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Letter pubs.acs.org/journal/ascecg

Direct Growth of Ultrathin NiCo2O4/NiO Nanosheets on SiC Nanowires as a Free-Standing Advanced Electrode for HighPerformance Asymmetric Supercapacitors Jian Zhao,† Zhenjiang Li,*,† Meng Zhang,† Alan Meng,*,‡ and Qingdang Li§ †

Key Laboratory of Polymer Material Advanced Manufacturing Technology of Shandong Provincial, Qingdao University of Science and Technology, 99 Songling Road, Qingdao, 266061 Shandong, People’s Republic of China ‡ State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao, 266042 Shandong, People’s Republic of China § College of Sino-German Science and Technology, Qingdao University of Science and Technology, 99 Songling Road, Qingdao, 266061 Shandong, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, we successfully employed SiC nanowires (SiC NWs) with splendid anticorrosion, antioxidation, heat-resistant properties, excellent conductivity, and large specific surface area directly deposited on carbon cloth (CC) as scaffolds to grow first the loose, porous and ultrathin NiCo2O4/NiO nanosheets (NiCo2O4/NiO NSs) via a facile hydrothermal technology coupled with annealing treatment to form a free-standing and stable hybrid electrode for asymmetric supercapacitor (ASC). Benefiting from the smart combination of SiC NWs and NiCo2O4/NiO NSs, illustrating a promising synergistic strategy, the electrode delivered an ultrahigh specific capacitance of 1801 F g−1 at 1 mA cm−2 as well as a remarkable rate capability of 1499 F g−1 at 10 mA cm−2. Furthermore, the additive-free functionalized SiC NWs@NiCo2O4/NiO NSs on CC acted as the positive electrode, assembled with the activated carbon (AC) on nickel foam (NF) negative electrode to fabricate an advanced ASC with intriguing electrochemical performances in terms of huge energy density (60 Wh kg−1 at 1.66 kW kg−1) in addition to exceptional cycling stability (90.9% capacitance retention after 2000 cycles). This novel strategy can not only further widen the application of SiC NWs-based materials but also provide new insight into the development of next-generation supercapacitors with high energy/power densities. KEYWORDS: NiCo2O4/NiO nanocomposites, SiC nanowires, Hydrothermal method, Hybrid electrode, Asymmetric supercapacitor

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INTRODUCTION

improve the energy density for the development of supercapacitors. Among various supercapacitor electrode materials, nickel oxide (NiO) and spinel nickel cobaltite (NiCo2O4) have recently been paid much attention due to their versatile merits, such as high theoretical capacitance, excellent redox property, favorable electrochemical activity, lower cost, high abundance and environmentally benign.6−12 These intriguing advantages are beneficial to its application in high-performance supercapacitors. Nevertheless, previous attempts to fabricate NiO or NiCo2O4 supercapacitor electrode materials encountered several major problems such as low capacitance,13,14 poor cycling stability,15 and inferior rate performance,16 which immensely limited their commercial attractiveness. The most likely reason is that the metal oxides generally suffer from poor electronic conductivity or easily agglomerate in substrates. To

Owing to the restricted usability of fossil fuel and the increasingly urgent concern over ecological environment influence of traditional energy technologies, hunting for ecofriendly and reproducible advanced energy storage devices is one of the most stringent challenges facing us today. Recently, as an alternative to batteries and traditional electrostatic capacitors, supercapacitors (also called electrochemical capacitors) with irreplaceable performances of enhanced power density, fast charge−discharge rate (in seconds) and superior cycling stability, have raised widespread concerns about potential high-power applications such as heavy transport, hybrid electric vehicles and a number of microdevices.1−4 However, supercapacitors possess relatively lower energy density compared with batteries, which seriously precludes their large-scale industrial utilizations in energy storage.5 Thus, to meet the industrial demand, novel nanostructured electrode materials should be rationally designed and synthesized to boost the operating voltage and specific capacitances and © XXXX American Chemical Society

Received: April 6, 2016 Revised: May 24, 2016

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DOI: 10.1021/acssuschemeng.6b00697 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

window as high as 1.8 V, and maximum energy and powder densities of 60 Wh kg−1 and 16.7 kW kg−1, respectively. In addition, the device also exhibits superior long-term stability, as 90.9% of the capacitances still retained after 2000 cycles at a high current density of 20 mA cm−2. The amazing results obtained in this work substantiate that the parepation of Ni− Co oxides on SiC NWs with distinct structures is an efficient method to enhance the electrochemical behaviors of electrodes.

address this vexing issue, an effective solution is the incorporation of these active materials into one-dimentional (1D) nanomaterials such as carbon nanotubes,17 Cu/CuOx nanowires,18 halloysite nanotubes19 with huge surface area, forming active materials−strong framework hybrid nanostructure electrodes, and which indeed exhibit relatively more favorable electrochemical characteristics than individual components. Inspired by the design concept, a novel and promising strategy has been proposed that make use of functional semiconductors 1D nanostructures or cermet nanowires such as zinc oxide nanorods (ZnO NRs), titanium nitride nanowires (TiN NWs), SiC nanowires (SiC NWs) and titanium dioxide nanotubes (TiO2 NTs) as a new class of supporters to grow electroactive materials for high-performance SCs, which have attracted considerable interest because these materials provide the advantages of low cost, good electrical conductivity and excellent chemical resistance.20−23 Among these, nanowire-type SiC is considered as a tremendous fascination to researchers because it possesses the high electron mobility, low band gap, large specific surface area, desirable electrical conductivity, outstanding mechanical strength as well as remarkable thermal and physicochemical stability.24−27 Moreover, Gu et al.28 and Alper et al.29 have prepared pure SiC NWs electrode materials for SCs, which exhibit long-term cycling stability, high areal capacitance, exceptional resistances to electrochemical etching and oxidation, and superior flexible properties. In consideration of the merits of SiC NWs described above, when they are selected as skeletons for depositing electroactive materials, the SiC NWs can not only allow electroactive materials to distribute uniformly on their surfaces, shortening the diffusion distance for electrolyte ions and facilitating electron transfer but also act as a buffering phase for the volume changes of electrodes and effectively impede the possible electrode structures corrosion and collapse during continuous charge− discharge process. In addition, as electroactive materials, NiCo2O4 is a class of p-type semiconductor material, doping it with NiO that is also semiconductor material can introduce impurity band effects, which improves its electrical conductivity, leading to easier electron transport. Futhermore, NiCo2O4 and NiO possess different reductive reaction potentials, thus the cooperation effect of the two kinds of material can extend their discharge plateau, contributing to the enhanced electrochemical energy storage.30,31 As a result, it is entirely reasonable to postulate that the sensible combination that NiCo2O4/NiO NSs are coated on the SiC NWs can illustrate distinguished electrochemical performances, making them very intriguing and promising as advanced SCs electrode materials. To the best of our knowledge, the reasonable development of a Ni−Co oxides decorated-SiC NWs composites electrode for SCs has scarcely been reported to date. Herein, for the first time, the loose, porous and ultrathin NiCo2O4/NiO NSs modified with SiC NWs grown on CC (SiC NWs@NiCo2O4/NiO NSs on CC) were successfully fabricated via a combination of simple hydrothermal and calcination treatments, which can be directly acted as a selfsupported electrode for ASC, exhibiting an ultrahigh specific capacitance of 1801 F g−1 at a current density of 1 mA cm−2 and the capacitance remained as high as 83.2% when the current density increased to 10 mA cm−2. In accordance with the above electrode material, a high-performance ASC based on SiC NWs@NiCo2O4/NiO NSs on CC and AC on NF is successfully assembled, which illustrates a stable voltage

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EXPERIMENTAL SECTION

Preparation of SiC NWs Grown on CC. SiC NWs grown on the CC have been successfully synthesized by chemical vapor reaction (CVR). Typical experiments were performed as follows: The milled analytically pure silicon and graphite powders (mixing molar ratio of 1:1.5) were employed as the starting raw materials. Carbon cloth was used as the substrate, Ni(NO3)2 (ethanol solutions of 0.01 mol/L) was used as the catalyst source. Initially, the substrate was immersed in the catalyst ethanol solutions for 10 min, and naturally dried in air. Second, a piece of carbon cloth, Si−graphite mixture powders and the substrate were orderly placed into a homemade graphite chamber, then the chamber was placed into a vacuum furnace. Before heating, the system was purged 2−3 times with high-purity argon (Ar) employing a rotary vacuum pump to eliminate oxygen from the furnace chamber. Third, the temperature of the furnace was raised to 1250 °C from room temperature at a mean rate of 350−400 °C h−1 and maintained at peak temperature for 13−16 min. Finally, the power was switched off and the furnace was naturally cooled down to room temperature. Fabrication of SiC NWs@NiCo2O4/NiO NSs on CC. The asprepared SiC NWs grown on the CC were used as the scaffold for the growth of NiCo2O4/NiO NSs through combining a hydrothermal reaction and a thermal annealing process. First, the reaction solution was obtained via mixing Ni(NO3)2·6H2O (0.052 mol/L), Co(NO3)2· 6H2O (0.013 mol/L) and hexamethylene-tetramine (0.064 mol/L) in 50 mL of H2O and 25 mL of ethanol under vigorous magnetic stirring. After stirring for 30 min, the as-prepared solution was transferred into a 100 mL polytetrafluoroethylene (PTFE) autoclave, and then a piece of CC (1 × 1 cm) with SiC NWs was immersed into the reaction solution. Second, the autoclave was sealed and maintained at 90 °C for 12 h in an electric oven. After cooling down to the room temperature naturally, the SiC NWs loaded with the Ni−Co precursors were washed with distilled water and ethanol for several times to remove the impurities, and dried at 60 °C for 12 h. Finally, the precursors were transformed into NiCo2O4/NiO NSs via the calcination at 300 °C for 2 h at a ramping rate of 1 °C min−1 in air atmosphere. In contrast, the pure NiCo2O4/NiO NSs were also fabricated directly on CC (1 × 1 cm) in a similar manner under the same conditions. The mass loading of the active materials (NiCo2O4/NiO NSs) on each electrode is around 0.6 mg cm−2, which is achieved by carefully weighing the samples before hydrothermal treatment and after thermal annealing using a high-precision electronic balance. Fabrication of Asymmetric Supercapacitor (ASC). An activated carbon (AC) on NF electrode was prepared by mixing 80 wt % AC, 10 wt % polyvinylidenedifluoride and 10 wt % carbon black in 1-methyl-2-pyrrolidinone. The resulting mixture was casted onto NF substrate and pressed at 10 MPa and then dried at 80 °C for 12 h. This electrode was selected as the negative electrode and combined with the positive electrode (SiC NWs@NiCo2O4/NiO NSs on CC) with 6 mol/L KOH as the electrolyte and filter paper as a separator to assemble an ASC device. In accordance with the well-known charge balance theory,32 the typical total mass loading of the positive and negative electrode active materials was about 2.7 mg cm−2. Materials Characterization. The morphology and chemical elements were characterized by using a JEOL JSM-6 field emission scanning electron microscopy (FESEM) equipped with an energy dispersive X-ray spectroscopy (EDS) equipment. Further detailed structural information was obtained by using a Hitachi (Tokyo, Japan) H-8100 transmission electron microscopy (TEM) instrument, the corresponding selected area electron diffraction (SAED) and highB


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Figure 1. Schematic illustration of the formation of SiC NWs@NiCo2O4/NiO NSs on carbon cloth via hydrothermal reaction and subsequent annealing treatment.

Ni 2 + + 2OH− → Ni(OH)2

resolution transmission electron microscopy (HRTEM). The X-ray powder diffraction (XRD) pattern of the products was recorded by using a Advance D8 X-ray diffractometer. In addition, X-ray photoelectron spectroscopy (XPS) analysis was performed to characterize further the chemical compositions on a Thermo ESCALAB 250Xi device with an Al Kα (hν = 1486.6 eV) excitation source. Electrochemical Measurements. In terms of electrochemical measurements of the as-prepared electrodes, the SiC NWs@ NiCo2O4/NiO NSs on CC or NiCo2O4/NiO NSs on CC was directly selected as the working electrode without any ancillary materials. The electrochemical tests were carried out in a standard three-electrode system at room temperature where Pt wire served as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. For the assembled ASC, a two-electrode configuration was employed to test its electrochemical performances in which the SiC NWs@NiCo2O4/NiO NSs on CC and AC on NF acted as the positive electrode and negative electrode, respectively. A 6 mol/L KOH solution was used as the electrolyte for all electrochemical measurements. The electrochemical performances were tested in a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Shanghai, China) by the techniques of cyclic voltammetry (CV), galvanostatic charge−discharge (GV) and electrochemical impedance spectroscopy (EIS).

Second, a simple calcination treatment is employed to convert the Ni−Co precursor into NiCo2O4/NiO NSs with wellretained morphology coated on the SiC NWs on CC, as expressed by the simple oxidation reaction as follows:

RESULTS AND DISCUSSION The SiC NWs@NiCo2O4/NiO NSs are fabricated on a conductive carbon cloth (CC) substrate through a two-step process, as schematically illustrated in Figure 1. Firstly, Ni−Co precursor NSs dispalyed in Figure S1 can be easily grown evenly on the entire surface of the SiC NWs directly deposited on CC under hydrothermal conditions, as described by the following equations:19,33,34 (1)

NH3 + H 2O → NH4 + + OH−

(2)

Ni 2 + + 2Co2 + + 6OH− → NiCo2(OH)6

(3)

NiCo2(OH)6 + 1/2O2 → NiCo2O4 + 3H 2O

(5)

Ni(OH)2 → NiO + H 2O

(6)

The as-obtained products are first characterized by SEM and TEM to identify their morphology and microstructure, respectively. It can be clearly observed from the lowmagnification SEM image of SiC NWs that a large amount of wire-like products are randomly distributed on the surfaces of CC, which are about several tens of micrometers in length as displayed in Figure S2 in the Supporting Information. The corresponding high-magnification SEM image in Figure 2a indicates that the long NWs with uniform diameters are dense. Figure 2b is a TEM image of an individual SiC NW, clearly revealing the diameter of the NW with smooth surface is approximately 30 nm. The top-right inset of Figure 2c depicts a typical HRTEM image of the SiC NW, which displays that it has a homogeneous crystalline structure with a lattice spacing of 0.25 nm, illustrating the NW grows along the [111] crystallographic direction. The corresponding SAED pattern is presented in the bottom-right inset of Figure 2b and suggests a set of sharp diffraction spots, revealing the single-crystal instinct of the cubic SiC (β-SiC). Figure 2c,d depicts the typical SEM images of the SiC NWs@NiCo2O4/NiO NSs at different magnifications, which display that the surfaces of SiC NWs are densely covered by the trimly aligned NiCo2O4/NiO NSs with a small thickness of 20−30 nm, forming a dense shell with highly loose and porous nanosheets that are interconnected. This particular morphology is conductive to the penetraton of the electrolyte ions into the surfaces of the whole electrode, which leads to extensive contact of the electroactive materials with the electrolyte ions for fast and reversible redox reactions, greatly enhancing the charge storage capacity. The specific elemental composition of the as-prepared sample is further investigated via EDS as displayed in the inset in Figure 2d,

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(CH 2)6 N4 + 6H 2O → 6HCHO + 4NH3

(4)

Because the additive amount of nickel resource was higher than cobalt in the experiment, so redundant nickel ion can also react with hydroxyl in the solution to generate nickel hydroxide, which can be expressed as follows: C


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Figure 2. (a) High-magnification SEM image of the SiC NWs; (b) TEM image of the SiC NWs, insets are the corresponding HRTEM image and SAED pattern of the selected wine region in panel b; (c) low-magnification and (d) high-magnification SEM images of the SiC NWs@NiCo2O4/ NiO NSs, (inset: the corresponding EDS spectrum); (e) low-magnification and (f) high-magnification TEM images of the SiC NWs@NiCo2O4/ NiO NSs; (g) HRTEM image taken from the blue circle in panel f; (h) XRD patterns of the SiC NWs (green line), pure NiCo2O4/NiO NSs (magenta line) and SiC NWs@NiCo2O4/NiO NSs (red line).

suggesting that the electrode surfaces contain C, Si, Ni, Co and O elements, which confirms the existence of Ni−Co oxides on the SiC NWs surfaces. For comparasion, the pure NiCo2O4/ NiO NSs are casually and irregularly deposited on the CC (see Figure S3 of the Supporting Information). To study further the microstructure of the SiC NW@NiCo2O4/NiO NSs more clearly, TEM images under different magnifications are taken and presented in Figure 2e,f, illustrating the homogeneous growth of NiCo2O4/NiO NSs on the SiC NWs supporter,

which is well consistent with the above SEM observation (Figure 2c,d). Through detailed observation of the microstrutures of the products, it is found that besides the porous features of the shell constituted by the ultrathin interconnected nanosheets, the NiCo2O4/NiO NSs themselves are also polyporous in nature as shown in an enlarged view (Figure 2f), and the diameter of the porous is 3−5 nm, which is further demonstrated by the corresponding HRTEM image shown in Figure 2g. This information indicates that more active materials D


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Figure 3. XPS spectra of the as-prepared SiC NWs@NiCo2O4/NiO NSs. (a) XPS survey scan; (b−d) high-resolution XPS spectra for Co, Ni and O.

attributed to Co3+, another two peaks observed at 782.9 and 796.8 eV are ascribed to Co2+,35 thus two kinds of cobalt species containing Co2+ and Co3+ can be detected. As illustrated in Figure 3c, the binding energies at 855.9 and 873.8 eV are assigned to Ni3+, whereas the peaks at 854.0 and 871.7 eV can be indexed to the signal of Ni2+ in the NiCo2O4/ NiO, which are in agreement with the previously reported values. The satellite peak at around 861.7 and 879.7 eV are two shakeup type peaks of nickel at the high binding energy side of the Ni 2p3/2 and Ni 2p1/2 edge.31 So, the Ni 2p spectra consist of two kinds of nickel species that are characteristic of Ni2+ and Ni3+ and two satellite peaks (indicated as “Sat.”). The highresolution spectra for O 1s (Figure 3d) reveal three oxygen species denoted as O1, O2 and O3. In accordance with previous studies,36,37 the peak of O1 at a binding energy of 529.6 eV is typical metal−oxygen bond, O2 at 531.4 eV is generally associated with the a large amount of defects with low oxygen, and O3 at 532.4 eV corresponds to adsorbed water on or within the surface. The XPS investigation results indicate that the sample possesses a mixed composition, containing Ni2+/Ni3+ and Co2+/Co3+, which can afford enough active sites for rich redox, contributing to elevating substantially the specific capacitance of electrode material. To study the electrochemical characteristics of the two freestanding functionalized as-fabricated electrodes, several electrochemical measurements were carried out in a threeelectrode configuration as presented in Figure 4. Figure 4a illustrates the CV curves of the SiC NWs on CC (the black curve), pure NiCo2O4/NiO NSs on CC (the red curve) and SiC NWs@NiCo2O4/NiO NSs on CC (the olive curve) at the same sweep rate of 20 mV s−1. Apparently, the CV curve of the SiC NWs on CC shows no redox peaks, in contrast, a pair of remarkable redox peaks of the CV curves of the SiC NWs@NiCo2O4/NiO NSs and pure NiCo2O4/NiO

can be exposed to the aqueous electrolyte, highly improving their utilization. Consequently, this kind of electrode with polyporous nanostructure is a significantly desired candidate for SC applications. Figure 2g illustrates the HRTEM image of the NiCo2O4/NiO NSs, in which the lattice fringes have different orientations, indicating that the sample is polycrystalline. Furthermore, the crystal lattice spacings are measured to be 0.279 and 0.148 nm, corresponding well with the distance of NiCo2O4 (220) and NiO (220) planes, respectively, which further substantiates that the as-prepared products contains NiCo2O4/NiO NSs. The crystal structures of the SiC NWs, pristine NiCo2O4/NiO NSs and the SiC NWs@NiCo2O4/NiO NSs scraped off from the CC are characterized by XRD. As shown in Figure 2h, the peaks indexed by the blue circle correspond to SiC NWs. From XRD profile, we can also clearly see that the peaks related to NiCo2O4 are (220), (311) and (511), and the peaks associated with NiO are (200), (220), (311) and (222). The experimentally obtained diffraction peaks of the sample are in accord with the standard XRD patterns of SiC (JCPDS Card No. 29-1129), NiCo2O4 (JCPDS Card No. 20-0781) and NiO (JCPDS Card No. 65-2901) without any collateral peaks, which reveals that the prepared sample possesses high purity. Taken the above together, the SiC NWs@NiCo2O4/NiO NSs on CC have been successfully synthesized for their use as electrode materials. To evaluate the composition and chemical bonding state of the as-prepared SiC NWs@NiCo2O4/NiO NSs, XPS measurements were recorded and the corresponding results are illustrated in Figure 3. Figure 3a shows the survey spectrum of the sample, mainly including not only Si 2p and C 1s signals deriving from the SiC NWs but also the typical signals of O 1s, nickel and cobalt species originating from the NiCo2O4/NiO NSs. In the Co 2p spectra (Figure 3b), the peaks at 780.2 and 795.2 eV can be E


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Figure 4. CV curves of (a) the SiC NWs on CC, NiCo2O4/NiO NSs on CC and SiC NWs@NiCo2O4/NiO NSs on CC at the same scan rate of 20 mV s−1, and (b) the SiC NWs@NiCo2O4/NiO NSs on CC at various scan rates; GV curves of (c) the NiCo2O4/NiO NSs on CC and SiC NWs@ NiCo2O4/NiO NSs on CC at 2 mA cm−2, and (d) the SiC NWs@NiCo2O4/NiO NSs on CC at various current densities; (e) comparison of specific capacitance calculated from GV curves as a function of current density for the NiCo2O4/NiO NSs on CC and SiC NWs@NiCo2O4/NiO NSs on CC; (f) Nyquist plot of the NiCo2O4/NiO NSs on CC and SiC NWs@NiCo2O4/NiO NSs on CC.

NSs can be observed within the voltage windows of −0.2−0.6 V in 6 mol/L KOH aqueous electrolyte, which mainly results from redox reactions related to X−O/X−O−OH,36 where X corresponds to Ni or Co, indicating the typical pseudocapacitive characteristics of the electrode materials. Moreover, the current signal the SiC NWs@NiCo2O4/NiO NSs is much higher than that of the pure NiCo2O4/NiO NSs and SiC NWs, suggesting that the SiC NWs@NiCo2O4/NiO NSs electrode can display larger energy storage capacity compared with the pure NiCo2O4/NiO NSs or SiC NWs because the capacitance is directly proportional to the integrated area under the CV curve. Notably, the shape of CV curve of the SiC NWs on CC tends to a straight line, namely the enclosed area of the CV curve is rather small, so the SiC NWs on CC contributes little to the capacitance. Figure 4b presents the CV curves of the SiC NWs@NiCo2O4/NiO NSs on CC at different scan rates ranging from 5 to 50 mV s−1. As can be seen, with the augment

of the scan rate, the redox current increases, whereas the oxidation and reduction peaks shift slightly toward higher and lower potential, respectively, and the peaks shape in these CV curves can maintain even at a large sweep rate of 50 mV s−1. This phenomenon reveals a lower resistance of the electrode and superior rate capability because of the close contact between electroactive SiC NWs@NiCo2O4/NiO NSs and conductive CC.6 To understand better the charge storage mechanism, the relationship plots of anodic and cathodic peak current versus scan rate are displayed in Figure S4 (Supporting Information). It can be clearly observed from the plots that there is an almost linear relationship between anodic (cathodic) peak current and square root of scan rate, suggesting that the reactions in the system are dominated by the diffusion process.38−40 Figure 4c displays the GV curves of the SiC NWs@NiCo2O4/NiO NSs and NiCo2O4/NiO NSs on CC with a voltage window of 0−0.4 V at 2 mA cm−2. Obviously, the F


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Figure 5. Capacitive performance of the SiC NWs@NiCo2O4/NiO NSs on CC//AC on NF asymmetric supercapacitor (ASC). (a) Schematic illustration of the ASC device composed of the SiC NWs@NiCo2O4/NiO NSs on CC and AC on NF; (b) CV comparison of the SiC NWs@ NiCo2O4/NiO NSs on CC and AC on NF at 50 mV s−1 in a three-electrode system; (c) CV and (d) GV curves of the ASC; (e) relationship between specific capacitance vs current density; (f) Nyquist plot of the ASC; (g) Ragone plot of different supercapacitors; (h) cycling test of the ASC at a high current density of 20 mA cm−2 for 2000 cycles in 6 mol/L KOH.

discharging time of the SiC NWs@NiCo2O4/NiO NSs is much longer than that of NiCo2O4/NiO, revealing the specific capacitance is highly improved in the SiC NWs@NiCo2O4/ NiO case. GV measurements of the SiC NWs@NiCo2O4/NiO NSs on CC at different current densities of 1 to 10 mA cm−2

were carried out as depicted in Figure 4d, which display symmetric charge−discharge processes at various current densities, suggesting the high Coulombic efficiency and excellent electrochemical features of the electrode materials. Moreover, the GV curves show a couple of voltage plateaus in G


Letter


Specifically speaking, first of all, the SiC NWs are directly deposited on the highly conductive CC with robust adhesion, avoiding the use of a polymer binder or conductive additive which commonly adds in extra contact resistance or weight. The SiC NWs offer a strong skeleton for NiCo2O4/NiO NSs, which can entwine together to form a typical high-surface-area conductive netty structure, effectively preventing the aggregation of the active materials, thus making the electrolyte easily contact with the active materials, shortening electrolyte ion diffusion pathway and facilitating electron transport. Second, the ultrathin NiCo2O4/NiO NSs with the loose and porous features have high specific surface area and numerous active sites, pledging the expedite ionic intercalation into the electroactive materials for rapid and reversible redox reactions, which results in more efficient charge transportation and large increment for specific capacitance. Third, the SiC NWs with outstanding resistances to oxidation and corrosion are able to avert the erosion and collapse of the whole electrode structures, highly enhancing their long-term cycling stability. Finally, the homogeneously distributed NiCo2O4/NiO NSs are tightly anchored on the surfaces of SiC NWs and retain the free space between the NSs, which can availably avoid the possible swelling and shrinking of the active materials during the electrolyte ion insertion/extraction process especially at a relative high current density, greatly boosting the rate capability of the composite electrode. In conclusion, the electrode composed of SiC NWs@NiCo2O4/NiO NSs has many merits that can largely contribute to the enhanced elctrochemical behavior for promising applications in high-energy storage systems. To estimate further the SiC NWs@NiCo2O4/NiO NSs on CC for practical applications, an asymmetric supercapacitor (ASC) was prepared by employing the SiC NWs@NiCo2O4/ NiO NSs on CC as the positive and the AC on NF as the negative in 6 mol/L KOH with one piece of filter paper as the separator as shown in Figure 5a. As we all know, AC has been broadly selected as the typical electrode material for asymmetric supercapacitor, particularly acting as an outstanding negative electrode in alkaline electrolytes.3,36 The electrochemical properties of AC were first characterized before the ASC assembly in a three-electrode measurement with 6 mol/L KOH as the electrolyte (for details, see Figure S7 in the Supporting Information). Figure 5b shows the comparative CV curves of the AC on NF (black line) and SiC NWs@NiCo2O4/NiO NSs on CC (red line) at the sweep rate of 50 mV s−1. The AC on NF illustrates a typical feature of electric double-layer capacity within a potential window of −1.2−0 V, whereas the SiC NWs@NiCo2O4/NiO NSs on CC exhibits superior electrochemical reversibility within the range of −0.2−0.6 V. Thus, the operating potential of the ASC device can be up to 1.8 V. Figure 5c shows a typical CV curves for the ASC device corresponding to different sweep rates. With the increment of sweep rate from 10 to 50 mV s−1, all the curves behave similar in shape, revealing the splendid high-rate charge−discharge performances of the device. Moreover, the shapes of the CV curves exhibit dual effect of electric double-layer capacitance and pseudocapacitance for the device. The GV curves of the device with no obvious voltage drops under various current densities indicate fast I−V response as displayed in Figure 5d. According to the total masses of the two electrodes, the calculated specific capacitance of the device as a function of the current densities is plotted in Figure 5e. The device capacitance is as high as 133.4 F g−1 at a current density of 5 mA cm−2,

each charge and discharge process, revealing the capacitance originates from typical redox reactions, which is in good agreement with the aforementioned CV analysis. In addition, the corresponding CV and GV curves of the pure NiCo2O4/ NiO NSs on CC are presented in Figure S5a,b (Supporting Information). On the basis of the GV curves of SiC NWs@ NiCo2O4/NiO NSs on CC and pure NiCo2O4/NiO NSs on CC (as shown in Figure 4d and Figure S4b, respectively), the specific capacitances of the electrodes were calculated and the specific calculating approach can be seen in the Supporting Information. The typical calculation results are summarized in Figure 4e, the specific capacitance of the SiC NWs@NiCo2O4/ NiO NSs on CC is determined to be 1801, 1728, 1633, 1565 and 1499 F g−1 at the current densities of 1, 2, 4, 6 and 10 mA cm−2, respectively. The specific capacitance gradually reduces with the increment of the current densities from 1 to 10 mA cm−2, but it still remains 83.2% of the initial capacitance even at 10 mA cm−2, indicating that it possesses the excellent rate capability. In contrast, the specific capacitance of the pure NiCo2O4/NiO NSs on CC is only 537 F g−1 at 10 mA cm−2, corresponding to approxiately 49.7% of the capacitance at 1 mA cm−2 as depicted in Figure 4e. To illustrate more clearly the electrochemistry properties of the two electrodes, their specific capacitances at the specific measurement value of the current are also displayed in Figure S6 in the Supporting Information. To the best of our knowledge, the above measurement results of the hybrid electrode greatly surpass previously published values of pure Ni−Co oxides or other skeletons@Ni−Co oxides supercapacitor electrodes (Supporting Information, Table S1). Furthermore, for the sake of investigating the electrical properties of the two electrodes, EIS measurements are carried out and their corresponding Nyquist plots are shown in Figure 4f. In the high-frequency region, the intersection of the plot at the x-axis exhibits the internal resistance of the electrochemical system, and the semicircle can be observed with the diameters representing the charge-transfer process at the interface between working electrode and electrolyte.18 The slope of the linear part in the low-frequency domain refers to the well-known Warburg impedance, which is assigned to the impedance of OH− ions diffusion.41 As can be evidently seen from the plots, the SiC NWs@NiCo2O4/NiO NSs on CC illustrates lower internal resistance, charge-transfer resistance and diffusion resistance than the NiCo2O4/NiO NSs on CC, which results from the fact that the conductive network formed by the entanglement of SiC NWs can offer efficient and rapid channels for ion and charge transport. Thus, the direct SiC NWs@NiCo2O4/NiO NSs to CC electrode possesses exceptional conductivity. In previous studies, researchers have confirmed that the electrode materials with the 3D structure features can possess a variety of advantages, which play a significant role in the applications of high-performance surpercapacitors and batteries.42−44 On the basis of the consideration, the excellent electrochemical performances of the SiC NWs@NiCo2O4/NiO NSs on CC with special 3D structure characteristics may be attributed to the cooperative effect of their distinctive geometry and the reasonable selection of component materials, namely the 3D architecture with considerable space between nanosheets can lead to fast and easy access of ions to the electrode/ electrolyte interface, and highly conductive SiC nanowire cores offer “superhighways” for charge transfer; meanwhile, the loose and mesoporous nature of NiCo2O4/NiO NSs can be expected to enhance the number of active sites for richer redox reations. H



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whereas it still retains 75.1 F g−1 when the current density is increased from 5 to 50 mA cm−2. The Nyquist plot of the device in Figure 5f displays lower internal resistance (1.38 Ω) and diffusion resistance, and from the plot, we can clealy see that it presents very small charge-transfer resistance, which indicates the device possesses excellent electrical conductivity. To elaborate further the energy and power densities of the ASC device, Ragone plot is plotted as illustrated in Figure 5g. The asymmetric device shows a high energy density of 60 Wh kg−1 at a power density of 1.66 kW kg−1, and even at an outstanding power density of 16.7 kW kg−1, the energy density can still remain as high as 33.8 Wh kg−1. This outperforms lots of previous reported ASCs, such as NixCo3−xO4//AC (37.4 Wh kg−1 at 0.163 kW kg−1),3 Cu/CuOx@NiCo2O4//AG (12.6 Wh kg−1 at 0.344 kW kg−1),18 CTN@NiO//PCPs (25.4 Wh kg−1 at 0.4 kW kg−1),34 NiCo2O4//AC (34.8 Wh kg−1 at 0.464 kW kg−1)36 and NiCo2O4//AC (19.5 Wh kg−1 at 0.2 kW kg−1).45 Such remarkable performances can be attributed to the joint contributions of the extended working potential and large specific capacitance of the electrodes materials. In addition, the long-term cycling property is identified as another significant factor for practical application of the device. The cycling was carried out via using GV test at a high current density of 20 mA cm−2 for 2000 cycles as presented in Figure 5h. In the initial cycles, the capacity of the device has a slight increase due to the activation of the electrode materials.46 After that, the specific capacitance gradually reduced with an increase in the cycle number, and it is still capable of maintaining 90.9% of the initial capacitance after 2000 cycles, exhibiting a superior stability toward long time usages. Moreover, the interconnected nanosheet morphology and structure of the as-synthesized electroactive material is well-retained after 2000 cycles at 20 mA cm−2 as displayed in Figure S8 of the Supporting Information, which further substantiates the excellent cycling performance of NiCo2O4/NiO nanosheets on SiC nanowires. Meanwhile, the above analysis results also suggest the SiC NWs indeed possess prominent anticorrosion and they play a significant role in preserving the integration of the hybrid electrode architectures of the ASC we fabricated, thus SiC NWs are perfect scaffolds for electrodes of high-performance SCs. Noteworthily, the detail calculated approach of the specific capacitance and energy and power density of the ASC device can be seen in the Supporting Information.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00697. Methods to calculate the specific capacitance, energy and power density, SEM image of the Ni−Co precursor (Figure S1), low-magnification SEM image of the SiC NWs (Figure S2), SEM images of the pure NiCo2O4/ NiO NSs (Figure S3), the relationship between peak current and the square root of scan rates for the SiC NWs@NiCo2O4/NiO NSs (Figure S4), CV curves and GV curves of the NiCo2O4/NiO NSs (Figure S5), specific capacitance as a function of specific measurement value of the current (Figure S6), the electrochemical performance of activated carbon on nickel foam (Figure S7), SEM and TEM images of the SiC NWs@NiCo2O4/ NiO NSs after 2000 cycles at 20 mA cm−2 (Figure S8) and comparison of the electrochemical properties of the as-fabricated SiC NWs@NiCo2O4/NiO NSs with the reported ones (Table S1) (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Z. Li. E-mail: [email protected]. *A. Meng. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work reported here was supported by the National Natural Science Foundation of China under Grant No. 51572137, 51502149, 51272117, 51172115, the Natural Science Foundation of Shandong Province under Grant No. ZR2015PE003, ZR2013EMQ006, the Research Award Fund for Outstanding Young Scientists of Shandong Province Grant No. BS2013CL040, the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant No. 20123719110003, the Tackling Key Program of Science and Technology in Shandong Province under Grant No. 2012GGX1021, the Application Foundation Research Program of Qingdao under Grant No. 13-1-4-117-jch, 15-9-1-28-jch, 142-4-29-jch, Shandong Province Taishan Scholar Project and Overseas Taishan Scholar Project. We express our grateful thanks to them for their financial support.

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CONCLUSIONS In conclusion, a simple and cost-effective hydrothermal approach combined with annealing treatment was successfully developed to deposit loose and porous NiCo2O4/NiO NSs on the surfaces of SiC NWs on conductive CC with strong adhesion as a freestanding electrode for an ASC. It has been demonstrated that the hybrid electrode exhibited ultrahigh specific capacitance, and splendid rate capability and conductivity. More importantly, an ASC device was successfully assembled, where the SiC NWs@NiCo2O4/NiO NSs on CC and AC on NF act as positive and negative electrodes, respectively, which delivered a simultaneous significant enhancement in terms of operating potential, energy and power densities, and long-term cycling stability. These amazing results can open up the possibility of combining inorganic nanowire with transition metal oxide composites for applications in ASCs with huge energy and power densities to satisfy the multiple demands in industry development.

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REFERENCES

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