One-step synthesis of Ni₃S₂ nanoplatelets on graphene for high performance supercapacitors

Abstract

Here, we present a one-step hydrogen reduction synthesis of Ni₃S₂ nanoplatelets on graphene surface by using NiSO₄·3N₂H₄/GO as precursor. In this process, we have demonstrated that hydrazine molecule, which can coordinate with NiSO₄ in the form of pink precipitation, not only contributes to the formation of Ni₃S₂ nanoplatelets structure, but also enhances the efficiency of SO₄²⁻ to S₂⁴⁻ conversion compared with NiSO₄/GO. Supercapacitors made from the obtained Ni₃S₂/rGO composite exhibits a specific capacitance of 912.2 F g⁻¹ at 2 mV s⁻¹ scanning rate, and 875.6 F g⁻¹ at galvanostatic discharge current density of 1 A g⁻¹, along with exceptional rate capability of 83.2% at discharge current density from 1 A g⁻¹ to 10 A g⁻¹ as well as good cycling stability. We attribute the excellent performance from the improved contact between graphene and the planar Ni₃S₂ structure, which strengthens the synergistic effect with graphene as conductive support and Ni₃S₂ nanoplatelets as the pseudocapacitive materials. This method allows the direct and efficient preparation of Ni₃S₂, and provides a simple route to integrate them with graphene for energy storage applications.

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Introduction

Transition metal oxides and chalcogenides have been intensively studied because of their broad applications such as energy storage and catalyst.^1–3 Among transition metal chalcogenides (TMCs), recently, Ni₃S₂ has attracted tremendous attention ascribed to their superior performance in energy storage. For example, it was reported that Ni₃S₂ nanorods prepared via hydrothermal method exhibits a specific capacitance of 1277 F g⁻¹ at 2 mV s⁻¹ scanning rate.⁴ Similar method is used to produce Ni_xCo_3−xS₄ hollow nanoprisms, which also perform enhanced pseudocapacitive behavior.⁵ However, those methods relies on wet chemistry through the sulfidation of Ni using H₂S decomposed from sulfur-containing molecules like thioacetamide (TAA) or thiourea that used in work mentioned above. Other methods like CVD method have also been reported, but the process involve many different steps thus are less efficient.⁶

On the other hand, due to the semiconducting property of TMCs, conductive materials like carbon materials are commonly used to combine with TMCs to improve their electron conductivity in order to fully exploit their pseudocapacitive capacity for energy storage application. However, to composite TMCs with carbon materials through wet chemistry is not efficient since transition metal nanoparticles should be integrated with the desired carbon materials firstly. Previously, we have developed a two-step method to prepare graphene supported droplet-shape structure of hollow Ni₃S₂/rGO and core–shell Ni@Ni₃S₂/rGO for supercapacitors application.⁷ Although enhanced pseudocapacitance has been achieved, the process is still not easy enough.

In this work, we have further developed a direct method to prepare Ni₃S₂ nanoplatelets through H₂ reduction of NiSO₄–hydrazine complex (NiSO₄·3N₂H₄). In addition, this method allows the convenient integration of Ni₃S₂ nanoplatelets with graphene through one-step H₂ reduction of NiSO₄·3N₂H₄/GO. Compared with wet chemistry through the sulfidation of transition metals adopted elsewhere, this presented method shows several obvious advantages. Firstly, Ni₃S₂ are reduced directly from NiSO₄·3N₂H₄, with no need to prepare nickel nanoparticles in advance as common methods. Secondly, Ni₃S₂ prepared from this method can be easily integrated with carbon materials like graphene avoiding the prior preparation of nickel/carbon material composites. Moreover, because of the interaction between Ni²⁺ and oxygen-containing groups on graphene oxide, the prepared Ni₃S₂ nanoplatelets have good contact with graphene, enabling better charge transfer between them. After made into electrodes and tested for supercapacitors, a specific capacitance of 912.2 F g⁻¹ at scanning rate of 2 mV s⁻¹ and 875.6 F g⁻¹ at 1 A g⁻¹ galvanostatic discharge current density are obtained for Ni₃S₂/rGO composite reduced from NiSO₄·3N₂H₄/GO. Compared with the two-step method we developed before,⁷ the main advantage of this presented method is the simple process but comparable performance.

Results and discussion

We have firstly investigated the hydrogen reduction of NiSO₄ and NiSO₄·3N₂H₄, respectively. Here, NiSO₄·3N₂H₄ is prepared according to previous reported method,⁸ and the corresponding XRD result is shown in Fig. S1.† During hydrogen reduction process, NiSO₄·7H₂O and NiSO₄·3N₂H₄ are separately loaded into a furnace under 200 sccm Ar and 20 sccm H₂ at 400 °C for 2 h. After hydrogen reduction, as shown in XRD results in Fig. S2,† peaks at 21.8° (101), 31.2° (110), 37.9° (003), 44.5° (202), 49.7° (211), 55.3° (300) of Ni₃S₂ (JCPDS no. 44-1418) are identified for both NiSO₄·7H₂O and NiSO₄·3N₂H₄, suggesting the formation of Ni₃S₂ and the successful conversion of SO₄²⁻ into S₂⁴⁻, and peaks at 51.9° (200), 76.5° (220) of Ni (JCPDS no. 04-0850) indicates partial reduction of Ni ions in NiSO₄ or Ni₃S₂ under H₂ environment. In a control experiment without introduction of H₂, after loading NiSO·7H₂O into furnace annealed under argon at 400 °C for 2 h, only peaks for NiSO₄ are observed (Fig. S3†), indicating that H₂ is a crucial reactant in this conversion. X-ray photoelectron spectroscopy (XPS) is then used to demonstrate the state change of sulphur element. From high resolution S2p XPS spectrum, after H₂ reduction of NiSO₄·3N₂H₄, peak at range of 167–172 eV corresponding to SO₄²⁻ has been almost vanished along with the appearance of a new peak for S₂⁴⁻ at range of 160.5–163.5 eV in Fig. 1,^9,10 indicating a complete conversion of SO₄²⁻ into S₂⁴⁻. While using NiSO₄·7H₂O as precursor, besides the new peak for S₂⁴⁻, high intensity SO₄²⁻ peak at range of 167–172 eV (Fig. S4†) is still observed. Compared with NiSO₄·7H₂O, it can be concluded that N₂H₄ indeed enhances the conversion of SO₄²⁻ to S₂⁴⁻ in NiSO₄·3N₂H₄. Previous methods to realize such conversion relies on the use of sulphate-reducing bacteria (SRB), a type of anaerobic microorganisms, for sulphate degradation,¹¹ however, hydrogen reduction of transition metal sulphate or transition metal sulphate–hydrazine complex has not been reported before. At current stage, the conversion mechanism of NiSO₄ and NiSO₄·3N₂H₄ into Ni₃S₂ is not clear. A possible explanation is that under H₂ environment at 400 °C, Ni²⁺ and SO₄²⁻ will be reduced to Ni and H₂S/SO₂, respectively, followed by reaction between them to form Ni₃S₂.^7,12,13 Since the precursor NiSO₄ is water-soluble, the method we have developed here makes the convenient integration of Ni₃S₂ with carbon materials possible.

Fig. 1 High-resolution S2p XPS spectrum of pure NiSO₄ (black line) and Ni₃S₂ reduced from NiSO₄·3N₂H₄ by H₂ (red line).

To integrate Ni₃S₂ with graphene for supercapacitors application, Fig. 2 displays the scheme for the preparation of Ni₃S₂/rGO from NiSO₄·3N₂H₄/GO. In detail, firstly, NiSO₄/GO (Fig. 2a) is prepared by simply mixing NiSO₄ solution with graphene oxide (GO) dispersion. After the observation of well mixing of NiSO₄ with GO, excess N₂H₄ solution is added dropwise. The key reason we use N₂H₄ here is that it can precipitate NiSO₄ in the form of NiSO₄·3N₂H₄. Ascribed to electrostatic interaction between Ni²⁺ and oxygen-containing group on GO, the formed NiSO₄·3N₂H₄ is supposed to sedimentate in situ and attach tightly onto graphene surface as shown in Fig. 2b. After that, through one-step H₂ reduction of NiSO₄·3N₂H₄/GO, Ni₃S₂/rGO are obtained and denoted as Ni₃S₂/rGO-2 (Fig. 2c), and the XPS result is shown in Fig. S5,† in which N element is detected due to the usage of N₂H₄. In another experiment, Ni₃S₂/rGO reduced directly from NiSO₄/GO by H₂ is denoted as Ni₃S₂/rGO-1. Fig. 3 shows the XRD results of Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2. Besides peaks for Ni₃S₂, graphene peak C(002) is also identified, confirming the successful integration of Ni₃S₂ with rGO. The lack of strong Ni₃S₂ diffraction peaks in sample Ni₃S₂/rGO-2 results from the well dispersion of Ni₃S₂ nanoplatelets on graphene surface and the small thickness of obtained Ni₃S₂ nanoplatelets, as confirmed by TEM results (shown later in Fig. 4). Fig. S6† shows the Raman spectrum of GO in comparison with reduced graphene oxide in Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2. The increase of I_D/I_G ratio from 0.733 (GO) to 1.051 for Ni₃S₂/rGO-1 and 1.066 for Ni₃S₂/rGO-2 indicates the reduction of GO.

Fig. 2 Schematic illustration for the synthesis of Ni₃S₂/rGO. (a) Graphene oxide (GO) and Ni²⁺ aqueous solution. (b) NiSO₄ precipitates in the form of NiSO₄·3N₂H₄ on GO after the introduction of hydrazine. (c) Ni₃S₂ nanoplatelets formed in situ on graphene surface after H₂ reduction, accompanied with the reduction of GO into rGO.

Fig. 3 XRD spectrum of Ni₃S₂/rGO-1 reduced by H₂ from NiSO₄/GO and Ni₃S₂/rGO-2 reduced from NiSO₄·3N₂H₄/GO, respectively.

Fig. 4 TEM results of Ni₃S₂/rGO reduced from NiSO₄·3N₂H₄/GO. (a) Ni₃S₂ nanoplatelets with uniform size distributed on graphene surface. (b) Particle size analysis histogram of Ni₃S₂ nanoplatelets (mean: 29.6 nm, standard deviation: 9.4 nm). (c) and (d) High resolution TEM of Ni₃S₂ nanoplatelets on graphene surface.

Morphology of Ni₃S₂/rGO is characterized by transmission electron microscopy (TEM), and shown in Fig. 4, for Ni₃S₂/rGO-2 (i.e. reduced from NiSO₄·3N₂H₄/GO). We can see small nanoplatelets with uniform size are distributed on the graphene surface without obvious aggregation. In contrast, TEM images of Ni₃S₂/rGO-1 (i.e. reduced from NiSO₄/GO) show round and irregular nanoparticles with broad diameter distribution, as depicted in Fig. S7.† The morphology difference observed should be resulted from the different precipitation process. In NiSO₄/GO, NiSO₄ deposits from solution and randomly anchors on the graphene surface during the drying-recrystallization process. On the contrary, for NiSO₄·3N₂H₄/GO, with the aid of N₂H₄, NiSO₄ is precipitated in the form of NiSO₄·3N₂H₄ and tightly fixed on the graphene surface due to the interaction between Ni²⁺ and oxygen-containing groups, similar to previous work for the preparation of NiO/Ni(OH)₂ nanosheets/nanoplates.^14,15 During the following H₂ reduction of NiSO₄·3N₂H₄/GO, Ni₃S₂ will be prepared in situ to form Ni₃S₂/rGO, while Ni₃S₂ is more likely to aggregate into different morphology and size in Ni₃S₂/rGO-1 (i.e. reduced from NiSO₄/GO). From the particle size analysis histogram in Fig. 4b, the average size of these Ni₃S₂ nanoplatelets is about 29.6 nm for Ni₃S₂/rGO-2. High resolution TEM image in Fig. 4c shows the crystal structure, and plane spacing of 2.84 Å corresponds to Ni₃S₂ (300). More importantly, these Ni₃S₂ nanoplatelets are in good contact with graphene as pointed out by white arrow in Fig. 4c, in agreement with our previous analysis. In addition, Ni domains from partial reduction of Ni ions in NiSO₄/Ni₃S₂ can be detected in some particles as illustrated in Fig. 4d, consistent with XRD results stated before.

To demonstrate their potential application in energy storage, we further use electrochemical methods to investigate capacitive property of Ni₃S₂/rGO-1 (i.e. reduced from NiSO₄/GO) and Ni₃S₂/rGO-2 (i.e. reduced from NiSO₄·3N₂H₄/GO), respectively. The electrochemical tests of those synthesized materials are carried out in a three-electrode setup, where Pt works as the counter electrode, Hg/HgO as the reference electrode and 2 M KOH as the electrolyte solution. Electrochemical results are shown in Fig. 5, in which cyclic voltammetry (CV) test is conducted between 0.1 V and 0.6 V, and galvanostatic charge/discharge test range is from 0.1 V to 0.55 V. Fig. 5a shows the CV curves for Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2 at scanning rate of 2 mV s⁻¹, and both of these two composites show a pair of reversible redox peaks at the range between 0.25 V and 0.5 V, demonstrating their pseudocapacitive properties. And a specific capacitance of 360 F g⁻¹ for Ni₃S₂/rGO-1 and 912.2 F g⁻¹ for Ni₃S₂/rGO-2 can be calculated from these CV curves (for detailed calculation see the ESI†). CV results at different scanning rate from 2 mV s⁻¹ to 50 mV s⁻¹ are presented in Fig. S8a and c† for Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2, respectively. Fig. 5b displays the galvanostatic discharge curves at 1 A g⁻¹, and a specific capacitance of 140.0 F g⁻¹ for Ni₃S₂/rGO-1 and 875.6 F g⁻¹ for Ni₃S₂/rGO-2 are obtained (see the ESI† for calculation method). The significantly increased performance for Ni₃S₂/rGO-2 is possibly due to several reasons. Firstly, with the aid of N₂H₄, the conversion of SO₄²⁻ into S₂⁴⁻ is more complete, as confirmed with XPS result. Thus after H₂ reduction, NiSO₄·3N₂H₄/GO leads to higher ratio of Ni₃S₂ active materials than that of NiSO₄/GO. Secondly, the Ni₃S₂ nanoplatelets in Ni₃S₂/rGO-2 have better contact with graphene substrate than round/irregular shape Ni₃S₂ nanoparticles with non-uniform size randomly deposited on graphene in Ni₃S₂/rGO-1. Thirdly, due to its highly reductive property, hydrazine can help reduce GO into rGO, contributing to improved electronic conductivity. Ni₃S₂/rGO-2 also delivers better rate capability than Ni₃S₂/rGO-1 as plotted in Fig. 5c. By increasing galvanostatic charge/discharge current density from 1 A g⁻¹ to 10 A g⁻¹, the calculated specific capacitance for Ni₃S₂/rGO-2 varies from 875.6 F g⁻¹ to 728.5 F g⁻¹, equal to 83.2% capacitance is retained. While for Ni₃S₂/rGO-1, specific capacitance decreases rapidly from 140 F g⁻¹ to 42.2 F g⁻¹, only 30.1% capacitance maintained. Fig. S8b and d† present the galvanostatic discharge curves for these two composites at different current density from 1 A g⁻¹ to 10 A g⁻¹. The CV and galvanostatic discharge curves for pure Ni₃S₂-2 reduced from NiSO₄·3N₂H₄ and pure rGO are presented in Fig. S9,† and a capacitance value of 333.8 F g⁻¹ for Ni₃S₂-2 and 78.2 F g⁻¹ for rGO are obtained at 1 A g⁻¹ discharge current density. Taking into account the rGO weight percentage of 55.6%, the faradaic capacitance is 832.1 F g⁻¹ for Ni₃S₂/rGO-2 at 1 A g⁻¹ current density. Electrochemical impedance spectroscopy (EIS) is also employed to measure the ion diffusion and charge transfer process at the frequency from 100 kHz to 10 MHz with amplitude of 5 mV, and results are shown in Nyquist plots in Fig. 5d, with inset equivalent circuit, where R_s is the ohmic resistance accounting for the intrinsic resistance of electrode, contact resistance between current collector and electrode materials as well as the electrolyte resistance, C_DL is the double layer capacitance, R_CT is the charge transfer resistance, W is the Warburg diffusion element and C_F is the faradaic capacitance.^7,16 The Nyquist plots fit well with the equivalent circuit by the equation:¹⁶

Fig. 5 Electrochemical results of Ni₃S₂/rGO. (a) CV at scanning rate of 2 mV s⁻¹ for Ni₃S₂/rGO reduced from NiSO₄/GO (Ni₃S₂/rGO-1) and NiSO₄·3N₂H₄/GO (Ni₃S₂/rGO-2) respectively. (b) Corresponding galvanostatic discharge results at 1 A g⁻¹ for Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2. (c) Rate capability comparison of Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2 at discharge current density from 1 A g⁻¹ to 10 A g⁻¹. (d) Nyquist plots at amplitude of 5 mV over frequency range from 100 kHz to 0.01 Hz. Inset are the equivalent circuit and high frequency part zoomed in.

The lower R_s of 0.53 Ω for Ni₃S₂/rGO-2 than Ni₃S₂/rGO-1 (0.78 Ω) should be mainly resulted from its better conductivity due to the introduction of N₂H₄. The R_CT for Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2 are 1.23 Ω and 1.01 Ω, respectively. Smaller R_CT value for Ni₃S₂/rGO-2 indicates better charge transfer, attributed to the good contact of Ni₃S₂ nanoplatelets with graphene. At last, we have conducted the cycling performance test for Ni₃S₂/rGO-2 (i.e. reduced from NiSO₄·3N₂H₄/GO) at current density of 5 A g⁻¹, as shown in Fig. S10.† After 1000 cycles at this current density, 91.3% capacitance is maintained, demonstrating its good cycling stability.

Experimental

Chemicals

Graphite (Grafguard 160-50N with the average particle size of 350 um) was expanded by microwave before use. K₂S₂O₈, P₂O₅, H₂SO₄, KMnO₄, H₂O₂, NiSO₄·7H₂O, N₂H₄ (35 wt% in water) and poly(vinylidene fluoride) (PVDF) were all purchased from Sigma-Aldrich with analytical grade and used without further purification.

Preparation of graphene oxide (GO)

Graphene oxide (GO) is prepared through modified Hummers method.^17,18 In brief, firstly, 30 mL concentrated sulfuric acid is added into a flask containing 1 g graphite, 5 g K₂S₂O₈ and 5 g P₂O₅, reacted at 90 °C for 4.5 h. Then filtrate the product through a 0.2 micron Nylon Millipore filter and wash the sample with excess DI until the pH is around 5.5. Secondly, after drying the sample at 60 °C, 150 mL concentrated sulfuric acid and 30 g KMnO₄ are added in sequential slowly and reacted at 35 °C for another 4 h. Thirdly, the mixture product is added to 1 L DI water and stirred for 2 h, followed by the addition of 50 mL 30% H₂O₂ until the solution turns into bright yellow. After stirred for another 2 h, settle down the solution for 24 h. Finally, discard the supernatant and use 1 L of 10% HCl solution and excess DI to wash the product until the pH ∼ 7.

Preparation of NiSO₄·3N₂H₄ complex

To 2 mL 0.1 M NiSO₄ solution, 1 mL hydrazine solution (35 wt% in H₂O) is added and stirred for 10 min. After filtration and drying the pink precipitate, NiSO₄·3N₂H₄ is obtained.

Preparation of NiSO₄/GO and NiSO₄·3N₂H₄/GO composites

To 20 mL 1 mg mL⁻¹ GO solution, 2 mL 0.1 M NiSO₄ is added slowly under vigorous stirring, and then stirred for another 1 h. After freeze-drying the mixture solution, NiSO₄/GO composite is obtained. To prepare NiSO₄·3N₂H₄/GO composite, 1 mL hydrazine solution (35 wt% in H₂O) is added into the above NiSO₄/GO solution. After filtration and drying the product, NiSO₄·3N₂H₄/GO composite is obtained.

Preparation of Ni₃S₂/rGO composites

Load the prepared NiSO₄/GO and NiSO₄·3N₂H₄/GO composite into a furnace under 200 sccm Ar and 20 sccm H₂ at 400 °C for 2 h, respectively. After cooling down to room temperature, the products are collected. rGO and Ni₃S₂ are prepared separately through a procedure similar to the preparation of Ni₃S₂/rGO, apart from the absence of NiSO₄ and GO respectively. The mass ratio of Ni₃S₂ in Ni₃S₂/rGO-1 and Ni₃S₂/rGO-2 are 63.1% and 55.6%, respectively.

Crystal structure and morphology characterization

All the characterization tests are done in materials characterization and preparation facility (MCPF) in Hong Kong University of Science and Technology (HKUST). XRD (PW 1830, Philips) with Cu Kα radiation (λ = 1.5406 Å) is used to test the phase structure. The state change of sulphur is checked with XPS (VG Micro Tech ESCA 2000). And transmission electron microscope (TEM, JEOL 2010) is used to characterize the morphology and crystal structure.

Electrode preparation and electrochemical measurement

90 wt% electrode materials and 10 wt% PVDF as the binder are put into an agate mortar to make slurry. Then coat the slurry onto Ni foam with area of 1 cm² uniformly. After drying at 80 °C for 12 h under vacuum environment, the prepared electrodes are pressed tightly under 10 MPa. Three-electrode system is employed here to measure the electrochemical properties of the electrodes. In this system, the prepared electrode is used as the working electrode, platinum plate as counter electrode and Hg/HgO as reference electrode. The electrolyte used here is 2 M KOH. Before the electrochemical test, the electrodes are dipped into the electrolyte for 6 h. The electrochemical test is carried out on an electrochemical workstation (CHI 660). Cyclic voltammetry test and galvanostatic charge/discharge test are conducted at the range from 0.1 V to 0.6 V and 0.1 V to 0.55 V, respectively. The electrochemical impedance spectroscopy (EIS) is done at the frequency range from 100 kHz to 0.01 Hz under amplitude of 5 mV.

Conclusions

In summary, we have synthesized Ni₃S₂ directly from NiSO₄–hydrazine complex (NiSO₄·3N₂H₄) through H₂ reduction, and integrated these Ni₃S₂ nanoplatelets with graphene for supercapacitors application. As a result, Ni₃S₂/rGO reduced from NiSO₄·3N₂H₄/GO shows a specific capacitance of 912.2 F g⁻¹ at scanning rate of 2 mV s⁻¹ and 875.6 F g⁻¹ at 1 A g⁻¹ discharge current density. We believe that hydrazine molecule introduced in NiSO₄·3N₂H₄ contributes to the enhancement of conversion of SO₄²⁻ into S₂⁴⁻, and also allows the formation of Ni₃S₂ nanoplatelets, ensuring better contact with the underlying graphene. The method presented here provides a new simple route for the preparation of Ni₃S₂ as well as Ni₃S₂/rGO composite for energy storage.

Acknowledgements

This project is supported by the Research Grant Council of Hong Kong SAR (Project number 623512 and DAG12EG05). Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The calculation method for specific capacitance, XRD, XPS, Raman, TEM and electrochemical test results. See DOI: 10.1039/c5ra22426h

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