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. 2025 Oct 31;15:38176. doi: 10.1038/s41598-025-22033-y

Electrochemical and dielectric properties of cobalt doped SnS on cotton based electrodes for potential use in supercapacitors

Mohd Arif Dar 1,2,3,✉, Safwat A Mahmoud 4,✉, S R Majid 1,✉, M Satgunam 2, L Guganathan 5, P Arularasan 6, M Karunakaran 7, S Sheik Fareed 8, Hilal Ahmad Rather 9,✉, Lamiaa Galal Amin 10
PMCID: PMC12578913  PMID: 41174078

Abstract

Herein, we synthesize Co-doped SnS nanoparticles (NPs) using a simple solvothermal method. The X-ray diffraction (XRD) spectra determine the orthorhombic structure of the Co-doped SnS NPs. The Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirm the rod-like structure of the Co-doped SnS NPs. The surface area of 31, 225, 256 and 383 (m2/g) of Co-doped SnS NPs determines their tendency for electrical and supercapacitive properties. The deconvolution curves show the binding energy peaks of Sn, S and Co, confirming the doping of Co ions in the lattice of SnS compound. The 20%Co:SnS electrode retains 94% of its capacitance with a coulombic efficiency of 78% even after 3000 cycles of GCD curves. The SnS XRD peaks observed even after the cyclic stability test of 20%Co:SnS electrode determine the stability of the electrodes. The conductivity values of the AN1, AN2, AN3 and AN4 cotton electrodes are 9.6 × 10−08, 3.5 × 10−07, 5.8 × 10−07 and 1.2 × 10−06, providing their tendency for future material in dielectric and microelectronic device applications.

Keywords: Electrochemical, Electrical, Capacitive retention, Coulombic efficiency

Subject terms: Materials science, Nanoscience and technology

Introduction

The rapid increase in global energy consumption, together with the transition toward renewable resources, has intensified the demand for efficient and reliable energy storage systems. Among various technologies, supercapacitors have emerged as highly promising due to their outstanding power density, ultrafast charge–discharge response, and prolonged cycling stability, which render them particularly suitable for applications requiring instantaneous energy delivery such as electric mobility, smart grids, and portable electronics1–5. Despite these advantages, their relatively low energy density compared to conventional batteries remains a critical limitation, necessitating the development of innovative electrode materials that can bridge this gap and extend device functionality. The electrode structure and composition dictate capacitance, durability, and rate characteristics of the material. For this purpose, significant research efforts are directed toward the design of materials that simultaneously exhibit superior electrochemical performance, sustainability, and scalability for practical deployment6–15. Furthermore, the growing need for environmentally benign and cost-effective fabrication strategies underscores the importance of developing electrode systems that can balance high efficiency with large-scale manufacturability, thereby accelerating the integration of supercapacitors into next-generation energy technologies.

Beyond their established role in rapid energy delivery, supercapacitors are also valued for their safety, wide operating temperature range, and superior power handling compared to batteries. Their ability to withstand millions of cycles without significant degradation makes them indispensable for applications where long-term reliability is crucial. Depending on the charge storage mechanism, they are broadly categorized into electrochemical double-layer capacitors (EDLCs), which rely on non-faradaic ion adsorption at the electrode–electrolyte interface, and pseudocapacitors, which utilize fast and reversible redox reactions to achieve higher capacitance16–19. A hybrid configuration that combines both mechanisms has also gained momentum, aiming to achieve a balance between power and energy density. This classification highlights the central importance of electrode materials, as their surface area, conductivity and structural stability determine the efficiency of charge storage and transfer processes. Consequently, tailoring material properties at the nanoscale, incorporating heteroatom doping and engineering hierarchical architectures are being actively explored to overcome existing limitations and advance supercapacitor technology for emerging energy-intensive applications20–22.

Also, dielectric properties are increasingly recognized as a decisive factor governing the electrochemical performance of supercapacitor electrodes, as they directly influence charge storage capacity, charge–discharge kinetics and coulombic efficiency23–25. In EDLCs, energy storage occurs through the formation of electric double layers at the electrode–electrolyte interface, where a high dielectric constant substantially enhances interfacial charge accumulation, thereby improving the attainable energy density26,27. Equally important is the dielectric strength, which reflects the resistance of a material to electrical breakdown, thereby, electrodes with high dielectric strength enable ultrafast cycling stability and efficient energy delivery, a requirement in advanced technologies such as electric transportation and smart grids. In hybrid supercapacitors that integrate double-layer and faradaic storage, fine-tuning dielectric characteristics becomes even more critical, as these parameters govern the kinetics and stability of charge transfer across the electrode–electrolyte boundary28–30. Thus, rational optimization of dielectric behavior in electrode materials is indispensable for enhancing energy density, accelerating charge–discharge processes and ensuring thermal robustness, these are key attributes required for the large-scale deployment of supercapacitors in high-performance and energy-intensive systems31,32.

Parallel to dielectric optimization, material selection plays a pivotal role in defining the overall storage capability of supercapacitors. Conventional systems have employed transition metal oxides, conductive polymers, and carbon-based nanostructures such as graphene, owing to their large surface area, electrical conductivity and tunable surface chemistry, which collectively contribute to enhanced charge accumulation and high energy density33–36. More recently, metal chalcogenides have attracted considerable attention due to their diverse structural phases, rich redox activity, and favorable electrical properties, with binary compounds such as SnS, NiS, MnS, PbS and SnSe emerging as promising candidates37–39. Despite their potential, pristine chalcogenide electrodes often suffer from limited specific capacitance and inferior cycling stability compared to oxide-based counterparts. To overcome these challenges, cationic doping strategies involving transition metals such as Co, Ni, Mn, Fe, Cu, and Zn have been employed to tailor the physicochemical properties and electrochemical response40. Among these, cobalt incorporation has proven particularly effective, as Co ions introduce additional redox-active sites, enhance electronic conductivity, and increase the electrochemically accessible surface area, thereby promoting rapid charge transport and higher energy storage capability. Such modifications significantly elevate the specific capacitance and energy density, underscoring the promise of doped metal chalcogenides as next-generation electrode materials for high-performance supercapacitors40–46.

Here, in this study, Co-doped SnS NPs are synthesized through the hydrothermal method. The synthesized NPs are characterized through the different characterization techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) studies. Finally, the electrodes of the Co-doped SnS NPs are prepared using Ni-foam, graphite substrate and white cotton pads. The Ni-foam electrode was made in the dimension (2 × 1 cm2), graphite and cotton pads were cut into coin cell size. The slurry of Co-doped SnS NPs formed was pasted on Ni-foam, graphite substrate and cotton using the doctor blade method. The electrodes were then analyzed using cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) tests. The Co-doped SnS cotton electrodes were further used for electrical studies. Lastly, XRD, FTIR, and Raman studies were conducted after the cyclic stability test of the Co-doped SnS graphite electrode to assess the electrode’s stability.

Experimental

The solvothermal method was used to synthesize nanocrystals of Sn1− xCoxS, where x is 0.05, 0.10, 0.15, and 0.20. The investigation utilized multiple types of raw materials of analytical grade without any additional purification. The tin chloride (SnCl2·2H2O), cobalt chloride (CoCl2·4H2O), and Thiourea (SC(NH2)2) purchased from Sigma Aldrich (99.9% analytical grade) were weighed based on the mmol ratio and chemical formula of the metal cation in Sn1− xCoxS. The heterogeneous components were dissolved in 5 mL of acetic acid and 50 ml of tartaric acid. The mixture was then agitated for approximately 30 min until a uniform solution was obtained. Ultimately, the solution was placed within a 50 ml stainless steel autoclave and subjected to a temperature of around 180 °C for 18 h in an electric oven. The resultant precipitates were repeatedly rinsed with deionized water and acetone to eliminate contaminants. The final product was dehydrated in an electric oven at a temperature of 60 °C for 12 h. The produced NPs of Sn1− xCoxS (x = 0.05, 0.10, 0.15, and 0.20) were utilized for various characterization experiments. The NPs were designated as 5%, 10%, 15%, and 20%Co:SnS NPs, respectively. Figure 1 displays the schematic image of the formation of 5%, 10%, 15%, and 20%Co:SnS NPs. The slurry for the formation of the electrode was prepared by using the active material (5%, 10%, 15%, and 20%Co: SnS NPs, separately), acetylene black, and PVDF in the ratio (85:10:05), as shown in Fig. 2. The substrates used are Ni foam (2 × 1 cm2), graphite, and cotton in the form of coin cells (Fig. 2). The specific capacitance (Cp) values are calculated using the formulas for CV and GCD studies, respectively46,47.

Fig. 1.

Fig. 1

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Schematic diagram of the synthesis of 5%, 10%, 15% and 20%Co:SnS NPs, respectively.

Fig. 2.

Fig. 2

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Schematic diagram of 5%, 10%, 15% and 20%Co:SnS cotton electrode formation.

Three-electrode system

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Two-electrode system

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Where A is the area beneath the CV curves, S is the scan rate (V/s), ΔV is the potential window (V), I is the current (A) and Δt is the discharge time (sec). The average mass of the active material in the Ni foam electrode (in between 5 and 6 mg/electrode), graphite substrate (in between 1.64–1.68 mg/electrode), and cotton pads (3–4 mg/electrode).

Results and Discussion

Structural and morphological studies

Figure 3a shows the XRD spectra of 5%, 10%, 15% and 20%Co:SnS NPs which exhibited diffraction peaks at 20.07o, 26.07º, 27.48o, 30.49o, 31.55o, 32.0º, 39.09o, 44.77o, 45.61o, 48.58o, 51.33o, 54.32o and 66.81o, corresponding to (110), (120), (021), (101), (111), (040), (131), (141), (150), (211), (151), (061) and (080) hkl planes, respectively. The peak positions of the 5%, 10%, 15%, and 20%Co:SnS NPs matched with JCPDS card No. 14–0620, having the orthorhombic structure of space group Pnma-62. Furthermore, the XRD spectra of 5%, 10%, 15%, and 20%Co:SnS NPs have not shown any additional crystal phases such as SnS2, S, and Sn2S3. Also, the intensity of high peaks decreases continuously from 5% to 15%Co:SnS NPs with increasing Co dopants (Fig. 3b). The decrease of the high-intensity peak occurs due to the difference in the ionic radius between Sn2+ (1.22 Aº) and Co2+ (0.65 Aº) ions48,49. This observation provides strong evidence that Co2+ ions are present in the lattice positions of SnS. Further, it is observed that when the doping concentration reaches up to 20%Co:SnS, the high-intensity peak increases, indicating the interstitial and substitutional doping of Co2+ ions in the SnS lattice sites. The pronounced increase in the (040) peak intensity at 20%Co doping can be primarily attributed to the preferred orientation of the rod-like crystallites and dopant-induced recrystallization, which enhances crystallinity and reduces lattice strain49. The decrease in the intensity of most peaks from 5% to 20%Co:SnS NPs has changed their morphology from rod-like to spherical nanospheres. The average crystallite size, microstrain, and dislocation density were calculated using the following equations50.

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Fig. 3.

Fig. 3

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(a) XRD spectra (b) Magnified diffraction (040) peak pattern of 5%, 10%, 15% and 20%Co:SnS NPs, respectively.

The calculated average crystallite size for 5%, 10%, 15%, and 20%Co-doped SnS nanoparticles was found to be 33, 30, 33, and 34 nm, respectively, with corresponding microstrain values of 1.1 × 10−3, 1.4 × 10−3, 1.4 × 10−3 and 1.3 × 10−3 and dislocation density 1.3 × 10−3, 2.0 × 10−3, 2.3 × 10−3 and 2.0 × 10−3, respectively. The marginal variations in crystallite size and microstrain suggest that the incorporation of Co ions into the SnS lattice does not cause significant distortion or phase transformation. This behavior can be attributed to the comparable ionic radii of Co²⁺ and Sn²⁺, which facilitates substitution without inducing major lattice strain. The relatively stable dislocation density further confirms that the introduction of dopant ions does not substantially disrupt the crystalline framework. The consistency of the XRD peak profiles across all compositions reinforces this observation, reflecting a robust crystal structure with only minor local perturbations induced by Co incorporation. Thus, the near invariance of the average structural parameters demonstrates that the Co dopant is effectively accommodated within the SnS lattice, preserving crystallinity while introducing only minimal strain and defect density. This structural stability is advantageous for maintaining the integrity of the material while tuning its physical and electrochemical properties.

The purity and bonding of elements in 5%, 10%, 15%, and 20%Co:SnS NPs were classified using FTIR spectroscopy. Figure 4 shows the FTIR spectrum of 5%, 10%, 15%, and 20%Co:SnS NPs recorded in the wavelength range between 400 and 4000 cm−1. The FTIR spectrum exhibited several absorption bands at 566, 570, 608, 669, 671,693, 1113, 1384, 1632, 2852, 2921 and 3443 (cm−1) for the 5%, 10%, 15%, and 20%Co:SnS NPs. The band observed at 3443 cm−1 belongs to the hydroxyl groups related to O-H stretching vibrations51. The band observed at 1632 cm−1 belongs to hydroxyl groups of O-H bending vibrational modes52. The bands observed at 2852 and 2921 (cm−1) are related to the C-H stretching vibrations52. The band pattern observed at 1113 and 1384 (cm−1) belongs to C-O and C=O functional groups53. The characteristic band observed at 608 cm−1 belongs to the vibrational modes of antisymmetric Sn-S stretching vibrations54,55. This band moves towards a higher wavelength with doping of Co ions and appears at 669, 671, and 693 (cm−1) for 10%, 15%, and 20%C:SnS NPs. Further, the intensity of this band decreases with Co ion concentration from 5% to 20%C:SnS NPs coincides with XRD results. Also, two more small peaks are seen at 566 and 570 (cm−1) for the 10% and 15%Co:SnS NPs belong to the doping of metal Co ions.

Fig. 4.

Fig. 4

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FTIR spectrum of 5%, 10%, 15% and 20%Co:SnS NPs, respectively.

The synthesized 5%, 10%, 15%, and 20%Co:SnS NPs were examined for pore size and surface area distribution using the N2 adsorption/desorption technique. Figure 5 (a-d) shows the BET studies of 5%, 10%, 15% and 20%Co:SnS NPs. From Fig. 5 (a-d), all the synthesized NPs show type IV hysteresis loops, indicating the presence of mesopores on the surface of 5%, 10%, 15% and 20%Co:SnS NPs. The surface area of the 5%, 10%, 15% and 20%Co:SnS NPs are 31, 225, 256 and 383 (m2/g), respectively. The surface area of the synthesized NPs increases with the rise in the concentration of Co ions. Figure 5 (a-d) shows the isothermal curves of the inserted 5%, 10%, 15% and 20%Co:SnS NPs. From Fig. 5 (a-d), the pore radius of the 5%, 10%, 15% and 20%Co:SnS NPs are 32.714 Å, 18.613 Å, 15.506 Å and 14.783 Å and the pore volume 0.186, 0.373, 0.393 and 0.582 (cc/g), respectively. All these values determine the porosity of the 5%, 10%, 15% and 20%Co:SnS increases with the rise in Co dopants. The 5%, 10%, 15% and 20%Co:SnS shows the mesoporous structure falls between 1 and 10 nm. The increase in the porosity of synthesized nanoparticles with the Co dopants provides sufficient space for the electrodes in the interaction of electrolyte ions during the electrochemical reaction, which may act as an advantage to enhance the supercapacitive behavior.

Fig. 5.

Fig. 5

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(a-d) BET curves with inserted isothermal curves of the 5%. 10%, 15% and 20%Co:SnS NPs.

Figure 6(a–l) displays the SEM images of 5%, 10%, 15%, and 20%Co:SnS NPs. At lower doping concentration (5%), the nanoparticles exhibit a pronounced rod-like morphology [Figure 6(a–c)]. However, with increasing Co content, a progressive morphological transformation occurs, in which the rods evolve into nearly spherical nanoparticles. This transition can be attributed to the influence of Co incorporation on the lattice dynamics and surface energy. The reduction in XRD peak intensity and the shift in the FTIR Sn–S vibrational band suggest that Co dopants induce subtle lattice distortions, which in turn alter the preferred growth directions. As a result, the anisotropic rod-like growth is suppressed, favoring isotropic nucleation and the formation of spherical particles. Moreover, the diameter of the spherical structures increases steadily beyond 10%Co doping [Figure 6(d–l)], reflecting dopant-driven modifications in crystallization and coalescence processes.To corroborate these findings, TEM analysis of 5%Co:SnS nanoparticles was performed. The TEM images [Figure 7(a, b)] clearly confirm the rod-like morphology, consistent with the SEM observations. HRTEM images [Figure 7(c–f)] provide further insights: the presence of fine spherical contrast spots within the rods indicates localized incorporation of Co dopants, generating minor inhomogeneities within the lattice. The lattice fringe image [Figure 7(e)] shows the (040) crystallographic plane with well-resolved spacing, while the SAED pattern [Figure 7(f)] exhibits sharp diffraction rings, confirming the crystalline nature of the doped nanoparticles.

Fig. 6.

Fig. 6

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SEM images (a-c) 5%, (d-f) 10%, (g-i) 15% and (j-l) 20%Co:SnS NPs, respectively.

Fig. 7.

Fig. 7

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(a, b) TEM images (c-e) HRTEM images (f) SAED pattern of the 5%Co:SnS NPs.

Thus, the introduction of Co not only modifies the crystallographic features but also governs the morphological transition from rods to spheres. This transformation is likely driven by surface energy minimization and strain relaxation, as spherical particles present lower surface energy and reduced lattice strain compared to anisotropic rods at higher doping levels. Importantly, such morphological evolution enhances the electrochemical properties of the Co:SnS system, since spherical nanostructures offer a larger electrochemically active surface area and improved ion accessibility compared to rod-like structures.

Figure 8(a) presents the XPS survey spectrum of the 5%Co:SnS NPs, confirming the presence of Sn, S, and Co elements in the sample. The high-resolution spectrum of Sn 3 d [Figure 8(b)] displays two distinct peaks at binding energies between 485 and 500 eV, corresponding to Sn 3d5/2 and Sn 3d3/2, which are characteristic of Sn species47. The S 2p spectrum [Figure 8(c)] exhibits peaks in the range of 155–175 eV, consistent with the binding energies of S atoms in the SnS lattice6. The Co 2p spectrum [Figure 8(d)] reveals well-defined peaks in the range of 770–810 eV, which can be assigned to the Co 2p3/2 and Co 2p1/2 states20. The successful detection of Co 2p features, along with the characteristic Sn and S peaks, confirms the incorporation of Co ions into the SnS lattice without the formation of secondary impurity phases.

Fig. 8.

Fig. 8

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(a) XPS images (b) Sn peaks (c) S peaks (d) Co peaks of the 5%Co:SnS NPs.

Electrochemical studies

The electrochemical studies of the 5%, 10%, 15% and 20%Co:SnS electrodes were checked out in a three and two-electrode system. Figure 9(a-d) shows the CV of 5%, 10%, 15%, and 20%Co:SnS electrodes in a three electrode system. All the electrodes show close to rectangular CV profiles, typical for EDLC behavior, with no visible faradaic redox peaks. The lack of redox features demonstrates that the charge storage mechanism is primarily controlled by non-faradaic electrostatic adsorption of ions at the electrode/electrolyte interface, instead of pseudocapacitive or battery-type reactions. The retention of this EDLC-type behavior at all doping levels indicates that the introduction of Co into the SnS lattice does not bring about substantial surface redox activity but rather preserves structural integrity and rapid ion adsorption/desorption kinetics. Significantly, the progressive development of the CV region with growing Co content indicates improved capacitive characteristics, which can be explained by the change in morphology (rod-like to spherical) and related gain in available electroactive surface area. The close-to-ideal rectangular characteristics of the curves, even under more elevated scan rates, also emphasize the good reversibility and fast charge transfer of the Co:SnS electrodes, evidencing their applicability for high-rate energy storage. Figure 9(e) shows the GCD curves of pure, 5%, 10%, 15%, and 20%Co:SnS electrodes at the current density of 1 A/g in a three electrode system. The values of specific capacitance derived from the GCD plots of pure, 5%, 10%, 15%, and 20%Co:SnS electrodes are 8, 11, 21, 41, and 115 F g⁻¹, respectively (Fig. 9f). This stepwise increase unequivocally illustrates that incorporation of Co greatly increases the charge storage ability of SnS, where greater doping concentrations offer greater electroactive sites, better surface area, and quicker ion mobility, which in turn increases the overall capacitance performance.

Fig. 9.

Fig. 9

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(a-d) CV curves of the 5%, 10%, 15% and 20%Co:SnS electrodes, (e) GCD curves and (f) specific capacitance versus of pure, 5%, 10%, 15% and 20%Co:SnS electrodes.

The cotton-based electrodes of 5%, 10%, 15%, and 20%Co:SnS NPs were fabricated by combining PVDF, acetylene black, and 5%, 10%, 15%, and 20%Co:SnS NPs individually in a pistol mortar using NMP to form the slurry. The cotton pad was cut into a circular shape with a radius of 0.8 × 0.8 cm2 and immersed in the slurry of 5%, 10%, 15%, and 20%Co:SnS NPs and named 5%, 10%, 15%, and 20%Co:SnS electrodes, respectively. The schematic diagram is shown in Fig. 2. The electrochemical studies were done in a two-electrode system in which each cell contains 5%, 10%, 15%, and 20%Co:SnS electrodes separately, AC, and a separator (i.e., microfiber filter) in which a few drops of 1 M KOH aqueous electrolyte are poured acting as an electrolyte. The coin cell arrangement is shown in Fig. 2. Figure 10(a) shows the CV curve of the pure cotton, 5%, 10%, 15%, and 20%Co:SnS electrodes. The figure clearly shows that the CV area increases with the rise in Co concentration. Figure 10(b-e) shows the CV curves of the 5%, 10%, 15%, and 20%Co:SnS electrodes at the scan rate of the 5, 10, 20, 30, 40, 50 and 100 mV/s, respectively. The CV curve increases with the rise of scan rates governing the capacitive nature of the 5%, 10%, 15%, and 20%Co:SnS electrodes, respectively. Furthermore, the CV curves do not show any reduction peaks, showing the EDLC nature of the 10%, 15%, and 20%Co:SnS electrodes, respectively56,57.

Fig. 10.

Fig. 10

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(a) CV curves of the pure 5%, 10%, 15% and 20%Co:SnS electrodes at the scan rate of 5 mV/s, (b-e) CV curves of the 5%, 10%, 15% and 20%Co:SnS electrodes at the scan rate of 5, 10, 20, 30, 40, 50 and 100 mVs−1 and (f) CV curves of the 20%Co:SnS electrode device at different potential windows ranging from − 0.2–1.0 V with a constant scan rate of 5 mVs−1.

Figure 10(f) shows the CV curves of the 20%Co:SnS electrode device at different potential windows ranging from − 0.2 to 1.0 V with a constant scan rate of 5 mV/s. It shows similar rectangular shapes, demonstrating that the EDLC nature confirms fast ion transportation/redox reaction kinetics in the asymmetrical cell device. Generally, the asymmetrical supercapacitor cell configuration achieves a higher voltage than the symmetrical supercapacitor cell configuration. Further, to gain good electrochemical performance of the fabricated asymmetrical 5%, 10%, 15% and 20%Co:SnS||AC device, the balance of charge between the two electrodes is essential (Inline graphic) which can be attained by the mass balance of the following relation58:

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where Inline graphicand Inline graphic are the charge stored at the negative and positive electrodes, respectively, C is the specific capacitance, m is the mass of the active electrode and ΔV is the voltage difference in volts, respectively.

Further, the electrochemical studies of the 5%, 10%, 15%, and 20%Co:SnS electrodes were analyzed by checking the CV, GCD, and cyclic stability tests in graphite substrates. For that purpose, 20%Co:SnS electrode using graphite as a substrate was analyzed for CV, GCD, and cyclic stability tests in a two-electrode cell configuration with reference to the AC electrode. Figure 11 (a, b) shows CV curves of the 20%Co:SnS electrode at the scan rate of 5, 10, 30, 50, and 100 mV/s and different potential ranges varying from 0 to 0.8 V, respectively. Both these figures show a similar trend of the CV curves as seen in the cotton-based electrodes. The shape of the CV curves provides insights into the electrochemical behavior and capacitance. The increase in current with higher scan rates indicates good rate capability and electrochemical performance59. Figure 11 (c, d) shows the GCD curves of the 20%Co:SnS electrode at the current density of 0.05, 0.06, 0.07, 0.08, 0.09, and 0.1 A/g and different potential ranges varying from 0 to 0.8, respectively. The calculated specific capacitance values are shown in Fig. 12 (a) and compared with the literature report in Table 1. Here, the decreasing discharge time with higher current density suggests that specific capacitance decreases as current density increases, which is typical for electrochemical capacitors. The specific capacitance decreases as current density increases, which is common due to limited ion accessibility at higher current densities. The values at lower current densities are higher, indicating better charge storage at slower rates. Figure 12 (b) Rangone plot of the 20%Co:SnS electrode. Figure 12 (c) shows the capacitive retention and coulombic efficiency of the 20%Co:SnS electrode. The capacitive retention and coulombic efficiency of the 20%Co:SnS electrode were calculated for 3000 cycles which determined that the electrode retains a capacitive retention of 94% and a coulombic efficiency of 78% even after 3000 cycles of GCD curves (last 25 GCD cycles Fig. 12d). These results indicate the remarkable long-term stability and performance of the 20%Co:SnS electrode under extended cycling conditions. The capacitive retention greater than 94% even after 3000 cycles suggests an improvement in performance over time, possibly due to electrode activation or structural adjustments that enhance charge storage capability. Maintaining such high retention is rare and points to the electrode’s strong architecture and synergistic properties of cobalt and SnS that prevent degradation over repeated cycles. Additionally, a coulombic efficiency of 78% after 3000 cycles highlights its efficient charge transfer and minimal capacity loss, demonstrating consistent cycling behavior. These attributes make the 20%Co:SnS electrode a promising candidate for high-performance energy storage applications, where durability and reliable efficiency are critical. Further, XRD and Raman spectra are done for 20%Co:SnS electrode after the cyclic stability tests (Fig. 13a-d). The XRD and Raman spectra show the peaks of the SnS NPs, confirming the stability of the electrode even after the cyclic stability tests. The major peaks observed at 26.37º in the XRD spectra and above 1000 cm−1 Raman spectra are the peaks of graphite used as the substrate for the formation of the electrode (Fig. 13a, c). Figure 14 shows the FTIR spectra of the 20%Co:SnS electrode before and after the cyclic stability test. Here, we can see the same peaks before and after the cyclic tests with small deflection. Thus, all these electrode characterizations determine the stability and future prospects of Co-doped electrodes for supercapacitors.

Fig. 11.

Fig. 11

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(a) CV curves of the 20%Co:SnS electrode at the scan rate of 5, 10, 30, 50 and 100 mV/s, (b) CV curves of the 20%Co:SnS electrode at the different potential windows ranging from 0.1–0.8 V, (c) GCD curves of the 20%Co:SnS electrode at the current density of 0.05, 0.06, 0.07,0.09 and 0.1 A/g, (d) GCD curves of the 20%Co:SnS electrode at the different potential windows ranging from 0.1–0.8 V with a constant current density of 0.05 A/g.

Fig. 12.

Fig. 12

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(a) Specific capacitance versus current density, (b) Rangone plot, (c) Capacitive retention and Coulombic efficiency and (d) last 25 cycles of GCD cycles of the 20%Co:SnS electrode.

Table 1.

Comparative studies of electrochemical properties to the present work.

Active material Preparation method Electrolyte Specific capacitance (F/g) Ref.
ZnS Solvothermal KOH 32 66
WS2 Chemical exfoliation - 40 67
MoSe2 Mechanical delamination PVA-LiOH gel electrolyte 89 68
δ-MnSe Hydrothermal - 96 69
20%Co:SnS Solvothermal KOH 62 Present work
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Fig. 13.

Fig. 13

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(a, b) XRD spectra, (c, d) Raman spectra of the 20%Co:SnS electrode after the cyclic stability test.

Fig. 14.

Fig. 14

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FTIR Raman spectra of the 20%Co:SnS electrode before and after the cyclic stability test.

Figure 15 (a) shows the Nyquist plots of pure, 5%, 10%, 15%, and 20%Co:SnS electrodes along with the equivalent circuit model employed for fitting. The spectra show a semicircle arc in the high-frequency region followed by a linear Warburg tail at low frequencies, characteristic of combined charge-transfer resistance and ion diffusion mechanisms, respectively. Here, it is seen that 20%, 15%, 10% and 5% doped samples of SnS have smaller semicircle diameters than pristine SnS, verifying a significant reduction of charge-transfer resistance by doping. Among all the compositions, the 20%Co:SnS electrode shows the lowest, implying augmented back electronic conductivity and more rapid interfacial charge transport. This enhancement is ascribable to the synergistic action of Co incorporation, which causes defect states, maximizes the lattice structure and facilitates efficient accessibility of the electrolyte ions. The approximately vertical slope in the low-frequency region for the 20%Co:SnS electrode still highlights superior capacitive behavior with lower diffusion resistance, consistent with the ideal EDLC-type response. Figure 15 (b) presents the Bode phase angle plots of the pure, 5%, 10%, 15% and 20%Co:SnS electrodes. The maximum phase angle for the pure SnS electrode is near 50°, whereas the phase angles of the Co-doped samples are lower, with 20%Co:SnS retaining a consistent response at a wide range of frequencies. The decreased phase angle at higher Co concentrations suggests fast ion dynamics and better capacitive kinetics with faster relaxation times and greater charge storage reversibility. This agrees with the Nyquist analysis, which supports the fact that Co doping efficiently accelerates electron and ion transport in the electrode–electrolyte system.

Fig. 15.

Fig. 15

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(a) Nyquist plot (b) Bode plot of the pure, 5%, 10%, 15% and 20%Co:SnS electrodes, respectively.

In general, the EIS outcomes reveal that Co doping considerably reduces charge-transfer resistance and aligns ion diffusion channels to the best extent, with 20%Co:SnS possessing the most effective kinetics. These observations are very clear-cut evidence that controlled doping may be a facile method to modify interfacial charge transfer and enhance the capacitive performances of SnS-based supercapacitor electrodes.

Dielectric and electrical studies

The cotton-based electrodes of 5%, 10%, 15% and 20% were represented by the names AN1, AN2, AN3, and AN4 NPs with the dimensions of 0.8 × 0.8 cm2 and average thickness of 3.3 mg. The schematic diagram is shown in Fig. 2. The cotton electrodes of AN1, AN2, AN3, and AN4 NPs were used for EIS measurement. The EIS studies give information about AN1, AN2, AN3, and AN4 cotton electrodes about their conduction behavior, dielectric relaxation, and polarization mechanism of a material and mainly depend upon electronic, ionic, dipolar, and space charge distribution60 Electronic polarization is the key factor in the variation of dielectric properties of crystalline materials in the bulk form, exhibited in the optical frequency range. ionic polarization occurs by the relative separation of positive and negative ions61. The permanent dipole moment displays space charge polarization and molecules reorient when exposed to the electric field. The dielectric constant is an important electrical property to explain the potential applications of AN1, AN2, AN3, and AN4 cotton electrodes as it expresses the nature of the electrical phenomenon. The dielectric constant ‘ε’ is represented by the relation:

graphic file with name d33e1236.gif 10

where Inline graphic is the real part of eccentricity and Inline graphic is the imaginary part of eccentricity and can be used to calculate the complex dielectric constant. The following equations are used to calculate the real part of the dielectric loss or dielectric constant.

graphic file with name d33e1256.gif 11
graphic file with name d33e1262.gif 12

where C, A, and d are the capacitance, cross-sectional area, and thickness of the cotton electrode. The Inline graphic is the permittivity in free space (Inline graphic = 8.85 × 10−12 F/m). The real and imaginary parts of the dielectric constant are plotted against the frequency and log frequency shown in Fig. 16 (a-d). The ε is high at low-frequency regions (up to log 1.6 Hz) owing to the reason of different types of polarization and continuously decreases (up to log 5.12 Hz), then finally saturates. The high dielectric constant (ε) observed at low-frequency regions (up to approximately 40 Hz, or log 1.6 Hz) can be attributed to multiple types of polarization mechanisms such as interfacial, dipolar, ionic, and electronic polarization. At lower frequencies, these mechanisms have sufficient time to align with the alternating electric field, leading to an enhanced dielectric response. The interfacial polarization, also known as Maxwell-Wagner polarization, is particularly significant in composite materials like Co-doped SnS, where distinct interfaces between the doped and undoped regions contribute to charge accumulation and enhanced polarization62. As the frequency increases beyond this range (up to log 5.12 Hz or approximately 130,000 Hz), the ability of these polarization mechanisms to keep pace with the rapidly alternating electric field diminishes. The heavier and slower polarization components, such as interfacial and ionic polarization, cannot respond as quickly, resulting in a decrease in ε. Eventually, at even higher frequencies, only the fastest electronic polarization remains active, and the dielectric constant reaches a steady state or saturation. This behavior reflects the frequency-dependent dielectric response intrinsic to AN1, AN2, AN3 and AN4 electrodes, demonstrating how the material’s microstructure and dopant distribution influence its overall polarization characteristics and electrical performance62. The low values of frequency might be due to the Cotton used for electrode formation however, the values of frequency increase from AN1 to AN4 NPs owing to the reason that Co ions concentration increases means that Co-doped SnS NPs can easily be polarized at low frequency and exhibit high relaxation time compared to undoped SnS NPs as evident from Fig. 16 (a, c). Figure 16 (b, d) shows the imaginary part of the dielectric constant plotted against the frequency and log frequency for AN1, AN2, AN3 and AN4 cotton electrodes, respectively. The imaginary part of the dielectric constant tells us the actual conduction phenomenon represents energy loss, which is greater at low frequency owing to free charge motion, indicating enhanced mobility of charge carriers, while with an increase in the frequency, the energy loss decreases due to a decrease in the space charge polarizations, and finally, it saturates above 5.12 Hz. The tangent loss is represented in Fig. 16e, which compares the dissipation factors and energy transmission, which is evaluated using the following formula62:

graphic file with name d33e1319.gif 13

Fig. 16.

Fig. 16

Open in a new tab

(a-d) real and imaginary parts of the dielectric constant versus frequency and log frequency (e) tangent loss versus log frequency (f) AC conductivity versus log frequency of the AN1, AN2, AN3 and AN4 NPs.

where υ and Rc are the frequency and capacitive time constant. The dielectric tangent loss increases with frequency and saturates after 5 MHz, that are attributed to the relaxation phenomenon. Further, for a better understanding of the conduction phenomenon of AN1, AN2, AN3, and AN4 NPs, ac conductivity σacis computed by using the following equation63:

graphic file with name d33e1339.gif 14

where Inline graphic is the permittivity in free space, Inline graphic is the relative permittivity and Inline graphic is the angular frequency. Figure 16 (f) shows the ac conductivity versus log frequency and Fig. 17 (a, b) shows the log ac conductivity versus frequency and log frequency. Both these figures of ac conductivity show that ac conductivity increases linearly at a low frequency up to 5 Hz, after that it increases drastically owing to the reason of dipole relaxation phenomenon64. The conductivity is low for the AN1 cotton electrode and increases with increasing the Co concentration. This means Co-ions increase the conduction of the charge migration of ions within the material. The Co-ions induce impurities, reduce the energy band gap, and increase the charge carrier concentration, which in turn enhances the conductivity65.

Fig. 17.

Fig. 17

Open in a new tab

(a, b) log AC conductivity versus frequency and log frequency (c) Nyquist plot (d) DC conductivity values versus Co concentration (e, f) real and imaginary parts of electrical modulus versus log frequency of the AN1, AN2, AN3 and AN4 cotton electrodes, respectively.

The electrical behavior and polarization of the electrode material are further explained by Nyquist plots of AN1, AN2, AN3, and AN4 cotton electrodes (Fig. 17e). The Nyquist plot of the AN1, AN2, AN3, and AN4 cotton electrodes reveals semicircles that decrease in diameter from AN1 to AN4. This suggests that an increase in Co dopant concentration results in a reduction of Rb values, which in turn enhances the DC conductivity. The following equation is used to calculate the DC conductivity of AN1, AN2, AN3, and AN4 cotton electrodes, respectively.

graphic file with name d33e1391.gif 15

where t and A are the thickness and area of the AN1, AN2, AN3 and AN4 cotton electrodes and Rb is the resistance of the AN1, AN2, AN3 and AN4 cotton electrodes, respectively and the calculated conductivity values are shown in Fig. 17f. The calculated conductivity values show a similar increasing trend from AN1 to AN4 cotton electrodes as observed from AC conductivity values owing to the similar reason that Co dopants enhance the conductivity of undoped SnS NPs.

Moreover, conducting thorough electrical modulus investigations can greatly enhance our understanding of the relaxation mechanisms and electrical conduction phenomena exhibited by dielectric materials. It distinguishes the microscopic mechanisms of the material that are responsible for dielectric relaxation. The complex electrical modulus is obtained by taking the inverse of complex dielectric permittivity, which may be expressed using the following relation:

graphic file with name d33e1416.gif 16
graphic file with name d33e1422.gif 17
graphic file with name d33e1428.gif 18

Figure 17 (c, d) shows the log frequency-dependent real and imaginary parts of the electrical modulus of AN1, AN2, AN3 and AN4 cotton electrodes, respectively. Both Inline graphic and Inline graphic is nearly zero and frequency independent in the low-frequency region up to 1 Hz. After that as frequency increases, the values of the Inline graphic and Inline graphic increases for AN1, AN2, AN3, and AN4 cotton electrodes and shows relaxation peaks at the high-frequency region in between 4 and 6 Hz. The region underneath the peak maximum corresponds to charge carrier long-range conduction. Charge carriers are constrained to potential wells by short distances above the peak maximum58. As the concentration of Co increases, the peaks of relaxation move and broaden towards the higher frequencies. The XRD data indicate that an augmentation in Co content leads to a decrease in strain, which can be correlated with the observed dielectric properties. Consequently, the conductivity of the AN1, AN2, AN3, and AN4 electrodes should increase due to the enhanced mobility of carriers inside the lattice and reduced resistance to conduction. Moreover, the observed increase in conductivity is supported by the reduced XRD crystallite size resulting from Co-doping.

Conclusion

In this research, Co-doped SnS NPs were successfully prepared by a simple solvothermal method, producing nanorods and nanospheres with orthorhombic crystal phases. The structural and morphological characterization (XRD, SEM, TEM) affirmed the uniform substitution of Co into the SnS lattice, whereas XPS deconvolution confirmed the existence of Sn, S, and Co binding states. The progressive enhancement in surface area from 31 to 383 m²/g upon increasing Co doping, as confirmed by BET studies, emphasizes the importance of dopants in maximizing electrochemical activity. The 20%Co:SnS electrode exhibited outstanding capacitive performance, maintaining 94% capacitance with 78% coulombic efficiency after 3000 cycles, thus expressing its long-term durability and stability. Furthermore, the enhanced electrical conductivity of the Co-doped SnS NPs at high Co content further supports the multifunctional applications of these nanostructures. Overall, these findings make Co-doped SnS NPs efficient candidates to be used as next-generation supercapacitors, dielectric devices, and microelectronic applications, and this provides a solid basis for their translation into useful energy storage devices.

Acknowledgements

This work was supported by Uniten for the BOLD Refresh Postdoctoral fellowship (Project code BOLDREFRESH; postdoc 10791206). The authors extend their appreciation to Northern Border University, Saudi Arabia, for supporting this work through project number (NBU-CRP-2025-2292).

Author contributions

Mohd Arif Dar; Writing-original draft, Conceptualization, Software, Methodology and visualization, Safwat A. Mahmoud; Resources and review and editing, S. R. Majid and M.Satgunam; Supervisor and review and editing, L. Guganathan; Resources, P. Arularasan; Resources, M. Karunakaran; Resources, S. Sheik Fareed; Resources, Hilal Ahmad Rather: Resources and review and editing, and Lamiaa Galal Amin; Resources.

Data availability

The data can be shared by the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Conflict of interest

The authors have no conflict of interest.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Mohd Arif Dar, Email: dararifphy@gmail.com.

Safwat A. Mahmoud, Email: Samahmoud2002@yahoo.com

S. R. Majid, Email: shana@um.edu.my

Hilal Ahmad Rather, Email: hilal@kiu.ac.ug.

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