Palani Raja Jothiab,
Rahul R. Salunkhe*a,
Malay Pramanika,
Shanthi Kannanb and
Yusuke Yamauchi*a
aWorld Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: SALUNKHE.Rahulraghunath@nims.go.jp; Yamauchi.Yusuke@nims.go.jp
bDepartment of Chemistry, Anna University, Chennai 600 025, India
First published on 12th February 2016
We report a simple soft-templating strategy for the synthesis of nanoporous crystalline nickel sulfide with two-dimensional (2-D) morphology. The nickel sulfide phases and morphologies are varied by changing the hydrothermal temperatures applied. Furthermore, the nanoporous nickel sulfide (PNS) flakes can be hybridized with reduced graphene oxide (rGO) sheets. As compared to bare PNS flakes, the PNS/rGO composite, containing 40% rGO, exhibits a superior electrochemical performance in terms of specific capacitance and cyclic stability. The specific capacitance of this composite is evaluated by a three-electrode system, and it shows the highest specific capacitance of 1312 F g−1 at a scan rate of 5 mV s−1. In addition, this composite is also assembled to form an asymmetric supercapacitor with zeolitic imidazolate framework (ZIF-8)-derived carbon as a negative electrode, which gives a highest specific capacitance of 47.85 F g−1 at 2 A g−1, a high energy density of 17.01 W h kg−1, and a high power density of 10 kW kg−1.
Nickel sulfide exists in various thermodynamically stable crystal structures and stoichiometric forms. Considering the very complex crystal structures, it is hard to prepare a single phase of nickel sulfide with a uniform morphology.7 There have been many research reports concerning the synthesis of nickel sulfide with different morphologies through various methods.8 Most of the reports have shown mixed crystal phases of nickel sulfide because the synthetic conditions are more sensitive to the reaction temperatures, pressures, and compositions. Some reports suggest possible control over the crystal phases of nickel sulfide by using different sulfur sources, temperatures, and reaction times; however, the experimental routes reported are not promising for large-scale synthesis.9
To date, numerous reports have been published on the synthesis of nickel sulfide for supercapacitor applications.10 For instance, Zhu et al. reported hierarchical nickel sulfide hollow spheres using an SiO2 template and showed the specific capacitance value of 927 F g−1 at a current density of 4.08 A g−1 with 70% cycling stability retention from its initial capacitance value.10a In a similar way, flower-like nickel sulfide was reported by Yang et al., and the electrodes showed 966 F g−1 at a current density of 0.5 A g−1.10b Pang et al. synthesized nickel sulfide nanocube electrodes having a large specific capacitance of 695 F g−1 at 1.25 A g−1 and excellent cycling performance maintaining 93.4% of initial specific capacitance after 3000 cycles.10c Yang et al. reported ion exchange synthesis of a mixed phase of nickel sulfide with reduced graphene oxides (rGO) and carbon nanotubes (CNT). The nickel sulfide/rGO obtained showed a highest specific capacitance value of 905.30 F g−1 at 0.5 A g−1 with good cyclic stability.10d Although graphene-based supercapacitors have been extensively investigated due to their high surface area and high conducting properties,11,12 re-stacking and residual defects of graphene during the synthesis steps sometimes hinder the significant properties of graphene. For practical electrochemical applications, the progressive reduction of oxygen-containing groups (rGO) leads to the re-stacking of graphene layers, which decreases the storage capacity over repeated cycles. For composite materials, graphene acts as a support to provide better conductivity, as well as better stability.11 The above-mentioned literature has mainly focused on the synthesis of various nickel sulfide structures. However, controlling the crystalline phases of nickel sulfide with nanoporous structures is still a challenge. This is a promising way to increase supercapacitor performance with nickel sulfide electrodes. Together with the interesting electrochemical properties of nickel sulfide, electrode stability also remains a big challenge.13
To overcome these issues, we propose that nanoporous nickel sulfide (PNS) flakes could be wrapped or covered with thin layers of rGO, which would increase electrode stability and the active surface area. To the best of our knowledge, such a controlled synthesis of PNS flakes with a single crystal phase has not yet been reported. We demonstrate a facile low temperature hydrothermal synthesis of PNS flakes using a soft-templating method. Crystalline phases, compositions, and shapes of the obtained nickel sulfide can be controlled simply by changing the reaction temperatures. The obtained nickel sulfide phase under optimized conditions is one of the most conducting phases of all nickel sulfide phases. Synthesized nanoporous nickel sulfide flakes and their composites with rGO show a much higher specific surface area as compared to those previously reported in the literature.
(1) |
The specific capacitance (C), specific energy (SE, W h kg−1), and specific power (SP, W kg−1) of the ASC cell were calculated from chronopotentiometric curves using eqn (2)–(4):
(2) |
(3) |
(4) |
Fig. 1 Wide-angle XRD patterns of (a) BNS, (b) PNS, (c) PNS/rGO20, (d) PNS/rGO40, and (e) PNS/rGO80 composites. |
TEM images of both PNS and PNS/rGO40 composites are shown in Fig. 2. The PNS that was prepared using P123 shows a well-defined flake morphology. The nanoporous structure is clearly visible inside the flakes, as indicated by the circles (Fig. 2b), although the nanopores are not well arranged. The average lateral sizes are 80–100 nm. The framework is highly crystallized, and the lattice fringes are clearly observed over the entire area. The observed lattice fringes show a distance of 0.23 nm, which corresponds to the (220) plane of the nickel sulfide millerite phase. For comparison, BNS prepared without surfactants was also checked using SEM and TEM. The nickel sulfide particles do not possess any porous nature, and the particle size is much larger (Fig. S3†). For PNS/rGO composites with low content of rGO sheets, the original flake-like morphology is basically confirmed; in some parts, the rGO sheets cover the nickel sulfide flakes (Fig. 3). When the loading amount of rGO is increased, the rGO sheets are completely encapsulated with nickel sulfide flakes. Introducing the nickel sulfide flakes as spacers prevents the restacking or aggregation of the rGO sheets. High-resolution TEM images of the PNS/rGO composites show two different types of lattice fringe values, as shown in Fig. 2d. Both the rGO sheets and nickel sulfide flakes are present in the composites. One lattice fringe with a d-spacing value of 0.38 nm corresponds to the (002) plane of rGO sheets, while the other lattice fringe with a d-spacing value of 0.23 nm can be indexed to the (220) plane of the trigonal nickel sulfide millerite phase. The surface coating and presence of rGO on the nickel sulfide flakes were further confirmed by elemental mapping (Fig. S4†).
The average pore sizes and the surface areas of the samples were measured using N2 adsorption–desorption isotherms (Fig. S5†). The PNS flakes and PNS/rGO composites show type IV isotherms with hysteresis loops in the range of 0.45–1.00. This behavior is due to the capillary condensation of N2 into the nanopores. PNS flakes prepared with surfactants show a high specific surface area (85.7 m2 g−1), as compared to BNS prepared without surfactants (18.2 m2 g−1) (Fig. S5c†). The isotherm of BNS shows a type III isotherm, which is typical for nonporous materials (Fig. S5a†).16 It was revealed that P123 serves as the template for forming the nanoporous structure. When the loading amount of rGO is increased to 40 wt%, the surface areas gradually increased. However, when the loading amount of rGO reaches 80 wt%, the surface area is decreased, due to the stacking of sheets. The isotherms for various composites of PNS/rGO are shown in Fig. S5b.† The surface area values are 101.7 m2 g−1, 146.8 m2 g−1, and 94.1 m2 g−1 for PNS/rGO20, PNS/rGO40, and PNS/rGO80, respectively. The surface areas obtained in this study are quietly higher than those of other related materials reported previously (Table S1†).17 Pore-size distribution of the PNS and the PNS/rGO composites calculated using the BJH method is very broad, in a range from 2 nm to 14 nm (Fig. S5d†). The pore-size distribution for all PNS/rGO composites is almost the same. These pores with broad size distributions are formed in the interlayer space. Actually, the N2 adsorption isotherm gradually increases without exhibiting any clear capillary condensation step, which has been observed for well-ordered mesoporous materials.18
The possible formation mechanism of PNS flakes is proposed. In the first step, Ni2+ ions dissolved in the aqueous solution are decorated with the hydrophilic poly(ethylene oxide) part of the P123. This is because coordinated water molecules near the Ni2+ center interact with the poly(ethylene oxide) group of P123 through hydrogen bonding.19 Then, S2− ions are introduced into the reaction mixture through a thiourea solution. The formed S2− ions react with the Ni2+ ions, and nickel sulfide nuclei are formed. The nickel sulfide framework develops during the aging stage, in which the P123 micelles (as porogens) are assembled.20 The surface charges of PNS flakes and rGO sheets play an important role in preparing for the hybridization of nickel sulfide with rGO. The zeta potentials of the PNS flakes and the rGO sheets were measured to be +19.79 mV and −22.09 mV, respectively (Fig. S6†). These positive and negative charges on the materials match with reports from previous literature.21 Therefore, positively charged nickel sulfide species can be effectively attached (or interacted) with the negatively charged rGO surface through electrostatic interaction. The zeta potential value of the PNS/rGO composite was +1.93 mV.
It is well known that crystal phases obtained are more sensitive to reaction temperatures, pressures, and reactant concentrations.22 In this study, the crystalline phases were found to be mainly dependent on the applied temperatures in hydrothermal conditions. The materials were prepared at four different temperatures (100 °C, 120 °C, 150 °C, and 180 °C). At 100 °C (i.e., typical condition), a single nickel sulfide phase with a 2D flake shape is obtained. The strong and sharp diffraction peaks confirm the high crystallinity of the obtained sample. When the synthetic temperature increased to above 100 °C (i.e., 120, 150, and 180 °C), the crystal phases are significantly changed to other phases, as shown in Fig. S2.† This is probably due to the following two reasons: (i) an uncontrolled release of S2− ions in the reaction mixture and (ii) an increased crystallization rate.23 The morphologies of the obtained materials are also greatly affected by the applied temperatures. When the temperature is increased from 100 °C to 180 °C, the final morphology is changed from flakes to spheres (Fig. S7†). As a result, a serious decrease in the specific area is confirmed. Consequently, the reaction temperatures in our case show a significant influence on the crystal phases and particle morphology.
Notably, the obtained trigonal millerite nickel sulfide phase has a highly conducting metallic character (∼2 × 104 ohms−1 cm−1 at room temperature), as compared to other nickel sulfide phases.24 This is because the Ni–S bond length in trigonal millerite nickel sulfide (2.25 Å) is relatively shorter than those of hexagonal nickel sulfide (2.39 Å) and nickel disulfide (NiS2) (2.40 Å).24 This metallic character delivers superior electrical conductivity, which enhances efficient electron/ion transport in the electrode.25 This feature is essentially needed for real energy storage and conversion devices. However, the use of this type of material for supercapacitors has not yet been fully explored. Therefore, in this study, the PNS flakes with a trigonal millerite nickel sulfide phase were prepared using a hydrothermal process at 100 °C; they were also hybridized with rGO sheets to prepare PNS/rGO composites.
The obtained CV curve is almost symmetric, signifying good reversibility of the redox processes. CV shows a pair of redox peaks due to the faradic reactions of NiII and NiIII, which are due to the following possible reaction mechanism:10e
NiS + OH− ↔ NiSOH + e−1 | (5) |
NiSOH + OH− ↔ NiSO + H2O + e−1 | (6) |
As seen in Fig. 4a–b, as the applied scan rate is increased, the current increases, and the oxidation peak shifts to the more positive potential side, while the reduction peak shifts to the more negative potential side. Thus, the PNS and PNS/rGO40 composite exhibit good capacitive behavior and stable operation over a potential range of 0.0 to 0.6 V. The specific capacitance of the PNS/rGO40 composite as a function of the scan rate is plotted in Fig. 4c. From the CV curves, the specific capacitance values are determined to be 1312, 772, 550, 433, 357, and 302 F g−1 at different scan rates of 5, 20, 40, 60, 80, and 100 mV s−1, respectively.
Additional advantages of these composites include stability in their CV shape due to the presence of rGO. The CV studies were carried out for 500-CV cycles at 20 mV s−1. Although the PNS sample also showed high capacitance value, capacitance retention after 500 cycles was 86%. On the other hand, the PNS/rGO40 composite showed good capacitance retention of 94% (Fig. 4d). This fact clearly highlights the advantage of PNS/rGO composites for retaining capacitance value over long cycle operation for practical applications. Furthermore, in order to identify the role of nanoporous structures on supercapacitor performance, we compared the specific capacitance of our PNS flakes with those from previous literature for various nickel sulfide nanostructures, which clearly confirms that our capacitance values are higher than those of previous reports (Table S2†).
To investigate the capacitive performance of PNS/rGO40 (showing a high performance in the three-electrode system mentioned above), an asymmetric supercapacitor (ASC) was fabricated using PNS/rGO40 as a positive electrode. In our previous studies, we demonstrated that ZIF-8-derived nanoporous carbon has a high specific surface area of ∼1500 m2 g−1 for supercapacitor applications.26 This nanoporous carbon (derived from ZIF-8) electrode has already shown its potential to substitute for activated carbon as a counter in supercapacitor applications. In the present study, ZIF-8-derived nanoporous carbon is used as a negative electrode. For the synthesis of ZIF-8 derived carbon and electrode fabrication, we followed the same process as in our previous reports.26b Fig. 5a shows the CV curves of ASCs in various higher potential window limits extending from 0.9 to 1.6 V at a scan rate of 20 mV s−1. The CV curves have a distorted rectangular shape with good reversibility, which may be due to the combined contributions of the electrodes. Thus, the maximum potential window of 0.0–1.6 V was chosen for the ASC investigation. When the upper potential exceeds 1.2 V, the presence of slight redox peaks indicates that the pseudocapacitive factor is coming from the sulfide reactions of the positive cell. The redox peak intensity increases with further increases in the potential window, similar to that seen in previous report.27 After 1.6 V, oxygen evaluation occurs.
Furthermore, the electrochemical performance of the asymmetric cell was tested by galvanostatic charge–discharge tests conducted at various current densities ranging from 2 A g−1 to 9 A g−1, which are shown in Fig. 5b. All of the discharge curves show a combined pseudo-EDLC character, indicating a well-balanced charge storage and electrochemical reversibility of the materials. Our asymmetric device shows a specific capacitance value of 47.85 F g−1 at 2 A g−1 and decreased to 37.76 F g−1 at 9 A g−1, showing good rate capability. Also, 78.91% specific capacitance retention is obtained when the current density values are increased from 2 A g−1 to 9 A g−1 (Fig. 5c).
Fig. 5d shows the energy and power density of our ASC cell, obtained from charge–discharge curves. The ASC cell shows a higher specific energy of 17.01 W h kg−1 at a specific power of 2285.36 W kg−1 at a current density of 2 A g−1. The specific power is increased to 10283 W kg−1 at a current density of 9 A g−1 while maintaining the specific energy of 13.42 W h kg−1. Furthermore, our ASC exhibits improved energy density at high power density as compared with those previously reported, as summarized in Table S3.† The high-rate performance and large-cell voltage of the aqueous electrolyte demonstrate the advantages of the PNS/rGO40 electrode for future supercapacitor applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26946f |
This journal is © The Royal Society of Chemistry 2016 |