Investigation on Carbonsphere@Nickel Cobalt Sulfide Core-shell Nanocomposite for Asymmetric Supercapacitor Application

Introduction

Consumer electronics and modern electronic gadgets have become the order of the day for every common man. Cellular phone, laptop, can-camera, and house-hold electronic goods are having tough competition in the consumer market. Each passing day, with new options and features, the market is flooded with newer devices and gadgets. These modern-day gadgets rely heavily on energy storage and delivery parameters. In this context, supercapacitors have revolutionized and in certain cases replaced the batteries for some electrochemical applications due to their good power density and quick charging/deliverance of power (watts = voltage times the current) so enormous interest and extensive attention has been given to supercapacitors by the researchers for the past two decades (Zhai et al. 2011; Jiang et al. 2012; Wang, Zhang, and Zhang 2012; Wu and Zhang 2017).

A variety of electrodes are investigated based on carbonaceous (Manaf, Bistamam, and Azam 2013; Hao, Li, and Zhi 2013), transition metal oxides (Snook, Kao, and Best 2011; Nguyen et al. 2015; Wei et al. 2018c), conducting polymers (Lokhande, Dubal, and Joo 2011; Lamiel, Kumar, and Shim 2017) and so on, however they have their drawbacks, i. e. carbon materials have lower specific capacitance, low electrical conductivity for transition metal oxides and very poor cyclability of conducting polymer (Yuan et al. 2014; Chen and Dai 2013; Shown et al. 2014; Wang et al. 2016). On the other hand, the transition metal sulfides exhibit good reversibility, cyclability and high theoretical capacitance (Pu et al. 2014; Lin, Tai, and Chou 2014; Yang, Chen, and Chang 2011; Chou and Lin 2013; Zhu et al. 2011; Hou et al. 2011; Wei et al. 2018a; Cheng et al. 2017). Amongst sulfides, the ternary metal sulfides (NiCo₂S₄) have attracted a lot of attention as electrodes for supercapacitors owing to their excellent electrochemical performance (Yang et al. 2016; Nguyen, Lamiel, and Shim 2015; Chen et al. 2013; Xiao et al. 2014; Hu et al. 2009; Zou et al. 2016). Albeit this appealing performance, ternary metal sulfides have shown low rate performance at higher current density and lower cycling stability (Beka, Li, and Liu 2017).

The researchers have carefully attempted to avoid the inherent low rate performance of NiCo₂S₄ by increasing the active surface area by synthesizing it in nano core-shell structures. The core-shell structure of NiCo₂S₄ has improved the low rate performance and cycling stability but is not stable (Beka, Li, and Liu 2017). So in order to improve stability on the rate performance and cycling stability NiCo₂S₄ is hybridized with carbon material (Lamiel et al. 2016; Justin, Vickraman, and Reddy 2018). The researchers found that Carbon in spherical symmetry (CS) with NiCo₂S₄ provides high surface area, excellent conductivity, and good mechanical as well as chemical stability for supercapacitors (Justin, Vickraman, and Reddy 2018). Based on the review of CS as a core templating with NiCo₂S₄ as a shell for the successful accomplishment of electrochemical performance the present study is focused on CS@NiCo₂S₄ core-shell composite. The synthesis and performance study of CS@NiCo₂S₄ had been carried out by Lamiel et al. via hydrothermal method and reported the specific capacitance of 724 F g⁻¹ at 2 A g⁻¹ with 86.1 % capacity retention for 2000 charge/discharge cycles using nickel foam as a substrate (Lamiel et al. 2016). But in this work, the investigation is based on synthesis of CS@NiCo₂S₄ via low temperature water-bath method and the performance will be discussed on the synthesizing route (Justin, Vickraman, and Reddy 2018).

Experimental

Result and Discussion

The XRD studies on as-prepared pristine CS, pristine NiCo₂S₄ and the ternary composite CS@ NiCo₂S₄ are carried out (Figure 1). In Figure 1(a), peak at 2θ = 20 ° corresponds to carbon sphere which confirms the carbonization of the glucose (Justin, Vickraman, and Reddy 2018). The Figure 1(b) shows the characteristics peaks of the pristine nano NiCo₂S₄ (scn0) appeared at 20 ° (111), 33.7 ° (220) and 60 ° (422) which match with JCPDS- 20–0782 of cubic structure. Even though the cobalt stoichiometry molar ratio is high in NiCo₂S₄, there is no trace of it in the composites. This may be due to the cobalt occupation in the nickel sites (possibly because of same ionic sizes) are not altered the structure (Mi et al. 2015). The Figure 1(c) shows the XRD pattern of CS@NiCo₂S₄ composite (scn1). The peak at 2θ = 20 ° in both CS and NiCo₂S₄ are merged and appeared as broadened hump, and no changes are observed in the peaks of scn0 at 33.7 ° and 60 ° in scn1. Further, increasing the amount of nano CS, the peak at 20 ° is highly broadened in scn2, scn3 and scn4 (Figure 1(d–f)), and also no change of position of other peaks are noted. Thus XRD studies has revealed that the nano composites with CS are in amorphousness which in turn has facilitated for improving the electrode/electrolyte interface causing better electrochemical performance (Beka, Li, and Liu 2017; Lamiel et al. 2016; Justin, Vickraman, and Reddy 2018).

Figure 1:

XRD diffractogram of (a) CS, (b) scn0, (c) scn1, (d) scn 2, (e) scn 3 and (f) scn 4.

FTIR spectra of the as-prepared materials are recorded to understand complexation of various constituents (Figure 2). The spectra show the vibration bands at 493 and 617 cm⁻¹ respectively represent the metal (Ni-Co) and sulfide (S) stretching’s (Zheng et al. 2018). This confirmed the molecular complexation of NiCo₂S₄. The vibration peak at 1024 cm⁻¹ indicates the C-O stretching mode. The stretching vibrational modes at 1698 cm⁻¹ and 1551 cm⁻¹ correspond to O-H group. The peak at 1375 cm⁻¹ confirms the presence of CO₃⁻² ions (Wu et al. 2004). While increasing the concentration of CS, the peak at 617 cm⁻¹ has been broadened which suggests the complexation of CS and NiCo₂S₄.

Figure 2:

FTIR spectra of (a) CS, (b) scn0, (c) scn1, (d) scn 2, (e) scn 3 and (f) scn 4.

The specific surface area and pore size distribution of pure NiCo₂S₄ and CS@NiCo₂S₄ are observed through N₂ adsorption/desorption isotherms, as shown in Figure 3. The type-IV H3 hysteresis loop is attributed to both pure (Figure 3(a)) and composite (Figure 3(b)) which are confirming the mesoporous structure (Thommes et al. 2015). The high surface area of 53.5 m² g⁻¹ is observed for scn2 compared to 42.8 m² g⁻¹ of scn0. The inset fig. gives the pore calculation of scn2 and scn0 using Barret-Joyner-Halenda (BJH) method. The pore size of mesoporous structure of scn2 is 7.15 nm whereas for scn0 it is 2.78 nm. The higher pore size seems to be good enough for electrolyte penetration unto act as a reservoir (Mohamed, Attia, and Hassan 2017) for creating sufficient path ways for ion transport and resulting in enhanced of the pseudocapacitive performance of scn2 (Justin, Vickraman, and Reddy 2018) in the present study.

Figure 3:

XPS analysis of nco3 nanocomposite sphere (a) Ni 2 p, (b) Co 2 p, (c) S 2 p and (d) C 1s.

The electronic state and elemental composition of CS@ NiCo₂S₄ core-shell are examined by XPS. The Figure 4(a) shows the Ni 2 p photoelectron spectrum. The doublet peaks of Ni 2p_3/2 and Ni 2p_1/2 are noted at 860 eV and 879 eV respectively. The peaks at 853 and 870 eV represent the Ni²⁺ and 855 and 873 eV are attributed to Ni³⁺. These intense satellite peaks indicate that the main valence state of nickel is at Ni²⁺ (Wen et al. 2017). The Figure 4(b) shows two peaks which are ascribed to Co 2p_3/2 and Co 2p_1/2 having two satellite peaks at 783 eV and 802 eV. These indicate that the most of Co ion are present in the Co³⁺ state (Chen et al. 2017). The deconvoluted peaks at 778 eV and 793 eV correspond to Co³⁺ (Chen et al. 2017) and peaks at 780 eV and 796 eV are assigned to Co²⁺ (Chen et al. 2017). In Figure 4(c), the S 2 p spectrum is fitted with one main peak and one satellite peak. The peak at 161 eV corresponds to S 2p_3/2 and at 162.5 eV assigned to metal – sulfur bond which corresponds to S 2p_1/2 reveals the existence of sulfur ions in low coordination onto the surface. The peak at 169 eV is attributed to the surface-adsorbed oxidized sulfur species (SO₄^2– and HSO^4–) (Khani and Wipf 2017). The Figure 4(d) shows C 1s spectrum wherein the peak at 284.6 eV corresponds to sp2 hybridized carbon and the peak at 285.6 eV is ascribed to C-heteroatom bond and a peak at 288 eV is assigned to O-C = O group (Meng et al. 2017).

Figure 4:

BET surface and pore size studies of (a) scn0 and (b) scn 2.

The TEM morphology of as-prepared pristine nano CS, nano NiCo₂S₄ and CS@NiCo₂S₄ are shown in Figure 5. In Figure 5(a), CS appears as a perfect shape of spheres in a bunch with smooth surface in different sizes. The Figure 5(b) image of pristine nano NiCo₂S₄ appears as a scattered nano flake with sponge like surface but CS@NiCo₂S₄ (scn2) nanocomposite core-shell (Figure 5(c)) looks like an interlinked bunch of grapes. The composite surface morphology clearly confirms the core-shell formation. The cause for the formation is due to the presence of carboxyl and hydroxyl groups (Justin, Vickraman, and Reddy 2018) of carbon sphere’s strong orientation with the metal sulfides which might be the reason for the formation of the stable core-shell. In other words, this stable core-shell configuration favors to entrap the electrolyte into the bulk of the electrode material and improving the electrode/electrolyte interaction which inturn leads to causing for better electrochemical performance (Lamiel et al. 2016). The Figure 5(d) shows existence of scn2 core-shell in the amorphous phase. Figure 5(e) represents the EDAX spectrum of core-shell nano composite.

Figure 5:

TEM morphological studies of (a) CS, (b) scn0 (c) scn2 and (d) SAED pattern of scn2 (e) EDS mapping of scn2.

The electrochemical studies of nano NiCo₂S₄ and nano core-shell composites CS@NiCo₂S₄ carried out in three electrode mode in 3 M KOH electrolyte. The cyclic voltametry (CV) study is carried at different scan rates (2 mVs⁻¹–100 mVs⁻¹) in potential window −0.1 V – 0.5 V. Irrespective of varying load of CS, a pair of redox process is noted in all electrode materials (Figure 6(a–e)), corresponding to the redox process of Co²⁺ /Co³⁺ /Co⁴⁺ and Ni²⁺ /Ni³⁺ (Peng et al. 2016).

Figure 6:

Cyclic Voltagram of (a) scn0, (b) scn1, (c) scn2, (d) scn3, (e) scn4 and (f) Comparison of CV curve at 2 mVs⁻¹.

It is observed that the shifting of oxidation peak at higher potential and as that of reduction peak at lower potential is noted by increasing the scan rate. This is because the diffusion rate of electrolyte is not sufficient for fulfilling the electrochemical reaction of active material (Peng et al. 2016; Chen et al. 2017; Wei et al. 2018b; Zhi et al. 2013). The Figure 6(f) presents the CV curves of all CS@NiCo₂S₄ electrode materials in scan rate of 2 mVs⁻¹, and it is found that the scn2 core-shell nanocomposite has larger area under CV curve which has provided high electrochemical performance.

The galvanostatic charge/discharge studies are carried out for both pristine NiCo₂S₄ as well as all CS@NiCo₂S₄ composite in −0.2 V – 0.4 V potential difference for different current densities of 1, 2, 5, 10 and 20 A g⁻¹ (Figure 7). The discharge curves clearly indicate that pesudocapacitance behavior of them. In other words, the two segment of discharge curve are observed i.e, internal resistance and change in energy within the capacitor (Xiao et al. 2014). The specific capacitances are calculated using the equation and is found to be 425, 365, 293, 207 and 147 F g⁻¹ for scn0 (Figure 7(a)); 483, 395, 315, 218 and 170 F g⁻¹ for scn1 (Figure 7(b)); 838, 783, 608, 416 and 330 F g⁻¹ for scn2 (Figure 7(c)); 541, 484, 419, 317 and 240 F g⁻¹ for scn3 (Figure 7(d)); and for scn4 (Figure 7(e)), 506, 456, 368, 287 and 213 F g⁻¹ at current densities mentioned earlier. The comparisons of calculated specific capacitance of all the electrode materials are depicted in Figure 7(f). Amongst the electrodes, scn2 has showed higher specific capacitance and rate capability than other electrodes, i. e. TEM morphology core-shell mesoporous structure favors for acting as a electrolyte reservoir not only the surface but also on the bulk of the material (Beka, Li, and Liu 2017; Cherusseri and Kar 2015). This internal reservoir is bringing the internal active surface layers directly in contact with electrolyte, facilitating for the enhancement of diffusivity rate of ion for fast ion transportation leading to better redox reactions to take place (Wang et al. 2012). The concentration dependency of CS is reflecting on the deterioration of the electrochemical performance is noted i. e. increasing thickness raises the inability of ion transfer in the electrode/electrolyte contact surface and as a result higher internal resistance and shorter charge/discharge cycle (Nguyen, Lamiel, and Shim 2015).

Figure 7:

Galvanostatic charge/discharge studies of (a) scn0, (b) scn1, (c) scn2, (d) scn3, (e) scn4 and (f) Specific capacitance of GCD curve at different current densities.

The electrochemical impedance studies (EIS) of as-prepared materials are carried out for understanding the favorable kinetics and rapid ion transport in the frequency range 0.01 Hz – 1 MHz. The impedance spectra are fitted with an equivalent series circuit which contain the bulk solution resistance, (R_s) charge−transfer resistance (R_ct), the double layer capacitance (C_dl), the pseudocapacitance (C_ps), and the Warburg impedance (W) (inset in Figure 8(a)). The analysis of EIS is related to equivalent series resistance (R_ESR), semi−circle at high frequency and slope at low frequency. The low R_ESR is observed for the core-shell scn2 (Table 1).The semi-circle in the high frequency region corresponds to low charge transfer resistance (Table 1) of core-shell nanocomposites CS@NiCo₂S₄ (scn2) (Figure 8(a)) for enhancing the ion transport (Sun et al. 2017). In other words CS increment at NiCo₂S₄ the scn2 core-shell has showed the better diffusion is observed.

Figure 8:

(a) Electrochemical impedance spectrum of the electrodes with fitting circuit and (b) Capacity retention and Coulombic efficiency of scn2.

The cycling stability of scn2 core-shell composite has been carried out in a constant current density 20 A g⁻¹ for 5000 charge/discharge cycles (Figure 8(b)). It is noted that the good capacity retention of 89 % and 95 % coulombic efficiency for 5000 cycles (equation). The core-shell structure showed the good cycling performance. The main attributing factor of core-shell composition is that the core template CS is fully covered by the nano flake NiCo₂S₄ shell, which directly interacts with the electrolyte improving the mechanical stability and good electrical conductivity (Beka, Li, and Liu 2017).

The electrochemical performance of as-prepared rGO has been examined in three electrode system for device fabrication (Ag/AgCl as a reference electrode; platinum wire as a counter electrode and rGO as a working electrode with the aqueous 3 M KOH as a electrolyte). The Figure 9(a) shows the CV performance of rGO in a rectangular shape which confirms the behavior of it as a double layer capacitor. The ramp shapes (Figure 9(b)) of the GCD curves have also confirmed the double layer capacitative behavior. The specific capacitance is calculated (eq. (1)) and is given as 370 F g⁻¹ at 3 A g⁻¹.

Figure 9:

Electrochemical performance of rGO (a). CV curve (b). GCD curve.

To examine the practical application of the core-shell nanocomposite CS@NiCo₂S₄ (scn2) the asymmetric supercapacitor (ASC) cell (CS@NiCo₂S₄//rGO) is fabricated wherein scn2 as a positive electrode, reduced graphene oxide (rGO- laboratory prepared) as a negative electrode and 6 M KOH as a electrolyte were used. The Figure 10 shows the electrochemical performance of the asymmetric capacitor. The Figure 10(a) depicts the CV curves of ASC over the potential window 0–1.4 V at the scan rate from 10 mVs⁻¹ – 200 mVs⁻¹. This is exhibiting redox peaks at all scan rates indicating the faradic process. The Figure 10(b) shows the GCD behavior of the ASC at the current densities of 5, 10 and 20 A g⁻¹. The non-linear shape of the charge/discharge profile has revealed the faradic type performance of ASC. The specific capacitances 103, 62.5 and 23.5 F g⁻¹ at current densities 5, 10 and 20 A g⁻¹ respectively are calculated using equation. The mass is calculated using the equation. The cycling stability of ASC is observed through the continuous charge/discharge cycle at constant current density of 20 A g⁻¹ with 92 % capacity retention for 3000 cycles (Figure 10(c)) is noted and indicates the good reversibility and stability of ASC. The energy and power densities are calculated 101 Wh kg⁻¹ and 6.3 k W kg⁻¹ respectively. These observed energy and power densities are quite high with reference to the recently reported NiCo₂S₄ (Wang et al. 2012; Li et al. 2014; Chen, Xia, and Alshareef 2014; Hu et al. 2014; Huang et al. 2016; Yan et al. 2016; Chen et al. 2014) based asymmetric capacitors.

Figure 10:

Electrochemical performance of scn2 // rGO asymmetric supercapacitor device (a) CV curves, (b) Charge/discharge profile (c) Impedance spectrum and (d) Capacity retention.

Conclusions

The pure NiCo₂S₄ and interlinked mesoporous core-shell nanocomposite CS@NiCo₂S₄ have successfully been prepared via low temperature water-bath method without any further physical and chemical treatment and confirmed by TEM studies. The XRD diffractogram reveals the cubic structure of the as-prepared materials. The surface area and the mesoporous network of the interlinked core-shell are examined by BET analysis. The XPS analysis suggests the elemental presence and the oxidation states of the element in the core-shell nanocomposite. The TEM morphological study confirms the interlinked core-shell structure. The electrochemical studies report that CS@NiCo₂S₄ (scn2, 1:1) composite exhibits the higher specific capacitance (838 F g⁻¹ at 1 A g⁻¹) with 89 % of capacity retention and 95 % of coulombic efficiency for 5000 cycles. The asymmetric supercapacitor CS@NiCo₂S₄//rGO has been fabricated and it shows the energy density of 101Wh kg⁻¹ and power density of 6.3 kW kg⁻¹ and the capacity retention of 92 % sustained for 3000 cycles at 20 A g⁻¹.