Ni7S6 composite with the inverse opal

C-IOP/NiO/Ni7S6 composite with the inverse opal lattice as an electrode for supercapacitors Nadezhda S. Sukhinina, Vladimir M. Masalov, Andrey A. Zhokhov, Irina I. Zverkova, Gennadi A. Emelchenko Institute of Solid State Physics RAS, 143432 Chernogolovka, Moscow District, Russia ABSTRACT In this work, we demonstrate the results of studies on the synthesis, the structure and properties of carbon inverted opal (C-IOP) nanostructures, the surface of which is modified by oxide and sulfide of nickel. It is shown that the modification of the matrix C-IOP by nickel compounds led to a decreasing the specific surface area more than three times and was 250 m2/g. The specific capacitance of the capacitor with the C-IOP/NiO/Ni7S6 composite as electrode has increased more than 4 times, from 130 F/g to 600 F/g, as compared with the sample C-IOP without the modification by nickel compounds. The significant contribution of the faradaic reactions in specific capacitance of the capacitor electrodes of the composites is marked. Keywords: inverse opal lattice, oxide and sulfide of nickel, composite electrode, supercapacitor

1. INTRODUCTION Nanostructured carbon materials are attractive from both fundamental and practical point of view, and find wide application in many fields of technology, including electrode materials for supercapacitors, batteries and fuel cells, sorbents for different purposes, materials for catalysis [1-3]. The most actively developed areas are connected with portable power sources in microelectronics, energy storages, power components of impulse devices and other devices where is required a high-speed energy source [4]. Supercapacitors are a kind of an intermediate energy storage device between batteries and capacitors, possessing a high power density, a long cycle life and the ability to quickly recharge [5]. A supercapacitor performance is primarily determined by the material from which the electrodes are made. Currently there are three types of electrodes, depending on their chemical composition: electrodes based on carbon structures, metal oxides and polymer electrodes [6-8]. Cycling properties of supercapacitors can be significantly improved by making the electrodes from carbon and metal oxides composites. Carbon has a high electron conductivity, and it may increase the conductivity of the active materials. It can also act as a barrier precluding the aggregation of the active particles and, hence, increasing the stability of the structure during the recharge process [9]. Electrodes based on carbon structures modified of metal oxides (MnO2, RuO2, NiO, etc.) possess so-called "pseudocapacity"[10]. Pseudocapacitive materials let achieve energy a density of batteries combined with a long cycle life and a power density of electrical double-layer capacitors. Thus the synthesis of the carbon composites is an efficient way to improve the electrochemical characteristics of the electrodes. In recent years, various metal sulfides as electrode materials are studied due to their advantages oxides over more conductivity and a mechanical stability [11, 12]. It is interesting to investigate the effect of a combination of oxide and sulfide of the same metal on the electrochemical parameters of the composite. Among methods for producing nanostructured carbon materials, the matrix synthesis (the template method) has the best abilities to control the porous structure of the material. This method is based on filling the lattice cavities in matricestemplates by various substances. Opal-like materials are convenient as a matrix to create nanostructures [13]. The system of interconnected micro-, meso- and macropores within inverted opal combined with high specific surface area increases sorption, catalytic and electrochemical properties of the material. An important advantage of nanostructures with inverted opal lattice is their three-dimensional regular arrangement. In [14] the influence of the thermochemical treatment of opal matrices filled with carbon-containing compounds, followed by the removal of silicon dioxide from them, and silica particle sizes in the initial opal matrix on the formation of the porous structure were described in detail. The sucrose carbonization was carried out by annealing samples with a size of the particle SiO2 from 10 to 300 nm in an argon flow at the temperature range of 600-1500 °C. It is noted that the specific surface area increases with decreasing silica particle size and lowering the processing temperature. A natural explanation of this dependence is that the increase of the treatment temperature leads to sintering nanoparticles, an increase in the fraction of macro- and mesopores in the Nanotechnology VII, edited by Ion M. Tiginyanu, Proc. of SPIE Vol. 9519, 95190N © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2179795

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sample, and, as a consequence, a reduction of the specific surface area. The influence of the temperature is most essential for SiO2 particles with the size less than 100 nm. In the present work, the results of studies on the synthesis, the structure and properties of carbon inverted opal (C-IOP) nanostructures, the surface of which modified by oxide and sulfide of nickel are presented.

2. EXPERIMENTAL SECTION 2.1 The preparation C-IOP structure Carbon inverted opal (C-IOP) nanostructures were synthesized by the template method using an opal matrix composed of spherical globules of amorphous silicon dioxide 260 nm in diameter which was used as a template in accordance with the technique of preparation of opal matrices is detailed in work [15].For the incorporation of carbon into the matrix, we used aqueous solution of sucrose, C12H22O11, with addition of sulfuric acid, with which samples were impregnated. The samples were placed into aqueous solution (per 1 g of SiO2, 1.25 g of C12H22O11, 0.14 g of H2SO4, and 5 g of H2O) and held with the solution at a temperature of 100 °C for about 5 h and, then dried up at a temperature of 160 °C for 18 h. After that, the samples were impregnated with a solution (1 g SiO2 : 0.8 g C12H22O11 : 0.09 g H2SO4 : 5 g H2O) and the thermal treatment was repeated. Then, the samples were carbonized by annealing in an argon flow at 900 °C, 3 h. The carbon–silica composite obtained by pyrolysis was etched with 40 wt% hydrofluoric acid for 24-120 h at room temperature to remove the silica. The template-free carbon product was washed with distilled water and dried at 100 °C. Fig. 1 shows the C-IOP microstructure obtained by the above method.

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Figure 1. С-IOP structures (SEM): typical view of C-IOP at small magnification and empty carbon hemispheres upon etching of SiO2 globules.

2.2 The synthesis of C-IOP/NiO/Ni7S6 composite The synthesis of the C-IOP/NiO/Ni7S6 composite was performed by follow method. Samples C-IOP were placed into an aqueous solution of nickel sulfate NiSO4×6H2O with additives urea СO(NH2)2, stirred and heated at 100 ° C for 5 hours. Then the precipitated material was washed with deionized water and ethanol and dried at 80 ° C in air overnight, calcined in a flow of argon at 350-500 ° C for 2 hours. 2.3 Material characterization The products were characterized using X-ray powder diffraction (XRD, Siemens D-500, Cu-Kα1 radiation), scanning electron microscopy (SEM, Zeiss Supra 50 VP) and transmission electron microscopy (TEM, JEM-2100). N2 adsorptiondesorption was determined by Brunauer-Emmett-Teller (BET) [16] measurements using a Quantachrome QuadraWin. Electrochemical data were measured by the laboratory of prof. Jun Wang in Harbin Engineering University, China [17]. All of electrochemical tests were carried out on a CHI660D electrochemical workstation. The electrodes were prepared by weighing 8 mg material (85 wt. % as-prepared samples, 10 wt. % conducting carbon, and 5 wt. % polyfluortetraethylene (PTEE)) and all the material were pressed onto the nickel foam (10 × 10 × 1 mm). The active


substance on the electrodes was about 7 mg. The electrochemical measurements were performed in a 6 M KOH electrolyte at the room temperature.

3. RESULTS AND DISCUSSIONS The phase composition of synthesized carbon composites with nickel compounds with different weight content of Ni, having a different thermal treatment, investigated by means of X-ray diffraction (Table 1). Table 1. The phase composition of composites.

Sample no.

Content of Ni, wt. %

Thermal treatment

Phase composition

1

5

100 °С, 5 h

Ni2(CO3)(OH)2; NiCO3; NiO

2

5

350 °С, 2 h

NiO; an amorphous phase

3

5

500 °С, 2 h

NiO; an amorphous phase

4

5

750 °С, 2 h

NiC; NiO; Ni7S6; Ni3S2

5

20

500 °С, 2 h

NiO; Ni3S2

6

40

350 °С, 6 h

NiO

7

40

500 °С, 2 h

NiO; Ni7S6

After a low-temperature treatment (100 °C) of the carbon structure C-IOP in an aqueous solution of nickel sulfate NiSO4×6H2O with additives urea СO(NH2)2 the initial nickel sulfate transformed into hydrocarbonate, carbonate and oxide of nickel (broad lines). The annealing in an argon flow at 350-500 °C led to the formation of nanocrystalline nickel oxide NiO on the surface of carbon. At an increase of the nickel concentration in the composite, as well as the treatment temperature was observed along with nickel oxide the emergence of a significant phase of nickel sulfide Ni7S6. With further increase of the annealing temperature to 750 °C (sample no. 4 in the table) nickel begins to interact with carbon and nickel carbide is formed. Fig. 2 shows the X-ray diffraction of composites C-IOP/NiO (sample no. 3, 5 wt. % of Ni) and C-IOP/NiO/Ni7S6 (sample no. 7, 40 wt. % of Ni).

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Figure 2. XRD of C-IOP/NiO (5 wt. % of Ni, left) and C-IOP/NiO/Ni7S6 composite (40 wt. % of Ni, right): * – nickel oxide phase; + – nickel sulfide phase.

The investigation of samples by electron microscopy (SEM and TEM) are shown in Fig. 1 and 3. The image of the initial structure C-IOP (Fig. 1) obtained at a low magnification reveals that the sample consists of agglomerates of tens of microns size (Fig. 1a). The agglomerate surface is covered with a cellular structure. High magnification reveals the


ordered structure of the void hemispheres formed by etching of SiO2 globules (Fig. 1b). The carbon shell surrounding the empty spheres is 10-15 nm thick. The diameter of the spheres corresponds to that of the silica globules (~ 260 nm). The composite C-IOP/NiO/Ni7S6 (sample no. 7) (Fig. 3) is a periodic structure of the carbon C-IOP, the surface of which is covered with nickel compounds. These images allow assessment of the nickel compounds size: small particles are ~ 10 nm, larger particles are ~ 35 nm.

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The characteristics of the porous structure were determined by gas adsorption-desorption (N2, 77 K). The specific surface area of the carbon sample C-IOP was 847 m2/g., and a specific surface area of the composite containing 40 wt. % of Ni (sample no. 7) decreased by more than three times and was 250 m2/g. The fraction of micropores (up to 2 nm) was 48 %, that of mesopores (from 2 to 50 nm) 45 % and that of macropores 7 %. The main contribution (80 %) to the specific surface area of the samples is made by pores sized ~ 1 nm, ~ 2 nm, ~ 3.5 nm and 4.7 nm. To understand the macroscopic electrochemical reaction at the electrode surface of supercapacitors during the process of the charging and discharging, measurements of the cyclic voltammetry (CV) were made. Fig. 4 shows the CV-curves of the composite C-IOP/NiO/Ni7S6 in a potential range of -0.2 to 0.7 V at various scan rates in the range of 5-100 mV/s. The shape of the curves indicates that a significant contribution to the observed capacitance makes pseudocapacity, which is based on reversible redox reactions with the transition between the states Ni2+ and Ni3+.

- 10 mV/s - 20 mV/s - 25 mV/s

0.1

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Potential (V)

0.4

0.6

08

0

200

400

600

800

1000

1200

Time (sec)

Figure 4. Cyclic voltammogram at different scan rates (left) and galvanostatic charge-discharge at various current densities (right) of C-IOP/NiO/Ni7S6 composite (40 wt. % of Ni).

On CV-curves two pairs of peaks, presumably related to redox reactions in the nickel oxide and nickel sulfide can be seen. Thus, at a scan rate of 10 mV/s, more intensive pair of peaks is detected at a voltage of 0.22 V (an anodic peak) and 0.44 V (a cathodic peak), less intense couple is revealed at a voltage of 0.17 V (the anodic peak) and 0.39 V (the


cathodic peak). Naturally to assume that less intense pair of peaks is caused by the redox reaction in the nickel sulfide according to X-ray data (Fig. 2). It can be seen that currents of the oxidation peak increase with an increasing of the scan rate and the oxidation peak is shifted toward positive potentials. Meanwhile, the reduction peak potential shifts into the negative region, indicating the kinetic limitation in the electrochemical processes of the composite electrode. Figure 4 also presents the measurement of the specific capacitance of the C-IOP/NiO/Ni7S6 composite by the galvanostatic method. The calculation of the capacitance was performed according to the following equation [18, 19]: C=

It . ΔVm

(1)

Where I, t, ΔV, and m are the constant current (A), discharge time (s), total potential deviation (V), and mass of active materials (g), respectively. Fig. 4 shows that the discharge curves contain two stages: a rapid potential drop (0.45-0.3 V) and a slow potential decline of (0.3-0.2 V) for the current density of 5 mA/cm2. The first stage is due to the internal resistance of the electrolyte and the electrodes, the second portion is a pseudocapacitive feature of the electrode. The specific capacitance of the capacitor with the C-IOP/NiO/Ni7S6 composite (the concentration of Ni ~ 40 wt. %) as the electrodes calculated by the equation (1) was 603 F/g and 660 F/g for the current density during discharge at 5 mA/cm2 and 10 mA/cm2, respectively. The C-IOP material without modifying by the oxide and the sulfide of nickel possessed the specific capacitance of 130 F/g at a current density of 5 mA/cm2. It indicates that the faradaic reactions connected with nickel compounds (pseudocapacity) have a significant contribution in the specific capacitance of the capacitor. It should be noted that the values of the specific capacitance even have an appreciable potential for a growth, since the internal resistance of the investigated composite estimated by the impedance measurement was about 4 ohms, which is considerably larger than quantities published in [19].

4. CONCLUSIONS Carbon nanostructures with the inverse opal lattice modified by nickel compounds have been synthesized. The influence of heat treatment on the phase composition of the composites has been studied. It has been shown that increasing of the nickel concentration in the composite and the annealing temperature lead to the formation of nickel sulfide Ni7S6 along with nickel oxide phase. With further increase of the annealing temperature to 750 °C, nickel begins to react with carbon and nickel carbide is formed. Morphologically the C-IOP/NiO/Ni7S6 composite is the carbon periodic structure of C-IOP, the surface of which is coated with nickel compounds. The particles of nickel compounds are represented by two sizes: small particle size of about 10 nm and a large faceted particle size of about 35 nm. The pore surface area of the C-IOP amounted to 847 m2/g, and that of the composite containing 40 wt. % of Ni, decreased by more than three times and was 250 m2/g. The evaluation of electrochemical parameters of the C-IOP/NiO/Ni7S6 composite has demonstrated a significant contribution of the faradaic reactions in the specific capacitance of the capacitor. The specific capacitance of the capacitor with the C-IOP/NiO/Ni7S6 composite as an electrode in comparison with that of the C-IOP without modification by nickel compounds has increased more than 4 times, from 130 F/g to 600 F/g.

ACKNOWLEDGEMENTS This work was partly supported by the Russian Foundation for Basic Research (project no. 13-02-00777). We thank Professor Jun Wang and his Colleagues from Harbin Engineering University, 150001 People's Republic of China for assistance in electrochemical characterization of samples and helpful discussions.

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