Ultra-high-rate, ultra-long-life asymmetric supercapacitors based on few-crystalline, porous NiCo2O4 nanosheet composites

Long Zhang ab, Lei Dong ab, Mengxiong Li ab, Peng Wang ab, Jiajia Zhang ab and Hongbin Lu *ab
aState Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Collaborative Innovation Center of Polymers and Polymer Composites, Fudan University, 220 Handan Road, Shanghai 200433, China. E-mail: hongbinlu@fudan.edu.cn
bShanghai Xiyin New Materials Corporation, 135 Guowei Road, Shanghai 200438, China

Received 23rd August 2017 , Accepted 6th November 2017

First published on 6th November 2017


NiCo2O4-based supercapacitors are promising for practical applications due to their high specific capacitance, good electrical conductivity and electrochemical activity. Significant advances are highly desired but few studies can simultaneously achieve good rate capability and long cycle life (>30[thin space (1/6-em)]000 cycles) while maintaining high specific capacitance, three factors that dominate practical applications. Here, we demonstrate that when few-crystalline, porous NiCo2O4 nanosheets grown on graphene and carbon nanotubes were assembled into asymmetric supercapacitors, they delivered an energy density of 38.1 W h kg−1 at 797.8 W kg−1 or 13.3 W h kg−1 at 58.1 kW kg−1. Even at a current density of 20 A g−1, they retained 104.5% and 81.2% of the initial capacitance after 20[thin space (1/6-em)]000 and 50[thin space (1/6-em)]000 cycles, respectively. Such performance arises from a new strategy that balances the surface active site, ion diffusion, charge transfer and structural stability of active materials during high-rate cycling. Different from well-crystalline or amorphous pseudocapacitive materials, the few-crystalline, porous nanosheets not only afford an effective charge transfer path when combining with conductive graphene and carbon nanotubes, but also prevent the aggregation of conductive components and boost the diffusion of electrolyte ions within the electrode. The presence of the amorphous domains enriches the surface active sites of nanosheets and enhances their adaptability to deformation during large-current charge/discharge. This strategy holds promise for addressing critical issues facing pseudocapacitive materials, enabling the optimization of the comprehensive performance of supercapacitors in a mild manner.


Introduction

Supercapacitors (SCs) have attracted considerable attention due to their high power density, excellent cycle life, fast charge–discharge rate, and good safety features.1–4 Based on their energy storage mechanisms, SCs can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store charges through adsorption of electrolyte ions on the surface of active materials like activated carbon, having high power density (≈10 kW kg−1) and long cycle life (>100[thin space (1/6-em)]000 cycles), but the energy density (5–10 W h kg−1) and specific capacitance are limited.5,6 By comparison, pseudocapacitors can afford higher specific capacitance and energy density through fast, reversible redox reactions on or near the surface of active materials like metal oxides.7 However, their power density, rate performance and cycle life remain far below those of EDLCs owing to the inherent low conductivities of metal oxides or hydroxides, as well as slow electrode kinetics.1,8 Improving the rate performance, that is, fast charge/discharge capability at large current densities, is a critical strategy that balances the power and energy densities of SCs.9 Despite substantial efforts, however, it is still an open issue that inhibits practical application of pseudocapacitive materials; especially at high current densities, drastic capacitance decaying is unacceptable. The fast charge/discharge process at large current densities usually requires fast charge transfer, short transfer pathway and superior structure stability to ensure the desired cycle life. However, it is difficult to simultaneously meet all these requirements; few studies have revealed fast charge/discharge at large current densities (>20 A g−1) while maintaining a high capacitance after >30[thin space (1/6-em)]000 cycles. We here demonstrated a new strategy, based on few-crystalline, porous NiCo2O4 nanosheets, to overcome the above difficulty. The assembled asymmetric SC not only exhibits a good energy/power density balance, but also reveals highly desirable rate capability and cycle life.

Cobalt–nickel oxide (NiCo2O4), a binary mixed-valence transition metal oxide, has excellent electrical and electrochemical properties [two orders of magnitude higher electrical conductivity than that of monometal oxides (NiO or Co3O4)], which makes it an ideal choice for developing high-performance SCs.10–12 To ensure high electrical conductivities, almost all of the reported pseudocapacitors or asymmetric SCs were developed on the basis of well-crystalline NiCo2O4. However, such SCs rarely reveal the desired electrochemical performance at high current densities when assembled to two-electrode devices (see ESI Table S1). Their specific capacitance decays drastically when charging/discharging at ≥20 A g−1 and the cycle life is mostly lower than 10[thin space (1/6-em)]000 cycles. The well-crystalline structure is beneficial for facilitating the charge transfer within electrodes; however, it is hard to accommodate the structural deformation during large-current charge/discharge.13–15 Moreover, the active sites on these particle surfaces (low specific surface areas (SSAs), typically 54–92 m2 g−1) are limited and the ion diffusion within electrodes is also restricted to some extent due to the lack of effective charge transfer channels.16,17 By comparison, an amorphous structure, especially porous amorphous structures, affords larger SSAs, more redox reactive sites, and insensitivity to structural deformation. However, it also generates large charge transfer resistance within electrodes, so that the electrochemical performance at large current densities is remarkably suppressed, with a limited cycling stability,18–20e.g., 81% of the initial capacitance after 10[thin space (1/6-em)]000 cycles.21 Nevertheless, the introduction of amorphous phase would contribute to enhance the number of surface active sites and avoid possible structural damage in high-rate cycling. Essentially, neither well-crystalline or amorphous structures can well fulfil the requirements of high-performance pseudocapacitors, that is, high capacitance, low charge transfer resistance, and good structural stability under high-rate charge/discharge.9 It has thus become a critical step for high-rate, long-life SCs to rationalize the microstructure of pseudocapacitive materials that can boost charge transfer within electrodes at large current densities, and ensure sufficient active sites and structural stability.

In this work, we propose a possible approach that addresses the critical issues facing pseudocapacitive materials in practical applications (limited rate capability and cycle life) through creating few-crystalline structures in NiCo2O4-based composites, which has rarely been reported. We demonstrate a facile, low-cost, eco-friendly strategy to prepare few-crystalline, porous NiCo2O4 nanosheets, which are deposited on the surface of reduced graphene oxide (RGO) and carbon nanotubes (CNTs) to form electrochemically active composites. Different from the previous reports,16,22–29 we employed refluxing growth of the precursor combined with low-temperature air calcination to construct active material networks (porous nanosheets), in which the crystalline domain of NiCo2O4 facilitates the charge transfer within the electrode and the amorphous domain affords sufficient surface active sites and adaptability to the deformation during large-current charge/discharge. The resulting composite electrode reveals a good balance between high capacitance, high charge/discharge rate and long cycle life. We exhibit that the asymmetric supercapacitor (ASC), assembled with activated carbon (AC) has a record rate performance (72.0% of capacitance retention at 10 A g−1, 61.7% at 20 A g−1, and 42.2% even at larger 80 A g−1.) and ultra-long cycle stability at large current densities (81.2% of capacitance retention after 50[thin space (1/6-em)]000 cycles at 20 A g−1). Moreover, the ASC can deliver 38.1 W h kg−1 at 797.8 W kg−1, and 13.3 W h kg−1 at 58.1 kW kg−1, which are the highest values compared with those of the reported NiCo2O4-based aqueous ASCs.

Experimental

Synthesis of few-crystalline porous NiCo2O4/RGO/CNTs composites

Multi-walled carbon nanotubes (MWCNTs, diameter range 10–20 nm) were treated by mixed acid (H2SO4/HNO3 = 3[thin space (1/6-em)]:[thin space (1/6-em)]1 in volume) for purification and functionalization following the reported method,30 resulting in carboxylic MWCNTs (cMWCNTs). Graphene oxide (GO) was synthesized from natural flake graphite based on our previous report;31 the as-prepared GO dispersion was diluted to 2 mg mL−1. 10 mg of cMWCNTs was added into 30 mL of deionized water with sonication for 10 minutes to form a cMWCNT solution, then 20 mL of GO dispersion was mixed with the above solution with stirring for 1 hour, resulting in solution A. Subsequently, 1 mmol of Ni(NO3)2·6H2O, 2 mmol of Co(NO3)2·6H2O and 20 mmol pf urea were successively added to 50 mL of deionized water with vigorous stirring, resulting in solution B. Subsequently, solution B was slowly mixed with solution A by 2 hours of stirring, then the obtained solution was transferred to a 250 mL round-bottom flask and refluxed at 100 °C for 12 hours. After quench cooling to ambient temperature, the precipitate was collected, washed several times with deionized water and alcohol, and dried at 60 °C under vacuum overnight. Finally, the as-prepared samples were annealed at 250 °C in air for 2 hours with a heating rate of 5 °C min−1, and black few-crystalline porous (FCP-) NiCo2O4/RGO/CNTs composites resulted. In control experiments, the pure NiCo2O4 and RGO/CNTs were synthesized through a similar procedure, without the addition of solution A or B.

Fabrication of asymmetric supercapacitors (ASCs)

The positive electrode was prepared by mixing the FCP-NiCo2O4/RGO/CNTs composite, acetylene black, and PTFE binder (60 wt%) in a mass ratio of 80[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10, and grinding with a small amount of alcohol. The obtained slurry was pressed onto a nickel foam collector (1.0 cm × 1.0 cm) under 10 MPa pressure for 1 minute and then dried at 60 °C under vacuum with a mass loading of 1.2 mg. The negative electrode was prepared by mixing commercial activated carbon (AC, YEC-8A, Fuzhou Yihuan Carbon Co. Ltd.), acetylene black, and PTFE following the same procedure to that used for the positive electrode. After being immersed in 6 M KOH solution for 12 hours, the two electrodes were separated using a cellulose membrane (pore diameter 0.45 μm) and assembled into a two-electrode ASC with a self-made mould. The optimal mass ratio of the positive and negative electrodes was decided by charge balance theory (q+ = q) according to the following equation:
 
image file: c7ta07449b-t1.tif(1)
where m is the mass of the active material, C is the specific capacitance of the electrode, and ΔV is the working potential excluding IR drop of the electrode. The mass ratio determined thereby is m+/m ≈ 0.35. Similarly, for the NiCo2O4-based ASC, the mass ratio was determined as m+/m ≈ 0.96.

Characterization

The morphology and structure of the samples were characterized by field emission scanning electron microscopy (FESEM, Ultra 55), transmission electron microscopy (TEM, Tecnai G2 20 TWIN) and high-resolution TEM (HRTEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) using an AXIS UltraDLD spectrometer (Shimadzu-Kratos) with Al Kα radiation (1486.6 eV) was employed to characterize the elemental composition and chemical state. X-ray diffraction (XRD) was used to characterize the crystal structure with a PANalytical X'Pert PRO diffractometer operating at 40 kV and 40 mA with monochromatic Cu Kα radiation (λ = 1.54 Å) ranging in 2θ from 10 to 90°. The SSA and pore volume were measured with a Quadrasorb evo analyzer (Quantachrome Instruments) using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

Electrochemical measurements

Cyclic voltammetry (CV), chronopotentiogram (CP), electrochemical impedance spectroscopy (EIS) and cycling stability for two-electrode and three-electrode systems were measured by a CHI 660E electrochemical workstation. EIS was measured in a frequency range of 100 kHz to 0.01 Hz at open circuit potential and an amplitude of 5 mV. All electrochemical measurements were conducted in 6 M KOH solution at room temperature. For three-electrode measurement, an Ag/AgCl electrode and Pt wire were used as the reference electrode and counter electrode, respectively. The specific capacitance was calculated according to the following equation:
 
image file: c7ta07449b-t2.tif(2)
where I (A) is the discharge current, Δt (s) is the discharge time, and ΔV (V) is the working potential excluding the IR drop. For the specific capacitance of the electrode materials (CS, F g−1) in three-electrode mode, m (g) is the mass loading of active materials on a single electrode. For the specific capacitance of ASC devices (CT, F g−1), m (g) is the total mass loading of active materials on the two electrodes. The energy density (E, W h kg−1) and power density (P, W kg−1) for the ASCs were calculated according to the following equations:
 
image file: c7ta07449b-t3.tif(3)
 
image file: c7ta07449b-t4.tif(4)

Results and discussion

The preparation process and growth mechanism of the FCP-NiCo2O4/RGO/CNTs composite are illustrated in Fig. 1a. GO and CNTs were firstly dispersed in deionized water to form solution A. Owing to the presence of oxygen-containing groups (negatively charged) on the GO and CNTs, they formed a stable, homogeneous suspension. When mixing with the solution B containing Ni(NO3)2, Co(NO3)2 and urea, positively charged Co2+ and Ni2+ were preferentially adsorbed on the GO surface because of its larger specific surface area (SSA) compared with CNTs.32 Subsequent refluxing and low-temperature calcination made the few-crystalline NiCo2O4 nanosheets grow along the planar direction of GO and resulted in the formation of porous nanosheet structures. In control experiments, NiCo2O4 nanobelts were also synthesized in the absence of GO and CNTs following the same procedure.
image file: c7ta07449b-f1.tif
Fig. 1 (a) Schematic illustration of the fabrication of the FCP-NiCo2O4/RGO/CNTs composite. (b–d) FESEM images and (e–g) TEM images of the FCP-NiCo2O4/RGO/CNTs composite at different magnifications.

FESEM and TEM were used to characterize the morphology and structure of the NiCo2O4 nanobelts and FCP-NiCo2O4/RGO/CNTs composites. As shown in Fig. S1a and b, the NiCo2O4 nanobelts obtained are mostly 20–40 nm wide and several hundreds of nanometers long, along with an aggregated morphology. TEM images clearly show their belt-like, porous morphology (see Fig. S1c and d). For the FCP-NiCo2O4/RGO/CNTs composite, no belt-like morphology was observed in FESEM images, besides loosely stacked nanosheets and some carbon nanotubes across different nanosheets (Fig. 1b and c). Although some sheet-like, porous NiCo2O4 particles can be identified in the magnified image (Fig. 1d), many of the NiCo2O4 nanosheets were found to adhere on the RGO surface (Fig. 1e and f, GO was partially reduced to graphene during the calcination, RGO). They typically have a lateral size of 100–200 nm and porous networks (Fig. 1f and g); the latter primarily arises from the decomposition of the Ni–Co precursor during annealing. Such porous structure is beneficial to enhance the charge storage capability of NiCo2O4 nanosheets.

We further investigated the elemental composition and chemical state of the NiCo2O4/RGO/CNTs composite with XPS, and the results are shown in Fig. 2. The full survey spectrum (Fig. 2a) indicates the presence of elements Ni, Co, C, and O. The high-resolution Ni 2p spectrum (Fig. 2b) consists of two spin–orbit doublets (Ni2+ and Ni3+) and two couples of shakeup satellites (Sat.).33 Two peaks at 854.2 and 871.6 eV are attributed to Ni2+ while the peaks at 855.6 and 873.1 eV come from Ni3+. The fitting peaks at 861.1 and 879.6 eV arise from two shakeup species.34 Similarly, two Co types were also detected in the high-resolution Co 2p spectrum (Fig. 2c). Two peaks at 779.5 and 794.6 eV are ascribed to Co3+, while the peaks at 781.0 and 796.1 eV are indexed to Co2+.35 The C 1s spectrum (Fig. 2d) can be deconvolved into three components. The peak at 284.6 eV is ascribed to sp2-hybridized carbon atoms, while two small peaks at 286.0 and 288.6 eV stem from the contribution of epoxy/alkoxy carbon (C–O) and carbonyl carbon (C[double bond, length as m-dash]O) bonds.36 The O 1s spectrum shown in Fig. 3b indicates that these NiCo2O4 nanosheets are few-crystalline. All these results suggest that the FCP-NiCo2O4/RGO/CNTs composite contain two electron couples of Co3+/Co2+ and Ni3+/Ni2+, which is expected to yield a synergy in electrochemical reactions. In this sense, NiCo2O4 can be expressed as Co1−x2+Cox3+[Co3+Nix2+Ni1−x3+]O4 (0 ≤ x ≤ 1).26,37


image file: c7ta07449b-f2.tif
Fig. 2 XPS spectra of the FCP-NiCo2O4/RGO/CNTs composite: (a) survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) C 1s.

image file: c7ta07449b-f3.tif
Fig. 3 Evidence of the few-crystalline structure in the NiCo2O4 nanosheets. (a) XRD patterns of the FCP-NiCo2O4/RGO/CNTs composite, RGO/CNTs, and NiCo2O4. (b) XPS spectra of O 1s for the FCP-NiCo2O4/RGO/CNTs composite and (c) HRTEM image of the NiCo2O4 nanosheets in the composite.

We next prove the few-crystalline nature of the NiCo2O4 nanosheets in composites. The XRD patterns of the as-prepared NiCo2O4 nanobelts and composite are shown in Fig. 3a, in which the characteristic diffraction peaks at 19.04°, 31.28°, 36.75°, 44.69°, 59.18°, and 65.03° coincide with the (111), (220), (311), (400), (511), and (440) crystal planes of the spinel NiCo2O4 (JCPDS card no. 73-1702), respectively. No impurity peaks appear in the patterns, indicating a high phase purity for NiCo2O4.38 In the XRD pattern of RGO/CNTs, a weak, broad peak around 25.1° is ascribed to the (002) crystal plane of graphite-like structures, indicating that re-aggregation of RGO sheets has been largely suppressed due to the presence of CNTs.39 For the FCP-NiCo2O4/RGO/CNTs composite, both CNTs and NiCo2O4 can prevent RGO sheets from re-aggregation, so that the similar diffraction become invisible. Notably, both NiCo2O4 nanobelts and the FCP-NiCo2O4/RGO/CNTs composite reveal weak characteristic peaks, indicating that the NiCo2O4 nanobelts or nanosheets formed under the present mild condition are not well-crystalline. This is in agreement with the XPS results. As shown in Fig. 3b, the high-resolution O 1s spectrum includes three O components, namely, O1, O2, and O3. The O1 component at 529.3 eV is a typical metal–oxygen bond (M–O, M[double bond, length as m-dash]Ni, Co), while the O2 peak (531.3 eV) relates to defect sites that originate from low oxygen coordination and small particle size. The components O1 and O2 are ascribed to oxygen ions (O2−) in NiCo2O4, while the O3 at 533.1 eV is primarily due to physically or chemically adsorbed water on the surface.27,40 Compared with the O1 peak, the stronger O2 peak indicates that the NiCo2O4 nanosheets contain a large amount of defects, a few-crystalline feature. In addition, this feature is also reflected in their HRTEM image. As shown in Fig. 3c, the NiCo2O4 nanosheets reveal a porous structure, and several lattice fringes with d spacings of 0.202, 0.243, and 0.287 nm, corresponding to the (220), (311), and (400) planes of spinel NiCo2O4, respectively, which is consistent with the XRD results and, some amorphous domains are also identified in Fig. 3c. The O 1s spectrum and HRTEM images of NiCo2O4 nanobelts (Fig. S2 and S3) exhibit the same few-crystalline feature, indicating that the mild preparation conditions play an important role in forming the few-crystalline structure.

The BET method was used to measure the specific surface area (SSA) and the corresponding nitrogen absorption/desorption isotherm curves are presented in Fig. S4. The isotherm of the NiCo2O4 nanobelts is a type IV curve with a H1 hysteresis loop, typical of mesoporous materials, while the isotherm of the FCP-NiCo2O4/RGO/CNTs composite reveals the feature of RGO or CNTs (slit-shaped pores).16 According to the above results, we determined the SSA and pore volume of the FCP-NiCo2O4/RGO/CNTs to be 148 m2 g−1 and 0.545 cm3 g−1, respectively, larger than those of the NiCo2O4 nanobelts, 129 m2 g−1 and 0.247 cm3 g−1, respectively. These values are larger than those of the reported NiCo2O4 crystals, 54–92 m2 g−1.16,17 Such porous structure contributes to facilitate the diffusion of electrolyte ions and, meanwhile, the presence of crystalline domains in the nanosheets is important for the charge transfer within the electrode. Different from the amorphous or well-crystalline metal oxide electrodes,13–21 we expect that such a few-crystalline, porous NiCo2O4 structure can better balance the specific capacitance, rate capability and cycle life of supercapacitors.

We first determined their electrochemical properties with a three-electrode system. Fig. 4a shows the cyclic voltammetry (CV) curves of the FCP-NiCo2O4/RGO/CNTs composite at 5 to 100 mV s−1 in the voltage range of −0.1–0.5 V. A distinct pair of redox peaks is identified, arising from the faradaic redox reactions related to M2+/M3+ (M[double bond, length as m-dash]Co or Ni ions) and Co3+/Co4+. The corresponding charge storage mechanism of NiCo2O4 in alkaline is expressed as follows,41

 
NiCo2O4 + OH + H2O ⇌ NiOOH + 2CoOOH + e(5)
 
CoOOH + OH ⇌ CoO2 + H2O + e(6)


image file: c7ta07449b-f4.tif
Fig. 4 Electrochemical behaviors of the FCP-NiCo2O4/RGO/CNTs composite in a three-electrode system. (a) CV curves of the composite electrode at scan rates from 5 to 100 mV s−1. (b) CV curves of the composite and NiCo2O4 electrodes at a scan rate of 100 mV s−1. (c and d) CP curves of the composite electrode at current densities from 1 to 50 A g−1. (e) Specific capacitance of the composite and NiCo2O4 electrodes at different current densities. (f) Nyquist plot of the composite and NiCo2O4 electrodes, the inset of which shows the amplified Nyquist plot.

With the increase of the scan rate to 100 mV s−1, all curves of the composite retain a similar shape, indicating its advantage in fast redox reactions.26 In addition, the large area in the CV curve at 100 mV s−1 implies a better specific capacitance, relative to the NiCo2O4 (Fig. 4b and S5).35 The NiCo2O4 electrode exhibits a large polarization issue at 100 mV s−1, reflecting the superiority of the FCP-NiCo2O4/RGO/CNTs composite electrode in charge transfer and ion diffusion. Fig. 4c and d show the CP curves of the composite electrode at current densities from 1 to 50 A g−1. The discharge time decreases with increasing current densities, and in terms of eqn (2), the composite electrode exhibits excellent specific capacitance (Cs) (1618.0 F g−1 at 1 A g−1) and ultra-high rate capability: 1469.2 (90.8%), 1361.7 (84.2%), 1283.2 (79.3%), 1225.3 (75.7%), 1164.2 (72.0%), 1079.7 (66.7%), and 917.8 F g−1 (56.7%) at 2, 5, 10, 15, 20, 30, and 50 A g−1, respectively. Such rate performance is greatly superior to the reported results of Ni and/or Co-based materials,2,11,14,16,27,32–35,42–50 and a detailed comparison is presented in Table S2. In contrast, the specific capacitances of the NiCo2O4 electrode only show 586.5, 528.1, 441.1, 372.2, 309.4, 217.4 F g−1 at 1, 2, 5, 10, 20, and 50 A g−1, respectively, with capacitance retention of 37% at 50 A g−1. The specific capacitances of both the composite and NiCo2O4 electrodes at different current densities are shown in Fig. 4e. We believe that the better electrochemical performance of the composite results from the synergy of the following factors: (1) the few-crystalline structure of the NiCo2O4 nanosheets contributes to the charge transfer within an individual nanosheet, which is more effective compared with those amorphous metal oxide nanoparticles, (2) RGO acts as a substrate, which prevents the aggregation of NiCo2O4 nanosheets and facilitates the formation of electrolyte ion-accessible porous structures (a large pore volume: 0.545 cm3 g−1), to increase the number of active sites, (3) CNTs construct a bridge between different nanosheets and form conductive networks that facilitate the charge transfer, and (4) the coadjacent few-crystalline, porous nanosheets benefit the diffusion of electrolyte ions and the occurrence of redox reactions.

The EIS results provide direct evidence of the above synergistic effect. The Nyquist plots of the composite and NiCo2O4 electrodes are shown in Fig. 4f. At high frequency, the equivalent series resistance (Rs, corresponding to the interception on the Z′ axis) and charge-transfer resistance (Rct, denoted by the diameter of the semicircle) of the composite electrode are much lower than those of the NiCo2O4 electrode, confirming the high electrical conductivity of the composite electrode. At low frequency, the diffusion resistance (Warburg resistance, inversely proportional to the slope of the inclined line) of the composite electrode is also lower than that of the NiCo2O4 electrode, which suggests better ion diffusion efficiency and ideal capacitive behavior. These results enable us to conclude that the synergy between the few-crystalline, porous structure, the substrate effect of RGO to induce the growth of NiCo2O4 nanosheets and the conductive network of CNTs is important for improving the rate performance and cycle stability of NiCo2O4-based electrodes. In particular, such a few-crystalline, porous structure not only ensured high specific capacitances and better structural adaptability in large-current charge/discharge, the mild preparation conditions also exhibit large application potential.

To verify this, we further fabricated asymmetric supercapacitors using the composite and NiCo2O4 as the positive electrode and activated carbon (AC) as the negative electrode (ASCs, denoted as FCP-NiCo2O4/RGO/CNTs//AC and NiCo2O4//AC, respectively). The electrochemical performance of AC was first tested with CV and CP (Fig. S6). The representative electrical double layer characteristic [rectangular CV curves and the good rate performance (listed in Table S3)] indicates that the AC is a competitive candidate as the negative electrode of ASCs. Fig. 5a presents CV curves of the AC electrode and the composite electrode at 50 mV s−1 in a three-electrode system. The working potential of the AC and composite electrodes is −1.1–0.1 V and −0.1–0.5 V, respectively, indicating a working potential as high as 1.6 V. Fig. 5b exhibits the CV curves of the FCP-NiCo2O4/RGO/CNTs//AC at scan rates from 10 to 100 mV s−1. It was found that at the scan rate of 100 mV s−1, the CV curve still retains the similar shape, suggesting an outstanding rate capability. The impedance spectra of both FCP-NiCo2O4/RGO/CNTs//AC and NiCo2O4//AC are presented in Fig. 5c. At low frequency, the diffusion resistance of the FCP-NiCo2O4/RGO/CNTs//AC is much lower than that of NiCo2O4//AC. From the inset of Fig. 5c, we found that the Rs (0.33 Ω) and Rct (1.36 Ω) of NiCo2O4/RGO/CNT//AC are also lower than those of NiCo2O4//AC (Rs is 0.52 Ω and Rct is 2.32 Ω), indicating that the FCP-NiCo2O4/RGO/CNTs//AC has better ion diffusion efficiency and higher electrical conductivity. In addition, the CP curves of FCP-NiCo2O4/RGO/CNTs//AC at current densities from 1 to 80 A g−1 (Fig. 5d and e) show that the specific capacitances (CT), based on the total loading mass of two electrodes and eqn (2), are 107.8, 100.1, 88.9, 79.9, 77.5, 66.5, 50.1, and 45.5 F g−1 at 1, 2, 5, 8, 10, 20, 50, and 80 A g−1, respectively, showing the ultra-high rate capability. Power density and energy density are two determinant parameters for practical applications and the corresponding results, calculated in terms of eqn (3) and (4), are presented in a Ragone plot (Fig. 5f). The highest energy density of FCP-NiCo2O4/RGO/CNTs//AC is 38.1 W h kg−1 at 797.8 W kg−1, comparable to that of lead–acid batteries (30–50 W h kg−1).51 It stays at 13.3 W h kg−1 even when the power density increases to ultra-high 58.1 kW kg−1, which fulfils the power demand of PNGV (Partnership for a New Generation of Vehicles).3 Fig. S7 exhibits that the highest energy and power densities of NiCo2O4//AC are 29.2 W h kg−1 (at 797.2 W kg−1) and 15.5 kW kg−1 (at 9.6 W h kg−1), respectively. They are all far lower than the results of FCP-NiCo2O4/RGO/CNTs//AC, reflecting the electrochemical performance advantage of FCP-NiCo2O4/RGO/CNTs composites. These values are also higher than those of most Ni and/or Co-based ASCs, including CQDs/NiCo2O4//AC (27.8 W h kg−1 and 10.2 kW kg−1),52 NiCo2O4@Au nanotubes (19.56 W h kg−1 and 2.12 kW kg−1),53 GF-CNT@NiO//G-CNT (23.4 W h kg−1 and 7.14 kW kg−1),54 Ni–Co hydroxide/graphene/Ni foam//AC (33.8 W h kg−1 and 7.5 kW kg−1),55 and Co3O4//carbon (36 W h kg−1 and 8 kW kg−1).56 Although the energy density of our composite ASC is slightly lower than that of a few reported ASCs, such as NiCo2O4/NGN/CNTs//NGN/CNTs (42.7 W h kg−1 at 775 W kg−1)57 and NiCo2O4 microspheres@nanomeshes (45.3 W h kg−1 at 533.3 W kg−1),58 it has the highest power density (58.1 kW kg−1) with decent energy density at 80 A g−1, significantly superior to the existing NiCo2O4-based aqueous ASCs. Such performance, especially its ultra-high rate capability, is desired for practical applications. A more detailed comparison is presented in Fig. 5f.44,50,52–57,59–62


image file: c7ta07449b-f5.tif
Fig. 5 Electrochemical behaviors of FCP-NiCo2O4/RGO/CNT//AC ASC. (a) CV curves of the FCP-NiCo2O4/RGO/CNT composite positive electrode and the AC negative electrode at a scan rate of 50 mV s−1. (b) CV curves of FCP-NiCo2O4/RGO/CNTs//AC ASC at scan rates from 10 to 100 mV s−1. (c) Nyquist plots of FCP-NiCo2O4/RGO/CNTs//AC and NiCo2O4//AC, the inset of which shows the amplified Nyquist plots. (d and e) CP curves of FCP-NiCo2O4/RGO/CNTs//AC at current densities from 1 to 80 A g−1. (f) Ragone plot of FCP-NiCo2O4/RGO/CNTs//AC ASC, which includes data for the Ni- and/or Co-based ASCs reported previously.

It is visually demonstrated in Fig. 6 that the FCP-NiCo2O4/RGO/CNTs//AC ASC possesses an ultra-high rate capability, and the capacitance retains 72.0% at large current density of 10 A g−1, 61.7% at 20 A g−1, and 42.2% even at larger 80 A g−1. In contrast, the specific capacitance of NiCo2O4//AC is 82.6 F g−1 at 1 A g−1 and only 18.5% (15.3 F g−1) at 50 A g−1 (detailed information is presented in Fig. S8). It also can be seen from Fig. 6 that the rate capability of our composite ASC is far superior to other reported Ni- and/or Co-based ASCs. These results afford direct evidence to the above analysis, that is, the synergy of the above four factors plays a critical role in the composite ASC, especially for rate performance.


image file: c7ta07449b-f6.tif
Fig. 6 Specific capacitance of FCP-NiCo2O4/RGO/CNTs//AC, NiCo2O4//AC, and the reported Ni- and/or Co-based ASCs at different current densities.

On the other hand, cycle stability has been a critical problem for most pseudocapacitive materials, especially at large current densities. The present few-crystalline, porous nanosheet strategy presents a possible solution to this issue. Given the rate advantage of the FCP-NiCo2O4/RGO/CNTs//AC, for example, only 5.37 s to achieve 61.7% of the total capacitance at 20 A g−1, we employed the same current density (20 A g−1) to evaluate its cycling stability. As shown in Fig. 7, its capacitance increases to 120.8% of the initial capacitance after 12[thin space (1/6-em)]000 cycles, which could be attributed to full activation of the electrode (through electrochemical reactions) during charge/discharge57 and good structural stability. Furthermore, it still keeps 104.8% and 81.2% after 20[thin space (1/6-em)]000 and 50[thin space (1/6-em)]000 cycles, respectively. Such ultra-long cycling stability again reflects the advantage of the few-crystalline, porous nanosheet structure. During the charge/discharge process, RGO and porous NiCo2O4 nanosheets can buffer the volume change at large current densities so that the NiCo2O4 could be fully activated and hold well the porous morphology, and no obvious pulverization occurred even after 50[thin space (1/6-em)]000 cycles (Fig. S9). In comparison, Fig. S10 exhibits the cycle performance of NiCo2O4//AC ASC at 20 A g−1. As can be seen there, the capacitance shows an increasing trend over the first 20 cycles because of the electrode activation. However, the capacitance decreases rapidly in the following cycles, and just retains 39.7% of the initial capacitance after 500 cycles. These results indicate the effectiveness of the novel structure in our composites, which ensures full activation of the pseudocapacitive electrode and structural stability and exhibits ultra-long cycle life. To demonstrate its practical application, the inset of Fig. 7 exhibits a red commercial light-emitting diode (LED, 3 mm, 2.4 V) connected to a close circuit with two FCP-NiCo2O4/RGO/CNTs//AC ASCs in series, and a light-emitting duration of up to 25 minutes or more. Based on the above results, we believe that such a few-crystalline, porous nanosheet strategy opens up a promising door for practical applications of pseudocapacitive materials such as NiCo2O4.


image file: c7ta07449b-f7.tif
Fig. 7 Cycling stability of the FCP-NiCo2O4/RGO/CNTs//AC at a large current density of 20 A g−1; the inset shows a lit red LED powered by two FCP-NiCo2O4/RGO/CNTs//AC ASCs connected in series, which can last for 25 minutes or more.

Conclusions

We demonstrated a novel strategy to achieve simultaneous improvements in specific capacitance, rate performance and cycle life of supercapacitors by constructing few-crystalline, porous NiCo2O4 nanosheets/reduced graphene oxide/carbon nanotube (FCP-NiCo2O4/RGO/CNTs) composite electrodes. The preparation process of FCP-NiCo2O4/RGO/CNTs composites is eco-friendly and can be completed at lower operation temperatures (refluxing at 100 °C and calcination at 250 °C) and under ambient pressure, milder conditions compared with those employed in the reported studies. The few-crystalline, porous structure has a specific surface area of 148 m2 g−1 and a pore volume of 0.545 cm3 g−1, and thus affords superior diffusion efficiency of electrolyte ions and adaptability to deformation at large current densities. When combining with reduced graphene oxide and carbon nanotubes to form FCP-NiCo2O4/RGO/CNTs composites, effective conductive networks were constructed so that the composite electrode reveals high specific capacitance of 1618.0 F g−1 at 1 A g−1 and 917.8 F g−1 at 50 A g−1. When assembling with an activated carbon electrode into the asymmetric supercapacitor (FCP-NiCo2O4/RGO/CNTs//AC), the latter delivered an energy density of 38.1 W h kg−1 (at 797.8 W kg−1) and a power density of 58.1 kW kg−1 (at 13.3 W h kg−1), the highest values among all the reported NiCo2O4-based aqueous ASCs. Furthermore, it also revealed an ultra-long cycle life at the large current density (20 A g−1): 104.5% and 81.2% of the initial capacitance retention after 20[thin space (1/6-em)]000 and 50[thin space (1/6-em)]000 cycles at 20 A g−1, respectively. It is believed that such a few-crystalline, porous nanosheet composite strategy will contribute to the development of industrially viable supercapacitor devices, especially with ultra-high rate performance and ultra-long cycle life.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support by the 973 project (2011CB605702), the National Science Foundation of China (51173027), and Shanghai key basic research project (14JC1400600).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta07449b

This journal is © The Royal Society of Chemistry 2018