Mitigation of Shape Evolution and Supercapacitive Performance of CuCo₂S₄ Electrodes Prepared via a Simple Solvent Variation Approach

Abstract

This work demonstrates the evolution of different architectures of Cu-Co bimetallic sulfide on Ni-foam. A simple solvent-changing strategy has been adopted to develop these architectures through a solvothermal approach. When water and ethylene glycol was used as a solvent, the surface of Ni-foam seemed to be covered with a snowflake-like architecture. On the addition of glycerol instead of ethylene glycol, the growth of spongy rectangular prisms from mud-like bricks was initiated. Analyzing electrochemically, both electrodes deliver excellent capacitance. The electrode developed with water and glycerol was found to be richer in terms of capacitive performance, which gives 1459.7 F/gm (5692.8 mF/cm²) at a higher current density of 5 mA/cm². An over-33% increment in capacitance was noted when compared with the specific capacitance (areal capacitance) of another electrode which can provide 986.6 F/gm (3157 mF/cm²) at the same current density. Both electrodes are richer in terms of providing higher rate capability above 75%, even if the current density is increased by three times. Furthermore, both electrodes deliver long-lasting storage capability, with above 80% capacitance retention over 10,000 charge–discharge cycles even at a higher current density of 60 mA/cm². This simple solvent-changing approach can be useful for developing electrode materials with outstanding capacitive performance.

1. Introduction

Renewable energy sources for the storage and conversion of energy are under extensive development to reduce dependency on fossil fuels and thereby control the worsening environmental pollution [1,2]. Along with batteries, fuel cells’ rechargeable supercapacitors have started gaining wide attention as a clean energy source for storing and converting energy. The combined features of the supercapacitors, such as superior power density, fast charging capability, long cycle stability, high specific capacitance, environmental safety, and low cost, make them extensively propitious in backup power supplies, emergency lighting, electric vehicles, and portable electronic devices [3,4,5]. In supercapacitors, the conversion and storage of energy take place through electrochemical reactions. The method of charge storage execution splits supercapacitors into electric double-layer capacitors (EDLC) and pseudocapacitors (faradaic supercapacitors) [6]. Charge storage in EDLC is based on double-layer ion adsorption normally associated with carbon-based electrode materials. In pseudocapacitors, charge storage takes place through reversible faradaic reactions at the surface of the electrode or inner surface [7,8]. Pseudocapacitive electrodes of various materials, including transition metal oxides (TMOs), hydroxides, sulfides, conducting polymers, and carbonaceous materials, have been prominently employed as a cathode in energy storage devices [9,10]. Their remarkable physical and chemical properties enriched electronic conductivity, and decent thermal and mechanical stability favor transition metal sulfides in overriding the electrochemical performance of the other materials [10,11]. Binary/ternary metal sulfides are the most favored materials over single-component sulfides in transition metal sulfides. Here, the synergistic effect between two metal cations results in the procurement of large redox reactions, which ultimately enhances the overall electrochemical activities compared to single-metal-component-based sulfides [11,12,13]. Recent research on supercapacitors implies that alongside a selection of superior material, the in situ development of hierarchical nanostructures of that material on a conductive substrate gives a fascinating electrochemical performance. Directly grown hierarchical nanostructures have various benefits, such as a large surface area, binder-free tight physical contact between the conductive substrate and as-grown nanostructure, and good electrical contact without any conducting additives, which constitute attractive electrochemical properties even in case of high mass-loading [14,15]. In recent years, hierarchical nanostructures of transition metal sulfides have been extensively used for various applications such as catalysis, photocatalytic, biomedical, and sensor technologies, and energy storage and conversion devices [11,16,17,18,19]. Various synthesis methods have been adopted with which to develop these materials with a tuned morphology. Hydrothermal (water-assisted) or solvothermal (solvent-based) methods are commonly used synthetic approaches with which to procure types of morphological tuning of the various materials to make them useful for tremendous applications [20]. In the past, many researchers adopted a two-step hydrothermal method to grow the different nanostructures of sulfide-based materials. In the first step, hydroxides of the metal cations were grown at a specific temperature, and sulfurization was conducted in the second step using the sulfur source and using water as a solvent. The two-step approach is a prolonged technique that involves multiple processing steps, requiring high energy consumption and a high cost to procure the final product. The fabrication of numerous sulfide-based nanostructures with a cost-effective, one-step hydrothermal/solvothermal method is very rare. Furthermore, from the family of multimetal sulfides, the electrochemical properties of nickel–cobalt-based bimetallic sulfide have been extensively investigated; however, the bimetallic sulfides with metal cations Cu and Co have not yet been much explored. The associated vital features such as richer availability at low cost, lesser toxic nature, excellent multifunctionality, and high charge carrier mobility make copper–cobalt-based sulfides promising candidates for energy storage and conversion applications.

In this work, a simple one-step solvothermal strategy has been adopted to directly fabricate CuCo₂S₄ on Ni-foam. Here, two samples of the CuCo₂S₄ were developed using mixed solvents, i.e., water + ethylene glycol and water + glycerol. The effect on morphological modulation, and hence the electrochemical properties, were analyzed in detail. Directly grown CuCo₂S₄ using a solvent of water and glycerol shows an excellent specific capacitance of 1459.7 F/gm. An increase of about 33% was noticed in the specific capacitance of CuCo₂S₄ grown using solvent water and ethylene glycol, showing a specific capacitance of 986.6 F/gm at a current density of 5 mA/cm². This increase refers to the tuned morphological features of transition metal sulfide, i.e., the growth initiation of the irregularly shaped snowflakes in a water and ethylene glycol medium, while the development of spongy mud-like bricks and rectangular prisms was noticed when the solvent was changed to water and glycerol.

2. Experimental Details

2.1. Chemicals

To develop the binder-free supercapacitor electrodes, analytical regents of copper nitrate pentahydrate, cobalt nitrate hexahydrate, thiourea, ethanol, and acetone were purchased from Sigma Aldrich (St. Louis, MO, USA). Here, Ni-foam was used as a current collector (substrate), which was purchased from MTI Korea Corporation (Seoul, Republic of Korea).

2.2. Synthesis of CuCo₂S₄@Ni

A simple one-step solvothermal strategy was adopted to grow CuCo₂S₄ on the Ni-foam. Before the actual experiment, ultrasonication treatment was employed on the Ni-foam in acetone, deionized water (DI), and ethanol for 15 min each and dried in an oven for 2 h. This process will help clean up the current collector’s surface impurities. Followed by the cleaning of the Ni-foam; two different 60 mL solvents were prepared, with one containing DI water (50 mL) and ethylene glycol (10 mL) while the other contained DI water (50 mL) and glycerol (10 mL). A mixture of both solvents was stirred for 15 min using a magnetic stirrer. AR regents of copper nitrate pentahydrate (2 mmol), cobalt nitrate hexahydrate (4 mmol), and thiourea (10 mmol) were poured in the above solvents with continuous stirring for 30 min. Here, the purchased chemicals were utilized without further treatment or purification process. The mixed solution was then transferred to the 125 mL Teflon vessel, and cleaned Ni-foam (2-cm × 5-cm) was aligned vertically to the walls of the Teflon vessel. The closed Teflon vessel was further placed in a stainless steel autoclave assembly, and the autoclave was placed in an electric oven. The electric oven temperature was raised to 120 °C and maintained for 8 h. After finishing the reaction time, CuCo₂S₄-loaded Ni-foams were carefully fetched from the autoclave assembly. The loaded Ni-foams were washed with DI water and ethanol and dried overnight at 80 °C. For simplification, the coated electrodes are referred to as CCS-EG and CCS-glycerol.

2.3. Characterization Techniques

X-ray diffraction (DIATOME-Pananlytical, Malvern, UK) of the CuCo₂S₄-deposited Ni-foams was measured with Cu K (1.54 Å) radiation to dissect the crystalline phase formation. Cu, Co, and S elemental states were investigated using X-ray photoelectron spectroscopy (XPS; Versaprobe II, ULVAC-PHI Inc., Chigasaki, Kanagawa, Japan). The evolution of the morphological properties was assessed using a field-emission scanning electron microscope (FE-SEM; S-4800, Hitachi, Ibaraki, Japan). The energy-dispersive X-ray spectroscopy (EDS) and elemental mapping were assessed using the same FE-SEM. The inside view of the morphology was evaluated using transmission electron microscopy (TEM; JEOL/JEM-F200, Akishima, Tokyo, Japan).

2.4. Electrochemical Measurements

The electrochemical properties i.e., cyclic voltammetry (CV), charge–discharge curves (GCD), and electrochemical impedance spectroscopy (EIS) of the CuCo₂S₄ electrodes prepared in different solvents were measured in a 2 mol per liter solution of KOH (potassium hydroxide). All the measurements were performed in a three-electrode cell assembly constituted od a working electrode of CuCo₂S₄@Ni (1 × 1 cm), a graphite plate as a counter electrode, and a reference electrode-saturated calomel electrode (SCI). ZIVE SP5 (Wona Tech; Seocho-gu, Seoul, Republic of Korea) potentiostat/galvanostat/impedance analyzer was used to measure the electrochemical properties. The mass of active material was 3.2 mg/cm² for CCS-EG and 3.9 mg/cm² for CCS-Glycerol electrode.

3. Results and Discussion

Figure 1 represents diffraction patterns of the copper–cobalt sulfide anchored on the Ni-foam in different solvents, i.e., CCS-EG and CCS-glycerol. X-ray diffraction was measured at room temperature between 2θ angle 10 to 80°. Diffraction patterns of the Ni-foam dominate over the crystal planes from the copper–cobalt sulfide. Highly intense patterns at 2θ angles 44.8° (111), 52.2° (200), and 76.8° (220) assigned by # symbol reflect the cubic crystal phase of the Ni-foams, with assigned planes as shown in bracket, respectively [1,4,18]. Furthermore, four low-intense crystal patterns as compared with the intensity of substrate crystal planes can be noticed which are centered at 31.2°, 38.2°, 49.9°, and 55.1°, respectively. Those peaks speak to the formation of the cubic crystalline phase of the Cu-Co-based bimetallic sulfide with space group Fd-3m without any contaminants. These four are the most intense among all diffraction patterns of the CuCo₂S₄ system, which are assigned to the (113), (004), (115), (044) crystal planes according to the JCPDS # 042-1450 [6,16]. Lattice parameters of this cubic system are a = b = c = 9.4740 Å, and the volume of a unit cell is V = 850.35 ų. The majority of the lower intense planes of this system are invisible and remain underneath in front of the dominant intensity of crystal planes arising from Ni-foam. Interestingly, no traces of any change in the diffraction patterns were rectified among the two samples fabricated in a different solvent.

Survey XPS spectra of the Cu-Co-based bimetallic sulfide prepared using solvent water and glycerol are demonstrated in Figure 2a. Well-established peaks of the encompassed elements, i.e., Cu, Co, and S, along with peaks of O and C, can be recognized through this spectrum. The emergence of peaks of C and O relate to air exposure of the sample during synthesis and/or measurements [7]. The in-detail emission spectrum of Cu 2p explicates two well-distinguished peaks of the spin orbitals Cu 2p_3/2 and Cu 2p_1/2, positioned at binding energies 935.3 eV and 952.4 eV, respectively. As shown in Figure 2b, each of these peaks elaborates two well-developed illustrious states of the Cu 2p, i.e., Cu⁺ and Cu²⁺. Spectrum-indicating Cu⁺ species were centered at 931.9 eV for Cu 2p_3/2 orbital and 952.2 eV for Cu 2p_1/2 [10]. Another characteristic spectrum associated with the Cu²⁺ oxidation state was located at a binding energy of 933.6 eV and 954.6 eV inside 2p_3/2 and 2p_1/2 spin orbitals, respectively [6,7,21]. The emissions originating at 941.9 eV and 961.9 eV, next to the 2p_3/2 and 2p_1/2 spin orbital, respectively, are the shake-up satellites [22]. Two well-defined satellite peaks are attributed to the strong interaction between Cu cations and S anions; this also confirms intense ligand–metal charge transfer (LMCT) [7,23]. Furthermore, the emission spectrum of the Co 2p element also bifurcates in two spin orbitals, Co 2p_3/2 and Co 2p_1/2, located at binding energies 781.5 eV and 797.10 eV, as represented in Figure 2c. Moreover, satellite emissions were found to be situated at 786.05 eV and 803.2 eV for both spin orbitals, respectively. Gaussian fitting confirms that each orbit has two species of Co cations viz Co³⁺ and Co²⁺. Co²⁺, and Co³⁺ species were centered at 780.8 eV and 782.1 eV in the case of the 2p_3/2 orbit, whereas in the 2p_1/2 orbit, these species were found at 796.8 eV and 798.5 eV. The existence of the sulfur ions with the S²⁻ state was confirmed from the S2p fitting, as shown in Figure 2d. The two orbitals 2p_3/2 and 2p_1/2 of the S²⁻ species were located at binding energies of 161.6 eV and 162.5 eV. The satellites of these orbitals were noted at 164.1 eV and 169.3 eV, respectively [6,10,19].

Tweaking of the morphology with a change in the solvent was noted through FE-SEM. Images taken at different magnifications for CuCo₂S₄ deposited on Ni foam using solvent water and ethylene glycol are represented in Figure 3a–c, and electrode material prepared with water and glycerol is represented in Figure 3d–f. From the FE-SEM images, it is noted that variation in the solvent during electrode fabrication leaves a great impact on the morphological characteristics of the electrode material. Close examination of FE-SEM images indicated that Ni-foam was covered with irregularly shaped mud-like bricks with big cracks on them in both cases. Here, the formation of snowflakes was noted to be initiated for the electrode material prepared with a mixed solvent of water and ethylene glycol. On the other hand, replacing ethylene glycol with glycerol makes a great impact on surface morphology. In this case, the growth of rectangular prisms initiates on the surface. The geometrical dimensions (width and length) of these rectangular prisms differ from each other; a few have shorter lengths and widths, while others have larger ones and some of them have square faces. In the case of CCS-EG, flakes are thick in size and interconnected to each other, leading to the formation of 3D architecture (Figure 3b,c), whereas for CCS-glycerol, the mud-like bricks appear to be crushed as the rectangular prism growth takes place from the inside out (Figure 3e,f). Elemental mapping (Cu—green; Co—red; and S—violet) and the energy dispersive x-ray spectrum (EDS) of the Cu-Co bimetallic sulfide prepared with water and ethylene glycol are represented in Figure 4a, and the sample prepared by replacing glycol with glycerol is shown in Figure 4b. Both samples only confirm the elements that are taken during preparation, i.e., Cu, Co, and S on Ni foam.

Additional morphological scrutiny was carried out using transmission electron microscopy (TEM) for both electrode materials. TEM images for CCS-EG and CCS-glycerol are represented in Figure 5a,b, respectively. The TEM image of CCS-EG reveals a flake with a width of about 2.2 μm and 3.2 μm in length. The mud-like brick is notable for the CCS-glycerol electrode. The estimated length of a brick is around 2 μm and the width is 1.5 μm.

To analyze the capacitive performance and find out the superior electrode among the two different architectures of CuCo₂S₄, electrochemical tests (CV, GCD, EIS) have been carried out in a 2 M aqueous electrolyte of KOH. Cyclic voltammetry profiles of the CCS-EG and CCS-glycerol are represented in Figure 6a,b, respectively. While measuring CV curves of both electrodes over the voltage range of −0.2 to 0.6 V, a set of strong faradaic oxidation (anodic) and deoxidization (cathodic) peaks are noted at all applied scan rates between 5 mV/s to 100 mV/s. This couple of faradaic peaks indicate the pseudocapacitive characteristics, and they most probably occur due to the reversible state transition of metal cations during the redox reaction process, such as Cu²⁺ ↔ Cu⁺ and Co²⁺ ↔ Cu³⁺ in our case [19]. Hence, the energy storage mechanism of these Cu-Co-based bimetallic sulfides can be illustrated as [7]:

$$\mathrm{C}\mathrm{u}{\mathrm{C}\mathrm{o}}_2{\mathrm{S}}_4+2{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{C}\mathrm{u}{\mathrm{C}\mathrm{o}}_2{\mathrm{S}}_4{\left({\mathrm{O}\mathrm{H}}^{-}\right)}_2+2{\mathrm{e}}^{-}.$$

(1)

Furthermore, the position of the anodic peak climbs to a higher potential; on the other hand, the cathodic peak inclines toward the lower potential side with the increase in the scan rate. This movement of redox peaks is ascribed to the rate of intercalation of electrolytic ions on the surface of the active material and indicates the quasi-reversible features of redox reactions for both electrodes [24,25]. In measuring CV from a low scan rate to a higher one (Figure 6a,b), no obvious change was recognized in the appearance, reflecting that both electrode materials are capable of providing excellent rate performance. Moreover, a linear path of the current with respect to scan rate implicates higher prosperity intercalation of OH⁻ ions on the surface of active material. Optimized performance electrode material for energy storage was indorsed by comparing the CV of CCS-EG and CCS-glycerol at one fixed scan rate of 50 mV/s. As seen in Figure 6c CV, the profile of CuCo₂S₄ prepared with water and glycerol has an optimal area under the curve and archives a higher current, reflecting richer energy performance than the CCS-EG. This was further proved by estimating the specific capacitance and rate capability of the CCS-EG and CCS-glycerol by analyzing the charge–discharge mechanism at various current densities (5 mA/cm² to 15 mA/cm²). A comparative GCD profile at a current density of 5 mA/cm² for CCS-EG and CCS-glycerol is represented in Figure 6d. Obviously, CCS-glycerol reflects a richer discharge period (797 s) than CCS-EG (442 s), suggesting that it has a predominant charge storage capacity. Measuring GCD over a different current density enables the discovery of the rate capability of the active material. The GCD profiles at different current densities for CCS-EG and CCS-glycerol were represented in Figure 6e,f, respectively. The specific capacity (C_s, F/g)/areal capacity (C_A, mF/cm²) of each electrode was estimated at each current density using the discharge time (t_d), a mass of active material (m)/area of the electrode (1 cm²), and a voltage range (ΔV) as [19] following:

$$C_s=\frac{I\times t_d}{m\times ∆V},$$

(2)

$$C_A=\frac{I\times t_d}{A\times ∆V}.$$

(3)

The CCS-EG electrode delivers the specific (areal) capacitance for 5, 7, 9, 11, 13, and 15 mA/cm² at 986.6 (3157), 931.2 (2980), 900 (2880), 869.2 (2781.4), 824.1 (2637.1), and 803.57 (2571.4) F/g (F/cm²), respectively. Compared to this, CCS-glycerol enables higher values of specific (areal) capacitance; the values for this electrode are 1459.7 (5692.8), 1400 (5460), 1345.05 (5245.7), 1277.3 (4981.4), 1185.7 (4624.3), and 1109.9 (4328.6), respectively. The CCS-EG electrode was found to be richer in rate capability than CCS-glycerol, which gives 81.4% of the initial capacity, even over the three-fold increase in the current density, i.e., from 5 mA/cm² to 15 mA/cm², while the later electrode retains 76.03%. This reflects that the diffusion rate of electrolytic ions on the surface of CCS-EG does not much vary with the current density, as in the case of CCS-glycerol. The GCD curves of both electrodes are almost symmetric in shape, suggesting a high level of coulombic efficiency. Moreover, distinct plateaus of GCD profiles give evidence of the large Faradaic redox reactions at the electrode–electrolyte interface, similar to that noted from CV curves [24]. Furthermore, in the case of both electrodes, the discharge time (t_d) estimated at the respective current density advances the charging time (t_c), indicating that both electrodes enable higher levels (above 100%) of coulombic efficiency. The predominant capacitive performance of CCS-glycerol reflects the rational design with spongy mud-like bricks and rectangular prisms, enabling larger interface coupling, abundant electroactive points, and a free path for ion insertion and desertion. Due to this, the rate of the redox reactions is heavier than that of the CCS-EG electrode; hence, CCS-glycerol gives a higher capacitive performance. Moreover, the capacitive performance of both electrodes prepared in this case overrides the capacitance values of many of the Cu-Co-based bimetallic sulfide electrodes studied previously. The electrochemical performance of the CuCo₂S₄-based electrodes with different architectures is summarized in

Table 1. Comparative analysis of the electrochemical performance of CuCo₂S₄-based electrodes with different morphologies.

Electrode	Morphology	Specific Capacity/Capacitance (Current Density)	Cyclic Stability (%) (Number of Cycles)	Ref.
CuCo2S4	Nanoneedle array	363 C/g (1 A/g)	-	[1]
CuCo2S4	Nanoparticles	1885 C/g (2 A/g)	98% (6000)	[2]
CuCo2S4	Flower-like	908.9 F/g (5 mA/cm2)	91.1% (2000)	[4]
CuCo2S4	Sheet with nanowires	2879 C/g (1 A/cm2)	87.26% (10,000)	[7]
CuCo2S4	Hollow spheres	839 F/g (1 A/g)	95.1% (10,000)	[10]
CuCo2S4	Nanoparticles	5030 F/g (20 A/g)	80.5% (2000)	[13]
CuCo2S4	Flake-like cordillera	2558 F/g (2 A/g)	99% (10,000)	[16]
CuCo2S4	Micro-nanostructure particles	90.6 mAh/g (2 A/g)	99.4% (2000)	[20]
CuCo2S4	Nanosheet-array	409.2 mAh/g (3 mA/cm2)	91.5% (10,000)	[22]
CuCo2S4	Flower-like	592 F/g (1 A/g)	90.4% (3000)	[24]
CuCo2S4	Snowflakes	986.6 F/g (5 mA/cm2)	87.8% (10,000)	This work
CuCo2S4	Mud-like bricks with rectangular cubes	1459.7 F/g (5 mA/cm2)	83.3% (10,000)	This work

.

An electrode surface that allows diffusion of K⁺ ions from an electrolyte with a higher rate has a superior charge-storing capability. Therefore, to discover which electrode has the greater ability of K⁺ diffusion, the diffusion coefficient of CCS-EG and CCS-glycerol have been evaluated using the CV profiles of both electrodes at different scan rates. The Randles–Sevcik equation was used to evaluate the diffusion coefficient values, which is as follows [26]:

$$i_p=0.4463\times n\times F\times C\times A\times \sqrt{\frac{nFvD}{RT}}.$$

(4)

Considering constant factors in the above equation (F—Faraday constant; R—molar gas constant; and T—temperature), this equation can be presented as follows:

$$\frac{i_p}{v^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}=2.69\times {10}^5\times A\times C\times D^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\times n^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}.$$

(5)

Placing the values of $i_p$—the peak (reduction and oxidation) current value; A—area of the electrode (1 cm²); v—respective scan rate of the CV curve; C—concentration of electrolyte; and n—number of electrons; the diffusion coefficient can be estimated. The slope value noted through the linear fitting of the plot i_p vs. v^1/2 will equalize the left side, i.e., $\left(\frac{i_p}{v^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}\right)$ in the above equation. The plot of the i_p vs. v^1/2 with linear fitting over peak points of reduction and oxidation for CCS-EG and CCS-glycerol is represented in Figure 7a. Considering slope values, the estimated value of the diffusion coefficient (reduction and oxidation) for CCS-glycerol is higher than that of the CCS-EG. CCS-EG has a diffusion coefficient of 2.81 $\times$ 10⁻⁴ cm²s⁻¹ for reduction and 2.46 $\times$ 10⁻⁴ cm²s⁻¹ for oxidation, respectively, whereas CCS-glycerol exhibits a diffusion coefficient of 2.96 $\times$ 10⁻⁴ cm²s⁻¹ (reduction) and 2.73 $\times$ 10⁻⁴ cm²s⁻¹ (oxidation), respectively. To analyze the governing of the charge-storage process (diffusion-controlled intercalation activity or surface-encouraged capacitive process) in the case of both electrodes, the power law relationship was utilized [27]:

$$i\left(v\right)=i_{surface}+i_{diffusion}={av}^b.$$

(6)

Here, a is the constant, while b is the power law exponent, and i represents the value of peak current which follows the scan rate v. The intercept and slope obtained through the linear fitting of plot log i vs. log v gives the values of a and b, respectively. The value of b recognizes that the charge storage is governed by diffusion-controlled intercalation activity or surface-encouraged capacitive process. Tending slope (b) toward 0.5 acknowledges that charge storage is from diffusion-controlled intercalation activity, and if it reaches 1.0, then the mechanism is governed by a surface-encouraged capacitive process [28]. The plot of log I vs. log v with linear fitting for both electrodes is represented in Figure 7b. It seems that charge storage in our electrodes originates from the contribution of both processes, as the b-value of CCS-EG is 0.59 and that of for CCS-glycerol electrode is 0.56. The b-value near 0.5 in both cases reflects that the charge storage based on diffusion-controlled intercalation activity is heavier than that of the capacitive process and more prominent in the case of the CCS-glycerol electrode. The involvement of each process in the total charge storage mechanism was evaluated as a percentage at each scan rate for both electrodes as per the following equation [26]:

$$i\left(V\right)=k\nu +k^{\prime }{\nu }^{0.5},$$

(7)

or,

$$\raisebox{1ex}{$i\left(V\right)$}\!\left/ \!\raisebox{-1ex}{$v^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$}\right.=k{\nu }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+k^{\prime }.$$

(8)

Here, k and k′ give the contribution arising from surface-encouraged capacitive process and diffusion-controlled intercalation activity, respectively. Figure 7c,d shows the contribution (stacked columns) of both processes at various scan rates for CCS-EG and CCS-glycerol electrodes, respectively. As noted from the b-value, the contribution from the diffusion-controlled intercalation process is dominant in the case of both electrodes. The contribution of this process is higher at a lower scan rate and decreases with a further increase in the scan rate. This process is more dominant in the case of CCS-glycerol, which gives about 87.83% at 5 mVs⁻¹ to the total charge contribution, which is 78.48% in the case of the CCS-EG electrode at the same scan rate. Even at a higher scan rate, the contribution arising from the diffusion-controlled process is higher than that of the capacitive process, reflecting that both CCS electrodes are pseudocapacitive [29]. Figure 7e,f shows the CV curve at 10 mVs⁻¹, which reflects the charge storage originating from the diffusion-controlled process and capacitive control process for CCS-EG and CCS-glycerol, respectively.

To assess the long-term feasibility of both electrodes, charge–discharge kinetics were measured up to 10,000 cycles at a current density of 60 mA/cm². Figure 8a reflects the capacitance retention and coulombic efficiency of CCS-EG and CCS-glycerol. When comparing the capacitance of the first cycle to the 10,000th cycle, the CCS-EG electrode shows a dominating capability, giving 87.8% capacitance over a huge number of cycles. In this case, the CCS-glycerol electrode can retain 83.3% of its capacitance, that noted for the first cycle. Interestingly, the coulombic efficiency remains over 100%, even after a large number of charge–discharge cycles on both electrodes. These features reflect that CCS-EG and CCS-glycerol are capable of use over a longer duration. Architecture collapse is the main reason why the CCS-glycerol is unable to retain capacitance in the manner of CCS-EG. To look over this, the FE-SEM was carried out after measurement of the GCD over 10,000 cycles. Figure 8b,c reflects the FE-SEM images after cyclic stability for CCS-EG and CCS-glycerol, respectively. As observed, CCS-EG retains the same morphology as before the stability, i.e., snowflakes with very slight changes. On the other hand, CCS-glycerol is able to retain the spongy mud-like bricks; however, the rectangular prisms disappear over the longer cycling stability. This erosion of architecture mainly arises due to the faster reversible redox (reduction–oxidation) reactions taking place on the surface of the electrode material and leading to a worsening performance.

To obtain insight into the details of intrinsic resistance (R_s) and charge transfer resistance (R_ct), electrochemical impedance spectroscopy (EIS) of both electrodes was studied at a fixed amplitude of 10 mV and with a frequency ranging from 0.1 Hz to 10⁵ Hz. Figure 9a,b shows the EIS spectrum (Nyquist plots) for the CCS-EG and CCS-glycerol electrodes, respectively. CCS-glycerol has a lower intrinsic resistance (R_s) of 0.3 Ω, which is 0.65 in the case of the CCS-EG electrode. Lower values of intrinsic resistance indicate a good balance between the active material and the current collector. Further, the charge transfer resistance (R_ct) is also lower for the CCS-glycerol electrode, which is 36.32 Ω, whereas for CCS-EG, it is 53.25 Ω. This confirms the prominent electrochemical features associated with the CCS-glycerol electrode [30]. The values of R_s and R_ct after the stability measurement for the CCS-EG electrode are 0.25 Ω and 80.27 Ω, while for the CCS-glycerol electrode, they are 0.23 Ω and 65.79 Ω, respectively.

4. Conclusions

Bimetallic sulfide electrodes composed of Cu and Co metal ions have been prepared through a straightforward solvothermal method. Altering the solvents during the fabrication of active electrode material significantly influences the architectural growth, leading to prominent variation in the electrochemical performance. Growth of snowflakes such as CuCo₂S₄ on the surface of Ni-foam was noted when electrode fabrication took place in the medium of water and ethylene glycol. On the other hand, the development of spongy mud-like bricks and rectangular prisms was seen to grow when the solvent was changed to water and glycerol. Enhanced electrochemical properties, with higher capacitance, were noted for the electrode fabricated using a solvent combined with water and glycerol. Interestingly, the capacitance of identical materials upsurges by about 33% when the solvent medium during the fabrication is altered. Through in-depth investigation of the charge storage mechanism, it is found that charge storage in both electrodes of CuCo₂S₄, fabricated in different solvents, takes place from diffusion-controlled intercalation process. Our study showcases an uncomplicated approach to the fabrication of highly capacitive electrode materials. Fabricating electrodes with the versatile architecture of the same material allows for the tailoring of materials for tremendously advanced, applications including energy storage and conversion.