One-Step Synthesis of Zirconium Sulfide Nanoparticles on Flexible Carbon Cloth for Supercapacitor Application

Abstract

Zirconium sulfide nanoparticles (Zr_xS_y) are prepared on a flexible substrate of carbon cloth (CC) via a one-step synthesis approach using the low-pressure chemical vapor deposition (LPCVD) technique. The scanning electron microscopy (SEM) image reveals that the particle sizes are in the range of ca. 3~23 nm with an average value of ~13.02 nm. The synthesized Zr_xS_y nanoparticles are composed of ZrS₃ and Zr₉S₂ phases, which is verified by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). By using the Zr_xS_y/CC as a supercapacitor flexible electrode, the capacitance extracted from the cyclic voltammetry measurement is 406 C g⁻¹ at a scan rate of 5 mV s⁻¹; the capacitance values obtained from GCD curves at a current density of 0.5 A g⁻¹ and 1 A g⁻¹ are 151 and 134 C g⁻¹, respectively. These results highlight the promising potential of Zr_xS_y as a supercapacitor material for future energy-storage technology.

1. Introduction

In recent years, with the increasing global emphasis on low carbon technologies and green energy, the inherent limitations of traditional energy storage systems, such as batteries, have become increasingly evident in terms of charge–discharge efficiency, power density, and energy density. In contrast, supercapacitors, with their unique combination of double-layer capacitance and pseudocapacitive effects, show great potential for various applications. This has attracted considerable attention, especially with the rising demand for fast-charging energy storage systems in fields such as medical equipment, electric vehicles, and electronic devices [1,2,3,4,5,6,7]. The advantages of supercapacitors include improving cycle life and enhancing power for major power systems such as fuel cells and batteries. Electrochemical supercapacitors can be classified into two types based on their energy storage mechanisms [8,9], i.e., electric double-layer capacitors (EDLC) and pseudo-capacitors (PC). For an EDLC, when a voltage is applied to the electrodes, positive and negative ions in the electrolyte move toward different sides, forming a tightly packed charge layer that produces the capacitive effect. Since this process involves only physical charge transfer, there is minimal change in the volume or shape of the electrode material, resulting in a longer cycle life for EDLCs. As for a PC [10,11,12], this is a capacitor-like device in which a reversible and fast Faradaic reaction occurs on or near the surface of electrode materials when voltage is applied, producing the capacitive effect. Because a Faradaic reaction can occur both within the electrodes and on the surface, a pseudo-capacitor typically exhibits higher energy density and specific capacitance as compared to an EDLC. More precisely, pseudo-capacitance is a Faradaic charge storage mechanism based on fast and highly reversible surface or near-surface redox reactions, and can be divided into the following three categories according to their charge storage mechanisms [8,13,14,15,16]: (i) underpotential deposition; (ii) surface redox pseudo-capacitance; and (iii) intercalation pseudo-capacitance. Surface redox pseudo-capacitance refers to the process of storing charge on the surface of the electrode material through continuous and reversible redox reactions.

Recently, the promising electrode materials used for supercapacitors [17] are: (i) carbon-based materials; (ii) conductive polymers; (iii) transition metal oxides; and (iv) transition metal dichalcogenides (TMDs) [18,19,20,21,22], among others. TMDs have recently been used as the electrode materials for battery-like supercapacitors or pseudo-capacitors due to their excellent properties, such as a large surface area that ensures ample active sites for efficient charge transfer kinetics and electrochemical reactions, as well as exemplary chemical stability in electrochemical environments, which guarantees prolonged cycling stability and device performance [23,24]. Transition metal sulfides [15,25,26,27,28,29,30], such as MoS2 [31,32,33,34], SnS₂ [1,2,4,35], FeS₂ [36,37,38], Co₃S₄ [39,40,41], and Ni₃S₄ [42,43,44], are commonly used as electrode materials because of their high redox activity, high specific capacity, good electrical conductivity, unique crystal structures, and low electronegativity [15,45,46]. Metal sulfides offer superior electrochemical properties over metal oxides due to the substitution of oxygen with sulfur [18]. Key advantages include higher electrical conductivity, as sulfur forms weaker bonds with metals, allowing for greater electron delocalization [47]. Metal sulfides also have larger specific surface areas, providing more active sites for reactions and improving capacitance. Additionally, their unique electronic structure reduces charge transfer resistance, leading to better rate capability and power density, making them potential candidates for high-performance supercapacitor electrodes [48]. Additionally, TMDs possess higher electrical conductivity, enhanced thermal and mechanical durability, and more diverse redox chemistry compared to the corresponding metal oxides and carbon-based nanomaterials, making them ideal electrode materials for supercapacitors [49].

When selecting electrode materials, several factors such as cost, toxicity, and abundance in the earth’s crust must also be considered. Up to date, research on zirconium sulfide-based supercapacitor electrodes is limited. To the best of our knowledge, only one study focused on ZrS₂-based electrodes for supercapacitor application. M. Habib et al. [50] successfully synthesized single crystal ZrS₂ nanoflakes using the chemical vapor transport method and investigated their performance as the electrode of a supercapacitor. Electrochemical tests revealed that the ZrS₂ electrode showed a remarkable specific capacitance of 18.8 C g⁻¹ at 10 mV s⁻¹. In this work, zirconium sulfide nanoparticles (Zr_xS_y) composed of ZrS₃ and Zr₉S₂ phases are prepared on the flexible substrate of carbon cloth (CC) via a one-step synthesis approach using the low-pressure chemical vapor deposition method. By using Zr_xS_y/CC as a supercapacitor flexible electrode, the capacitance extracted from the cyclic voltammetry measurement shows 406 C g⁻¹ at a scan rate of 5 mV s⁻¹. As compared to the previous report, the Zr_xS_y-based electrode composed of ZrS₃ and Zr₉S₂ phases exhibits much higher capacitance than that of the electrode with single crystal ZrS₂ nanoflakes. These results highlight the promising potential of Zr_xS_y as a supercapacitor material for energy-storage technology.

2. Materials and Methods

2.1. Materials

Sulfur powder (99.5% purity) and zirconium tetrachloride (ZrCl₄, 98.0%) were obtained from Alfa Aesar and Acros organics, respectively. Flexible carbon cloth substrate (CC, W0S1009) was obtained from CeTech Co., Ltd., Taichung, Taiwan. Argon (Ar, 99.9995%) and hydrogen (H₂, 99.9995%) gases were purchased from Shenyi Gas Co. Ltd., Taipei, Taiwan.

2.2. Synthesis of Zirconium Sulfide (Zr_xS_y) Nanoparticles

An LPCVD system, equipped with a tube furnace and a quartz tube, is used to prepare the Zr_xS_y/CC electrode (Figure 1). The sulfur (S) and zirconium (Zr) precursors were sulfur powder and zirconium tetrachloride, respectively. During the growth process, sulfur was placed in an upstream alumina boat (i.e., Zone I) at a temperature of approximately 220 °C. A tungsten boat containing the Zr precursor was positioned at the center of the heating furnace (i.e., Zone II), and the vertically standing carbon cloth (CC) substrate was placed at the edge of the heating furnace (i.e., Zone III). The temperature program of Zone II was designed to increase from room temperature (RT) to 950 °C, hold at 950 °C for 30 min, and then cool down to RT. The process was conducted under a pressure of 30 torr with the gas flow of Ar (150 sccm) and H₂ (5 sccm). When Zone II reached 700 °C, Zone I was heated to 220 °C (5.5 °C min⁻¹) to facilitate the vaporization of the sulfur powder.

2.3. Material Characterization

To investigate the basic material properties, the composition and structure of the synthetic materials will be verified by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F, Taipei, Taiwan), field emission scanning electron microscope (FE-SEM, JEOL 7900F, Taipei, Taiwan), energy-dispersive X-ray spectroscopy (EDS, OXFORD X-MaxN TSR, Taipei, Taiwan), X-ray diffraction (XRD, D2 Phaser with XFlash, Bruker, Taipei, Taiwan), and Brunauer–Emmett–Teller (BET, NOVAtouch LX2, Quantachrome Instruments, Taipei, Taiwan).

2.4. Electrochemical Characterizations

Using a three-electrode cell system, all electrochemical measurements are performed by using an Autolab 204 potentiostat (Metrohm Autolab, Utrecht, The Netherlands). Without the use of any polymer binders or conductive fillers, a 3 × 1 cm² electrode with 1 × 1 cm² active area (Zr_xS_y loading mass: ~0.1 mg) was used directly as a working electrode. The surface area of Zr_xS_y/CC and pristine CC are 47.58 m² g⁻¹ and 39.01 m² g⁻¹, respectively; these values are obtained via BET analysis. A Ag/AgCl electrode and platinum sheet are used as a reference electrode and counter electrode, respectively. The electrolyte is 0.5 M H₂SO_4(aq.). Within the applied voltage range of −0.2 to 1.0 V, the cyclic voltammogram (CV) measurement was performed under various scan rates. The specific capacitance values are calculated by integrating the CV curve area using the following equation [51]:

$$\mathrm{C}\mathrm{a}={\displaystyle \frac{1}{\left[2m\mathsf{\upsilon }\right({\mathrm{V}}_{\mathrm{a}}-{\mathrm{V}}_{\mathrm{b}}\left)\right]}}·\int \mathrm{I}\left(\mathrm{V}\right)\mathrm{d}\mathrm{V}$$

where m is the mass of the active material (i.e., Zr_xS_y), ${\mathrm{V}}_{\mathrm{a}}-{\mathrm{V}}_{\mathrm{b}}$ is the potential range, υ is the scan rate and $\int \mathrm{I}\left(\mathrm{V}\right)\mathrm{d}\mathrm{V}$ is the area within the curve.

3. Results and Discussions

3.1. The Morphology of the Zr_xS_y/CC

The scanning electron microscope (SEM) primarily uses a finely focused electron beam to scan the surface of a sample. It can observe the morphology and structure of the sample surface while also performing size observations. The SEM images illustrate the surface morphology and nanostructure of CC and Zr_xS_y/CC. Figure 2a shows that the CC surface contains several grooves, which serve as sites for deposition. Figure 2b presents the morphologies of Zr_xS_y nanoparticles directly synthesized on the fibers of the CC electrode. It demonstrates that the Zr_xS_y nanoparticles grow densely and uniformly on the fibers of the CC electrode. Figure 2c shows the distribution of particle size. It suggests that the Zr_xS_y nanoparticles have an average particle size of approximately 13.02 nm. The particle sizes mostly lie in the range from 3 nm to 23 nm.

TEM is a technique that utilizes a high-energy electron beam to image ultra-thin samples. Its imaging resolution can reach atomic levels, up to 0.1 nanometers, making it an ideal instrument for observing the nanostructure of materials or analyzing lattice defects. Figure 3a presents the TEM images of Zr_xS_y nanoparticles. The homogeneous elemental distribution of Zr and S in the Zr_xS_y nanoparticles is observed by using elemental mapping profiles, as shown in Figure 3b,c. It shows that the Zr and S elements are uniformly distributed in the sample prepared for TEM analysis. HR-TEM can achieve atomic-level resolution, allowing for the observation of crystal structures, defects, and interfaces. To further investigate the crystal structure of the Zr_xS_y nanoparticles, HR-TEM images and selected area electron diffraction (SAED) patterns are provided in Figure 3d,e, respectively. Figure 3d demonstrates that the lattice spacings between two adjacent layers are approximately 0.235 nm, 0.259 nm, 0.298 nm, and 0.443 nm, which correspond to the (201), (200), (110), and (002) planes of ZrS₃, respectively. On the other hand, the interlayer spacings of 0.235 nm, 0.246 nm, and 0.259 nm correspond to the (008), (400), and (314) lattice planes of Zr₉S₂, respectively. The corresponding SAED pattern (Figure 3e) identifies and assigns the interplanar spacings corresponding to the diffraction rings.

XRD analysis is a technique that utilizes the interaction between X-rays and matter to study the structure of materials. It is primarily used to analyze crystal structures, phase compositions, and the nanostructure of materials. Figure 4 displayed the XRD patterns of the Zr_xS_y sample. The peaks of 20.5°, 30.3°, 34.6°, and 37.5° could be assigned to ZrS₃ (Monoclinic, PDF card No. 04-2864), and the corresponding crystal planes were (201), (200), (110), and (002), respectively. The other peaks of 30.6°, 35.4°, and 37.5° could be assigned to Zr₉S₂ (Tetragonal, PDF card No. 07-1763), and the corresponding crystal planes were (008), (400), and (314), respectively. The XRD patterns showed that the prepared Zr_xS_y sample mainly comprised ZrS₃ and Zr₉S₂ phases. These results further confirm the HR-TEM analysis, indicating that the Zr_xS_y nanoparticles are composed of ZrS₃ and Zr₉S₂ phases.

3.2. Electrochemical Performance of Zr_xS_y/CC Electrode

To investigate the performance of the Zr_xS_y/CC electrode, we first characterized its electrochemical properties in a three-electrode system using 0.5 M H₂SO₄ aqueous solution as the electrolyte, Pt foil as the counter electrode, and a Ag/AgCl electrode as the reference electrode. Cyclic voltammetry is a highly sensitive electrochemical testing method with a wide range of applications. In qualitative analysis, it is commonly used to study redox processes, electron transfer dynamics, and other related phenomena. Figure 5a shows the variation in mass capacitance at different scan rates for the Zr_xS_y/CC electrode, where the CC and Zr_xS_y act as the conducting substrate and active material, respectively. All capacitance values were extracted from the CV curves, as shown in the inset of Figure 5b. As the scan rate increased, the current density also increased; however, all CV curves maintained a similar shape, even at high scan rates, indicating that the Zr_xS_y continues to contribute to the capacitance. The calculated mass capacitance values were 406, 211, 88, 65, 57, and 46 C g⁻¹ at scan rates of 5, 10, 50, 100, 150, and 200 mV s⁻¹, respectively. On the other hand, the specific surface area values of Zr_xS_y/CC and pristine CC are 47.58 m² g⁻¹ and 39.01 m² g⁻¹, respectively. Thus, the area specific capacitance based on the specific surface area of Zr_xS_y can be estimated to be 47.37, 24.62, 10.27, 7.58, 6.65, and 5.37 C m⁻² at scan rates of 5, 10, 50, 100, 150, and 200 mV s⁻¹, respectively.

In galvanostatic charge–discharge (GCD) experiments, the electrode is subjected to a constant current during both charging and discharging processes, while the voltage across the electrode is continuously monitored within a defined voltage range and over a set duration. Figure 6a shows the GCD curves of the Zr_xS_y/CC electrode; the capacitance values measured at a current density of 0.5 A g⁻¹ and 1 A g⁻¹ (within 1.0 V~−0.2 V) are 151 C g⁻¹ and 134 C g⁻¹, respectively. The long-term stability of the Zr_xS_y/CC electrode was evaluated by cycling its charge–discharge behavior in a 0.5 M H₂SO₄ solution at a current density of 5 A g⁻¹, as shown in Figure 6b. After 1200 GCD cycles, the Zr_xS_y/CC electrode’s capacitance retention is 63.88%.

4. Conclusions

Zirconium sulfide nanoparticles (Zr_xS_y) composed of ZrS₃ and Zr₉S₂ phases are prepared on the CC substrate via a one-step synthesis approach using the low-pressure chemical vapor deposition (LPCVD) technique. The SEM image reveals that the particle sizes are in the range of ca. 3~23 nm with an average value of ~13.02 nm. HR-TEM images and XRD patterns reveal Zr_xS_y nanoparticles possess the (201), (200), (110), and (002) planes of ZrS₃ and the (008), (400), and (314) planes of Zr₉S₂. By using the Zr_xS_y/CC as a supercapacitor flexible electrode, the capacitance value obtained from CV measurement shows 406 C g⁻¹ at a scan rate of 5 mV s⁻¹; the capacitance values obtained from GCD curves at a current density of 0.5 A g⁻¹ and 1 A g⁻¹ are 151 and 134 C g⁻¹, respectively. After 1200 GCD cycles (at 5 A g⁻¹), the Zr_xS_y/CC electrode’s capacitance retention is 63.88%. To the best of our knowledge, this is the best performance of a Zr_xS_y-based supercapacitor electrode to date. These results highlight the promising potential of Zr_xS_y as a supercapacitor material for energy-storage technology.