Ni2Mn1Ox composites on Ni foam for high performance battery-supercapacitor hybrid devices

Journal of Alloys and Compounds 818 (2020) 153350

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis of hierarchical porous Co(OH)2/Ni2Mn1Ox composites on Ni foam for high performance battery-supercapacitor hybrid devices Haicheng Xuan*, Ting Liang, Guohong Zhang, Yayu Guan, Hongsheng Li, Rui Wang, Peide Han, Yucheng Wu College of Materials Science and Engineering, Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 September 2019 Received in revised form 8 December 2019 Accepted 9 December 2019 Available online 10 December 2019

In this work, nickel-manganese metal oxide (NiMnOx) composites with different Ni to Mn molar ratios onto Ni foam (NF) were fabricated by one-pot hydrothermal method. The influence of Ni to Mn molar ratio on the electrochemical performance was investigated and the optimal value was determined to be 2/1 (Ni2Mn1Ox/NF). A Co(OH)2/Ni2Mn1Ox/NF nanocomposite with novel porous honeycomb network structure was designed by electrodeposition Co(OH)2 on the surface of Ni2Mn1Ox/NF. The specific capacity value of Co(OH)2/Ni2Mn1Ox/NF composite electrode material reaches 949.02 C g
1 at 1 A g
1, which is obviously superior to those of the Ni2Mn1Ox/NF electrode. A battery-supercapacitor hybrid (BSH) device was assembeld by developing Co(OH)2/Ni2Mn1Ox/NF with activated carbon (AC), which possesses a promising energy of 47.34 Wh kg
1 at 424.69 W kg
1 power density, cycling retention 100% even after 5000 cycles over a voltage window of up to 1.7 V. This approach generates a strategy to fabricate nanocomposite electrode materials and also provides a promising contender for batterysupercapacitor hybrid devices in energy storage. © 2019 Elsevier B.V. All rights reserved.

Keywords: Molar ratios Hydrothermal Electrodeposition Composites Battery-supercapacitor hybrid devices

1. Introduction In recent years, binary metal oxides synthesized from two costeffective transition metal oxides, combining the advantages of the two components to form mixed transition metal oxides with higher electrochemical performance, have been assumed to be the new excellent electrode materials for a series of energy-related applications [1]. For instance, MnCo2O4 [2], CoFe2O4 [3], CoMoO4 [4] and NiMn2O4 [5] have been proved to be the attractive and promising battery-type electrode materials for battery-supercapacitor hybrid (BSH) device. As an important spinel binary metal oxide, nickelmanganese metal oxide (NiMnOx) has attracted considerable attention as an electrode material in electrochemical reactions for its lower cost, environmental benignity and good electrochemical reactivity [6]. Yang et al. fabricated porous NiMn2O4 nanosheets with high surface area, which exhibited a specific capacitance value of 1321.6 F g
1 at 1 A g
1 [7]. Wang et al. synthesized murdochitetype Ni6MnO8 nanospheres loaded on reduced graphene oxide

* Corresponding author. E-mail address: [email protected] (H. Xuan). https://doi.org/10.1016/j.jallcom.2019.153350 0925-8388/© 2019 Elsevier B.V. All rights reserved.

nanosheets, reaching a better capacitance value of 862.5 F g
1 (0.5 A g
1) [8]. Nevertheless, the application of NiMnOx in energy storage systems is often impeded by its inferior electrical conductivity, lower specific surface area and enlargement in volume during the charge-discharge process [9,10]. It seems that designing composite architecture which combines NiMnOx with metal hydroxides is a brilliant way to enhance its electrochemical performance. Some metal hydroxides such as Co(OH)2 and Ni(OH)2 are perceived as the promising candidates. Among these materials, Co(OH)2 has been proposed as a competitive battery-type materials for BSH devices, because of its welldefined electrochemical redox activity, hierarchical structure and large interlayer space, nature abundance and large theoretical capacity (~3500 F g
1) [11,12]. Thus, Co(OH)2 is often used to form a composite with transition metal oxides (TMO) to boost the electrochemical performance of the composite material. For instance, Ren et al. successfully fabricated NiMoO4@Co(OH)2 core-shell structure by hydrothermal treatment and electrochemical deposition method, which exhibited a high specific capacity of excellent of 2.335 F cm
2 (5 mA cm
2) [13]. Xu et al. reported the synthesis of NieCoeS/Co(OH)2 nanocomposite for SCs with a higher specific capacitance value of 1560.8 F g
1 (1 A g
1) [14]. Therefore, if the

2

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

composite electrode materials can be synthesized by NiMnOx and Co(OH)2, and the full potential of the electrode materials may be realized because the unique properties of individual material can be combined through the synergistic effect [6]. Herein, we designed a low-cost and easy method to construct Co(OH)2/Ni2Mn1Ox (Ni/Mn ¼ 2/1) on Ni foam (NF) as a battery-type materials for BSH device. Firstly, NiMnOx/NF electrode materials with various Ni to Mn molar ratios were prepared by a facile onepot hydrothermal reaction. Interestingly, the morphology and electrochemical properties of the electrode materials vary significantly with the Ni to Mn molar ratios. The highest specific capacity was attained in the molar ratio of 2/1 (Ni2Mn1Ox/NF). Then Co(OH)2 was deposited on the surface of Ni2Mn1Ox/NF by potentiostatic electrodeposition. It is worth mentioning that the highest specific capacity value of 949.02 C g
1 (1 A g
1) and the cycling performance with 83% cycling retention after 5000 cycles (10 A g
1) are achieved in Co(OH)2/Ni2Mn1Ox/NF composite. In addition, the assembled BSH device with Co(OH)2/Ni2Mn1Ox/NF and activated carbon (AC) as positive/negative electrodes exhibits a extraordinary energy density of 47.34 Wh kg
1 at the power density of 424.69 W kg
1, and 100% capacity retention over 5000 cycles. 2. Experimental 2.1. Preparation of NiMnOx/NF In a typical process, Ni foam was washed in acetone, HCl, deionized (DI) water followed by ethanol under sonication for 15 min, respectively. Then, urea (12 mmol), PSS (0.4 g) and different molar ratios of NiCl2$6H2O and MnCl2$6H2O were dispersed in DI water (60 mL) under continuous stirring to form a homogeneous precursor solution. After that, the cleaned Ni foam (2*1 cm2) and precursor solution were poured into a Teflon-lined stainless autoclave (100 mL), followed by a heating at 100  C for 10 h. After a 10 h reaction, the autoclave was taken out and cooled down to the ambient temperature. The products were rinsed by DI water and dried at 60  C for 12h. To obtain the final composites, the precursor was annealed at 300  C for 2 h. The loading of NiMnOx was calculated by the mass difference between NiMnOx/NF and NF, which is about 1e1.2 mg cm
2. The Ni to Mn molar ratios were set as 1/4, 1/2, 1/1, 2/1, 4/1, and the corresponding products were denoted as Ni1Mn4Ox/NF, Ni1Mn2Ox/NF, Ni1Mn1Ox/NF, Ni2Mn1Ox/ NF, and Ni4Mn1Ox/NF, respectively.

2.4. Electrochemical measurements The electrochemical tests including cyclic voltammograms (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectra (EIS) were measured in 2 M KOH electrolyte with a three-electrode system on a CHI660E electrochemical station. In the three-electrode system, the prepared composite (2*1 cm2), Hg/HgO, and platinum were employed as the working electrode, reference electrode, and counter electrode, respectively. The CVs were obtained in a voltage range of 0e0.7 V. GCD tests were measured at the potential window ranging from 0 to 0.5 V. EIS was measured in the frequency range from 0.01 Hz to 100 kHz. 2.5. Fabrication of Co(OH)2/Ni2Mn1Ox/NF//AC BSH device To fabricated the BSH device, the Co(OH)2/Ni2Mn1Ox/NF and AC were employed as the anode and cathode, respectively. The cathode was fabricated by mixing the AC, acetylene black and polyvinyld ifluoride (mass ratio of 8 : 1: 1) to form a uniform slurry. After coating the above slurries on Ni foam, the electrode was dried at 80  C. The relation of the mass loading for positive and nagtive electrodes can be calculated via the following formula [15]:

mþ C
 DV
¼ m
Cþ  DVþ

where m, C and DV represent the mass (g), specific capacity (C g
1), and potential window (V) of the positive and negative materials, respectively. 3. Results and discussion To determine the structure of NiMnOx/NF electrodes with different Ni to Mn molar ratios, the samples were investigated by XRD measurements. As exhibited in Fig. 1, the strong diffraction peaks at 2q of 44.51, 51.85 and 76.37 in all patterns are attributed to the (111), (200) and (220) reflections of the crystalline peaks of metallic Ni from the Ni foam substrate (JCPDS no. 04e0850) [16]. The other peaks are well indexed to the XRD patterns of Mn2O3 (JCPDS no. 76e0150) and Ni2O3 (JCPDS no. 14e0481). Interestingly, the XRD patterns are similar for all the samples with different Ni/ Mn molar ratios. This phenomenon shows that the crystal structure of the composites cannot be changed by tuning the molar ratio of

2.2. Preparation of Co(OH)2/Ni2Mn1Ox/NF Co(OH)2/Ni2Mn1Ox/NF was synthesized by the potentiostatic electrodeposition of Co(OH)2 onto the surface of Ni2Mn1Ox/NF. The electrodeposition process was performed in
1.0 V (vs. SCE) in 0.1 M Co(NO3)2 aqueous electrolyte (50 mL) for 80 s. Then, the prepared composites were carefully washed by DI water, followed by dried in an oven at 60  C for 12 h. Similarly, the loading mass of the Co(OH)2 was calculated by the mass difference of Co(OH)2/ Ni2Mn1Ox/NF and NiMnOx/NF. The loading mass of Co(OH)2 was about 0.4 mg cm
2. 2.3. Material characterizations The structure of the samples was determined using X-ray diffractometer (XRD, TD-3500) with Cu Ka radiation. The surface morphologies were analyzed by SEM (TESCAN, MIRA3) and TEM (JEOL, JEM-2100f). X-ray photoelectron spectroscopy (XPS) was collected on a K-Alpha electron spectrometer with Al Ka source to examine the composition and valence of the composites.

(1)

Fig. 1. XRD patterns of NiMnOx/NF with different Ni/Mn molar ratios.

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

Fig. 2. SEM images of Ni1Mn4Ox/NF (a and b), Ni1Mn2Ox/NF (c and d), Ni1Mn1Ox/NF (e and f), Ni2Mn1Ox/NF (g and h), and Ni4Mn1Ox/NF (i and j) composites.

3

4

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

Ni/Mn, which would be attributed to the invariance of Ni2þ or Mn2þ carbonate hydroxides formed by hydrolysis of urea under hydrothermal conditions [17]. In addition, no impurity peak is detected, confirming the well-defined NiMnOx/NF with high purity. The SEM images of the NiMnOx/NF composites with various Ni/ Mn molar ratios are presented in Fig. 2. Evidently, the morphology of the composites varies greatly with different Ni/Mn molar ratios. When the Ni/Mn molar ratio is low (1/4), the Ni1Mn4Ox/NF displays an ellipsoid structure with a rough surface. As shown in Fig. 2b, these ellipsoids are distributed as isolated islands with a diameter of 10.66 mm. On the basis of previous reports, the surface morphology of MnO2 are composed of a number of outward stretching nano-films that form a layered irregular spherical structure [18]. Hence, the ellipsoid structure would be mainly influenced by the more Mn element of the Ni1Mn4Ox/NF composite. In this case, Ni foam is far from uniformly covered by the Ni1Mn4Ox/ NF composite and the electrolyte ion is difficult to contact with the active material completely, which is unfavorable to the electrochemical performance of the electrode material. With the decrease of the content of Mn element in the complex, the morphology of Ni1Mn2Ox/NF changes towards irregular particles (Fig. 2c). Fig. 2d shows that the particles are randomly distributed on the substrate with the particle size of 0.61e0.91 mm. When the Ni/Mn molar ratio is 1/1 (Fig. 2e and f), the ellipsoid is no longer presented in the

Ni1Mn1Ox/NF sample, forming a large number of flocculating particles with a particle size reduced to 0.29e0.37 mm. At the higher Ni element content, i.e. Fig. 2g (Ni/Mn ¼ 2/1), it is showed that the sample consists of porous agglomerates built by the as-formed particles. Furthermore, the size of the agglomerated particles increases to 0.41e0.42 mm, forming an open flake structure (Fig. 2h). Under this circumstances, Ni2Mn1Ox/NF composite can offer more migration paths for electrolyte ions in the redox process, resulting in higher specific capacity [19]. As the Ni/Mn molar ratio reaches 4/ 1 (Fig. 2i), the lamellar particles began to become thinner and shorter, leading to the porous spherical structure because of the increase of Ni element content in the complex [20]. Fig. 2j presents the obvious high-density spherical products, and the particle size is increased to 1.03e1.32 mm, which would hinder the efficient utilization of Ni4Mn1Ox/NF to some degree [21]. Hence, it is confirmed that the Ni/Mn molar ratios play a crucial role in the morphology of these products. The influence of Ni/Mn molar ratios on the electrochemical properties of the materials was analyzed by a series of electrochemical tests. Fig. 3a shows the CV curves of NiMnOx/NF composites with various Ni/Mn molar ratios at 20 mV s
1. One pair of typical redox peaks are clearly observed in all CV curves, which would be ascribed to the Faradaic redox reactions related to M-O/ M-O-O-H (M refers to Ni and Mn) [22]. Moreover, the enclosed area

Fig. 3. (a) Comparison CV curves of NiMnOx/NF with different Ni/Mn molar ratios at 20 mV s
1, (b) comparison GCD curves of NiMnOx/NF with different Ni/Mn molar ratios at 1 A g
1, (c) specific capacity of NiMnOx/NF with different Ni/Mn molar ratios at various current density, (d) EIS spectrum of NiMnOx/NF with different Ni/Mn molar ratios (the inset is the enlargement of the curves in high frequency region).

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

5

Fig. 4. SEM images of Co(OH)2/NF (a and b) and Co(OH)2/Ni2Mn1Ox/NF (c and d) composites.

of the CV curves and the redox peaks are gradually enlarged as the Ni/Mn molar ratio increases, indicating that the richer redox reactions and the enhanced electrochemical activities. The charge storage capability of the composites can be represented by the surrounded area of the CV curve [23]. Generally, the sample exhibits the larger area of the CV curve represents the higher charge storage ability. Obviously, the Ni2Mn1Ox/NF composite displays the largest area of CV curve, indicating the highest specific capacity and charge storage capability, which is also in accord with the GCD tests. As illustrated in Fig. 3b, Ni2Mn1Ox/NF exhibits the longest discharge time compared to other composites with different Ni/Mn molar ratios. According to the discharge time of GCD curve, the specific capacity of Ni2Mn1Ox/NF is 535.88 C g
1 (1 A g
1), which is higher than that of Ni4Mn1Ox/NF (420.62 C g
1), Ni1Mn1Ox/NF (479.96 C g
1), Ni1Mn2Ox/NF (398.63 C g
1), and Ni1Mn4Ox/NF (126.26 C g
1) composites. These results are consistent with the enclosed area of the CV curves. Moreover, the specific capacity of the NiMnOx/NF electrode with various Ni/Mn molar ratios is compared and displayed in Fig. 3c. In addition, the specific capacity value of the composites declines with the current density increases. As the current density increases from 1 to 30 A g
1, the specific capacity of Ni1Mn4Ox/NF, Ni1Mn2Ox/NF, Ni1Mn1Ox/NF, Ni2Mn1Ox/ NF, and Ni4Mn1Ox/NF remains about 19%, 65%, 54%, 67%, and 48% of its initial specific capacity, respectively. The electrochemical properties of NiMnOx/NF composites are significant influenced by the different Ni/Mn molar ratios, which can be ascribed to the different contents of active metal ions and the changes of morphology. When the content of the Mn element in the composite is higher than that of Ni, the charge transfer is mainly dominated by the Mn element, and the ellipsoid structure reduces the charge transfer rate, resulting in a lower Cs value. As the concentration of Ni element increases, the highly active Ni2þ plays a major role in the Faradaic reaction. For Ni2Mn1Ox/NF, the open flake structure provides a mass of channels for electrolyte penetration and ion intercalation, and increases the amount of highly

electrochemical active sites, improving the utilization of active materials [24]. For Ni4Mn1Ox/NF, although the Ni content is higher, the dense structure hinders the ion transfer, which would result in a relatively low specific capacity. The corresponding EIS spectrum is displayed in Fig. 3d. All samples show a similar plot which contains a nearly straight line in the low frequency range and a semicircle in the high frequency

Fig. 5. (a) TEM image of Co(OH)2/Ni2Mn1Ox/NF, (b) HRTEM image of Ni2Mn1Ox, (c) HRTEM image of Co(OH)2, (d) SAED pattern of Co(OH)2/Ni2Mn1Ox/NF.

6

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

region. The straight line represents the Warburg impedance (ZW), which indicates the diffusive impedance of electrolyte ions in the composites [25]. And the semicircle diameter of EIS is assigned to the charge-transfer resistance (Rct), while the intersection between the curve and the x - axis indicates the resistance of equivalent series (Rs) [26]. From the inset of Fig. 3d, the Ni2Mn1Ox/NF owns lower value, indicating its lower intrinsic resistance. Meanwhile, the diameter of the semicircle region of Ni2Mn1Ox/NF tends to be much smaller, suggesting the lower Rct value, which is beneficial for a better rate capability [27]. Moreover, the slope of the Ni2Mn1Ox/ NF is also larger than that of NiMnOx/NF composites with other Ni/ Mn molar ratios, which means that it is much easier for OH
to spread to the surface of active material [28]. The above EIS analysis further proves the excellent electrochemical performance of the Ni2Mn1Ox/NF composite. To further improve the electrochemical property of the Ni2Mn1Ox/NF composite, Co(OH)2 was successfully deposited on the surface of the composite to form Co(OH)2/Ni2Mn1Ox/NF complex. SEM images of the deposited pure Co(OH)2 electrode are displayed in Fig. 4a and b. It is observed that the Ni foam skeletons are uniformly covered by the nanosheets (Fig. 4a), which is a typical cobalt hydroxides films prepared by electrodeposition [29,30]. These nanosheets are firmly attached to the scaffold, connected to each other to form an orderly open structure (Fig. 4b). In Fig. 4c, the surface of Ni2Mn1Ox/NF composite is covered by a fair amount of Co(OH)2 nanosheets, which are almost vertically grown on the substrate and interconnected together, forming a honeycomb percolation network. Additionally, because of the merits of electrodeposition technique, Co(OH)2 is adhered to the substrate robustly, which would lead to the lower contact resistance between Co(OH)2 nanosheets and the substrate [11]. The porous honeycomb network structure is also beneficial to the effective electron transmission and the diffusion of electrolyte ions, leading to the enhanced charge storage activity [31]. As shown in Fig. 4d, there are a number of voids distributed on the surface of crumpled nanosheets, which is conductive to the valid contact with the electrolyte

and consequently brings more active centers for further shortening the ion transport pathway [32]. Energy dispersive X-ray spectroscopy (EDS) was conducted to investigate the distribution of element in the Co(OH)2/Ni2Mn1Ox/NF composite, as shown in Fig. S1. The elemental mappings exhibit that Ni, Mn, Co, O elements are evenly dispersed on the selected area of the Co(OH)2/Ni2Mn1Ox/ NF composite, revealing the well-distributed of Co(OH)2 and Ni2Mn1Ox. The detailed morphologies of Co(OH)2/Ni2Mn1Ox/NF composite were further examined by TEM. The Co(OH)2 exhibits the typical wrinkled nanosheets and evenly distributed on the substrate (Fig. 5a), consisting with the SEM results. Fig. 5b depicts the HRTEM images of Ni2Mn1Ox in the composite. Obviously, the distribution of pore structure (red circles) can effectively increase the specific surface area and transport channel of ions, thus obtaining better capacity [33]. The lattice fringes with the diatance of 0.28 nm and 0.33 nm correspond to the (002) and (101) lattice planes of Ni2O3, and 0.24 nm represents the (400) lattice plane of Mn2O3, respectively. The HRTEM in Fig. 5c shows clear lattice fringes with interplanar spacing of 0.27 nm and 0.38 nm, which agree well with the (100) and (006) planes of Co(OH)2. The annular patterns displayed in the selected area electron diffraction (SAED) definitely indicate the polycrystallinity of Co(OH)2/Ni2Mn1Ox/NF composite. As shown in Fig. 5d, the rings from inside to outside correspond to the (200) plane of Mn2O3, (101) plane of Ni2O3, and (100) plane of Co(OH)2, respectively. Fig. 6a shows the XRD patterns for comparison of Co(OH)2/NF and Co(OH)2/Ni2Mn1Ox/NF composites. For Co(OH)2/NF, the diffraction peaks appeared at 2q values of 11.54 , 33.54 , 34.47, 59.81 can be assigned to (003), (100), (102), (110) planes of Co(OH)2 (JCPDS no. 46e0605), respectively [34]. The diffraction peaks of Co(OH)2/Ni2Mn1Ox/NF indicate a complex of the Co(OH)2, Ni2O3 and Mn2O3, confirming the successful integration of Ni2Mn1Ox with Co(OH)2. XPS was performed to explore the chemical states and the detailed composition information of the Co(OH)2/ Ni2Mn1Ox/NF composite. The XPS survey spectrum (Fig. 6b)

Fig. 6. (a) XRD patterns of Co(OH)2/NF and Co(OH)2/Ni2Mn1Ox/NF, XPS spectra of Co(OH)2/Ni2Mn1Ox/NF (b) survey, (c)Ni 2p, (d) Mn 2p, (e) Co 2p, and (f) O 1s.

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

7

Fig. 7. (a) Comparison CV curves of Co(OH)2/NF, Ni2Mn1Ox/NF and Co(OH)2/Ni2Mn1Ox/NF at 20 mV s
1, (b) comparison GCD curves of Co(OH)2/NF, Ni2Mn1Ox/NF and Co(OH)2/ Ni2Mn1Ox/NF at 1 A g
1, (c) CV curves of Co(OH)2/Ni2Mn1Ox/NF at various scanning rates, (d) GCD curves of Co(OH)2/Ni2Mn1Ox/NF at various current densities, (e) logarithmic relationship between the peak current and scan rates for Co(OH)2/Ni2Mn1Ox/NF, (f) relative proportions of capacitive-controlled and diffusion-controlled contribution at various scanning rates for Co(OH)2/Ni2Mn1Ox/NF.

demonstrates the existence of Ni, Mn, Co as well as O element. The Ni 2p spectrum (Fig. 6c) exhibits two spin-orbit doublets characteristics of Ni2þ and Ni3þ states and two shake-up statellites with the high binding energy at 865.89 eV for Ni 2p3/2 and 884.12 eV for Ni 2p1/2 [35]. The fitted peaks at 856.95 and 875.87 eV are related with Ni2þ, while other two peaks at 860.05 and 878.80 eV are

related to Ni3þ. Similarly, the Mn 2p spectrum (Fig. 6d) displays two main peaks assigning to the Mn 2p3/2 and Mn 2p1/2 [36]. After fitting, the Mn2p spectrum composes of four peaks. The peaks at the binding energies of 645.77 and 657.56 eV are related with Mn2þ, and the other two peaks at 650.52 and 659.12 eV are assigned to Mn3þ, respectively. The spectra of Co 2p shown in

8

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

Fig. 6e reveals that the peaks at 785.83 and 801.62 eV are correspond to Co 2p3/2 and Co 2p1/2, respectively, with a splitting of 15.79 eV, which are due to the existence of the Co2þ chemical state, indicating the formation of Co(OH)2 [12]. In terms of O 1s (Fig. 6f), it is observed that the O 1s can be deconvoluted into two oxygen contributions at 533.53 and 535.49 eV, which are correspond to the metal-oxygen bonds (M-O-M) and hydroxyl ions (M-O-H) in Co(OH)2 [37]. The above XPS analysis confirms that the product has a hybrid chemical composition of Mn2þ/Mn3þ and Ni2þ/Ni3þ, which is of great importance to enhance the whole electrochemical performance of the composite effectively because it can guarantee sufficient redox reaction in the process of charge-discharge. After successfully combining Ni2Mn1Ox/NF with Co(OH)2, the electrochemical performance of the composite has been greatly improved. For comparison, the CV curves of Co(OH)2/NF, Ni2Mn1Ox/ NF and Co(OH)2/Ni2Mn1Ox/NF are illustrated in Fig. 7a at the scan rate of 20 mV s
1. It is noticeable that the CV curve of Co(OH)2/NF composite presents one anodic peak at 0.19 V, associated with an oxidation reaction as Co(OH)2 þ OH
/ CoOOH þ H2O þ e
[38]. Compared with Ni2Mn1Ox/NF, the area integrations of Co(OH)2/ Ni2Mn1Ox/NF within the CV curves are considerably larger than single material of Ni2Mn1Ox/NF and Co(OH)2/NF, suggesting that the synergistic effect of Ni2Mn1Ox and Co(OH)2 can significantly enhance the electrochemical performance. The corresponding GCD curves are shown in Fig. 7b. Remarkably, Co(OH)2/Ni2Mn1Ox/NF composite shows the relative longer charging and discharging times, indicating its superior electrochemical properties, which is consistent with the CV tests. Fig. 7c displays the CV curves of the Co(OH)2/Ni2Mn1Ox/NF composite at various scanning rates. It is can be seen that the anodic and cathodic peaks shift to positive/negative potential as the scan rate increases from 5 to 50 mV s
1, and the shape of the CVs is maintained without obvious changes, confirming the low polarization and a good rate capability. The GCD curves of Co(OH)2/ Ni2Mn1Ox/NF at various current densities are shown in Fig. 7d. On the basis of the discharge time, the specific capacity value of the Co(OH)2/Ni2Mn1Ox/NF is as high as 949.02 C g
1 at 1 A g
1, and reaches 675.73 C g
1 at 30 A g
1 with a rate capability of 71%, which is a better performance than that of many reported electrodes (Table S1), indicating the great potential in high performance supercapacitor applications. In the Co(OH)2/Ni2Mn1Ox/NF composite, the intrinsic properties of high specific capacity and high conductivity of Co(OH)2 are advantageous for fast electron transport, which can effectively promote the full utilization of active

material [13]. Moreover, the interconnected Co(OH)2 nanosheets are tightly encapsulated on the surface of Ni2Mn1Ox/NF, resulting in the short ion-transport pathway and effective electrolyte diffusion [39,40]. In addition, porous honeycomb percolation network can not only increase the effective active sites for Faradaic energy storage, but also provide more electroactive areas to accelerate the diffusion of electrolyte, facilitating faster kinetics and leading to large charge/discharge capacity even at high current densities [14]. Hence, combing the respective advantages of Ni2Mn1Ox and Co(OH)2 offers an effective way for improving the electrochemical performance of the electrode materials. The CV curves are an effective tool for identifying and analyzing electrochemical processes of electrodes. In order to better elucidate diffusion-controlled capacity and capacitance capacity in the electrochemical reaction process, kinetic analysis was applied in the CV results. Fig. 7e shows the plots of log (peak current) vs. log (sweep rate) of cathodic and anodic peaks for Co(OH)2/Ni2Mn1Ox/NF. Generally, the peak current (i) response of the electrode at various scan rate (mV s
1) is determined by the following relationship [41]:

i ¼ avb

(2)

in which a is a constant. When b ¼ 0.5, it indicates that the redox reaction is under the control of diffusion-dominated reactions, while the value of b is 1 indicating capacitive behavior controlled by a surface faradic redox reaction [42,43]. The b values of cathodic and anodic current are calculated to be 0.56 and 0.67, respectively, which are closer to 0.5, indicating more diffusion-dominated reactions of the Co(OH)2/Ni2Mn1Ox/NF. The capacitive-controlled (k1 v) and diffusion-controlled (k2 v1=2 ) electrochemical processes for electrodes are based on the following equations [44]:

iðVÞ ¼ k1 v þ k2 v1=2 iðVÞ

.

v1=2 ¼ k1 v1=2 þ k2

(3) (4)

here, iðVÞ and v represent the current density and scanning rate, k1 and k2 are constants, respectively. According to the linear fitting of iðVÞ=v1=2 and v1=2 at various scanning rates, k1 and k2 can be obtained. Fig. 7f exhibits the capacitive and diffusion-controlled contributions at various scan rates for Co(OH)2/Ni2Mn1Ox/NF on the basis of the Eqs. (3) and (4). It can be seen that the overall

Fig. 8. (a) Cycling stability and coulombic efficiency of Co(OH)2/Ni2Mn1Ox/NF, (b) EIS plots before and after 5000 cycles.

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

9

Fig. 9. (a) Schematic illustration of the BSH device, (b) CV curves of Co(OH)2/Ni2Mn1Ox/NF and AC at 10 mV s
1, (c) CV curves of BSH device at various scan rates, (d) GCD curves of BSH device at various current densities, (e) cycling stability and coulombic efficiency of BSH device, (f) Ragone plot of BSH device.

contribution of Co(OH)2/Ni2Mn1Ox/NF is diffusion-controlled at the low scan rate, which is consistent with the above results. The capacitive contribution increases gradually with increasing sweep rate since the high sweep rate inhibits the diffusion of ions [45]. The high ratio of capacitive contribution further proves the excellent rate performance of Co(OH)2/Ni2Mn1Ox/NF, indicating that the material exhibits rapid redox reaction and rate-independent behavior in the electrochemical process [46]. Cycling performance plays a key role in the practical applications of electrode materials. As shown in Fig. 8a, the GCD measurement of Co(OH)2/Ni2Mn1Ox/NF was performed for 5000 cycles

at 10 A g
1. After 5000 consecutive charging-discharing cycles, the Co(OH)2/Ni2Mn1Ox/NF exhibits good cycling performance with 83% of the initial capacity. The reduce of capacity may be stems from the volume variation in the redox reaction and the shedding of active material during the long-time cycling process [47,48]. In addition, 99% coulombic efficiency is achieved in the cycling tests. To examine the kinetic mechanism and inherent electrochemical behavior, EIS tests before and after cycles were performed and the Nyquist plots are presented in Fig. 8b. From the inset of Fig. 8b, the change of Rs and Rct at high frequency is quite small after 5000 cycles, which could be attributed to the lower inherent and charge

10

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350

transfer resistance, suggesting the excellent stability properties at the high rate charging-discharging process. To investigate the potential application of the Co(OH)2/Ni2Mn1Ox/ NF composite for energy storage, a BSH device was fabricated by using Co(OH)2/Ni2Mn1Ox/NF as the anode and AC as the cathode, as shown in Fig. 9a. It is necessary to balance the charges relationship qþ ¼ q
to obtain the excellent performance for BSH device [49]. According to Eq. (1), it is calculated that the optimal mass ratio is 0.26 for positive/negative materials and the total mass is around 7.2 mg. The CV measurements were carried out on the anode and cathode materials to estimate the stable potential window of the BSH. As exhibited in Fig. 9b, the CV curve of the AC exhibits a quasirectangular shape without redox peaks under a voltage window of
1.0-0 V, revealing a typical EDLCs behavior of carbon based materials [50]. As for Co(OH)2/Ni2Mn1Ox/NF, a pair of redox peaks is founded in the potential window of 0e0.7 V. Therefore, it can be concluded that the whole voltage window of BSH device could be broadened to 1.7 V by the combination of two electrode materials. Fig. 9c exhibits the obtained CV curves of BSH device at different sweep rates. It is clearly observed that the CV curves possess the Faradaic and non-Faradaic contributions in all the scan rates, confirming the asymmetric behavior of the assembled BSH device. The anodic and cathodic peaks move to more positive and negative potentials as the sweep rate increases, which is consistent with the CV curves observed in the three-electrode system. Moreover, the CV profiles still remain a quasi-rectangular shape without obvious distortion even at the high sweep rates, suggesting the outstanding charge and discharge performance for power device. It is required that an ideal BSH device should provide high specific capacity and large energy density at the high charge/discharge rates. GCD curves of the BSH device at different current densities are exhibited in Fig. 9d. The corresponding value of specific capacity reaches 200.50, 177.01, 157.22, 142.51, 130.29, 128.10 and 115.82 C g
1 at the current density of 0.5, 1, 2, 3, 5, 7 and 10 A g
1, respectively. The cycling stability of the BSH was further evaluated by 5000 cycles at 3 A g
1. As exhibited in Fig. 9e, an obviously increasing trend of capacity is observed within the first 3000 cycles, which would be ascribed to the activation process of electrode material at the beginning of the test [51]. Obviously, the final specific capacity is maintained at 100% after 5000 cycles, indicating the perfect cycle stability. Compared with the single Co(OH)2/Ni2Mn1Ox/NF electrode, the cycling performance of Co(OH)2/Ni2Mn1Ox/NF//AC BSH device is significantly enhanced, largely due to the utilization of AC negative electrode and reasonable assembly of the device. Specifically, the outstanding cycling stability of AC negative electrode is helpful to improve the capacity retention of BSH devices assembled with Co(OH)2/Ni2Mn1Ox/NF as positive electrode. Moreover, the reasonable assembly of BSH device can effectively reduce the shedding of active materials [52]. In addition, a stable coulombic efficiency was calculated as 98%, with no obvious fluctuation during the 5000 cycles. To demonstrate the overall electrochemical properties of the assembled device, the Ragone plot of Co(OH)2/Ni2Mn1Ox/NF//AC (energy density vs. power density) is presented in Fig. 9f. The energy density (E) and power density (P) of the BSH device are calculated by the formulae given blow [33,53]:

E¼

P¼

Cm DV 2 E

Dt

density of 47.34 Wh kg
1 at a power density of 424.69 W kg
1 and still retains a energy density of 27.34 Wh kg
1 under a high power density of 9651.11 W kg
1, which are higher than that of many previously reported devices (Table S2), implying its promising and feasible application for energy storage fields. 4. Conclusions In summary, a facile hydrothermal method was adopted to synthesis of NiMnOx/NF with different Ni/Mn molar ratios. The Ni/ Mn molar ratio is of great important to the morphology and electrochemical performance of the NiMnOx/NF composites. The Ni2Mn1Ox/NF (Ni/Mn ¼ 2/1) shows the largest specific capacity of 535.88 C g
1 (1 A g
1) when the Ni/Mn molar ratio is 2/1. Moreover, the Co(OH)2/Ni2Mn1Ox/NF complex exhibits the enhanced electrochemical properties for BSH device after deposited Co(OH)2 on the surface of Ni2Mn1Ox/NF. Benefiting from the advantages of the complex, Co(OH)2/Ni2Mn1Ox/NF exhibits superior performances with a specific capacity of 949.02 C g
1 at 1 A g
1 compared with pure Ni2Mn1Ox/NF electrode. Besides, an BSH device based on Co(OH)2/Ni2Mn1Ox/NF and AC achieves a relative large energy density of 47.34 Wh kg
1 at the power density of 424.69 W kg
1 and a long cycle life (100% after 5000 cycles). The excellent electrochemical properties demonstrate the promising potential applications of the Co(OH)2/Ni2Mn1Ox/NF in energy storage and conversion devices. Author contributions section Haicheng Xuan and Ting Liang conceived the idea of the study and wrote the paper; Guohong Zhang and Yayu Guan analyzed the data; Hongsheng Li and Rui Wang interpreted the results; all authors discussed the results and revised the manuscript. Declaration of competing interest We would like to declare on behalf of my co-authors that the work is an original research and has not been considered for published elsewhere. All authors of this manuscript have directly participated in planning, execution, and/or analysis of this experiment. And all the authors listed have approved the manuscript that is enclosed. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51401140), the Natural Science Foundation of Shanxi Province, China (Grant No. 201801D121100), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (OIT), the Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (STIP) (Grant No. 201802033), and the Collaborative Innovation Center for Shanxi Advanced Permanent Magnetic Materials and Technology (2016-06). Appendix A. Supplementary data

(5) Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153350.

(6)

where Cm (C g
1) represents the specific capacity of BSH, DV (V) is the operating potential window of BSH, Dt is the discharge time. It is worth nothing that the BSH device achieves an excellent energy

References [1] S. Sahoo, S. Zhang, J.-J. Shim, Porous ternary high performance supercapacitor electrode based on reduced graphene oxide, NiMn2O4, and polyaniline, Electrochim. Acta 216 (2016) 386e396.

H. Xuan et al. / Journal of Alloys and Compounds 818 (2020) 153350 [2] M. Li, W. Yang, J. Li, M. Feng, W. Li, H. Li, Y. Yu, Porous layered stacked MnCo2O4 cubes with enhanced electrochemical capacitive performance, Nanoscale 10 (2018) 2218e2225. [3] G. Liu, B. Wang, T. Liu, L. Wang, H. Luo, T. Gao, F. Wang, A. Liu, D. Wang, 3D self-supported hierarchical core/shell structured MnCo2O4@CoS arrays for high-energy supercapacitors, J. Mater. Chem. 6 (2018) 1822e1831. [4] Z. Yin, Y. Chen, Y. Zhao, C. Li, C. Zhu, X. Zhang, Hierarchical nanosheet-based CoMoO4-NiMoO4 nanotubes for applications in asymmetric supercapacitors and the oxygen evolution reaction, J. Mater. Chem. 3 (2015) 22750e22758. [5] M. Zhang, S. Guo, L. Zheng, G. Zhang, Z. Hao, L. Kang, Z.-H. Liu, Preparation of NiMn2O4 with large specific surface area from an epoxide-driven sol-gel process and its capacitance, Electrochim. Acta 87 (2013) 546e553. [6] Z. Wang, Z. Zhu, C. Zhang, C. Xu, C. Chen, Facile synthesis of reduced graphene oxide/NiMn2O4 nanorods hybrid materials for high-performance supercapacitors, Electrochim. Acta 230 (2017) 438e444. [7] H. Yan, T. Li, K. Qiu, Y. Lu, J. Cheng, Y. Liu, J. Xu, Y. Luo, Growth and electrochemical performance of porous NiMn2O4 nanosheets with high specific surface areas, J. Solid State Electrochem. 19 (2015) 3169e3175. [8] L. Wang, G. Duan, J. Zhu, S.-M. Chen, X. Liu, High capacity supercapacitor material based on reduced graphene oxide loading mesoporpus murdochitetype Ni6MnO8 nanospheres, Electrochim. Acta 219 (2016) 284e294. [9] R.B. Rakhi, W. Chen, D. Cha, H.N. Alshareef, Substrate dependent selforganization of mesoporous cobalt oxide nanowires with remarkable pseudocapacitance, Nano Lett. 12 (2012) 2559e2567. [10] Y.T. Ngo, L. Sui, W. Ahn, J.S. Chung, S.H. Hur, NiMn2O4 spinel binary nanostructure decorated on three-dimensional reduced graphene oxide hydrogel for bifunctional materials in non-enzymatic glucose sensor, Nanoscale 9 (2017) 19318e19327. [11] T. Peng, H. Wang, H. Yi, Y. Jing, P. Sun, X. Wang, Co(OH)2 nanosheets coupled with CNT arrays grown on Ni mesh for high-rate asymmetric supercapacitors with excellent capacitive behavior, Electrochim. Acta 176 (2015) 77e85. [12] U.M. Patil, M.S. Nam, J.S. Sohn, S.B. Kulkarni, R. Shin, S. Kang, S. Lee, J.H. Kim, S.C. Jun, Controlled electrochemical growth of Co(OH)2 flakes on 3D multilayered graphene foam for high performance supercapacitors, J. Mater. Chem. 2 (2014) 19075e19083. [13] W. Ren, D. Guo, M. Zhuo, B. Guan, D. Zhang, Q. Li, NiMoO4@Co(OH)2 core/shell structure nanowire arrays supported on Ni foam for high-performance supercapacitors, RSC Adv. 5 (2015) 21881e21887. [14] T. Xu, G. Li, L. Zhao, Ni-Co-S/Co(OH)2 nanocomposite for high energy density all-solid-state asymmetric supercapacitors, Chem. Eng. J. 336 (2018) 602e611. [15] Y. Ouyang, R. Huang, X. Xia, H. Ye, X. Jiao, L. Wang, W. Lei, Q. Hao, Hierarchical structure electrodes of NiO ultrathin nanosheets anchored to NiCo2O4 on carbon cloth with excellent cycle stability for asymmetric supercapacitors, Chem. Eng. J. 355 (2019) 416e427. [16] X. Nie, X. Kong, D. Selvakumaran, L. Lou, J. Shi, T. Zhu, S. Liang, G. Cao, A. Pan, Three-Dimensional carbon-coated treelike Ni3S2 superstructures on a nickel foam as binder-free bifunctional electrodes, ACS Appl. Mater. Interfaces 10 (2018) 36018e36027. [17] X. Qian, H. Li, L. Shao, X. Jiang, L. Hou, Morphology-Tuned synthesis of nickel cobalt selenides as highly efficient Pt-free counter electrode catalysts for dyesensitized solar cells, ACS Appl. Mater. Interfaces 8 (2016) 29486e29495. [18] Q. Lu, L. Liu, S. Yang, J. Liu, Q. Tian, W. Yao, Q. Xue, M. Li, W. Wu, Facile synthesis of amorphous FeOOH/MnO2 composites as screen-printed electrode materials for all-printed solid-state flexible supercapacitors, J. Power Sources 361 (2017) 31e38. [19] U.M. Patil, J.S. Sohn, S.B. Kulkarni, S.C. Lee, H.G. Park, K.V. Gurav, J.H. Kim, S.C. Jun, Enhanced supercapacitive performance of chemically grown cobaltnickel hydroxides on three-dimensional graphene foam electrodes, ACS Appl. Mater. Interfaces 6 (2014) 2450e2458. [20] M.H. Tahmasebi, A. Vicenzo, M. Hashempour, Nanosized Mn-Ni oxide thin films via anodic electrodeposition: a study of the correlations between morphology, structure and capacitive behaviour, Electrochim. Acta (2016) 143e154. [21] S. Liu, S.C. Lee, U. Patil, I. Shackery, S. Kang, K. Zhang, J.H. Park, K.Y. Chung, S. Chan Jun, Hierarchical MnCo-layered double hydroxides@Ni(OH)2 coreeshell heterostructures as advanced electrodes for supercapacitors, J. Mater. Chem. 5 (2017) 1043e1049. [22] H. Nan, W. Ma, Z. Gu, B. Geng, X. Zhang, Hierarchical NiMn2O4@CNT nanocomposites for high-performance asymmetric supercapacitors, RSC Adv. 5 (2015) 24607e24614. [23] S. Chen, H. Chen, C. Li, M. Fan, Tuning the electrochemical behaviour of CoxMn3-x sulfides by varying different Co/Mn ratios in supercapacitor, J. Mater. Sci. 52 (2017) 6687e6696. [24] Q. Li, Y. Li, H. Peng, X. Cui, Layered NH4CoXNi1-XPO4$H2O nanostructures finely tuned by Co/Ni molar retios for asymmetric supercapacitor electrodes, J. Mater. Sci. 51 (2016) 9946e9957. [25] B. Kirubasankar, P. Palanisamy, S. Arunachalam, V. Murugadoss, S. Angaiah, 2D MoSe2-Ni(OH)2 nanohybrid as an efficient electrode material with high rate capability for asymmetric supercapacitor applications, Chem. Eng. J. 355 (2019) 881e890. [26] J.-J. Zhou, Q. Li, C. Chen, Y.-L. Li, K. Tao, L. Han, Co3O4@CoNi-LDH core/shell nanosheet arrays for high-performance battery-type supercapacitors, Chem. Eng. J. 350 (2018) 551e558. [27] D. Liu, Q. Wang, L. Qiao, F. Li, D. Wang, Z. Yang, D. He, Preparation of nanonetworks of MnO2 shell/Ni current collector core for high-performance supercapacitor electrodes, J. Mater. Chem. 22 (2012) 483e487.

11

[28] Z. Li, L. Zhang, B.S. Amirkhiz, X. Tan, Z. Xu, H. Wang, B.C. Olsen, C.M.B. Holt, D. Mitlin, Carbonized chicken eggshell membranes with 3D architectures as high-performance electrode materials for supercapacitors, Adv. Energy Mater. 2 (2012) 431e437. nio, T.M. Silva, M.J. Carmezim, M.F. Montemor, a[29] R. Della Noce, S. Euge Co(OH)2/carbon nanofoam composite as electrochemical capacitor electrode operating at 2 V in aqueous medium, J. Power Sources 288 (2015) 234e242. [30] A.D. Jagadale, V.S. Jamadade, S.N. Pusawale, C.D. Lokhande, Effect of scan rate on the morphology of potentiodynamically deposited b-Co(OH)2 and corresponding supercapacitive performance, Electrochim. Acta 78 (2012) 92e97. [31] J. Zhao, Z. Li, X. Yuan, T. Shen, L. Lin, M. Zhang, A. Meng, Q. Li, Novel core-shell multi-dimensional hybrid nanoarchitectures consisting of Co(OH)2 nanoparticles/Ni3S2 nanosheets grown on SiC nanowire networks for highperformance asymmetric supercapacitors, Chem. Eng. J. 357 (2019) 21e32. [32] J. Liu, M. Zheng, X. Shi, H. Zeng, H. Xia, Amorphous FeOOH quantum dots assembled mesoporous film anchored on graphene nanosheets with superior electrochemical performance for supercapacitors, Adv. Funct. Mater. 26 (2016) 919e930. [33] S. Li, W. Huang, Y. Yang, J. Ulstrup, L. Ci, J. Zhang, J. Lou, P. Si, Hierarchical layer-by-layer porous FeCo2S4@Ni(OH)2 arrays for all-solid-state asymmetric supercapacitors, J. Mater. Chem. 6 (2018) 20480e20490. [34] A. Roy, H.S. Jadhav, G.M. Thorat, J.G. Seo, Electrochemical growth of Co(OH)2 nanoflakes on Ni foam for methanol electro-oxidation, New J. Chem. 41 (2017) 9546e9553. [35] W. Kang, Y. Tang, W. Li, X. Yang, H. Xue, Q. Yang, C.-S. Lee, High interfacial storage capability of porous NiMn2O4/C hierarchical tremella-like nanostructures as the lithium ion battery anode, Nanoscale 7 (2015) 225e231. [36] K. Vijaya Sankar, S. Surendran, K. Pandi, A.M. Allin, V.D. Nithya, Y.S. Lee, R. Kalai Selvan, Studies on the electrochemical intercalation/de-intercalation mechanism of NiMn2O4 for high stable pseudocapacitor electrodes, RSC Adv. 5 (2015) 27649e27656. [37] Y. Liu, X. Teng, Y. Mi, Z. Chen, A new architecture design of Ni-Co LDH-based pseudocapacitors, J. Mater. Chem. 5 (2017) 24407e24415. [38] Y. Kou, J. Liu, Y. Li, S. Qu, C. Ma, Z. Song, X. Han, Y. Deng, W. Hu, C. Zhong, Electrochemical oxidation of chlorine-doped Co(OH)2 nanosheet arrays on carbon cloth as a bifunctional oxygen electrode, ACS Appl. Mater. Interfaces 10 (2018) 796e805. [39] J.-K. Chang, C.-M. Wu, I.W. Sun, Nano-architectured Co(OH)2 electrodes constructed using an easily-manipulated electrochemical protocol for highperformance energy storage applications, J. Mater. Chem. 20 (2010) 3729. [40] S. Chou, J. Wang, H. Liu, S. Dou, Electrochemical deposition of porous Co(OH)2 nanoflake films on steinless steel mesh for flexible supercapacitors, J. Electrochem. Soc. 155 (2008) A926eA929. [41] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, High-rate electrochemical energy storage through Liþ intercalation pseudocapacitance, Nat. Mater. 12 (2013) 518e522. [42] X. Wang, G. Shen, Intercalation pseudocapacitive TiNb2O7@carbon electrode for high-performance lithium ion hybrid electrochemical supercapacitors with ultrahigh energy density, Nano Energy 15 (2015) 104e115. [43] M. Liang, M. Zhao, H. Wang, Q. Zheng, X. Song, Superior cycling stability of a crystalline/amorphoous Co3O4 core-shell heterostructure for aqueous hybrid supercapacitors, J. Mater. Chem. 6 (2018) 21350e21359. [44] Y. Yang, W. Huang, S. Li, L. Ci, P. Si, Surfactant-dependent flower-and grasslike Zn0.76Co0.24S/Co3S4 for high-performance allsolid-state asymmetric supercapacitors, J. Mater. Chem. (2018) 22830e22839, 2018. [45] L. Ye, Y. Zhou, Z. Bao, Y. Zhao, Y. Zou, L. Zhao, Q. Jiang, Sheet-membrane Mndoped nickel hydroxide encapsulated via heterogeneous Ni3S2 nanoparticles for efficient alkaline battery-supercapacitor hybrid devices, J. Mater. Chem. 6 (2018) 19020e19029. [46] Y. Lan, H. Zhao, Y. Zong, X. Li, Y. Sun, J. Feng, Y. Wang, X. Zheng, Y. Du, Phosphorization boosts the capacitance of mixed metal nanosheet arrays for high performance supercapacitor electrodes, Nanoscale 10 (2018) 11775e11781. [47] Y. Zou, Y. Cui, Z. Zhou, P. Zan, Z. Guo, M. Zhao, L. Ye, L. Zhao, Formation of honeycomb-like Mn-doping nickel hydroxide/Ni3S2 nanohybrid for efficient supercapacitive storage, J. Solid State Chem. 267 (2018) 53e62. [48] G.P. Kamble, A.A. Kashale, S.S. Dhanayat, S.S. Kolekar, A.V. Ghule, Binder-free synthesis of high-quality nanocrystalline ZnCo2O4 thin film electrodes for supercapacitor application, Bull. Mater. Sci. 42 (2019) 272. [49] R. Li, S. Wang, Z. Huang, F. Lu, T. He, NiCo2S4@Co(OH)2 core-shell nanotube arrays in situ grown on Ni foam for high performances asymmetric supercapacitors, J. Power Sources 312 (2016) 156e164. [50] X. Bai, Q. Liu, H. Zhang, J. Liu, Z. Li, X. Jing, Y. Yuan, L. Liu, J. Wang, Nickel-cobalt layered double hydroxide nanowires on three dimensional graphene nickel foam for high performance asymmetric supercapacitors, Electrochim. Acta 215 (2016) 492e499. [51] X. Li, R. Ding, W. Shi, Q. Xu, D. Ying, Y. Huang, E. Liu, Hierarchical porous Co(OH) F/Ni(OH)2 : a new hybrid for supercapacitors, Electrochim. Acta 265 (2018) 455e473. [52] Y. Du, G. Li, M. Chen, X. Yang, L. Ye, X. Liu, L. Zhao, Hollow nickel-cobaltmanganese hydroxide polyhedra via MOF templates for high-performance quasi-solid-state supercapacitor, Chem. Eng. J. 378 (2019) 122210. [53] Z. Fang, S.u. Rehman, M. Sun, Y. Yuan, S. Jin, H. Bi, Hybrid NiO-CuO mesoporous nanowire array with abundant oxygen vacancies and a hollow structure as a high-performance asymmetric supercapacitor, J. Mater. Chem. 6 (2018) 21131e21142.