Controlled Electrochemical Synthesis of Nickel Hydroxide Nanosheets

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Controlled Electrochemical Synthesis of Nickel Hydroxide Nanosheets Grown on Non-Woven Cu/PET Fibers: A Robust, Flexible and Binder-Free Electrode for High-Performance Pseudocapacitors Sung Min Cha, Goli Nagaraju, and Jae Su Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04611 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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The Journal of Physical Chemistry

Controlled Electrochemical Synthesis of Nickel Hydroxide Nanosheets Grown on Non-Woven Cu/PET Fibers: A Robust, Flexible and Binder-Free Electrode for High-Performance Pseudocapacitors Sung Min Cha,‡ Goli Nagaraju,‡ and Jae Su Yu* Department of Electronics and Radio Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea

*Address correspondence to [email protected] Tel: +82-31-201-3820; Fax: +82-31-206-2820 ‡ These authors contributed equally to this work 1 ACS Paragon Plus Environment


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ABSTRACT Using a simple electrochemical deposition (ECD) method, β-nickel hydroxide (β-Ni(OH)2) nanosheets (NSs) were facilely integrated on non-woven conductive textile substrate (NWCTs). The NWCTs with copper-plated polyethylenterephthalate (Cu/PET) fibers serves as a highly conductive and low-cost working substrate for the ECD method. With an aid of direct current (DC) voltage in the growth solution, the β-Ni(OH)2 NSs well adhere to NWCTs fibers with good uniformity. The surface morphology and crystallinity properties of the synthesized β-Ni(OH)2 NSs on NWCTs are characterized by various physico-chemical techniques. Furthermore, the hierarchical β-Ni(OH)2 NSs on NWCTs reveal superior energy storage performance in 1 M KOH electrolyte solution as a binder-free electrode for pseudocapacitors. Compared to the conventional conductive foil-based electrode, the pseudocapacitive electrode in this work exhibit a relatively higher electrochemical performance (specific capacitance of 2185.6 F g-1 at 5 A g-1) and stable capacitance retention (95%) after long-term cycling process, which is attributed to the hierarchical nanonetwork of β-Ni(OH)2 NSs and three-dimensional fibrous configuration of NWCTs electrode. This cost-effective growth of metal hydroxide/oxide nanostructures on flexible textiles may be useful for advanced energy storage device applications.

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INTRODUCTION

Recently, the development of flexible and wearable energy conversion and storage devices (e.g., nanogenerators, lithium ion batteries, supercapacitors, etc.) has attracted tremendous research interest for their potential applications in consumer electronics.1-4 Among these devices, supercapacitors have been mainly employed as an efficient energy source because of their high power density, good cycling stability, safe operation and high efficiency.5 All of these advantageous properties allow supercapacitors to play an important role in several application fields such as portable electronics and automotive mobiles.6-8 Particularly, textile-based supercapacitors can serve dual functions of effectively perceiving stimuli from the mechanical motions due to beneficial properties including relatively light weight and high flexibility as compared to rigid and inflexible devices.9-11 Accordingly, research efforts to develop novel textile-based supercapacitors for smart and flexible energy storage applications are increasing rapidly.12-13 Various textiles such as carbon cloths, graphene-coated fabrics, carbon nanotubes (CNTs)/textiles and carbon fiber papers have been widely investigated for flexible supercapacitor electrodes.13-16 The low conductivity in carbon textiles, expensive materials and complicated fabrication procedure could be needed to manufacture these substrates, which may hamper the large scale production and make it not suitable for the use in clothing devices. Therefore, it is important to develop lightweight, durable and flexible textile electrodes by combining the high conductive property and scalable fabrication process. To address this challenge, recently, electroless plating of metallic layers, such as nickel (Ni), copper (Cu), cobalt (Co), etc., on polyester fabrics is fascinating significant attention, because this process is extremely simple, and cost-effective materials can be used and scaled up for large scales with high conductivity.17-18 Normally, textile fibers could be weaved into woven and non-woven 3 ACS Paragon Plus Environment


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fibrous texture. Compared to the woven textiles, the non-woven textile fibers provide a highsurface area owing to the intertwined and disorderly-arranged fibrous network. Thus, the nonwoven conductive textile substrate (NWCTs) consisting of metallic Cu thin film on polyethylenterephthalate fibers can be expected to serve as a flexible and highly conductive electrode owing to the low resistivity, three-dimensional (3D) fibrous framework, good porosity and cost effectiveness.19 These conductive electrodes are facilely prepared using a green autocatalytic coating method in Cu based aqueous solutions, which is recognized as a low-cost approach for the preparation of current collectors for supercapacitors.14 Such high conductive nature and porous fibrous-structure framework current collector offer a rapid electron transport to enhance charge storage properties and probably enable the use for wearable energy storage devices.20-21

On the other hand, the choice of electroactive material is one of important factors, which strongly affects the energy storage properties of supercapacitors.15 With the choice of electroactive materials and energy storage mechanism in supercapacitors, they can be categorized into two types: one is electric double layer capacitors (EDLCs) and the other is pseudocapacitors.16-17 For EDLCs, electroactive materials including activated carbon, carbon nanotubes and reduced grapheme oxide nanostructures are generally employed as an electroactive material. Herein, the electric double layer is formed by the opponent charges which are physically separated at the electrode-electrolyte interface. 18-19 In fact, ELDCs usually suffer from relatively low energy storage properties due to the absence of chemical reactions within the electroactive material. In contrast, pseudocapacitors utilize transition metal oxide/hydroxides and conducting polymers as electroactive materials, which possess higher energy storage properties owing to their charge storage mechanism of faradaic redox reactions. 4 ACS Paragon Plus Environment

20-22

Therefore, transition

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metal oxide/hydroxides with different nanostructures are intensively examined as electroactive materials for enhanced energy performance in pseudocapacitors.23-25 Of various transition metal hydroxide/oxides, nickel hydroxide (Ni(OH)2) with different structures such as nanowires, nanoflakes and flower-shaped nanostructures are considered to be the most promising materials and have been extensively used as electroactive materials due to their high theoretical capacitance, large surface area, low toxicity to environment and low cost.26-28 Generally, Ni(OH)2 is available in two phases: one is α-Ni(OH)229 and the other is β-Ni(OH)2.30 Compared to the α-Ni(OH)2, the β-Ni(OH)2 is more stable in alkaline electrolyte solution.31 Additionally, during the electrochemical process, the Ni atom is able to participate in rapid redox reactions with electrolyte ions through the redox transition between +2 and +3 oxidation states in βNi(OH)2, which helps to improve the electrochemical performance in pseudocapacitors.32-33 For preparing the β-Ni(OH)2 nanostructures, different growth methods including hydrothermal method, microwave-assisted synthesis and chemical bath deposition method have been employed.33-36 Despite these complicated and time-consuming growth methods, electrochemical deposition (ECD) process is known to be a simple and versatile method to grow metal oxide/hydroxide nanostructures on conductive electrodes with various morphologies in short growth time.37-38 In addition to the ECD method, binder- and conductive carbon-free electrode can be easily fabricated on pseudocapacitive current collectors for high-performance energy storage devices.

Herein, ultrathin β-Ni(OH)2 nanosheets (NSs) were facilely deposited on NWCTs using a simple ECD method. The influence of direct current (DC) voltage and growth time on morphological properties of β-Ni(OH)2 NSs were also studied. As a flexible electrode for



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pseudocapacitors, the as-grown β-Ni(OH)2 NSs on NWCTs exhibited excellent energy storage properties compared to the conventional metal foil based electrodes.

EXPERIMENTAL DETAILS

Synthesis of β-Ni(OH)2 NSs on NWCTs: Hierarchical β-Ni(OH)2 NSs were facilely prepared using a low-cost ECD method on flexible NWCTs. Initially, the NWCTs pieces with sizes of ∼ 2 × 3 cm2 were prepared and washed with ethanol and deionized (DI) water followed by nitrogen gas drying at room temperature (RT). Then, the electrode setup for ECD process was made up using a platinum coated titanium mesh as anode (counter electrode) and NWCTs as cathode (working electrode) in Teflon plate support.39-40 Meanwhile, a glass beaker containing 500 ml of DI water was heated on hotplate at the temperature of 75-77 °C. After that, the growth solution was prepared by mixing 12 mM of nickel nitrate hexahydrate (Ni(NO3)2.6H2O, Sigma-Aldrich Corp.,) and 2 mM of ammonium fluoride (NH4F, Sigma-Aldrich Corp.,) into the above glass beaker under a constant magnetic stirring . After a while, 24 mM of hexamethylenetetramine (C6H12N4, Sigma-Aldrich Corp.,) was further added into the above solution, leading to the formation of green colour homogeneous solution in the ECD beaker. For depositing the βNi(OH)2, the electrodes setup (both anode and cathode) was dipped into the growth solution and a DC voltage of -1.2 V was applied to the cathode for 20 min with a DC power supply. After ECD process, the NWCTs coated β-Ni(OH)2 was separated from the growth setup and was washed repeatedly with DI water followed by drying in a stream of nitrogen at RT. The loaded weight of β-Ni(OH)2 on NWCTs was measured to be about 0.3 ± 0.02 mg cm-2. In addition, the influence of ECD parameters such as growth voltage (-0.8 to -1.4 V) and growth time (5 to 30 min) also varied to examine the surface morphologies of β-Ni(OH)2 NSs on NWCTs. To further 6 ACS Paragon Plus Environment

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explore the advantage of NWCTs, the stainless steel (SS) foil (size: 2 × 3 cm2) was also utilized to synthesize the β-Ni(OH)2 NSs in similar procedure.

Characterizations: The surface morphologies of the prepared β-Ni(OH)2 NSs on NWCTs were observed with a field-emission scanning electron microscope (FE-SEM, LEOSUPRA 55, Carl Zeiss) field-emission transmission electron microscope (FE-TEM, JEOL, JEM-2100F), fitted with the energy dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) pattern of the synthesized sample was recorded on Mac Science (M18XHF-SRA) X-ray powder diffractometer with CuKα = 1.54178 Å. The Fourier transform infrared (FT-IR) analysis was measured in the frequency range of 450-4000 cm-1 with a Perking Elmer spectrum-100 to investigate the surface functional groups of the grown material. To perform the FT-IR analysis, the sample was scrapped off from the NWCTs, mixed with a potassium bromide and made into a thin pellet of particular thickness at room temperature. X-ray photoelectron spectroscopy (XPS) of the deposited sample was recorded using K-Alpha model (Thermo Electron) system to examine the surface chemical compositions and oxidation states.

Electrochemical measurements: All the electrochemical tests were carried in a conventional three-electrode system using IVIUMSTAT electrochemical interface instrument (Ivium Technologies) in 1 M KOH (potassium hydroxide, DaeJung Chemicals Ltd.) electrolyte solution at room temperature. Hierarchical β-Ni(OH)2 NSs on NWCTs as binder-less electrode was directly used as a working electrode (0.5 × 2 cm2), while a Ag/AgCl and platinium wire were used as the reference and counter electrodes, respectively. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 0.01 to 100 kHz at open circuit potential with an alternating current (AC) perturbation of 5.0 mV. 7 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION

Figure 1 illustrates the preparation procedure of hierarchical β-Ni(OH)2 NSs on NWCTs via a cost-effective ECD method under low-temperature: (a) preparation of NWCTs for ECD method and (b) growth of β-Ni(OH)2 NSs on NWCTs. Herein, we chose a commercially available and highly flexible NWCTs as the working electrode for ECD method. As schematically shown in Figure 1(a)(i), the NWCTs was layered with a Cu thin film on polyethylenterephthalate (Cu/PET) fibers. Thus, the NWCTs exhibited a very low resistance of 0.035 Ω/sq and it was beneficial for the ECD method. Also, the good conductivity of the NWCTs is possible to be used as a low-cost current collector for supercapacitor devices. As shown in the microscopic image of Figure 1(a)(ii), the NWCTs was weaved by the randomly distributed Cu/PET fibers, exhibiting a large specific surface area and porous property. When the DC voltage was provided to the cathode using DC power supply in growth solution, the β-Ni(OH)2 NSs were uniformly distributed on NWCTs fibers (Figure 1(b)). The β-Ni(OH)2 NSs were deposited on NWCTs by rapid electrochemical reactions and subsequent precipitation of generated nickel (Ni2+) and hydroxyl (OH-) ions in the growth solution. At first, the Ni(NO3)2.6H2O was dissolved into H2O, which leads to the formation of Ni2+ and NO3- ions in the growth solution. By applying a DC voltage to the working electrode, the NO3- ions were electrochemically reduced with H2O. As a result, negatively charged OH- ions were generated and they were accumulated onto the surface of working electrode. Also, the growth solution comprised of copious C6H12N4, which was also decomposed with H2O and slowly released the OH- ions. Ultimately, the liberated OH- ions on the surface of working electrode reacted with the Ni2+ ions by Columbic attraction and were precipitated as hierarchical β-Ni(OH)2 NSs on NWCTs.39,

41


The involved electrochemical

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kinetics and subsequent decomposition of C6H12N4 in whole process in the preparation of βNi(OH)2 NSs via the OH- ions were given by the equations given below.42

NO3- + H2O + 2e- → NO2- + 2OH-

(1)

C6H12N4 + 6H2O → 6HCHO + 4NH3

(2)

NH3 + H2O → NH4+ + OH-

(3)

Ni+2 + 2OH- → Ni(OH)2

(4)

To understand the importance of DC voltage and C6H12N4, we have also conducted the ECD method separately without electric field in the presence of Ni(NO3)2.6H2O and C6H12N4 growth solution; without C6H12N4 under the presence of electric field in Ni(NO3)2.6H2O-contained growth solution, which only showed the pristine NWCTs fibers (Figure S1). This means that the both applied electric field and C6H12N4 play a significant role in generating the OH- ions for the growth of β-Ni(OH)2 NSs on NWCTs.

Figure 2 displays the photographic and FE-SEM images of the synthesized β-Ni(OH)2 NSs on NWCTs under a DC voltage of -1.2 V for 20 min. From the photographic image of Figure 2(a), after the ECD method, the surface of the NWCTs turns to greenish-white color, indicating that the β-Ni(OH)2 NSs were uniformly coated onto whole part of NWCTs with strong attachment. Under the flexed condition, when subjected to bending test, no apparent surface cracks were observed for the deposited material on NWCTs, as shown in the inset of Figure 2(a). This confirms the excellent flexible property of the as-grown sample on NWCTs. From low-



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magnification view of FE-SEM image in the inset of Figure 2(b), the disorderly arranged Cu/PET micro-fibrils were homogeneously coated with β-Ni(OH)2. It is also clear that each Cu/PET micro-fiber was fully covered with β-Ni(OH)2 NSs. As shown in the increased magnification view of FE-SEM image in Figure 2(c), the appearance of β-Ni(OH)2 showed to be numerous NSs were closely entwined with each other to form a hierarchical and 3D nanonetwork on NWCTs. Such hierarchical and 3D nanonetwork of β-Ni(OH)2 NSs on NWCTs could be useful to deliver the higher electrochemical performance as a pseudocapacitive electrode. Each of these NS has a thickness of about ∼ 10-30 nm. Clearly, the β-Ni(OH)2 NSs were perpendicularly incorporated into the Cu/PET fibers with a height of ∼280-300 nm (Figure 2(d)). Using SS foil as working substrate, the morphologies of the β-Ni(OH)2 NSs were also prepared under the same ECD conditions (see Figure S4).

The XRD patterns and FT-IR spectrum of the synthesized β-Ni(OH)2 NSs on NWCTs under a DC voltage of -1.2 V for 20 min were shown in Figure 3. In the XRD pattern of Figure 3(a)), the bare NWCTs shows intense diffraction peaks at 2θ degree of 43.3, 50.45 and 74.1° which are related to the crystal indices of (110), (200) and (220) of Cu layer (JCPDS card #851326) on PET fibers. Apart from the diffraction peaks of NWCTs, the other peaks located at 2θ values of 19.2, 33, 38.5, 52.1, 62.7 and 72.7° correspond to the (001), (100), (101), (102), (111) and (201) crystal planes, respectively. These peak positions were well indexed with the β-phase hexagonal Ni(OH)2 crystallographic data (JCPDS card# 14-0117) without any impurities.43 The functional groups presented in the β-Ni(OH)2 NSs on NWCTs confirmed by the FT-IR spectroscopy. Here, the spectrum was analyzed in the wavelength range of 450~4000 cm-1 as shown in Figure 3(b). The intense peak observed at 3637 cm-1 corresponds to the ν(OH) stretching vibration, indicative of the hydroxyl groups bonded with Ni species in free configuration.44 The 10 ACS Paragon Plus Environment

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broad band located around ∼ 3432 cm-1 is assigned to the νO-H vibration of hydrogen-bonded water molecule. Furthermore, other bands observed around 1648 are attributed to the bending vibrational modes of water molecules. Also, the band observed at1383 cm-1 because of the formed co-ordination bond between nitrogen atoms of the C6H12N4 and Ni2+ ions.45 Moreover, the bands positioned below ∼ 550 cm-1 were due to the metal-OH and metal-O (i.e., Ni-OH and Ni-O) stretching lattice vibration modes. From these results, it can be confirmed that the βNi(OH)2 NSs were grown on NWCTs with high purity.

Further investigation into morphological and crystallinity nature of the β-Ni(OH)2 NSs, TEM analysis were carried out. The preparation process of the sample in FE-TEM measurement is followed as; first the β-Ni(OH)2 NSs were separated from the NWCTs using scalpel and then the obtained powder was dispersed in ethanol using sonication for 10 min. After that, FE-TEM grid was dipped into the obtained colloidal solution and dried at RT before loading into the FETEM measurement system. From the FE-TEM image shown in Figure 4(a), clearly reveals that these NSs were intercalated with one another to form a porous and 3D nanonetwork. Such porous nanonetwork of β-Ni(OH)2 NSs enables the rapid penetration of electrolyte solution and increases the rich electroactive sites for electrochemical process.14 Accordingly, enhanced charge storage properties would be observed as an electroactive material for pseudocapacitors. Moreover, from the magnified view of FE-TEM image in Figure 4(b), each NS has an average thickness of ~ 15-30 nm. The high-resolution FE-TEM image taken from an edge of the βNi(OH)2 NSs revealed that the lattice fringes have an average d-spacing value of 0.232 nm, which corresponds to the (101) crystal plane separation of hexagonal β-Ni(OH)2. The elemental mapping images in Figure 4(d) and 4(e) indicates that the Ni and O elements were uniformly disseminated according to the FE-TEM image of β-Ni(OH)2 NSs. In addition, the corresponding 11 ACS Paragon Plus Environment


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elements presented in β-Ni(OH)2 NSs clearly were displayed in the EDX spectrum as shown in Figure 4(f).

An influence of growth parameters (DC voltage and growth time) on morphology of βNi(OH)2 NSs on NWCTs was also examined. Figure 5 shows the morphologies of β-Ni(OH)2 NSs on NWCTs synthesized at different DC voltages of -0.8 to -1.4 V under the constant ECD time of 20 min (Figure 5). In fact, the applied electric field in growth solution plays a vital role in determining the structural and morphological properties of the various metal hydroxide/oxide nanostructures during the ECD method.37, 41 With an increased DC voltage, excessive amount of OH- ions were produced by the rapid electrolysis process in growth solution. The as-obtained OH- ions were strongly coupled with the oppositely charged metal ions, forming different sizes, shapes, uniformities and densities of the nanostructures on conductive electrodes. It can be seen in Figure 5(a), the β-Ni(OH)2 structure with hexagonal NSs morphologies was partially distributed on the Cu/PET fiber surface at a DC voltage of -0.8 V. However, without the DC voltage (i.e., 0 V), β-Ni(OH)2 NSs were not grown on the NWCTs surface (see Figure S1) due to the absence of electrolysis process. With the increased DC voltage of -1.0 V (Figure 5(b)), the βNi(OH)2 NSs could be deposited on the surface of NWCTs with smaller NS thickness. Under the optimum DC voltage of -1.2 V, as shown in the FE-SEM images of Figure 2(b-d), the β-Ni(OH)2 NSs with good adhesion with adequate amounts of mass were uniformly distributed over the whole surface of NWCTs. Meanwhile, when the applied DC voltage was further extended to -1.4 V, the thicker and denser β-Ni(OH)2 NSs film was immensely deposited on NWCTs, thus leading to the formation of surface cracks on NWCTs as shown in Figure 5(c). For pseudocapacitors, an adhesion between the current collector and electroactive material is an impact factor, which determines the charge storage performance during the electrochemical tests. 12 ACS Paragon Plus Environment

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Therefore, the sample prepared at a DC voltage of -1.2 V is expected to deliver superior energy storage properties for pseudocapacitors. Moreover, optimization of growth time is another important factor which strongly influences the surface morphology of β-Ni(OH)2 NSs on NWCTs. Accordingly, we carried out the growth time-dependent ECD method at the constant DC voltage of -1.2 V in the same growth solution. Figure S2 displays the FE-SEM images of the synthesized β-Ni(OH)2 NSs on NWCTs at different growth times of (a) 5 min, (b) 10 min and (c) 30 min under the optimized voltage of -1.2 V. For 5 min of ECD method (Figure S2(a)), the βNi(OH)2 NSs were partially distributed on NWCTs with very small mass loading, which indicates that this growth time is not enough for the proper formation of β-Ni(OH)2 NSs. After the growth time of 10 min, β-Ni(OH)2 NSs began to be grown on NWCTs with somewhat increased sheet thickness (Figure S2(b)). However, when the β-Ni(OH)2 NSs were synthesized at the growth time of 20 min (as shown in Figure 2), it is noticeable that the NSs were fully covered on NWCTs fibers with high uniformity and good porosity. Further increasing the growth time to 30 min as shown in Figure S2(c), the β-Ni(OH)2 NSs were hugely deposited on NWCTs in the form of big lumps. During the ECD method under extended growth time, indeed, the continuous electrochemical process and gradual decomposition of C6H12N4 result in the rapid generation of OH- ions, which intends to immediately precipitate with Ni2+ ions, leading to the growth of lump shaped β-Ni(OH)2 NSs on NWCTs.

To test the feasibility for pseudocapacitors, the β-Ni(OH)2 NSs grown on NWCTs at different DC voltages were directly used as the working electrodes in a conventional threeelectrode cell beaker and their electrochemical performance was characterized by cyclic voltammetry (CV) and galvanic charge-discharge (CD) measurements. Figure 6(a) shows the CV curves of the bare NWCTs and the β-Ni(OH)2 NSs synthesized on NWCTs under different DC 13 ACS Paragon Plus Environment


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voltages under a scan rate of 30 mV s-1. In comparison with the bare NWCTs, the β-Ni(OH)2 NSs samples exhibited larger CV integral areas, demonstrating that the involvement of pristine NWCTs for the electrochemical performance is negligible. Additionally, it can be clearly identified that the CV shapes were different from the ideal rectangular CV curves, confirming the pseudocapacitive characteristics of the β-Ni(OH)2 NSs on NWCTs. All the samples showed the pair of redox peaks such as anodic (positive current) and cathodic (negative current) peaks. This is ascribed to the following faradaic redox reactions within the active material in electrolyte solution as follows;46

Ni(OH)2 + OH- ⇋ NiOOH + H2O + e-.

(5)

Noticeably, the β-Ni(OH)2 NSs prepared with the DC voltage of -1.2 V exhibited a larger integrated CV area than the samples at the other growth voltages of -0.8, -1.0 and -1.4 V. Also, of the samples synthesized at different growth times of 5, 10, 20 and 30 min under the constant DC voltage of -1.2 V, the sample at 20 min clearly exhibited the larger CV integral area and redox peak current values compared to the samples at the other growth times (Figure S3). Thus, the β-Ni(OH)2 NSs synthesized at a DC voltage of -1.2 V for 20 min of growth time are more suitable for the enhanced electrochemical energy storage. Figure 6(b) displays the CV curves of β-Ni(OH)2 NSs on NWCTs (-1.2 V sample) at different scan rates of 5 to 70 mV s-1. With the extended scan rates (from 5 to 70 mV s-1), it can be clearly seen that the redox peak positions were shifted with increasing the peak current values. The oxidation and reduction peaks from the CV curves were also observed under higher scan rates, signifying the excellent electrochemical reversibility nature of the β-Ni(OH)2 NSs on NWCTs. The resulting redox peak current values


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versus square root of the applied scan rates revealed a linear relationship (Figure 6(c)), which means the diffusion-controlled process of electrolyte ions in β-Ni(OH)2 NSs on NWCTs. To further examine the energy storage properties of the hierarchical β-Ni(OH)2 NSs on NWCTs, the CD measurements were carried out in aqueous electrolyte (1 M KOH) solution under various current density values of 5 to 30 A g-1. Figure 7(a-c) shows the CD curves of the βNi(OH)2 NSs on NWCTs prepared with the different applied DC voltages of -1.0, -1.2 and -1.4 V, respectively. Clearly, all the CD plateaus were not ideal straight lines, exhibiting the pseudocapacitive behavior of the β-Ni(OH)2 NSs on NWCTs, which is consistent with the above CV results. Under the increased current densities, a small initial potential drop was observed for all the samples, which may be due to the internal resistance of the electrode material. Moreover, the charge-discharge times of the sample grown with a DC voltage of -1.2 V confirm the superior energy storage capabilities of the β-Ni(OH)2 NSs on NWCTs. Based on the CD curves in all the samples, the specific capacitance values can be calculated using the following formula:47

ூ×∆௧

Csc =

.

௠×∆௏

(6)

Here, Csc = specific capacitance (F/g), I = applied current (A), ∆t = discharge time (s), m = mass of the active material (g) and ∆V = potential window (V). From the calculation, the β-Ni(OH)2 NSs on NWCTs at -1.2 V show higher Csc values than the other growth samples as shown in Figure 7(d). With the applied current density of 5 A g-1, the Csc values obtained for the samples synthesized under the DC voltages of -1.0, -1.2 and -1.4 V were about 1770.4, 2185.6 and 1156.0 F g-1, respectively. Meanwhile, at a maximum current density of 30 A g-1, the samples grown



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with the DC voltages of -1.0, -1.2 and -1.4 V exhibited the Csc values of 1310.1, 1685.3 and 821.3 F g-1, respectively. The rate capabilities under high current density were 73.9, 77.1 and 71%, respectively. Noteworthy, the synthesized β-Ni(OH)2 NSs on SS foil substrate with an ED voltage of -1.2 V exhibited the relatively lower Csc of 414.5 F g-1 at a current density of 5 A g-1 than the β-Ni(OH)2 NSs on NWCTs (Figure S4(f)). Herein, hierarchical nanonetwork of βNi(OH)2 NSs on NWCTs (-1.2 V sample) allows the electrolyte ions into its interior parts rapidly and accelerate the faradaic redox reactions. On the other hand, the porous and 3D fibrous framework of NWCTs allows the growth of β-Ni(OH)2 NSs with strong adhesion, which provides the rapid electronic transfer channels and exhibits more active reaction sites with the electrolyte solution. Therefore, the β-Ni(OH)2 NSs on NWCTs participated in fast redox reactions with increased energy storage properties compared to the rigid or bulky surfaceconsisted conductive foil electrodes. Furthermore, the obtained energy storage properties of the β-Ni(OH)2 NSs on NWCTs in the present work are higher or comparable to the previous reports based on Ni(OH)2 nanostructures for pseudocapacitors (Table 1S).48-57

To examine the electrochemical conductivity behaviors of β-Ni(OH)2 NSs synthesized under various DC voltages, the EIS analysis was also investigated. Here, the EIS analysis was performed in 1 M KOH electrolyte solution with an AC perturbation of 5.0 mV in a frequency range of 0.01 to 100 kHz. As shown in Figure 8(a), all the spectral curves display two distinct parts: one is a negligible semicircle in the higher frequency region and the other is a sloped line in the lower frequency region. This semicircle may be ascribed to the faradaic redox reactions in the electroactive material and it gives the information about charge transfer resistance (Rct) at the electrode/electrolyte interface. And, the straight line from the sample shows the Warburg impedance (Zw), which is due to the ionic diffusion or penetration of electrolyte within the 16 ACS Paragon Plus Environment

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electroactive material. The real axis intercept represents bulk solution resistance (Rs) which includes the electrolyte resistance, intrinsic resistance of current collector and contact resistance of the electroactive material with the current collector. It can be clearly seen that the hierarchically grown β-Ni(OH)2 NSs on NWCTs at -1.2 V DC voltage exhibited relatively lower Rs and Rct values than those of other samples, confirming the superior electrochemical conductivity of the optimized sample. For practical applications, the long-life span under the extended cycles is also one of the main parameters that determine the suitability of the electroactive material in energy storage devices. Figure 8(b) shows the cycling stability of the βNi(OH)2 NSs on NWCTs under a current density of 20 A g-1 in 1 M KOH electrolyte solution. It is noted that the β-Ni(OH)2 NSs on NWCTs exhibited the capacity retention of 95% after 1000 cycles. From the inset of Figure 8(b), the initial 20 cycles of CD test further confirm the symmetrical charge-discharge behavior without any deviation. Such good cyclic stability is probably ascribed to the strong contact of β-Ni(OH)2 NSs on NWCTs during the cycling process. In order to elucidate in detail about the excellent cyclic stability of the β-Ni(OH)2 NSs on NWCTs, we performed the FE-SEM and XPS measurements after 1000 cycles (Figure S5). From the FE-SEM images in Figure S5(a), it can be clearly noticeable that there are no significant variation in the surface morphology of β-Ni(OH)2 NSs on NWCTs even after longterm cycling, which designates no dissolution or aggregation of the active electrode material. Additionally, the XPS analysis is also carried out to determine the chemical compositions of the β-Ni(OH)2 NSs on NWCTs after cycling. The XPS survey scan spectrum displayed in Figure S5(b), Ni and O atoms were only observed, indicative of no further impurities or unknown reactions of β-Ni(OH)2 with electrolyte solution during the cycling process. Thus, the superior pseudocapacitive properties of the β-Ni(OH)2 NSs on NWCTs even under high current density



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can be attributed to the following advantageous properties as shown in Figure 8(c). (i) The lowcost NWCTs which was weaved with disorderly arranged Cu/PET fibrous framework allows the facile and fast growth of β-Ni(OH)2 NSs by ECD method and acts as a flexible current collector for the fast electron transportation. (ii) The as-prepared pseudocapacitive electrode eliminates the use of conductive carbon and polymer binders. (iii) The highly porous and hierarchical nanonetwork of β-Ni(OH)2 NSs on NWCTs promotes the efficient accessibility of electrolyte ions within its interior parts, which results in the rapid electrochemical reactions, thus leading to the outstanding charge storage properties. Concerning the above merits, the β-Ni(OH)2 NSs on NWCTs can be effectively used as a promising candidate for high-performance flexible energy storage applications.

CONCLUSIONS

Hierarchical β-Ni(OH)2 NSs on NWCTs have been facilely prepared by using a low-temperature based ECD method. By applying a DC voltage of -1.2 V for 20 min to the working electrode in the growth solution, the highly conductive NWCTs allows to grow the β-Ni(OH)2 NSs with reliable adhesion. The as-grown β-Ni(OH)2 NSs on NWCTs exhibited the 3D hierarchical porous nanonetwork. Such preferential morphologies of the fabricated flexible electrode enabled to deliver the superior electrochemical properties, indicating an ultrahigh Csc of 2185.6 F g-1 at 5 A g-1, superior cycling stability of 95% at 20 A g-1 and a good electrochemical conductivity. This facile and fast growth of metal hydroxide/oxide nanostructures on low-cost conductive textilebased electrode is desirable for the fabrication of flexible and wearable energy storage devices.

Acknowledgments


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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068407). Authors Contribution: ‡ These authors contributed equally to this work. Supporting Information Available: Supplementary material (FE-SEM images of the NWCTs and β-Ni(OH)2 NSs on SS foil, electrochemical properties of β-Ni(OH)2 NSs on SS foil, comparative electrochemical performance of β-Ni(OH)2 NSs coated NWCTs with previous reports and XPS analyses of the material after cycling test) is available in the online version of this article at http://pubs.acs.org. References 1. Gwon, H.; Kim, H.-S.; Lee, K. U.; Seo, D.-H.; Park, Y. C.; Lee, Y.-S.; Ahn, B. T.; Kang, K., Flexible Energy Storage Devices Based on Graphene Paper. Energy Environ. Sci. 2011, 4, 1277-1283. 2. Liao, Q.; Zhang, Z.; Zhang, X.; Mohr, M.; Zhang, Y.; Fecht, H.-J., Flexible Piezoelectric Nanogenerators Based on a Fiber/Zno Nanowires/Paper Hybrid Structure for Energy Harvesting. Nano Res. 2014, 7, 917-928. 3. Yu, H.; Zhu, C.; Zhang, K.; Chen, Y.; Li, C.; Gao, P.; Yang, P.; Ouyang, Q., ThreeDimensional Hierarchical Mos2 Nanoflake Array/Carbon Cloth as High-Performance Flexible Lithium-Ion Battery Anodes. J. Mater. Chem. A. 2014, 2, 4551-4557. 4. Liu, L.; Yu, Y.; Yan, C.; Li, K.; Zheng, Z., Wearable Energy-Dense and Power-Dense Supercapacitor Yarns Enabled by Scalable Graphene-Metallic Textile Composite Electrodes. Nat Commun. 2015, 6. 5. Zhang, Y.-Z.; Zhao, J.; Xia, J.; Wang, L.; Lai, W.-Y.; Pang, H.; Huang, W., Room Temperature Synthesis of Cobalt-Manganese-Nickel Oxalates Micropolyhedrons for HighPerformance Flexible Electrochemical Energy Storage Device. Scientific Reports. 2015, 5, 8536. 6. Cheng, Y.; Zhang, H.; Lu, S.; Varanasi, C. V.; Liu, J., Flexible Asymmetric Supercapacitors with High Energy and High Power Density in Aqueous Electrolytes. Nanoscale 2013, 5, 1067-1073. 7. Lai, L., et al., Preparation of Supercapacitor Electrodes through Selection of Graphene Surface Functionalities. ACS Nano 2012, 6, 5941-5951. 8. Armaroli, N.; Balzani, V., Towards an Electricity-Powered World. Energy Environ. Sci. 2011, 4, 3193-3222. 9. Lu, X.; Yu, M.; Zhai, T.; Wang, G.; Xie, S.; Liu, T.; Liang, C.; Tong, Y.; Li, Y., High Energy Density Asymmetric Quasi-Solid-State Supercapacitor Based on Porous Vanadium Nitride Nanowire Anode. Nano Lett. 2013, 13, 2628-2633.



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Figure 1. Schematic diagram for the fabrication of hierarchical β-Ni(OH)2 NSs on NWCTs by ECD method.



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Figure 2. (a) Photographic image of the synthesized β-Ni(OH)2 NSs on NWCTs, demonstrating its high flexibility without any surface damages, and (b-d) FE-SEM images of the hierarchical βNi(OH)2 NSs on NWCTs at a DC voltage of -1.2 V for 20 min.


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(a)

β -Ni(OH)2 #14-0117

(1 1 1)

Cu #85-1326

(2 0 0)

β -Ni(OH)2 NSs

on CTs (2 2 0)

(2 0 1)

(1 1 1)

(1 0 2)

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PET

Bare NWCTs

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1648 cm-1 1383 cm-1 3432 cm-1

551 cm-1

3637 cm-1 4000 3500 3000 2500 2000 1500 1000

500

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Wavenumber (cm )

Figure 3. (a) XRD patterns and (b) FT-IR spectrum of the β-Ni(OH)2 NSs on NWCTs.



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Figure 4. (a-c) TEM images, (d-e) elemental mapping images and (f) EDX spectrum of the βNi(OH)2 NSs after separating from the NWCTs.


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Figure 5. FE-SEM images of the synthesized β-Ni(OH)2 NSs on NWCTs under different DC voltages of (a) -0.8 V, (b) -1.0 V and (c) -1.4 V for 20 min.



0.06

(a)

-1.2 V

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R = 0.99821

0.03

Cathodic Andic Lenior fit Lenior fit

0.00 -0.03 -0.06 R = 0.99832 -0.09 0.05

0.10

0.15

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-1/2 -1/2

(V

s

)


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Figure 6. (a) Comparison of CV curves for the prepared samples under different applied DC voltage with a constant scan rate of 30 mV s-1 and (b) CV curves of the β-Ni(OH)2 NSs on NWCTs (-1.2 V) at different scan rates of 5 to 70 mV s-1. (c) Plot of the anodic and cathodic peak current values vs. square root of the scan rates.

(a)

-1

-1.0 V

5Ag -1 7Ag -1 10 A g -1 15 A g -1 20 A g -1 25 A g -1 30 A g

0.4 0.3

(b)

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Figure 7. CD curves of the sample synthesized under different DC voltages of (a) -1.0 V, (b) 1.2 V and (c) -1.4 V for 20 min. (d) Calculated Csc values as a function of current density for the corresponding samples.



1800 (b)

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900 600 300

(a)

-1

I = 20 A g

0.4 0.3 0.2 0.1 0.0 0

250

500

4

8

12

16

20

1000

1250

1500

1750

Time (s)

0 0

750

0

200

400

600

800

1000

Cycle Number

Z' (ohm)

Figure 8. (a) Nyquist plots of the β-Ni(OH)2 NSs on NWCTs under various DC voltages of -1.0, -1.2, -1.4 V and (b) long-term cycling process of the optimized growth condition sample (-1.2 V) at a current density of 20 A g-1. (c) Schematic diagram showing the penetration of electrolyte ions and transfer of electrons through the β-Ni(OH)2 NSs on NWCTs for electrochemical energy storage.


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Controlled Electrochemical Synthesis of Nickel Hydroxide Nanosheets

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