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DOI: 10.1039/C3RA46387G (Paper) RSC Adv., 2014, 4, 6324-6329

Conformal coating of ultrathin Ni(OH)2 on ZnO nanowires grown on textile fiber for efficient flexible energy storage devices†

Imran Shakirac, Zahid Alia, Jihyun Baeb, Jongjin Park*a and Dae Joon Kang*a
aDepartment of Physics, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea. E-mail: djkang@skku.edu; jongjin00.park@samsung.com; Fax: +82-31-290-5947; Tel: +82-31-290-5906
bFrontier Research Laboratory, Samsung Advanced Institute of Nanotechnology, Yongin, Republic of Korea
cThe Sustainable Energy Technologies Center, King Saud University, PO-BOX 800, Riyadh, Saudi Arabia

Received 4th November 2013 , Accepted 23rd December 2013

First published on 3rd January 2014

Abstract

A highly flexible supercapacitor is fabricated through a simple solution-based method in which conformal ultrathin (2 nm) nickel hydroxide (Ni(OH)2) layer is deposited on vertically grown zinc oxide (ZnO) nanowires on a three-dimensional, highly conductive textile substrate. The conformal ultrathin Ni(OH)2 layer enables a fast and reversible redox reaction which improves the specific capacitance by utilizing the maximum number of active sites for the redox reaction, while vertically grown ZnO nanowires on wearable textile fibers effectively transport electrolytes and shorten the ion diffusion path. The Ni(OH)2 coated ZnO nanowire electrodes show a high specific capacitance of 3150 F g−1 in a 1 M LiOH aqueous solution. Moreover, the asymmetric electrochemical capacitors with Ni(OH)2-coated ZnO nanowires as the positive electrode and multiwall carbon nanotubes-textile as the negative electrode exhibit promising characteristics with a maximum power density of 110 kW kg−1, an energy density of 54 W h kg−1, and excellent cycling performance of ∼96% capacitance retention over 5000 cycles.

1. Introduction

The rapid development in flexible and stretchable electronic devices during last few years poses new challenges for the design and fabrication of compatible wearable energy storage devices.1–6 Among the various energy storage devices, electrochemical supercapacitors have attracted much attention in the field of light weight, ultrathin energy management devices for wearable electronics due to their high power density, long life cycles, and high efficiency.3–5 Various carbon based materials such as carbon nanotubes (CNTs) or graphene nanosheets, and polymerization of conducting polymers such as polypyrrole have been employed to make conductive papers and textiles which can act as electrodes and/or current collectors for flexible supercapacitors.6–8 However, these polymers have low mechanical strength, and more importantly, low conductivity and current carrying capacity compared to CNTs. On the other hand, CNTs-incorporated papers and textiles made by low cost and high-speed solution-based processes exhibit exciting properties,9–11 but the amount of CNTs impregnated into the fabric vary from section to section, limiting the types of fabrics that can be used as textile electrodes to realize flexible energy storage applications. The future development of higher performance energy storage devices not only depends on the employment of new and lighter electrode materials, but also on the development of new materials to use as substrates.12,13 Here we highly conductive textile substrate based on LBL deposited metal on non-woven wiper because of its soft and robust mechanical characteristics. Furthermore, we attempted to solve the major technical challenges of attaining high capacitance and energy density of metal hydroxides by designing a simple and scalable approach to deposit ultrathin metal hydroxide Ni(OH)2 (∼2 nm thick) on vertically grown metal oxide ZnO nanowires on a wearable conducting textile fiber. This approach enabled us to achieve high specific capacitance and a long-term life cycle for flexible supercapacitor electrode applications. The use of ZnO nanowires provides several advantages over commonly used CNTs such as easy synthesis with extremely low cost on any desired substrate, biocompatibility and being environmentally friendly. ZnO nanowires enable the full utilization of ultrathin Ni(OH)2, allowing fast electric and ionic conduction through the electrode due to its relatively good electrical conductivity.

2. Experimental methods

For the growth of ZnO nanowires, gold coated fabric was dip coated with 5 mM zinc acetate dehydrate in ethanol three to four times and then heated on a hot plate at 170 °C for 10 minutes. Next, the seeded fabric was hung in a glass bottle containing 200 mL aqueous solution of 12 mM zinc nitrate hexahydrate and 12 mM hexamethylenetetramine (HMTA). The bottle was then heated in a furnace at 95 °C. After the hydrothermal reaction, the fabric was rinsed with deionized (DI) water and dried on a hot plate. Conformal coating of Ni(OH)2 at a controlled thickness on vertically grown ZnO nanowires was achieved as follows. For a coating of 2 nm-thick Ni(OH)2, 0.1 mM of a Ni(NO3)2 aqueous solution was prepared and stirred for 20 minutes. Stirring continued during the addition of 0.1 mL of 0.01 mM of NH4(OH)2 at regular intervals to allow a very slow hydrolysis of nickel nitrate. The solution was continuously stirred for three days and then a textile fiber substrate containing ZnO nanowires was immersed in the solution for 4 hours. Finally, the Ni(OH)2-coated ZnO nanowire substrate was washed several times with DI water, dried at 80 °C for one day, and further characterized. To achieve a 5 nm or 20 nm-thick coating of Ni(OH)2 on ZnO nanowires, 0.5 or 1 mM Ni(NO3)2 was used respectively as an aqueous solution. Electrochemical measurements were carried out in a conventional three-electrode cell with a CHI-660B electrochemical station (CH Instruments Inc., TX, USA). Ni(OH)2-coated ZnO nanowires as a working electrode while Ag/AgCl (saturated with KCl)electrode and a Pt wire were used as reference and counter electrode, respectively, in an aqueous electrolyte solution of 1 M LiOH. The specific capacitance (Cs, F g−1) of the electrode was calculated from the CVs and the discharge capacitance from galvanostatic charge–discharge curve according to the following equations:
Cs = i/(V × m)
Where i is the current density (A) in the CV anodic branch; V is the scanning rate (V s−1) and m is the mass of active material on the electrode.
Cs = (i × t)/(ΔV × m),
where i is the discharge current, t is the discharge time, ΔV is the potential window, m is the mass of the active materials.

Energy and power density (E, P) of the electrodes were calculated according to the following equations

E = 1/2CtotalV2 and P = 2E/unit time,
where Ctotal is the total capacitance of the cell.

3. Results and discussion

To prepare a flexible supercapacitor electrode, a polyester woven fabric was chosen as a flexible substrate. Electroless plating was employed to deposit Ni/Cu/Ni/Au layers sequentially on the polyester woven textile fiber fabric which was used as a flexible substrate. The polyester textile fiber cloth possesses features such as (a) low water absorbability and retention; (b) a high degree of softness, cleanliness, and resistance of the surface to scratching; (c) absence of free and chemical binders. To confer conducting properties, Ni/Cu/Ni/gold was deposited on the polyester fabric surface (as the large electrical conductivity of gold along with ample mechanical strength makes metal coated textile fiber an ideal candidate for a substrate for flexible energy storage devices). We believe that metal-coated textile fiber provides the surface with a uniform aperture which ensures easy transfer in the fabrication process, leading to good conductivity of ions and electrons. The flexible supercapacitor was fabricated through a two-step approach. First, ZnO nanowires were grown on conducting textile fiber substrate through a simple and low-cost hydrothermal method to support the large-area deposition of Ni(OH)2. We selected ZnO as the host material for the deposition of Ni(OH)2 because it can be easily fabricated at an extremely low cost14,15 and has relatively good electrical conductivity, providing a natural pathway for electron transport. In the second step, conformal coating of ultrathin layers of Ni(OH)2 were deposited on the surface of the ZnO nanowires via a co-precipitation method (experimental details are provided in the Methods section) resulting in ZnO nanowire/Ni(OH)2 composite nanostructures. A schematic illustration of the deposition of conformal coating of ultrathin Ni(OH)2 on ZnO nanowires is shown in Fig. 1.
image file: c3ra46387g-f1.tif
Fig. 1 Schematic illustration of the fabrication process of ultrathin Ni(OH)2 onto ZnO nanowires grown on Ni/Cu/Ni/gold-coated porous textile fiber for a flexible supercapacitor.

Fig. 2(a–c) presents a low-magnification field emission scanning electron microscopy images showing conducting textile fiber covered with ZnO nanowires, with a close-up view in Fig. 2(d). The textile fiber substrate is highly flexible and the ZnO nanowires synthesized on the textile fiber have a typical hexagonal flat-ended morphology. As grown, ZnO nanowires have an average diameter of 20 nm with lengths (height) of about 2 μm.


image file: c3ra46387g-f2.tif
Fig. 2 (a and b) Low-resolution SEM images of textile fiber substrate covered with vertically grown ZnO nanowires, (c) low-resolution SEM image of one fiber covered with ZnO nanowires, and (d) higher-magnification SEM image of ZnO nanowires grown on the fiber.

The crystallinity of the ZnO and Ni(OH)2 coated ZnO nanowires was confirmed by X-ray diffraction (XRD), as shown in Fig. 3(a). XRD patterns of the Ni(OH)2/ZnO nanowires show three well-resolved peaks that can be indexed as (100), (110), and (211) reflections associated with the hexagonal crystal structure of ZnO. The additional peaks at 19.6° and 38.7°, attributed to the (001) and (101) planes of the hexagonal phase of typical Ni(OH)2 (JCPDS-14-0117), indicate the presence of nickel hydroxide. To examine the uniformity, thickness, and crystal structure of the Ni(OH)2 layer deposited on ZnO nanowires, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) analysis were employed. The electron diffraction pattern in Fig. 3(b) is composed of two sets of SAED patterns. The bright diffraction pattern spots confirm the hexagonal single crystalline nature of the ZnO nanowires, which is in good agreement with our XRD results. An additional weak ring pattern is attributed to the polycrystalline Ni(OH)2 nanoparticle layer on the surface of the ZnO nanowire. HRTEM images for ZnO nanowires subjected to 0.1 mM and 0.5 mM NiNO3 solutions are shown in Fig. 3(c and d). These images confirm the uniform and ultrathin 2 nm deposition of Ni(OH)2 on ZnO nanowires for the 0.1 mM NiNO3 sample, and the uniform 5 nm coating of Ni(OH)2 for the 0.5 mM NiNO3 sample. These results confirm that we can easily control the thickness of Ni(OH)2 deposited on ZnO nanowires by varying the ratio of starting precursor. The surface area of the pristine and Ni(OH)2 deposited ZnO nanowires was determined from BET measurement (Coulter SA3100) using nitrogen adsorption. The pristine ZnO nanowires showed a surface area of 15 m2 g−1, whereas Ni(OH)2 coated ZnO nanowires exhibited 23 m2 g−1. We believe that the higher surface area of Ni(OH)2 coated ZnO nanowires as compared to pristine ZnO nanowires are ascribed to the excellent electrochemical performance as compared to bare ZnO nanowires.


image file: c3ra46387g-f3.tif
Fig. 3 (a) XRD patterns of textile fiber substrate, ZnO nanowires, and Ni(OH)2-coated ZnO nanowires, (b) SAED with the inset showing TEM of nanowires, and (c and d) HRTEM images of Ni(OH)2-coated ZnO nanowires.

The electrochemical redox process in Ni(OH)2-coated ZnO nanowires was examined through cyclic voltammetry using 1 M LiOH as an electrolyte at different scan rates. Fig. 4(a) shows the cyclic voltammograms (CVs) of the various thicknesses of Ni(OH)2 deposited on ZnO nanowire electrodes at a scan rate of 5 mV s−1. A rapid increase in current level was observed for 2 nm Ni(OH)2 deposited on the ZnO nanowires, which gradually decreased to its minimum value at a thickness of 20 nm. A very thin layer of Ni(OH)2 (2 nm) on conducting ZnO nanowires gives rise to a very high surface-area-to-volume ratio which results in the full utilization of Ni(OH)2, resulting in very high pseudocapacitive behavior of the Ni(OH)2 layer. As we increased the thickness of Ni(OH)2, its utilization lessened, which consequently decreased the specific capacitance of the electrode material. The electrode with a 2 nm-thick layer of Ni(OH)2 on ZnO nanowires showed the highest specific capacitance of 3150 F g−1 at a scan rate of 5 mV s−1 (Fig. 4(c)) S3 compared to just 1450 F g−1 for a 20 nm Ni(OH)2 electrode under the same conditions (based on Ni(OH)2 weight). This increase in capacitance as compared to reported values was due to the uniform coating of Ni(OH)2 on ZnO nanowires as schematically shown in Fig. 1. This was effective in increasing the mass loading of the Ni(OH)2 considerably, while maintaining contact with ZnO and providing a good electrical conduction path as it was deposited along the conducting path which allowed intrinsically good contact with the ZnO. For comparison, we also studied the electrochemical properties of pristine ZnO nanowires under the same conditions and found that ZnO nanowires showed the specific capacitance of 35 F g−1 at a scan rate of 5 mV s−1.


image file: c3ra46387g-f4.tif
Fig. 4 (a) Cyclic voltammograms (CV) of Ni(OH)2-coated ZnO nanowires of various thicknesses ranging from 2 nm to 20 nm at a scan rate of 5 mV s−1 in a 1 M aqueous LiOH solution at room temperature, (b) CV of 2 nm Ni(OH)2-coated ZnO nanowire electrode at various scan rates from 5 to 100 mV s−1 in a 1 M aqueous LiOH solution at room temperature, (c) specific capacitance variation of 2 nm Ni(OH)2-coated ZnO nanowire electrodes at different scan rates, and (d) CV curves of 2 nm Ni(OH)2-coated ZnO nanowire electrodes under normal test and bending conditions, showing no apparent changes for the electrode that was mechanically bent at different angles such as 0, 30, 60 degrees.

To understand the role of the metal coated textile fiber substrate, a simple test was performed to compare Ni (OH)2-coated ZnO nanowires on fiber and Indium tin oxide (ITO) electrodes. At a scan rate of 50 mV s−1, the electrochemical performance of Ni(OH)2 coated ZnO nanowires on textile fiber substrate outperforms that of an ITO electrode in terms of wider voltage range and higher capacitance. For 2 nm Ni(OH)2 deposited on ZnO nanowires grown on gold-coated textile fiber, a very high specific capacitance of 3150 F g−1 can be achieved at a scan rate of 5 mV s−1. However, the value is just 1510 F g−1 for an ITO electrode under the same conditions (see Fig. S3(a)†). This control experiment further confirms the outstanding electrochemical performance of Ni(OH)2-coated ZnO nanowires on a gold-coated textile fiber electrode. This increase in the specific capacitance with respect to the mass of Ni(OH)2 at the same scan rate is likely mainly due to the high utilization of well-dispersed Ni(OH)2 on the large surface area metal coated textile current collector which leads to good conductivity of ions and electrons. For the same mass loading, the thickness of Ni(OH)2 on textile fibers is much smaller than on the ITO substrate, which largely facilitates ion and electron transport in the electrode materials. This study clearly illustrates that the Ni(OH)2-coated ZnO nanowires grown on textile fiber is better than a flat metal current collector for pseudocapacitor applications. To quantify the rate performance and redox processes, cyclovoltammetry experiments were carried out at various sweep rates for Ni(OH)2-deposited ZnO electrodes. The current increased slowly with increasing potential and the CV curve of the Ni(OH)2-deposited ZnO electrode at high scan rate was highly distorted, showing a poor rate capability (Fig. 4(b)).

This phenomenon may be due to the slow diffusion of OH− ions into the Ni(OH)2 on the ZnO nanowire electrode. At high scan rates, diffusion limits the movement of OH− ions due to time constraints, and only the outer active surface is utilized for charge storage, whereas at low scan rates all the active surface area can be utilized for charge storage, leading to higher specific capacitance. It can be seen from Fig. 4(b) that, at a lower scan rate, the CV curve of the 2 nm Ni(OH)2 on ZnO showed charging and discharging taking place at a constant rate over the applied voltage. However, the specific capacitance of the electrode was found to decrease with increasing scan rate, and this phenomenon may be due to the slow diffusion of OH− ions into the Ni(OH)2 on the ZnO nanowire electrode. At high scan rates, diffusion limits the movement of OH− ions due to time constraints, and only the outer active surface is utilized for charge storage, whereas at low scan rates the entire active surface area can be utilized for charge storage, leading to higher specific capacitance.16–19 Consistently, we have also found that the specific capacitance of 2 nm-thick Ni(OH)2 deposited on ZnO nanowires is much higher (3150 F g−1) than that of the 20 nm thick sample (1450 F g−1) at the same scan rate. Furthermore, the former shows good rate capability in a 1 M LiOH aqueous solution, as shown in Fig. 4(a). The capacitance of the 2 nm-thick Ni(OH)2 on ZnO nanowire sample is much higher than Ni(OH)2 nanoflakes deposited on carbon nanotube networks.20,21 These results indicate that our electrode of 2 nm-thick Ni(OH)2 deposited on ZnO nanowires is a highly flexible, low cost and environmentally friendly material as compared to CNTs, which can be applied even in harsh environments such as folding/twisting conditions. The fabricated supercapacitors were flexible and the flexible nature of Ni(OH)2-deposited ZnO nanowire electrode was investigated under folding/twisting conditions. It was found that there are no significant variations in specific capacitance, power and energy density of the supercapacitor with or without bending. The CVs in Fig. 4(d) of the normal test and bending conditions were the same and no apparent changes were observed even when the Ni(OH)2-deposited ZnO nanowire electrode composite was mechanically bent to different angles. These results may indicate that mechanical bending has little influence on the ionic transport in electrolyte solution or the quality of interface between the electrolyte layer and the Ni(OH)2-coated ZnO nanowires at least up to 60 degree. Long-term chemical and electrochemical stability of the Ni(OH)2-deposited ZnO electrode was examined by CV at a scan rate of 20 mV s−1 for 5000 cycles, and the corresponding results are presented in Fig. 5(a). The decay in capacity was only 2% after 5000 cycles, indicating that the supercapacitor is very stable, which is a requirement for industrial application of flexible energy storage devices. To further study the specific capacitance of Ni(OH)2-deposited ZnO through this simple solution-based approach, galvanostatic charge–discharge curves were recorded in the same working cells. Fig. 5(b and c) shows that the 2 nm Ni(OH)2 sample had an ultrafast charge/discharge rate with good electrochemical reversibility at a specific current of 10 A g−1 compared to other electrodes, indicating again that the composite has a good electrochemical capacitive characteristic and superior reversible redox reaction. Rate capability is another important factor for industrial use of supercapacitors in power applications. Therefore, the effects of changes in current density on the specific capacitance of the samples prepared via a bottom up approach were also measured, as shown in Fig. 5(d). We observed that the 2 nm-deposited sample retained 80% of its specific capacitance as the current density increased from 10 to 50 A g−1. However, the specific capacitance values of other samples with thicker layers of Ni(OH)2 are not only much lower than that of the 2 nm sample, but also decreased significantly with increased current densities as shown in Fig. 5(d). This superior rate capability of the 2 nm-deposited Ni(OH)2 electrode can be attributed to the ultra-thin film of Ni(OH)2, which greatly reduces the diffusion and migration paths of OH− ions, thereby facilitating ion insertion/extraction during the rapid charge/discharge process due to complete electrochemical utilization of the Ni(OH)2 layer. Consistently, when the thickness of Ni(OH)2 was increased to 20 nm, the electrochemical performance of the electrode was found to be decreased, which can be attributed to lower conductivity, lower accessibility to the electrolyte solution, and higher electron transport resistance. Another result that demonstrates the superior electrochemical performance of the Ni(OH)2-coated ZnO nanowire textile electrodes is its cyclic stability behavior compared to the flat ITO substrate. There was only a 2% decrease in capacitance after 6000 cycles for the textile electrode, compared to 15% for the ITO electrode, which may be due to significant peeling of the rigid ITO substrate.


image file: c3ra46387g-f5.tif
Fig. 5 (a) Cyclic voltammetry of the first cycle and after 5000 cycles of 2 nm Ni(OH)2-coated ZnO nanowire electrode with a 20 mV s−1 scan rate in a 1 M aqueous LiOH solution, (b) galvanostatic charge–discharge curves of various Ni(OH)2 thicknesses ranging from 2 nm to 20 nm at a current density of 10 A g−1 in a 1 M aqueous LiOH solution at room temperature, (c) the first few cycles of charge–discharge for 2 nm Ni(OH)2-coated ZnO nanowires at a current density of 10 A g−1, and (d) specific capacitance variation of 2 nm Ni(OH)2-coated ZnO nanowire electrode at different current densities.

To increase the working voltage window of the fabricated supercapacitor, we constructed an asymmetric supercapacitor using a Ni(OH)2-coated ZnO nanowire-based anode and an AC-based cathode. The ratio of the mass of the anode and cathode was kept fixed around 4.3 to obtain maximum energy density.15 The charge discharge curve of the asymmetric supercapacitor shows a rectangular shape with a large potential range from 0 to 2 V which is one of the most critical parameters to enhance the energy density of the device (Fig. S3(b)†). Electrochemical performance of the electrochemical cells was further investigated by calculating the power density and energy density of the as-prepared electrodes. The energy density reached 54 W h kg−1 at a power density of 110 W kg−1, and was reduced to 15 W h kg−1 at a power density of 1 kW kg−1, confirming that Ni(OH)2 deposited on ZnO nanowires through this simple solution-based approach is a promising electrode material for supercapacitors. We compared the energy density of the 2 nm-thick Ni(OH)2 electrode to that of other reported CNTs composites and metal oxide electrodes and found that the energy density obtained in our study is comparable or higher for supercapacitors using CNT composites and metal hydroxide electrodes.16,19–21 This improvement in energy density without compromising power density suggests that this supercapacitor, made by a simple and straightforward bottom-up approach, is an ideal candidate for a wide range of device applications such as connected health care devices, outdoor sportswear, and monitors and information processing devices for military, police and firemen.

4. Conclusions

In summary, a simple and cost-effective bottom-up approach was developed to uniformly coat an ultrathin film of Ni(OH)2 on vertically grown ZnO nanowires on a conducting textile fiber substrate, providing drastic improvement in electrochemical storage device performance. The conducting textile fiber provides a platform for dense growth of ZnO nanowires which leads to a high surface area for the deposition of conformal coatings of ultrathin Ni(OH)2 layers, and also improves ionic conductivity and mechanical stability of the metal hydroxide thin film. The 2 nm-deposited Ni(OH)2 sample exhibits a record-high specific capacitance, cycling stability, ultrafast charge discharge rate, and excellent energy and power density, making it one of the most promising electrodes for high performance large-scale energy storage applications.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0094023).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: EDXA of substrate, element mapping, cyclic voltammetry (CV) of 2 nm Ni(OH)2-coated ZnO nanowires electrode on ITO substrate and galvanostatic charge–discharge curves of Ni(OH)2-coated ZnO nanowires based asymmetric supercapacitor at discharge current density of 1 A g−1 in a 1 M aqueous LiOH solution at room temperature can be found. See DOI: 10.1039/c3ra46387g
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