Sulfur-Doped TiO2 Nanotube

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Surfaces, Interfaces, and Applications

Interfacial Engineered Polyaniline/Sulfur-doped TiO2 Nanotube Arrays for Ultralong Cycle Lifetime Fiber-Shaped, Solid-State Supercapacitors Chun Li, Zhuanpei Wang, Shengwen Li, Jianli Cheng, Yanning Zhang, Jingwen Zhou, Dan Yang, Dong-Ge Tong, and Bin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01160 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Interfacial Engineered Polyaniline/Sulfur-doped TiO2 Nanotube Arrays for Ultralong Cycle Lifetime Fiber-Shaped, Solid-State Supercapacitors Chun Li1+, Zhuanpei Wang1+, Shengwen Li3,4, Jianli Cheng1*, Yanning Zhang4*, Jingwen Zhou1, Dan Yang1, Dong-Ge Tong2, Bin Wang1* 1

Institute of Chemical Materials, China Academy of Engineering Physics, Chengdu,

Sichuan 621900, China 2

College of Materials and Chemistry & Chemical Engineering, Chengdu University

of Technology, Chengdu 610059, China 3

Chengdu Green energy and green manufacturing technology R&D center, Chengdu,

Sichuan, 610200, China 4

School of Energy Science and Engineering, University of Electronic Science and

Technology of China, Chengdu, 611731, China +

These authors contributed equally to this work.

Corresponding Authors *E-mail:

[email protected];

[email protected];

[email protected];

Fax:(+86)28-6572 6198.

Keywords: interfacial engineering, sulfur-doped TiO2 nanotube, fiber-shaped supercapacitors, ultralong lifetime,

1

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Abstract Fiber-shaped supercapacitors (FSCs) have great promises in wearable electronics applications. However, the limited specific surface area and inadequate structural stability caused by the weak interfacial interactions of the electrodes result in relatively low specific capacitance and unsatisfactory cycle lifetime. Herein, solid-state FSCs with high energy density and ultralong cycle lifetime based on polyaniline (PANI)/sulfur-doped TiO2 nanotubes array (PANI/S-TiO2) are fabricated by interfacial engineering. The experimental results and ab initio calculations reveal that S doping can effectively promote the conductivity of titania nanotubes and increase the binding energy of PANI anchored on the electrode surface, leading to much stronger binding of PANI on the surface of the electrode and excellent electrode structure stability. As a result, the FSCs using the PANI/S-TiO2 electrodes deliver a high specific capacitance of 91.9 mF cm-2, a capacitance retention of 93.78% after 12,000 charge/discharge cycles, and an areal energy density of 3.2 µWh cm-2, respectively. Meanwhile, the all-solid-state FSC device retains its excellent flexibility and stable electrochemical capacitance even after bending 150 cycles. The enhanced performances of FSCs could be attributed to the large surface area, reduced ion diffusion path, improved electrical conductivity and engineered interfacial interaction of the rationally designed electrodes.

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1. Introduction With the rapid development of wearable electronics, flexible fiber-shaped energy storage devices have attracted interest due to their potential multifunctionality and wearability.1-3 Among them, supercapacitors have received more attention because of their high power density, excellent cycle lifetime, and short charge-discharge time4-6. Compared

with

traditional

planar

supercapacitors,

flexible

fiber-shaped

supercapacitors (FSCs) possess advantages, such as a small volume, high flexibility and wearability, and these advantages are convenient for integration with wearable electronics7, 8. Although FSCs are in high demand, designing suitable component materials and assembling them into a device are not easy. Recently, efforts have been devoted to fabricating reliable electrodes with satisfactory electrochemical properties and the ability to be knotted, bent, or woven into different shapes. Typically, nanostructured composite electrodes have been prepared by introducing high specific pseudocapacitance materials, such as metal nitrides9,

10

, metal oxides11-13 and

electroactive polymers,14, 15 into carbon materials with a high specific surface area by sol-gel

and

hydrothermal

methods.

Nevertheless,

the

previously

reported

nanocarbon-based fiber-shaped composite electrodes have problems of low specific capacitances and limited long-term cyclability. These problems can be attributed to the limited specific surface area of the electrode, the structural instability of the electrode nanostructure and weak interfacial interactions between the electrode components.16-19 Therefore, the energy densities and mechanical properties of FSCs still fail to satisfy the requirements for smart, wearable electronics, i.e., flexible and long-term, durable power supply. Highly ordered titania nanotube arrays (TNTAs) have a high specific surface area, excellent chemical stability and a wide potential window as supercapacitor electrodes.20,

21

However, when TNTAs are separately used as the electrode for

supercapacitors, the supercapacitors suffer from limited flexibility and a low specific capacitance due to the brittle phase and poor conductivity caused by wide bandgap of 3



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TiO2 (3.2 eV for anatase and 3.0 eV for rutile phase). Besides carbon coating,22, 23 heterogeneous element doping is another method to effectively manipulate the physical properties of TNTAs.24 Hydrogenated TiO2 Nanotube shows improved rate capability due to the enhanced carrier density by donor doping as well as by increasing the density of hydroxyl group on TiO2 surface.25 Nevertheless, the specific capacitances of H-TiO2 TNTAs are still significantly smaller than that of other metal oxides, thus the specific capacitances need to be further improved by high pseudocapacitance materials integration.26-28 Conductive polymers (e.g., polyaniline (PANI)) have a high conductivity and typical pseudocapacitance behaviors.29,

30

However, the weak interfacial bonding of PANI with the substrate due to interfacial incompatibility results in an undesired capacitance loss when using composite electrodes of PANI deposited on TNTAs.31, 32 Very recently, interfacial engineering is now proven as a novel method to improve the electrochemical performance and structural integrity of electrode materials27. Covalent-coupled H-TiO2 nanocrystals/ nitrogen-doped graphene composite materials showed excellent cycling stability as supercapacitor

electrodes

due

to

high

structural

stability

by

interfacial

hydrogen-bonding interaction.26 Meanwhile, the doped nitrogen is proved vitally important for immobilizing lithium polysulﬁdes in lithium-sulfur batteries to improve the cycling stability through the strong LiSx--N interfacial interaction.33, 34 If high pseudocapacitance polymers can be tightly and homogeneously anchored on the surface of highly conductive TNTAs by strong surface interaction, a high specific capacitance and robust long-term cycling stability can be obtained for fiber-shaped electrodes, while simultaneously endowing the device with flexibility. Herein, we reported a strategy of interfacial engineering to fabricate polyaniline (PANI)/sulfur-doped TNTA composites electrodes (denoted PANI/S-TiO2/Ti) for ultralong cycle lifetime fiber-shaped supercapacitors (FSCs). Sulfur-doped TNTAs (denoted S-TiO2) have a much higher conductivity than TNTAs due to the isovalent substitution doping, which narrows the TiO2 band gap, enhances the charge carriers in the conduction band and boosts electron movement within the nanotubes.35 Our 4


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designed PANI/S-TiO2/Ti electrodes combine the high specific surface area and improved conductivity of sulfur-doped TNTAs with the high capacitance of PANI, demonstrating excellent electrochemical performances and mechanical stability. Both the experimental analyses and ab initio calculation results demonstrate that interfacial engineering on TNTAs by S doping is feasible to fabricate ultra-stable long-term cycling electrodes for FSCs. S-doping can significantly improve the electronic conductivity of TNTAs and enhance the PANI binding capability with TiO2 surface which is beneficial to high energy density and cycling stability. Meanwhile, the nanotube array in TNTAs provides large surface area and short ion diffusion path which benefits for high rate capability. These characteristics of PANI/S-TiO2/Ti fibers result in a high specific capacitance of 91.9 mF cm-2, a high areal energy density of 3.2 µWh cm-2 (volumetric energy density of 2.1 mWh cm-3), and an ultra-stable, long cycle lifetime with a capacitance retention of 93.78% after 12,000 cycles.

2. Experimental Section Materials: Ti wires (diameter of 60 µm, 99.8% in purity) were obtained from Anping Shengzhuo Technology company, China. The alcohol (99%), acetone (99.5%)， ethylene glycerol (99.0%), ammonium fluoride (98%), concentrated sulfuric acid (99.8%) and aniline (99.9%) were directly used without any treatment (AR). Fabrication of the S-TiO2 nanotube arrays: TNTAs were prepared by electrochemical anodization (DH1722A-1, Beijing Dahua) in a two-electrode system with a Ti wire and Pt sheet as the anode and cathode, respectively. The whole process proceeded in a 0.3 wt% NH4F/ethylene glycol solution containing 8 vol% H2O at a voltage of 60 V for 0.5-3 h. Prior to the anodization, the Ti wires were cleaned by ultrasonication in acetone and alcohol, rinsed with distilled water, and then dried at 80 °C for 8 h. After the anodization process, the samples were rinsed with deionized water and dried at 80 °C for 8 h. The S-TiO2/Ti fiber was mixed with sublimed sulfur and annealed at 5



400 °C for 1 h and 500 °C for 2 h with a heating rate of 2 °C per minute under a N2 atmosphere. Then, the S-TiO2/Ti fiber was washed with deionized water and dried at 80 °C for 8 h. Fabrication of the PANI/S-TiO2 hybrid composites: The electropolymerization process was carried out in a three-electrode system (Ag/AgCl electrode, Pt sheet and S-TiO2/Ti fiber electrode used as the reference electrode, counter electrode and working electrode, respectively) in a H2SO4 solution of 0.25 M with 0.1 M aniline monomers by CV from -0.3 V to 0.9 V for several cycles. After the electrodeposition, the PANI/S-TiO2/Ti fibers were washed with deionized water and dried at 80 °C for 8 h. The mass loading of the PANI was calculated from mass difference before and after depositing PANI on sulfur-doped TiO2 nanotube arrays. Analytical Balance (METTLER TOLEDO XS105DU, Readability 0.1 mg / 0.01 mg) was used to calculate the mass difference before and after depositing PANI on sulfur-doped TiO2 nanotube arrays. The average mass loading of the PANI on sulfur-doped TiO2 nanotube arrays is 7.5 µg cm-1. Preparation of the PVA solid electrolyte and assembling the FSCs: The solid electrolyte was a PVA-H3PO4 gel prepared by the following steps. First, 10 g of PVA was added into 90 ml of deionized water in a glass bottle at 95 °C with vigorous stirring to obtain a clear gel. Then, 10 g of H3PO4 was added to the above gel and stirred for 60 min at room temperature. To assemble the FSCs, the PANI/S-TiO2/Ti fiber electrodes were placed in parallel, covered with PVA-H3PO4 and dried at room temperature. Characterization of the PANI/S-TiO2/Ti fiber electrode: The morphology of the samples were tested using scanning electron microscopy (SEM, ZeissUltra 55) and transmission electron microscopy (TEM, Zeiss Libra 200FE). X-ray photoelectron spectroscopy was carried out to investigate the surface electronic states of the samples by using monochromatic Al Ka radiation (XPS, Perkin-Elmer PHI 5000C ESCA). BRUKER Optik GmbH model TENSOR 27 FTIR spectrometer was used for Fourier 6


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transform infrared spectroscopy (FTIR). First-principles computational details: The density functional theory (DFT) studies were performed by using the Vienna Ab initio Simulation Package (VASP)36, 37 along with the projector augmented wave (PAW) method38. The generalized gradient approximation (GGA) functional with the Perdew-Burke-Emzerhof formulation was choosed to investigate the exchange-correlation interaction among the electrons39, and the nonlocal Van der Waals (vdW) correction (optB86b-vdW) was included for the dispersion interactions.40, 41 An on-site Hubbard U of 3.5 eV was added for Ti 3d to correct the Coulomb repulsion between the localized Ti 3d orbitals.42-45 An energy cutoff of 400 eV was used for the expansion of plane wave, and the Brillouin zone was sampled with a size-dependent Monkhorst-pack k-points method, i.e., 7×7×3 and 3×3×1 for an anatase phase TiO2 unit cell and a TiO2(101) surface with (1×3) supercells in the lateral plane (144 atoms), respectively. The crystal constant and positions of the ions were fully relaxed until the final force on each atom was smaller than 0.01 eV/Å. The formation energy for S doping on the TiO2 (101) surface was obtained by Ef(OS) = (ETiO2_S – ETiO2 + nOEO - nSµS)/nS

(S1)

where ETiO2_S and ETiO2 are the energies of the surfaces with and without S doping, respectively, nO and nS are the number of oxygen and sulfur atoms that are removed or added, respectively, EO is the energy of an oxygen atom in one oxygen molecule, and µS is the chemical potential of a sulfur atom using a single S atom (S-rich conditions), S8, H2S and SO2 (S-poor conditions) as the sulfur reservoirs. The binding of PANI molecules on different TiO2 (101) surfaces was evaluated using the adsorption energy, which was calculated by Ead = Esurf+PANI - Esurf - EPANI

(S2)

where Esurf+PANI, Esurf and EPANI are the energies of the PANI-adsorbed surface, clean surface, and PANI adsorbate, respectively. 7



Electrochemical measurement sand calculations of the specific capacitances of the FSCs: The CV curves and the GCD curves are used to calculate the specific capacitances C (F g-1, F cm-1, F cm-2, F cm-3) of the electrodes. The related calculations can been found from our previous papers.6,14,46 The energy density (E) and power density (P) of the solid-state supercapacitor based on two fiber electrodes can be obtained from E = 0.125 × C × U2 and P = E × t-1. CV and EIS measurements were obtained on an electrochemical workstation (VMP3, Bio-Logic, France). Galvanostatic charge/discharge (GCD) measurements of the supercapacitors were carried out on an Arbin Instruments testing system (Arbin BT-2000). The EIS measurements were conducted in the frequency range of 100 kHz to 100 mHz.

3. Results and discussion Schematic illustration of the preparation processes of the PANI/S-TiO2/Ti fiber electrodes is shown in Figure 1a. The vertically grown TiO2 nanotubes are aligned on the surface of the Ti fiber after the anodization. Highly ordered S-doped TNTAs are fabricated on a Ti fiber by a subsequent sulfidation. The S atomic ratio in the surface of the sulfur-doped TiO2 nanotube arrays is around 2.03 at% from XPS results. In addition, PANI is coated on the surface of S-TiO2/Ti fibers by an electropolymerization process. Scanning electron microscopy (SEM) images clearly show the grown TiO2 nanotubes on the surface of the Ti fiber, and the nanotubes are vertically aligned with the fiber in the S-TiO2/Ti and PANI/S-TiO2/Ti composites (Figure 1b). The S-TiO2/Ti fiber has a diameter of approximately 55∼65 µm with a TiO2 length of ∼3.5 µm and an interior diameter of approximately 100∼150 nm (Figure 1b-d). After coating the fiber with PANI, the SEM images of the PANI/S-TiO2/Ti fiber indicate that the surface of the S-TiO2/Ti fiber is uniformly coated by PANI, which has a diameter of approximately 60 µm (Figure 1e-g). The transmission electron microscopy (TEM, Figure 2a and b) images show that the S-TiO2/Ti fiber is composed of crystallized, anatase TiO2 with lattice fringe spacing of 8


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0.35 nm, corresponding to the (101) facets of the anatase TiO2.47, 48 The TEM images (Figure 2c and d) of the PANI/S-TiO2/Ti fiber show the lattice fringes of the anatase TiO2 and disordered films on the surface, which indicate that PANI is well deposited on the S-TiO2/Ti fiber. To further clarify the electrodeposition of PANI on the S-TiO2/Ti fiber, energy dispersive X-ray spectroscopy (EDS) was tested (Figure 2e). The elemental mapping distribution in the PANI/S-TiO2/Ti fibers confirms that the elements of C, N, O, S and Ti are homogeneously distributed in the fibers, indicating that S is well doped in the fiber. However, PANI hardly electrodeposits on the TiO2/Ti fiber due to its poor conductivity, as confirmed by SEM study (Figure S1). To investigate the MSy crystal of the S-TiO2/Ti fiber, X-ray diffraction (XRD) was conducted. The diffraction peaks in Figure 2f at approximately 25.3°, 48.9°, 53.8°, 55.1° and 62.5° can be attributed to the anatase TiO2 (PDF card 21-1272), and the diffraction peaks at 34.8°, 38.2°, 39.9°, 53°, 70.7° and 76.3° can be attributed to Ti (PDF card 44-1294). The XRD patterns of the undoped TNTA/Ti and S-TiO2 /Ti have almost the same peaks, indicating that S doping results in subtle changes in the crystal morphology of TiO2. Fourier transform infrared spectra (FTIR) of the PANI/S-TiO2/Ti fiber, PANI/TiO2/Ti fiber and PANI/Ti fiber are shown in Figure S2. The peaks at 1566 and 1489 cm-1 are attributed to the C=C stretching of the quinoid ring and the C=C stretching vibration mode of the benzenoid rings, respectively.49, 50 The bands at 1295 and 1227 cm-1 are assigned to the C-N stretching vibration of the secondary aromatic amine, while the N-Q-N stretching at 1106 cm-1 is the characteristic band of polyaniline salts.51, 52 The C-C and C-H in a benzenoid unit associated with the peak of 799 cm-1. The peaks between 800 and 500 cm-1 are related to TiO2 and are also observed in the PANI-TiO2/Ti composite.53 The FTIR spectra of the PANI/TiO2/Ti and PANI/S-TiO2/Ti fibers show that PANI was deposited on the S-TiO2/Ti fiber electrode but not on the TiO2/Ti fiber electrode, which are consistent with the SEM results (Figure S1) and indicate that S doping helps the PANI deposition.

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X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the PANI/S-TiO2/Ti and S-TiO2/Ti fibers (Figure 3a and S3a). The survey XPS spectra of the PANI/S-TiO2/Ti fiber show the corresponding peaks for Ti 2p, O 1s, S 2p, C 1s, and N 1s. As shown in Figure 3b and S3b, the Ti 2p 3/2 peak located at 458.7 and Ti 2p 1/2 peak at 464.4 eV suggest the Ti4+ state is predominant. Compared to the O 1s spectrum of the S-TiO2/Ti fiber (Figure S3c), the O 1s spectrum of the PANI/S-TiO2/Ti composite can be deconvoluted into three bands at 533.5 eV (O-S), 532.1 eV (O-Ti-S), and 530.1 eV (O-Ti) 31(Figure 3c) with a little bit shift to higher binding energy, which may be attributed to different surface states such as more defects and less electron density. Meanwhile, Figure 3d and S3d show the S 2p spectrum, which confirmed that S was doped into the TiO2 nanotubes and exists in two chemical states. The S2- species (161.7 and 164.0 eV) can be assigned to S-Ti and S-Ti-O, respectively, whereas the S4+ ions (168.8 eV) can be attributed to the linking of S-O, which shows that S4+ ions replace Ti4+ ions in the TiO2 nanotubes and S2- ions replace O2- on the disordered surface.35 At nonequilibrium states, such as high temperatures or sulfur-rich compositions, the substitution of S for O in TiO2 is feasible. Therefore, the formation of the S-doped TiO2 may be related to the high defect concentration and oxygen-deficit structure that formed in the annealing process with an Ar flow, which can facilitate more substitution of O2- with S2- by providing additional energy and sufficient space.54 The deconvolution of the C 1s XPS spectrum of the PANI/S-TiO2/Ti composite (Figure 3e) reveals four peaks at 284.6, 285.2, 286.5, and 288.8 eV, which can be assigned to the carbon atoms in C=C, C-C, C-O/C-N+, and HO-C=O55, respectively. In Figure 3f, the N 1s spectrum of the PANI/S-TiO2/Ti composite has four peaks, which correlate to the different electronic states of PANI. The peaks with binding energies of 398.9, 399.6, 400.5 and 401.5 eV are related to the quinoid amine groups (-N=), the amine (-NH–) in the benzenoid amine or amide groups, the positively charged nitrogen cationic radical (-NH+=) and other types of nitrogen cationic radicals (-NH2+-)55, 56, respectively. Therefore, the above results demonstrate that the TNTAs 10


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were well doped with sulfur, and the PANI polymer was successfully deposited on the S-TiO2/Ti fiber. To characterize the electrochemical performances of the PANI/S-TiO2/Ti fibers, FSCs were assembled by placing two PANI/S-TiO2/Ti fiber electrodes in parallel. For comparison, the PANI/TiO2/Ti fiber, S-TiO2/Ti fiber and PANI/Ti fiber were also studied. The synthesis parameters, including the anodizing time, deposition cycles and deposition rate, were investigated to determine their effect on the capacitive performances of the devices (Figure S4). The PANI/S-TiO2/Ti fiber electrodes that were prepared with a 2 h anodization time and a PANI deposition rate of 5.6 mV s-1 for 11 cycles showed the best comprehensive performance, including a higher specific capacitance and much stable cycling behaviors. Cyclic voltammetry (CV) curves of the PANI/S-TiO2/Ti electrodes were tested in a voltage window from -0.2 to +0.8 V at a scan rate of 500 mV s-1. Compared with the PANI/TiO2/Ti and S-TiO2/Ti electrodes, the PANI/S-TiO2/Ti and PANI/Ti electrodes exhibited enhanced capacitive behaviors with semi-rectangular CV shapes and higher current responses as shown in Figure 4a and Figure S5, which indicated that PANI contributes to the specific capacitance of the composite electrode. However, the PANI/TiO2/Ti and TiO2/Ti fiber materials had poor electrochemical performances, which may be caused by the weak bonding of PANI on the electrode surface due to the non-wettability of TiO2. Furthermore, the peak current in the CV curves of the PANI/S-TiO2/Ti electrodes continued to increase with the scanning rate increased from 10 to 1000 mV s-1, as shown in Figure 4b. The redox peaks can still be clearly seen at a scanning rate of 1000 mV·s-1, which indicated fast diffusion kinetics. Compared with the performances of the PANI/TiO2/Ti and TiO2/Ti electrodes, the PANI/S-TiO2/Ti hybrid composite has higher specific areal capacitance of 139.6 mF cm-2 at 10 mV s-1, demonstrating that the electrochemical performance is enhanced by interfacial interaction engineering.

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The electrochemical impedance spectroscopy (EIS) was used to further analyze the fiber electrode materials. The Nyquist plot with a blue line in Figure 4c consists of a semicircle in the high to middle frequency region and a vertical line in the low frequency region, which can be attributed to charge transfer behaviors and electron diffusion processes, respectively (Figure 4c inset). The impedance spectra were ﬁtted by the equivalent circuit as shown in Figure S6. The solution resistance of the PANI/S-TiO2/Ti, PANI/Ti, PANI/TiO2/Ti, and TiO2/Ti are 38.8, 25.1, 226, and 386.8 Ω, respectively. The charge-transfer resistance of the PANI/S-TiO2/Ti, PANI/Ti, PANI/TiO2/Ti and TiO2/Ti are 65.7, 2.79, 6984 and 33660 Ω, respectively. The EIS measurements show that the PANI/S-TiO2/Ti electrode has a smaller electrical resistance and solution resistance than PANI/TiO2/Ti electrode and TiO2/Ti electrode, indicating that PANI and sulfur doping could enhance the conductivity of composite electrodes. The galvanostatic charge/discharge (GCD) curves with different cutoff potential windows, from 0.2 to 1 V, show similar electrochemical behaviors with good charge-discharge curve symmetry (Figure 4d), indicating excellent reversibility and wide potential windows. The rate capability of the PANI/S-TiO2/Ti fiber electrode was also measured by GCD (Figure 4e). The charge-discharge curves in the GCD profiles display excellent symmetry at different current densities ranging from 0.01 to 2 mA cm-2, indicating the good reversibility of the PANI/S-TiO2/Ti fiber electrode during the charge/discharge process. Meanwhile, a negligible voltage drop (IR drop) could be observed in the curves, which indicated a low internal resistance. Figure 4f and S4f show the long-term cycling performances of the PANI/S-TiO2/Ti and PANI/Ti fiber electrodes. The PANI/S-TiO2/Ti exhibited excellent long-term cycling performances with a capacitance retention of 93.78% after 12,000 cycles. However, the PANI/Ti fiber showed a quickly capacitance decay. The inset of Figure 4f shows that the GCD curves of the 12,000th cycle and the second cycle are nearly identical, indicating excellent cycling stability during the long-term test. The cycling performance of the PANI/S-TiO2/Ti electrode is much better than that of the PANI/Ti 12


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and PANI/TiO2/Ti electrodes, showing the synergistic effect of PANI and S-doped TiO2 in the composite fiber electrode. CV curves of the PANI/S-TiO2/Ti FSCs in Figure S7 were obtained at different current densities after long term cycling. The CV curves in Figure S7 show semi-rectangular shapes with the clear redox reaction peaks even at a scan rate of 1000 mV s-1, which also confirms the stable cycling performances of the FSCs. Meanwhile, the FTIR spectra of PANI/S-TiO2/Ti hybrid fibers before and after 5000 cycles did not show obviously change (Figure S8). Moreover, it can be known from the SEM images after 5000 cycles (Figure S9) that the PANI still coated on the surface of the ordered titania nanotube arrays, demonstrating the outstanding stability. At the same time, the specific areal and volumetric capacitances were calculated from the GCD curves, as shown in Figure 4g and Figure S10. As the current density changed from 0.01 to 2 mA cm-2, the specific areal capacitance of the PANI/S-TiO2/Ti fiber electrode changed from 91.9 (61.3 F cm-3) to 29.5 mF cm-2 (19.7 F cm-3). In contrast, the specific capacitance of the PANI/Ti fiber electrode ranged from 121.3 (80.8 F cm-3) to 8.9 mF cm-2 (5.9 F cm-3) as the current density increased from 0.01 to 2 mA cm-2. The specific areal and volumetric capacitances of the PANI/S-TiO2/Ti fiber electrode are much larger than those of the S-TiO2/Ti fiber electrode and other previously reported results, such as yarn supercapacitors created by dry-spinning aligned polyaniline (PANI)/CNT fibers (38 mF cm-2)57 and solid-state symmetrical supercapacitors based on TiO2@C core-shell NWs (0.125 F cm-3).58 For FSC applications, the energy density and power density are two critical parameters. The areal energy density of the PANI/S-TiO2/Ti FSC was 3.2 µWh cm-2 in Figure 4h, which is higher than that of the wet-spinning PANI/CNT FSCs (~0.56 µWh cm-2)59 and a stainless-steel wire supercapacitor with electrolytically deposited PANI (~0.95 µWh cm-2)60. The maximum volumetric energy density of the PANI/S-TiO2/Ti FSC was 2.1 mWh cm-3, which is much higher than that of symmetrical supercapacitors with an aqueous electrolyte system based on TiO2@C core–shell NWs (0.011-0.005 mWh cm-3)58, asymmetric supercapacitors based on 13



H-TiO2@MnO2// H-TiO2@C (0.3 mWh cm-3) 61, and paper-based graphite/polyaniline network hybrid symmetric supercapacitors (0.32 mWh cm-3)62 and is comparable to that of a hybrid CC/GP/PANI supercapacitor (3.4 mWh cm-3). 63 The maximum areal power density and volumetric power density were 0.5 mW cm-2 and 0.33 W cm-3, respectively. The good electrochemical performance results are from the synergistic effect of PANI and the S-TiO2 nanotubes. To better understand the effect of S doping, systematic ab initio calculations were carried out to investigate the structural and electronic properties of the TiO2(101) surfaces before and after the S doping. The optimized structures in Figure 5a and b show that the substitution of monolayer-S (S-TiO2 (101)) for the top O layer on TiO2 (101) does not result in surface distortions, but it does increase the Ti-S bond length by >20%. The formation energy of the S-doping monolayer obtained from Equation (S1) is -0.862 eV under S-rich conditions, which is only 15 meV higher than that of single-S doping. This shows that doping S on the TiO2 (101) surface is easy, proven by the experimental results above. The total density of states (TDOS) of these two surfaces, i.e., the black lines with shadows in Figure 5c and d, show that a broad peak exists just below the Fermi level for S-TiO2 (101) as a result of the strong hybridization between the Ti-d and S-p orbitals. Thus, the band gap of S-TiO2 (101) narrows from 1.92 eV for clean TiO2 (101) to 0.87 eV. This benefits the excitation of the charge carriers to the conduction band and improves the conductivity of TiO2. The adsorption structures of PANI on different TiO2 (101) are showed in Figure 5 and include the nonlocal van de Waals correction in the calculations. The dehydrogenated N atom in PANI directly connects to an O or S atom on the top surface and slightly shifts toward the position of the nearest Ti atom. PANI on a non-doped TiO2 (101) surface tilts with a N-O-Ti6f angle of 69.1º, a N-O bond length (bNO) of 1.420 Å and a N-C bond length (bNC) of 1.398 Å. In contrast, the distance between PANI and the S-doped TiO2 (101) surface is slightly larger with a N-S-Ti6f angle of 105.8º, a N-S bond length (bNS) of 1.634 Å and a slightly decreased bNC of 1.383 Å. The calculated adsorption energy (Ead) obtained from Equation (S2) is -1.266 eV for PANI on a clean 14


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TiO2 (101) surface and -1.759 eV for PANI on a S-TiO2(101) surface. Clearly, PANI has a much stronger binding affinity for the latter surface. The total and partial density of states for PANI/TiO2 (101) and PANI/S-TiO2 (101) in Figure 5c and d, respectively, clearly illustrate that the adsorption of PANI on clean TiO2 (101) does not change the DOS profiles, but it does induce a sharp peak between the surface valence and conduction bands because of the hybridization of the N-p orbitals in PANI and the Ti6f-d and O2f-p states on the substrate. For PANI/S-TiO2(101), the adsorption peak at -0.75 eV below the Fermi level only consists of the N-p and S-p states. Strikingly, the interactions between the N-p orbitals and Ti6f-d states cause a small peak just below the Fermi level, which endows the PANI/S-TiO2(101) system with semimetallic properties. These properties can improve the conductivity and further boost the electrochemical performance, which is consistent with the experimental results. The PANI/S-TiO2/Ti FSCs can bend at different angles, i.e., 30º, 60º, 90º, 120º, 150º and 180º, and be tied into a knot in Figure 6a, which are desirable characteristics for use in portable devices. The electrochemical performances of the freestanding FSCs under a bending angle of 180º were tested to determine the potential applications for wearable devices, as shown in Figure 6b and c. Only slight changes were observed in the charge-discharge curves after bending the FSCs for different times, which indicated that the structures of the FSCs were well maintained (Figure 6b). After bending 150 times, the capacitance retention of the PANI/S-TiO2/Ti FSC was 73.5% of the initial capacitance after 1,000 cycles at a current density of 0.2 mA cm-2, as shown in Figure 6c, illustrating that the flexible PANI/S-TiO2/Ti FSCs are tough even after thousands bending. To meet the different operational voltage and/or power requirements for practical applications, we further investigated the electrochemical performances of the integrated PANI/S-TiO2/Ti FSCs connected in series or parallel. The GCD curves of one, two, three and four series-connected FSCs are shown in Figure 6d. The one, two, three and four FSCs connected in series can provide a maximum potential of 1, 2, 3 15



and 4 V, respectively, with almost the same charge/discharge time, demonstrating the scalability of the PANI/S-TiO2/Ti FSCs. In addition, the duration time for the four parallel-connected FSCs is approximately four times that of one device at the same current density (Figure 6e). These results indicate that the integrated PANI/S-TiO2/Ti FSC groups are stable and can provide high voltage, energy and power. To further show the potential of the PANI/S-TiO2/Ti FSCs as power devices in wearable electronics, we weaved four series-connected FSCs into the shape of an uppercase letter ‘M’ on a cotton fabric surface to demonstrate their potential application, as shown in Figure 6f. The four FSCs can run at a potential of 4 V and easily light up green light-emitting diodes, as shown in Figure 6g, showing their potential for wearable power supply applications.

4. Conclusions In summary, interfacial engineering was used to fabricate PANI/S-TiO2/Ti fiber electrodes with good flexibility and excellent performance for ultralong cycle lifetime FSCs. As revealed by the experimental results and ab initio calculations, sulfur doping can effectively improve the conductivity and PANI binding affinity of the TiO2 electrode surface. When used as FSC electrodes, the PANI/S-TiO2/Ti fiber electrodes show excellent electrochemical performances, including an ultra-stable, long cycle lifetime, a high energy density and outstanding flexibility. The areal energy density of the PANI/S-TiO2/Ti fiber-based FSC reaches 3.2 µWh cm-2. The good performances can be attributed to the high conductivity and favorable ion diffusion kinetics by S-doped TiO2 and the enhanced electrochemical performances as the results of the PANI growth. Meanwhile, the FSCs can be easily knitted into a textile to light up light-emitting diodes and show excellent electrochemical stability after long-term bending tests. Interfacial engineering shows an alternate way of designing

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nanostructured hybrid fiber electrode with long cycle lifetime and high energy density for wearable electronics. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21501160 and 51741305), the Science Foundation for Distinguished Young Scholars of Sichuan Province (2017JQ0036 and 2016JQ0025), the “1000plan” from the Chinese Government, the “QianYingBaiTuan” Plan of China Mianyang Science City and the Science Foundation of Institute of Chemical Materials (No.011100301). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FTIR spectra of the PANI/Ti fiber, PANI/TiO2/Ti and PANI/S-TiO2/Ti hybrid fiber; XPS spectra of the S-TiO2/Ti fiber; specific capacitance and cyclic performance plots of different FSCs at different conditions; The magnified CV curves of the electrode with/without PANI; CV curves of FSCs at different scan rates after long-term cycling test; FTIR spectra of the PANI/S-TiO2/Ti hybrid fiber before and after 5000 cycles; the magnified areal specific capacitance and volumetric specific capacitance plots of different FSCs. ■AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. 17



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Figure 1 (a) Schematic illustration of the synthetic process of the PANI/S-TiO2/Ti fiber. (b-d) SEM images of the top and cross-section of the S-TiO2/Ti fiber; (e-g) SEM images of the top and cross-section of the PANI/S-TiO2/Ti fiber.

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Figure 2 (a) Low-magnification and (b) high-magnification TEM images of the S-TiO2/Ti fiber; (c) Low-magnification and (d) High-magnification TEM images of the PANI/S-TiO2/Ti hybrid fiber; (e) Elemental mapping of C, N, O, S and Ti in the PANI/S-TiO2/Ti fibers; (f) XRD patterns of the S-TiO2/Ti and TiO2/Ti fibers.

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Figure 3 XPS spectra of the (a) survey spectra and deconvolution spectra of (b) Ti 2p, (c) O 1s, (d) S 2p, (e) C 1s and (f) N 1s for the PANI/S-TiO2/Ti hybrid fiber.

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Figure 4 (a) CV curves of the PANI/S-TiO2/Ti, PANI/TiO2/Ti, PANI/Ti and S-TiO2/Ti electrodes at 500 mV s-1. (b) CV curves at different scan rates in a 1 V potential 27



window. (c) Nyquist plots of the fiber-shaped PANI/S-TiO2/Ti FSC, PANI/Ti FSC, PANI/TiO2/Ti FSCs, S-TiO2/Ti FSC and TiO2/Ti FSC; the inset shows the magnified Nyquist plots of the PANI/S-TiO2/Ti FSC and PANI/Ti FSC. (d) GCD curves of the PANI/S-TiO2/Ti obtained from different voltages ranging from 0.2 to 1 V at a current density of 0.2 mA cm-2. (e) GCD curves of the PANI/S-TiO2/Ti at different current densities from 0.02 to 2 mA cm-2 in a potential window of 1 V. (f) Cyclic performance of the FSCs; the inset is the GCD curves for the second cycle and 12000th cycle of the FSCs. (g) Areal specific capacitance and volumetric specific capacitance plots of the fiber-shaped PANI/S-TiO2/Ti FSC, PANI/Ti FSC and S-TiO2/Ti FSC. (h) Ragone plots based on the two-fiber supercapacitor.

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Figure 5 (a) and (b) Side- and top-views of PANI on the TiO2(101) and S-doped TiO2(101) surfaces, respectively, through first-principles studies. The red, gray, blue, dark gray, cyan and yellow balls represent O, Ti, N, C, H and S atoms, respectively; (c) and (d) the TDOS of the TiO2(101) and S-TiO2(101) surfaces before (black) and after (red) PANI adsorption. The insets show the partial density of states (PDOS) of the adsorption peak, and the zero value corresponds to the Fermi level.

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Figure 6 (a) Digital photographs of the FSCs at different bending angles and a tied FSC; (b) GCD curves at a current density of 0.2 mA cm-2 in a potential window of 1 V after different bending cycles. (c) Cycling performance of the FSCs after bending 150 times at a current density of 0.2 mA cm-2. The inset is a digital photograph of the FSCs at a bending angle of 180 degrees. (d) and (e) GCD curves of the four FSC group devices connected in series and parallel and measured at a current density of 0.2 mA cm-2. (f) Digital photographs of an uppercase letter ‘M’ woven on a cotton fabric surface. (g) Digital photographs of the all-solid-state FSCs woven on cotton fabric lighting up a green LED.

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Sulfur-Doped TiO2 Nanotube

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