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Oct 5, 2016 - High performance hybrid supercapacitors using granule Li 4 Ti 5 O 12 /Carbon nanotube anode. Byung-Gwan Lee , Seung-Hwan Lee , Hyo-Jin ...
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Applications of Novel Carbon/AlPO4 Hybrid-Coated H2Ti12O25 as a High-Performance Anode for Cylindrical Hybrid Supercapacitors Jeong-Hyun Lee† and Seung-Hwan Lee*,‡ †

Department of Electronics Materials Engineering, Kwangwoon University, Seoul 01897, Korea Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, United States

‡

S Supporting Information *

ABSTRACT: The hybrid supercapacitor using carbon/AlPO4 hybrid-coated H2Ti12O25/activated carbon is fabricated as a cylindrical cell and investigated against electrochemical performances. The hybrid coating shows that the conductivity for the electron and Li ion is superior and it prevented active material from HF attack. Consequently, carbon/AlPO4 hybrid-coated H2Ti12O25 shows enhanced rate capability and long-term cycle life. Also, the hybrid coating inhibits swelling phenomenon caused by gas generated as decomposition reaction of electrolyte. Therefore, the hybrid supercapacitor using carbon/AlPO4 hybridcoated H2Ti12O25/activated carbon can be applied to an energy storage system that requires a long-term life. KEYWORDS: carbon/AlPO4 hybrid coating, H2Ti12O25, hybrid supercapacitor, HF attack, swollen phenomenon

1. INTRODUCTION As the environmentally friendly industry was issued with the development of the economy, the energy paradigm was also becoming environmentally friendly. Among them, new and renewable energy such as solar, wind, and geothermal energy was promising for alternative energy storage devices because fossil fuels were not environmentally friendly and gradually become exhausted. However, new and renewable energy was limited in its application in energy storage systems which sustainably needs a supply of energy. So a new concept for energy storage devices is required because current energy devices (Li ion batteries and supercapacitors) have respective drawbacks.1,2 The proposed hybrid supercapacitor of this paper can be regarded as a next-generation energy storage device with uninterruptible power supply, emergency light, signal lamp, pitch of wind power generation, and so forth. To achieve high performance with a hybrid supercapacitor, a study of novel electrode material design and optimization was performed.3 First of all, this is important because the selection of electrode materials have an effect on performance of a hybrid supercapacitor. Carbon-, oxide-, and metal-based materials have received attention as electrode materials. In this paper, the hybrid supercapacitor is designed with carbon-based positive electrode material for the supercapacitor and oxide-based negative electrode material for the Li ion battery.1 Carbon© XXXX American Chemical Society

based materials have superior performance such as low cost, chemical stability, and high conductivity. Among them, activated carbon is selected as a positive electrode because it has high specific surface (1000−2000 m2/g), superior conductivity, and adjustable pore structure (2−5 nm).4−6 Recently, there are many promising applicants for negative electrode, such as carbon nanotube (CNT)/carbon nanofiber (CNF) based composites such as MnO2,7 TiO2,8 and Ni(OH)2.9 Although they can improve the conductivity using CNT/CNF, there are disadvantages to application of the actual product due to difficulty to manufacture and rise in cost. The H2Ti12O25 has not only high initial discharge capacity of 229 mA·h/g compared to that of Li4Ti5O25 (175 mA·h/g) with comparable cycle stability but also excellent rate capability.1,10 Compared to TiO2(B), H2Ti12O25 shows superior cycle stability. These comparative advantages can be elucidated by a tunnel structure.10 As a result, it can be concluded that H2Ti12O25 is appropriate to apply to negative electrodes. In addition, the performance of electrode material can be enhanced by surface coating or being in nanosize form. Although the performance of hybrid supercapacitors is Received: July 1, 2016 Accepted: October 5, 2016

A

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the fabrication process for hybrid-coated H2Ti12O25 (HTO).

Figure 2. (a) XRD pattern of the pristine and surface-coated HTO negative electrode materials. (b) TEM image and (c) EDS mapping of carbon/ AlPO4 hybrid coating layer. EDS mapping of (d) C element and (e) Al element.

affects the improvement of electronic conductivity, it limits ionic conductor. Also, the thick carbon layer causes low diffusivity of Li ion. Ultimately, the AlPO4 coating layer is an easy, highly effective, safe, and cost-effective approach to suppress the swelling phenomenon without performance degradation in the cell.13−15 To improve the coating layer function, the AlPO4 is mixed with carbon because although the ion conductivity of the carbon coating is less than its electron conductivity, AlPO4 can enhance the ion conductivity of the carbon coating.1,12 Therefore, hybrid-coated H2Ti12O25 is employed as negative electrode material for hybrid supercapacitors, which demand

improved since the distance of Li ion diffusion is decreased by the nanosize form, it induces undesirable side reaction, for example, agglomeration by improved surface energy, increase of resistance by agglomerated particle, nongas generation suppression, difficult process, and high cost.11 With other methods, the surface coating not only are low in cost but also are a simple process. Among them, a carbon coating has many advantages such as suppression of grain growth, improvement of conductivity, stability during cycling, and gas generation suppression. However, carbon coating has been greatly studied previously and therefore it needs to be improved because it has the disadvantage to be solved.12 Although the carbon layer B

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Initial charge−discharge curves for cell balancing of hybrid supercapacitor with different electrode thicknesses. (b) Enlarged initial charge−discharge curves between 2.6 and 2.85 V.

high power density and long-term cycling stability rather than Li ion batteries. In this paper, we investigated electrochemical performances of cylindrical hybrid supercapacitors using H2Ti12O25 negative electrode with different coating materials.

ances of all samples were measured using an Arbin BT 2042 battery test system at various current densities with a cut-off voltage of 1.5 V. The electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 10−1 to 10−6 Hz.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION Figure 2a shows XRD patterns of the prepared pristine H2Ti12O25 (HTO), carbon-coated HTO (C-HTO), AlPO4 coated HTO (A-HTO) and hybrid AlPO4/C coated HTO (H-HTO) powder. All XRD patterns are indicated that the pristine and surface coated HTO powders are well synthesized according to previous studies.10,16,17 Although surface coating does not change HTO structure, it is difficult to identify in XRD patterns, as will be discussed by TEM. The morphology of all powders is similar and particle size is under 1 μm as shown in Figure S1. In order to investigate surface coating, TEM and energy-dispersive spectroscopy (EDS) mapping were carried out on surface-coated HTO powder, as shown in Figure 2b−e and Figure S2. The coating materials were uniformly coated as approximately 7 nm thickness over HTO. To confirm existence of AlPO4 and carbon in coating layer, EDS mapping of H-HTO was conducted, as shown in Figure 2c. AlPO4 and carbon are evidenced by the Al and C element, respectively. As a result, it is obvious that AlPO4 and carbon are randomly mixed in hybrid carbon/AlPO4 coating layer. The existence of each coating layer can also be detected by FT-IR. The FT-IR spectroscopy was used to prove each coating layer, as shown in Figure S2c. From the FT-IR analysis, we can confirm that all samples have common peaks of HTO. Two absorption peaks at approximately 2370 and 3447 cm−1 were observed, indicating O−H stretching vibration.9 As shown in C-HTO, there are two main peaks around 1706 and 1612 cm−1, corresponding to C O and CC vibrations, respectively.18 For the A-HTO, three strong bands were observed at 1027 (PO4), 593 (O−P−O), and 538 (PO4) cm−1, corresponding to characteristic of PO43− vibration.19 These results indicate the existence of AlPO4 and carbon layer, respectively. From the FT-IR analysis of H-HTO, we can observe that the peaks of AlPO4 and carbon layer existed together, indicating that hybrid coating was successfully prepared on the surface of HTO. There was little change for all samples regardless of coating, indicating no significant interaction between coating layer and HTO. The hybrid supercapacitor is designed to have asymmetric structure of negative and positive electrodes. Also, the hybrid supercapacitor is reported that the negative and positive electrodes generate

First, to prepare hybrid-coated H2Ti12O25 negative electrode material, Na2CO3 (99.5%) and TiO2 were mixed at the molar ratio of 1:3 and then this mixture was heated at 800 °C for 20 h in air. The resultant Na2Ti3O7 via ion-exchange reaction using 1 M HCl solution for 3 days at 60 °C was changed into H2Ti3O7. The fabricated H2Ti3O7 was washed with deionized water and then dried at 100 °C for 24 h in air. By heating the H2Ti3O7 sample at 300 °C for 5h, H2Ti12O25 was obtained. As a first step, the produced H2Ti12O25 powders were mixed with Super P carbon black used as carbon source in distilled water and then ball-milled for 3 h. After that, Super P carbon black solution was mixed with gelatin (amphoteric surfactant) solution, prepared by dissolving gelatin in distilled water at 50 °C for 2 h. As a second step, AlPO4 suspension (of AlPO4 nanoparticles) was fabricated by mixing distilled water, Al(NO3)3·9H2O (0.3 g), and (NH4)2HPO4 (0.1 g). The mixed H2 Ti12 O25 powders were slowly added to AlPO4 suspension solution and then were mixed until the final viscosity of the slurry reached approximately 100 P. The weight ratio of AlPO4 suspension and carbon-mixed H2Ti12O25 powder was 5:95 mass ratio and then dried at 100 °C for 24 h. Then dried powders were calcined at 300 °C for 3 h. The process of hybrid-coated H2Ti12O25 powder is illustrated in Figure 1. The hybrid-coated H2Ti12O25 powder was prepared using a first and second step, respectively. An activated carbon was used as the positive electrode. For the negative electrode, hybrid-coated H 2Ti12O25 was mixed with conductive carbon black binder (Super P) and polyvinylidone fluoride in the weight ratio 83:7:10 in N-methylpyrrolidinone (NMP) solvent. The masses were 2.9 and 3.1 g for positive and negative electrodes, respectively. The obtained slurry was spread to a thickness of 125 μm of aluminum foil using a bar coater and then dried at 100 °C to remove the NMP solvent. To make a negative electrode of 70−80 μm thickness, the covered aluminum foil was pressed. The widths and heights of positive and negative electrodes were 55 cm × 3.4 cm and 60 cm × 3.4 cm, respectively. Also, the diameter and length of cylindrical cell were 2.2 and 5.2 cm, respectively. Subsequently, the cell was dried in a vacuum oven for 48 h to remove the moisture in the cell and was impregnated with a 1.5 M LiPF6 solution in 1:1 ethylene carbonate (EC):dimethyl carbonate (DMC) as the electrolyte. Lastly, in an argon gas-filled glovebox, the positive electrode, separator, and negative electrode were assembled as a cylindrical cell. The structural properties of the hybrid-coated H2Ti12O25 particles were measured by X-ray diffraction (XRD), a transmission electron microscope (TEM), a scanning electron microscope (SEM), and Fourier transform infrared (FT-IR). The electrochemical performC

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Cyclic voltammetric curves of pristine and surface-coated HTO (a) at 5 mV s−1 and (b) 20 mV s−1. (c) Initial charge−discharge curves of hybrid supercapacitors using the pristine and surface-modified HTO. (d) Rate capabilities of the hybrid supercapacitors using pristine and surfacemodified HTO.

different reactions.20 The capacitance was decided by Li-ion insertion/extraction reaction at negative electrode and BF4− ion adsorption/desorption reaction at positive electrode. At negative electrode, Li-ion kinetics is slower than that of the BF4− ion. Therefore, the balance between positive electrode and negative electrode using controlling thickness of HTO in hybrid supercapacitor is the one of the most important factors. In the HTO, Li ion can react at only the upper part due to slow kinetics. This phenomenon causes performance degradation for hybrid supercapacitor.21 Therefore, optimized hybrid supercapacitor can be fabricated by thickness control of negative electrode.22 To optimize the cell balance, the electrode thickness is controlled because it is the major factor influencing the electrochemical performances.22 Figure 3 shows the initial charge−discharge curves of hybrid supercapacitors using HTO negative electrode having different thicknesses at 0.1 A g−1. All initial charge−discharge curves are asymmetric in shape and the capacitance is 36, 52, 67, and 73 F g−1, respectively. According to the table, the Coulombic efficiency of the 90−240 μm electrode is lower than that of the 70−240 μm electrode. On the other hand, the IR drops decreased by increased electrode thickness; however, the IR drop of 90−240 μm electrode increased. The IR drop is 0.109, 0.059, 0.023, and 0.037 Ω, respectively. The cell balance of 90−240 μm electrode is unsuitable because the Coulombic efficiency is decreased by an increase of IR drop at the electrode thickness over 70−240 μm. Consequently, the 70−240 μm electrode can be decided as optimized cell balance electrode owing to reversible Coulombic efficiency and low IR drop.

To better understand intercalation/extraction reactions of each different surface-coated HTO, the CV curves of individual electrodes were measured in the potential ranges 1.0−3.0 V at a scan rate of 5 mV s−1, as shown in Figure S3. Compared with pure HTO, the polarization between anodic and cathodic peaks of surface-coated HTO decreased. Among them, the H-HTO shows the smallest polarization value, indicating fast electrochemical reactions. As a result, it can be inferred that the hybrid coating plays an important role in enhancing the reversibility. Figure 4a,b shows cyclic voltammetric (CV) curves of a hybrid supercapacitor using pristine and each different surface-coated HTO negative electrodes at (a) 5 and (b) 20 mV s−1, respectively. Although the increased scan rate reportedly could distort the CV curve from the prior CV curve, it did not change because the surface coating is designed to facilitate stable reaction between electrode and electrolyte.23 From the CV curves, the redox reaction of the hybrid supercapacitor also indicated that it occurs by Li+ ion insertion/extraction (H2Ti12O25 + xLi+ + xe− ↔ LixH2Ti12O25) corresponding to Ti3+/Ti4+ redox couple in the HTO.24,25 By the report of Akimoto et al.,11 an 8.6 of Li+ ion is inserted/extracted in the HTO and this phenomenon is indicated broadly at the 2.4 and 2.7 V of CV curves. As is well-known, the carbon and AlPO4 play important roles in electron and Li ion conductivity, respectively. However, the opposite case is unfriendly. Unlike these single-coating layers, H-HTO shows not only superior electron conductivity but also Li-ion conductivity due to the synergetic effect. It can be inferred that smooth movement of electrons and Li ions on H-HTO can yield good electroD

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) Photographs of cylindrical hybrid supercapacitor of HTO and H-HTO. (b) Cycling performance of the hybrid supercapacitors using pristine and surface-modified HTO. EIS curves of pristine and surface-modified HTO after (c) 1 and (d) 3000 cycles.

dissolution of transition metal to occur.28,29 At the 3 A g−1, the retention of C-HTO is lower than that of A-HTO. Although carbon has electronic conductivity superior to the coating material, it having a thick coating layer that disturbs Li-ion diffusion at a high rate makes it an inferior ionic conductor.11,12 On the other hand, the retention of H-HTO shows the best rate capability of all scan rates. As shown in Figure 1, the hybrid coating layer has two paths which is Li ion and electron because carbon is a good electronic conductor and AlPO4 of suitable thickness is a good ionic conductor.30 At 3 A g−1, the hybrid supercapacitor using H-HTO shows the superior retention of 51%, applicable to actual product. This result means that HHTO can be utilized at a high speed. Figure 5b shows cycle performance of a hybrid supercapacitor using pristine and each different surface-coated HTO negative electrode at 3 A g−1. After 3000 cycles, the discharge retention of HTO, A-HTO, CHTO, and H-HTO is 47.6, 72.4, 58.5, and 93%, respectively. The HTO indicates bad cycle performance because this result is attributed to HF attack as mentioned in Figure 4d. The CHTO has a tendency to decrease obliquely since 1000 cycles. This is because although the surface coatings improve electrode reaction kinetics at high rates, the carbon coating has poor Liion conductivity. It is indicated that with A-HTO there is no obliquely decreased region during 3000 cycles. The reason is that AlPO4 coating layer having appropriate thickness shows superior Li-ion conductivity and facilitates tunneling of electrons.1 Also, AlPO4 is transformed as AlF3 through reaction with HF.31 However, the produced AlF3 layer can be led to an increase of the coating thickness and then the tunneling of electrons can be hindered by a thick coating layer. In contrast, H-HTO represents superior cycle performance among them.

chemical performance. The initial charge−discharge curves of a hybrid supercapacitor using pristine and each different surfacecoated HTO negative electrodes are shown in Figure 4c. The initial charge−discharge process is carried out at 1 A g−1, and the capacitances of HTO, C-HTO, A-HTO, and H-HTO are 51.21, 55.32, 53.43, and 58.19 F g−1, respectively. The capacitance of the hybrid supercapacitor using surface-coated materials is increased compared to that using pristine HTO. The HTO having tunnel structure represents high capacity due to inserting/extracting many Li ions. However, as Li ion is concentrated on an electrode surface during the charge process, this phenomenon results in a decrease of Li-ion diffusion and decline of capacitance by increase of cell resistance.26 Therefore, the surface coating is effective for improving capacitance by reducing the resistance of the cell.27 As shown in Figure 4b, by increase of the electrode reaction kinetics, the capacitance can be regarded as improved because the electrochemical reaction is enhanced by surface coating. Among the surface-coated HTOs, the H-HTO negative electrode shows superior performance owing to synergetic effect between carbon and AlPO4. The rate capabilities are investigated to indicate effect of surface coating on active material as shown in Figure 4d. The rate capabilities evaluation is performed as rates from 0.5 to 5 A g−1. At relatively slow rates (0.5 and 1 A g−1), the retention is represented that it is slightly different. In contrast, the retention of all cells shows remarkable difference at rapid rates (3 and 5 A g−1). In a previous report, the HF is caused by hydrolysis of LiPF6 decomposition reaction (LiPF6 + H2O → 2HF + POF3 + LiF). The retention of HTO is extremely decreased because the structure of HTO is collapsed by HF attack which causes E

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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caused by gas generated as decomposition reaction of electrolyte (i.e., EC + 2Li+ + 2e− → Li2CO3↓ + C2H4↑ and DMC + 2Li+ + 2e− → CH3OLi↓ + CO↑).31 To investigate the applicability of an energy storage system (ESS), hybrid supercapacitors using hybrid-coated HTO negative electrode

The hybrid coating has two paths through synergetic effect between carbon and AlPO4. Even if the coating layer is slightly thicker due to generated AlF3, the electron can be moved enough in the active material. Also, generated AlF3 prevented active material from HF attack and thus it can be helped as a reaction stabilizer between electrode and electrolyte.32,33 Likewise, the AlPO4 having stable structure by strong P−O bond can act as a reaction scavenger between electrode and electrolyte.34,35 Figure S4 shows the XRD patterns of various coated HTOs after 3000 cycles. All samples before cycling showed identical XRD patterns regardless of coating materials. However, we could confirm that the position of the peaks shifts to lower angles after 3000 cycles. These phenomena were explained by two causes: (1) The continuous expansion of the lattice during cycling led to the deformation of the original HTO structure, resulting in shifted peak. (2) The brokenness of the particle led to a broadened peak.21 Compared with other coating materials, the pattern of H-HTO was almost unchanged. Therefore, we can conclude that H-HTO shows structural stability superior to others. Figure 5c,d shows the EIS using pristine and each different surface-coated HTO negative electrode after charging at 2.8 V. At the first cycle, the chargetransfer resistance (Rct) of HTO, A-HTO, C-HTO, and HHTO is 0.048, 0.038, 0.032, and 0.026 Ω, respectively. After 3000 cycles, the Rct of HTO, A-HTO, C-HTO, and H-HTO is 0.205, 0.093, 0.102, and 0.053, respectively. The Rct of C-HTO is increased more than that of A-HTO. In addition, The Li-ion diffusion coefficients (DLi) are calculated by the following equation,36 DLi =

2 1 ⎛ RT ⎞ ⎜ ⎟ 2 ⎝ An2F 2Cσ ⎠

Figure 6. Ragone plots of hybrid supercapacitors using H-HTO negative electrode.

is compared with others as shown in Figure 6. The power density (P) and energy density (E) were calculated as1 ΔE =

(1)

where R is the gas constant, T is the absolute temperature, A is the surface area of the negative electrode, n is the number of electrons, F is Faraday’s constant, C is the concentration of Li+, and σ is the Warburg impedance coefficient.30 Table 1 shows

pristine HTO A-HTO C-HTO H-HTO

DLi (cm2 s−1) 7.865 8.243 8.521 8.883

× × × ×

−7

10 10−7 10−7 10−7

(2)

I M

(3)

P = ΔE ×

E=P×t

(4)

where Emax is the potential at the starting discharge, Emin is the potential at the end of discharge, I is the charge and discharge current, t is the discharge time in the hybrid supercapacitor, and m is the weight of active materials in both the negative and positive electrodes. The energy density and power density were calculated based on the total mass of two active materials. The energy and the power density show a trade-off relationship. The energy and power density of H-HTO ranged from 9.1 to 46.3 W h kg−1 and 184.7 to 12223.9 W kg−1, respectively. From the above results, the hybrid supercapacitor using H-HTO/AC can be applied to ESS, which requires a long-term life because its performance is as good as anything in this field.39−44

Table 1. DLi and ΔEp−p′ of Hybrid Supercapacitors Using Pristine and Surface-Modified HTO sample

Emax + Emin 2

ΔEp−p′ (V) 0.31 0.20 0.26 0.13

the polarization electrode and Li-ion diffusion coefficients. As a result, it means that the electrode reaction kinetics is improved by property of surface-coating materials.1,36 Therefore, this result can be inferred that the surface coating affects the rate capability of the hybrid supercapacitor.37,38 At the high rate, the concentration of Li ion on the electrode surface is raised as mentioned in Figure 4c, and thus it can be considered that the carbon having poor ion conductivity at this phenomenon increases its resistance. Among them, the HHTO shows the best performance. This is because the hybrid coating is a superior electron and ionic conductor as shown in Figure 1. In addition, the AlF3 that is known as Lewis acid can be transformed to AlF4−, leading to increased conductivity and decreased impedance.20 Photographs of the cylindrical hybrid supercapacitor using (i) pristine HTO and (ii) hybrid-coated HTO after 3000 cycles are shown in Figure 5a. Photographs indicate that the hybrid coating inhibits swelling phenomenon

4. CONCLUSION The H2Ti12O25 having tunnel structure represents a high capacity (225 mA h g−1) among oxide electrode materials and thus the H2Ti12O25 has attracted attention as a negative electrode material for a hybrid supercapacitor. The hybrid supercapacitor is fabricated with an activated carbon positive electrode and a carbon/AlPO 4 hybrid-coated H 2Ti12 O25 negative electrode. The hybrid-coated H2Ti12O25 shows superior rate capability and long-term cycle life through improved conductivity for electron and Li ion and it prevented active material from HF attack. Also, the hybrid-coated H2Ti12O25 shows that the superior energy and power density ranged from 9.1 to 46.3 W h kg−1 and 184.7 to 12223.9 W kg−1, respectively. In addition, it shows the extraordinary retention of F

DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces 93% after 3000 cycles at 3 A g−1. Consequently, the hybrid supercapacitor using H-HTO/AC is eligible for applicability to an actual energy storage device.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08032. Morphology (SEM and TEM images) and structure (XRD and FT-IR spectrum) and CV curves for pristine and modified HTO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-H.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.6b08032 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX