Enhanced Electrochemical Performance of Supercapacitors via Two-Dimensional Indium Sulfide Heterostructure on Carbon Nanotubes

Abstract

This study reports on the synthesis and characterization of a novel electrode material for supercapacitor applications based on a clustered heterostructure of indium sulfide (In₂S₃) and single-walled carbon nanotubes (SWCNTs). The In₂S₃-SWCNT (INSC) sample was prepared using the facile successive ionic layer adsorption and reaction (SILAR) method and demonstrated a higher specific capacitance (258 Fg⁻¹ at 1 Ag⁻¹) compared to the bare indium sulfide (In₂S₃) electrode. The enhanced electrochemical performance is attributed to the synergistic effect between the In₂S₃ and SWCNTs, which improves electron transportation, catalytic nature, and specific capacitance. Moreover, the cyclic stability of the INSC electrode was significantly improved, retaining 96.8% of the initial capacitance after 3000 cycles. The high voltage holding capacity and high cyclic efficiency of the fabricated INSC-based supercapacitor devices suggest their potential for next-generation energy storage devices. Additionally, the INSC electrode-based supercapacitor devices exhibit excellent flexibility and bendability, retaining similar performance even at a bending angle of 180°, making them suitable for flexible energy storage applications.

1. Introduction

Recently, supercapacitors (SCs) with high performance, such as higher specific capacitance, faster charging and discharging, and a sustainable cyclic lifespan, have become highly necessary owing to the rapidly growing market for wearable electronics. The bendability and sustainable performance of the SCs are essential features in wearable devices. The design of novel electrode materials and assembly of the SCs could be the strategy to provide sufficient flexibility in the SCs. Transition metal sulfides (TMSs) have been attractive for SC applications due to their excellent conductivity, mechanical robustness, thermal stability, and great redox reaction activity [1,2]. As a prominent TMS nanomaterial, indium sulfide (In₂S₃) could be chosen as an excellent electrode material as it possesses a layered structure with stable structural form and high electrical conductivity [3]. In our previous report, In₂S₃ was prepared as an electrode material via a solvothermal method. However, the solvothermal method involves high temperatures during synthesis, and thereby, when adopted for the direct growth of electrode materials, NF substrates often become brittle, limiting their durability and adaptability in wearable applications [4]. Recently, 2D layered nanostructure materials have been the focus of research towards advanced electrode materials as they have a high surface-interface area, facilitating better electrochemical kinetics and charge storage. However, the material’s stability in itself is a major concern for its applicability as an electrode material.

SWCNTs are well explored toward enhancing the mechanical robustness and providing an efficient charge transfer channel, leading to more exposure of active sites while decreasing the risk of stacking or aggregation. Herein, we report the bare INS and SWCNTs-In₂S₃ heterostructure-based electrode materials prepared by a facile, room-temperature successive ionic layer adsorption and reaction (SILAR) method. This room-temperature synthesis route yields the layered SWCNT-In₂S₃ heterostructure directly grown on NF, which enables it to accommodate better robustness and flexibility. Furthermore, the SILAR method has benefits for controlling minute film thickness due to the consequential nature of film growth [5].

Here, we report (i) SWCNTs incorporated into a 2D In₂S₃ network heterostructure via a SILAR method of growth for SC applications, (ii) higher electrochemical performance of the SWCNT-In₂S₃ heterostructure in terms of specific capacitance, excellent cyclic retention, and unvarying performance even in 180° bending, demonstrating remarkable flexibility. (iii) Also, commercial reliability is tested by voltage holding, leakage current, and self-discharge. Interestingly, our SWCNTs incorporated into a 2D In₂S₃ heterostructure exhibited outstanding electrochemical performance, excellent cyclic stability, and 180° mechanical flexibility. Furthermore, the voltage holding capability, high cyclic efficiency, and operating power range of fabricated SSC devices demonstrate that the prepared electrode material can be an excellent candidate for next-generation energy storage devices.

2. Materials and Methods

The binder-free In₂S₃-SWCNT heterostructure-based electrodes were grown on NF by utilizing a two-step cyclic SILAR deposition method. All the chemicals utilized were of analytical grade and were used without any further treatment or purification. The chemicals indium chloride pentahydrate and thiourea were purchased from Sigma Aldrich (St. Louis, MI, USA). The single-walled carbon nanotube solutions (SWCNT) were procured from KH Chemicals Inc. The average diameter and length of SWCNT were 1.1–1.4 nm and 5 µm, respectively, with a G/D ratio of more than 100 [6]. The detailed method for electrode synthesis is as follows: A two-step cyclic SILAR process is employed for the deposition of electrode material. As a typical procedure for a one-cycle SILAR process, in which the nickel foam substrate (NF) was first immersed in an anion-rich solution (0.05 M Na₂S) and then immersed in a solution containing metal precursors (0.05 M) as the cationic source. This was followed by a dip in DI water in a beaker (25 mL volume) to remove any loosely bound corpuscles from the substrate. In each beaker, an NF substrate was dipped for 30 s after the previous beaker, and 20 cycles with the same methods were repeated consecutively for homogeneous, binder-free deposition on the surfaces of the substrates. Following that, the samples were dried in an oven at 60 °C for 12 h. For composite material preparation, one more beaker (1 mL SWCNT in 25 mL water) is kept between each SILAR cycle (after DI water).

2.1. Fabrication of an SSC Device

For the fabrication of solid-state symmetric (SSC) devices, a material-coated electrode sample on a NF substrate with a deposition area of (2 × 3) cm² was used. Three grams of PVA and three grams of potassium hydroxide were dissolved in thirty milliliters of deionized water to form a semi-solid electrolyte gel. Using 0.45-micron filter paper (Millipore) (procured from Merck Millipore) as a separator, the electrodes were sandwiched together and held at room temperature for two minutes after being soaked in electrolyte gel for two minutes. A protective coating of paraffin film is applied to the fabricated SSC device to prevent leaking and short-circuiting, and it is allowed to solidify at room temperature overnight to ensure optimal operation.

2.2. Characterization Techniques

The X-ray Diffraction (XRD) measurements were performed on a Rigaku SmartLab [Cu K] radiation (1.54 Å) source. The morphology of as-synthesized electrode material and EDS (energy-dispersive X-ray spectroscopy) were investigated using the JEOL/JSM-7800F field emission scanning electron microscopy instrument. The FTIR spectra for electrode material on NF substrate were acquired using an FT-IR microscope, the Hyperion 2000. Electrochemical studies on a fabricated two-terminal supercapacitor device were carried out using an IVIUM Technologies Compactstat.h electrochemical workstation. On NF, the mass loadings on INS and INSC are 1.8 and 2.2 mg/cm², respectively. The electrochemical performance of prepared, direct-grown electrode samples was investigated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques.

The following equation can be used to determine the specific capacitance (Cs) of electrode samples based on CV curves [7].

$$C_S=\frac{1}{ms\left(V_2-V_1\right)}{{\displaystyle \int }}_{V_1}^{V_2}I\left(V\right)dv$$

(1)

where “m” denotes the mass of electrode material deposited on the substrate as determined by the mass difference method, “s” denotes the scan rate, “I(V)” denotes the response current, and “(V₂ − V₁)” denotes the measurement potential window.

The specific capacitance can be determined from the GCD study.

$$C_S=Q/\left(\Delta V\times m\right)=\left(I\times \Delta t\right)/\left(\Delta V\times m\right)$$

(2)

I, Δt, ΔV, and m denote the utilized current, discharge time, accepted potential window of the GCD curve, and active mass deposited on the substrate.

The following formulae are utilized to calculate both parameters, energy density and power density, from GCD curves.

$$E=1/2\left(C_S\times V_{max}^2\right)$$

(3)

$$P=\left(E\times 3600\right)/\Delta t$$

(4)

V_max, E, and P are used to denote the maximum voltage, energy density, and power density, respectively.

3. Results and Discussion

A schematic representation of the SILAR process for the preparation of electrode materials is depicted in Figure 1a. INS nanostructures with stratified, yet interconnected clumps resembling tree reef corals’ morphology are shown in Figure 1b, grown on NF. On the other hand, INSC grown on NF substrate with an even more integrated layered clump nanostructure exhibiting cauliflower-like morphology is shown in Figure 1c. This SWCNT can improve the electrode surface properties, charge transport, and performance consistency of the electrode sample. The well-separated layered nanostructure enables better electrochemical reactions and provides a higher active electrode surface area than the as-prepared electrode samples.

The Substrate Preparation: The substrate is first cleaned and dried to remove any impurities or contaminants from its surface. This is important to ensure that the deposited film adheres well to the substrate.

Immersion in the Precursor Solution: The substrate is then immersed in a precursor solution containing a dissolved metal or metal compound. The substrate is kept in the solution for a predetermined amount of time to allow the precursor ions to adsorb onto its surface. In our case, the indium precursor is attached to the substrate via physical adsorption, which is mainly driven by Vander Waals force.

Rinsing: After the adsorption step, the substrate is rinsed with a suitable solvent to remove any excess precursor solution that may be on its surface.

Reaction: The rinsed substrate is then immersed in a second solution (a sulfide dioxide precursor), typically a reducing agent, which reacts with the adsorbed precursor ions to form a thin film of the desired material on the substrate surface. The reducing agent reacts with the precursor ions in a controlled manner to ensure the growth of a uniform and conformal film.

Drying: Finally, the substrate is dried to remove any remaining solvent or water molecules, leaving behind a thin film of the desired material on its surface.

XRD (X-ray diffraction) was used to examine the crystalline phase structure of both electrode materials, and the resulting spectra are shown in Figure 1d (Standard ICDD pattern 32-0456), which corresponds to the cubic structure of In₂S₃, which is found in the XRD spectrum of prepared samples [8]. In spectra, Miller indices are used to index all peaks. All peak positions have confirmed a cubic phase of In₂S₃ consistent with previously reported values. The molecular structure and functional group present in In₂S₃ were analyzed using the FTIR (Fourier-transform infrared spectroscopy) spectra, as shown in Figure 1e. The vibration of the O-H groups in chemisorb water molecules is characterized by a broad peak near 3224 cm⁻¹, the bending vibration of H₂O occurs at 1656 cm⁻¹, and the sulfonic group is characterized by signature peaks between 800 and 1500 cm⁻¹ [9].

3.1. Electrochemical Characterization

In Figure 2a, the electrochemical performance of the as-synthesized electrode materials (INS and INSC) is compared via cyclic voltammetry (CV) curves at a fixed scan rate of 100 mVs⁻¹. The INSC-based sample exhibits a larger enclosed area under the CV curve than the INS sample, indicating better electrochemical kinetics [10]. The calculated specific capacitance values of INS and INSC at various scan rates ranging from 10 mVs⁻¹ to 100 mVs⁻¹, are shown in Figure 2b. The normalized CV curves for INS and INSC are compared in Figure S9, which shows clearly the increase in enclosed area under the CV curve for INSC compared to INS [11,12,13]. Notably, the obtained specific capacitance for the INSC sample (210 Fg⁻¹) is significantly higher than the bare INS (112.7 Fg⁻¹), which clearly shows the effect of the SWCNT framework [6]. This improvement in the specific capacitance of INS with SWCNT heterostructure is attributed to the improved usage of redox species, enhancement of electrochemically available active sites, and quick charge transfer kinetics. The synergistic and cooperative contribution of SWCNT with INS materials boosted dopant (counter-anions) diffusion and improved conducting pathways for better electrolyte accessibility, which may be the reason for boosted electrochemical performance [14]. The CV measurements of the INSC-fabricated SSC device (Figure 2c) were performed at various potential windows ranging from 0.6 to 1.2 V, leading to an increase in the active integrated area under the curve. By scanning the CV data at various potential windows, the behavior of the supercapacitor is estimated under different operating conditions. For example, scanning at a wider potential window provides information on the capacitance and energy density of the device at high voltages, while scanning at a narrower potential window can reveal the capacitance and power density at low voltages. Overall, scanning the CV data of an INSC-based SSC device at various potential windows provides valuable information on its performance characteristics, leading to the optimization of its design for specific applications.

Figure 2d shows comparative galvanostatic charge-discharge (GCD) curves for INS and INSC samples. Higher discharge times observed in sample INSC (258 Fg⁻¹) show better capacitance performance and match the CV result. Figure 2e shows the calculated specific capacitance values of the sample at different current densities. Figure 2f shows GCD curves of sample INSC at different potential windows, ranging from 0.6 to 1.6 V, indicating the wide operating area of the prepared SSC device. Redox reactions in the presence of hydroxide counter-electrolyte anions can be formulated as follows for the ionic charge storage behavior of In₂S₃ [4].

SWCNTs possess unique electronic, optical, and mechanical properties, making them a popular component in heterostructures. Incorporating SWCNTs into an In₂S₃ (INS) heterostructure can lead to various effects, such as band alignment, charge transfer, optical properties, and mechanical properties. The resulting enhancement in the electrochemical performance and mechanical stability of INS in the heterostructure is significant.

The electronic properties of the heterostructure can be influenced by the SWCNTs’ band alignment, which creates a built-in electric field that affects the flow of charge carriers. Furthermore, the high mechanical strength and flexibility of SWCNTs can improve the durability and flexibility of the device. The SWCNTs can also interact with INS through charge transfer, which can alter the electronic properties of the device and the valence state of the materials. The effect of SWCNTs on the valence state of the INS heterostructure is complex and dependent on various factors, including the nature of the interaction, the energy level alignment between the materials, relative positions in the heterostructure, and the chemical properties of the materials. According to density functional theory (DFT) analysis, the work function of SWCNTs is higher than that of In₂S₃, resulting in a downward band bending at the interface and a potential barrier for electron transfer. However, the study found that the barrier height is relatively low, allowing for efficient electron transfer across the heterojunction [15].

In₂S₃ + OH⁻↔ In₂S₃OH + e⁻

(E1)

Figure 2g illustrates the Ragone plot (a graph between power density and energy density) of samples INS and INSC. For SC applications, the Ragone plot can analyze the functionalities and operational material efficiency [16]. Sample INSC exhibits a high energy density of 22.93 Whkg⁻¹ at a power density of 400 Wkg⁻¹. CV measurements at 5 mVs⁻¹ were performed to evaluate its cyclic stability (Figure 2h) for 3000 cycles. Interestingly, sample INSC (96.8%) displays outstanding cyclic retention compared to the INS sample (90%). Swelling and shrinkage are speculated to be two main factors that influence the stability of electrode materials. Thus, significant enhancement of cyclic stability is attributed to SWCNTs enabling physical reinforcement, leading to a robust morphology of the heterostructure. In addition, the insertion of SWCNTs into electrode materials significantly increases the stability behaviors of the electrodes by forming π−π bonds [17]. This increases the number of active sites and creates a more conductive channel, allowing greater electrode-electrolyte interaction. SWCNTs establish cross-linking with layers, which increases conductivity and, as a result, improves the electrochemical performance [18].

Electrochemical impedance spectroscopy (EIS) is used to examine the charge transfer and estimate the impedance parameters of the electrode material. Figure 2i shows that the EIS spectra of the fabricated SSC device range from 100 kHz to 0.01 Hz with a bias voltage of 0.1 V. The nearly identical shape of the EIS curves implies that the charge is channelized comparably in each material sample. A smaller semicircle, smaller intercept, and more vertical line toward the low-frequency side suggest a greater mass transfer limit, faradic charge transport, and reduced inter-facial charge resistance in the sample INSC. The estimated impedance parameters for both the electrode samples are tabulated in Table S2 in Supporting Information.

3.2. Self-Discharge Study

Self-discharge (SD) behavior and voltage holding (VH) capabilities are essential performance factors to consider when evaluating the applicability of supercapacitors for practical applications. After charging, the rapid self-discharging of SCs limits their employability on an industrial scale. Figure 3a shows four 100-cycle GCD measurements for SSC devices from INSC samples, followed by 2 h VH and 2 h SD tests. Figure 3b shows a sequence of GCD (100 cycles), followed by a 2-h VH test for an INSC SSC device. The GCD cycling was done at 1 Ag⁻¹ from 0 to 1.2 V, and furthermore, VH tests were performed at 1.2 V for 2 h, and SD tests were performed at OCP (open circuit potential), that is, in floating condition. Over two hours, a four-cycle VH test showed almost no internal resistance at the device level. Resistance and diffusion charge redistribution are important factors determining SD in SC devices [19]. The device was examined for SD, VH, and leakage currents, and the results showed that SD mitigation was due to improved nanostructural integrity in the fabricated SSC device. We exhibited a fabricated SSC device for LED illumination to further validate its practical application.

Two SSC devices, connected in series, are extremely efficient when used to illuminate red commercial LED lights (Figure 3c). The charge transfer behavior of ions in a layered nanostructure for charge storage is depicted in the scheme in Figure 3d. Excellent electrochemical performances may indeed result from (i) a well-defined layer structure; (ii) the provision of highly accessible interfacial contacts due to the layered facets of 2D In₂S₃, which allows for greater use of the ion accumulation; and (iii) decreased internal material resistance due to the incorporation of conducting SWCNTs, which results in a shorter electron diffusion length during the charge/discharge process [20]. All electrochemical results substantially support the merits of the developed heterostructure embedded with INSC electrode material as a high-performance supercapacitor application.

Mechanical flexibility testing is a crucial step in ensuring the overall performance, robustness, and longevity of a device. This study evaluates the ability of the SC device to maintain its functionality under various bending angles. In particular, the mechanical flexibility testing was performed on a SSC device, and the results are shown in Figure 4a–e. For comparing the electrochemical performance, the CV curves of the SSC device were plotted in Figure 4f at a scan rate of 100 mVs⁻¹, which revealed the capacitance performance of the device. Essentially, it was observed that the capacitance performance of the SSC device did not degrade even when the bending angle was increased from 0° to 180°, which is a significant deviation from the flat (0°) condition.

This finding demonstrates the excellent flexible nature of the fabricated INSC-based SSC device and indicates that the device’s capacitance performance remains unaffected even under extreme bending conditions. This, in turn, ensures the device’s long-term durability and robustness, as it can maintain its functionality even in challenging environments. Thus, the test of mechanical flexibility plays a vital role in evaluating the performance, robustness, and longevity of a device. The results of the mechanical flexibility testing performed on the MSC device show that it has excellent flexible properties and does not degrade even under extreme bending conditions, making it an ideal choice for applications that require flexibility and robustness.

4. Conclusions

The newly developed 2D In₂S₃ heterostructure electrode material exhibits high capacitance, high power density, and exceptional cycle retention properties. INSC heterostructure offers efficient charge transfer and a stable structure with increased active sites and reduced stacking or aggregation, contributing to its impressive electrochemical and stability performance. The 2D INSC material demonstrates its potential as a next-generation electrode material for supercapacitor applications, offering improved performance and stability over traditional electrode materials. The results suggest this material could be used in energy storage and power conversion systems. Overall, the 2D In₂S₃ heterostructure is a promising option for supercapacitor applications due to its high capacitance, power density, and cycle retention properties.