Performance Assessment of Fe_0.5Cu_0.5S/rGO Hybrid Composite as Potential Material for Advanced Energy Storage Applications ^†

Abstract

Transition metal sulfides have found a popular spot in research for super capacitive materials due to their enhanced power density and conductivity. This study reports the preparation of a hybrid iron copper sulfide, Fe_0.5Cu_0.5S/rGO, composite via the co-precipitation method. The structural and morphological characterization was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM), which confirmed the successful integration of Fe_0.5Cu_0.5S with rGO. The composite exhibited a high specific capacitance of 416.91 F/g at 1 A/g, 330.65% higher than 96.81 F/g of Fe_0.5Cu_0.5S and outstanding cyclic stability. The enhanced performance can be attributed to the synergistic effects of Fe_0.5Cu_0.5S and rGO, facilitating efficient charge transfer kinetics, ion diffusion, and structural stability, making it a promising candidate for high-performing supercapacitor applications.

1. Introduction

In the development of the energy storage system, demand for efficient energy storage has become necessary. Supercapacitors have become a renowned energy storage device due to their rapid charge–discharge capability, high power density, and great cyclic stability compared to the batteries [1,2]. As a supercapacitor combines both battery and capacitor-like properties in one device, it has become a research hub for energy storage. There are three main types of supercapacitors: electric double layer capacitors (EDLCs), pseudo capacitors, and hybrid supercapacitors. EDLCs store charge electrostatically at the electrode–electrolyte interface while pseudo capacitors store charges through faradaic reactions, which is reduction and oxidation, and hybrid supercapacitors combine both EDLCs and pseudo capacitor behavior, which stores charge electrostatically and through redox reactions, increasing its overall energy density and maintaining high power density [3]. These properties have made hybrid supercapacitors a prime focus [4].

Globally, transition metal sulfides have been researched because of their good conductivity and high specific capacitance, as well as their high abundance [5]. Compared to other materials used, such as oxides and hydroxides, they have shown better results. Singhal et al. developed a CuS/rGO composite exhibiting a specific capacitance of 207 F/g at a scan rate of 0.5 A/g [6]. In another work Sandhya et al. reported an FeS₂ electrode having a specific capacitance of 206 F/g at 1/g [7]. However, using a single metal comes with a challenge due to its low stability. Incorporating different metal ions aids in capturing the synergistic benefits of differing metals, allowing several transition metal ions to take part in the oxidation–reduction reaction, which increases the specific capacitance and conductivity of the metal sulfides. The incorporation of other metals into sulfide compounds helps address the limitations of single-phase materials including inadequate redox activity, weak electrochemical performance, low conductivity, short cycle life, and enhances the composite materials’ properties [8]. Furthermore, the electrode can be optimized by compositing these sulfides with reduced graphene oxide (rGO). rGO may act to provide an improved conductivity and shorter diffusion paths for the electrolyte ions. This will allow for an efficient ion transport and rapid interaction of electrolyte ions with the electroactive sulfides/rGO layers [6].

Despite the growing interest in bimetallic sulfide-based composites, the specific combination of Fe-Cu in 1:1 ratio coupled with rGO remains largely unexplored. In this study, we have synthesized a novel transition metal sulfide, an Fe_0.5Cu_0.5S/rGO hybrid composite, through co-precipitation method followed by solid state mixing with rGO to have an advanced electrode material for energy storage applications. The composite electrode exhibited impressive specific capacitance of 416.91 F/g, which is 330.65% greater than 96.81 F/g of bare Fe_0.5Cu_0.5S. This positions our material as a competitive not only among rGO-based systems but also in comparison with CNT containing compounds where some popularly reported capacitances were 130.8 F/g at 5 A/g for a PIM/CuS@CNT electrode [9] and 219 F/g at 1 A/g for a CuFeS₂ electrode [10]. The structural, morphological analysis was performed using X-ray diffraction (XRD) and a scanning electron microscope (SEM), whereas the electrochemical analysis was performed on a potentiostat.

2. Experimental

2.1. Materials

Material used were iron chloride (FeCl₃·6H₂O; > 76%) from Sigma-Aldrich, copper nitrate (Cu(NO₃)₂·6H₂O; >98.0%) from Sigma-Aldrich, and sodium sulfide (Na₂S·9H₂O; >99.9%) from Duksan; nickel foam was utilized as substrate and lab-grade rGO was used without further modification.

2.2. Synthesis

Taking two separate beakers, in beaker A 2.95 g of iron chloride (FeCl₃·6H₂O) and 2.63 g copper nitrate (Cu(NO₃)₂·6H₂O) were added in 50 mL of DI water and stirred at 800 rpm for 20 min. In beaker B, 5.25 g of sodium sulfide (Na₂S·9H₂O) was added in 30 mL of DI water and stirred at 800 rpm for 20 min. Beaker A solution was heated at 70 °C and stirred at 1200 rpm, beaker B solution was then added drop by drop into beaker A and stirred at 1200 rpm for 1 h at 70 °C. The solution was then centrifuged at 6000 rpm, alternatively using DI water and dilute ethanol for 3 cycles each. The sample was then air dried in an oven at 90 °C and calcined at 400 °C for four hours maintaining a constant heating rate of 2 °C/min.

This was followed by slurry preparation of iron copper sulfide (Fe_0.5Cu_0.5S) and the nanocomposite, respectively. For A1 (Fe_0.5Cu_0.5S electrode), 12 mg of prepared binary sulfide was added into a slurry tube containing 30 µL polyvinylidene fluoride (PVDF) and 40 µL N-Methyl-2-pyrrolidone (NMP) and stirred at 360 rpm for 8 h. For A2 (Fe_0.5Cu_0.5S/rGO, 10:1 electrode), 10.8 mg of active material and 1.2 mg of reduced graphene oxide (RGO) were added into a slurry tube containing 30 µL PVDF and 40 µL NMP and stirred at 360 rpm for 8 h. Each slurry was deposited on separate nickel foam and dried at 80 °C for 10 h (as shown in Figure 1).

The characterization of prepared Fe_0.5Cu_0.5S and Fe_0.5Cu_0.5S/rGO electrode included X-ray diffraction (XRD, AXRD LPD, Proto, ON, Canada), scanning electron microscopy, and energy dispersive X-ray spectroscopy (Carl Zeiss Evo 15 SEM, Jena, Germany), and electrochemical analysis (Gamry Potensiostat, Reference 3000, Warminster, PA, USA).

3. Results

3.1. X-Ray Diffraction

The X-ray diffraction results show the successful synthesis of different phase constituents in the Fe_0.5Cu_0.5S/rGO composite. The characteristic peaks such as 29.34° (111), 33.11° (200), 40.87° (210), 54.05° (220), and 64.0° (311) are available in the reference chart JCPDS card No. 35-0752 of the iron copper sulfide phase (Figure 2a). These peaks relate to the crystalline structure of Fe_0.5Cu_0.5S. The broad peak at 25.2° (002) can be related to the graphitic (002) plane of reduced graphene oxide (Figure 2b) as per JCPDS card No. 75-1621, which suggests successful inclusion of rGO into the composite. Moreover, some extra and unidentified peaks can be attributed to residues and/or surface oxidation of particles inside the furnace during annealing.

3.2. Scanning Electron Microscopy

The surface morphology was attained through SEM. Figure 3a1, b1, a2, b2 depict the surface morphology of Fe_0.5Cu_0.5S and Fe_0.5Cu_0.5S/rGO, respectively. It can be observed that Fe_0.5Cu_0.5S has been aggregated and has a spherical morphology with an average particle size of approximately 107.46 nm (Figure 3). As for the composite structure, it can be noticed that iron copper sulfide particles are dispersed onto the rGO sheets. The spherical particles tend to provide a much uniform surface area and better pathways for ion diffusion. These spherical particles also tend to attach better with the rGO sheets, leading to faster ion movement during charge/discharge cycles and an improved overall performance.

3.3. Energy Dispersive Spectroscopy

The distribution of S, Fe, and Cu in Fe_0.5Cu_0.5S (Figure 4b1–d1) appears to be uniform, and a similar pattern can be observed in Fe_0.5Cu_0.5S/rGO (Figure 4b2–e2) for S, Fe, Cu, and C, which indicates the successful synthesis of electroactive materials. The uniform distribution of C after rGO addition substantially proves that its presence may add to conductivity without effecting the homogeneity of Fe, Cu, and S. The uniformity as a whole suggests that a more effective charge transfer pathway is present leading to enhanced electrochemical performance.

As from the EDX analysis of the elemental composition of Fe_0.5Cu_0.5S (Figure 5a) and the Fe_0.5Cu_0.5S/rGO composite (Figure 5b), an observation may be drawn that both samples comprise Fe, Cu, and S. The presence of carbon peak in the composite ascertains the successful incorporation of rGO into the composite. A high atomic percentage of C (46.83%) attests the uniform distribution of rGO in the composite and might contribute as structural support for charge storage and enhances of the electric conductivity.

3.4. Cyclic Voltammetry (CV)

Herein, the electrochemical behavior of A1 (Fe_0.5Cu_0.5S NPs) and A2 (Fe_0.5Cu_0.5S/rGO; 10:1) was analyzed by the cyclic voltammogram obtained from a three-electrode system in 1M KOH solution ranging from 3 to 100 mv/s scan rates (Figure 6). The CV curves indicated the pseudo capacitive behavior of A1 and A2 by exhibiting distinct redox peaks confirming faradic charge storage. The Fe_0.5Cu_0.5S/rGO composite generally exhibited an increased area under the CV curve, indicating enhanced capacitance. A comparison was drawn between A1 and A2 at 10 mV/s with +29.9 mA and +69.26 mA as peak oxidation currents and −23.79 mA and −51.82 mA as peak reduction currents. The higher redox current values of the composite thus confirmed its superior electrochemical properties.

3.5. Galvanostatic Charge Discharge (GCD)

The GCD cycles have been obtained for A1 and A2 using a three-electrode system in 1M KOH solution (Figure 7). The deviation from perfectly rectangular discharge curve suggests the contribution of both an electrostatic double layer and Faradic charge storage mechanism over a current density ranging from 0.5 to 6 A/g and a potential window of 0–0.55 mV/s. The graph also suggests discharge time shifting from 122 s in A1 to 227 s in A2, validating the superior performance of the composite. Also, from the GCD curves, the specific capacitance was calculated using the following equations:

$$Q_s={\displaystyle \frac{(I\times ∆t)}{m}}$$

(1)

$$C_S={\displaystyle \frac{Q_s}{∆V}}$$

(2)

The specific capacitance values, as from the GCD curves, were calculated to be 96.81 F/g and 416.91 F/g for A1 and A2, respectively, at a charge density of 1A/g, respectively.

3.6. Nyquist Plots

In the electrochemical analysis, internal resistance were measured (Figure 8). EIS was conducted, which is a powerful technique to determine the properties of supercapacitors. As the figure indicates, the impedance is decreasing in A2 compared to A1 due to improved conductivity behavior by rGO addition.

4. Conclusions

In this work, the preparation of the Fe_0.5Cu_0.5S/rGO composite, by taking the co-precipitation route for Fe_0.5Cu_0.5S synthesis, is shown to be an effective strategy. The structural and morphological characterization confirmed the presence of spherical Fe_0.5Cu_0.5S particles on the rGO sheets. The introduction of rGO tremendously improved the electrochemical properties with specific capacitance enhancement by 330.65% from 96.81 F/g to 416.91 F/g at 1 A/g and discharge time increased from 122 s to 227 s, hence indicating enhanced charge storage and retention capacity. It can thus be concluded that Fe_0.5Cu_0.5S/rGO can serve as a potential material for next-generation supercapacitors with combined high capacitance and long-term stability.