Synthesis and Characterization of Fe_0.5Co_0.5S/Ag-Citrate for Energy Storage Applications ^†

Abstract

Supercapacitors are widely recognized for their high power and energy densities. This study explores Fe_0.5Co_0.5S and its Ag-citrate composite for supercapacitors. Synthesized via coprecipitation, the composite was characterized using SEM and XRD, confirming its formation. Electrochemical tests revealed enhanced performance: CV oxidation current rose from 16.5 mA to 33.89 mA, GCD discharge time increased from 44.8 s to 129 s, and specific capacitance jumped from 37.3 F/g to 107.5 F/g—nearly threefold. EIS results also improved. The Ag-citrate addition boosted conductivity and capacitance, making the composite a promising supercapacitor material.

1. Introduction

With the increasing demand for efficient and sustainable energy storage solutions having their application in portable and large appliances including vehicles, hospital equipment. The electrode materials gained a vide interest in research [1,2]. FeCoS-based materials have gained significant attention due to their excellent magnetic, electrical, and catalytic properties [3,4]. These materials exhibit high electrical conductivity and mechanical stability, making them suitable for energy storage applications, particularly in supercapacitors and batteries [5]. However, their electrochemical performance is often hindered by agglomeration and surface oxidation [6,7].

Functionalization with Ag-Citrate has been explored as a means to enhance the electrochemical properties of Fe_0.5Co_0.5S particles [6,7]. Silver nanoparticles provide superior conductivity, while citrate acts as a stabilizing agent, preventing aggregation and improving dispersion [7]. Prior studies have demonstrated that citrate-stabilized nanoparticles exhibit improved charge transfer kinetics, resulting in enhanced capacitance and energy storage efficiency [1,2,3,4]. The primary objective of this research is to synthesize Fe_0.5Co_0.5S/Ag-Citrate nanoparticles and investigate their structural and electrochemical properties to assess their potential for energy storage applications [7].

The development of anode material is crucial for supercapacitor application. FeCoS played vital role due to their high theoretical capacity and favorable electrochemical properties but their significant volume expansion during charge discharge cycle and low electrical conductivity have limited their practical application. However, researchers have explored their integration with hollow structures like carbon, graphene and other hollow structures. Bimetallic Sulfides are renowned for their high specific capacity, high redox activity and friendly nature for their use in supercapacitor application. However, their flower like structure synthesized with one step hydrothermal method coated with RGO found with cycling stability of 371 mAh g⁻¹ at 100 mA g⁻¹ after 150 cycles and rate performance of 428 mAh g⁻¹ at 50 mA g⁻¹ and 248 mAh g⁻¹ at 1000 mA g⁻¹ [8]. Further, 3D nanocomposites of bimetallic sulfides based on FeCoS₂ Prepared by coaxial electrospinning method and nitrogen doped hollow nanocarbon, high temperature calcination and solvothermal process. As a binder free anode material for potassium ion batteries found promising cycling stability superior cycling stability (132.6 mAh g⁻¹ at 3200 mA g⁻¹ after 600 cycles) [9]. To cater for the agglomeration problem of Sulfides, FeCoS₄ nanoparticles were embedded in S-doped hollow carbon composites were synthesized by combination of hydrothermal process and sulfidation treatment. Prepared composite showed excellent structural stability with specific area of 303.7 m² and pores volume of 0.93 cm³ g⁻¹ and exhibit efficient specific capacity of 970.2 mA·h·g⁻¹ at 100 mA·g⁻¹ and enhanced cycling stability (574 mA·h·g⁻¹ at 0.2 A·g⁻¹ after 350 cycles) [10].

Here in this work, we synthesised Fe_0.5Co_0.5S using the coprecipitation method and subsequently composited it with silver citrate (Ag-citrate) to enhance energy storage performance. Its SEM and XRD analyses confirmed the successful formation of the composite. In addition, the electrochemical analysis revealed an increase in oxidation current from 16.5 mA to 33.89 mA, discharge time from 44.8 s to 129 s, and specific capacitance from 37.3 F/g to 107.5 F/g. These findings underscore the enhanced electrical conductivity and capacitance of the composite, establishing its potential for supercapacitor applications.

2. Experimental

2.1. Synthesis of Fe_0.5Co_0.5S

Fe_0.5Co_0.5S nanoparticles were synthesized via the ultrasonication method. Initially, 1.386 g of hydrated FeCl₂ was dissolved in 50 mL of deionized (DI) water, followed by the addition of 2.617 g of CoCl₂·6H₂O in the same beaker. The mixture was placed on a magnetic stirrer for thorough mixing. Separately, 5.52 g of Na₂S·5H₂O, serving as the sulfur source, was dissolved in 30 mL of DI water. Once this solution was fully homogenized, it was added dropwise into the first beaker under continuous stirring. The combined solution was then stirred at 80 °C to facilitate the reaction. The resulting mixture was washed sequentially using DI water, ethanol, acetone, and again DI water through centrifugation at 6000 RPM for 20 min. Finally, the product was dried in a vacuum oven at 80 °C for 12 h, yielding Fe_0.5Co_0.5S particles.

2.2. Synthesis of Fe_0.5Co_0.5S/Ag Citerate

For Ag-Citrate functionalization, an aqueous solution of silver nitrate (AgNO₃) was prepared and mixed with a trisodium citrate solution under stirring. The Fe_0.5Co_0.5S particles were then added, and the reaction mixture was maintained at 80 °C to facilitate functionalization. The final product was collected by centrifugation, washed, and dried for further analysis. Fe_0.5Co_0.5S particles and Ag-citrate particles were mixed in a 10:1 weight ratio using solid-state grinding in a mortar and pestle. The resulting mixture yielded the Fe_0.5Co_0.5S/Ag-citrate composite. For the preparation of electrodes for electrochemical testing, both Fe_0.5Co_0.5S and Fe_0.5Co_0.5S/Ag-Citrate were dropped cast onto individual nickel foam and left to dry for 12 h at 80 °C in an oven.

2.3. Characterization

The surface morphology and elemental composition, crystal structure and phases, and electrochemical measurements of potential samples were performed using energy-dispersive X-ray spectroscopy (EDX)-coupled scanning electron microscopy (SEM, ZEISS EVO15, Cambridge, UK), X-ray diffractometer (XRD, Proto, AXRD LPD, London, UK) and galvanostat (Gamry Instruments, Reference 3000, Warminster, PA, USA), respectively.

3. Results and Discussion

3.1. Morphology Analysis

Figure 1a–c presents SEM images of Fe_0.5Co_0.5S at different magnifications, displaying predominantly spherical particles with a relatively uniform size distribution in the range of ~80–150 nm. The morphology suggests a well-controlled coprecipitation process. Figure 1d–f depict the Fe_0.5Co_0.5S/Ag-citrate composite, where Ag-citrate particles are clearly observed as a uniform coating over the Fe_0.5Co_0.5S surface, indicating strong physical attachment and good interfacial contact. This effective surface coverage is likely to facilitate improved charge transport and redox activity, as reflected in the enhanced electrochemical performance discussed in Section 3.4. Additionally, the presence of slight agglomeration is noticeable, which may be attributed to electrostatic interactions or Van der Waals forces among the particles. Despite this, no significant structural damage or surface degradation was observed; both pristine and composite samples retained relatively smooth morphologies. The preservation of structural integrity, combined with effective surface modification, highlights the potential of Ag-citrate functionalization in boosting the electrochemical efficiency of Fe_0.5Co_0.5S-based materials.

3.2. Elemental Analysis

Elemental area mapping of Fe_0.5Co_0.5S, as shown in Figure 2a–c, reveals a uniform distribution of Fe, Co, and S elements across the sample. Minor variations in the atomic percentages of these elements are likely attributed to particle agglomeration or slight surface oxidation. Similarly, Figure 2d–i demonstrates the homogeneous dispersion of C, O, S, Ag, Fe, and Co, clearly confirming the successful integration of Ag-citrate with Fe_0.5Co_0.5S. This uniform elemental distribution serves as strong evidence of effective composite formation.

The Energy Dispersive Spectroscopy (EDS) spectrum in Figure 3, with energy on the x-axis and intensity on the y-axis, confirms the presence of characteristic X-ray peaks for Fe (K, L), Co (K, L), and S (K), indicating the successful synthesis of Fe_0.5Co_0.5S. Additionally, the presence of Ag (L), C (K), and O (K) in Figure 3 confirms the formation of the Ag-citrate/Fe_0.5Co_0.5S composite. The relatively high oxygen peak may be attributed to surface oxidation or residual by-products during the curing process.

3.3. Phase and Crystal Structure Analysis

X-ray diffraction (XRD) analysis confirmed the successful synthesis of Fe_0.5Co_0.5S and its composite with Ag-citrate. The resulting peaks were obtained by setting the X-ray tube at an angle ranging from 10° to 90°; see Figure 4a,b. The diffraction peaks matched JCPDS card No. 01-075-0253, validating the formation of the Fe_0.5Co_0.5S phase. Additional peaks at ~40° and 65° 2θ, corresponding to the (111) and (220) planes of metallic silver, confirm the incorporation of Ag-citrate. The absence of peak shifts or distortions indicates physical attachment rather than alloying, preserving the crystallinity and phase purity of both components—factors expected to enhance the composite’s electrochemical performance. In addition, some unidentified peaks might be due to the surface oxidation of particles and some background noise.

3.4. Electrochemical Analysis

3.4.1. Cyclic Voltammetry (CV)

For electrochemical analysis, a Gamry setup was employed. To conduct the subsequent measurements, a 1 M potassium hydroxide (KOH) solution was prepared in a 50 mL beaker. Three electrodes were immersed in the KOH electrolyte: the prepared (working) electrode, a reference electrode (Hg/HgO), and a counter electrode.

For cyclic voltammetry (CV) analysis, the prepared electrodes were initially stabilized at scan rates of 10 mV/s and 20 mV/s within a potential window of 0 to 0.7 V for 20 cycles; see Figure 5a–c. Subsequently, CV measurements were conducted over a range of scan rates from 2 mV/s to 50 mV/s. The cyclic voltammetry (CV) curves confirm the hybrid behavior of the particles, as seen in the distinct curve shapes. The oxidation and reduction peaks for Fe_0.5Co_0.5S are at 16.51 mA and −10.4 mA, while for Fe_0.5Co_0.5S/Ag-Citrate, they are at 33.89 mA and −25.76 mA. The CV curve shape, showing a mix of sharp and broad peaks, indicates hybrid capacitive and pseudo-capacitive behavior. This enhances charge storage, reflecting the synergistic effect of Fe_0.5Co_0.5S and Ag-citrate. The increased current for the composite suggests improved electrochemical performance due to Ag-citrate’s enhanced conductivity and redox activity.

3.4.2. Galvanostatic Charge Discharge (GCD)

For the GCD (Galvanostatic Charge-Discharge) analysis, the electrodes were initially charged at constant current densities ranging from 0.5 A/g to 4.0 A/g, with an increment of 0.5 A/g, up to a maximum potential of 0.6 V. In this section, only the discharge times were analyzed to evaluate the retention capacity. Figure 6a,b presents the galvanostatic charge-discharge (GCD) curves of Fe_0.5Co_0.5S and Fe_0.5Co_0.5S/Ag-Citrate composites at various current densities. As the current density increases, the discharge time decreases due to rapid depletion of stored charge and elevated internal resistance, leading to a faster voltage drop. Figure 6c illustrates a significant improvement in discharge time upon compositing with Ag-citrate, increasing from 44.8 s to 129 s at 0.5 A/g. Furthermore,

Table 1. Specific capacity and specific capacitance calculations for Fe_0.5Co_0.5S.

Sr. No	Potential Window (V)	Current Density (A/g)	Discharge Time (s)	Specific Capacity (C/g)	Specific Capacitance (F/g)
1	0.6	0.5	44.8	22.4	37.3
2	0.6	1	14.5	14.5	24.2
3	0.6	1.5	7.2	10.8	18
4	0.6	2	4.1	8.2	13.7
5	0.6	2.5	2.4	5.9	9.8
6	0.6	3	1.4	4.2	6.9
7	0.6	4	0.3	1.2	2.1

summarizes the specific capacity and specific capacitance values for Fe_0.5Co_0.5S/Ag-Citrate, showing maximum values of 22.4 C/g and 37.3 F/g at 0.5 A/g. In comparison,

Table 2. Specific capacity and specific capacitance of Fe_0.5Co_0.5S/Ag-Citrate.

Sr. No	Potential Window (V)	Current Density (A/g)	Discharge Time	Specific Capacity (C/g)	Specific Capacitance (F/g)
1	0.6	0.5	129	64.5	107.5
2	0.6	1	43	43	71.7
3	0.6	1.5	22.5	11.2	18.8
4	0.6	2	14	14	23.3
5	0.6	2.5	10	15	25
6	0.6	3	7	14	23.3
7	0.6	4	3	7.5	12.5

presents the corresponding data for the Fe_0.5Co_0.5S/Ag-Citrate composite, which exhibits enhanced values of 64.5 C/g and 107.5 F/g at the same current density. These results clearly underscore the beneficial impact of Ag-citrate integration on the electrochemical performance of Fe_0.5Co_0.5S.

3.4.3. Electrochemical Impedance Spectroscopy (EIS)

EIS was performed to evaluate prepared electrodes to evaluate charge storage behavior and interfacial kinetics. In Figure 7a–c, the Nyquist plots and electrochemical impedance spectroscopy (EIS) data for Fe_0.5Co_0.5S and Fe_0.5Co_0.5S/Ag-Citrate composites are presented. The Fe_0.5Co_0.5S/Ag-citrate composite exhibits a reduced charge transfer resistance (R_ct) compared to Fe_0.5Co_0.5S, evident from the smaller semicircular arc in the high-frequency region. Enhanced charge transport is further supported by the sharper low-frequency slope, indicating improved ion diffusion. The incorporation of Ag-citrate boosts electrical conductivity, leading to lower overall impedance and better electrochemical performance. These findings align with the improved specific capacitance and charge storage properties observed in the composite material.

4. Conclusions

The Fe_0.5Co_0.5S/Ag-Citrate composite, synthesized via coprecipitation and functionalization, exhibits superior electrochemical performance for supercapacitor applications. Structural characterization (SEM, XRD, EDS) confirmed successful composite formation with uniform elemental distribution. Electrochemical tests demonstrated a twofold increase in oxidation current (16.5 mA to 33.89 mA), threefold longer discharge time (44.8 s to 129 s), and enhanced specific capacitance (37.3 F/g to 107.5 F/g). Reduced charge transfer resistance and improved ion diffusion, revealed by EIS, highlight Ag-citrate’s role in boosting conductivity. These results position the composite as a promising high-performance electrode material, paving the way for advanced energy storage solutions. Further studies on cycling stability and scalability are recommended.