Hierarchical Mesoporous ZincâNickelâCobalt Ternary Oxide

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Hierarchical Mesoporous Zinc-Nickel-Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High Performance Electrodes for Supercapacitors Chun Wu, Junjie Cai, Qiaobao Zhang, Xiang Zhou, Ying Zhu, Peikang Shen, and Kaili Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07607 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 20, 2015

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ACS Applied Materials & Interfaces

Hierarchical Mesoporous Zinc-Nickel-Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High Performance Electrodes for Supercapacitors Chun Wu a, Junjie Cai a, Qiaobao Zhang a, Xiang Zhou a, Ying Zhu a, Pei Kang Shen b, *, Kaili Zhang a,* a

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong b Department of Physics and Engineering, Sun Yat-sen University, Guangzhou, 510275, China

* Corresponding authors, E-mail address: [email protected] and [email protected]

Abstract Nickel foam supported hierarchical mesoporous Zn-Ni-Co ternary oxide (ZNCO) nanowire arrays are synthesized by a simple two-step approach including hydrothermal method and subsequent calcination process and directly utilized for the supercapacitive investigation for the first time. The nickel foam supported hierarchical mesoporous ZNCO nanowire arrays possess an ultrahigh specific capacitance value of 2481.8 F g-1 at 1 A g-1 and excellent rate capability of about 91.9% capacitance retention at 5 A g-1. More importantly, an asymmetric supercapacitor with a high energy density (35.6 Wh kg-1) and remarkable cycle stability performance (94% capacitance retention over 3000 cycles) is assembled successfully by employing the ZNCO electrode as positive electrode and activated carbon as negative electrode. The remarkable electrochemical behaviors demonstrate that the nickel foam supported hierarchical mesoporous ZNCO nanowire array electrodes are highly desirable for application as advanced supercapacitor electrodes.

Keywords: Zn-Ni-Co ternary oxide; Nanowire arrays; Asymmetric supercapacitor; Electrode materials; Energy storage 1

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1. Introduction Among various energy storage devices, supercapacitors have been extensively studied thanks to the wonderful cycle stability, high power density, and fast charge/discharge rate.1-6 In particular, considerably greater attention has been focused on faradaic capacitors, as faradaic capacitance is higher than double layer capacitance and the energy density associated with Faradaic reaction is larger than that of electrical double-layer capacitors (EDLCs).7 Especially, more attention has been paid on asymmetric supercapacitor with a battery-type positive electrode and an activated carbon negative electrode because of their wider operating potential windows and larger specific capacitances. These properties can allow the asymmetric supercapacitor to satisfy the needs of emerging applications.8,9 Recently, numerous attempts have been conducted to develop transition metal oxides (MnO2, NiO, Co3O4, NiCo2O4, ZnCo2O4 et al.) as battery-type positive electrodes used in asymmetric supercapacitors due to the low price, abundance in resources, and facile and scalable preparation.10-19 Li et al. have reported hierarchical porous NiCo2O4 nanowires by a simple scalable strategy. These nanowires showed excellent rate performance and cycling stability.16 However, several drawbacks of the transition metal oxides, including the low electron conductivities, slow ion diffusion rates and big volume change of the electrode materials, limit the applications of high-performance supercapacitor. Recently, three-dimensional (3D) hierarchical architectures that offer various advantages including better permeabilities and large specific surface areas resulting in reactive sites, have been studied for practical applications in biomedical science, catalysis, sensors, lithium ion batteries and supercapacitors.20,21 Moreover, studies about the electrode materials that are directly grown on the conductive current collectors including nickel foam and carbon cloth as supercapacitor electrodes have shown that the approach can significantly boost the 2


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electrochemical properties of the electrode materials.22,23 Therefore, the 3D substrate supported hierarchical architectures are interesting because of their large surface areas, easy electrolyte access to electrode, efficient electron transfer, fast ion transport, and good strain accommodation.24-32 This way can not only achieve high contact between substrate and electrode active materials resulting in great improvement of ion transportation, but also provide channels for the transportation of the charge. The above properties are beneficial to maximize the utilization of electrochemically active material. Although numerous previous studies related to transition metal oxides have been reported, only a few reports are related to ternary oxides, which are used as supercapacitor electrodes. Luo et al. have synthesized nanostructured Mn-Ni-Co oxide composites (MNCO), and the results showed that a maximum capacitance of 1260 F g-1 could be achieved within -0.1 to 0.4 V potential range.33 Another report by Li et al. demonstrated a facile hydrothermal method to obtain the aligned spinel Mn-Ni-Co ternary oxide (MNCO) nanowires. The resulting MNCO nanowires show a specific capacitance of 638 F g-1 at 1 A g-1 and exhibited excellent cycling stability.34 However, the electrochemical behaviors of the above ternary oxides have yet to be improved. Furthermore, until recently, there are very few studies about Zn-Ni-Co ternary oxides (ZNCO) and their application in supercapacitors. Compared with pure Co3O4 electrode materials, the ZNCO electrode materials would greatly reduce the cost and possess better safety performance at the same time. Meanwhile, ZNCO electrode materials (including contributions from cobalt, nickel, and zinc ions) are expected to offer a synergistic effect on redox reactions, relative to corresponding single component oxides. This effect is due to the fact that Ni provides a high capacity and can improve the active site density, conductivity and roughness, Co offers increased electronic conductivity and Zn possesses good electrical conductivity that can result in 3



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the improvement of electrical conductivity and capacitive performance.3,5 Moreover, the incorporation of several metal ions may yield multi-phase metal oxides and introduce abundant structural defects. This approach can also achieve improved stability and cycle life of the metal oxide electrode. Herein, we report a facile and environmental-friendly strategy to prepare 3D nickel foam supported hierarchical mesoporous ZNCO nanowire arrays (ZNCO electrode) and investigate their physical and electrochemical characteristics as binder-free electrode material for supercapacitors. To acquire the supercapacitors with high energy density, an asymmetric supercapacitor (ASC) using the ZNCO electrode materials as positive electrode and activated carbon (AC) materials as negative electrode has been successfully prepared. This ASC device exhibits a high specific capacitance value of 113.9 F g-1 with corresponding energy density and power density of 28.5 Wh kg-1 and 150.1 W kg-1, respectively. In addition, the ASC device shows the high coulombic efficiency of almost 100% and remarkable cycle stability with 94.1% of its initial capacity retention during the cycling process. All these excellent behaviors in terms of high specific capacitance, good rate capability as well as wonderful cycle stability make the nickel foam supported hierarchical mesoporous ZNCO nanowire arrays as a member of the most attractive and promising candidates for supercapacitor electrode materials.

2. Experimental 2.1. Synthesis of nickel foam supported hierarchical mesoporous ZNCO nanowire arrays Via a simple hydrothermal method and subsequent calcination procedure, the hierarchical mesoporous ZNCO nanowire arrays grown on nickel foam were prepared. A piece of nickel foam with 3.5 cm × 6 cm size was treated in an ultrasound bath in pure acetone solution for 1 h 4


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and rinsed with deionized (DI) water for several times. In a typical preparation process, 6 mmol cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 3 mmol zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 3 mmol nickel nitrate hexahydrate (Ni(NO3)2·6H2O), 12 mmol urea (CO(NH2)2) and 4 mmol ammonium fluoride (NH4F) were mixed with 80 mL DI water. The reaction mixture and the cleaned nickel foam were put into a 100 mL Teflon-lined stainless steel autoclave and kept at 130 oC for 5 h. After the autoclave was cooled down, the samples were washed in an ultrasound bath for several times and dried at 60 oC for 12 h. Finally, the obtained samples were put into the furnace and annealed at 350 oC in air for 2 h to obtain nickel foam supported hierarchical mesoporous ZNCO nanowire arrays. For comparison, the nickel foam supported ZnCo2O4 and Co3O4 electrode material were prepared by the same method and named as ZCO and CO. The mass of ZNCO, ZCO, and CO loaded on the nickel foam substrate are 1.5, 1.8, and 1.7 mg cm−2 on average.

2.2. Characterization of materials In order to gain the morphology and microstructure information of the ZNCO electrode materials, field emission scanning electron microscopy (FESEM Hitachi S4800) and transmission electron microscopy (TEM JEM-2010HR) were used. By using X-ray diffraction (Rigaku SmartLab XRD), the crystalline structure of the ZNCO electrode materials can be obtained. Meanwhile, to examine the hierarchical mesoporous structure of the ZNCO electrode material, a Quantachromeautosorb automated gas sorption system was utilized and adsorption/desorption isotherms of nitrogen were measured at 77 K.

2.3. Electrochemical measurements 5



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Via a three-electrode system with the electrolyte of 6 M KOH, the electrochemical behaviors of ZNCO electrodes were tested. A piece of nickel foam was chosen as counter electrode and the Hg/HgO electrode was selected as reference electrode. All the electrochemical characterizations of the single electrode, including cyclic voltammetry measurements (CV), galvanostatic charge/discharge tests as well as electrochemical impedance spectroscopy (EIS), were proceeded on a electrochemical workstation system CHI660. Electrochemical tests of the ASC device were carried out in a two-electrode electrochemical cell that was impregnated using 6 M KOH as electrolyte. The scheme of the ASC device is shown in Figure S1 (see Supporting Information). Particularly, the negative electrode material of the supercapacitor consisted of 80 wt % activated carbon bought from Kejing Co. Ltd., 10 wt% acetylene black and 10 wt% polyvinylidene fluoride. The mixture was coated onto nickel foam after being well dispersed in 1-methylpyrrolidone solvent. Finally, the fabricated electrode was dried and then pressed under a pressure of 1.6×107 Pa. The electrochemical behaviors of the fabricated ASC device were tested through the above methods in ambient conditions and the voltage range varied from 0 to 1.5 V. The electrochemical performance of supercapacitor is largely influenced by the charge balance between positive and negative electrodes. The charge balance should follow the equation q+=q-, in which q+ and q- represent the charges stored by the positive and negative electrodes, respectively. The following equation shows the relationship of the mass between positive and negative electrodes:35

m+ C− × ∆E− = m− C+ × ∆E+

6


(1)

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where m represents the mass (g) of electrode, C stands for the specific capacitance (F g-1) and ∆E shows the potential range (V) during the charge/discharge procedure.

3. Results and discussion 3.1. Structure analysis Figure 1 shows the formation process of the hierarchical mesoporous ZNCO electrode materials. First, under the hydrothermal condition of 130 oC for 5 h, the raw materials react and form the precursor of hierarchical mesoporous ZNCO electrode materials. Then, the precursor of the hierarchical mesoporous ZNCO electrode materials are calcined in air at 350 oC for 2 h and transformed into the final products. The hierarchical mesoporous ZNCO electrode materials grown on the nickel foam provide high contact between the substrate and the electrode active material, which would shorten the diffusion path lengths of electrons and ions and obtain a low contact resistance, resulting in outstanding electrochemical behaviors of the electrode materials.

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Figure 1 Scheme of the formation process of the hierarchical mesoporous ZNCO electrode materials

The morphology and the microstructure of hierarchical mesoporous ZNCO electrode materials are characterized by SEM and TEM. From the low magnification SEM image in Fig. 2(a), ZNCO nanowire arrays with needle-like shapes are grown homogeneously on the substrate to form a 3D nanostructure, which can provide a 3D interconnected network of both electron and ion pathways, allowing for efficient charge and mass exchange during faradic redox reactions.32 In addition, the length of each nanowire is approximately 2 to 3 µm, and diameter of the nanowire is about 10 to 30 nm. The hierarchical structures of ZNCO nanowires arrays resulting from this procedure can also be prepared on different substrates as shown in Fig. S2, including Si wafer, Cu foam and carbon cloth, and the morphologies on these substrates do not differ from that of nickel foam supported hierarchical mesoporous ZNCO nanowire arrays. More interestingly, this sample, efficient, and versatile strategy can be used to prepare different nanostructures, such as mesoporous microspheres, nanowalls and so on (see Fig. S3), by using different metal salts, indicating that the high efficiency and broad applications of this synthesis strategy. The morphology and structure of nickel foam supported hierarchical mesoporous ZNCO nanowire arrays are further characterized by TEM. Fig. 2(d) presents a TEM image of an individual nanowire. The porous structure of the ZNCO nanowire can be evidently observed. These highly porous structures are composed of nanocrystallites approximately 5 nm in size. Furthermore, numerous mesopores with size ranging from 2-10 nm in these areas can be clearly seen. This unique mesoporous morphological characteristic is proven to be beneficial for electrolyte penetration and rapid ion/electron transfer, thereby leading to enhanced 8


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electrochemical reactivity [36, 37]. From the HRTEM image of the ZNCO nanowire in Fig. 2(e), interplanar spacing, which is measured to be 0.462 nm, corresponds to the (111) lattice planes of the ZNCO electrode materials. The SAED pattern in Fig. 2(f) shows the well-defined rings of the ZNCO nanowire, demonstrating that the ZNCO materials are of polycrystalline nature. The calculated d values of 0.28, 0.24, 0.20, 0.15 and 0.14 nm corresponds to the (220), (311), (400), (511) and (440) Miller indices, respectively.

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(a)

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(b)

5 µm

10 µm

(d)

(c)

1 µm

50 nm

(e)

(f)

(311) (220) (400)

d=0.462 nm (111)

porous

(511)

(440)

5 nm

51/nm

Figure 2 (a, b and c) SEM images of nickel foam supported hierarchical mesoporous ZNCO nanowire arrays, (d) TEM image and (e) HRTEM image of an individual ZNCO nanowire, (f) SAED pattern of an individual ZNCO nanowire

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Figure 3 (a-e) shows the TEM and related elemental mapping results of ZNCO nanowire. The images present Zn, Ni, Co and O elements, which are distributed uniformly on each nanowire. The composition of the ZNCO electrode material scratched from the nickel foam is investigated by EDX analysis, which confirms the existence of Zn, Ni, Co and O in Fig. 3(f). The atom ratio of Zn : Ni : Co is 0.62 : 1.01 : 2.20, demonstrating that cobalt atoms are partially replaced by both zinc and nickel atoms.

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(a)

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(b)

700 nm

700 nm

(c)

Zn

(d)

Ni

700 nm

700 nm O

(e)

Co

(f)

700 nm

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Figure 3 (a-e) TEM mapping results of individual ZNCO nanowire arrays, (f) EDX spectrum of hierarchical mesoporous ZNCO nanowire arrays

The structure of nickel foam supported hierarchical mesoporous ZNCO nanowire arrays are characterized by XRD analysis after being peeled off from the nickel foam before the test to exclude influences generated from the substrate. All XRD diffraction peaks of the as-prepared ZNCO electrode materials in Fig. 4(a) can be indexed as the spinel structure phase of the space group Fd3m.19 Meanwhile, the diffraction peaks of the ternary oxide slightly shift in comparison with those of the spinel Co3O4 (PDF No. 74-2120),32 it is due to the differences of the metal ionic radii of Co, Zn and Ni. However, the substitution of Zn and Ni does not significantly affect the crystal structure of spinel Co3O4.34 Furthermore, no additional peaks for other phases are observed. The microstructures of hierarchical mesoporous ZNCO electrode materials are investigated through the nitrogen adsorption tests. Figure 4(b) presents the nitrogen adsorption-desorption isotherms and pore size distribution curves. Notably, the nitrogen adsorption-desorption isotherm of the ZNCO electrode material is close to that of IUPAC type-IV with distinct hysteresis loops,38 indicating the existence of mesopores. The inset of Fig. 4(b) shows the pore size distribution of the as-prepared ZNCO electrode material, which is centered at about 4.5 nm. This result further confirms the existence of the mesoporous structure. The pore volume and BET specific surface area of the hierarchical mesoporous ZNCO electrode material are tested to be 0.29 cm3 g-1 and 84.2 m2 g-1, respectively. The large specific surface area could significantly result in increased electrochemical reactive sites and better penetration of the electrolyte into the whole electrode materials, which will generate the high supercapacitive performance. 13



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Figure 4 (a) XRD pattern and (b) N2 absorption-desorption isotherm of the hierarchical mesoporous ZNCO electrode materials, the inset is the pore distribution

3.2. Electrochemical behaviors of the ZNCO electrode Capacitive behavior of the ZNCO electrode is studied by CV with the voltage range varied from 0 to 0.55 V. Comparisons of CV curves between of the CO, ZCO, and ZNCO electrodes at a scan rate of 2 mV s-1 are presented in Fig. 5(a). Evidently, the area surrounded by the CV curve of ZNCO electrodes is considerably larger and the redox current is higher than that of the ZCO and CO electrode, indicating the much more superior capacitive behavior of the ZNCO electrode. The reason may be that the addition of nickel element, which can provide high capacity of the ZNCO electrode. The peak positions of the three electrode materials present slightly shifted redox peaks in the CV plot, which can be ascribed to the different polarization behavior of the different electrode materials.17 The CV curves of the ZNCO electrode under 1 to 10 mV s-1 are presented in Fig. 5(b). The shapes of the CV curves obviously show the typical faradaic behavior of the ZNCO electrode. As can be clearly observed, a pair of redox peaks exists in the CV curves, thereby demonstrating that the electrochemical behaviors of ZNCO electrode generate from their 14


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faradaic reactions. To acquire more detailed information about the reactions occurring during electrochemical tests, X-ray photoelectron spectroscopy measurements are carried out on the electrode taken out from the charge/discharge test and the corresponding spectra are presented in Fig. S4. The results indicate that the energy storage mechanism of the hierarchical mesoporous ZNCO electrode originates mainly from Faradic redox reactions assigned to the M-O/M-O-OH,39 where the M represents both Ni and Co ions. In addition, the redox peak positions progressively shift with increasing scan rate, the reason may due to the increase of scan rate leading the existence of polarization.40 In addition, the specific capacitances from CV curves can be calculated as follows (2):41

C=

Q idt =∫ V ∆V

(2)

Here i represents a sampled current value(A), dt stands for a sampling time span (s), and ∆V shows the total potential deviation of the voltage window during the testing process (V). The specific capacitances of the ZNCO electrode are calculated to be about 2015.2, 1969.7, 1818.2, and 1538.8 F g-1 at the scan rates of 1, 2, 5, and 10 mV s-1, respectively.

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Figure 5 (a) Comparisons among CV curves of the CO, ZCO and ZNCO at 2 mV s-1 and (b) CV curves of ZNCO under different scan rates

Figure 6(a) shows the comparison of charge/discharge curves among CO, ZCO and ZNCO electrodes at 1 A g-1 with the voltage range varied from 0 to 0.5. It can be seen that an obvious discharge voltage plateau in the discharge process, which demonstrates that the electrode exhibits typical faradaic behavior. Significantly, the discharge time of the ZNCO electrode is evidently longer than that of the ZCO and CO electrode, demonstrating that the ZNCO electrode possesses higher specific capacitance. The charge/discharge curves of the ZNCO electrode at 1-5 A g-1 are displayed in Fig. 6(b). Notably, all curves are symmetric in shape, indicating the excellent electrochemical behaviors of the ZNCO electrode. The specific capacitances are calculated according to the following equation (3):41

Cm =

it m ∆V

(3)

Where, Cm represents the specific capacitance value (F g-1), i shows the charge/discharge current value during the testing process (A), ∆V stands for the potential window (V), td represents the discharge time (s), and m is the mass of electrode active material (g). Specific capacitances of the ZNCO electrode are calculated to be about 2481.8 (higher than the theoretical faradaic capacitance of 2434.8 F g-1 within 0.5V in SI, possibly because the obtained capacitance is contributed by faradaic capacitance from redox reaction at interfaces and EDLC from high surface area of this porous material), 2391.2, 2329.8, 2303.2, and 2281.1 F g-1 at 1, 2, 3, 4, and 5 A g-1, respectively. The above specific capacitances can be calculated to the form of specific capacitance per unit surface area of 4.2, 4.0, 3.9, 3.8, and 3.7 F m-2 at 1.7, 3.3, 5.0, 6.7,

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and 8.3 mA cm-2, which are higher than those of the reported values in published researches, including Co3O4-NiO nanowire arrays (1.35 F cm-2 at 6 mA cm-2),42 NiO-TiO2 nanotube arrays (3 F cm-2 at 0.4 mA cm-2),43 nickel foam supported CoO (1.23 F cm-2 at 1 mA cm-2),44 NiCo2O4 grown on nickel foam (1.5 F cm-2 at 5 mA cm-2),45 and nickel foam supported CoMoO4 (1.26 F cm-2 at 4 mA cm-2).46 Specific capacitance of the CO, ZCO and ZNCO electrodes at 1-5 A g-1 are presented in Fig. 6(c). The specific capacitances of the ZNCO electrode are higher than that of the ZCO and CO electrode at the same current density, even at 5 A g-1 the capacitance retention remains 91.9%, which shows the wonderful rate capability of the ZNCO electrode. Furthermore, specific capacitances of this work and other metal oxides in the literature are presented in Table S1. From the comparison values in the table, the specific capacitance of the ZNCO electrode is considerably higher than those of the other metal oxide electrodes at 1 A g-1, demonstrating that the ZNCO electrode is efficient for supercapacitor applications. To further investigate the electrochemical behaviors of ZNCO electrode, the EIS measurement ranging from 105 Hz to 10−2 Hz with amplitude of 5 mV is conducted and the comparisons between CO, ZCO, and ZNCO curves are shown in Fig. 6(d). The curves consist of a small semicircle and a straight line. Owing to Warburg impedance existence, the curve presents a straight line in the low frequency region. This phenomenon can be ascribed to the result of the frequency dependence of ion diffusion to the electrode interface.47-50 As depicted in the high frequency range, the first intersection point represents the internal resistance (Rs), which is consisted of the contact resistance generated from the interface between the electrode active material and the substrate, the intrinsic resistance of electrode active materials and the ionic resistance in the electrolyte solution. Meanwhile, the semicircle corresponds to the Faradic reactions, and the diameter represents the interfacial charge transfer resistance (Rct).51 It can be 17



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seen that the Rs and Rct values of ZNCO electrode are lower than those of the CO and ZCO electrodes, indicating that the best supercapacitive behavior of ZNCO electrode. The capacitance retention and coulombic efficiency of the ZNCO electrode at 10 A g-1 is presented in Fig. S5. The coulombic efficiency remains 100 % and the capacitance retention maintains at 80.9 % after 6000 cycles, indicating the wonderful cycling behaviors of the ZNCO electrode materials. Furthermore, comparisons between CV and charge/discharge testing curves of ZNCO electrode prepared at different reaction times are presented in Fig. S6. Obviously, the area surrounded by the ZNCO electrode CV curve is considerably larger than those of the other two electrodes. This phenomenon demonstrates that the ZNCO electrode possesses much higher specific capacitance than those of the other two electrodes. Moreover, the curves during the charge/discharge tests represent that the ZNCO electrode prepared under 5 h at 130 oC possesses longer discharge time and shows the best supercapacitive performance. The difference in the electrochemical performance of the samples may result from different morphologies. As shown in Fig. S7, ZNCO-3 shows the coarse surface on the nickel foam, and the obtained curly nanosheets may be the seed for the growth of the nanowires. When the reaction time was increased to 7h, nanowires aggregate severely, and numerous nanoparticles also aggregate on the surface of the nanowires, which would not be beneficial for electrolyte transportation and may lead to poor electrochemical performance.

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Figure 6 (a) Comparisons between charge/discharge curves of the CO, ZCO and ZNCO electrode at 1 A g-1, (b) charge/discharge curves of ZNCO electrode under different current densities, (c) specific capacitance of CO, ZCO and ZNCO electrodes at different current densities and (d) Nyquist plots of the CO, ZCO and ZNCO electrode

3.3. Electrochemical behaviors of the ZNCO//AC ASC device To evaluate the electrochemical properties of ZNCO electrode in supercapacitors, an ASC device is fabricated, in which the ZNCO electrodes act as the positive electrode and AC electrodes act as the negative electrode. These two electrode materials are tested by CV in 6 M KOH solution prior to ASC device testing and the CV curves at 1 mV s-1 are depicted in Fig. 7(a). 19



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The detailed electrochemical performances of this AC electrode are presented in Fig. S8. As evidently seen from Fig. 7(a), the CV curve of the AC electrode is nearly a rectangular shape with the potential window from -1 to 0 V, presenting the characteristics of the double-layer capacitors. Meanwhile, within a potential window ranging from 0 to 0.5 V, a pair of redox peaks can be seen for the ZNCO electrode. This typical CV curve shape evidently demonstrates the faradaic behavior characteristics of the ZNCO electrode. According to the above results, the faradaic behavior and electrical double-layer capacitive performance can corporately make contributions to the specific capacitance of the ASC devices. A potential window of 0-1.5 V can be selected to characterize the electrochemical properties of the ASC device. Figure 7(b) presents the CV curves of the ASC device at 1-10 mV s-1. The shapes of CV curves almost retain the same with the increase of the scan rates, demonstrating wonderful capacitive behaviors of the ASC device. The charge/discharge curves of the ZNCO//AC ASC device are shown in Fig. 7(c). The specific capacitances under different current densities are displayed in Fig. 7(d). A high specific capacitance of 113.9 F g-1 is reached at 1 A g-1 of the ZNCO//AC ASC device. Furthermore, it can be noted that the shapes of all charge/discharge curves are symmetrical during the testing procedure, indicating the good electrochemical behaviors of the ZNCO//AC ASC device. Energy density (E) and power density (P) represented in Fig. 7(e) can be calculated as follows:52,53

E = 1 / 2 × C m × ( ∆V ) 2

(4)

P = E / ∆t

(5)

Here, E represents the energy density (Wh kg-1), Cm stands for the specific capacitance of ZNCO//AC ASC device (F g-1), ∆V shows the potential drop during the discharge process (V), P is the power density (W kg-1), and ∆t represents the time of the discharge process (s). From the 20


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Ragone plot, the ASC device delivers a high energy density of 35.6 Wh kg-1 at a power density of 187.6 W kg-1 and the energy density retains 19.1 Wh kg-1 even at a high power density of 938.1 W kg-1. Comparisons have also been conducted between our work and the previously reported studies, such as NiO//carbon (15 W h kg-1 at a power density of 447 W kg-1),54 Co3O4@MnO2//AC-ASC (17.7 W h kg-1 at a power density of 600 W kg-1),55 hierarchical porous NiO//carbon (~10 W h kg-1 at a power density of 30 W kg-1),56 NiCo2O4 NSs@HMRAs//AC ASC (15.4 W h kg-1 at a power density of ~700 W kg-1),

57

and NiCo2O4@MnO2//AC-ASC

device (35 W h kg-1 at a power density of 163 W kg-1).58 In addition, the cycling performance of the ZNCO//AC ASC device is measured at 3 A g-1 within the potential window 0-1.5 V for 3000 cycles and the curves are displayed in Fig. 7(f). Coulombic efficiency remains at 100 %, and the capacitance retention maintains at 94 % after 3000 cycles. Furthermore, the capacitance retention and coulombic efficiency of more than 3000 cycles of ZNCO//AC ASC device is presented in Fig. S9. The coulombic efficiency keeps 100 % and the capacitance retention maintains at 71.2 % after 6000 cycles. The above results demonstrate the good stable behaviors of the ZNCO//AC ASC device.

21



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Figure 7 (a) CV curves of AC and ZNCO electrode tested at 2 mV s−1, (b) CV curves of the ZNCO//AC ASC device at different scan rates, (c) and (d) charge/discharge curves and specific capacitance of the ZNCO//AC ASC device under different current densities, (e) Ragone plots of the ZNCO//AC ASC device and (f) The capacitance retention and coulombic efficiency during the charge/discharge cycle of the ZNCO//AC ASC device at 3 A g-1 22


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Several aspects contribute considerably to the excellent supercapacitive performance of the ZNCO electrodes: Firstly, the promising synergistic effect among Co, Ni, and Zn ions may be attributed to the excellent properties of ZNCO nanowire arrays. Ni provides a high capacity. Co offers increased electronic conductivity. Zn possesses excellent electrical conductivity, resulting in electrical conductivity enhancement and capacitive behaviors improvement of the electrode materials. Secondly, the direct growth of freestanding ZNCO nanowire arrays on nickel foam eliminates use of conductive additives and polymer binders, thereby substantially leading to the a very low “dead volume” in the whole electrode.51 Thirdly, the direct growth of ZNCO nanowire arrays on nickel foam can induce high interfacial contact between the electrode active materials and the substrate, which will enhance ion transportation, thereby leading to high rate capability of the electrode. Fourthly, ZNCO nanowire arrays grown on the nickel foam substrate can supply an effective path for high electron transport and largely remain the original structures of the electrode material during the cycling, thus resulting in wonderful cycle stability. Finally, the hierarchical mesoporous ZNCO electrode materials possess large specific surface area and highly porous structures. These properties could significantly enhance the electrochemical active area and allow electrolyte to penetrate in the whole electrode, resulting in superior electrochemical performance.

4. Conclusions Nickel foam supported hierarchical mesoporous ZNCO nanowire arrays have been successfully prepared through a simple two-step approach that involves hydrothermal method and subsequent thermal annealing treatment. The use of ZNCO electrode for supercapacitors is studied for the first time, and it shows remarkable electrochemical behaviors. A high specific 23



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capacitance value of 2481.8 F g-1 at 1 A g-1 and excellent rate capability of 91.9 % even at 5 A g1

can be achieved. Furthermore, the as-prepared ZNCO//AC ASC device with a wide potential

window of 1.5 V reaches a specific capacitance of 113.9 F g-1 at 1 A g-1 with a high energy density of 35.6 Wh kg-1, and a good cycling stability of 94 % over 3000 charge/discharge cycles. The excellent electrochemical behaviors demonstrate that nickel foam supported hierarchical mesoporous ZNCO nanowire arrays electrode is highly desirable for application as advanced supercapacitor electrode.

ASSOCIATED CONTENT Supporting Information Available: Schematic illustration of asymmetric supercapacitor, SEM images of various ZNCO nanostructures, XPS spectra, electrochemical performance of the ZNCO and AC electrode, capacitance retention and coulombic efficiency of the ZNCO//AC ASC device. A comparison of specific capacitance of ZNCO electrode with other reported metal oxide electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by a grant from the Hong Kong Research Grants Council (project no. CityU 125412) and a grant from the City University of Hong Kong (project no. 7004248)

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