activated carbons

Electrochimica Acta 305 (2019) 403e415

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Systematic study on hybrid supercapacitor of Ni-Co layered double hydroxide//activated carbons Qingqing Qin a, Dawei Ou a, Changjing Ye a, Lingxue Chen a, Binbin Lan a, Jian Yan a, b, *, Yucheng Wu a, b, ** a b

School of Materials Science and Engineering & Institute of Industry & Equipment Technology, Hefei University of Technology, Hefei, 230009, China Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei, 230009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2018 Received in revised form 4 March 2019 Accepted 12 March 2019 Available online 15 March 2019

Hybrid supercapacitors (HSCs) with high energy density have caused great interest. However, it is still lack of general principle to optimize the electrochemical performance. Hereby, an insightful study on HSCs has been carried out using devices based on flower-like NiCo layered double hydroxide (NiCo-LDH, 777 C g
1) and activated carbon (AC) in KOH. It has been found that the upper potential limit of NiCo-LDH is about 0.5 V (vs Ag/AgCl) while the lower potential limit of AC is about
1.1 V to
1.3 V. The stable potential window of HSC is 1.6 V. The key to boost cell specific capacitance is to enlarge the potential window of AC by adjusting the Zero Voltage through tuning the mass ratio between the two electrodes. The optimized mass ratio is found to be lower than that calculated by traditional method. We have developed a model to disclose the effect of materials performance, cell potential window and mass ratio on the cell performance. A parameter of match-coefficient (0.8e0.85, in our case) is proposed to estimate the optimized mass ratio. Following the optimized cell parameters, a demo of all-solid-state HSC (optimized mass ratio of 2) has been assembled using solid polymer electrolyte. It exhibits a high specific energy of 69.5 Wh kg
1 at 450 W kg
1 and good cycling stability with 95.7% capacitance retention after 5000 cycles within 1.8 V. This work sheds light on comprehensive understanding towards design and fabrication high performance HSCs. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Hybrid supercapacitor Ni-Co layered double hydroxide Mass ratio Electrochemical potential window Cell design

1. Introduction With the development of hybrid electric vehicles, portable electronic equipment, memory backup systems, aerospace electronics, power grids and energy-storage technology, new efficient energy-storage devices are highly desirable [1e3]. As a new type of energy-storage device, supercapacitors present multifold advantages of high power density, fast charging-discharging and long life [4,5]. However, supercapacitors suffer from low energy density compared to batteries [1,6]. Among various type supercapacitors, hybrid supercapacitors (HSCs) are very promising due to their

* Corresponding author. School of Materials Science and Engineering & Institute of Industry & Equipment Technology, Hefei University of Technology, Hefei, 230009, China. ** Corresponding author. School of Materials Science and Engineering & Institute of Industry & Equipment Technology, Hefei University of Technology, Hefei, 230009, China. E-mail addresses: [email protected] (J. Yan), [email protected] (Y. Wu). https://doi.org/10.1016/j.electacta.2019.03.082 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

remarkable advantages of wide operation voltage window and high specific capacitance for achieving higher energy density [7,8]. HSCs mostly consist of a battery-type electrode (cathode) as the energy source and a capacitor-type electrode (anode) as the power source [7,9]. However, to meet the needs of the rapid development of electronic equipment, it is necessary to optimize the cell design and electrode materials performance to further boost the energy density of HSCs [6,10,11]. Energy density of aqueous HSCs can be enhanced by increasing the electrochemical capacitance of the electrode materials. In general, battery-type faradaic electrode materials with high specific capacitance values are used as positive electrodes. Among various battery-type materials, transition metal based layered double hydroxides (LDHs) are regarded as emerging high energy positive electrode materials due to their high theoretical specific capacitance, relatively low cost and eco-friendliness [12e15]. LDHs with sufficient interlayer space can effectively improve anion exchange capability because of the intercalated anions and water molecules [12,13]. However, LDH electrode materials usually suffer from poor

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rate capability or limited cycle life [14,16]. Herein, we report a simple route to prepare flower-like NiCo-LDH with interconnected nanosheets, which distinctly enhanced electrochemical properties due to the synergistic effects between Ni and Co elements and numerous electroactive sites arising from the multiple oxidation states [14,17]. Cell designing is very important to achieve higher energy density. One of the methods is to enlarge the cell voltage of the device. Normally, the stable voltage window for water is thermodynamically limited to 1.23 V, making it difficult to increase the cell working voltage [18]. Numerous attempts have been made to increase the cell voltage of aqueous supercapacitor [19e21]. Recently, Fu et al. have developed an aqueous symmetric supercapacitor consisting of Longan seed-derived activated carbon electrodes and the alkaline-acidic electrolyte, which can realize a high stable working voltage of 2 V [22]. Lee et al. have expanded the cell voltage of asymmetric supercapacitors using Mn3O4 and VO2 electrodes in neutral electrolytes to 2.2 V. However, the cell specific capacitance is low, which limited the specific energy (42.7 Wh kg
1) [18]. As to the KOH based supercapacitors, much higher cell specific capacitance could be achieved. But, the upper limit of alkaline based cell voltages is from 1.5 V to 1.8 V [23,24]. It is important to reveal the limiting cell voltage of the devices and to detect what is the highest cell voltage in KOH solution-based supercapacitor. On the other hand, the detailed effect of electrode materials performance, mass ratio of negative/positive electrode and the upper cell voltage on the capacitive properties of HSCs are necessary to be disclosed [25]. Currently, systemic study in this field is lacking [25,26]. In this study, we focus on the analysis of the correlation between the potential windows, the mass ratio, materials performance and the cell performance using two electrodes cell design based on NiCo-LDH and AC in KOH solution [27,28]. We have detected the potential limits of both electrodes and the side reactions beyond these limits. Coulombic efficiency is suggested to evaluate the stability of the cell potential window. The mass ratio plays an important role on controlling the cell performance by adjusting the real working voltage of the two electrodes, which could be reflected by a parameter of Zero Voltage (E0). E0 is the electrode voltage vs Ag/AgCl reference electrode when the cell discharges to 0 V. A model has been developed to reveal the general principle in designing the HSCs. An equation with a factor of match-coefficient (MC) is proposed to evaluate the optimized mass ratio. Using the optimized cell parameters, we have assembled an all-solid-state NiCo LDH//AC HSC with optimized mass ratio of 2, which shows a high specific energy of 69.5 Wh kg
1 at 450 W kg
1. This work provides valuable clues in designing HSCs towards high electrochemical performance.

24 h. The autoclave was then naturally cooled to room temperature and the precipitate was collected by centrifuge and washed with DI-water for more than five times. Finally, it was dried in a vacuum oven at 60  C for 10 h and collected for further characterization. For contrast, the molar ratios of Ni(Ac)2$4H2O and Co(Ac)2$4H2O were adjusted from 2 : 1, 1 : 1 and 1 : 2, and the other conditions were not changed. The corresponding precipitates were marked as Ni2Co1LDH, NiCo-LDH and Ni1Co2-LDH. 2.2. Materials characterization The phase structure of the sample was identified on X-ray diffractometer (XRD, D/MAX2500V, Rigaku Corporation, Japan) using Cu Ka (l ¼ 0.15406 nm) radiation, operating at 40 kV and 40 mA in the range of 5 e80 (2q). The morphology and structure of NiCo-LDH were observed by Field-emission scanning electron microscope (FESEM, SU8020, Hitachi, Japan). Transmission electron microscope (TEM, JEM-2100F) was further performed to characterize the microstructures of the samples. 2.3. Preparation of the electrode The electrodes of NiCo-LDH and AC were prepared by simple drop casting method. An AC electrode was prepared by mixing 70% AC with 15% Super-P (SP) and 15% PBI, then dropped on an area of 1.0 cm  1.0 cm on graphite paper (GP) substrate (1.0 cm  2.0 cm) followed by drying at 80  C in air for 10 h. The electrodes of NiCoLDH (70% NiCo-LDH, 15% SP and 15% PBI) was also prepared in the same way. 2.4. Electrochemical measurements The electrochemical measurements of as-synthesized sample were carried out in a standard three-electrode system, consisted of Ag/AgCl as the reference electrode, platinum foil as the counter electrode and GP coated with electroactive materials as the working electrode. The measurements were carried out in 1 M KOH solution at room temperature. The mass loading of active material of a single electrode was about 0.7e1 mg. Cyclic voltammetry (CV) and galvanostatic charge-discharge (CD) measurements were performed using an electrochemical analyzer (Autolab Potentiostat, PGSTAT101), while cyclic performance was tested by LAND BT2013S supercapacitor test system. CV and CD measurements were recorded in a potential window between
0.1-0.5 V and 0e0.45 V, respectively. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 100 kHz to 0.01 Hz at the open circuit voltage with a 5 mV disturbance signal using electrochemical station (IM6, Zahner, Germany).

2. Experimental 2.5. Fabrication of aqueous HSC and all-solid-state HSC 2.1. Fabrication of NiCo-LDH nano-flowers All the powders (99.9%) for this current investigation were of analytical grade without further purification and purchased from Sinopharm Chemical Reagent Co., Ltd (China) unless otherwise specified. Activated carbon (AC) and polybenzimidazole (PBI) used in this work are the same in our previous report [29]. The NiCo-LDH nano-flowers was prepared by a one-pot chemical coprecipitation way. In a typical experiment, 1.5 mmol Ni(Ac)2$4H2O and 1.5 mmol Co(Ac)2$4H2O were dissolved in 20 mL of deionized (DI) water and 100 mL of ethylene glycol with vigorous stirring to form a homogenous mixture. Afterwards, 7 mmol sodium dodecyl sulfate was added to the above solution and stirred for 1 h to form a uniform mixture. The resultant product was transferred to a 200 mL Teflon lined autoclave and kept in an electrical oven at 140  C for

To analyze the effect of mass ratios of negative/positive electrodes and electrode potential window on the electrochemical properties of the supercapacitor, the NiCo-LDH//KOH//AC HSC was fabricated. AC, NiCo-LDH and 1 M KOH solution were used as negative electrode, positive electrode and electrolyte, respectively. The all-solid-state HSC was assembled in a similar manner with PBI doped with KOH (PBI-KOH) replacing the separator and liquid electrolyte. PBI separator prepared by a drop casting method using PBI dissolved in dimethylacetamide (5 wt%). The integrated supercapacitors were assembled by pressing two electrodes and PBI film with both sides applied with PBI solution (5 wt%) followed by drying at 80  C for 10 h. The working area of the HSC was 1.0 cm2. The device was then soaked in 6 M KOH solution for more than 3 days. After doping process, a Ni strip (50 mm) was adhered to the

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device. Finally, the HSC was sealed by a plastic bag.

3. Results and discussions 3.1. Material characterization The structure of NiCo-LDH was detected by XRD with results shown in Fig. 1a, which shows an evident layered structure with a basal spacing of 0.78 nm (2q ¼ 11.5 ). In addition, the characteristic peaks can be indexed to a typical LDH phase, similar to those reported in the literature [13,30]. Fig. 1bed shows FESEM images of samples at different magnifications. Fig. 1b shows flower-like structures with a size range of several mm, which are composed of ultrathin nanosheets with smooth surface (Fig. 1c and d). In addition, the nanosheets are interconnected with each other and interlaced like petals. Such nano-sheets based hierarchical structure with multiple characteristic dimensions allows high accessibility of electrolyte [31]. Fig. 1e and f shows typical TEM images of NiCo-LDH nano-flowers. It can be clearly seen that the sheets are very thin, which indicates a low diffusion distance of ions and more exposed surface area. This is of benefit to achieve better rate capability. Meanwhile, thin sheets are also favorable for structural stability of active materials during cycling test [32,33]. Similarly, the Ni2Co1LDH and Ni1Co2-LDH also exhibit flower-like morphology as indicated in Fig. S1.

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3.2. Electrochemical performance of NiCo-LDH The electrochemical measurements of the samples were evaluated by a three-electrode configuration using 1 M KOH electrolyte with Ag/AgCl and platinum foil as reference and counter electrode, respectively. The electrochemical performance was evaluated by CV and CD method. As shown in Fig. 2a, the CV curves of NiCo-LDH electrode with a mass loading of 0.7 mg cm
2 exhibit a typical pair of redox peaks at different scan rates, which is related to the faradic redox reactions between M-OH and M-O-OH within a potential window of
0.1e0.5 V, where M represents both the Ni and Co ions. This is consistent with previous studies [16,17]. Due to the polarization in the CV measurements, the oxidation and reduction peaks are shifting towards higher and lower voltages with the increase of scan rates, respectively. At high scan rate, more severe polarization could be observed, which might be due to the internal resistance of the electrode [34,35]. Fig. 2b shows the CD curves at different current densities within the potential range of 0e0.45 V. Unlike the linear characteristic of electric double layer capacitance electrodes, the plateaus in CD curves indicate that NiCo-LDH electrodes exhibit typical battery behavior [36,37]. The CD curves at 2 A g
1 and the specific capacity obtained at different current densities with various Ni/Co ratios were presented in Fig. 2c and d, respectively. The NiCo LDH with Ni:Co ¼ 1:1 shows the best performance, which can be ascribed to

Fig. 1. (a) XRD pattern of NiCo-LDH; (bed) Low and high-magnification FE-SEM images of the NiCo-LDH; (eef) TEM images of the NiCo-LDH.

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Fig. 2. Electrochemical performance of active materials measured in a three-electrode test. (a) The CV curves of the NiCo-LDH electrode tested at different scan rates; (b) The CD curves of the NiCo-LDH electrode at different current densities; (c) CD curves of NiCo-LDH, Ni2Co1-LDH and Ni1Co2-LDH electrode at a current density of 2 A g
1; (d) Specific capacity as a function of current density for the NiCo-LDH electrodes with various Ni/Co ratios obtained at different current densities; (e) Cycling performance and coulombic efficiencies of NiCo-LDH at a constant current density of 10 A g
1 for 5000 cycles; (f) Nyquist plots of NiCo-LDH electrode before and after cycling test (insets are the magnified view of the Nyquist curves and equivalent circuit).

the synergistic effect between the two different components [17,38]. The specific capacity is 777, 760, 718, 668 and 582 C g
1 obtained at current density of 1, 2, 5, 10 and 20 A g
1, respectively. The capacity retention of 75% at 20 A g
1 indicates good rate capability. It could be attributed to the open space between neighboring nano-flakes, which allows an easy access of ions to the electrolyte/electrode interface and facilitates the occurrence of faradic reactions during the energy conversion process. NiCo-LDH with the ratio of 1:1 presents the best electrochemical performance among all NiCo-LDHs samples, which might be ascribed to the synergistic effect between the two different elements [9]. In NiCo based electrode materials, Co can reduce the charge transfer resistance and improve the stability of the compounds while Ni

promotes the electrochemical activity of the materials, leading to an improved specific capacity and rate capability [39]. Controlling composition is determinative to their electrochemical performance because the redox reactivity and stability directly relate to the nature of transition metal components. In addition, morphology and nanostructure of an electrode material greatly influence the electrochemical performance [39]. The best ratio is also related to different synthesis conditions and the morphology of product. Many researches also have different optimal ratios, such as Ni:Co ¼ 4:1 [40], Ni:Co ¼ 2:1 [35], Ni:Co ¼ 3:2 [16] and Ni:Co ¼ 1:1 [13]. Therefore, NiCo-LDH with good electrochemical performance were used as the positive electrode material for further study in the present study.

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Cycling stability is also one of the crucial factors for evaluating supercapacitors. In this work, the cycling stabilities of NiCo-LDH electrode were examined by repeating the CD process. As depicted in Fig. 2e, the capacity retention of the initial capacitance after 5000 cycles at 10 A g
1 is about 92% indicating good cyclic stability compared with recently reported results [31,41e43]. Fig. S2 presents the FESEM images of NiCo-LDH electrode before and after the cycling examination. Inspiringly, the structural integrity was well maintained without obvious deformation or destruction. It suggests the excellent long-term stability of the NiCo-LDH electrode. Furthermore, EIS analysis was also performed to investigate the electrochemical performance of NiCo-LDH nano-flower at an opencircuit potential in the frequency range of 100 kHze0.01 Hz. Fig. 2f shows Nyquist plots before and after 5000 cycles with the expanded views of the high frequency region and the equivalent circuit diagram used for fitting impedance spectra. An equivalent circuit for fitting the EIS plots is composed of the series resistance (Rs), a charge transfer resistance (Rct), a double-layer capacitance (Cdl), a faradic capacitance (CF) and Warburg impedance (W). Generally, Rs represents the resistance related to the ionic conductivity of the electrolyte and electronic conductivity of the electrodes and current collectors, Cdl is accounting for a doublelayer capacitance, Rct is the charge-transfer resistance associated with the Faradic reactions, CF is the faradic capacitance and W is the Warburg resistance arised from the ion diffusion and transport in the electrolyte [16,44]. The fitting Rct in Table S1 (0.01 U cm
2) of NiCo-LDH electrode shows small value, indicating the few charge transfer processes. After 5000 cycles, the more apparent semicircle shape is observed, which may be due to the increase of electrical resistance between the NiCo-LDH and the electrode matrix caused by volume change of active materials during cycling [29,45]. Additionally, the slight decrease of the slope for the straight line at low frequency region after 5000 cycles further verify the remarkable stability and highly reversible redox reactions of the NiCo-LDH electrode [46]. Furthermore, the electrochemical performance of NiCo-LDH nano-flower with different loading density is also evaluated with specific capacitances shown in Fig. S3. With increase of loading mass, the specific capacity decreases. It could be illustrated by the typical loading density effect [47,48]. Encouragingly, the NiCo-LDH material with a mass loading of 2.7 mg cm
2 can still deliver a relative high specific capacity of 643 C g
1 at 1 A g
1, suggesting the good capacitive performance. 3.3. Cell designing of aqueous electrolyte based HSCs Considering that the NiCo-LDH exhibits a relative high specific capacitance and good rate capability, HSC based on NiCo-LDH and AC in 1 M KOH solution is assembled in the form of two electrodes cell to reveal the principle for device designing. 3.3.1. Potential limits of positive and negative electrodes Fig. 3 shows the CD curves of the aqueous HSC with different mass ratios between negative and positive electrodes (mN/mP) at 1 A g
1. The real-time voltage curves of positive and negative electrodes are also presented. The upper potential limit of NiCoLDH electrode increases from 0.4 to 0.5 V (vs Ag/AgCl) as the mN/ mP changed from 1.5 to 3 when the device worked at 1.5 V (Fig. 3). With the cell voltage up to 1.6 V or higher, the upper potential limit of NiCo-LDH electrode keeps at about 0.5 V. However, when the mass ratio is above 2.5 or the cell voltage at 1.7 V or 1.8 V, clear side reaction of NiCo-LDH electrodes could be observed during charge process. This is also reflected in the plateaus tail of cell charging curves. It suggests that the positive electrodes should work below 0.5 V. Otherwise, there will be oxygen evolution reaction. Normally, it causes low coulombic efficiency, which is harmful to the stability

407

of devices. Correspondingly, the lower potential limit of AC electrode is roughly
1.0 V,
1.1 V,
1.2 V and
1.3 V (vs Ag/AgCl) with cells working at 1.5 V, 1.6 V, 1.7 V and 1.8 V, respectively. In this voltage range, no clear plateaus indicating the hydrogen generation is not severe. However, the absolute slope of AC discharge curve still becomes smaller in the range from
1.2 V to
1.3 V. As to the AC electrodes, the specific capacitance increases when the lower limit of voltage from
1.0 V to
1.3 V (Fig. S4). However, the stability of AC in such voltage ranges is not good. In addition, if the upper limit of voltage close to 0.2 V, there are side reaction(s) as indicated by the plateaus in the charge curves (Fig. 3a1-a3), indicating a poor stability of AC electrode (Fig. S4). However, the upper potential limit of 0.1 V is acceptable for AC electrode. Roughly, it can be seen that at low current density, the potential window of the cell is stable up to 1.6 V with mN/mP below 2. This is suitable for the charge (Q) balance and avoiding the emergence of platform. Considering the polarization at high current density, the potential window of the cell could be enlarged to about 1.7 V or more in aqueous electrolytes. 3.3.2. Effect of mass ration on cell performance Fig. 4 shows the specific capacitances calculated from the CD curves for the aqueous HSCs with various mass ratios and cell voltages. The HSC with mass ratio of 2.5 delivers a very high specific capacitance up to 176 F g
1 at 1 A g
1 within 1.5 V potential window. However, with increase of cell voltage, its capacitive performance decreases. Differently, the HSC with mass ratio of 2 shows increase of specific capacitance with when the cell working potential from 1.5 V to 1.8 V. Within cell voltage of 1.7 V and 1.8 V, the HSCs with mass ratio of 2.5 and 2 both exhibit high specific capacitance. At the potential window of 1.8 V, the HSC with mass ratio of 2 exhibits better rate capability. As to the HSCs with mass ratio of 1.5 or 3, the specific capacitances are clearly lower. At 1.8 V cell voltage, the HSC with high mass ratio of 3 exhibit worse rate capability. Thus, the HSC with mass ratio of 2e2.5 gives the best performance. Traditionally, the optimized mass ratio between the positive and negative electrodes could be estimated by the equation below:

mp Cp ¼ mn SCn 1V

(1)

where mp, mn, Cp and SCn are the positive electrode mass, negative electrode mass, specific capacity of positive electrode and specific capacitance of negative electrode, respectively. Considering that the specific capacitance of AC is only ~200 F g
1 within 0 ~
1 V (Fig. S4), the optimized electrode mass ratio should be above 3 in the cell based on NiCo-LDH//AC. However, in our case, the best mass ratio is about 2e2.5. In order to disclose the reason causing such difference, further detailed analyze on the CD curve and capacitive performance of both electrodes have been carried out in the following. Here, a parameter of E0 is introduced. When a supercapacitor is fully discharged, its voltage is zero, the potentials of positive electrode and the negative electrode are equal (En ¼ Ep). The E0 is the voltage of both electrodes vs. Ag/AgCl when the cell voltage is 0 V. The E0 is of advantage to detect the real potential window for each electrode. The capacitive performance of both electrodes could be estimated by the equation below from the CD curves of the aqueous HSC at 1 A g
1 in Supplementary Material. Fig. 5a depicts the E0 of HSCs based on NiCo-LDH//AC with different electrode mass ratios and cell potential window. It exhibits a very clear trend that with the increase of mass ratio or potential window, the E0 becomes lower. As to the mass ratio of 1.5, the E0 is about 0.17 V when the cell voltage increased from 1.5 V to 1.7 V. Even when the cell reaches 1.8 V, the E0 is still around 0.14 V. It suggests that the

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Fig. 3. The CD curves of the aqueous HSC at 1 A g
1 and real-time voltage curves of positive and negative electrodes. (a1-a4) The curves obtained with different negative and positive ratios at 1.5 V; (b1-b4) The curves obtained with different negative and positive ratios at 1.6 V; (c1-c4) The curves obtained with different negative and positive ratios at 1.7 V; (d1-d4) The curves obtained with different negative and positive ratios at 1.8 V.

Fig. 4. Specific capacitance as a function of current density for the aqueous HSC with various negative and positive ratios obtained at different voltages: 1.5 V, 1.6 V, 1.7 V, 1.8 V.

electrochemical utility of NiCo-LDH is low (see Fig. 3). The specific capacity is only about 450 C g
1 as indicated in Fig. 5b. Accordingly,

the upper limit of voltage of AC electrode is E0, which is clearly higher than 0 V. The work voltage of AC electrode is larger than

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409

Fig. 5. (a) Potential of zero voltage for aqueous HSC obtained at different voltages; (b) Capacity contribution of positive electrode during the discharging process for aqueous HSC obtained at 1 A g
1; (c) Capacitance contribution of negative electrode during the discharging process for aqueous HSC obtained at 1 A g
1; (d) Capacity contribution of negative electrode during the discharging process for aqueous HSC obtained at 1 A g
1.

0 ~
1 V. However, the specific capacitance of AC is not high as shown in Fig. 5c. As to the HSC with electrode mass ratio of 3, the E0 is about 0 V when the cell potential window is 1.5 V. However, E0 decreases very fast from 0 V to
0.4 V when the cell potential window increases. It shrinks the work voltage of AC electrode leading to a low electrochemical utility of electrode materials as indicated in Fig. 5bed. Obviously, the mass ratio of 3 is not a good one. For the cells with mass ratios of 2 and 2.5, the E0 is in the range of 0.7e0.15 V the specific capacity of NiCo-LDH in the HSC (mass ratio of 2.5) is very high with value up to 900 C g
1. In the cell with mass ratio of 2, the specific capacity of NiCo-LDH is 708, 727, 786 and 814 C g
1 at 1.5, 1.6, 1.7 and 1.8 V, respectively. Meanwhile, the specific capacitance of AC electrodes also exhibits high values (300e350 F g
1, see Fig. 5c and d). When the mass ratio is optimized, high electrochemical utility could be achieved both in positive and negative electrodes. This is a very interesting phenomenon, which is different from common understanding that the electrodes deliver a relatively stable specific capacitance/capacity within particular potential window. For the cell working at 1.8 V, the E0 of HSC with mass ratio of 2 and 2.5 dropped to
0.06 V and
0.25 V, respectively. It shrinks the working potential window of AC in the HSC with mass ratio of 2.5, which might be the reason causing the lower specific capacitance compared with the HSC with mass ratio of 2. In addition, the E0 of
0.25 V is not good for the stability of HSCs since side reactions may occur. The above results indicate that E0 is very important to the specific capacitance of HSC. E0 is better to be 0.1 ± 0.05 V in our case. Note that this value is determined by the discharge curves of

NiCo-LDH electrode. Based on E0 and the potential limits of both electrodes, it is possible to estimate the optimized mass ratio between the positive and negative electrode. In the following, a model of HSCs is developed using the proposed equation to estimate the optimized electrode mass ratio. 3.3.3. Coulombic efficiency and stable potential window of HSCs The stable potential window is also critical to HSCs. The coulombic efficiency is good factor for evaluate the stability of the potential window of HSCs. Fig. 6 shows the coulombic efficiencies of aqueous HSCs with different mass ratios obtained at different current densities from 1 A g
1 to 20 A g
1 and various voltages from 1.5 V to 1.8 V. At high current densities such as 10 and 20 A g
1, all of the HSCs deliver high coulombic efficiencies close to 100%, which is due to the high polarization. At low current densities, the coulombic efficiencies decreased with the increase of cell voltages from 1.5 V to 1.8 V, especially at 1 A g
1. As to cells working at 1.7 V and 1.8 V, the coulombic efficiencies of HSCs with different mass ratios are all below 90%. For the cell working at 1.6 V, the HSC with mass ratio of 2 exhibits a best coulombic efficiency of 95%. As to the HSC (mass ratio of 2.5), it shows much lower coulombic efficiency (85%) at 1.6 V. In general, these results indicate that the HSC with mass ratio of 2 delivers the best electrochemical performance within a stable potential window of 1.6 V. It can be seen that coulombic efficiency is an effective parameter to evaluate the stability of potential window, and coulombic efficiency above 90% of HSC is suggested. In previous papers, there are many groups reporting HSCs working at 1.7 V or 1.8 V using Ni-Co

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Fig. 6. Coulombic efficiencies of aqueous HSC obtained at different current densities from 1 A g
1 to 20 A g
1 and various voltages from 1.5 V to 1.8 V. (a) mN/mP ¼ 1.5; (b) mN/ mP ¼ 2; (c) mN/mP ¼ 2.5; (d) mN/mP ¼ 3.

based positive electrodes and carbon based negative electrodes [41,49]. Considering that discharge plateaus might be different in various Ni and/or Co based materials, HSCs with 1.7 Ve1.8 V potential window might be stable. However, the coulombic efficiency of such devices with high potential windows is encouraged to be provided strongly. On the other hand, supercapacitor is supposed to deliver high power and allow high charging rate. For the HSCs working at 1.7 V or 1.8 V, it would be of help to reduce the negative effect of large polarization.

x is the normalized discharge depth and could be written follow the equation below:

x ¼ Qp




Cp *mp



(3)

where Qp is the charge of positive electrode during discharge, Cp is the specific capacity of NiCo-LDH, mp is the mass of electrodes. It can be seen that the red line fits well with the green line. As to the AC electrode, it follows the equation below:

3.4. Model development In order to disclose the principles in designing HSCs, a simple model is developed as shown in Fig. 7a. The green line denotes the normalized discharge curve of NiCo-LDH at 2 A g
1. The depth of discharge is set to be 0.9. The red line shows the simulated discharge curves with a proposed equation:

Vp ¼ 0:22
0:06x
x23

(2)

where Vp (vs Ag/AgCl) is the voltage of NiCo-LDH during discharge,

Qn ¼ SCn *ðVn
Vmin Þ*g*mp

(4)

where Qn is the charge of negative electrode during discharge, SCn is the specific capacitance of AC, Vn (vs Ag/AgCl) is the voltage of negative electrode, Vmin is the negative potential limit, g (g ¼ mn/ mp) is mass ratio. For simplicity, the SCn is set to be a constant. When the cell discharges to 0 V, Qp is equal to Qn and Vp is equal to Vn. Then the value of x could be obtained at given g and Vmin by solving the following equation below:

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411

Fig. 7. A model of HSC based on Ni-LDH//AC system.

1

0 0 ¼ 0:22
Vmin
@0:06 þ 

1 n g SC Cp

Ax
x23

(5)

Accordingly, the specific capacitance of the cell could be calculated using the following equation:


SCcell ¼ x  Cp ð0:5
Vmin Þ=ðg þ 1Þ

(6)

In Fig. 7b, it shows the simulated specific capacitance of the cell using NiCo-LDH (850 C g
1) and AC electrode (320 F g
1) with different mass ration. With cell voltage of 1.5 V, the specific capacitance exhibits a peak value of about 160 F g
1 at mass ratio of about 2. It matches very well with specific capacitance of HSC (mass

ratio of 2) shown in Fig. 3. According to the traditional method, the optimized mass ratio should be ~2.4. With the cell voltage increases from 1.5 V to 1.8 V, the maximum specific capacitance drops slightly. Meanwhile, the optimized mass ratio shifts to lower sider. In a real HSC, the specific capacitance of AC electrode working at more negative side becomes larger (Fig. 3). Therefore, the specific capacitance of cell does not show decrease in case of the HSC with mass ratio of 2 in Fig. 4. Fig. 7c depicts the influence of specific capacity of positive electrode on the cell specific capacitance (cell voltage of 1.5 V). The specific capacitance of negative electrode (320 F g
1) is fixed. With the increase of specific capacity from 650 C g
1 to 1050 C g
1, the cell specific capacitance only increases from 147 F g
1 to 173 F g
1. It does not show very significant enhancement by using high

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performance positive electrode materials. This is due to the reason that in a capacitor, the cell capacitance is limited by the one with low specific capacitance, namely, the AC electrode. Fig. 7d presents the influence of specific capacity of negative electrode on the cell specific capacitance with working voltage of 1.5 V. The specific capacity of positive electrode (850 C g
1) is fixed. With the increase of specific capacitance from 150 F g
1 to 450 F g
1, the cell specific capacitance shows a fast increase from 93 F g
1 to 200 F g
1. The optimized mass ratio decreases from 4 to about 1.4. In order to achieve high specific capacitance of HSCs, it is more important to enhance the specific capacitance of negative electrodes rather than the positive electrodes. Above results suggest that the mass ratio and the capacitance ratio between the two electrodes are the two major factors determining the cell specific capacitance. Fig. 7e depicts the plots of the cell specific capacitance normalized to the specific capacity of positive electrode vs the mass ratio and capacitance ratio with cell voltage of 1.5 V. The optimized mass ratios at various capacitance ratios are plotted in Fig. 7f. According to the traditional equation (formula 1), the value of mass ratio timing capacity ratio should be about 1 with AC working at 0 ~
1.0 V. However, it is only about 0.7e0.75 in our case as shown by the red spheres in Fig. 7f. Further considering that the discharge depth of positive electrode is only 0.9, the value is revised to about 0.8e0.85 as shown by the blue spheres in Fig. 7f. Here, this value is defined as MC, which could serve as a parameter to evaluate the optimized mass ratio using the follow equation:

mp Cp MC ¼ mn SCn 1V

(7)

Roughly, the MC is about 0.8e0.85. The mass ratio calculated using equation (7) normally gives a higher cell specific capacitance compared with the mas ratio calculated using equation (1). It gives a reasonable reason that in many reports, the cell specific capacitance is higher than expected values [41,49e51]. If the cell working at higher voltage (1.6 Ve1.8 V), MC could be a little lower. Considering that the Ni and Co based cathodes normally show high specific capacity, the mass of cathode is lower than that of anode (normally AC) in the device. In addition, Ni and Co based cathode exhibits lower conductivity and rate capability. It suggests that the mass ratio between the anode and cathode is relative larger at low current density than the one at high current density. Roughly, a little lower mass ratio, compared with the value calculated by equation (7), might be of help to achieve better rate capability in the device. This could be supported by comparing the performance of devices at 1.8 V potential windows with mass ratio of 2 and 2.5 in Fig. 4. Based on this model, we can estimate the highest achievable specific energy of cells using Ni-Co oxides/hydroxides and AC. Normally, the Ni-Co based electrode materials could exhibit a very high specific capacity of about 1000 C g
1. In some reports, the high specific capacitance close to 500 F g
1 could be achieved in carbon based materials [52]. Accordingly, a cell specific capacitance of 223 F g
1 could be obtained within a 1.8 V potential window. It gives a highest specific energy of about 100 Wh kg
1. To further enhance the specific energy of HSCs, it is necessary to develop AC with specific capacitance more than 500 F g
1. However, the specific capacitance of mostly used AC is below 400 F g
1, which may offer a highest specific energy of about 88 Wh kg
1 in an HSC working at 1.8 V. Comparatively speaking, it might be an effective strategy to boost the specific energy of HSC by enlarging the potential window.

3.5. Demo cell of all-solid-state HSC In order to test and verify the revealed principles for designing HSCs, an all-solid-state HSC has been assembled using NiCo-LDH as positive electrode (0.7 mg) and AC as negative electrode (1.4 mg) with mass ratio of 2. Aqueous KOH solution is replaced with KOH doped PBI (PBI-KOH) solid state electrolytes. We performed a series of CV and CD measurements with increasing voltage windows to estimate the best operating potential of the designed HSC. Fig. 8a displays the CV curves collected at different voltage windows for the all-solid-state HSC at scan rate of 50 mV s
1. With the cell working voltage up to 1.8 V, no severe side reaction could be observed. In the CD curves shown in Fig. 8b, no distortion is observed with the operating cell voltage from 1.5 to 1.8 V at 1 A g
1. The columbic efficiency is 96.6%, 95.9%, 94.4% and 91.3% at 1.5 V, 1.6 V, 1.7 V and 1.8 V, respectively. The coulombic efficiency is slightly lower than 100% should be derived from the negative electrode. As indicated in Figure S4, the discharge time of AC is longer than the charge time, especially at low current density. Consequently, the charging time of device would be little longer than the discharge time. Compared with the aqueous HSC, it suggests that all-solid-state HSC could operate at a high potential window of 1.8 V. This should be attributed to the PBI-KOH solid state electrolytes [24,31,41]. In PBI-KOH, there is only limited free water, which may supress the side reactions. In addition, our pervious work demonstrates that the all-solid-state HSC shows advantages of robust mechanical properties due to the tough PBIKOH and integration design [29]. Therefore, solid electrolytes are strongly suggested for assembling NiCo based HSCs. The electrochemical performance of the HSC was evaluated within a 1.8 V window. A combination of pseudo-capacitive and electrical double layer type features was clearly observed in the CV curves in Fig. S5a. With the scan rate increasing from 5 to 100 mV s
1, the shape of CV curves does not show obvious distortion, indicating fast charge/discharge property for all-solid-state HSC. The CD curves in Fig. 8c are different from that of linear characteristic of electric double layer capacitance, indicating coexistence accumulation of electrostatic and redox reaction at electrode/electrolyte interfaces. Fig. 8d depicts the specific capacitance calculated based on the total mass of NiCo-LDH and AC obtained at different voltages. The specific capacitance are almost same with the voltage increasing from 1.5 V to 1.8 V. The device exhibits the highest specific capacitance of 154 F g
1 at 0.5 A g
1 within 0e1.8 V and remains at 64 F g
1 at 20 A g
1, indicating a good rate capability. The specific capacitance is also close to the one obtained in the two electrodes cell using aqueous electrolyte as indicated in Fig. 4. Moreover, our all-solid-state devices also show good long-term cycling stability with capacitance retention of about 95.7% after 5000 cycles measured at 10 A g
1 (Fig. 8e). The Nyquist plots of the all-solid-state device before and after 5000 cycles are shown in Fig. S5b. The litter lower slope of a line in the low-frequency region after cycles demonstrates the good cycling stability. For comparison, the HSC with mass ratio of 1.5 and 2.5 were fabricated and investigated as shown in Fig. S6. Considering that PBI-KOH exhibits a little lower ionic conductivity, a little lower mass ratio is of benefit to achieve better rate capability as indicated by device with mass ratio of 1.5 shown in Fig. S6a. Meanwhile, the specific capacitance is also better compared with the one (mass ratio of 1.5 in aqueous electrolytes) shown in Fig. 4. Fig. 8f shows the Ragone plot of the all-solid-state device calculated from the discharge curves. Notably, the HSC achieves a maximum specific energy of 69.5 Wh kg
1 at a specific power of

Q. Qin et al. / Electrochimica Acta 305 (2019) 403e415

413

Fig. 8. Electrochemical performance of all-solid-state HSC with mass loading of 2.1 mg and mN/mP ¼ 2 (a) The CV curves of the HSC tested at 50 mV s
1 with the voltage from 1.5 V to 1.8 V; (b) The CD curves of the HSC at 1 A g
1with the voltage from 1.5 V to 1.8 V; (c) The CD curves of the HSC at different current densities with the voltage of 1.8 V; (d) Specific capacitance as a function of current density for the HSC obtained at different voltages; (e) Cycling performance and coulombic efficiencies of all-solid-state HSC at a constant current density of 10 A g
1 for 5000 cycles. (f) Ragone plots of the assembled device and recently reported values for comparison.

450 W kg
1, and can still retain 29.0 Wh kg
1 even at a high specific power of 17987 W kg
1, suggesting the superior capacitive performance of our device. In recent reports, Liu et al. reported a specific energy of 57.1 Wh kg
1 achieved in the supercapacitor based on NiCoAl-LDH//3D RGO with potential range of 0e1.7 V in KOH/PVA electrolyte [24]. Zhao et al. fabricated Ni,Co-OH/rGO//HPC asymmetric supercapacitor, which shows high energy densities of 56.1 Wh kg
1 at 1.6 V [14]. Though just commercial AC is used as negative electrode material in this work, performance of the HSC in this work is among the good devices reported recently [14,24,40,41,43,53e55].

4. Conclusions In this work, we have synthesized NiCo-LDH with a relative high specific capacitance of 777 C g
1. A comprehensive study has been carried out to disclose the principles in cell designing of HSCs based on NiCo-LDH//AC. It shows that the stable potential window in aqueous electrolyte is about 1.6 V with the upper potential limit of ~0.5 V for NiCo-LDH electrode and lower potential limit of
1.1 V for AC electrode. Higher potential window might cause low coulombic efficiency below 90%. The optimized mass ratio is lower than expected value based on traditional method. A model has been

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developed to reveal the influence of the performances of positive electrode and negative electrode, cell voltage and mass ratio on the electrochemical performance of HSCs. A parameter of MC (0.8e0.85, in our case) is proposed for the calculation of the optimized mass ratio. According to the optimized parameters, an allsolid-state HSC with mass ratio of 2 has been assembled showing a higher electrochemical working window of 1.8 V, which is due to the better stability of solid electrolytes (PBI-KOH). The device exhibits a maximum specific energy 69.5 Wh kg
1 and specific power of 450 W kg
1. It also exhibits a good cycling stability up to 5000 cycles. This work provides a detailed understanding on the cell designing of HSCs and useful clues for optimizing the cell performance.

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[17] [18]

[19] [20] [21]

[22]

Acknowledgements The authors acknowledge the financial support of the Natural Science Foundation of China (grant No. 21503065, 51672065) and the General Financial Grant from the China Postdoctoral Science Foundation (Grant No.2015M571924). The authors thank the staff in the Analytical and Testing Center of HFUT for their assistance in the materials characterization.

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[24]

[25] [26]

Appendix A. Supplementary data [27]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.03.082.

[28]

References

[29]

[1] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828. [2] Y. Huang, Y. Zeng, M. Yu, P. Liu, Y. Tong, F. Cheng, X. Lu, Recent smart methods for achieving high-energy asymmetric supercapacitors, Small Methods 2 (2018) 1700230. [3] M.R. Lukatskaya, B. Dunn, Y. Gogotsi, Multidimensional materials and device architectures for future hybrid energy storage, Nat. Commun. 7 (2016) 12647. [4] S.C. Sekhar, G. Nagaraju, J.S. Yu, Conductive silver nanowires-fenced carbon cloth fibers-supported layered double hydroxide nanosheets as a flexible and binder-free electrode for high-performance asymmetric supercapacitors, Nano Energy 36 (2017) 58e67. [5] Q. Qu, Y. Zhu, X. Gao, Y. Wu, Core-Shell structure of polypyrrole grown on V2O5 nanoribbon as high performance anode material for supercapacitors, Adv. Energy Mater. 2 (2012) 950e955. [6] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Q. Zhou, Y. Wu, W. Huang, Latest advances in supercapacitors: from new electrode materials to novel device designs, Chem. Soc. Rev. 46 (2017) 6816e6854. [7] B. Zhao, D. Chen, X. Xiong, B. Song, R. Hu, Q. Zhang, B.H. Rainwater, G.H. Waller, D. Zhen, Y. Ding, A high-energy, long cycle-life hybrid supercapacitor based on graphene composite electrodes, Energy Storage Materials 7 (2017) 32e39. [8] S. Dai, B. Zhao, C. Qu, D. Chen, D. Dang, B. Song, J. Fu, C. Hu, C.-P. Wong, M. Liu, Controlled synthesis of three-phase NixSy/rGO nanoflake electrodes for hybrid supercapacitors with high energy and power density, Nano Energy 33 (2017) 522e531. [9] B. Zhao, L. Zhang, Q. Zhang, D. Chen, Y. Cheng, X. Deng, Y. Chen, R. Murphy, X. Xiong, B. Song, Rational design of nickel hydroxide-based nanocrystals on graphene for ultrafast energy storage, Adv. Energy Mater. 8 (2018) 1702247. [10] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Flexible solid-state supercapacitors: design, fabrication and applications, Energy Environ. Sci. 7 (2014) 2160. [11] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater. 29 (2017). [12] X. Li, D. Du, Y. Zhang, W. Xing, Q. Xue, Z. Yan, Layered double hydroxides toward high-performance supercapacitors, J. Mater. Chem. 5 (2017) 15460e15485. [13] D. Zha, H. Sun, Y. Fu, X. Ouyang, X. Wang, Acetate anion-intercalated nickelcobalt layered double hydroxide nanosheets supported on Ni foam for highperformance supercapacitors with excellent long-term cycling stability, Electrochim. Acta 236 (2017) 18e27. [14] H. Ma, J. He, D.B. Xiong, J. Wu, Q. Li, V. Dravid, Y. Zhao, Nickel cobalt Hydroxide@Reduced graphene oxide hybrid nanolayers for high performance asymmetric supercapacitors with remarkable cycling stability, ACS Appl. Mater. Interfaces 8 (2016) 1992e2000. [15] X. He, R. Li, J. Liu, Q. Liu, R. chen, D. Song, J. Wang, Hierarchical FeCo2O4@NiCo layered double hydroxide core/shell nanowires for high performance flexible

[30]

[31]

[32]

[33]

[34]

[35]

[36] [37] [38]

[39]

[40]

[41]

[42]

[43]

all-solid-state asymmetric supercapacitors, Chem. Eng. J. 334 (2018) 1573e1583. X. Cai, X. Shen, L. Ma, Z. Ji, C. Xu, A. Yuan, Solvothermal synthesis of NiColayered double hydroxide nanosheets decorated on RGO sheets for high performance supercapacitor, Chem. Eng. J. 268 (2015) 251e259. Y. Liu, X. Teng, Y. Mi, Z. Chen, A new architecture design of NieCo LDH-based pseudocapacitors, J. Mater. Chem. 5 (2017) 24407e24415. R. Sahoo, D.T. Pham, T.H. Lee, T.H.T. Luu, J. Seok, Y.H. Lee, Redox-driven route for widening voltage window in asymmetric supercapacitor, ACS Nano 12 (2018) 8494e8505. Z. Dai, C. Peng, J.H. Chae, K.C. Ng, G.Z. Chen, Cell voltage versus electrode potential range in aqueous supercapacitors, Sci. Rep. 5 (2015) 9854. M. Yu, Y. Lu, H. Zheng, X. Lu, New insights into the operating voltage of aqueous supercapacitors, Chemistry 24 (2018) 3639e3649. F. Wang, X. Wang, Z. Chang, X. Wu, X. Liu, L. Fu, Y. Zhu, Y. Wu, W. Huang, A quasi-solid-state sodium-ion capacitor with high energy density, Adv. Mater. 27 (2015) 6962e6968. C. Li, W. Wu, P. Wang, W. Zhou, J. Wang, Y. Chen, L. Fu, Y. Zhu, Y. Wu, W. Huang, Fabricating an aqueous symmetric supercapacitor with a stable high working voltage of 2 V by using an alkaline-acidic electrolyte, Adv. Sci. 6 (2019) 1801665. J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater. 4 (2014) 1300816. X. Bai, Q. Liu, J. Liu, Z. Gao, H. Zhang, R. Chen, Z. Li, R. Li, P. Liu, J. Wang, All-solid state asymmetric supercapacitor based on NiCoAl layered double hydroxide nanopetals on robust 3D graphene and modified mesoporous carbon, Chem. Eng. J. 328 (2017) 873e883. Y.Y.X.Y.G. Wang, Hybrid aqueous energy storage cells using activated carbon and lithium-intercalated compounds, J. Electrochem. Soc. 153 (2006). J. Zhang, X.S. Zhao, On the configuration of supercapacitors for maximizing electrochemical performance, Chem. Sus. Chem. 5 (2012) 818e841. Q. Zhang, K. Han, S. Li, M. Li, J. Li, K. Ren, Synthesis of garlic skin-derived 3D hierarchical porous carbon for high-performance supercapacitors, Nanoscale 10 (2018) 2427e2437. ~ ero, F. Leroux, F. Be guin, A high-performance carbon for E. Raymundo-Pin supercapacitors obtained by carbonization of a seaweed biopolymer, Adv. Mater. 18 (2006) 1877e1882. Q. Qin, J. Liu, W. Mao, C. Xu, B. Lan, Y. Wang, Y. Zhang, J. Yan, Y. Wu, Ni(OH)2/ CNTs hierarchical spheres for a foldable all-solid-state supercapacitor with high specific energy, Nanoscale 10 (2018) 7377e7381. L. Zhi, W. Zhang, L. Dang, J. Sun, F. Shi, H. Xu, Z. Liu, Z. Lei, Holey nickel-cobalt layered double hydroxide thin sheets with ultrahigh areal capacitance, J. Power Souces 387 (2018) 108e116. T. Li, G.H. Li, L.H. Li, L. Liu, Y. Xu, H.Y. Ding, T. Zhang, Large-scale self-assembly of 3D flower-like hierarchical Ni/Co-LDHs microspheres for high-performance flexible asymmetric supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 2562e2572. B. Guan, Y. Li, B. Yin, K. Liu, D. Wang, H. Zhang, C. Cheng, Synthesis of hierarchical NiS microflowers for high performance asymmetric supercapacitor, Chem. Eng. J. 308 (2017) 1165e1173. X. Wang, C. Yan, A. Sumboja, J. Yan, P.S. Lee, Achieving high rate performance in layered hydroxide supercapacitor electrodes, Adv. Energy Mater. 4 (2014) 1301240. Y.-M. Wang, D.-D. Zhao, Y.-Q. Zhao, C.-L. Xu, H.-L. Li, Effect of electrodeposition temperature on the electrochemical performance of a Ni(OH)2 electrode, RSC Adv. 2 (2012) 1074e1082. G. Nagaraju, G.S. Raju, Y.H. Ko, J.S. Yu, Hierarchical Ni-Co layered double hydroxide nanosheets entrapped on conductive textile fibers: a cost-effective and flexible electrode for high-performance pseudocapacitors, Nanoscale 8 (2016) 812e825. P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210e1211. T. Brousse, D. Belanger, J.W. Long, To be or not to be pseudocapacitive? J. Electrochem. Soc. 162 (2015) A5185eA5189. Y. Jiang, C. Tang, H. Zhang, T. Shen, C. Zhang, S. Liu, Hierarchical walnut-like Ni0.5Co0.5O hollow nanospheres comprising ultra-thin nanosheets for advanced energy storage devices, J. Mater. Chem. 5 (2017) 5781e5790. T. Wang, H.C. Chen, F. Yu, X.S. Zhao, H. Wang, Boosting the cycling stability of transition metal compounds-based supercapacitors, Energy Storage Mater. 16 (2019) 545e573. X. Meng, M. Feng, H. Zhang, Z. Ma, C. Zhang, Solvothermal synthesis of cobalt/ nickel layered double hydroxides for energy storage devices, J. Alloys Compd. 695 (2017) 3522e3529. T. Wang, S. Zhang, X. Yan, M. Lyu, L. Wang, J. Bell, H. Wang, 2-Methylimidazole-Derived Ni-Co layered double hydroxide nanosheets as high rate capability and high energy density storage material in hybrid supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 15510e15524. Y. Song, X. Cai, X. Xu, X.-X. Liu, Integration of nickelecobalt double hydroxide nanosheets and polypyrrole films with functionalized partially exfoliated graphite for asymmetric supercapacitors with improved rate capability, J. Mater. Chem. 3 (2015) 14712e14720. F. Lai, Y.E. Miao, L. Zuo, H. Lu, Y. Huang, T. Liu, Biomass-derived nitrogendoped carbon nanofiber network: a facile template for decoration of ultrathin nickel-cobalt layered double hydroxide nanosheets as high-performance

Q. Qin et al. / Electrochimica Acta 305 (2019) 403e415 asymmetric supercapacitor electrode, Small 12 (2016) 3235e3244. [44] G. Nagaraju, S. Chandra Sekhar, L. Krishna Bharat, J.S. Yu, Wearable fabrics with self-branched bimetallic layered double hydroxide coaxial nanostructures for hybrid supercapacitors, ACS Nano 11 (2017) 10860e10874. [45] J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, Nanoporous Ni (OH) 2 thin film on 3D ultrathin-graphite foam for asymmetric supercapacitor, ACS Nano 7 (2013) 6237e6243. [46] D. Zha, Y. Fu, L. Zhang, J. Zhu, X. Wang, Design and fabrication of highly open nickel cobalt sulfide nanosheets on Ni foam for asymmetric supercapacitors with high energy density and long cycle-life, J. Power Souces 378 (2018) 31e39. [47] K.-T. Lee, C.-B. Tsai, W.-H. Ho, N.-L. Wu, Superabsorbent polymer binder for achieving MnO2 supercapacitors of greatly enhanced capacitance density, Electrochem. Commun. 12 (2010) 886e889. [48] J. Yan, A. Sumboja, X. Wang, C. Fu, V. Kumar, P.S. Lee, Insights on the fundamental capacitive behavior: a case study of MnO2, Small 10 (2014) 3568e3578. [49] Z. Tang, C.-h. Tang, H. Gong, A high energy density asymmetric supercapacitor from nano-architectured Ni(OH)2/Carbon nanotube electrodes, Adv. Funct. Mater. 22 (2012) 1272e1278.

415

~ ero, F. Be guin, Adjustment of electrodes [50] L. Demarconnay, E. Raymundo-Pin potential window in an asymmetric carbon/MnO2 supercapacitor, J. Power Souces 196 (2011) 580e586. [51] S.G. Krishnan, M. Harilal, B. Pal, I.I. Misnon, C. Karuppiah, C.-C. Yang, R. Jose, Improving the symmetry of asymmetric supercapacitors using battery-type positive electrodes and activated carbon negative electrodes by mass and charge balance, J. Electroanal. Chem. 805 (2017) 126e132. [52] J. Yan, T. Wei, W. Qiao, Z. Fan, L. Zhang, T. Li, Q. Zhao, A high-performance carbon derived from polyaniline for supercapacitors, Electrochem. Commun. 12 (2010) 1279e1282. [53] L. Ye, L. Zhao, H. Zhang, B. Zhang, H. Wang, One-pot formation of ultra-thin Ni/ Co hydroxides with a sheet-like structure for enhanced asymmetric supercapacitors, J. Mater. Chem. 4 (2016) 9160e9168. [54] F. Wang, S. Sun, Y. Xu, T. Wang, R. Yu, H. Li, High performance asymmetric supercapacitor based on Cobalt Nickle Iron-layered double hydroxide/carbon nanofibres and activated carbon, Sci. Rep. 7 (2017) 4707. [55] Y. Bai, M. Liu, J. Sun, L. Gao, Fabrication of Ni-Co binary oxide/reduced graphene oxide composite with high capacitance and cyclicity as efficient electrode for supercapacitors, Ionics 22 (2016) 535e544.