Hierarchical NiCo2O4@NiCo2S4 Nanocomposite on Ni Foam as an Electrode for Hybrid Supercapacitors.

In this work, NiCo2O4@NiCo2S4 nanocomposite with a hierarchical structure is prepared by a multistep process. First, NiCo2O4 nanowires array on Ni foam is prepared by a hydrothermal and a subsequent calcination process. Then, the NiCo2O4 nanowires array is converted to NiCo2O4@NiCo2S4 nanocomposite through a vapor-phase hydrothermal process. The NiCo2O4@NiCo2S4/Ni foam electrode exhibits a specific capacitance of 1872 F g-1 at 1 A g-1, a capacitance retention of 70.5% at 10 A g-1, and a retention ratio of 65% after 4000 charge-discharge cycles. The capacitance of NiCo2O4@NiCo2S4 nanocomposite is much higher than that of the NiCo2O4 nanowires array. The excellent electrochemical capacitive performances of the NiCo2O4@NiCo2S4 nanocomposite can be attributed to the hierarchical nanostructure, which can provide large surface areas and short diffusion pathways for electrons and ions. By using the NiCo2O4@NiCo2S4/Ni foam as the positive electrode and activated carbon/Ni foam as the negative electrode, a hybrid supercapacitor device is fabricated. The device achieves an energy density of 35.6 W h kg-1 and a power density of 1.5 kW kg-1 at 2 A g-1.

Introduction

In recent years, with the rapid development of electric vehicles and consumer electronic products, the requirements for electrical energy storage devices are ever-growing. Supercapacitors and secondary batteries are regarded as major electrical energy storage devices. Compared with the widely used secondary batteries such as lithium-ion batteries, supercapacitors possess the advantages including high power density, long cycle life, and improved safety,[1−8] but suffer from drawbacks such as low energy density. To meet the requirement of actual applications, much effort is still needed to improve the energy density of supercapacitors while maintaining a high power density and a long cycle life.

Electrode materials are decisive on the performance of supercapacitors. Transition metal oxides and sulfides such as NiCo2O4 and NiCo2S4 have been widely studied as electrode materials for supercapacitors.[9−16] Unlike carbon materials, which store charges via electric double-layer mechanism, transition metal oxides and sulfides store charges via reversible faradic redox reactions, and can achieve much higher specific capacitances than that of carbon materials.[17−20] As the faradic redox reactions mainly occur on the surface of electrode materials, it is necessary to develop electrode materials with large surface areas and short diffusion pathways for electrons and ions. Nanostructures such as nanosheets and porous nanowires have been well studied for supercapacitors because they can provide large surface areas and short diffusion pathways for electrons and ions.[21−24] Recently, some hierarchical nanocomposite materials have been developed for supercapacitor applications.[25−29] In the hierarchical nanocomposite materials, different materials with various nanostructures are combined together. The synergistic effects result in improved electrochemical performances than that of the individual constituents.

The energy density of supercapacitors is proportional to the square of working voltage, and a higher working voltage means a higher energy density.[30] However, the working voltage of symmetric supercapacitor is restricted by the stable potential window of water when aqueous electrolyte is used. To increase the working voltage of supercapacitors, hybrid supercapacitors are developed. In hybrid supercapacitors, a faradic-type material acts as the positive electrode and a capacitor-type material acts as the negative electrode.[31−35] Because of the different working mechanism of the positive and negative electrode materials, the working voltage of hybrid supercapacitors can be obviously increased. The development of high-performance electrode materials for hybrid supercapacitors is still of great interest.

In this work, a hierarchical core–shell NiCo2O4@NiCo2S4 nanocomposite is synthesized by a multistep process. In the NiCo2O4@NiCo2S4 nanocomposite, NiCo2S4 nanosheets are supported on NiCo2O4 porous nanowires. Owing to the excellent structural advantages such as large specific surface areas, short diffusion pathways, and absence of aggregation, the NiCo2O4@NiCo2S4 nanocomposite shows enhanced supercapacitor performances over NiCo2O4 porous nanowires. The hierarchical NiCo2O4@NiCo2S4 nanocomposite can be used as a high-performance electrode material for supercapacitor applications.

Results and Discussion

The synthesis of hierarchical NiCo2O4@NiCo2S4 nanocomposite was realized through a multistep process, as illustrated in Figure . First, a precursor was grown on Ni foam by using Ni(NO3)2, Co(NO3)2, and CO(NH2)2 as raw materials through a hydrothermal process. Then, the precursor was converted to NiCo2O4 through calcination under ambient atmosphere. Finally, NiCo2O4@NiCo2S4 nanocomposite was obtained by treating NiCo2O4 in the atmosphere of thioacetamide solution under hydrothermal condition. The composition of the products obtained at different stages is studied by X-ray diffraction (XRD) technique. The diffraction peaks in the XRD pattern of the precursor (Figure S1a, Supporting Information) can be indexed to Co(CO3)0.5(OH)·0.11H2O (JCPDS card no. 48-0083) and Ni2(OH)2CO3·H2O (JCPDS card no. 38-0714). The XRD pattern of the product obtained by calcining the precursor under ambient atmosphere (Figure S1b) can be indexed to spinel NiCo2O4 (JCPDS card no. 73-1702). The formation of NiCo2O4 is a result of the decomposition and oxidation of the precursor through calcination. The XRD pattern of the final NiCo2O4@NiCo2S4 sample is shown in Figure . In addition to the diffraction peaks of NiCo2O4, some weak diffraction peaks of NiCo2S4 are also present. This can be attributed to the low content and poor crystalline of NiCo2S4 in the NiCo2O4@NiCo2S4 sample.

Figure 1

Schematic illustration of the fabrication process of the NiCo2O4@NiCo2S4 nanocomposite.

Figure 2

XRD pattern of the NiCo2O4@NiCo2S4 nanocomposite.

The morphology of the products obtained at different stages is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image (Figure S2) of the precursor at low magnification shows that a layer of precursor is evenly covered on the surface of Ni foam. A SEM image of the precursor at higher magnification is shown in Figure a, and many nanowires with uniform size can be seen. A TEM image of the precursor is shown in Figure b. The nanowire is solid and its surface is smooth. Figure c shows the SEM image of the NiCo2O4 obtained by calcining the precursor under ambient atmosphere. The nanowire morphology of the precursor is retained after calcination. However, the TEM image of the NiCo2O4 nanowire shown in Figure d reveals a porous structure. The formation of the porous structure can be ascribed to the volume shrink and the release of gaseous species during the decomposition of the precursor.

Figure 3

(a, b) SEM and TEM images of the precursor and (c, d) SEM and TEM images of the NiCo2O4 nanowires.

Figure a,b shows the typical SEM images of the NiCo2O4@NiCo2S4 nanocomposite at different magnifications. The NiCo2O4@NiCo2S4 nanocomposite shows a hierarchical structure in which NiCo2S4 nanosheets are attached on NiCo2O4 nanowires. The TEM image of the NiCo2O4@NiCo2S4 nanocomposite in Figure c further illustrates the hierarchical structure. A high-resolution transmission electron microscopy (HRTEM) image of the outer nanosheet is shown in Figure d. The interplanar spacing of the lattice fringes is about 0.23 nm, which corresponds to the (400) lattice plane of NiCo2S4.

Figure 4

(a, b) SEM, (c) TEM, and (d) HRTEM images of the NiCo2O4@NiCo2S4 nanocomposite.

The chemical states of the component elements in NiCo2O4@NiCo2S4 nanocomposite are assessed by X-ray photoelectron spectroscopy (XPS) technique. The peaks at 169.7, 529.5, 781.5, and 855.7 eV in the XPS survey spectrum (Figure S3) correspond to S 2p, O 1s, Co 2p, and Ni 2p, respectively, which indicates the presence of S, O, Co, and Ni elements in the sample. Figure shows the high-resolution XPS spectrum of Ni, Co, O, and S elements. These peaks were fitted by using Gaussian fitting method. The Ni 2p XPS spectrum shown in Figure a contains Ni 2p1/2, Ni 2p3/2, and two shake-up satellite peaks. Both Ni 2p1/2 and Ni 2p3/2 peaks are fitted into two peaks. The peaks at 856.4 and 874.3 eV belong to Ni3+, whereas the peaks at 855.4 and 872.9 eV belong to Ni2+.[36] This result suggests the coexistence of both Ni2+ and Ni3+ in the sample. Figure b shows the Co 2p XPS spectrum. Similar to the Ni element, the Co 2p3/2 and Co 2p1/2 peaks are fitted into peaks corresponding to Co3+ and Co2+, revealing the coexistence of both Co2+ and Co3+ in the sample.[36] The O 1s XPS spectrum is shown in Figure c, the peak at 529.2 eV is typical of metal–oxygen binding,[37] the peak at 531.2 eV is attributed to hydroxyl that come from water vapor in the air, and the peak at 531.7 eV is attributed to oxygen in the low-valence state.[38,39] As compared to the O 1s XPS spectrum of NiCo2O4 nanowires (Figure S4), the peak at 529.2 eV is much weaker. This can be explained by the fact that in the NiCo2O4@NiCo2S4 nanocomposite, the NiCo2O4 are encapsulated by NiCo2S4. Figure d shows the S 2p XPS spectrum, which contains a main peak and a satellite peak. The main peak at 162.7 eV can be attributed to the sulfur ions that bonded with metal ions.[36]

Figure 5

XPS spectra of the NiCo2O4@NiCo2S4 nanocomposite: (a) Ni 2p, (b) Co 2p, (c) O 1s, and (d) S 2p.

The specific surface area and pore size distribution of the NiCo2O4@NiCo2S4 nanocomposite are measured by N2 adsorption–desorption isotherms, as shown in Figure a. The isotherm with significant hysteresis loops can be attributed to type IV isotherms. The Brunauer–Emmett–Teller (BET) surface area of the NiCo2O4@NiCo2S4 nanocomposite is 58.849 m2 g–1, which is higher than that of the NiCo2O4 nanowires (53.892 m2 g–1, Figure S5). The higher specific surface area of the NiCo2O4@NiCo2S4 nanocomposite can be attributed to the existence of thin NiCo2S4 nanosheets in the NiCo2O4@NiCo2S4 nanocomposite. Figure b shows the pore size distribution of the NiCo2O4@NiCo2S4 nanocomposite calculated by the Barrett–Joyner–Halenda (BJH) method. The pore diameter is mainly around 8 nm. The above results confirm the porous nature of the NiCo2O4@NiCo2S4 nanocomposite. When the NiCo2O4@NiCo2S4 nanocomposite was applied as the electrode material for supercapacitors, the porous structure can provide a large surface area for faradic redox reactions, and the pore size is suitable for ion transportation.

Figure 6

(a) N2 adsorption and desorption isotherms of the NiCo2O4@NiCo2S4 nanocomposite. (b) BJH pore size distribution curve.

The unique vapor-phase hydrothermal process in the second step is crucial for the formation of the hierarchical structure of NiCo2O4@NiCo2S4 nanocomposite. If the NiCo2O4 nanowires were immersed in the thioacetamide solution in the second step, only NiCo2S4 nanotubes can be obtained (Figure S6). To understand the formation mechanism of the hierarchical structure of the NiCo2O4@NiCo2S4 nanocomposite, a series of time-dependent experiments were done. Figure shows the SEM images of the products obtained by treating NiCo2O4 nanowires in the atmosphere of thioacetamide solution under hydrothermal condition at 120 °C for different periods of time. After treating for only an hour, no noticeable change can be seen, as shown in Figure a. This is possibly due to the slow temperature rise of the autoclave. When the reaction time reaches 2 h, some nanosheets start to evolve on the surface of NiCo2O4 nanowires, as indicated from Figure b. With the reaction time further prolonging, the nanosheets grow larger (Figure c). After 6 h, the NiCo2O4@NiCo2S4 nanocomposite with an obvious hierarchical structure can be obtained (Figure d). According to the above results, we believe that the reaction between NiCo2O4 and H2S in the atmosphere of thioacetamide solution is slower than that in the solution. During the reacting process, the outer layer of the NiCo2O4 nanowires is gradually converted to NiCo2S4 nanosheets. Finally, the NiCo2O4@NiCo2S4 nanocomposite with a hierarchical structure is obtained.

Figure 7

SEM images of the NiCo2O4@NiCo2S4 nanocomposite obtained at 120 °C for different reaction times, (a) 1 h, (b) 2 h, (c) 4 h, and (d) 6 h.

The NiCo2O4@NiCo2S4 nanocomposite is applied as an electrode material for supercapacitors. The electrochemical performances are evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) methods. Figure a shows a series of CV curves of the NiCo2O4@NiCo2S4 electrode measured in a three-electrode cell with the scan rates in the range of 10–100 mV s–1. All of the CV curves show significant redox peaks, indicating the pseudocapacitive characteristics of the NiCo2O4@NiCo2S4 active material. As the scan rate increases, the magnitude of the current response increases, and the anodic and cathodic peaks shifts to more a positive and negative potential, respectively. The GCD curves of the NiCo2O4@NiCo2S4 electrode at current densities in the range of 1–10 A g–1 are shown in Figure b. The GCD curves appear as obvious platforms during the charging and discharging process, which can be attributed to the redox reactions occurring in the process of charging and discharging. The IR drop in the GCD curves can be attributed to the internal resistance of the electrode. The capacitances of the electrode can be calculated by using the following equations.[40]where Cs (F g–1) is the specific capacitance, I (A) is the current during the discharge process, Δt (s) is the discharge time, ΔV (V) is the potential window, and m (g) is the mass of the active materials. When the current densities are 1, 2, 4, 6, 8, and 10 A g–1, the specific capacities are 1872, 1808, 1720, 1632, 1456, and 1320 F g–1, respectively. With increase in the current density from 1 to 10 A g–1, the capacitance retention of the NiCo2O4@NiCo2S4 electrode is about 70.5%, suggesting a good rate capability. Figure c shows the comparative GCD curves of the NiCo2O4@NiCo2S4 electrode and the NiCo2O4 electrode at a current density of 1 A g–1. It is obvious that the discharging time of the NiCo2O4@NiCo2S4 electrode is much longer than that of the NiCo2O4 electrode. The specific capacitance of the NiCo2O4 electrode is only 722 F g–1 at 1 A g–1.

Figure 8

(a) CV curves of the NiCo2O4@NiCo2S4 electrode in 3 mol L–1 of KOH at different scan rates. (b) GCD curves of the NiCo2O4@NiCo2S4 electrode at different current densities. (c) Comparative GCD curves of the NiCo2O4@NiCo2S4 electrode and NiCo2O4 electrode at 1 A g–1. (d) Cycling behavior of the NiCo2O4@NiCo2S4 electrode at 5 A g–1 for 4000 cycles. (e) Nyquist plots of the NiCo2O4@NiCo2S4 and NiCo2O4 electrodes.

Cycling stability of the NiCo2O4@NiCo2S4 electrode was tested at a current density of 5 A g–1 for 4000 GCD cycles, and the results are displayed in Figure d. With the increase in cycles, the capacitance slowly deteriorates. After 4000 cycles, a capacitance retention of about 65% is achieved. The deterioration of capacitance during the cycling process can be attributed to the oxidation of NiCo2S4 in alkaline electrolytes.[41] The electrochemical impedance spectra of the NiCo2O4@NiCo2S4 and NiCo2O4 electrodes were tested in a frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. The Nyquist plots are shown in Figure e. All of the plots contain a negligible semicircle in the high-frequency region and a straight line in the low-frequency region. The semicircle reflects the charge-transfer resistance in the electrode–electrolyte interface.[42] The negligible semicircle in the plots indicates that the charge-transfer resistance of the two electrodes is very low. This result can be attributed to the direct growth of the electrode materials on the Ni foam. The sloping straight line in the low-frequency region reflects the Warburg impedance. The slope of the lines reveals the diffusion rates of the electrolyte ions into the electrode materials.[43] The slopes of about 49.1 and 67.2° of the straight lines implies an efficient electrolyte and proton diffusion in the electrode materials.

To evaluate the NiCo2O4@NiCo2S4 electrode for practical application, a hybrid supercapacitor device was assembled by using the NiCo2O4@NiCo2S4 nanocomposite as the positive electrode material, activated carbon (AC) as the negative electrode material, and a 3 mol L–1 KOH aqueous solution as the electrolyte. As indicated from the CV curves of the positive and negative electrode materials measured in the three-electrode cell (Figure a), the working potential windows of the positive and negative electrode materials are in different regions. Therefore, the working potential window of the hybrid supercapacitor can be extended because of the complementary of the working potential windows of the positive and negative electrode materials. Figure b displays a series of CV curves of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor in a potential range of 0–1.6 V at the scan rates of 10, 20, 40, 60, 80, and 100 mV s–1. With the increase in the scan rate, the CV curves retain the similar shape, indicating that the hybrid supercapacitor could stably work in such a wide potential window. Figure c displays the GCD curves of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor at the current densities from 2 to 10 A g–1. The shape of the charge and discharge curves is nonlinear due to the occurrence of faradic redox reactions. The specific capacitances of the hybrid supercapacitor are 114, 83, 64, 50, and 46 F g–1 at the current densities of 2, 4, 6, 8, and 10 A g–1. The cycling stability of the device was measured at the current density of 4 A g–1 for 2000 GCD cycles (Figure d). The specific capacitance reaches a maximum of 131 F g–1 after 200 cycles, and then slowly decreases. After 2000 cycles, the specific capacitance of the NiCo2O4@NiCo2S4//AC device is 103 F g–1, which is about 78% of the maximum value. The energy density (E) and the power density (P) of the NiCo2O4@NiCo2S4//AC device can be calculated according to the following equations.[44]Figure e shows the Ragone plot of the NiCo2O4@NiCo2S4//AC device. At a current density of 2 A g–1, the energy density and the power density of the NiCo2O4@NiCo2S4//AC device are 35.6 W h kg–1 and 1.5 kW kg–1, respectively. At the current density of 10 A g–1, the energy density and the power density of the device are 14.4 W h kg–1 and 7.5 kW kg–1, respectively. Finally, a light-emitting diode (LED) is successfully illumed by two series connected NiCo2O4@NiCo2S4//AC devices (Figure f), which further proves the possibility of the NiCo2O4@NiCo2S4 nanocomposite for practical charge storage applications.

Figure 9

(a) Comparative CV curves of activated carbon (AC) and NiCo2O4@NiCo2S4 at a scan rate of 100 mV s–1 in a three-electrode cell. (b) CV curves of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor at different scan rates. (c) GCD curves of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor at different current densities. (d) cycling performances of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor. (e) Ragone plot of the NiCo2O4@NiCo2S4//AC hybrid supercapacitor. (f) Digital photograph of a LED lighted by the hybrid supercapacitor.

Conclusions

In conclusion, the NiCo2O4@NiCo2S4 nanocomposite with a hierarchical structure was prepared by a multistep process. The vapor-phase hydrothermal process in the second step is crucial for the formation of NiCo2S4 nanosheets on the surface of NiCo2O4 porous nanowires. The hierarchical NiCo2O4@NiCo2S4 nanocomposite has excellent structural advantages such as large specific surface areas and short diffusion pathways for electrolyte ions. When acting as an electrode material for supercapacitor, the NiCo2O4@NiCo2S4 nanocomposite shows much enhanced supercapacitor performances than that of the NiCo2O4 porous nanowires. The experimental results indicate that the hierarchical NiCo2O4@NiCo2S4 nanocomposite is suitable for high-performance supercapacitor applications.

Experimental Section

All of the chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. The purity of these chemical reagents was of analytical grade.

Synthesis of NiCo2O4 Nanowires Array on Ni Foam

A piece of Ni foam with a size of 2 cm × 2 cm was washed with 3 mol L–1 hydrochloric acid, acetone, ethanol, and deionized water in sequence. Ni(NO3)2·6H2O (0.291 g, 1 mmol), Co(NO3)2·6H2O (0.582 g, 2 mmol), and CO(NH2)2 (0.420 g, 7 mmol) were together dissolved in 40 mL deionized water in a beaker. The solution was transferred into a stainless-steel autoclave with a Teflon liner of 60 mL capacity, and the Ni foam was immersed in the solution. After that, the autoclave was sealed and heated at 120 °C for 6 h. The precursor-loaded Ni foam was fully washed with deionized water, dried under vacuum at 50 °C for 2 h, and then calcined in an ambient atmosphere at 380 °C for 2 h.

Synthesis of NiCo2O4@NiCo2S4 Nanocomposite on Ni Foam

Thioacetamide (0.380 g, 5 mmol) was dissolved in 25 mL deionized water in a beaker. Then, the solution was transferred into a 60 mL Teflon-lined stainless-steel autoclave. The NiCo2O4/Ni foam was placed on a Teflon support above the solution. After that, the autoclave was sealed and heated at 120 °C for 6 h. The NiCo2O4@NiCo2S4/Ni foam was washed with deionized water and ethanol in sequence, and then dried under vacuum at 50 °C for 2 h. The mass loading of NiCo2O4@NiCo2S4 on Ni foam is about 2.0 mg cm–2.

Characterizations

The composition of the samples was determined by X-ray powder diffraction (XRD, Bruker D8 Advance, Cu Kα radiation), X-ray photoelectron spectroscopy (XPS, ESCALab MKII, Al Kα radiation), and energy-dispersive X-ray spectra. The morphology of the samples was observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Hitachi HT-7700). The surface area of the sample was measured by N2 adsorption/desorption (Micromeritics ASAP 2020M) at 77 K. The electrochemical performances were measured on a CHI-660D electrochemical workstation (Chenhua Corp., Shanghai, China).

Electrochemical Measurements

The electrochemical performances were tested in a three-electrode cell. The working electrode was the NiCo2O4/Ni foam or NiCo2O4@NiCo2S4/Ni foam, the counter electrode was a platinum plate, and the reference electrode was an Hg/HgO electrode. The electrolyte was a 3 mol L–1 KOH aqueous solution.

The hybrid supercapacitor device was assembled by using NiCo2O4@NiCo2S4/Ni foam as the positive electrode, activated carbon/Ni foam as the negative electrode, and cellulose paper as the separator. The activated carbon was prepared according to literature report.[45] A 3 mol L–1 KOH solution, which was soaked in the separator, served as the electrolyte. The electrodes and separator were packed in a CR2032 battery case.