Wrinkle-Shaped Nickel Sulfide Grown on Three-Dimensional Nickel Foam: A Binder-Free Electrode Designed for High-Performance Electrochemical Supercapacitor Applications

Abstract

Recently, three-dimensional nickel foam (3D-Nf) has been increasingly studied; however, further modifications in nanoscale surface modification are necessary for particular applications. In this work, three-dimensional hierarchically porous nanogranular NiS (NiS-3D-Nf) and wrinkle-shaped NiS (w-NiS-3D-Nf) structures were fabricated directly on nickel foam by a simple one-step solvothermal process using two different solvents. Several characterization techniques, including X-ray diffraction pattern, X-ray photoelectron spectroscopy, and scanning electron microscopy, were used to characterize the samples’ properties. To prove their applicability, supercapacitor electrodes were tested directly in a three-electrode assembly cell. The resulting w-NiS-3D-Nf electrodes exhibited greater capacitive activity than the NiS-3D-Nf electrodes. The optimized w-NiS-3D-Nf electrodes delivered an excellent specific capacitance of 770 Fg⁻¹, at a current density of 1 Ag⁻¹, compared with the NiS-3D-Nf electrodes (162.0 Fg⁻¹ @ 1 Ag⁻¹), with a cyclic stability of over 92.67% capacitance retention after 2200 cycles. The resultant unique structure with integrated hierarchical three-dimensional configuration can not only enhance abundant accessible surface areas but also produce strong adhesion to the 3D-Nf, facilitating the fast transportation of ions and electrons for the electrochemical reaction via the conductive 3D-Nf. This set of results suggests that the modification of 3D-Nf surfaces with a suitable solvent has highly significant effects on morphology, and ultimately, electrochemical performance. Additionally, the current preparation approach is simple and worthwhile, and thus offers great potential for supercapacitor applications.

1. Introduction

The increasing current demand for efficient energy storage devices with high energy/power density has encouraged researchers to develop novel materials and methods, especially in electrochemical energy-storage devices [1]. In this regard, supercapacitors (also known as electrochemical capacitors), are receiving significant attention as prominent power sources and effective substitutes for rechargeable batteries, owing to their superior power density, rapid charge and discharge process, long cyclic stability, and cost-effectiveness, although they suffer from low energy density. Depending on the change storage process, SCs are broadly divided into electric double-layer capacitors (EDLCs), in which energy is stored in collecting charges at the interaction of the electrode and the electrolyte, and pseudocapacitors in which, in addition to the EDLC, the charge storage mechanism is stored through a faradaic redox reaction on the electrode surface [2,3]. It is well known that electrode materials with electrochemical activity and kinetic features are among the key factors in SC performance, along with electrolytes and assembly techniques. In order to obtain effective properties [4], superior electrochemical activity is generally achieved by means of a proper connection between the active materials and the electrolytes, and rapid ion or electron transportation in the electrode and at the electrode/electrolyte interface. Hence, the design and preparation of electrode materials with high conductivity, specific surface area, and porosity is clearly required [5]. Various metal oxides and hydroxides are broadly investigated as high-performance electrode materials for SCs owing to their rich redox/faradic reactions [6]. Although these types of electrodes exhibit high energy density, they suffer from low inertness and rate capability, and, in particular, their electronic conductivity is insufficient. Nevertheless, the relevant experimental results are generally less acceptable for practical use due to short performance rates and high fabrication costs [5,7]. Therefore, it is necessary to synthesize innovative electrodes that possess suitable electrochemical activity supercapacitors. Considerable efforts have been made to develop the construction of electrodes with high specific capacitance, in order to fabricate supercapacitors with higher energy density.

In order to determine an appropriate structure for the electrodes, researchers’ focus has generally been placed on the fabrication of nanostructured materials with various sizes and morphologies, such as nanoflower, nanosheets, nanowires, nanorods, etc., because of their distinctive physico-chemical assets [8,9]. Recently, nanostructured metal sulfides were proven to be among the electrode materials with the greatest potential, and the best alternatives to SCs, because of their highly specific capacitance, suitable semiconductor properties, rich redox capability, cost-effectiveness, and higher electronic conductivity than metal oxides [10,11]. These features of metal sulfides mark them out as suitable electrodes for SC materials, through which they interact with OH ions, leading to reversible redox reactions in electrolytes: MS + OH⁻↔MSOH + e⁻ (M: Co, Ni, etc.) [8]. The reversible redox process enables charge storage and the transmission of electrons/OH ions in active materials.

Among the various metal sulfides, nickel sulfides have attracted immense attention for SC electrodes, owing to their high specific capacitance value, better electrical conductivity, and versatile chemical configuration, along with the fact that they are economical and safe [12,13]. Specifically, their specific capacitance, which is greater than that of carbonaceous-based materials, makes them different from other active materials. The growth/deposition of the electrode material directly on the current collectors is a novel approach to fabricating active electrodes for supercapacitors in order to obtain free-standing, additive- and binder-free hierarchical nanostructure-based electrodes [6,12,13]. In addition, the method (hydrothermal/solvothermal) used for the fabrication of the electrode is stable compared to other methods. For example, at high-temperature fabrication, the crystalline phase is not stable. Materials with high vapor pressure can also be prepared by the hydrothermal method, which offers advantages over other methods [1,9]. This method involves a highly electroactive area, which increases the electrochemical reaction, and it reduces the electrode fabrication process, resulting in the enhancement of the specific capacitance and energy density. Additionally, supercapacitors’ activities are also manipulated through the shape/size of electrode materials, as well as their structure. Binder-free supercapacitors have been developed over a decade with a variety of conductive substrates, including carbon fiber paper, stainless steel, titanium foil, silicon foil, and Ni foam [13,14,15].

Three-dimensional nickel foam is a well-defined and cross-linked network current collector and can be considered a good strategy for the above requirements. Due to its attractive three-dimensional structure and high electronic conductivity, 3D-Nf is widely accepted as a current collector for electrode materials, since it promotes the penetration of electrolytes and ion transport, and improves electrochemical performance. In other words, 3D-Nf fulfils all the requirements of the current collectors and electrodes used to assemble a range of energy-storage devices [13,16]. The use of electrode pastes, which include conductive agents and polymer binders, is an outdated electrode material paste method that hampers electrochemical performance [11]. Hence, growing metal sulfides directly on current collectors is therefore an effective way of enhancing electrochemical performance.

Directly growing NiS on a current collector such as 3D-Nf foam is a viable way to overcome the problems related to poor conductivity and other side effects of binders. In view of the above findings, a NiS that featured two different solvents in order to obtain two different morphologies was successfully grown directly on nickel foam (NiS-3D-Nf and w-NiS-3D-Nf) by using a simple one-step hydrothermal route to act as a binder-free electrode for pseudocapacitors; the w-NiS-3D-Nf exhibited a high specific capacitance of 770.0 Fg⁻¹, whereas the NiS-3D-Nf delivered a specific capacitance of 162.0 Fg⁻¹ at a current density of 1.0 Ag⁻¹, with better cyclic stability (92.67%). We studied the structures and morphologies of the crystals using X-ray diffraction and a scanning electron microscope, respectively. The distinctive wrinkle-shaped morphology of the w-NiS-3D-Nf electrodes and the direct contact between the current collector and the active materials contributed to boosting the electrochemical performance of the prepared electrode.

2. Experimental Procedure

2.1. Materials

Nickel chloride, ethylene glycol, and thiourea were acquired from Sigma Aldrich, whereas the three-dimensional nickel foam was obtained through the MTI Corporation, USA.

2.2. Methods

An X-ray diffractometer (PRO-MPD, Netherlands) was used to examine the phases and crystal structures of the as-synthesized w-NiS-3D-Nf and NiS-3D-Nf. The chemical structure and its interaction with the w-NiS-3D-Nf and NiS-3D-Nf were observed by X-ray photoelectron spectroscopy. The surface morphologies of the w-NiS-3D-Nf and NiS-3D-Nf were examined through field emission scanning electron microscopy (Tecnai; F20-FEI; USA).

2.3. Electrochemical Measurements

The electrochemical measurements, such as cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD), of the w-NiS-3D-Nf and NiS-3D-Nf electrodes were examined on a VersaSTAT 3 potentiostat electrochemical workstation equipped with PC. The Ag/AgCl electrode was used as reference electrode and Pt plate was used as counter electrode, whereas the w-NiS-3D-Nf and NiS-3D-Nf electrodes were used as working electrodes inside the three-electrode assembly.

2.4. Fabrication of the w-NiS-3D-Nf and NiS-3D-Nf

The fabrication of three-dimensional hierarchically porous wrinkle-shaped NiS@Ni foam using a simple solvothermal method was as follows. The required moles of nickel chloride and thiourea were dissolved in a mixture of methanol and ethylene glycol solvent at a ratio of 20:5 mL. The mixture was stirred for 20 min. Next, two Ni foams were added and sonicated for a further 5 min. The solution and Ni foam were transferred into a Teflon lined autoclave at 115 °C for 10 h. Subsequently, reaction was completed and autoclave was cooled down at room temperature naturally. The synthesized electrodes were washed with DI water and ethanol several times to remove all ions and contamination remaining in fabricated electrode. The washed samples were dried in an oven at 80 °C for 24 h. The NiS grown on the 3D-Nf using methanol as solvent is abbreviated as NiS-3D-Nf. The NiS grown on the 3D-Nf using methanol and glycol as solvent mixture is abbreviated as w-NiS-3D-Nf (Figure 1).

3. Results and Discussion

3.1. XRD Analysis

For synthesizing the nickel sulfides, we employed the one-step hydrothermal method, since it is an adaptable and appropriate method for controlling the material's morphology, structure, and phase, especially for inorganic materials. The optical images clearly show the change in the surface color of the 3D-Nf before and after hydrothermal treatment from slivery white to black, which was indicative of the new moieties successfully grown on the 3D-Nf. The crystallographic information of the as-prepared w-NiS-3D-Nf nanostructures was confirmed through an XRD analysis in the range of 2theta from 20 to 80 to identify the crystal and phase, as revealed in Figure 2. The XRD pattern peaks were observed at 2theta angles of 30.25, 34.85, 45.89, and 53.69, which can be well indexed to the crystal planes of (100), (101), (102), and (110). No extra diffraction peaks from the other phases were observed, suggesting the purity of the prepared w-NiS-3D-Nf sample. The prepared nanostructure was well-matched with the standard plane position of JCPDS card 10-075-0613.

3.2. SEM Analysis

The morphologies of the two-nickel sulfides grown directly onto the nickel foam using the different solvents in the solvothermal process are shown in Figure 3. In both cases, the active materials were uniformly deposited onto the conducting 3D nickel foam. Figure 3a,c,e,g represents the granular-shaped morphology of the NiS (NiS-3D-Nf) at different magnifications. Although the granular-shaped NiS was uniformly coated onto the surface of Ni foam, the low porosity and the presence of large cracks in the morphology may have resulted in the falling off the thin film during the charge–discharge processes, resulting in capacitance decay and decline in the cycling stability. On the other hand, a porous structure with interconnected channels was found to be highly desirable for high-performance, binder-free electrodes [17,18,19]. Figure 3b,d,f,h represents the wrinkle-shaped NiS grown directly onto the nickel foam. The uniformly deposited 3D film with porosity and interconnected structure enabled the electrolyte ions to penetrate rapidly and smoothly to the active electrode materials, resulting in enhanced electrochemical performance [10,12,20]. The wrinkle-shaped porous and continuous crack-less structure provided extra stability to the binding of the active electrode material to the nickel foam and, therefore, the w-NiS-3D-Nf electrode outperformed the NiS-3D-Nf electrode.

3.3. XPS Analysis

A further confirmation of the good synthesis of the NiS electrode material wa performed by XPS analysis. The XPS technique was performed to determine the elemental composition and valence of the constituents in the finally synthesized material. Figure 4a represents the high-resolution XPS spectra of the nickel in the 2p state, centered at 855.30 eV for the Ni 2p_3/2 state and 872.20 eV for the Ni 2p_1/2 state. Furthermore, the two satellite peaks at 862.0 eV and 879.0 eV were in good agreement with the previous reported work on NiS [20,21]. Figure 4b shows the high-resolution XPS spectra of S, with the two peaks situated at 160.0 eV and 162.0 eV relating to the states 2p_3/2 and 2p_1/2, respectively, representing the presence of metal-sulfur bonds. In addition, the extra peak situated at 169.0 eV was due to the presence of sulfur on the surface of the material in the form of sulfates [22,23].

3.4. Electrochemical Analysis

The supercapacitance performances of the w-NiS-3D-Nf and NiS-3D-Nf electrodes were evaluated through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements using a three-electrode cell-test system in a 2M aqueous KOH electrolyte medium. The CV test helps to determine the mechanism of the charge storage, while GCD technique determines the time taken by an electrode to become completely charged and discharged [17,18,19,20,21,22,23,24]. Figure 5a represents a comparison of the CV performances of the w-NiS-3D-Nf and NiS-3D-Nf electrodes at a fixed scan rate of 10 mV s⁻¹ under the potential window of −0.1 to 0.4 V compared with the Ag/AgCl reference electrode. The CV curve profile demonstrates clearly that both the materials possessed well-defined redox peaks, indicating the pseudocapacitive nature of both the electrodes [25,26]. The redox reactions of the nickel sulfide in the aqueous KOH electrolyte is given as [27]:

NiS + OH⁻ ↔ NiSOH + e¯

(1)

The area under the CV curves represents the real specific capacitance of the electrodes [28,29]. In this study, among the two binder-free electrodes, the area enclosed by the CV curve of w-NiS-3D-Nf was larger than the NiS-3D-Nf electrode. Furthermore, the oxidation peak for the w-NiS-3D-Nf electrode had a higher potential than that of the NiS-3D-Nf electrode, showing the good reversibility and enhanced pseudocapacitance of the w-NiS-3D-Nf electrode compared to the NiS-3D-Nf electrode. Moreover, the GCD test was carried out to determine the gravimetric specific capacitance of the fabricated 3D binder-free electrodes [30]. Figure 5b shows the GCD data plot of the w-NiS-3D-Nf and NiS-3D-Nf electrodes at a fixed current density of 1.0 Ag⁻¹. The non-linear GCD data profiles of both the samples show the resulting charge storage arising from faradaic redox reactions [31,32]. Although the chemical compositions of both the binder-free electrodes were the same, the different precursor solutions resulted in the NiS-based electrodes exhibiting different morphologies and, consequently, resulted in different electrochemical performances [1,12,20]. It is clear that the w-NiS-3D-Nf electrode took more time to discharge compared to the NiS-3D-Nf electrode and, therefore, possessed a higher gravimetric specific capacitance than the NiS-3D-Nf-based electrode. The specific capacitance of the electrodes is given by the following formula [12,13]:

$$C=\frac{Idt}{mdV}$$

(2)

where C represents the specific capacitance in Fg⁻¹, I is the current, t is the discharge time, m is the mass of the coated or loaded active materials, and dV represents the operating potential window [13]. From Equation (2), the gravimetric specific capacitance calculated for the w-NiS-3D-Nf binder-free electrode was found to be 770 Fg⁻¹ at 1 Ag⁻¹ of the current density, while for the NiS-3D-Nf, it was found to be 162.0 Fg⁻¹ at the same current density (1 Ag⁻¹). The GCD data results also favored the CV data, which specified the enhanced electrochemical behavior of the w-NiS-3D-Nf electrode compared with the NiS-3D-Nf electrode. The obtained specific capacitance for the w-NiS-3D-Nf was found to be better than that of other NiS-based electrodes [33,34,35,36,37]. This could be attributed to the three-dimensional hierarchically porous wrinkle-shaped morphology having a large electroactive surface area, helping the electrolyte ions to access the maximum active area of the electrodes [38,39]. To further explore the charge–discharge behavior of the w-NiS-3D-Nf and NiS-3D-Nf electrodes, the GCD tests were carried out at different current densities, as shown in Figure 5c,d. The specific capacitance values determined for the NiS-3D-Nf electrode from Figure 5c at current densities of 1.0, 1.50, 2.0, 4.0, 6.0, and 8.0 Ag⁻¹ were found to be 162.0 Fg⁻¹, 135.0 Fg⁻¹, 128.0 Fg⁻¹, 120.0 Fg⁻¹, 108.0 Fg⁻¹, and 64.0 Fg⁻¹, respectively, while for the w-NiS-3D-Nf electrode, from Figure 5d, at the similar current densities, the values were found to be 770.0 Fg⁻¹, 630.0 Fg⁻¹, 532.0 Fg⁻¹, 400.0 Fg⁻¹, 336.0 Fg⁻¹, and 112.0 Fg⁻¹, respectively. The remarkably enhanced specific capacitance of the w-NiS-3D-Nf electrode compared with the NiS-3D-Nf electrode calculated from the charge–discharge profiles, with obvious hysteresis, were due to the unique porosity of the wrinkled structure and the good adhesion of the active electrode material to the 3D nickel foam [40]. The porosity provided enough space for the electrolyte ions during the electrochemical reaction, whereas the three-dimensional structure facilitated faster ion and electron transport for the electrochemical reactions to occur via the conductive nickel foam.

A plot of the specific capacitance compared with the current density is shown in Figure 6a, which represents the higher rate capability of the w-NiS-3D-Nf binder-free electrode compared with the NiS-3D-Nf binder-free electrode. Cyclic stability over a large number of charge–discharge cycles is the most important parameter used to determine the performance of SC electrodes [41]. The consecutive charge and discharge cycling performance of the w-NiS-3D-Nf electrode is shown in Figure 6b, for 2200 consecutive cycles. The wrinkle-shaped, binder-free electrode retained 92.67% of its initial capacitance, demonstrating good capacitance retention over a long run. A slight fluctuation was observed during the consecutive charge and discharge cycling processes, which is normal in the case of the any developed electrodes. Thus, the cyclic performance also suggested the potential use of w-NiS-3D-Nf as a high-performance SC electrode. The wrinkle-shaped, unique w-NiS-3D-Nf electrode possesses an integrated hierarchical porous three-dimensional (3D) configuration, which facilitates faster ion and electron transport for electrochemical reactions to occur via the conductive nickel foam. The results obtained from the electrochemical tests also signify that the particular morphologies of active electrode materials has a direct influence on electrochemical performance [42,43]. Thus, the w-NiS-3D-Nf binder-free electrode with a novel morphology provides better supercapacitance performance than the NiS-3D-Nf electrode.

4. Conclusions

A three-dimensional, hierarchically porous, wrinkle-shaped NiS and a nanogranular NiS were grown directly on a 3D-Nf substrate using a one-step solvothermal process. The developed electrodes, i.e., w-NiS-3D-Nf and NiS-3D-Nf, were characterized by the X-ray diffraction pattern, X-ray photoelectron spectroscopy, and scanning electron microscopy techniques. The electrochemical performances of the individual electrodes were evaluated through the three-electrode assembly cell, in which the optimized w-NiS-3D-Nf electrode delivered an excellent specific capacitance, of 770 Fg⁻¹, compared with the NiS-3D-Nf electrode (162.0 Fg⁻¹), at a current density of 1.0 Ag⁻¹ and with a cyclic stability of over 92.67% capacitance retention after 2200 cycles. The resultant unique structure, with an integrated hierarchical three-dimensional configuration can not only enhance abundant accessible surface areas but also produce strong adhesion to the 3D-Nf, facilitating the fast transportation of ions and electrons for electrochemical reactions via the conductive substrate. These results suggest that the modification of 3D-Nf surfaces with suitable solvents has highly significant effects on morphology, and ultimately, electrochemical performance.