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Flexible fiber-shaped supercapacitor based on nickel-cobalt double hydroxide and pen ink electrodes on metallized carbon fiber Libo Gao, Surjadi James Utama, Ke Cao, Hongti Zhang, Peifeng Li, Shang Xu, Chenchen Jiang, Jian Song, Dong Sun, and Yang Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16101 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Flexible fiber-shaped supercapacitor based on nickel-cobalt double hydroxide and pen ink electrodes on metallized carbon fiber

Libo Gao1, Surjadi James Utama1, Ke Cao1, Hongti Zhang1,2, Peifeng Li1, Shang Xu1,2, Chenchen Jiang1, Jian Song1, Dong Sun1 and Yang Lu1,2* 1

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong SAR, Kowloon 999077, Hong Kong;

2

Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, China.

ABSTRACT：：Flexible fiber-shaped supercapacitors (FSSCs) are recently of extensive interest for portable and wearable electronic gadgets. Yet the lack of industrial-scale flexible fibers with high conductivity and capacitance and low cost greatly limits its practical engineering applications. To this end, we here present pristine twisted carbon fibers (CFs) coated with a thin metallic layer via electroless deposition route, which exhibits exceptional conductivity with ～ 300% enhancement and superior mechanical strength (~1.8 GPa). Subsequently, the commercial available conductive pen ink modified high conductive composite fibers, on which uniformly covered ultrathin nickel-cobalt double hydroxides (Ni-Co DHs) was introduced to fabricate flexible FSSCs. The synthesized functionalized hierarchical flexible fibers exhibit high specific capacitance up to 1.39 F·cm-2 in KOH aqueous electrolyte. The asymmetric solid-state FSSCs show maximum specific capacitance of 28.67 mF·cm-2 and energy density of 9.57 µWh·cm-2 at corresponding power density as high as 492.17 µW·cm-2 in PVA/KOH gel electrolyte, with demonstrated high flexibility during stretching, demonstrating their potentials in flexible electronic devices and wearable energy systems.

KEYWORDS：：flexible supercapacitor; nickel-cobalt double hydroxides; wearable electronics; pen-ink electrode; carbon fiber

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INTRODUCTION Flourishing portable and lightweight energy storage devices have increasingly attracted much attention in order to meet the urgent demands of modern flexible and wearable gadgets, such as the smartphone, roll-up displays and photovoltaic cells.1–4 Among various energy storage devices, flexible fiber-shaped supercapacitors (FSSCs) have recently been considered as one of the most popular candidates, borne from their capability of easily assembling into other different structures for design innovation in flexible electronics, owing to its tiny volumes, mechanical flexibility and high capacitance density.5–9 Metal wires (MWs) are typically introduced as the substrate and scaffold for growing electrode materials for FSSCs mainly based on their high conductivity.4,10–12 Among them, nickel wire, titanium wire, and copper wire have already been extensively explored. Zhu et al. have recently used Fe-Co-Ni (Kovar) alloy MWs as substrates and catalysts for carbon nanofiber growth with pre-deposition of NiO nanowalls. The assembled FSSCs reached a high value of 12.5 mF·cm-2 at a current density of 10 µA·cm-1.12 Yang et al. also have reported that through dealloying of the Cu-Ni alloy (contra) MWs, the FSSC based on 3D porous nickel MW exhibits a high specific capacitance and high stability.13 Presently alloy MWs are seemingly promising candidates for FSSCs compared to conventional pure MWs. However, they still face the awkward situation that it is too difficult to easily knit them into textile simultaneously to guarantee the softness and flexibility of fabrics, due to their intrinsic rigidity and bulky heavy.5 In this regard, the CFs have been judged as optimal alternative current collectors because of their desirable flexibility, relatively high conductivity, low-cost and large-scale productivity in manufacturing applications.14,15 Additionally, as known, electrode materials can be directly deposited on CFs by electrochemical deposition (ELD) or a hydrothermal route. Senthilkumar et al. fabricated a novel FSSC using NiCo2O4@CF as the positive electrode and it is easily woven into a textile.15 Gao et al. reported a coaxial yarn electrodes composed of CFs grown MoS2, showing an improved capacity and an enhanced cycle life.16 Nevertheless, there is still much room for the improvement of the conductivity of CFs in comparison with the MWs and conventional single CF filament only offer a limited cylindrical surface area platform and poor porosity for higher material loading.17 In recent times, these problems have been partially addressed: Zhao et al. deposited the branching


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carbon nanotubes on the CFs, which dramatically increase the specific surface area compared with that of blank CFs.17 Wang et al. proved that a 1733-fold enhancement of areal capacitance with remarkable specific surface areas than that of pristine CFs via electrochemical H2SO4-HNO3 activation.18 Limitedly, these above methods require elaborate apparatuses and accurate operation technologies, which is difficult to achieve large-scale fabrication and more importantly the issues of conductivity still exist. To this end, it is still necessary and of great significance to unceasingly improve the conductivity and surface areas of CFs to achieve high supercapacitors performance while still intrinsically possessing high flexibility and mechanical property. To address this challenge, here, we report for the first time fabrication of a novel flexible FSSC owning high supercapacitor performance (28.67 mF·cm-2) with high energy (9.57 µWh·cm-2) and power density (492.17 µW·cm-2) based on Ni-Co DHs and pen ink electrodes on metallized CF. In this rational design, the nickel-coated CF with high conductivity and flexibility was regarded as current collectors while mature industrialized pen ink with porous structure provide large surface areas and skeleton supporting for growing Ni-Co DHs. Interestingly, the pen ink composed of graphite carbon nanoparticles also exhibit impressive capacitance,2,4,19 which was further used as the negative electrode material. The low-cost, high-performance FSSCs provide an alternative strategy toward efficient flexible storage devices and wearable energy management.

MATERIALS AND METHODS Fabrication of nickel-coated carbon fibers. The nickel-coated CF was prepared via the ED

methodology which is similar to the report elsewhere.20 Briefly, prior to deposition nickel layer, the CF thread with an average diameter of 300 µm was sequentially treated with acetone, alcohol and distilled (DI) water for 15 min in ultrasonication, respectively. The treated CF thread was then immersed into 10 g·L-1 of SnCl2 for 20 min and washed thoroughly with abundant of DI water before being dried at 70 ℃. After drying, the CF thread was immersed into an activation solution containing 0.25 g·L-1 of PdCl2 and 10 mL·L-1 of hydrochloric acids for 20 min. Finally, after being rinsed with DI water and dried at 70 ℃, the activated treating CF thread was immersed into the home-made plating bath kept at 90 ℃ for



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15 min to obtain the metallic structure in a water bath. The nickel plating solution bath contained 40 g·L-1 of NiSO4·5H2O, 20 g·L-1 of sodium citrate, 10 g·L-1 of lactic acid, and 1 g·L-1 of dimethylamine borane in water. Note that for the final electroless deposition, the prepared plating solution above would be in the dilution by 5 folds. Integration of Pen ink/Nickel/CF. The facile dip-coating method was introduced to fabricate

the Pen ink/nickel/CF.19 In a typical procedure, the prepared nickel-coated CF was immersed into pen ink (“HeroTM” carbonic ink, commercially available from Shanghai Ink Factory, China) solution for 5 min (the average thickness of the ink film is about 600 nm). It was then carefully taken out from the ink solution and put on a hot stage at 60℃ for 3 h. The expected thickness or mass of the pen ink on nickel-coated CF can be obtained through repeating this procedure several times. Coating of Ni-Co DHs on Pen ink/Nickel/CF. The Pen ink/Nickel/CF coated with Ni-Co DHs

was synthesized in a standard three-electrode system at 25 ℃, with the above prepared Pen ink/Nickel/CF as the working electrode, Pt wire as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode. The electrolyte contained 100 mL of 0.1 M metal ion solution with Ni2+/Co2+ in a concentration ratio of 1:2. The Ni-Co DHs nanosheets were deposited at the potential static with -1.0 V for 10 min, corresponding curves has been shown in Figure S1. The Ni-Co DHs/Pen ink/Nickel/CF can then be obtained after carefully washing it with DI water and drying for 30 min in air at 60 ℃. Assembly of solid fiber supercapacitor. The PVA/KOH gel electrolyte was synthesized as the

following procedure in Figure S2. 3 g of PVA was initially dissolved in 20 mL of DI water at 100 ℃ with vigorous stirring for 1 h, followed by adding 10 mL of KOH solution with concentration of 0.3 g·mL-1 drop by drop slowly. Subsequently, the PVA/KOH gel electrolyte was obtained with continuous stirring at 80 ℃ for 1 h. The positive and negative electrodes, which were the Ni-Co DHs/Pen ink/Nickel/CF and Pen ink/Nickel/CF respectively, were immersed in the prepared transparent gel electrolyte for 5 min, before being taken out and dried at room temperature. Finally, they were assembled together with PVA/KOH gel electrolyte and protected in the silicone tube and heated at 60 ℃ for 2 h, removing excess water in the electrolyte. No separator was used in this all-solid fiber supercapacitor. Microstructural and electrochemical characterization. The element analysis and morphology


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of the products were investigated by field-emission scanning electron microscope (FESEM, Quanta 450), transmission electron microscope (JEOL JEM 2100) equipped with energy dispersive spectroscopy (EDS) and select area electron diffraction (SAED), and X-ray powder diffractometer (XRD, RigakuSmartLab) with monochromatic Cu Ka radiation (l¼1.5418 Å). Tensile strength was measured by a micro-test machine (DEBEN) with an elongation speed of 1 mm·min-1. The conductivity of the fiber was conducted using the electrochemical workstation in two electrode mode. All of the electrochemical tests were performed on the electrochemical workstation (CHI 7660E, Chenhua). The electrochemical performances of the positive and negative electrode were both tested in 6 M KOH aqueous as the electrolyte. Pt wire and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. The solid fiber supercapacitor was tested via two electrode system. The capacitance of the electrode and solid fiber device can be calculated according to the following equation:

Ca =

I ∆t S ∆V

(1)

Cm=

I ∆t m∆V

(2)

where Ca (F·cm-2) and Cm (F·g-1) are the areal and specific capacitance, respectively. In equation (1), I (A) is the discharge current, ∆ t is the discharge time and ∆ V is the potential window. S is the efficient surface area (cm-2) and m (g) is the mass of the electrode materials in equation (2). The energy density and its corresponding power density was obtained according to the following equation:4,21

E = ∫ UIdt = I ∫ Udt

(3)

3600 × E t

(4)

P=

where E (Wh·kg-1) denotes the energy density of the FSSC while the P (W·kg-1) is the average power density, U (V) is the potential window of the supercapacitor, I (A) is the



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constant discharge current and t (s) is the discharge time.

RESULTS AND DISCUSSIONS Fabrication of Ni-Co DHs/Pen ink/Nickel/CF

Figure 1a schematically illustrates the procedure of fabrication of the solid-state asymmetric FSSC. In our work, CF thread with a small diameter of 200-400 µm was chosen as the primary backbone supporting and subordinate electrode, owing to its high stiffness, light weight, and conductivity.14 Given the conductivity of the CF thread is still inferior as compared to the pure metal yarn,22 the CF thread was conformably coated with a thin nickel layer via ED methodology. The resulting nickel-coated CF thread exhibited a drastic enhancement in conductivity as compared to the pristine CF shown in Figure 1b, increasing it by a factor of 3.3 while being much lighter than previously explored pure metal yarns.4,10,23 This is because the nickel layer deposited is too thin (820 nm) to result in a significant mass increase of the electrode. Additionally, after being coated with a nickel layer, they show more excellent knittability than the pristine CF thread, i.e., the nickel coated CF can be successfully knotted while the pristine CF thread breaks easily with many burrs, as shown in Figures S3. The tensile strength of the Nickel/CF dropped from 2.05 to 1.47 GPa, which may be due to the adverse effect caused by the interaction between the surface of carbon filament and nickel deposit,24 as shown in Figure 1c. Afterwards, Ni-Co DHs were electrodeposited on the highly conductive fiber thread. Prior to deposition, pen ink, which mainly consist of graphite carbon nanoparticles were deposited on the nickel coated CF through a facile dipping-coating route.25 There are several benefits of growing Ni-Co DHs on pen ink film. It allows the conductive graphite carbon nanoparticles to form a porous morphology and coarse surface, enhancing the surface area to facilitate faster charge transportation, as increasing the active area eases the flow of electrolyte ions.11,19 It can greatly increase the loading mass of the electrochemically activated materials and offer a robust adhesion for Ni-Co DHs to enhance the cycling ability as well. Furthermore, the Nickel/CF coated with pen ink film can almost still retain its original conductivity as shown in Figure 1b. A slight decline may be due to the water molecules in the pen ink. Intriguingly, the tensile strength (1.8 GPa) of the pen ink film coated Nickel/CF has been improved (Figure 1c), which can be explained by the strong bonding of the graphite


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carbon nanoparticles and robust adhesion of hydrogen bonding between adjacent microfibrils conveyed by the water molecule in pen ink.26 The mechanical strength of the Pen ink/Nickel/ CF was found to be superior as compared to previously reported fibers.27–29 And the ink-coated Nickel/CF can still remain good stitchability as shown in Figures S3c. The electrodeposition of the Ni-Co DHs greatly enhances the overall performance of the supercapacitor, while the pen ink is also responsible for the enhancement of the supercapacitor to some extent.19 Succinctly, these synergistic effects are responsible for the superior overall performance of the supercapacitor. Structural characterization of hierarchical composite fiber

Figure 2 successively shows the morphology of the hierarchical fiber structure. Figure 2a exhibits the single CF filament of 6.93 µm with a smooth surface while Figure 2b shows that the CF filament has been uniformly coated a smooth thin nickel layer with a thickness of 820 nm. Due to the thorough penetration of chemicals into the inner space of CF, the nickel can be homogenously deposited onto the CF thread bundle, as shown in Figure 2c. Figure 2d-e shows the uniform deposition of the pen ink film on the Nickel/CF thread, the liquid pen ink can guarantee the thorough coating of the Nickel/CF (Figure S4). Figure 2f is the corresponding enlarged view of the pen ink film, indicating that numerous carbon graphite nanoparticles bond together to form porous structure and ravine morphology, which can greatly enhance the specific surface area and absorption of electrolyte ions.2 Figure 2g-i presents the FESEM images of the Ni-Co DHs on Pen ink/Nickel/CF. As shown in Figure 2h, a thin nearly transparent interconnected and continuous nanosheets can be obviously observed growing on the ink film, forming a decent conductive network with the porous ink film and abundant open space for electroactive surface sites. To further reveal its microstructure, Figure 2i shows the high magnification FESEM images of the region marked by the red dash rectangle in Figure 2h. A nanosheet microstructure with rippled silk morphology caused by its ultrathin feature can be seen evidently. Additionally, XRD was employed to characterize the hierarchical fiber structure and no other peaks arising from impurities can be found, indicating the composite materials were successfully synthesized (Figure S5). This is also further confirmed by the SEM EDS-mapping as shown in Figure S6.



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For TEM observation, Ni-Co DHs was carefully peeled off from the substrate by surgery blade and then diluted in ethanol with ultrasonic treatment for 5 min. Figure 3a shows the morphological characteristics of Ni-Co DHs, which is in accordance with the FESEM result. Figure 3b exhibits the high resolution TEM (HRTEM) image taken from the marked red rectangle, a lattice spacing of around 0.21 nm, which is in agreement with the (107) lattice plane of Ni–Co DHs.30 The selected area electron diffraction (SAED) pattern indicates the polycrystalline nature of Ni–Co DH nanosheets (inset of Figure 3b). Figure 3c-g shows its corresponding EDS mapping. Carbon, oxygen, cobalt and nickel element can be found evenly distributed throughout the whole structure. Also, the EDS spectrum shows that the pattern peaks of Co, Ni, C, O and Cu are clearly found in Figure S7, revealing the existence of these elements, accompanied with intense Cu signals originated from TEM copper mesh and C signals mainly from the pen ink. Specially, the atom proportion of the Co and Ni detected is 15.8% and 7.7%, respectively, which is very close to 2:1 atomic ratio. This is in accordance with our experiment, further confirming the successful preparation of Ni–Co DHs. Figure S8 revealed that the Ni–Co DHs directly grown on the carbon nanoparticles. Figure 3h shows the structural features of the pen ink film, where the porous structure produced by the interconnected nanoparticles can be clearly observed. The average size of carbon nanoparticles is mainly centered at 40 nm (inset of Figure 3h). It interestingly can be seen in Figure 3i that the ink nanoparticle was composed of graphite sheets and the ordered lattice fringe is 0.34 nm, in line with the d-spacing of (002) planes of hexagonal graphite (JCPDF# 41-1487), like the onion-like carbon, which is similar to other reports.18,19,25,31,32 The corresponding FFT images further verify the feature of the carbon nanoparticles. Both the ultrathin feature of Ni-Co DH nanosheets and the porous texture of the pen ink film composed of graphite facilitates electrolyte penetration and transportation during the electrochemical reaction.33,34 Electrochemical characterization of the fiber electrode

Electrochemical performance of the Ni-Co DHs/Pen ink/Nickel/CF was performed in 6 M KOH electrolyte in a three-electrode system. Figure S9 shows the cyclic voltammetry (CV) curves of Ni-Co DHs/CF, Ni-Co DHs/Nickel/CF and Ni-Co DHs/Pen ink/Nickel/CF at a scan rate of 100 mV·s-1. As expected, the Ni-Co DHs/Pen ink/Nickel/CF has a larger current


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density than the other two electrodes, demonstrating an improved electrochemical capacitance through ink coating.13 Apparently, the supercapacitor produced by the bare Nickel/CF can be negligible. To evaluate the capacitance performance of the Ni-Co DHs/Pen ink/Nickel/CF, CV curves at different scan rates ranged from 5 to 100 mV·s-1 were investigated, as shown in Figure 4a. The well-defined redox peaks observed mainly originate from the Faradaic reactions of the surface oxycation species based on the following equations in KOH solution.35

Co ( OH ) 2 +OH - ↔ CoOOH+H 2O+e -

(5)

CoOOH+OH - ↔ CoO2 +H 2O+e-

(6)

Ni ( OH )2 +OH - ↔ NiOOH+H 2O +e-

(7)

Obviously, all curves show the similar shape and the current density increases with the growing scan rate. Even at a scan rate of 100 mV·s-1, the CV redox peaks still can be clearly seen, suggesting the hybrid structure is favorable to fast redox reactions.36 Galvanostatic charge/discharge (GCD) profiles were also employed to characterize its capacitance performance at different current densities ranging from 2.5 to 30 A·g-1 between the potential of -0.2-0.4 V, as shown in Figure 4b. Near symmetric curves for all current densities were obtained and over 85% for coulombic efficiency at different current densities can be obtained (Figure S9c), indicating high charge-discharge coulombic efficiency and low polarization. The specific capacitance can be calculated according to equation (2), which has been shown in Figure 4c. With increasing current density from 2.5 to 100 A·g-1, the specific capacitance (Cm) changes from 1395.8 to 666.67 F·g-1, suggesting that a 47.8% of the capacitance is remains even if the current density is enlarged by 40 times. To the best of our knowledge, nearly no reports about Ni-Co binary hydroxide has been observed to exhibit such high rates so far.37–39 Areal capacitance was always used to characterize the micro-scale supercapacitor performance. The areal capacitance (Ca) is 1.39 F·cm-2 at a current density of 2.5 mA·cm-2 according to equation (1), which is comparable to other micro supercapacitors.3,6,19 Cycling ability is another important parameter to consider for its practical application. As shown in Figure S10, after 5000 charge-discharge cycles at a current density of 20 A·g-1, the



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capacitance retention still remains at 95.6%, demonstrating a superior recyclability. The electrochemical impedance spectroscopy (EIS) spectra were also tested at a frequency range between 0.01 Hz to 100 k Hz with a 5 mV potential window as shown in Figure 4d. A vertical curve can be seen in the low frequency region, while no obvious semicircle curve is observed in high frequency, indicating the fast charge transfer behavior inside the electrode,40 which originated from the unique hybrid hierarchical structure and excellent conductivity of the fiber electrode. The EIS spectra shows no distinct change after 5000 cycles, further indicating good maintenance. Based on above discussion, the high performance for Ni-Co DHs/Pen ink/Nickel/CF can be attributed to the mechanism illustrated in Figure 4e. Firstly, the ultrathin Ni-Co DH nanosheets with rippled silk morphology can give rise to extremely large surface area thus, providing abundant electroactive sites for redox reaction. Additionally, the conductive graphite carbon nanoparticles can form a porous morphology and coarse surface, efficiently further enhancing the specific surface area and charge transport speeds, as improving the active area to allow easy access for electrolyte ions. Moreover, it can greatly increase the loading mass and offer a robust adhesion for Ni-Co DHs to enhance the cycling ability.32 The 3-D hierarchical conductive network (CF/Nickel/Pen ink/Ni-CoDHs) can greatly improve the charge transfer and strengthen the diffusion kinetics within the electrode. Last but not least, the graphite carbon nanoparticles can play an important role in the overall capacitance performance as well.32,41 Electrochemical performance of the solid asymmetric FSSCs

Although the Ni-Co DHs/Pen ink/Nickel/CF was observed to exhibit high supercapacitor in terms of specific capacitance, rate ability, and cycling ability, the weak potential window (-0.2-0.55 V) still limits its practical application as an energy device with high energy and power density. Therefore, the ink-coated Nickel/CF was employed as the negative electrode to assemble with Ni-Co DHs/Pen ink/Nickel/CF (positive electrode) to enlarge the potential window. The high electrochemical performance of Pen ink/Nickel/CF in three-electrode in 6 M KOH was shown in Figure S11, where an electrical-double-layer capacitance feature can be clearly observed. Charge balance matches the relationship: q+ = q-, where q+ (C), q- (C) denotes the total charge of the positive and electrode, respectively.


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q = C × ∆E × m

(8)

where C (F·g-1) is specific capacitance, ∆E (V) is charge-discharge potential window and m is electrode mass (g). Thus the theoretical mass ratio of positive to negative electrode is

m + C − ×∆E − = m − C + ×∆E +

(9)

Which can be chosen by the ∆E value, as shown in Figure S12, 0.12 can be obtained in this design.13 Figure 5a-b exhibits the CV and GCD curves of the solid asymmetric fiber supercapacitor at different potential windows ranging from 0-0.8 V to 0-1.55 V, respectively, indicating that the electrochemical potential window up to 1.55 V is stable. Figure 5c shows CV curves of the FSSC at a different scan rates between 5 to 150 mV·s-1 in PVA/KOH gel electrolyte. From this figure, we can observe that even the scan rate up to 150 mV·s-1, the redox reaction peaks can still be clearly seen, which indicates a good reaction kinetics. Furthermore, both the pseudocapacitance and electrical-double-layer capacitance feature can be obviously seen as well, which further can be demonstrated by the GCD curves as shown in Figure 5d. The specific capacitance ranges from 22.94 F·g-1 (28.67 mF·cm-2) to 7.9 F·g-1 (10.2 mF·cm-2) when the current density was between 0.5 to 2 A·g-1 while the coulombic efficiency kept over 86% at various current densities (Figure S12b). Figure 5e shows the cycling ability of the FSSC at a constant current density of 1 A·g-1 for 5000 cycles. Even after 5000 cycles, a capacitance retention of 86% can still be obtained and comparing the initial and 5000th GCD curves (inset figure), no obvious change can be observed. The FSSC exhibits both high energy density and power density, as shown in Figure 5f. As the current density was increased from 0.5 to 2 A·g-1, the specific energy decreases from 7.66 Wh·kg-1 (9.57 µWh·cm-2) to 3 Wh·kg-1(3.58 µWh·cm-2). Simultaneously, the specific power increases from 393.94 W·kg-1 (492.17 µW·cm-2) to 1542.85 W·kg-1 (1841.1 µW·cm-2). It is worth mentioning that the energy and power density of the FSSC fabricated by this facile method using the commercial productive pen ink as a coating film are higher (or comparable with) than some other reported values for solid supercapacitor. For instance, NiCo2O4



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nanograss@CF||porous carbon (9.46 µWh·cm-2 at power density of 608.4 uW·cm-2,15 MnO2 ||CNT/carbon paper (5.4 µWh·cm-2),42 Ni(OH)2||ordered mesoporous carbon (10 µW·cm-2),43 Carbon nanotube||Carbon nanotube/CF (9.8 µWh·cm-2).44 The digital photographs (insets of Figure 5f) show the initial and bending status of the FSSC, indicating the excellent flexibility of the FSSC fabricated. The superior performance of the asymmetric FSSC supercapacitor can be mainly attributed to the synergetic effect between the positive and negative electrode. In other words, the integration of the positive with negative electrode can not only widen the working potential window efficiently but also fully utilize the high specific capacitance, thus enhancing the energy density output. The ink composed of graphite carbon nanoparticles play a variant role in the positive and negative electrodes. Specifically, for the positive electrode, the ink film is able to act as the mechanical support and conductive network for Ni-Co DHs, owing to its porous structure and coarse surface. As for the negative electrode, the ink film with characteristic feature is capable of supplying large capacitance. Most importantly, the nickel layer, which acts as the scaffold and current collector for the flexible FSSC, is capable of greatly improving the overall conductivity of the electrode thus facilitate the faster electron transportation, ensuring good rate performance. Finally, the unique hierarchical Pen ink/Nickel/CF structure can guarantee the high strength and flexibility of the FSSC without the rigidity of the pure metal fibers.5

CONCLUSIONS In conclusion, an asymmetric flexible solid FSSC has been developed successfully via using Ni-Co DHs/ nickel/pen ink/CF and pen ink coated on highly conductive Nickel/CF as positive and negative electrode, respectively. The asymmetric FSSCs exhibit high specific capacitance, superior energy density, cyclability and flexibility in PVA/KOH gel electrolyte, owing to their expanded operating potential window, high conductivity, and sufficiently synergistic effects of Ni-Co DHs, pen ink and nickel coated CF. Notably, the nickel and pen ink are both extremely low-cost materials compared with graphene or carbon nanotubes, therefore our approach can be easily scale-up to meet commercial viability in practical applications, and can be extend to fabricate other hierarchically nanostructured metal oxides/hydroxides with low cost, based on


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this innovative design concept.

ASSOCIATION CONTENT Supporting information

Electrochemical deposition curves for Ni-Co DHs; Process of preparation of the solid fiber supercapacitor using the PVA/KOH gel electrolyte; Digital optical images of (a) CF thread (b) Nickel/CF and (c) Pen ink/Nickel/CF. Knot test demonstrates the good knittability and flexibility; Cross-sectional FESEM image of (a) Nickel/CF and (b)(c) Pen ink/Nickel/CF; XRD patterns of CF, Nickel/CF, Pen ink/Nickel/CF and Ni-Co DHs/Pen ink/Nickel/CF; (a)-(e), SEM-EDS elemental mapping of the Ni-Co DHs/Pen ink/Nickel/CF; STEM-EDS spectrum images of the Ni–Co DHs; (a)-(d), TEM images of the Ni-Co DHs coated on pen ink; (a)The cyclic voltammetry curves of Ni-Co DHs/CF, Ni-Co DHs /Nickel/CF and Ni-Co DHs/Pen ink/Nickel/CF at a scan rate of 100 mV·s-1 and (b) EIS spectra of the Ni-Co DHs/CF, Ni-Co DHs /Nickel/CF and Ni-Co DHs/Pen ink/Nickel/CF (c) is coulombic efficiency of the Ni-Co DHs/Pen ink/Nickel/CF; Cycling performance of the Ni-Co DHs/Pen ink/Nickel/CF; (a)-(f) Electrochemical performance of Pen ink/Nickel/CF in three-electrode in 6 M KOH; (a) CV curves of the positive electrode and negative electrode at the scan rate of 10 mV·s-1(b) is coulombic efficiency of the FSSC. The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding author *Y. L. E-Mail: [email protected]

Author Contributions

L.G. designed and conducted experiments, analyzed data, and wrote the main manuscript. J.U.S. analyzed data and wrote part of the manuscript. K.C. performed the TEM experiments and H.Z. conducted the STEM-EDS of the samples. Y.L. supervised the research and revised the manuscript. All authors have given approval to the final version of the paper.

Notes

The authors report no conflict of interest.

ACKNOWLEDGEMENTS The authors gratefully thank the funding supports from Research Grants Council of the Hong Kong Special Administrative Region of China (GRF No. CityU11216515), City University of Hong Kong (Project Nos. 9667117 and 9680108), as well as Shenzhen basic research grant JCYJ20160401100358589. D.S. acknowledges the funding from RGC under the project GRF CityU11210315.


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Figure Captions Figure 1. (a) Schematic illustration of fabrication for the flexible fiber-type solid-state

asymmetric fiber supercapacitor device. (b) shows the two electrode test and (c) shows the tensile test of the CF、Nickel/CF and Pen ink/Nickel/CF. Figure 2. SEM images of the hierarchical structure fiber: (a) Carbon fiber (CF) filament with

smooth surface. (b) CF filament uniformly coated with a smooth nickel layer. (c) The top is the CF bundle while bottom is the CF bundle coated with nickel, indicating the homogeneous depositing. (d)(e) is the Nickel/CF uniformly deposited a pen ink film. (f) shows the porous structure and ravine morphology of the Pen ink/Nickel/CF. (g)(h) exhibit that the Ni-Co DHs forms a decent conductive network with the porous ink film and abundant open space for electroactive surface sites. (i) High magnification FESEM images show that the rippled silk nanosheet is ultrathin. Figure 3. (a) TEM images and (b) HRTEM images of Ni-Co DHs. Inset is corresponding

SAED pattern. (c)-(g) STEM-EDS mapping of the Ni-Co DHs. (h) TEM images and (i) HRTEM images of pen ink film. Inset is the corresponding particle size distribution and FFT images, respectively. Figure 4. Electrochemical performance of Ni-Co DHs/Pen ink/Nickel/CF in 6 M KOH

electrolyte in a three-electrode system. (a) CV curves at different scan rates from 5 to 100 mV·s-1. (b) GCD profiles at different current densities ranging from 2.5 to 30 A·g-1. (c) Specific (Areal) capacitance as a function of various current densities. (d) Nyquist plots in the frequency range of 100 kHz to 0.01 Hz for Ni-Co DHs/Pen ink/Nickel/CF before and after 5000 charging-discharging cycles. (e) Schematic illustration of the charge storage advantages of Ni-Co DHs/Pen ink/Nickel/CF. Figure 5. Electrochemical characterization of the asymmetric FSSC in solid PVA/KOH gel

electrolyte. (a) CV curves at various potential windows at a scan rate of 100 mV·s-1 (inset: schematic diagram of a fiber FSSC). (b) GCD curves at different operating voltage window at a constant current density of 1.25 A·g-1. (c) CV curves of different scan rates of 5-150 mV·s-1. (d) GCD curves at different current densities of 0.5-2.0 A·g-1. (e) Cycling performance of the assembled device at a current density of 1 A·g-1, inset is corresponding GCD curves of the devices before and after 5000 cycles test, respectively. (f) The Ragone plots associated with


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the energy density to power density while the inset is the optical images of flexible asymmetric FSSC.



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Table of Contents


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(a)

(b) 0.15 0.10

(c)2.5

CF Nickel/CF Pen ink/Nickel/CF

CF Nickel/CF Pen ink/Nickel/CF

2.0

0.05

Stress (GPa)

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

0.00

1.0

-0.05

0.5

-0.10 -0.15 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Potential (V)

0.0

0

Figure 1 ACS Paragon Plus Environment

1

2 Strain (%)

3

4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2


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Figure 3 ACS Paragon Plus Environment


(a)

250

(b)

-1

5 mVs

-1

200

10 mVs

-1

0.4

2.5 Ag

-1

5 Ag

20 mVs

150

40 mVs

-1

15 Ag

60 mVs

-1

80 mVs

50

-1

100 mVs

0 -50

-1

20 Ag

0.2

-1

30 Ag

0.1 0.0

-100 -0.1 -0.2 -0.1

0.0

0.1 0.2 0.3 Potential (V)

0.4

0.5

1500 Specific capacitance Areal capacitance

1400

1.4 1.2

(d)

1100

0.8

1000

0.6

900

0.4

800

0.2

700

0.0

600 0

20

40 60 80 -1 Current density(Ag )

100

100

200

300

400

500

600

Time (s)

700

800

500 450

Fresh 5000th cycling

350

-Z''(Ω)

1.0

1200

0

400

-2

1300

0.6

20

300

18

250

14

16

-Z''(Ω)

-0.2

Areal capacitance (Fcm )

-200

-1

10 Ag

-1

100

-150

(c)

-1

0.3

-1

Potential (V)

Current density(Ag-1)

-1

Specific capacitance (Fg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 150

12 10 8 6

100

4

50

0

0

2 0

2

4

6

8

10

12

14

16

18

20

Z'(Ω)

0

50

100 150 200 250 300 350 400 450 500

Z'(Ω)

(e)

Figure 4


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5

2 1

0.8 0.6 0.4

-2

0.2 0.2

0.4


1.2

1.4

2 1


1.2

1.4

1.6

0

30

60

Fresh 5000th cycling

1.4

1.0

Potential (V)

1.2

60 50 40

0.8 0.6 0.4

30

0.2

20

0.0

0

0

10

1000

20

35

30 40 Time (s)

2000

50

3000

60

70

4000

40

45

50

-1 0.50 Ag -1 1.00 Ag -1 1.25 Ag -1 1.50 Ag -1 1.75 Ag -1 2.00 Ag

(f)

90

10

0.0

1.6

100

70

25 30 Time (s)

0.6

0.2

80

20

0.8

-2 0.4

15

1.0

0.4

0.2

10

1.2

-1

0.0

5

1.4

0

-3

0

1.6

Potential(V)

3

0.0

1.6

(d)

-1

150 mVs -1 100 mVs -1 80 mVs -1 60 mVs -1 40 mVs -1 20 mVs -1 10 mVs -1 5 mVs

5 4

-1

1.0

-1

(c) Current density (Ag )

1.2

0

0.0

0-1.55 V 0-1.5 V 0-1.4 V 0-1.3 V 0-1.2 V 0-1.1 V 0-1.0 V 0-0.9 V 0-0.8 V

1.4

Potential (V)

4

-3

(e)

1.6

(b)

0-1.55 V 0-1.5 V 0-1.4 V 0-1.3 V 0-1.2 V 0-1.1 V 0-1.0 V 0-0.9 V 0-0.8 V

3

-1

Current density (Ag )

(a)

Capacitance retention (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5000

Cycles

Figure 5


90 Time(s)

120

150

180

Flexible Fiber-Shaped Supercapacitor Based on ... - ACS Publications

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