Enhanced Capacitance of TiO2 Nanotubes with a Double-Layer

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Enhanced capacitance of TiO2 nanotubes with doublelayer structure fabricated in NH4F/H3PO4 mixed electrolyte Lizhen Wu, Ke Zhang, Xufei Zhu, Shikai Cao, Dongmei Niu, and Xiaojie Feng Langmuir, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Enhanced capacitance of TiO2 nanotubes with double-layer structure fabricated in NH4F/H3PO4 mixed electrolyte Lizhen Wua, Ke Zhangb, Xufei Zhua,*, Shikai Caoa, Dongmei Niua, Xiaojie Fenga aKey

Laboratory of Soft Chemistry and Functional Materials of Education Ministry,

Nanjing University of Science and Technology, Nanjing 210094, China bNanjing

Research Institute of Electronics Technology, Nanjing 210039, China

*Corresponding author. E-mail address: [email protected] (X. Zhu)

Abstract TiO2 is an attractive electrode material in fast charging/discharging supercapacitors because of its high specific surface area. However, the low capacitance of TiO2 nanotubes as-anodized in classical electrolyte restricts their further application in supercapacitors. Here, we study the performances of larger diameter nanotubes with double-layer structure fabricated in NH4F/phosphoric acid (H3PO4) mixed electrolyte. Results show that the double-layer structure increased the specific surface area of nanotubes owing to the cavities between the double layers and porous structure on walls. After soaking in H3PO4 aqueous solution for 40 min, the nanotubes anodized in mixed electrolyte containing 6 wt% H3PO4 show the specific capacitance of 13.89 mF cm-2, ~3.11 times that of the pristine nanotubes in classical electrolyte. The specific surface area of the soaked nanotubes is up to 113.2 m2 g-1, which is ~2.94 times that of the pristine nanotubes. The values of specific surface area of the anodized nanotubes and the soaked nanotubes fabricated in mixed electrolyte containing 6 wt% H3PO4 are 1

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roughly equal. It demonstrated that the specific surface area increased mainly due to the double-layer structure. Double-layer structure reveals a new strategy to enhance the specific capacitance of TiO2 nanotubes. Keywords: TiO2 nanotubes; mixed electrolyte; double-layer; soaking; supercapacitors

Introduction Anodic TiO2 nanotubes arrays (ATNAs) have attracted the interest of researchers for their wide applications such as photocatalysis [1], water splitting [2] and supercapacitors [3-8] owing to their high specific surface area. It is well known that the energy storage mechanism of TiO2 nanotubes is mainly that of electrical double-layer capacitor [9,10]. However, the capacitance of TiO2 nanotubes is limited by their low conductivity and poor electrochemical activities [10]. So many studies have reported effective ways to improve the performances of TiO2 nanotubes including electrochemical hydrogenation doping [11], nitridation [12], high temperature hydrothermal treatment [13-15], post-treatment methods [16,17] and the preparation of TiO2 composite electrode [18-21]. For instance, Cao et al. [15] prepared the hydrogenated TiO2 by annealing as-anodized TiO2 nanotubes in hydrogen atmosphere at high temperatures and for a composite electrode hydrogenated at 600 °C, the specific capacitance reaches 630.1 F g-1 at a scan rate of 10 mV s-1. Zhou et al, [17] prepared self-doped TiO2 nanotubes by a simple cathodic polarization treatment on the pristine TiO2 nanotubes to achieve improved conductivity and capacitive properties of TiO2 nanotubes. However, Chen et al. [22] fabricated anodic TiO2 nanotubes with novel 2


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double-layer structure in NH4F/phosphoric acid (H3PO4) mixed electrolyte. The double-layer structure may increase the capacitance of the anodized TiO2 nanotubes. But their work has not yet carried out a study about the performances of TiO2 nanotubes with double-layer structure. Herein, we fabricated and characterized TiO2 nanotubes with novel double-layer structure directly by one-step anodization in NH4F/H3PO4 mixed electrolyte [22]. Our results show that the double-layer structure can significantly increase the specific surface area of nanotubes and enhance the capacitance of TiO2 nanotubes. Both of the outer wall of the inner layer and the inner wall of the outer layer of the double-layer structure show the porous structure, which greatly increases the specific surface area of nanotubes. After soaking in H3PO4 aqueous solution for 40 min, ATNAs obtained in mixed electrolyte containing 6 wt% H3PO4 (called H3PO4-6wt% mixed electrolyte, below) show the specific capacitance of 13.89 mF cm-2 at the current density of 0.5 mA cm-2. The specific capacitance shows ~3.11 times that of the pristine ATNAs obtained in pure NH4F electrolyte without H3PO4 (called classical electrolyte, below). The specific surface area of the soaked nanotubes is up to 113.2 m2 g-1, which is ~2.94 times the specific surface area of the pristine nanotubes. BET results reveals that the specific surface area is mainly attributed to the double-layer structure. Experimental ATNAs were prepared by galvanostatic anodization of Ti foils (100 μm thickness, purity 99.5%) in ethylene glycol mixed electrolyte containing 0.5 wt% NH4F, 2 vol% H2O and various concentration of H3PO4 (0~12 wt%) at the current density of 2.5 mA 3


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cm-2 for 1 h at 30 °C with a Pt mesh as a cathode. Subsequently, Ti foils topped with TiO2 nanotubes were anodized at 60 V for 5 min in an ethylene glycol electrolyte containing 5 wt% H3PO4 to reinforce the adhesion of Ti substrate and nanotubes [6]. Prior to anodization, Ti foils were polished with a mixed solution of HNO3, HF and H2O (1:1:2 by volume) for about 10 s. The samples were soaked in 6 wt% H3PO4 aqueous solution for 0, 20, 40 and 60 min, respectively, and rinsed thoroughly by deionized water for 5 min and dried in air. Afterwards, all samples were thermal annealed at 450 °C for 3 h in air. The electrochemical performances of all samples were investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) methods. All the electrochemical experiments were carried out in a three-electrode electrochemical configuration (electrochemical workstation, CHI440C, CH Instrument Co., China) with platinum sheet and saturated calomel electrode (in 1 M KCl) as counter electrode and reference electrode. The samples with a fixed geometric area of 2.0 cm2 (1.0 cm × 1.0 cm, double face) were used as the working electrode. CV tests were performed using different scan rates of 20, 50, 100 and 200 mV s-1 and GCD tests were carried out at different current densities from 0.2 to 2 mA cm-2 over the range from -0.3 to 0.6 V in 0.5 M H2SO4 aqueous solution as the electrolyte. EIS were performed over a frequency range of 100 kHz to 0.1 Hz with AC signal amplitude of 10 mV at 0 V bias potential. The cycling stability of nanotubes was evaluated by GCD test of 5000 cycles. Prior to the electrochemical experiments, electrochemical hydrogenation doping of the ATNAs was carried out in 4


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0.5 M Na2SO4 aqueous solution at -1.5 V for 60 s. The morphology of all samples was characterized using field-emission scanning electron microscope (FESEM, FEI Quanta 350FEG). The N2 adsorption/desorption measurements were carried out at ~196 °C after drying in vacuum at 90 °C for 8 h on a ASAP 2020, and the average Brunauer-Emmett-Teller (BET) surface areas of ATNAs were calculated using BET equation. All experiments were repeated three times. Results and discussion Figure 1a-f show the FESEM images of ATNAs fabricated in various electrolytes after annealing. It is obvious that the diameter of ATNAs obtained in NH4F/H3PO4 mixed electrolyte greatly increased compared to the diameter (~102.7 nm) of ATNAs in classical electrolyte [22]. And the double-layer structure of nanotubes obtained in NH4F/H3PO4 mixed electrolyte appeared, which is originated from oxygen bubble mold [23-27]. The double-layer structure would increase specific surface area of nanotubes owing to the cavities between double layers and the porous structure of the double walls, as shown in Figure 1b-f. ATNAs with double-layer structure would show increased areal capacitance due to the enlarged specific surface area. This can be confirmed by CV voltammograms of ATNAs obtained in various electrolytes at a scan rate of 100 mV s-1. Clearly, as seen in Figure 1g, the cyclic voltammograms of ATNAs obtained in NH4F/H3PO4 mixed electrolytes exhibit larger intergrated areas. Data from capacitance values obtained from these curves are presented in Figure 1g, inset. Among all samples, ATNAs obtained in H3PO4-6wt% mixed electrolyte have the largest intergrated area and highest specific capacitance. This should be related to the presence of the double5


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layer structure, which significantly increased the contact of electrolyte with the surface of electrode. GCD curves of all samples recorded at 0.5 mA cm-2 are shown in Figure 1h. As determined by CV, ATNAs obtained in H3PO4-6wt% mixed electrolyte have the longest duration among all samples. The highly symmetric profiles of these GCD curves revealed the excellent reversibility and outstanding capacitive characteristics of nanotubes anodized in NH4F/H3PO4 mixed electrolyte. The calculated areal capacitances by Equation (1) [28] from the discharge curves at the current density of 0.5 mA cm-2 are 4.47, 6.88, 9.54, 7.74, 8.48 and 5.70 mF cm-2 for ATNAs obtained in mixed electrolytes containing 0, 4, 6, 8, 10 and 12 wt% H3PO4, respectively (Figure 1h, inset). Consequently, ATNAs obtained in H3PO4-6wt% mixed electrolyte have a maximum specific capacitance of 9.54 mF cm-2, which is ~2.13 times that of the pristine ATNAs obtained in classical electrolyte. idis∆t

CS = (V0 - VIR - drop)S

(1)

where CS (mF cm-2) is areal specific capacitance, idis (A) is constant discharge current, Δt (s) is discharge time, V0 (V) is the specified potential change, VIR-drop (V) is the IR drop for more accurate result and S (cm2) is the active area of samples.

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Figure 1. FESEM images of ATNAs obtained in various electrolytes containing different concentrations of H3PO4 after annealing: (a-f) 0, 4, 6, 8, 10 and 12 wt%, respectively. (g) Cyclic voltammograms at a scan rate of 100 mV s-1 and (h) GCD curves at a current density of 0.5 mA cm-2 of ATNAs obtained in various electrolytes. To confirm the double-layer structure and porous cavities in the large area of the 7


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anodized surface, the more convincing FESEM images are shown in Figure 2a and 2b. Figure 2a shows the cross-section image of TiO2 nanotubes fabricated in H3PO4-6wt% mixed electrolyte with the outer layer detached and the inner layer exposed. It is clearly observed that the outer wall of the inner layer of double-layer structure shows porous structure and the existence of double-layer structure is also confirmed. Figure 2b shows the cross-section image of TiO2 nanotubes with the outer layer of double-layer structure peeled off in some portions of nanotubes in a large area where the double-layer structure can be observed. Figure 2c shows a schematic of the formation mechanism of doublelayer structure and porous cavities of nanotubes. At the beginning of the anodization (stage I in Figure 2c), a barrier oxide layer of TiO2 is formed on the surface of Ti substrate. Since the top of the barrier oxide layer is exposed to the electrolyte, the upper portion is contaminated by the anions of electrolyte and the anion contaminated layer is formed (stage II in Figure 2c). With the progress of the anodization, the thickness of barrier oxide layer reaches a certain value. At this time, electronic current is generated and oxygen appears on the interface of barrier oxide layer and anions contaminated layer (stage II in Figure 2c) [23-27]. In the classical electrolyte, the anions concentration is low and the thickness is also smaller. All the produced oxygen bubbles break through the anion contaminated layer and TiO2 nanotubes with single-layer structure are formed (stage III in Figure 2c). In the electrolyte containing H3PO4, the thickness of anions contaminated layer increases due to the addition of H3PO4 compared to classical electrolyte. Some oxygen bubbles cannot break through the anion contaminated layer and porous cavities are produced on the interface of the barrier oxide layer and anions 8


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contaminated layer for the plastic flow of barrier oxide (stage IV in Figure 2c). The wall thickness of TiO2 nanotubes fabricated in electrolyte containing H3PO4 is thicker than that of TiO2 nanotubes fabricated in classical electrolyte due to the thicken anion contaminated layer. It is consistent with the experimental results (Figure 1).

Figure 2. (a, b) FESEM images of double-layer structure and porous cavities in the large area of TiO2 nanotubes fabricated in H3PO4-6wt% mixed electrolyte and (c) a schematic of formation mechanism of the double-layer structure and porous cavities of nanotubes. To further increase the specific capacitance of nanotubes, ATNAs anodized in H3PO4-6wt% mixed electrolyte were soaked in 6 wt% H3PO4 aqueous solution for different times. Figure 3a-d show the morphology of the ATNAs soaked for 0, 20, 40 and 60 min, respectively. It can be seen that the surface of all samples still exhibits nanopores rather than nanograss [29,30] owing to the larger nanotubes diameter (Figure 1c). The specific capacitance of nanotubes is attributed to the appearance of nanograss 9


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in Ref 29 and Ref 30. However, in this work, nanograss didn’t appear on the top of nanotubes after soaking in H3PO4 aqueous solution and the length of the soaked ANTAs for different times didn’t change too much (Figure 3a-d, inset) owing to the large nanotube diameter. It indicated that H3PO4 as a medium-strong acid is not very corrosive and nanograss wasn’t formed on the top of larger diameter nanotubes. Therefore, the increase of specific capacitance is caused by the enlarged specific surface area of nanotubes owing to the double-layer structure or attributed to the partial hydrogenation doping resulted from the soaking in aqueous solution [29-31]. Figure 3e and 3f show cyclic voltammograms at a scan rate of 100 mV s-1 and GCD curves at a current density of 0.5 mA cm-2 of the soaked ATNAs anodized in H3PO4-6wt% mixed electrolyte comparing with the pristine ATNAs obtained in classical electrolyte. As shown in Figure 3e, the soaked ATNAs exhibit higher current responses and larger intergrated areas comparing with the pristine ATNAs. This is consistent with the GCD results of longer duration of the soaked ATNAs. The calculated areal capacitances from the discharge curves by Equation (1) at 0.5 mA cm-2 are 9.54, 10.54, 13.89, and 11.48 mF cm-2 for the soaked ATNAs for 0, 20, 40 and 60 min, respectively. The specific capacitance of the soaked ATNAs for 40 min with largest duration is ~3.11 times that of the pristine ANTAs (4.47 mF cm-2).

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Figure 3. FESEM images of the soaked ATNAs for different times: (a-d) 0, 20, 40 and 60 min, respectively. (e) Cyclic voltammograms at a scan rate of 100 mV s-1 and (f) GCD curves at a current density of 0.5 mA cm-2 of the soaked ATNAs comparing with the pristine ATNAs. Figure 4a displays cyclic voltammograms of the soaked ATNAs for 40 min at various scan rates from 20 to 200 mV s-1. The CV curves with a nearly unchanged shape at various scan rates depicts good capacitive behavior and high rate capability. Similarly, GCD curves (Figure 4b) also show the good rate capability of the soaked ATNAs for 11


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40 min when the current density increases from 0.2 to 2 mA cm-2 and the good coulombic efficiency of the soaked ATNAs for 40 min at all operated current densities. Figure 4c shows the Nyquist plots of the anodized ATNAs and the soaked ATNAs for 40 min anodized in H3PO4-6wt% mixed electrolyte and the pristine ATNAs. For quantitative analysis, the experimental data have been fitted to the model depicted by the equivalent circuit shown in the inset of Figure 4c [11]. In the model here, Re is the resistance of the electrolyte (0.5 M H2SO4 aqueous solution) and R2 and C are the resistance and Helmholtz double-layer capacitance of the electrode. The interface charge transfer resistance (R1) and the constant phase element (CPE) are also included in the model. The value of Re is the same for all samples for the same electrolyte during measurement. The electrode resistances R2 for the pristine ATNAs, the anodized ATNAs and the soaked ATNAs in 6 wt% H3PO4 aqueous solution for 40 min anodized in H3PO4-6wt% mixed electrolyte are 6782.4, 1573.0 and 435.7 Ω cm-2. The results are consistent with the enhanced capacitance of the three samples. The EIS studies also demonstrate that the capacitance of ATNAs can be strongly enhanced by anodizing in electrolyte containing H3PO4 and soaking in H3PO4 aqueous solution. Figure 4d shows the GCD curves of the soaked ATNAs for 40 min. As shown in the inset of Figure 4d, after 5000 charge-discharge cycles the specific capacitance of nanotubes still retain 77.3% of the initial capacitance. The cross-section image of the soaked ATNAs for 40 min (Figure 4e) shows that the double-layer structure run through the entire nanotubes from up to bottom. And to investigate deeply the soaking in 6 wt% H3PO4 aqueous solution and specific surface area of nanotubes, BET gas sorptometry 12


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measurement was conducted. The N2 adsorption/desorption isotherms of the anodized ATNAs and the soaked ATNAs for 40 min anodized in H3PO4-6wt% mixed electrolyte and the pristine ATNAs are plotted in Figure 4f. The specific surface area of the soaked ATNAs for 40 min calculated by BET equation is up to 113.2 m2 g-1, a value representing an increase by a factor of 2.94 over the value for the pristine ATNAs. And the specific surface area of the anodized ATNAs in H3PO4-6wt% mixed electrolyte is 105.8 m2 g-1 and it is roughly equal to that of the soaked ATNAs for 40 min. It shows that the soaking in 6 wt% H3PO4 aqueous solution does not promote the increase of the specific surface area of nanotubes. It also explains from another aspect that the increase of capacitance resulted from soaking may be due to the partial hydrogenation doping rather than the increase of surface area. It could be ascribed to the double-layer structure and the porous structure of the walls.

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Figure 4. (a) Cyclic voltammograms at different scan rates, (b) GCD curves at different current densities, (d) GCD curves at 0.5 mA cm-2 (the inset shows the capacitance retention vs. cycle number up to 5000 cycles) and (e) FESEM images of the soaked ATNAs for 40 min. (c) Nyquist plots and (f) N2 adsorption/desorption isotherms of the anodized ATNAs and the soaked ATNAs for 40 min anodized in H3PO4-6wt% mixed electrolyte and the pristine ATNAs. Conclusions In this work, we studied the performances of larger diameter nanotubes with doublelayer structure in NH4F/H3PO4 mixed electrolyte. The double-layer structure increased 14


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the specific capacitance of nanotubes for the cavities between the double layers compared to the single-layer structure. In addition, after soaking in H3PO4 aqueous solution, ATNAs prepared in H3PO4-6wt% mixed electrolyte still show nanopores on the top of nanotubes owing to larger nanotube diameter but show higher capacitance. The soaked nanotubes for 40 min show the specific capacitance of 13.89 mF cm-2 at 0.5 mA cm-2. The specific capacitance shows ~3.11 times that of the pristine nanotubes. The specific surface area of the soaked ATNAs for 40 min is up to 113.2 m2 g-1, a value representing an increase by a factor of 2.94 over the value for the pristine ATNAs. The values of specific surface area of the anodized ATNAs and the soaked ATNAs fabricated in H3PO4-6wt% mixed electrolyte are roughly equal. It demonstrated that the specific surface area increased mainly due to the double-layer structure. The present results reveal that the double-layer structure is a sufficient way to improve the specific surface area of TiO2 nanotubes. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51577093, 51777097, 51377085), the Project Funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions (AD20537). References [1] Chen, H. H.; Nanayakkara, C. E.; Grassian, V. H. Titanium Dioxide Photocatalysis in Atmospheric Chemistry. Chem. Rev. 2012, 112, 5919–5948. [2] Abe, T.; Fukui, F.; Kawai, Y.; Nagai. K.; Kato, H. A Water Splitting System Using 15


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an Organo-photocathode and Titanium Dioxide Photoanode Capable of Bias-free H2 and O2 Evolution. Chem. Commun. 2016, 52, 7735–7737. [3] Li, Z.; Ding, Y. T.; Kang, W. J.; Li, C.; Lin, D.; Wang, X. Y.; Chen, Z. W.; Wu, M. H.; Pan, D. Y. Reduction Mechanism and Capacitive Properties of Highly Electrochemically Reduced TiO2 Nanotube Arrays. Electrochim. Acta 2015, 161, 40–47. [4] Cao, C.; Zhang, G.; Ye, J.; Hua, R.; Sun, Z.; Cui, J. Fast Growth of Self-aligned Titania Nanotube Arrays with Excellent Transient Photoelectric Responses. Electrochim. Acta 2016, 211, 552–560. [5] Zheng, L.; Dong, Y.; Bian, H.; Lee, C.; Lu, J.; Li, Y. Self-ordered Nanotubular TiO2 Multilayers for High-performance Photocatalysts and Supercapacitors. Electrochim. Acta 2016, 203, 257–264. [6] Yu, D.; Zhu, X.; Xu, Z.; Zhong, X.; Gui, Q.; Song, Y.; Zhang, S.; Chen, X.; Li, D. Facile Method to Enhance the Adhesion of TiO2 Nanotube Arrays to Ti Substrate. ACS Appl. Mater. Interfaces 2014, 6, 8001–8005. [7] Hang, R.; Zhao, Y.; Bai, L.; Liu, Y.; Gao, A.; Zhang, X.; Huang, X.; Tang, B.; Chu, P. Fabrication of Irregular-layer-free and Diameter-tunable Ni-Ti-O Nanopores by Anodization of NiTi Alloy. Electrochem. Commun. 2017, 76, 10–14. [8] Zhang, H.; Chen, Z.; Song, Y.; Yin, M.; Li, D.; Zhu, X.; Chen, X.; Chang, P.; Lu, L. Fabrication and Supercapacitive Performance of Long Anodic TiO2 Nanotube Arrays Using Constant Current Anodization. Electrochem. Commun. 2016, 68, 23– 27. 16


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[9] Wu, H.; Geng, J.; Wang, Y. H.; Wang, Y. L.; Peng, Z.; Zheng, G. F. Bias-free, Solar-charge Electric Double-layer Capacitors. Nanoscale 2014, 6, 15316–15320. [10] Mole, F.; Wang, J.; Clayton, D. A.; Xu, C. L.; Pan, S. L. Highly Conductive Nanostructures C-TiO2 Electrodes with Enhanced Electrochemical Stability and Double Layer Charge Storage Capacitance. Langmuir 2012, 28, 10610–10619. [11] Wu, H.; Li, D. D.; Zhu, X. F.; Yang, C. Y.; Liu, D. F.; Chen, X. Y.; Song, Y.; Lu, L. F. High-performance and Renewable Supercapacitors Based on TiO2 Nanotube Array Electrodes Treated by an Electrochemical Doping Approach. Electrochim. Acta 2014, 116, 129–136. [12] Samiee, M.; Luo, J. A Facile Nitridation Method to Improve the Rate Capability of TiO2 for Lithium-ion Batteries. J. Power Source 2014, 245, 594–598. [13] Fan, H.; Zhang, H.; Luo, X.; Liao, M.; Zhu, X.; Ma, J.; Song, Y. Hydrothermal Solid-gas Route to TiO2 Nanoparticles/Nanotube Arrays for High-performance Supercapacitors. J. Power Sources 2017, 357, 230–240. [14] Liu, N.; Chen, X. Y.; Zhang, J. L. A Review on TiO2-based Nanotubes Synthesized via Hydrothermal Method: Formation Mechanism, Structure Modification, and Photocatalytic Applications. Catal. Today 2014, 225, 34–51. [15] Cao, X. Y.; Xing, X.; Zhang, N.; Gao, H.; Zhang, M. Y.; Shang, Y. C.; Zhang, X. T. quantitative investigation on the effect of hydrogenation on the performance of MnO2/H-TiO2 composite electrodes for supercapacitors. J. Mater. Chem. A 2015, 3, 3785–3793. [16] He, X.; Yang, C.; Zhang, G.; Shi, D.; Huang, Q.; Xiao, H.; Liu, Y.; Xiong, R. 17


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Supercapacitor of TiO2 Nanofibers by Electrospinning and KOH Treatment. Mater. Des. 2016, 106, 74–80. [17] Zhou, H.; Zhang, Y. Electrochemically Self-doped TiO2 Nanotube Arrays for Supercapacitors. J. Phys. Chem. C 2014, 118, 5626–5636. [18] Yang, S. N.; Cheng, K.; Huang, J. C.; Ye, K.; Xu, Y.; Cao, D. X.; Zhang, X. M.; Wang, G. L. High-capacitance MnO2 Nanoflakes on Preformed C/TiO2 Shell/Core Nanowire Arrays for Electrochemical Energy Storage. Electrochim. Acta 2014, 120, 416–422. [19] Cheng, K.; Yang, F.; Zhang, D. M.; Yin, J. L.; Cao, D. X.; Wang, G. L. Pd Nanofilm Supported on C@TiO2 Nanocone Core/Shell Nanoarrays: A Facile Preparation of High Performance Electrocatalyst for H2O2 Electroreduction in Acid Medium. Electrochim. Acta 2013, 105, 115–120. [20] Kim, J. Y.; Choi, S. B.; Noh, J. H.; Yoon, S. H.; Lee, S.; Noh, T. H.; Frank, A. J.; Hong, K. S. Synthesis of CdSe-TiO2 Nanocomposites and Their Applications to TiO2 Sensitized Solar Cells. Langmuir 2009, 25, 5348–5351. [21] Li, C. X.; Lou, Z. R.; Yang, Y. C.; Wang, Y. C.; Lu, Y. F.; Ye, Z. Z.; Zhu, L. P. Hollowsphere Nanoheterojunction of g-C3N4@TiO2 with High Visible Light Photocatalytic Property. Langmuir, 2019, 35, 779–786. [22] Chen, S. Y.; Chen, Y.; Li, C. Y.; Ouyang, H. J.; Qin, S.; Song, Y. Double-walled Structure of Anodic TiO2 Nanotubes in H3PO4/NH4F Mixed Electrolyte. Mater. Res. Express 2018, 5, 045039(6pp). [23] Yu, M.; Chen, Y.; Li, C.; Yan, S.; Cui, H.; Zhu, X.; Kong, J. Studies of Oxide 18


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Growth Location on Anodization of Al and Ti Provide Evidence Against the Fieldassisted Dissolution and Field-assisted Ejection Theories. Electrochem. Commun. 2018, 87, 76–80. [24] Yu, M.; Cui, H.; Ai, F.; Jiang, L.; Kong, J.; Zhu, X. Terminated Nanotubes: Evidence Against the Dissolution Equilibrium Theory. Electrochem. Commun. 2018, 86, 80–84. [25] Zhao, S.; Li, C.; Wei, T.; Li, C.; Yu, M.; Cui, H.; Zhu, X. A Mathematical Model for Initiation and Growth of Anodic Titania Nanotube Embryos Under Compact Oxide Layer. Electrochem. Commun. 2018, 91, 60–65. [26] Zhao, S.; Wu, L.; Li, C.; Li, C.; Yu, M.; Cui, H.; Zhu, X. Fabrication and Growth Model for Conical Alumina Nanopores-Evidence Against Field-Assisted Dissolutio Theory. Electrochem. Commun. 2018, 93, 25–30. [27] Yu, M.; Li, C.; Yang, Y.; Xu, S.; Zhang, K.; Cui, H.; Zhu, X. Cavities Between the Double Walls of Nanotubes: Evidence of Oxygen Evolution Beneath An Anioncontaminated Layer. Electrochem. Commun. 2018, 90, 34–38. [28] Zhang, S.; Pan, N. Supercapacitors Performance Evaluation. Adv. Energy Mater. 2015, 5, 1401401(19pp). [29] Du, K.; Liu, G.; Li, M.; Wu, C.; Chen, X.; Wang, K. Electrochemical Reduction and Capacitance of Hybrid Titanium Dioxide-nanotube Arrays and “nanograss”. Electrochim. Acta 2016, 210, 367–374. [30] Zhao, S.; Chen, Y.; Zhao, Z.; Jiang, L.; Zhang, C.; Kong, J.; Zhu, X. Enhanced Capacitance of TiO2 Nanotubes Topped with Nanograss by H3PO4 Soaking and 19


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Hydrogenation Doping. Electrochim. Acta 2018, 266, 233–241. [31] Chen, W.; Wang, Y.; Chan, H. Hydrogen: A Metastable Donor in TiO2 Single Crystals. Appl. Phys. Lett. 2008, 92, 112907(3pp).

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Figure 1. FESEM images of ATNAs obtained in various electrolytes containing different concentrations of H3PO4 after annealing: (a-f) 0, 4, 6, 8, 10 and 12 wt%, respectively. (g) Cyclic voltammograms at a scan rate of 100 mV s-1 and (h) GCD curves at a current density of 0.5 mA cm-2 of ATNAs obtained in various electrolytes.


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Figure 2. (a, b) FESEM images of double-layer structure and porous cavities in the large area of TiO2 nanotubes fabricated in H3PO4-6wt% mixed electrolyte and (c) a schematic of formation mechanism of the double-layer structure and porous cavities of nanotubes.


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Figure 3. FESEM images of the soaked ATNAs for different times: (a-d) 0, 20, 40 and 60 min, respectively. (e) Cyclic voltammograms at a scan rate of 100 mV s-1 and (f) GCD curves at a current density of 0.5 mA cm-2 of the soaked ATNAs comparing with the pristine ATNAs.


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Figure 4. (a) Cyclic voltammograms at different scan rates, (b) GCD curves at different current densities, (d) GCD curves at 0.5 mA cm-2 (the inset shows the capacitance retention vs. cycle number up to 5000 cycles) and (e) FESEM images of the soaked ATNAs for 40 min. (c) Nyquist plots and (f) N2 adsorption/desorption isotherms of the anodized ATNAs and the soaked ATNAs for 40 min anodized in H3PO4-6wt% mixed electrolyte and the pristine ATNAs.


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Enhanced Capacitance of TiO2 Nanotubes with a Double-Layer

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