Phase- and interlayer spacing-controlled cobalt hydroxides for high performance asymmetric supercapacitor applications

Journal of Power Sources 422 (2019) 9–17

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Phase- and interlayer spacing-controlled cobalt hydroxides for high performance asymmetric supercapacitor applications

T

Milan Janaa,1, Periyasamy Sivakumara,1, Manikantan Kotaa, Min Gyu Junga, Ho Seok Parka,b,c,∗ a

School of Chemical Engineering, Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon, 440-746, South Korea Department of Health Sciences and Technology, Samsung Advanced Institute for Health Sciences and Technology (SAIHST), Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon, 440-746, South Korea c SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon, 440-746, South Korea b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

phases and interlayer dis• Crystalline tance of Co(OH) are controlled. α- and β-Co(OH) reveal flake-like and • nanorod-like structures. shows 1066 F g at • 2β-Co(OH) Ag . β-Co(OH) reveals 80% of its initial • capacitance at 20 A g . supercapacitor achieves • Asymmetric and 10,000 cycle sta20.05 W h kg 2

2

−1

−1

2

2

−1

−1

bility.

A R T I C LE I N FO

A B S T R A C T

Keywords: Cobalt hydroxide Nanostructure Redox capacitor Hybrid supercapacitors Ionic liquid Asymmetric device

A facile and selective hydrothermal synthesis is performed to control the crystalline phases of cobalt hydroxides into α-Co(OH)2 and β-Co(OH)2: α-Co(OH)2, consisting of both octahedral and tetrahedral Co sites, is produced without ionic liquids, whereas β-Co(OH)2, containing octahedral Co sites, is synthesized in the presence of ionic liquids. The ionic liquids play significant role as co-solvent and template to tune the morphology of Co(OH)2. αCo(OH)2 reveals flake-like structure, whereas β-Co(OH)2 exhibits nanorod-like network structure. The interlayer spacing of α-Co(OH)2 is 8.24 Å, which is larger than 4.63 Å of β-Co(OH)2 due to the expansion of interlayer by the precursor Cl─ anions. The presence of Cl─ anions hinders the insertion of hydroxide ion into α-Co(OH)2 interlayers, which shows the specific capacitance of 613 F g─1 less than 1066 F g─1 of β-Co(OH)2 at 2 A g─1. When the current density increases up to 20 A g─1, the capacitance retention of β-Co(OH)2 is 80%, greater than 70% of α-Co(OH)2. Configuring β-Co(OH)2 and reduced graphene oxide as positive and negative electrodes, asymmetric supercapacitor delivers the maximum energy and power densities of 20.05 W h kg─1 and 13.40 kW kg─1 with the capacitance retention of 93% over 10,000 cycles.

1. Introduction Supercapacitors (SCs) have received significant attention as a promising energy storage device owing to their fast charging rate, elevated

power density, favourable safety features, low maintenance costs, and long life cycle [1–3]. In spite of these several advantages, the low energy density of SC is a bottleneck to fulfil the ever rising energy demands required for emerging applications into electrical vehicles,

∗

Corresponding author. School of Chemical Engineering, Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon, 440-746, South Korea. E-mail address: [email protected] (H.S. Park). 1 Milan Jana and Periyasamy Sivakumar contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.03.019 Received 20 December 2018; Received in revised form 9 February 2019; Accepted 7 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

2. Experimental

renewable energy storage, and future electronic devices [3–5]. A way to increase energy density of SC is to develop high capacitance electrode materials. The SCs can be classified into electrical double layer capacitor (EDLC) and pseudocapacitor according to their charge storage processes [5–7]. Since EDLCs store charges at the electrical double layer (EDL) through a non-Faradic process [5,6], the capacitance is dependent on the available interfacial area and the characteristics of electrolyte solutions. Accordingly, carbon materials such as activated carbon (AC), graphene, carbon nanotube (CNT) have been extensively studied as an EDLC electrode owing to their large surface area, high electrical conductivity, and electrochemical stability [6,8–12]. On the other hand, pseudocapacitors store charges at the electrode surface through a reversible Faradaic process, which involves the charge transfer across the EDL [5–7]. Examples include transition metal oxides such as RuO2 and MnO2, which show pseudocapacitive characteristics of a quasi-rectangular cyclic voltammetry (CV) curve and non-linear galvanostatic charge/discharge (GCD) profile [6–8,11]. Considering the dependence of energy on the square of cell voltage, the other way for high energy density is to enlarge cell potential window. When SCs are configured in an asymmetric manner using two electrodes with different operating potentials, cell voltage can be enlarged if electrolyte is not decomposed. In order to achieve this goal, high capacitance pseudocapacitor or battery-like redox capacitor electrode has been used as a counter part of EDLC electrode. Battery-like redox capacitors have higher specific capacitance compared to EDLC and pseudocapacitors in distinct potential window, showing the well-separated redox peaks in CV curve [6–8]. Cobalt hydroxides are very attractive electrode materials owing to their high theoretical capacitance achieved by the charge storage mechanism Co(OH)2 + OH− ↔ CoOOH + H2O + e− [14–18]. With the same layered structure of cobalt hydroxides, Co(OH)2 and CoOOH show a pair of well-separated redox peaks through the ion intercalation, corresponding to battery-like characteristics of redox capacitor [12–16]. In particular, the crystalline phase of Co(OH)2 is classified into α and β polymorphs [19]. On one hand, α-phase consists of positively charged Co(OH)2-x forming hydrotalcite-like compound and its anions occupy the interlayer position for charge balancing [14,16,18,19]. On the other hand β-form of Co(OH)2 is composed of the brucite-like structure, where Co (II) occupies alternative rows of octahedral sites [14,16,19]. The α-Co(OH)2 exhibits larger interlayer spacing, which is dependent on the inserted anions, as compared to the β-form [19]. Ma et al. synthesized α-Co(OH)2 by N-Methyl-2-pyrrolidone (NMP) assisted electrodeposition method for SC applications [15]. The α-Co(OH)2 electrode exhibited the specific capacitance of 651 F g─1 at 2 A g─1in the working potential of −0.1–0.45 V using 1 M KOH electrolyte, showing a capacitance retention of 76% after 500 charge-discharge cycles [15]. Ranganatha et al. reported one-pot sol-gel process to synthesize mesoporous α-Co(OH)2 with the specific capacity of 477 F g─1 at 1 A g─1 [20]. Lokhande et al. demonstrated the potentiodynamical deposition of β-Co(OH)2 on stainless steel substrate [18]. The resulting β-Co(OH)2 revealed the specific capacitance of 890 Fg─1 at 5 mV s─1 in 1 M KOH electrolyte and 84% capacitance retention after 10,000 cycles. Despite these extensive researches in Co(OH)2, the control in the crystalline phases and interlayer spacing for the improved SC performances has yet to be explored. In this study, we demonstrated the phase- and interlayer spacingcontrolled synthesis of Co(OH)2 using the ionic liquid (IL) of 1-butyl-3methylimidazolium tetrafluoroborate ([BMim][BF4]) via a hydrothermal reaction. The SC performances of α-Co(OH)2 and β-Co(OH)2 are systematically compared in aqueous 6 M KOH electrolytes. In order to overcome the limited potential window of aqueous electrolyte, the asymmetric SC (ASC) devices were configured using β-Co(OH)2 and reduced graphene oxide (RGO) as the positive and negative electrodes, respectively. The IL-assisted controlled synthesis of Co(OH)2 and the configuration of ASC with RGO are expected to improve the energy density of SC in aqueous electrolytes.

2.1. Materials Cobalt chloride (CoCl2, 6H2O), and ammonia solution, polyvinylidene fluoride (PVDF), [BMim][BF4] IL, N-Methyl-2-pyrrolidone (NMP) were purchased from Sigma Aldrich. Conducting carbon black and nickel foam (NF) were obtained from Alfa Aesar and Shanghai Winfay New Material Co., Ltd, China, respectively. 2.2. Synthesis of materials Synthesis of β-Co(OH)2: β-Co(OH)2 has been synthesized by simple and facile IL-assisted hydrothermal procedure. In brief, 2.14 g CoCl2 was dissolved in 15 ml of DI water followed by addition of 0.50 ml of [Bmim][BF4]. The pH of the solution was controlled to be around 9 by adding NH4OH solution drop wise. The resultant solution was transferred to a 25 ml teflon lined stainless steel autoclave and placed inside a preheated hot air oven at 80 °C for 12 h. The resulted pink colour product was collected by repeated centrifugation with DI water and ethanol followed by freeze drying and designated as β-Co(OH)2. Synthesis of α-Co(OH)2: α-Co(OH)2 was synthesized through a hydrothermal process in the absence of [Bmim][BF4]. In brief, 2.14 g CoCl2 was dissolved in 15 ml of DI water followed by addition of NH4OH solution drop wise to adjust the pH of the solution to be around 9. The resultant solution was transferred to a 25 ml teflon lined stainless steel autoclave and placed inside a preheated hot air oven at 80 °C for 12 h. The resulted green colour sample was designated as α-Co(OH)2. 2.3. Materials characterization X-ray diffraction (XRD) of the samples was analysed by a D/Max 2500 V/PC (Rigaku Corporation, Tokyo, Japan) at a scan rate of 1° min−1 at room temperature. Fourier transform infrared (FT-IR) spectra of β-Co(OH)2 and α-Co(OH)2 were recorded with Nicolet iS10, Thermo Scientific, USA. The chemical environment of the prepared samples was evaluated by an x-ray photoelectron spectroscopy (XPS) analyser (Thermo multiLab 2000 system). The transmission electron microscopy (TEM) images were recorded by HR-TEMⅢ (Corrected Spherical Aberration Transmission Electron Microscope (Cs-TEM)). The field emission scanning electron microscopy (FE-SEM) images were collected with Ʃigma HD, Carl Zeiss, Germany. The Brunauer-Emmet-Teller (BET) surface area and pore size distribution of β-Co(OH)2 and α-Co (OH)2 were obtained using a BEL Sorp Mini, Microtrac surface area detecting instrument. 2.4. Electrochemical measurements In order to investigate the electrochemical performance of the Co (OH)2 samples, the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were analysed with a Bio-Logic Science instrument-VSP electrochemical workstation using 6 M KOH as the electrolyte. The active material (cobalt hydroxide), PVDF and carbon black were mixed homogenously in NMP solution to make a slurry with a mass ratio of 8: 1: 1. The areal loading of Co(OH)2 electrode is 2 mg cm−2. The slurry was then drop casted on NF to fabricate SC electrodes. The three electrode measurements were carried out using sample coated NF as working electrode, Pt wire as counter electrode, and AgCl/Ag as reference electrode. The specific capacitance (CS) was calculated from the GCD cycle according to the equation [21,22].

CS =

2 mV 2

∫ iV (t ) dt

(1)

where i, t and V are the applied current, discharging time and working potential, respectively. ‘m’ is the mass of the active material deposited 10

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

α-Co(OH)2 generally achieves less crystalline and more disordered structures compared to β-Co(OH)2 [14]. The IL-assisted hydrothermal procedure offered a soft template route to synthesize β-Co(OH)2 [16,27–29]. Ligands or functional groups by adsorbing on the building blocks regulate the structure formation varying the surface energy of specific crystalline plane [27,28]. The IL aggregates, which is driven by π-π interactions between the imidazolium rings, can interact with Co (OH)2 nuclei via hydrogen bonding to minimize the surface energy [27,28,30]. Therefore, the co-operative interaction with ILs induced the formation of anisotropic rod-like β-Co(OH)2. The precursor anions play significant role to determine the interlayer spacing of Co(OH)2 [14,19,25]. The precursor anions such as NH3, NO32─, and CO32─ are incorporated into the α-Co(OH)2 interlayers, which are expanded from 7.3 to 9.4 Å [14,31]. In our study, the interlayer spacing of α-Co(OH)2 and β-Co(OH)2 were found to be 8.24 and 4.63 Å, respectively, as previously demonstrated [14,18,19,25,26,31]. The positively charged imidazolium rings of IL attracted the Cl─ ion to hinder the insertion in the interlayer gallery of β-Co(OH)2. Therefore, compact interlayer spacing of more crystalline βCo(OH)2 was formed by the IL-assisted hydrothermal reaction as illustrated in Fig. 1. On the other hand, the enlarged interlayer spacing of α-Co(OH)2 suggested the insertion of Cl─ ions in the interlayer gallery [16,32]. FT-IR spectroscopy was investigated to understand the chemical environment of the as-prepared materials (Fig. 2b). α-Co(OH)2 exhibited characteristic peaks at 950, 765, 640 and 504 cm─1 related to Co-OH bonds [13,14,17,18,23,26,27]. For the case of β-Co(OH)2, the broad peak of Co-O-H stretching appeared at a low frequency ranging from 500 to 600 cm─1 indicating the formation of brucite like structure [19,35]. The sharp peak at 421 cm─1 was assigned to Co-O bonds of βCo(OH)2 octahedron [34]. Moreover, the characteristic peak of β-Co (OH)2 at 3620 cm─1, which was not observed in α-Co(OH)2, was associated with the -OH stretching modes of free -OH in brucite-like structures [19,35]. Surface adsorbed water molecule showed a transmittance peak at 3440 cm─1 [33,34] and thus, α-Co(OH)2 revealed more intense and broader peak at 3300–3500 cm─1 as compared to β-Co (OH)2. It means that the Cl─ anions accommodate with water molecule into the interlayers of α-Co(OH)2 to expand the interlayer distance. In order to identify the coordination states of the Co, the UV–Vis absorption spectra of the Co(OH)2 were further investigated. A broad hump at 400 nm suggested the Co3+/2+ octahedral (Oh) sites for both Co(OH)2 as shown in Fig. 2(c) [25,36,37]. In addition, two peaks nanorod-like network appeared at 640 and 675 nm, suggesting the

on NF. An ASC device was fabricated in a two electrode configuration, using β-Co(OH)2 and RGO as the positive and negative electrode, respectively. The charge balancing of β-Co(OH)2 and RGO was carried out by analyzing the CV curves at 50 mV s−1 following equation (2) and (3). The mass ratio of β- Co(OH)2 and RGO was 1–2.39. The charge storage capacity of SC electrode can be explained by the equation [22,23].

q = m × CS × ΔV

(2) +

−

In order to obtain q = q , the mass of the electrodes were adjusted by equation (3).

m+ C × ΔV− = S− m− CS + × ΔV+

(3)

where “+” and “-” represent the positive and negative electrodes, respectively. The β-Co(OH)2 and RGO electrodes have been sandwiched in ASC, where 6 M KOH and glass fiber (Whatman) were used as electrolyte and separator, respectively. The energy (E) and power (P) densities of the ASC were calculated according to equation (4) and (5), respectively [22,24].

E=

1 m

∫ iV (t ) dt

P = E/ Δt

(4) (5)

3. Results and discussions As shown in Fig. 1, Co(OH)2 with different phases were prepared by a facile one-step hydrothermal method using Co2+ (CoCl2) as precursor material. The IL-assisted hydrothermal reaction at 80 °C resulted pink colour product. Green colour product was obtained when the hydrothermal reaction was carried out in absence of [Bmim][BF4]. The phase structures of the prepared materials were determined by XRD analysis as shown in Fig. 2(a). In the case of the IL-assisted hydrothermal method, the diffraction peaks of Co(OH)2 at 2θ = 19.2, 32.2, 35.6, 38.5 and 52.2° were indexed to (002), (100), (101), (011) and (015) planes, corresponding to brucite like β-Co(OH)2 [14,18,19,25,26]. In the absence of IL, the green colour product appeared, showing 2θ values of 10.4, 22.2, 33.4, 59.3, and 60.5° corresponding to the (003), (006), (012), (110) and (113) planes of hydrotalcite phase for α-Co(OH)2 [14,15,19,20,25]. In particular, β-Co(OH)2 exhibited an additional peak at 2θ = 21.3°, which is ascribed to the self-assembled aggregates of the imidazolium rings of [Bmim][BF4] [16]. It has been known that

Fig. 1. Schematic of the processes involved in the synthesis of α and β cobalt hydroxides. 11

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

Fig. 2. (a) XRD patterns (b) FT-IR, (c) UV–Vis spectra and (d) BJH pore size distribution curves of α-Co(OH)2 and β-Co(OH)2.

presence of tetrahedral (Td) Co2+ in α-Co(OH)2 [25,36]. Hence, the UV–Vis analysis indicates that α-Co(OH)2 provided both Oh and Td cobalt sites. On the other hand, β-Co(OH)2 contained only Oh cobalt sites. The different coordination sites of α-Co(OH)2 and β-Co(OH)2 obtained from UV–Vis data are in line with the literature [25,36]. The XRD, FT-IR, and UV–Vis spectra confirmed the phase- and interlayercontrolled synthesis of α-Co(OH)2 and β-Co(OH)2 by IL-assisted hydrothermal reaction. The specific surface area and pore characteristics of α-Co(OH)2 and β-Co(OH)2 were analysed using N2 adsorption-desorption isotherms (Fig. S1). The specific surface area were found to be 25.2 and 35.8 m2 g─1 for α-Co(OH)2 and β-Co(OH)2, respectively. As derived from pore size distributions in Fig. 2(d), the α-Co(OH)2 exhibited pore volume and average pore size of 0.17 cm3 g─1 and 28.3 nm, respectively, while β-Co(OH)2 showed 0.26 cm3 g─1 and 29.7 nm. Larger surface area, pore size, and pore volume of rod-like β-Co(OH)2 with narrow pore size distribution are expected to contribute to short ion diffusion path and easy access of ions relative to α-Co(OH)2 for a better rate capability [1,2,17,21]. The chemical structures of α-Co(OH)2 and β-Co(OH)2 were investigated by XPS analyses. The main peaks can be indexed to Co 2p, O 1s, and C 1s core regions as shown in Fig. 3(a). The Co 2p spectrum revealed two peaks centered at around 781 and 798 eV, corresponding to Co 2p3/2 and Co 2p1/2 spin orbits, respectively [34]. After deconvolution, Co 2p3/2 spectrum of α-Co(OH)2 showed a strong intense peak at 782.0 eV ascribed to Co2+ state of Co(OH)2 [39,40]. The appearance of peak at 783.9 eV suggested the presence of cobalt oxyhydroxide (CoO (OH)) along with α-Co(OH)2. The existence of strong “shakeup” peaks further confirmed the formation of Co(OH)2. In the case of β-Co(OH)2, Co 2p3/2 spectrum also revealed two peaks at 781.2 and 783.2 eV related to β-Co(OH)2 and CoO(OH), respectively (Fig. 3e) [39,40]. The O 1s spectrum of α-Co(OH)2 was deconvoluted into two peaks: one peak at 536.2 eV was assigned to -OH group of Co(OH)2 and the other at

528.2 eV was associated with Co-O bonding [14,35]. On the other hand, the O 1s spectrum for β-Co(OH)2 revealed only one peak at 531.6 eV corresponding to -OH group as shown in Fig. 3(f). A small peak at 200.5 eV corresponding to Cl 2p1/2 appeared due to the inorganic chloride ions for the case of α-Co(OH)2 [38]. The XPS results suggested the formation of α-Co(OH)2 and β-Co(OH)2 along with CoO(OH). Additionally, the presence of Cl─ ions for α-Co(OH)2 was confirmed by XRD and FT-IR analyses. In order to investigate the typical morphological features of Co (OH)2 materials, TEM images were recorded. The flake-like structure for α-Co(OH)2 was presented in Fig. 4(a), while β-Co(OH)2 exhibited rod like network structure as shown in Fig. 4(d). The nanorods are organized to form open porous network structure. The formation of the specific morphology can be explained by the surfactant-induced fiber formation (SIFF) followed by kinetic-to-dynamic growth mechanism [28,42–44]. According to the SIFF mechanism, the IL aggregates might act as template to induce the anisotropic assembly of primary β-Co (OH)2 crystals into a specific direction by reducing the surface energy [27,30]. As the size of the hexagonal becomes larger, the internal stress of the (002) plane can increase, resulting in the exfoliation of the (002) plane to form nanorod-like network structure [37–39,44]. The α-Co (OH)2 nanoplatelets were highly crystalline in nature and the inter planar spacing was calculated to be 0.44 nm corresponding to (012) plane as shown in Fig. 4(b) [45]. The lattice spacing was 0.23 nm for βCo(OH)2, which is assigned to (101) crystal plane as shown in Fig. 4(e). The dotted line in the SAED pattern further confirmed the highly crystalline nature for both the Co(OH)2. As revealed in Fig. 4(c), elemental mapping image suggested that the Co, O and Cl─ were uniformly dispersed for α-Co(OH)2. However, β-Co(OH)2 showed the presences of Co and O in the elemental mapping as displayed in Fig. 4(f). The structure and morphology of α-Co(OH)2 and β-Co(OH)2 have been further analysed by FE-SEM analysis (Fig. S2). Large size platelets were obtained for α-Co(OH)2. The open structure octahedral geometry for β12

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

Fig. 3. (a) XPS survey scan of α-Co(OH)2 and β-Co(OH)2, core level (b) Co 2p, (c) O 1s, (d) Cl 2p XPS of α-Co(OH)2. Core level (e) Co 2p and (f) O 1s of β-Co(OH)2.

redox peaks are assigned to the oxidation of Co (II) to Co (III) and Co (III) to Co (IV), respectively [25,46]. It notes that two types of Co sites are possessed by Co(OH)2: Co2+ in the Td and Co3+/Co2+ in the Oh site. As evidenced from the UV–Vis spectroscopy, α-Co(OH)2 contained both the Td (Co2+ Td) and Oh (Co3+/Co2+ Oh) sites, whereas β-Co (OH)2 exhibited a single Oh Co3+/Co2+ site. Hence, variable oxidation states (Co2+ Td and Co3+/Co2+ Oh) of α-Co(OH)2 contributed to two redox peaks of P1 and P2 [25,46]. In the case of β-Co(OH)2, the redox characteristic was dominated by Co3+ Oh sites. The Faradaic reactions can be expressed in the following equations:

Co(OH)2 was further confirmed by the FE-SEM image analysis. Given by the structural and morphological analysis, the phase and morphology of Co(OH)2 can be tuned by the IL-assisted hydrothermal reaction. In addition, XPS and TEM analysis confirmed the presence of Cl─ ions, resulting larger interlayer spacing for α-Co(OH)2 compared to β-Co (OH)2 as evidenced by XRD analysis. The SC performances of α-Co(OH)2 and β-Co(OH)2 were analysed using CV, EIS and GCD. Fig. 5(a) and (b) show the CV curves at various scan rates of 10–200 mV s─1 in the range of 0–0.4 V. The symmetrical CV curves of both electrodes showed prominent redox peaks with peak separation of greater than 0.1 V. A careful observation of the CV curves indicated the appearance of two oxidation peaks at 0.19 V (P1) and 0.31 V (P2) for the α-Co(OH)2 as shown in Fig. 5(c). In case of β-Co (OH)2, only one oxidation peak at 0.30 V (P2) was observed. P1 and P2

Co(OH)2 + OH− ↔ CoOOH + H2O + e− −

CoOOH + OH

↔ CoO2 + H2O + e

−

(6) (7)

Fig. 4. (a) TEM, (b) HR-TEM (inset shows the SAED pattern image), (c) elemental mapping image of α-Co(OH)2. (d) TEM, (e) HR-TEM (inset shows the SAED pattern image) and (f) elemental mapping image of β-Co(OH)2. 13

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

Fig. 5. CV curves of (a) α-Co(OH)2, (b) β-Co(OH)2 at different scan rates of 10–200 mV s−1, (c) comparative CV curves at 25 mV s−1 and (d) Nyquist plots (inset shows the corresponding equivalent circuit). GCD curves of (e) α-Co(OH)2, (f) β-Co(OH)2, (g) specific capacitance at different current densities, (h) GCD cyclic stability profile up to 5000 cycles in three electrode configuration and (i) comparative CV curves of RGO, β-Co(OH)2 and ASC at 50 mV s−1.

process [14]. In order to understand properly the Nyquist plot of the samples were simulated with Z-Sim software as shown in Fig. S5 and Fig. S6. The best fitted curve exhibited an equivalent circuit including equivalent series resistance (Rs), constant phase element (Q), charge transfer resistance (Rct), and Warburg element (Wo) (Table S1) [5,49,50]. The Rs values were around 3.20 and 0.81 Ω, respectively, for α-Co(OH)2 and β-Co(OH)2. Q was expressed by Q-T and Q-P, indicating the capacitance and the phase exponent, respectively [40,41]. Although the Q-T values were similar to each other, Q-P values revealed more conductive nature of β-Co(OH)2 in comparison to the α-Co(OH)2. The Rct values were found in the order of α-Co(OH)2 (4.5 Ω) > β-Co(OH)2 (3.4 Ω). The Wo signifies the route of ion diffusion from the electrolyte and is expressed by two parts: Ohmic resistance (W-R) and diffusion time constant (W-T). The W-R value of 25 Ω for β-Co(OH)2, which was lower than 125 Ω of α-Co(OH)2. Moreover, the W-T value of 6 s indicates longer diffusion time constant for α-Co(OH)2 compared to 3 s of β-Co(OH)2. Thus, the open porous network structure of β-Co(OH)2 facilitated charge transfer kinetics. In order to understand properly, the diffusion co-efficient (D) has been determined for the α-Co(OH)2 and βCo(OH)2 using the following equations [51].

The anodic and cathodic peaks of both electrodes shifted to right and left side, respectively, with increasing the scan rates from 10 to 200 mV s─1. The less peak shifts and separations of β-Co(OH)2 were indicative of more reversible and faster charge-transfer process compared to α-Co(OH)2. The area of β-Co(OH)2 encircled by the CV curves was larger than that of α-Co(OH)2, implying a higher capacitance. In order to observe the redox characteristics with scan rates, the current density of cathodic peak vs. the square root of the scan rate was plotted for each electrode (Fig. S3 and S4). The current densities of the electrodes linearly increased with the square roots of the scan rates. The slope (γ) value of β-Co(OH)2 was 11.37, which was larger than 9.41 of α-Co(OH)2, suggesting faster redox kinetics of the former electrode [47,48]. EIS analysis is considered as the principal method to characterize the fundamental behaviour of SC electrodes [5,16]. The Nyquist plots of the Co(OH)2 electrodes were measured in the frequency range from 0.01 to 105 Hz with 10 mV sinusoidal bias as shown in Fig. 5(d). Nyquist plots of α-Co(OH)2 electrode revealed two characteristic regions: a semicircle arc at the high and medium frequency and a vertical line at the low frequency. β-Co(OH)2 showed a depressed semicircle and a straight line vertical to X axis, indicating fast charge transfer and ion diffusion in open porous networked structure [5,16]. The large semicircle of α-Co(OH)2 was associated with slow electrode kinetics due to the presence of Cl─ ions in the interlayer inhibiting charge transfer

D=

L2 W−T

(8)

where, L and W-T are the diffusion length and diffusion time constant, 14

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

Fig. 6. (a) CV curves of the ASC device at different scan rates of 10–100 mV s−1, (b) Nyquist plot of the ASC device (inset shows the high frequency intersection), (c) GCD plots at different current densities of 2–8 A g−1, (d) specific capacitance vs. current density, (e) Ragone plot and (f) stability curve up to 10,000 cycles for the ASC device (inset shows the first and last 10 GCD cycles).

respectively. The L for α-Co(OH)2 and β-Co(OH)2 were found to be 24 and 22 μm, respectively. The D values were calculated to be around 9.60 e─7 and 1.61 e─6 cm2 s─1, for α-Co(OH)2 and β-Co(OH)2, respectively. The lower D value of α-Co(OH)2 signifies lower diffusion of ions [5,50,51], which was resulted by the presence of Cl─ ions as evidenced from TEM and XPS analysis. As shown in the GCD curves at current densities of 2–20 A g─1 in Fig. 5(e) and (f) (Figs. S7 and S8), α-Co(OH)2 and β-Co(OH)2 exhibited the characteristic nonlinear regions in the discharge curves. The voltage plateaus were consistent with the prominent redox waves of the associated CV curves, indicating battery-like Faradic reaction of the redox couple Co3+ and Co2+ [14–16,25,46]. The specific capacitance of α-Co (OH)2 was 613 F g─1 at 2 A g─1. In case of β-Co(OH)2, the specific capacitance was calculated to be 1066 F g─1 at 2 A g─1. The specific capacitance vs. current density of the electrode were plotted in Fig. 5(g). The specific capacitance values decreased with increasing the current density. The capacitance retention of β-Co(OH)2 was 86% (919 F g─1) retained at 10 A g─1 of initial value at 2 A g─1. Even at 20 A g─1, the specific capacitance was retained to be 80% of its initial value (Fig. 5g). By contrast, α-Co(OH)2 revealed the capacitance retentions of 80 and 70% at 10 and 20 A g─1, respectively, which were lower than those of βCo(OH)2. Considering that the rate capability depends on the ion diffusion onto the active sites of electrode and fast charge transfer kinetics of β-Co(OH)2 was associated with the porous characteristics and rodlike network structure [5,7–9]. The cyclic stability of each electrode was evaluated by repeated GCD cycles at 5 A g─1 for 5000 cycles as shown in Fig. 5(h). The capacitance of β-Co(OH)2 was retained to be 86% over 5000 cycles, which was greater than 70% of α-Co(OH)2. The good cyclic stability of β-Co (OH)2 was attributed to the robust crystalline structure and the absence of Cl─ anion. Since the ACS can elevate the cell voltage combining high capacitance electrode and conductive EDLC-type electrode, the energy density can be further improved [7,21,52]. In this study, we used high capacitance β-Co(OH)2 and conductive RGO as two electrodes of the ASC. The RGO displayed a quasi-rectangular CV shape in a voltage window

from 0 to -1 V (Fig. S9), suggesting fast kinetics of the EDLC feature. The charge balance of the positive and negative electrodes was obtained from the CV curves at 50 mV s─1 as shown in Fig. 5(i). Thus, 1.4 V was chosen as the cell voltage of the given ASC. The CV curves of the ASC at the rates of 10–100 mV s─1 in the working voltage of 1.4 V are shown in Fig. 6(a). Unlike the sharp Faradic peaks in the CV curves of β-Co(OH)2 electrode, the ASC exhibited quasi-rectangular CV curves, having two broad distinct humps. This observation indicates the hybridization of EDLC-type RGO and Faradic β-Co(OH)2 electrodes with proper chargebalance in the ASC device [6,7,52]. Even at a high rate of 100 mV s─1, the CV profile still maintained the quasi rectangular shape. The electrochemical performance of the ASC was further examined by EIS analysis (Fig. 6(b)). The ESR was 0.69 Ω, suggesting the contribution of conductive RGO to the fast kinetics of the ASC device [14,49,53]. The existence of a small semicircle arc at the high frequency region suggested fast charge transfer kinetics of the ASC [19,40,41], as demonstrated by the CV and GCD results. The GCD curves of the ASC were obtained varying current densities from 2 to 20 A g─1 as shown in Fig. 6(c) (Fig. S10). The device capacitance of the ASC was calculated to be 74 F g─1 at 2 A g─1, and this value was retained to be 79% of its initial value at 20 A g─1. This high rate capability was consistent with the fast kinetic feature of the ASC device as demonstrated by EIS data. The ASC delivered the maximum energy density of 20.05 W h kg─1 at power density of 1.22 kW kg─1 as shown in Fig. 6(e) [21,52,54]. Even at the maximum power density of 13.40 kW kg─1, the device still maintained an energy density of 19.22 W h kg─1. The energy and power density values are comparable to previous ASC devices using cobalt-based materials [14,55–57,57–63]. In a comparison to previous SC performances using Co(OH)2 based-electrodes (Table S2), our ASC based on β-Co(OH)2 and RGO was superior or comparable to them [14–16,18,20,34,35,55–66]. The long-term cyclic stability of the ASC was measured repeating GCD cycles for 10,000 cycles at 10 A g─1 as shown in Fig. 6(f). The device capacitance of the ASC started to increase slowly at the beginning due to the surface activation and proper wetting of the electrode by electrolyte [67,68]. A careful observation revealed that the specific 15

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

capacitance slowly increased and again decreased in between 200 and 500 cycles as shown in Fig. S11. The specific capacitance was preserved up to 93% after 10,000 cycles. Consequently, the ASC achieved good cyclic stability of the capacitance retention of 93% over 10,000 chargedischarge cycles with the maximum energy and power densities of 20.05 W h kg─1 and 13.40 kW kg─1.

https://doi.org/10.1039/C5CS00580A. [9] 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) 6816–6854, https://doi.org/10.1039/ C7CS00205J. [10] B.G. Choi, Y.S. Huh, W.H. Hong, D. Erickson, H.S. Park, Electroactive nanoparticle directed assembly of functionalized graphene nanosheets into hierarchical structures with hybrid compositions for flexible supercapacitors, Nanoscale 5 (2013) 3976, https://doi.org/10.1039/c3nr33674c. [11] Z. Song, L. Li, D. Zhu, L. Miao, H. Duan, Z. Wang, W. Xiong, Y. Lv, M. Liu, L. Gan, Synergistic design of a N, O co-doped honeycomb carbon electrode and an ionogel electrolyte enabling all-solid-state supercapacitors with an ultrahigh energy density, J J. Mater. Chem. A 7 (2019) 816–826, https://doi.org/10.1039/ C8TA10406A. [12] Z. Song, D. Zhu, L. Li, T. Chen, H. Duan, Z. Wang, Y. Lv, W. Xiong, M. Liu, L. Gan, Ultrahigh energy density of a N, O codoped carbon nanosphere based all-solid-state symmetric supercapacitor, J. Mater. Chem. 7 (2019) 1177–1186, https://doi.org/ 10.1039/C8TA10158B. [13] B.G. Choi, Y.S. Huh, W.H. Hong, H.J. Kim, H.S. Park, Electrochemical assembly of MnO2 on ionic liquid–graphene films into a hierarchical structure for high rate capability and long cycle stability of pseudocapacitors, Nanoscale 4 (2012) 5394, https://doi.org/10.1039/c2nr31215h. [14] T. Deng, W. Zhang, O. Arcelus, J.-G. Kim, J. Carrasco, S.J. Yoo, W. Zheng, J. Wang, H. Tian, H. Zhang, X. Cui, T. Rojo, Atomic-level energy storage mechanism of cobalt hydroxide electrode for pseudocapacitors, Nat. Commun. 8 (2017) 15194, https:// doi.org/10.1038/ncomms15194. [15] T. Zhao, H. Jiang, J. Ma, Surfactant-assisted electrochemical deposition of α-cobalt hydroxide for supercapacitors, J. Power Sources 196 (2011) 860–864, https://doi. org/10.1016/j.jpowsour.2010.06.042. [16] B.G. Choi, M. Yang, S.C. Jung, K.G. Lee, J.-G. Kim, H. Park, T.J. Park, S.B. Lee, Y.K. Han, Y.S. Huh, Enhanced pseudocapacitance of ionic liquid/cobalt hydroxide nanohybrids, ACS Nano 7 (2013) 2453–2460, https://doi.org/10.1021/nn305750s. [17] C. Nethravathi, C.R. Rajamathi, M. Rajamathi, X. Wang, U.K. Gautam, D. Golberg, Y. Bando, Cobalt hydroxide/oxide hexagonal ring–graphene hybrids through chemical etching of metal hydroxide platelets by graphene oxide: energy storage applications, ACS Nano 8 (2014) 2755–2765, https://doi.org/10.1021/nn406480g. [18] A.D. Jagadale, V.S. Jamadade, S.N. Pusawale, C.D. Lokhande, Effect of scan rate on the morphology of potentiodynamically deposited β-Co(OH)2 and corresponding supercapacitive performance, Electrochim. Acta 78 (2012) 92–97, https://doi.org/ 10.1016/j.electacta.2012.05.137. [19] Z. Liu, R. Ma, M. Osada, K. Takada, T. Sasaki, Selective and controlled synthesis of α- and β-cobalt hydroxides in highly developed hexagonal platelets, J. Am. Chem. Soc. 127 (2005) 13869–13874, https://doi.org/10.1021/ja0523338. [20] S. Ranganatha, N. Munichandraiah, Sol–gel synthesis of mesoporous α-Co(OH) 2 and its electrochemical performance evaluation, ACS Omega 3 (2018) 7955–7961, https://doi.org/10.1021/acsomega.8b00760. [21] S. Zhu, L. Li, J. Liu, H. Wang, T. Wang, Y. Zhang, L. Zhang, R.S. Ruoff, F. Dong, Structural directed growth of ultrathin parallel birnessite on β-MnO 2 for highperformance asymmetric supercapacitors, ACS Nano 12 (2018) 1033–1042, https://doi.org/10.1021/acsnano.7b03431. [22] P. Sivakumar, M. Jana, M. Kota, M.G. Jung, A. Gedanken, H.S. Park, Controllable synthesis of nanohorn-like architectured cobalt oxide for hybrid supercapacitor application, J. Power Sources 402 (2018) 147–156, https://doi.org/10.1016/j. jpowsour.2018.09.026. [23] P. Wen, M. Fan, D. Yang, Y. Wang, H. Cheng, J. Wang, An asymmetric supercapacitor with ultrahigh energy density based on nickle cobalt sulfide nanocluster anchoring multi-wall carbon nanotubes hybrid, J. Power Sources 320 (2016) 28–36, https://doi.org/10.1016/j.jpowsour.2016.04.066. [24] L.-Q. Mai, A. Minhas-Khan, X. Tian, K.M. Hercule, Y.-L. Zhao, X. Lin, X. Xu, Synergistic interaction between redox-active electrolyte and binder-free functionalized carbon for ultrahigh supercapacitor performance, Nat. Commun. 4 (2013), https://doi.org/10.1038/ncomms3923. [25] F. Lyu, Y. Bai, Q. Wang, L. Wang, X. Zhang, Y. Yin, Phase-controllable synthesis of cobalt hydroxide for electrocatalytic oxygen evolution, Dalton Trans. 46 (2017) 10545–10548, https://doi.org/10.1039/C7DT01110E. [26] Y. Hou, H. Kondoh, M. Shimojo, T. Kogure, T. Ohta, High-yield preparation of uniform cobalt hydroxide and oxide nanoplatelets and their characterization, J. Phys. Chem. B 109 (2005) 19094–19098, https://doi.org/10.1021/jp0521149. [27] H.S. Park, Y.S. Choi, Y.J. Kim, W.H. Hong, H. Song, 1D and 3D ionic liquid–aluminum hydroxide hybrids prepared via an ionothermal process, Adv. Funct. Mater. 17 (2007) 2411–2418, https://doi.org/10.1002/adfm.200600935. [28] H.Y. Zhu, J.D. Riches, J.C. Barry, γ-Alumina nanofibers prepared from aluminum hydrate with poly(ethylene oxide) surfactant, Chem. Mater. 14 (2002) 2086–2093, https://doi.org/10.1021/cm010736a. [29] H.S. Park, Y.S. Choi, Y.M. Jung, W.H. Hong, Intermolecular interaction-induced hierarchical transformation in 1D nanohybrids: analysis of conformational changes by 2D correlation spectroscopy, J. Am. Chem. Soc. 130 (2008) 845–852, https:// doi.org/10.1021/ja073074k. [30] P.H. Hoang, H. Park, D.-P. Kim, Ultrafast and continuous synthesis of unaccommodating inorganic nanomaterials in droplet- and ionic liquid-assisted microfluidic system, J. Am. Chem. Soc. 133 (2011) 14765–14770, https://doi.org/10. 1021/ja2054429. [31] Y. Zhu, H. Li, Y. Koltypin, A. Gedanken, Preparation of nanosized cobalt hydroxides and oxyhydroxide assisted by sonication, J. Mater. Chem. 12 (2002) 729–733, https://doi.org/10.1039/b107750c. [32] V.V. Chaban, O.V. Prezhdo, Mixtures of 1-butyl-3-methylimidazolium

4. Conclusions In this research, we have demonstrated the control in the crystalline phase, interlayer spacing, and morphology of the Co(OH)2 varying the chemical environment through a hydrothermal reaction by ILs. The hydrothermal reaction of Co2+ precursor in ammonia environment resulted in synthesizing α-Co(OH)2 flakes with the interlayer spacing of 8.24 Å. By contrast, the IL-assisted hydrothermal reaction led to the formation of open porous networked structure β-Co(OH)2 nanorods with the interlayer spacing of 4.63 Å. The XPS results suggested the formation of CoO(OH) along with both the α and β phases. Both α and β phases of Co(OH)2 revealed battery-like redox behaviour, but different redox features due to distinctive oxidation states of Co2+/Co3+. The presence of precursor anions prevented the insertion of hydroxide anions into the active sites of α-Co(OH)2 electrode and thus, a low specific capacitance of 613 F g─1 was observed at 2 A g─1. By contrast, nanorod-like β-Co(OH)2 provided a high specific capacitance of 1066 F g─1at 2 A g─1 and a good rate capability of 80% due to the fast charge transfer kinetics. The ASC combining β-Co(OH)2 and RGO electrodes delivered a large energy density of 20.05 W h kg─1at the power density of 1.22 kW kg─1. Moreover, the ASC maintained the moderate energy density of 19.22 W h kg─1 at a high power density of 13.40 kW kg─1. The capacitance retention of 93% after 10,000 GCD cycles suggested long term stability of the ASC combining β-Co(OH)2 and RGO. Therefore, this research provides the facile strategy for the design of cobalt-based electrode materials and for the improvement of the energy density of SC. Acknowledgements This research was supported by both Korea Electric Power Corporation (Grant number: R17XA05-52) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF- 2017R1D1A1B03036362). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.03.019. References [1] T.M. Gür, Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage, Energy Environ. Sci. 11 (2018) 2696–2767, https://doi.org/10.1039/C8EE01419A. [2] M. Ye, Z. Zhang, Y. Zhao, L. Qu, Graphene platforms for smart energy generation and storage, Joule 2 (2018) 245–268, https://doi.org/10.1016/j.joule.2017.11. 011. [3] J. Vatamanu, D. Bedrov, Capacitive energy storage: current and future challenges, J. Phys. Chem. Lett. 6 (2015) 3594–3609, https://doi.org/10.1021/acs.jpclett. 5b01199. [4] L. Kouchachvili, W. Yaïci, E. Entchev, Hybrid battery/supercapacitor energy storage system for the electric vehicles, J. Power Sources 374 (2018) 237–248, https://doi. org/10.1016/j.jpowsour.2017.11.040. [5] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer US, Boston, MA, 1999http://public.eblib. com/choice/publicfullrecord.aspx?p=3085608 , Accessed date: 6 October 2018. [6] T. Brousse, D. Bélanger, J.W. Long, To Be or not to Be pseudocapacitive? J. Electrochem. Soc. 162 (2015) A5185–A5189, https://doi.org/10.1149/2. 0201505jes. [7] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210–1211, https://doi.org/10.1126/science.1249625. [8] Y. Wang, Y. Song, Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications, Chem. Soc. Rev. 45 (2016) 5925–5950,

16

Journal of Power Sources 422 (2019) 9–17

M. Jana, et al.

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51] T.Q. Nguyen, C. Breitkopf, Determination of diffusion coefficients using impedance spectroscopy data, J. Electrochem. Soc. 165 (2018) E826–E831, https://doi.org/10. 1149/2.1151814jes. [52] Y. Shao, M.F. El-Kady, J. Sun, Y. Li, Q. Zhang, M. Zhu, H. Wang, B. Dunn, R.B. Kaner, Design and mechanisms of asymmetric supercapacitors, Chem. Rev. 118 (2018) 9233–9280, https://doi.org/10.1021/acs.chemrev.8b00252. [53] A. Singh, A. Chandra, Significant performance enhancement in asymmetric supercapacitors based on metal oxides, carbon nanotubes and neutral aqueous electrolyte, Sci. Rep. 5 (2015), https://doi.org/10.1038/srep15551. [54] J. Zhu, youlong Xu, J. Hu, L. Wei, J. Liu, M. Zheng, Facile synthesis of MnO 2 grown on nitrogen-doped carbon nanotubes for asymmetric supercapacitors with enhanced electrochemical performance, J. Power Sources 393 (2018) 135–144, https://doi.org/10.1016/j.jpowsour.2018.05.022. [55] H. Tabassum, A. Mahmood, Q. Wang, W. Xia, Z. Liang, B. Qiu, R. zhao, R. Zou, Hierarchical cobalt hydroxide and B/N Co-doped graphene nanohybrids derived from metal-organic frameworks for high energy density asymmetric supercapacitors, Sci. Rep. 7 (2017), https://doi.org/10.1038/srep43084. [56] L.-B. Kong, M. Liu, J.-W. Lang, Y.-C. Luo, L. Kang, Asymmetric supercapacitor based on loose-packed cobalt hydroxide nanoflake materials and activated carbon, J. Electrochem. Soc. 156 (2009) A1000, https://doi.org/10.1149/1.3236500. [57] Y. Tang, Y. Liu, S. Yu, S. Mu, S. Xiao, Y. Zhao, F. Gao, Morphology controlled synthesis of monodisperse cobalt hydroxide for supercapacitor with high performance and long cycle life, J. Power Sources 256 (2014) 160–169, https://doi.org/ 10.1016/j.jpowsour.2014.01.064. [58] A.D. Jagadale, G. Guan, X. Li, X. Du, X. Ma, X. Hao, A. Abudula, Ultrathin nanoflakes of cobalt–manganese layered double hydroxide with high reversibility for asymmetric supercapacitor, J. Power Sources 306 (2016) 526–534, https://doi.org/ 10.1016/j.jpowsour.2015.12.097. [59] M. Jing, H. Hou, C.E. Banks, Y. Yang, Y. Zhang, X. Ji, Alternating voltage introduced NiCo double hydroxide layered nanoflakes for an asymmetric supercapacitor, ACS Appl. Mater. Interfaces 7 (2015) 22741–22744, https://doi.org/10.1021/acsami. 5b05660. [60] X. Sun, G. Wang, H. Sun, F. Lu, M. Yu, J. Lian, Morphology controlled high performance supercapacitor behaviour of the Ni–Co binary hydroxide system, J. Power Sources 238 (2013) 150–156, https://doi.org/10.1016/j.jpowsour.2013.03.069. [61] G. Godillot, P.-L. Taberna, B. Daffos, P. Simon, C. Delmas, L. Guerlou-Demourgues, High power density aqueous hybrid supercapacitor combining activated carbon and highly conductive spinel cobalt oxide, J. Power Sources 331 (2016) 277–284, https://doi.org/10.1016/j.jpowsour.2016.09.035. [62] J. Zhu, J. Jiang, Z. Sun, J. Luo, Z. Fan, X. Huang, H. Zhang, T. Yu, 3D carbon/cobaltnickel mixed-oxide hybrid nanostructured arrays for asymmetric supercapacitors, Small 10 (2014) 2937–2945, https://doi.org/10.1002/smll.201302937. [63] D. Lee, Q.X. Xia, R.S. Mane, J.M. Yun, K.H. Kim, Direct successive ionic layer adsorption and reaction (SILAR) synthesis of nickel and cobalt hydroxide composites for supercapacitor applications, J. Alloy. Comp. 722 (2017) 809–817, https://doi. org/10.1016/j.jallcom.2017.06.170. [64] Y. Xu, Z. Liu, D. Chen, Y. Song, R. Wang, Synthesis and electrochemical properties of porous α-Co(OH)2 and Co3O4 microspheres, pro, Nat. Sci. Mater. 27 (2017) 197–202, https://doi.org/10.1016/j.pnsc.2017.03.001. [65] X. Lin, H. Li, F. Musharavati, E. Zalnezhad, S. Bae, B.-Y. Cho, O.K.S. Hui, Synthesis and characterization of cobalt hydroxide carbonate nanostructures, RSC Adv. 7 (2017) 46925–46931, https://doi.org/10.1039/C7RA09050A. [66] X. Cai, S.H. Lim, C.K. Poh, L. Lai, J. Lin, Z. Shen, High-performance asymmetric pseudocapacitor cell based on cobalt hydroxide/graphene and polypyrrole/graphene electrodes, J. Power Sources 275 (2015) 298–304, https://doi.org/10.1016/ j.jpowsour.2014.10.204. [67] C. Xiang, M. Li, M. Zhi, A. Manivannan, N. Wu, A reduced graphene oxide/Co3O4 composite for supercapacitor electrode, J. Power Sources 226 (2013) 65–70, https://doi.org/10.1016/j.jpowsour.2012.10.064. [68] W. Ai, W. Zhou, Z. Du, Y. Du, H. Zhang, X. Jia, L. Xie, M. Yi, T. Yu, W. Huang, Benzoxazole and benzimidazole heterocycle-grafted graphene for high-performance supercapacitor electrodes, J. Mater. Chem. 22 (2012) 23439, https://doi.org/10. 1039/c2jm35234f.

tetrafluoroborate ionic liquid and acetonitrile: a molecular simulation, Cond. Mat. Mtrl. Sci. (2011) 1–17. Y. Meng, Synthesis and adsorption property of SiO2@Co(OH)2 core-shell nanoparticles, Nanomaterials 5 (2015) 554–564, https://doi.org/10.3390/ nano5020554. Material and nuclear research school, nuclear science and technology research institute (NSTRI), P.O. Box 14395-834, tehran, Iran, M. Aghazadeh, cobalt hydroxide nanoflakes prepared by saccharide-assisted cathodic electrochemical deposition as high performance supercapacitor electrode material, Int. J. Electrochem. Sci. (2017) 5792–5803, https://doi.org/10.20964/2017.06.48. G. Nagaraju, Y.H. Ko, J.S. Yu, Self-assembled hierarchical β-cobalt hydroxide nanostructures on conductive textiles by one-step electrochemical deposition, CrystEngComm 16 (2014) 11027–11034, https://doi.org/10.1039/C4CE01696C. R. Ma, Z. Liu, K. Takada, K. Fukuda, Y. Ebina, Y. Bando, T. Sasaki, Tetrahedral Co (II) coordination in α-type cobalt hydroxide: rietveld refinement and X-ray absorption spectroscopy, Inorg. Chem. 45 (2006) 3964–3969, https://doi.org/10. 1021/ic052108r. X. Ge, C.D. Gu, X.L. Wang, J.P. Tu, Ionothermal synthesis of cobalt iron layered double hydroxides (LDHs) with expanded interlayer spacing as advanced electrochemical materials, J. Mater. Chem. 2 (2014) 17066–17076, https://doi.org/10. 1039/C4TA03789H. P.F. Liu, S. Yang, L.R. Zheng, B. Zhang, H.G. Yang, Electrochemical etching of αcobalt hydroxide for improvement of oxygen evolution reaction, J. Mater. Chem. 4 (2016) 9578–9584, https://doi.org/10.1039/C6TA04078K. T.A. Mulinari, F.A. La Porta, J. Andrés, M. Cilense, J.A. Varela, E. Longo, Microwave-hydrothermal synthesis of single-crystalline Co3O4 spinel nanocubes, CrystEngComm 15 (2013) 7443, https://doi.org/10.1039/c3ce41215f. J. Yang, H. Liu, W.N. Martens, R.L. Frost, Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs, J. Phys. Chem. C 114 (2010) 111–119, https://doi.org/10.1021/jp908548f. M.A. Sayeed, T. Herd, A.P. O'Mullane, Direct electrochemical formation of nanostructured amorphous Co(OH) 2 on gold electrodes with enhanced activity for the oxygen evolution reaction, J. Mater. Chem. 4 (2016) 991–999, https://doi.org/10. 1039/C5TA09125J. C. Gu, T.-Y. Zhang, Electrochemical synthesis of silver polyhedrons and dendritic films with superhydrophobic surfaces, Langmuir 24 (2008) 12010–12016, https:// doi.org/10.1021/la802354n. Z.L. Wang, Transmission electron microscopy of shape-controlled nanocrystals and their assemblies, J. Phys. Chem. B 104 (2000) 1153–1175, https://doi.org/10. 1021/jp993593c. X. Ge, C.D. Gu, Y. Lu, X.L. Wang, J.P. Tu, A versatile protocol for the ionothermal synthesis of nanostructured nickel compounds as energy storage materials from a choline chloride-based ionic liquid, J. Mater. Chem. 1 (2013) 13454, https://doi. org/10.1039/c3ta13303f. X. Ge, C. Gu, X. Wang, J. Tu, Deep eutectic solvents (DESs)-derived advanced functional materials for energy and environmental applications: challenges, opportunities, and future vision, J. Mater. Chem. 5 (2017) 8209–8229, https://doi. org/10.1039/C7TA01659J. C. Xu, J. Sun, L. Gao, Controllable synthesis of triangle taper-like cobalt hydroxide and cobalt oxide, CrystEngComm 13 (2011) 1586–1590, https://doi.org/10.1039/ C0CE00311E. S. Xu, F. Wang, L. Mai, L. Wang, P.K. Chu, Electrochemical analysis of nickel electrode deposited on silicon microchannel plate, Electrochim. Acta 90 (2013) 344–349, https://doi.org/10.1016/j.electacta.2012.12.044. T.-W. Lin, C.-S. Dai, K.-C. Hung, High energy density asymmetric supercapacitor based on NiOOH/Ni3S2/3d graphene and Fe3O4/graphene composite electrodes, Sci. Rep. 4 (2015), https://doi.org/10.1038/srep07274. M. Jana, S. Saha, P. Samanta, N.C. Murmu, N.H. Kim, T. Kuila, J.H. Lee, Growth of Ni–Co binary hydroxide on a reduced graphene oxide surface by a successive ionic layer adsorption and reaction (SILAR) method for high performance asymmetric supercapacitor electrodes, J. Mater. Chem. A. 4 (2016) 2188–2197, https://doi.org/ 10.1039/C5TA10297A. Impedance Spectroscopy Theory, Experiment, and Applications, Wiley, 2005, https://doi.org/10.1002/0471716243.

17