Introduction

Being the future, electric vehicles employ energy storage devices such as batteries and supercapacitors, among which supercapacitors offer high PD, specific capacitance, and cycle life. While batteries can offer superior ED owing to the presence of redox reactions in the bulk1. Hence, to improve the ED parameter of supercapacitors, it is suggested to incorporate redox-active metal oxides, sulphides, hydroxides, or polymers into the electrode material. Similarly, the incorporation of carbonaceous materials could simultaneously enhance the PD, leading to a composite electrode material of EDLC and pseudo-type2.

Graphene oxide (GO), multiwall carbon nanotubes (MWCNT), reduced graphene oxide (rGO), carbon nanotubes (CNT), and other electrodes based on carbonaceous materials display electric double layer capacitance (EDLC) properties. The charge buildup at the electrode/electrolyte interface is solely electrostatic, meaning that the electrochemical storage process in the EDLC is non-faradic. Based on materials such as transition metal sulphides and transition metal oxides, the electrodes exhibit pseudocapacitor type (PC) behaviour. Such materials use a faradic reaction as their charge storage mechanism, meaning that charges intercalate and deintercalate at electrochemically active regions3,4,5,6. The synergy of EDLC and pseudo components in a single electrode material guarantees high energy density, long-term stability, and high-power density; hence, it is inevitable to combine both pseudo and EDLC into a single supercapacitor device7,8.

It has been discovered that transition metal sulphides, such as NiS, CuS, CoS, and NiCo2S4, provide promise as electrode materials for redox electrochemical capacitors. NiCo2S4 has garnered interest due to its superior electrical conductivity and increased capacity over its individual and oxide counterparts. Since the multicomponent sulphides can self-dope to increase electrochemical activity and electrical conductivity9. Recent results suggest that nanostructured NiCo2S4 is a great option for creating new nanomaterials with carbonaceous CNT to enhance capacitive performance and ED. However, compared to activated carbons, CNTs have a modest specific surface area, and their tubular shape may be used to effectively diffuse electrolytic ions and disperse active species10. Li et al. in 2016 investigated the effect of CNT incorporation of NiCo2S4 and obtained superior performance for NiCo2S4/CNT composite. Also, they suggested the use of NiCo2S4 as an effective electrode material for electrochemical capacitor devices11. In 2024, Jafar et al. incorporated rGO into NiCo2S4 to increase the effective surface area and the conductivity of NiCo2S4. Further integration of the composite into an asymmetric system resulted in a capacitance retention of 85%12.

In the literature, no one has yet developed a solid-state device with NiCo2S4 and CNT composite. Hence, in this study, a simple hydrothermal approach was used to synthesise the NiCo2S4, and the resulting powder was then mixed with a PVDF binder and a CNT conductive matrix to form the novel NiCo2S4/CNT/PVDF composite. Using a CHI-7007E electrochemical workstation, electrode characterisations were carried out in 2 M KOH using cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and ac impedance (EIS). The characterised electrode was integrated into a symmetric 2-electrode device to test the possibility of a practical device.

Experimental

Materials

Thermo Fisher Scientific provided poly ethylene glycol; Spectrochem provided 1-methyl-2-pyrrolidinone; AD Nanotechnologies provided multiwalled carbon nanotubes (CNT); Nice Chemicals provided Ni(NO3)2·6H2O and Co(NO3)2·6H2O; SRLchem provided potassium hydroxide (KOH); and FTO glass substrate were among the materials used in this study.

Synthesis

Initially, a homogenous solution of 20 mL of PEG 200 with 2.5 M thiourea is prepared by agitation for 1 h. After that, 0.25 M Ni(NO3)2·6H2O and Co(NO3)2·6H2O were added to the mixture above and agitated until they were completely dissolved, where the molar ratio between Co: Ni: S is unity. The resulting mixture was poured into an autoclave with a Teflon lining that held 100 mL. After 18 h of treatment at 160 °C, the autoclave spontaneously cooled to ambient temperature. After centrifuging the resultant black precipitate, ethanol and deionised water were used three times to wash it. Ultimately, the material was heated for an entire night at 70 °C to create stacked nanostructures of NiCo2S4 for electrode fabrication. The active mass loading for the 3-electrode and 2-electrode systems was taken as 200 µg and 2 mg, respectively.

Characterisation

The X-ray diffraction patterns in the 2θ range of 10⁰−90⁰ were obtained using PANalytical-X|Pert3 powder XRD with monochromatic Cu-Kα as the radiation source. Energy dispersive X-ray (EDX) spectroscopy and scanning electron microscopy (SEM) with a resolution of 2 μm were done using a Carl Zeiss microscope with model number EVO/18. Lastly, the electrochemical tests were carried out using a CHI 7007E electrochemical workstation. It had AC impedance (EIS), cyclic voltammetry (CV), and chronopotentiometry (GCD) installed.

Results and discussions

X-ray diffraction investigations were used to examine the structural properties of the synthesised NCS and the CNT; the resulting XRD spectra are displayed in Fig. 1A and B. The XRD data with maximum intensity in the (311) direction confirms the existence of amorphous NCS with a cubic crystalline structure, corresponding to the Fd-3 m space group and group number 227 (ref: 024–0334). Compared to crystalline phases, amorphous NiCo2S4 performs better electrochemically because of its disordered structure, which offers many active sites, speeds up ion/electron transport, and increases specific capacitance13. The presence of the MWCNT phase is confirmed by the reflections seen at about 26.21⁰ of the (002) plane and that at 43.4⁰ corresponding to the (100) plane, which perfectly aligns with ref. 26–1076. The additional peak observed around 19⁰ has been attributed to the (002) reflection of the graphitic structure of C-dots14. The morphology analysis using scanning electron microscope (SEM) confirmed a non-uniform surface of irregular-shaped particles of layered structure and nanoscale dimensions, having an average diameter of 290 nm with distributed irregular-shaped pores over the surface, having an average diameter of 107 nm. (see Fig. 1C and D).

Significant information on the redox behaviour of the chemical species present in the electrode may be obtained using cyclic voltammetry, while the resulting cyclic voltammogram at varying scan rates from 10mV/s to 50mV/s is shown in Fig. 2A. The voltage window was found to be 1.4 V (−1 V to 0.4 V), while the extension to the negative window can be attributed to the presence of CNT in the composite.

In NiCo2S4, nickel is anticipated to be in the + 2 oxidation state and cobalt in the + 3 state. The occurrence of both surface adsorption (EDLC) and the pseudo-type redox behaviour in the electrode material may be attributed to the presence of various oxidation states15. It is discovered that the addition of CNT to the NiCo2S4 crystallographic structures has resulted in a weakening of the redox behaviour and the observation of a rectangular-like pattern. This is explained by the enhancement of EDLC behaviour and the suppression of active sites by conductive CNTs on the surface. Put another way, because of the CNTs’ exceptional electrical conductivity, the electrochemical system’s impedance can be lowered, perhaps suppressing the redox peaks because of the reactions happening more swiftly and effectively16. The electrochemical activity of the NiCo2S4 electrode has generally been explained by three mechanisms. The first is the intercalation of cations into the electrode material during oxidation, when Ni2+ is transformed to Ni3+, and the second is the deintercalation following reduction of Ni3+ to Ni2+. The redox reaction between Co’s 3 + and 4 + oxidation states is the second mechanism. The last process is the cations’ adsorption and desorption on the electrode surface, which occurs on the surface phenomena without any EDLC. Equations 1, 2, 3, and 4 below illustrate the types of surface reactions and redox that exist in the electrode materials15,17.

$$\:NiS+{OH}^{-}\leftrightarrow\:NiSOH+{e}^{-}$$
(1)
$$\:CoS+{OH}^{-}\leftrightarrow\:CoSOH+{e}^{-}$$
(2)
$$\:CoSOH+{OH}^{-}\leftrightarrow\:CoSO+{H}_{2}O+{e}^{-}$$
(3)
$$\:{\left(CNT\right)}_{Surface}+{K}^{+}+{e}^{-}\leftrightarrow\:{(CNT-{K}^{+})}_{surface}$$
(4)
Fig. 1
figure 1

XRD plot of A) NiCo2S4 and B) CNT and SEM images of NiCo2S4 at C) 10 μm and D) 2 μm.

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The charge-discharge behaviour of the electrode material was analysed using the galvanostatic charge-discharge technique, and plots of the composite electrodes at varying current densities are displayed in Fig. 2B. The electrode material displays mild quasi-triangle curves and roughly symmetric behaviour, indicating a reversible faradic redox reaction and increased pseudocapacitive capability. Moreover, the specific capacitance values of the NCS electrode have been calculated using the equations given in the electrochemical study by Abin et al.18. The specific capacitance values calculated from CV measurements of the NCS electrode in a three-electrode system at 10mV/s were found to be 384.6 F/g, while it was 142 F/g from GCD at 1 A/g. The superior voltage window and the excellent specific capacitance suggest the possibility of device fabrication with the NCS electrode.

Fig. 2
figure 2

Plots showing A) CV, B) GCD, and C) EIS studies using three three-electrode system.

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Figure 2C displays the Nyquist plot, from which the real axis intercept of the straight line in the low frequency domain may be used to compute the equivalent series resistance (ESR)19. Moreover, the series resistance (Rs), charge transfer resistance (Rct), and equivalent series resistance (ESR) were found to be 20.23Ω, 8.38Ω, and 28.61Ω, respectively.

Fabrication of a symmetric solid-state device (SSD)

To derive maximum specific capacitance and ED from the system, a symmetric two-electrode system was developed using NCS/CNT/PVDF-coated FTO as the working and reference/counter electrode with an active mass loading of 2 mg, while the separator consists of Whatman No. 1 filter paper dipped in 2 M KOH electrolyte for 10 s. The real-time image of the fabricated device is shown in Fig. 3E, which delivered an open circuit potential of 1.12 V after being fully charged. The values of Cs, ED, and PD were calculated using Eqn. S1-S4.

Cyclic voltammetry studies were conducted to investigate the electrochemical behaviour of the SSD, from which pseudo behaviour with good reversibility has been confirmed, and the resulting CV plot is shown in Fig. 3A. While the potential window was found to be 3.1 (−1.6 V to 1.5 V), which was double that obtained in the three-electrode system. It might be explained by the absence of a conventional reference electrode in a two-electrode setup. The device delivered a superior specific capacitance of 206.5 F/g at a scan rate of 10mV/s due to its amorphous behaviour, which is superior to devices in recent literature. The GCD studies (shown in Fig. 3B) can be employed to calculate the ED and PD of the device using the equation mentioned in18, where the ED was 64.53 Wh/Kg and the PD was 0.54 KW/Kg, and both were calculated at 10mV/s and 1 A/g, respectively. Moreover, the specific capacitance calculated from GCD curves at 1 A/g was found to be 204.8 F/g, where a large discharge time of 430 s can be explained by the presence of a chemical reaction in the electrode.

One important factor to consider when assessing a device’s economic usefulness is its longevity. The stability tests were carried out using a current density of 7 A/g for 2500 cycles. Figure 3C displays the GCD curves for the first and last five cycles of the stability experiments. The specific capacitance from the CV signature before (206.5 F/g) and after (108.6 F/g) the stability at a scan rate of 10mV/s was analysed, and the capacitive retention was found to be 52.5% (see Fig. 3D). Moreover, the GCD technique was studied before (204.8 F/g) and after (115 F/g) the stability studies confirm a retention of 54%, where the results from both CV and GCD are in good agreement. The device’s sluggish electrochemical stability is caused by the increase of its volume and the subsequent depletion of redox active sites because of repeated ion intercalation and de-intercalation20.

Fig. 3
figure 3

Plots showing A) CV, B) GCD, C) stability studies, D) CV before and after, (E) fabricated device, and (F) EIS before and after using a two-electrode system.

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The depletion of redox pathways (active sites) is always associated with an increase in charge transfer resistance (Rct) and the total internal resistance (ESR); hence, the EIS was performed before and after the stability studies to confirm the exact cause for low cyclic stability and the resulting plot is shown in Fig. 3F21. After cyclic stability studies, the EIS analysis confirmed an increase in Rct from 10.88 ohms to 21.65 ohms and the ESR from 41.28Ω to 50.64Ω (The fitted curve and equivalent circuit are shown in Fig. S1 and S2). Thus, an increase in both Rct and ESR confirms the increased resistance to ion intercalation and decreased conductivity of the device upon stability studies, together responsible for the reduction in capacitive performance. In addition, following the cyclic stability testing, the ED values decreased to 33.75 Wh/Kg at 10mV/s, and the PD did not change much (0.5 KW/Kg at 1 A/g). We can once more verify the depletion of redox processes inside the electrode material while maintaining the capacity to transmit power based on the lowering of ED values without affecting PD. The comparison of the present work with recent literature is shown in Table 1.

Table 1 Table showing comparison of the present work with recent literature.
Full size table

Conclusion

To the best of our knowledge, this is the first study on the electrochemical performance of layered amorphous NiCo2S4 that incorporates PVDF and CNT. The exceptional specific capacitance, voltage window, and ED of each electrode can be attributed to the amorphous phase, which facilitates the effortless intercalation and deintercalation of ions. Due to the electrode’s exceptional performance, a symmetrical two-electrode solid-state device was used to study it. This device produced a wide potential window of 1.5 V (−1.6 V to 0 or 0 to 1.5 V), an exceptional PD of 0.54 KW/Kg, an ED of 64.53 Wh/Kg at 1 A/g, and a specific capacitance of 206.5 F/g. Additionally, a test of the device’s longevity after 2500 cycles revealed a 54% capacitive retention.