting polymer will be broken into small pieces to loss of active materials, leading into the degradation of capacitance.
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21706092); the Hubei Province Natural Science Foundation of China (Grant No. 2018CFB520). References
Fig. 11. (a) Specific capacitance of PANI/FTO and PANI/TiO2/FTO electrodes as a function of the cycle number at a scan rate of 10 mV/s. The CV curves of PANI/FTO and PANI/ TiO2/FTO electrodes before and after cycle can be found in the inset. (b) The EIS curves of PANI/FTO and PANI/TiO2/FTO electrodes. (c) Schematic of the assumed ion diffusion path in PANI/FTO and PANI/TiO2/FTO electrodes. The PANI/TiO2/FTO is synthesized at 0.16 mol of ANI.
The cycling stability of PANI/TiO2 and PANI/TiO2/FTO electrodes is carried out in 1 M H2SO4 aqueous electrolyte, as shown in Fig. 11a. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value, while the PANI/TiO2 electrode keeps only 65% of its initial value after 2000 cycles. Obviously, The PANI/TiO2 array as electrode material for supercapacitor exhibits much higher stability than that of pristine PANI film. It is attributed to that most mechanical deformation is assigned to TiO2 array in the redox process, which avoided the structural pulverization in the electrode material and is in favor of obtaining a good stability. In addition, the typical electrochemical impedance spectroscopy (EIS) for the PANI/TiO2 and PANI/TiO2/FTO electrodes is given in Fig. 11 b. It can be seen that a single semicircle in the high frequency region and a sloped line in the low frequency region are obtained in the impedance curves. Moreover, the semicircle of PANI/TiO2/FTO electrode is smaller than that of the PANI/FTO electrode, indicating PANI/TiO2/FTO electrode has lower impedance on electrode/electrolyte interface. Previous study showed that the vertically aligned nanoarray is benefitful for ion diffusion from an electrolyte solution to the surface of materials of nanoarray [34]. As illustrated in Fig. 11c, the electrolyte ions can enter or leave the surface of PANI/ TiO2 array easily and fast compared with pristine PANI film. Moreover, the array architecture can shorten the charge transport distance during charge-discharge process in the PANI materials [68]. Therefore, the regular array structure can optimize the ionic diffusion which reduces the resistance of ionic diffuse and charge transfer to obtain lower impedance and high capacitance performance. 4. Conclusion In summary, an array structure of PANI/TiO2 shell/core composite has been successively prepared by hydrothermal synthesis of tetrabutyl titanate and the oxidative polymerization of ANI on the conducting plane of FTO. The as-prepared PANI/TiO2/FTO can act as an electrode material for supercapacitor and its SC value is about 633 F/g and 781 F/g within the potential window 0.8 to 0.4 V vs. SCE at the rate of 10 mV/s and at the current density of 1 A/g, respectively. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value, while the PANI/TiO2 electrode keeps only 65% of its initial value after 2000 cycles. The electrochemical behaviors of all as-prepared electrodes prove that the PANI/TiO2/ FTO electrode possesses a synergistic effect of PANI and TiO2 exhibiting higher SC value and better cycling stability compared with that of each individual component. Acknowledgements The authors are gratefully acknowledged the support of the National Natural Science Foundation of China (Grant No.
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Growth of polyaniline on TiO2 tetragonal prism arrays as electrode materials for supercapacitor Shaoyun Chen a, Ben Liu a, Xingying Zhang a, Fang Chen a, Hong Shi a, Chenglong Hu a, *, Jian Chen b a
Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan, 430056, China Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou, 510275, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2018 Received in revised form 8 January 2019 Accepted 19 January 2019 Available online 23 January 2019
In this study, the method of hydrothermal synthesis is introduced to prepare the titanium dioxide (i.e.,TiO2) tetragonal prism array on the conducting plane of fluorine-doped tin oxide (i.e., FTO), and then the polyaniline (i.e., PANI) is coated on the surface of TiO2 array by normal chemical oxidation to form the PANI/TiO2 shell/core nanoarray. The array architecture of PANI/TiO2 composite is further confirmed by SEM, XPS, XRD, UVevis and Raman spectroscopy. The regular array structure of PANI/TiO2 can reduce the resistance of ionic diffuse and charge transfer via optimizing the ionic diffusion to obtain a lower impedance and higher specific capacitance. Compared with the individual component (PANI or TiO2), the PANI/TiO2/FTO electrode possesses a higher specific capacitance as the supercapacitor electrode material. The maximum specific capacitances of 633 F/g at the rate of 10 mV/s and 781 F/g at the current density of 1 A/g are obtained in the PANI/TiO2/FTO electrode and it is superior or close to individual PANI or PANIbased materials. In addition, the TiO2 array can undertake some mechanical deformation in the redox process without destroying the electrode material to enhance the cyclic stability of PANI/TiO2/FTO electrode. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value which is higher than that of the PANI/FTO electrode (65%) after 2000 cycles. Therefore, such array architecture composite can be promisingly used as the electrode material for electrochemical capacitive energy storage. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Titanium dioxide Polyaniline Array architecture Supercapacitors
1. Introduction Electrochemical capacitors (also called supercapacitors) as one type of energy storage device are gaining much attention because of their fast charge-discharge property, long life cycle and high power density [1e3]. To date, the developments of supercapacitors can be summarized: (i) Design of novel carbon structures for carbon-based electrodes to increase the specific capacitance (i.e., SC) [4e11]; (ii) Synthesis of transition-metal oxides [12e15] and conducting polymers [16e21] with structures or new concepts for pseudocapacitive materials to develop pseudocapacitors; (iii) Development of new electrolytes to enlarge the cell voltage [22e27]. Among the various electroactive materials, polyaniline (i.e., PANI) has been received as pseudocapacitive material due to
* Corresponding author. E-mail address: [email protected] (C. Hu). https://doi.org/10.1016/j.electacta.2019.01.110 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
its multiple redox states, high conductivity and largest theoretical pseudocapacitance [17,28e31]. For example, the intrinsic PANI with nanobelts [32], cabbage [33], nanorods arrays [34], nanowire arrays [35] has been exploited to act as promising electrode materials for electrochemical capacitors. Moreover, the PANI-based nanocomposites, such as PANI/graphene [36e38], PANI/carbon nanofiber [39,40], PANI/carbon nanotube [41,42], PANI/graphite nanosheet [43] and PANI/transition-metal oxide [44] have been widely designed and optimized for application in supercapacitors. Recently, TiO2 shows great potential in supercapacitors research due to its large specific surface area and charge transport property. For instance, (i) The TiO2 nanotube arrays can be hydrogenated to form HeTiO2, which combines with MnO2 nanoparticles to achieve high specific capacitance (912 F/g) [45]; (ii) The self-doped TiO2 nanotube arrays can be fabricated by a cathodic polarization treatment on the pristine TiO2 to improve the conductivity and capacitive property [46,47]; (iii) The graphene/TiO2 composite has
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been fabricated by in-situ preparation or microwave-assisted technique to improve electrochemical capacitive performance [48,49]. However, the semiconductor nature of TiO2 often leads to poor conductivity and low electrochemical activity, which impedes its applications in supercapacitors. Previous study showed that TiO2 can act as an electron passageway to form electron acceptor/donor pairs when it combines with conducting polymers. Therefore, TiO2 may provide electrical channel for electrons and increase the electrons or ions transport rate during the charge-discharge process. Xie et al. showed reported that the SC value was as high as 732 F/g at 1 A/g when PANI nanowire array encapsulated in TiO2 nanotubes [50]. Su et al. showed that a high specific capacitance of 1020 F/g at 2 mV/s was obtained in the PANI/TiO2/GO nanocomposite [51]. Fu et al. reported that the porous PANI nanoparticles were coated onto the surface of TiO2 nanorods array to achieve improved charge transfer ability compared with individual PANI film [52]. Based on above discussion and our previous studies [29,33,53,54], in this paper, the TiO2 tetragonal prism array is firstly prepared by the hydrothermal method on fluorine-doped tin oxide (i.e., FTO). A hydrothermal route for the preparation of TiO2 tetragonal prism array directly onto FTO electrode represents a simple method for the synthesis of high quality samples with excellent electrical contact to a substrate, which is significant for further electrochemical performance testing. Successively, the aniline monomers adsorb onto the surface of TiO2 tetragonal prism array to polymerize long chain macromolecule to form PANI/TiO2 shell/core structure. It is well known that the mechanical property of PANI and electronic conductivity of TiO2 are poor. However, the good electronic conductivity of PANI and mechanical property of TiO2 can be well combined to select as high performance and stable energy materials when PANI fenced onto the surface of TiO2 tetragonal prism array to form shell/core structure. The PANI/TiO2 shell/core structure is characterized by scanning electron microscope (i.e., SEM), X-ray photoelectron spectroscopy (i.e., XPS), UVevisible spectroscopy and Raman spectroscopy. The electrochemical performances of the PANI/TiO2/FTO electrode are carried out by the electrochemical workstation. The as-prepared electroactive PANI/TiO2 possesses a loose structure for electrolyte access, which enhances the specific capacitance of the electrodes. The specific capacitance of PANI/TiO2/FTO electrode reaches as high as 781 F/g at a current density of 1 A/g. The experiment provides the evidence for potential application of PANI/TiO2 in electrochemical capacitors. 2. Experimental 2.1. Chemicals Ammonium sulfate ((NH4)2S2O8, APS), aniline (i.e., ANI) and hydrochloric acid (HCl wt%: 37%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetrabutyl titanate was obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). FTO (<15 U/sq) was obtained from Shenzhen south China xiangcheng technology Co., Ltd. (Shenzhen, China). 2.2. Fabrication of TiO2 tetragonal prism array electrode (i.e., TiO2/ FTO) The FTO was firstly cleaned in acetone, 2-propanol and ethanol (volume rations of 1:1:1) mixed solvent by sonication, subsequently rinsed with redistilled water, and then dried in N2. 24 mL of redistilled water and 24 mL of hydrochloric acid (wt%: 37%) were mixed by magnetic stirring under ambient condition for 10 min, and then 0.8 g tetrabutyl titanate was added into the mixed
solution. The mixture solution was moved into a Teflon-lined stainless steel autoclave (100 mL volume) after stirring for 10 min. The FTO substrate was placed at an angle (45 ) against the wall of the Teflon-liner. It notes that the conducting plane of FTO must face down within a sealed autoclave. The autoclave was sealed and placed in an oven to react for 5 h at 150 C under autogenetic pressure. The obtained product was washed 4e5 times with redistilled water, and then dried in N2. 2.3. Growth of PANI on TiO2 tetragonal prism array (i.e., PANI/TiO2/ FTO) The PANI was grown onto the TiO2 tetragonal prism array by chemical oxidation. Briefly, 10 mL hydrochloric acid (1.0 M) combines with 0.16 mol ANI monomer to form solution A. 10 mL hydrochloric acid (1.0 M) combines with 0.16 mol APS to form solution B. Subsequently, the solution A and B were mixed by dramatically vibration for 30 s, and then put it into 50 mL beaker. Finally, the TiO2/FTO electrode was vertically putted into the beaker to react for 2 h at room temperature. PANI/TiO2/FTO with various morphologies were synthesized by adjusting the molar masses of ANI monomer (CANI ¼ 0.04, 0.08, 0.16, 0.32 and 0.64 mol) under the same conditions (molar ratio of ANI to APS was a constant). In contrast, the PANI coated onto bare FTO electrode was also prepared under the same conditions (i.e., PANI/FTO). 2.4. Electrochemical measurement The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed by CHI 660E electrochemical workstation (CHI Instruments, Shanghai Chenhua Inc., and China). A saturated calomel electrode (SCE), a platinum sheet and as-prepared FTO electrode acts reference electrode, counter electrode and working electrode, respectively, to form a three-electrode configuration. The electrochemical performances of as-prepared TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes were measured in 1 M H2SO4 aqueous solution. 2.5. Characterization The morphologies of as-prepared samples were characterized by scanning electron microscopy (SEM, Hitachi, SU8000). The elemental analysis of as-prepared samples was obtained by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, KALPHAþ). The Raman spectrum was measured at 532 nm excitation wavelength (inVia Laser, Renishaw). The X-ray diffraction was carried out by the Panalytical X'pert powder diffractometer. The water contact angle of as-prepared samples was recorded by surface Tensiometer (K12, Germany Krüss). The transmittance spectra of as-prepared samples were carried out by a PerkinElmer Lambda 25 spectrophotometer (Perkin Elmer, Waltham, MA). 3. Results and discussion Fig. 1a shows the formation process of TiO2 array and PANI/TiO2 array. The FTO conducting glass with conducting plane is chosen as electrode for in-situ growth of nanoarrays. The thetetrabutyl titanate is widely selected as a precursor solution to hydrolyze at high temperature under hydrothermal conditions for the growth of TiO2 nanoarray, as shown in Fig. 1b. The TiO2 tetragonal prism array directly grows onto the conducting plane of FTO electrode with excellent electrical contact to a substrate. Subsequently, the ANI monomers are adsorbed onto the surface of the TiO2 array to form a small granular structure in the initial chemical synthesis stage, and
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Fig. 1. (a) The formation process of TiO2 tetragonal prism array growing onto the FTO and PANI coated onto the TiO2 array. (b) The formation of TiO2 array by hydrolysis. (c) The optical images of TiO2/FTO and PANI/TiO2/FTO.
then the granular structure grows further to polymerize long macromolecule chain to form the PANI shell fenced onto the surface of the TiO2 nanoarray, as shown in Fig. 1a. The optical images of TiO2 array and PANI/TiO2 array can be found in Fig. 1c. The uniform and white TiO2 array film is obtained by hydrothermal method, while the prepared PANI/TiO2 shell/core array is intensive green and formed a uniform film on TiO2 array substrate. Fig. 2 shows top view and tilted 45 SEM images of the TiO2 and PANI/TiO2 shell/core array. Fig. 2a and b shows that the uniform TiO2 array covers onto the entire surface of the FTO electrode. The surface shape of TiO2 array is a square that indicates pure TiO2 array is tetragonal prism with tetragonal crystal. Also, the TiO2 tetragonal prisms are nearly perpendicular to the conducting plane of FTO with diameters of 40e60 nm and lengths of 450e500 nm (Fig. 2c). Fig. 2d and e shows that the PANI is uniformly grown onto the surface of TiO2 tetragonal prisms, and PANI/TiO2 shell/core array with cylindrical shape and rounded top facets is obtained. Interestingly, the lengths of PANI/TiO2 shell/core array are still keeping the same as TiO2 tetragonal prisms (Fig. 2f and the inset of Fig. 2f). Moreover, the morphology and the alignment of TiO2 tetragonal prisms are preserved after coating. It means the TiO2 tetragonal prisms as the core can exhibit good durability. The XRD patterns of the TiO2 tetragonal prism array and PANI/ TiO2 shell/core array films can be found in Fig. 3. The marked\is assigned to the diffraction peaks of FTO glass, while the markedArepresents the diffraction peaks of TiO2, which reveals that the TiO2 tetragonal prism has been indexed as the tetragonal rutile structure [55,56]. It means that the TiO2 tetragonal prism as one kind of crystalline metal oxides exhibits good cyclic stability. In addition, the diffraction peaks of PANI are not observed due to the amorphous nature of PANI. Previous study showed that the
Fig. 2. Morphologies of TiO2 and PANI/TiO2 array: (a) and (b) the top view SEM images of TiO2 tetragonal prism array at low and high magnification. (c) The tilted 45 SEM image of TiO2 tetragonal prism array. (d) and (e) the top view and SEM images of PANI/ TiO2 array at low and high magnification. (f) The tilted 45 SEM image of PANI/TiO2 shell/core array. The inset is the cross-sectional SEM image of PANI/TiO2 array. The PANI/TiO2 is synthesized at 0.16 mol of ANI.
Fig. 3. XRD patterns of as-prepared TiO2 tetragonal prism array and PANI/TiO2 shell/ core array (prepared at 0.16 mol of ANI).
electrochemical stability of PANI is poor compared with the crystalline metal oxide, but it exhibits good pseudocapacitive performance. Therefore, the TiO2 tetragonal prism array incorporating with amorphous PANI to form PANI/TiO2 shell/core array film is a feasible way to enhance the cyclic stability and electrochemical performance. To compare with the properties of all as-prepared electrodes, the PANI grows directly onto the conducting plane of FTO substrate to form PANI/FTO film under the same condition. The SEM images of TiO2/FTO, PANI/FTO and TiO2/PANI/FTO can be found in Fig. 4a. The result shows that the interconnected network structure is obtained when PANI is directly coated on the FTO substrate. Specially, the thickness of PANI coated onto the surface of FTO is controlled 450e500 nm, keeping the same length of TiO2/PANI array. Fig. 4b shows XPS spectra of TiO2/FTO. The survey spectra of the TiO2/FTO contain the Si2p, C1s, Ca2p, Ti2p, Sn3d and O1s peaks, as shown in Fig. 4a. The Si, Ca and Sn element exists in TiO2/FTO which is originated from FTO glass. The Ti and O element come from the TiO2. The inset of Fig. 4a shows that the Ti2p3/2 and Ti2p1/2 spinorbital splitting photoelectrons for TiO2/FTO are appeared at binding energies of 464.51 and 458.58 eV, respectively [57]. The separation value of Ti2p1/2 and Ti2p3/2 signals is about 5.93 eV in good agreement with previous study [58]. The XPS survey scan of
Fig. 4. (a) The SEM images of TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes (prepared at 0.16 mol of ANI). The XPS wide-scan spectra of (b) TiO2/FTO (the inset is Ti2p core-level spectra of TiO2), (c) PANI/FTO (the inset is N1s core-level spectra of PANI) and (d) PANI/TiO2/FTO electrodes (the inset is N1s core-level spectra of PANI and Ti2p core-level spectra of TiO2, respectively).
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PANI/FTO sample shows the presence of oxygen (O1s: 532.35 eV), nitrogen (N1s: 399.92 eV), carbon (C1s: 284.81 eV) and silicon (Si2p: 103.22 eV), as shown in Fig. 4c. The Si element existing in PANI/FTO is also originated from FTO glass. For the PANI/TiO2/FTO, the XPS survey scan is the same as the PANI/FTO and the Ti2p peak can not be found in the spectra. It means that the PANI is entirely coated on the TiO2 tetragonal prism array. Moreover, the N1s spinorbital splitting photoelectron for PANI/FTO and PANI/TiO2/FTO are appeared at binding energies of 399.22 and 399.57 eV, respectively, indicating each of as-prepared PANI has the same structure at the molecular level. The Raman spectra of the TiO2/FTO, PANI/FTO and PANI/TiO2/ FTO can be found in Fig. 5a. The bands at 240, 444, and 610 cm1 can be indexed to pure rutile TiO2, according with previous studies [59]. The Raman spectra of PANI growing directly onto the conducting plane of FTO substrate clearly show no obvious changes in both peak position and shape compared with the Raman spectra of PANI/TiO2/FTO. It further confirms that each of as-prepared PANI has the same structure at the molecular level, in accordance with the results of XPS spectra. The band at 1595 cm1 is assigned to CC stretching vibration in the quinonoid ring. The bands at 1500 and 1162 cm1 are associated with CN stretching vibration and CH bending vibration in quinonoid units, respectively. The bands at 811 and 420 cm1 are related to the C H deformation. The band at 520 cm1 is assigned to benzene deformation in aromatic rings [29,33,55]. The transmittance of bare FTO, TiO2/FTO, PANI/FTO and PANI/ TiO2/FTO electrodes is measured ranging from 200 to 800 nm in the visible region, as shown in Fig. 5b. The transmittance of FTO (82%) decreases after TiO2 tetragonal prism array grows onto the surface of FTO (73%), and the transmittance of PANI/TiO2/FTO film (26%) is lower than that of PANI/FTO film (56%) in the green gap spectral range (450e650 nm) because the TiO2 tetragonal prism array presents in the as-prepared film. Additionally, the water contact angle of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes is about 92 , 94.5 , 73.5 and 73.0 , respectively (Fig. 5c). It can be found that the hydrophilicity of as-prepared electrodes can be enhanced when the PANI is coated on the bare FTO or TiO2/FTO. It can be helpful to form the intimate contact between aqueous electrolyte and PANI to accelerate the exchange rate of Hþ ions during charge-discharge process.
Fig. 5. (a) The Raman spectra of as-prepared TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes. (b) UVevis spectra of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes. (c) The water contact angle of bare FTO, TiO2/FTO, PANI/FTO and PANI/TiO2/ FTO electrodes. The PANI/TiO2 is synthesized at 0.16 mol of ANI.
Fig. 6. (a) CV curves and (b) GCD curves of the TiO2/FTO, PANI/FTO and PANI/TiO2/FTO electrodes (prepared at 0.16 mol of ANI).
To compare with the electrochemical properties of TiO2/FTO, PANI/FTO and PANI/TiO2/FTO (prepared at 0.16 mol of ANI) as a supercapacitor electrode, the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) are characterized in Fig. 6. The CV curves show that there is nearly no peaks for TiO2/FTO and two pairs of redox peaks for PANI/FTO and PANI/TiO2/FTO, which is ascribed to the leucoemeraldineemeraldine and the emeraldinepernigraniline redox transitions of PANI (as shown in Fig. 6a). Therefore, the capacitance of PANI/TiO2/FTO electrode mainly comes from Faradaic reactions of PANI at the electrode/electrolyte surface [60]. In addition, the GDC curves of the PANI/FTO and PANI/ TiO2/FTO at 1 A/g in Fig. 6b show that a capacitive behavior with almost symmetric charge-discharge curves can be observed which is a typical of pseudocapacitive contribution [61]. Moreover, the CV loop and discharge time of PANI/TiO2/FTO electrode are bigger than those of PANI/FTO electrode, which means larger SC values of the materials when the as-prepared electrodes are tested under the same condition [62]. Therefore, PANI/TiO2/FTO electrode exhibits outstanding electrochemical property compared with PANI/FTO electrode. It indicates that the TiO2 tetragonal prism array can effectively increase the capacitance of PANI. Fig. 7a shows the CV response of PANI/TiO2/FTO electrode (prepared at 0.16 M of ANI) at different scan rates from 10 to 200 mV/s, and the potential window is carried out from 0.8 to 0.4 V. The relationship between the voltammetric currents and scan rates can be regarded as nearly directly proportional, indicating an ideal capacitive behavior (black line in Fig. 7b) [63]. The curve of SC value plots of the scan rate is also demonstrated in Fig. 7b (blue line). The SC value of PANI/TiO2/FTO electrode calculated from CV curve is about 633 F/g at scan rate of 10 mV/s [62], and its value decreases to 201 F/g when the scan rate increases from 10 to 200 mVs1. It is attributed to the fact that ions difficultly diffuse within the inner of electrode material, leading the inner active sites of PANI/TiO2/FTO cannot complete the redox transitions at higher scan rates because the electrode material can be considered full utilization to get high specific capacitance at the slowest scan rate. The SC value of 633 F/g of PANI/TiO2/FTO electrode reported here is
Fig. 7. (a) CV curves of the PANI/TiO2/FTO electrode (prepared at 0.16 mol of ANI) at a scan rate from 10 to 200 mV/s (b)The relationship between the peak current densities and scan rates (black line) and the SC value of PANI/TiO2/FTO electrode at different scan rates (blue line).
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Fig. 8. GCD curves of the PANI/TiO2/FTO electrode at a current density from 1 to 5 A/g. The PANI/TiO2/FTO is synthesized at 0.16 mol of ANI.
much bigger than those of PANI nanofibers (122 F/g) [64], PANI porous nanowires (700 F/g) [65], PANI/Ag composite(512 F/g) [66], PANI arrays/Graphene oxide sheets (555 F/g) [60] and PANI arrays/ Reduced graphene oxide (590 F/g) [67]. The GCD curves of the PANI/TiO2/FTO electrode at different current density are performed to better understand the capacitive behavior. Typical GCD curves of the PANI/TiO2/FTO electrode at different current density are shown in Fig. 8. It can be seen that all of the curves are not symmetric or not ideal straight lines, revealing the involvement of a faradaic reaction process. The result indicates the PANI/TiO2/FTO electrode is feasible for the development of supercapacitors. In addition, the “IR drop” in potential can be easily observed which is caused by internal resistance of PANI/TiO2 composite. The SC value calculated from the charging-discharging curve is about 781 F/g at current density of 1A/g. The inset of Fig. 8 shows the relationship between the specific capacitance and current density. The SC value of PANI/TiO2/FTO electrode decreases with the increase of the current density. It suggests that the PANI/ TiO2/FTO electrode has a good electrochemical reversibility and a large SC value of 781 F/g at a current density of 1A/g, according with the result obtained from CV curve. The effects of ANI concentration on morphologies of PANI/TiO2 array are also investigated in Fig. 9aee. It can be seen that the microstructures of the PANI coated on TiO2 array are significantly influenced by the concentration of ANI. At low concentration of aniline (ANI: 0.04 and 0.08 mol), few PANI is coated on the TiO2 array and the diameter of PANI/TiO2 array is small (Fig. 9a and b). The diameter of PANI/TiO2 array increases with the increase of aniline concentration (ANI: 0.16 mol) to form perfect nanoarrays with diameters of 40e60 nm and lengths of 450e500 nm (Fig. 9c). However, the polymerized PANI inclines to merge together to form irregular nanoarrays when the further increase of concentration of ANI (Fig. 9d). Furthermore, the redundant PANI is obviously produced on the surface of TiO2 array in the synthetic process (Fig. 9e). The transmittance (Fig. 9f) and the water contact angle (Fig. 9g) of PANI/TiO2/FTO film gradually decease with the increase of the concentration of ANI, revealing that the thickness and hydrophilicity of PANI/TiO2 array film increases along with the concentration of ANI, in agreement with the results of SEM. Based on above discussion, the composite material of PANI/TiO2 array may show excellent performance as supercapacitor electrode materials. Therefore, the influence of ANI concentration on the specific capacitance is investigated by cyclic voltammetry (10 mV/ s). The similar shape of all of the CV curves can be found in Fig. 10,
Fig. 9. (a)~(e) PANI/TiO2/FTO with various morphologies are prepared by adjusting the molar masses of ANI monomer. (f) UVevis spectra of as-prepared PANI/TiO2/FTO samples. (g) The water contact angle of as-prepared PANI/TiO2/FTO samples (ANI: 0.04, 0.08, 0.16, 0.32 and 0.64 mol).
Fig. 10. CV curves of as-prepared PANI/TiO2/FTO electrode (ANI: 0.04, 0.08, 0.16, 0.32, and 0.64 mol). The inset is the relationship between SC values and the concentration of ANI.
and the SC value of PANI/TiO2/FTO electrode increases with the concentration of ANI from 0.04 to 0.64 mol, with a higher SC value of 633 F/g at 0.16 mol ANI (the inset in Fig. 10). It is caused by that the coated PANI on the TiO2 arrays grow denser under the higher concentration of ANI monomer. However, the SC value PANI/TiO2/ FTO electrode decreases to 538 F/g when the concentration of ANI further increased to 0.64 mol. Fig. 9d and e shows that the random and the redundant PANI is also produced besides aligned PANI/TiO2 array at the concentration of 0.032 and 0.64 mol. These redundant PANI may act as barrier to reduce the specific capacitance property of the PANI/TiO2/FTO electrode. It is well known that the conducting polymer as the electrode material for supercapacitor can be decomposed during repeated charge-discharge processes because its volume will swell when the ions from electrolyte enter into polymers during charging and shrink when the ions go back to electrolyte during discharging. As a result, the conducting polymer will be broken into small pieces to loss of active materials, leading into the degradation of capacitance.
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21706092); the Hubei Province Natural Science Foundation of China (Grant No. 2018CFB520). References
Fig. 11. (a) Specific capacitance of PANI/FTO and PANI/TiO2/FTO electrodes as a function of the cycle number at a scan rate of 10 mV/s. The CV curves of PANI/FTO and PANI/ TiO2/FTO electrodes before and after cycle can be found in the inset. (b) The EIS curves of PANI/FTO and PANI/TiO2/FTO electrodes. (c) Schematic of the assumed ion diffusion path in PANI/FTO and PANI/TiO2/FTO electrodes. The PANI/TiO2/FTO is synthesized at 0.16 mol of ANI.
The cycling stability of PANI/TiO2 and PANI/TiO2/FTO electrodes is carried out in 1 M H2SO4 aqueous electrolyte, as shown in Fig. 11a. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value, while the PANI/TiO2 electrode keeps only 65% of its initial value after 2000 cycles. Obviously, The PANI/TiO2 array as electrode material for supercapacitor exhibits much higher stability than that of pristine PANI film. It is attributed to that most mechanical deformation is assigned to TiO2 array in the redox process, which avoided the structural pulverization in the electrode material and is in favor of obtaining a good stability. In addition, the typical electrochemical impedance spectroscopy (EIS) for the PANI/TiO2 and PANI/TiO2/FTO electrodes is given in Fig. 11 b. It can be seen that a single semicircle in the high frequency region and a sloped line in the low frequency region are obtained in the impedance curves. Moreover, the semicircle of PANI/TiO2/FTO electrode is smaller than that of the PANI/FTO electrode, indicating PANI/TiO2/FTO electrode has lower impedance on electrode/electrolyte interface. Previous study showed that the vertically aligned nanoarray is benefitful for ion diffusion from an electrolyte solution to the surface of materials of nanoarray [34]. As illustrated in Fig. 11c, the electrolyte ions can enter or leave the surface of PANI/ TiO2 array easily and fast compared with pristine PANI film. Moreover, the array architecture can shorten the charge transport distance during charge-discharge process in the PANI materials [68]. Therefore, the regular array structure can optimize the ionic diffusion which reduces the resistance of ionic diffuse and charge transfer to obtain lower impedance and high capacitance performance. 4. Conclusion In summary, an array structure of PANI/TiO2 shell/core composite has been successively prepared by hydrothermal synthesis of tetrabutyl titanate and the oxidative polymerization of ANI on the conducting plane of FTO. The as-prepared PANI/TiO2/FTO can act as an electrode material for supercapacitor and its SC value is about 633 F/g and 781 F/g within the potential window 0.8 to 0.4 V vs. SCE at the rate of 10 mV/s and at the current density of 1 A/g, respectively. The capacitance retention of PANI/TiO2/FTO electrode keeps 75% of its initial value, while the PANI/TiO2 electrode keeps only 65% of its initial value after 2000 cycles. The electrochemical behaviors of all as-prepared electrodes prove that the PANI/TiO2/ FTO electrode possesses a synergistic effect of PANI and TiO2 exhibiting higher SC value and better cycling stability compared with that of each individual component. Acknowledgements The authors are gratefully acknowledged the support of the National Natural Science Foundation of China (Grant No.
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