Facile Hydrothermal Synthesis and Supercapacitor Performance of Mesoporous Necklace-Type ZnCo₂O₄ Nanowires

Abstract

In this work, mesoporous ZnCo₂O₄ electrode material with necklace-type nanowires was synthesized by a simple hydrothermal method using water/ethylene glycol mixed solvent and subsequent calcination treatment. The ZnCo₂O₄ nanowires were assembled by several tiny building blocks of nanoparticles which led to the growth of necklace-type nanowires. The as-synthesized ZnCo₂O₄ nanowires had porous structures with a high surface area of 25.33 m² g⁻¹ and with an average mesopore of 23.13 nm. Due to the higher surface area and mesopores, the as-prepared necklace-type ZnCo₂O₄ nanowires delivered a high specific capacity of 439.6 C g⁻¹ (1099 F g⁻¹) at a current density of 1 A g⁻¹, decent rate performance (47.31% retention at 20 A g⁻¹), and good cyclic stability (84.82 % capacity retention after 5000 cycles). Moreover, a hybrid supercapacitor was fabricated with ZnCo₂O₄ nanowires as a positive electrode and activated carbon (AC) as a negative electrode (ZnCo₂O₄ nanowires//AC), which delivered an energy density of 41.87 Wh kg⁻¹ at a power density of 800 W kg⁻¹. The high electrochemical performance and excellent stability of the necklace-type ZnCo₂O₄ nanowires relate to their unique architecture, high surface area, mesoporous nature, and the synergistic effect between Zn and Co metals.

1. Introduction

An electrochemical supercapacitor is one of the most effective techniques to store electrical energy, owing to its high power density, high charge/discharge rate, long operation time, higher rate capability, mechanical stability, and environmental friendliness [1,2]. In general, the performance of electrochemical supercapacitors mainly depends on the electrode material types and properties [3,4]. It is well known that supercapacitor electrode materials are classified into three main categories: electrical double layer electrode materials, pseudocapacitor electrode materials, and battery-type electrode materials [5,6]. In the past decade, tremendous efforts have been undertaken to improve the electrochemical performance of supercapacitor electrode materials, including various transition metals-based oxides, hydroxides, sulfides, selenides, nitrides, carbides, phosphides, carbon-based materials with high surface areas, and polymeric materials [7,8,9,10,11,12,13].

Among the various supercapacitor materials, transition metal oxides, particularly AB₂O₄-type cubic spinel binary transition metal oxides (BTMOs), have been extensively utilized in the growth of new functional materials, due to the facile synthesis methods, controllable morphologies, high surface to volume ratios, varied oxidation states, lower cost, and better electronic conductivity [14,15]. Generally, cubic spinel BTMOs have high surface to volume ratios, which is ultimately applied in the fields of lithium-ion batteries, supercapacitors, electrocatalysts, solar water splitting, and degradation of organic pollutants [14,15,16,17,18,19,20]. Among these, cubic spinel ZnCo₂O₄ is a promising energy storage material, owing to its various valence states, excellent electrochemical redox properties, better electrical conductivity, low cost, non-toxicity, and different morphologies [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Currently, a variety of preparation methods are used to synthesize ZnCo₂O₄ nanomaterials, including hydrothermal [22,24,27,28,30,31,35,36], solvothermal [21,23,25,29], combustion [26], reflexing [32], and electrospinning methods [34]. Similarly, a series of ZnCo₂O₄ morphologies have been successfully synthesized, such as microspheres [21,30,32,33], nanorods [22,28], nanotubes [34], nanowires [24,31], nanosheets [22,27,29], nanobelt [24], nanoparticles [26], urchin-like [23], quasi cubes [25], and bipyramid nanoframes [35], and utilized widely in energy storage applications, mostly in electrochemical supercapacitors [21,22,23,24,25,26,27,28,30,31,34]. For instance, Bhagwan et al., prepared ZnCo₂O₄ nanoparticles via a combustion method for supercapacitors, showing a high specific capacitance of 843 F g⁻¹ at a current density of 1 A g⁻¹ [26]. Wang et al. reported the synthesis of porous ZnCo₂O₄ microspheres by a hydrothermal method, exhibiting a high specific capacitance of 953.2 F g⁻¹ at 4 A g⁻¹ [21]. Cheng et al. synthesized mesoporous ZnCo₂O₄ nanoflakes hybrids on nickel foam to enhance the specific capacitance (1220 F g⁻¹ at 2 A g⁻¹) [37].

In this work, we illustrate an easy way to synthesize another kind of cubic spinel ZnCo₂O₄ nanomaterial for effective supercapacitor application. Mesoporous necklace-type ZnCo₂O₄ nanowires were successfully synthesized by a facile water/ethylene glycol (EG)-mediated hydrothermal method. Various physicochemical characterization techniques revealed that the as-prepared ZnCo₂O₄ nanowires were built by several tiny building blocks of nanoparticles which led to the growth of necklace-type nanowires. Due to the high surface area, mesoporous structure, unique architecture, and synergistic effect, as-prepared necklace-type ZnCo₂O₄ nanowires delivered a high specific capacity (439.6 C g⁻¹ at a current density of 1 A g⁻¹), excellent rate performance (208.0 C g⁻¹ at 20 A g⁻¹), and outstanding cyclic stability (84.82 % capacity retention after 5000 cycles). Furthermore, a hybrid supercapacitor (HSC) device was fabricated with ZnCo₂O₄ nanowires as a positive electrode and activated carbon (AC) as a negative electrode (ZnCo₂O₄ nanowires//AC), which delivered an energy density of 41.87 Wh kg⁻¹ and a maximum power density of 4000 W kg⁻¹.

2. Results and Discussion

The crystalline phase of the as-prepared ZnCo₂O₄ sample is examined by XRD measurement. The XRD pattern in Figure 1 reveals that all peaks correspond to the characteristic peaks of spinel phase. All of the identified XRD peaks at around 18.95°, 31.08°, 36.70°, 38.54°, 44.71°, 55.42°, 59.18°, 65.01°, 73.41°, and 76.85° 2θ correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (620), and (533) planes of spinel-type ZnCo₂O₄ (JCPDS No: 23–1390), respectively [21,22,23,24,25]. Moreover, no other crystalline phases were identified, which indicates the pure phase formation of ZnCo₂O₄ sample.

The chemical valence states of the ZnCo₂O₄ sample were verified by XPS (Figure 2a–d). Figure 2a shows the survey scan spectrum, which confirms the presence of Zn, Co, and O elements. The Zn 2p spectrum (Figure 2b) was deconvoluted into two spin–orbit doublets at 1022.1 and 1045.2 eV, which belong to the Zn 2p3/2 and Zn 2p1/2 peaks, respectively [23,25,26,27,29,30,31,38]. Two main peaks at binding energies of 781.3 and 796.3 eV were detected in the Co 2p spectrum (Figure 2c), corresponding to the Co 2p3/2 and Co 2p1/2 labels, respectively [23,25,26,27,29,30,31]. Further, peak deconvolution and fitting were employed for Co 2p3/2 and Co 2p1/2 labels. Two deconvoluted peaks at 781.2 and 796.3 eV were ascribed to Co³⁺, while another set of deconvoluted peaks situated at 782.9 and 797.9 eV were assigned to Co²⁺ [23,25,27]. Moreover, two shakeup satellite peaks (sat.) were also observed at 790.1 and 805.4 eV. The Co 2p XPS data indicate that both Co(II) and Co(III) oxidation states were present in the as-prepared ZnCo₂O₄ nanomaterial [29,30,31]. In the O 1s spectrum (Figure 2d), the peaks located at 530.4 and 531.8 eV were assigned to a metal–oxygen bond and typical chemisorbed oxygen, respectively [23,25,27].

The morphology of the synthesized ZnCo₂O₄ sample was observed by FE–SEM and FE–TEM measurements. Figure 3 shows the FE–SEM images of the necklace-type ZnCo₂O₄ nanowires at different magnification. A low-magnification SEM image in Figure 3a shows the flower-like structures in the synthesized material. A closer view of the flower-like structure (Figure 3b,c) reveals that the ZnCo₂O₄ sample is built by several nanowires. The high-magnification SEM image (Figure 3d) clearly indicates that each ZnCo₂O₄ nanowire was composed of numerous nanoparticles and that these nanoparticles were interconnected to each other to grow necklace-type nanowires. The EDS spectrum in Figure 3e confirms the presence of Zn, Co, and O elements within the necklace-type ZnCo₂O₄ nanowires.

The detailed microstructure of the necklace-type ZnCo₂O₄ nanowires was further characterized by TEM. Figure 4a shows a low-magnification TEM image of a typical flower-like ZnCo₂O₄ structure. Obviously, this flower-like structure is assembled by numerous nanowires, and the higher magnification TEM image (Figure 4b) clearly indicates that the flower-like structure was composed of many ZnCo₂O₄ nanowires. Figure 4c,d show the enlarged sections of the regions marked in yellow and red in Figure 4b, illustrating that the necklace-like structures were built by many primary nanoparticles which were interconnected to form a nanowire structure. Such a morphology is highly beneficial for the penetration of electrolyte ions during the charge/discharge process [31,34,39]. The inset image in Figure 4c is the selected area electron diffraction (SAED) pattern taken from the marked region in Figure 4c, which indicates the polycrystalline nature of the prepared necklace-type ZnCo₂O₄ nanowires [30,31,37]. Lattice fringes of a ZnCo₂O₄ nanowire were also illustrated in Figure 4d, and the distance between the two adjacent lattice fringes was 0.24 nm, which corresponds to the (311) plane of the cubic spinel AB₂O₄-type ZnCo₂O₄ [30,31,33,37].

To determine the average BET surface area and pore size distribution of the necklace-type ZnCo₂O₄ nanowires, a N₂ adsorption/desorption measurement was carried out. As shown in Figure 5a,b, a representative type IV isotherm and a H3 hysteresis loop were observed, which can be assigned to the mesoporous characteristic of these necklace-type ZnCo₂O₄ nanowires [23,25,30,31,33,34,35]. The calculated average surface area and pore size distribution of the ZnCo₂O₄ nanowires were 25.33 m² g⁻¹ and 23.13 nm, respectively. In general, AB₂O₄-type cubic spinel structures with mesopores showed improved electrochemical supercapacitor properties, due to their high specific surface areas and increased electrolyte ion transport [14,15,23,25]. Therefore, we believe that our mesoporous necklace-type ZnCo₂O₄ nanowires also provide more electroactive sites and a facile diffusion of electrolyte ions during the intercalation/deintercalation process.

The electrochemical supercapacitor properties of as-prepared necklace-type ZnCo₂O₄ nanowires were studied by CV and GCD techniques in a traditional three-electrode system using 2M KOH aqueous solution. Figure 6a shows the CV curves of the necklace-type ZnCo₂O₄ nanowires electrode at various scan rates from 5 to 50 mV s⁻¹ in a voltage window of –0.1~0.5 V. From the CV curves, we can clearly see a pair of oxidation and reduction peaks within 0.1~0.5 V, which are mainly due to the Co–O/Co–O–OH redox reactions during the intercalation and deintercalation process of OH⁻ ions [30,31,34,37]. The possible Faradaic redox reactions of the necklace-type ZnCo₂O₄ nanowires electrode can be given as follows [24,26]:

$${\mathrm{ZnCo}}_2{\mathrm{O}}_4+{\mathrm{OH}}^{-}+{\mathrm{H}}_2\mathrm{O} \rightleftharpoons \mathrm{ZnOOH} + 2\mathrm{CoOOH} + {\mathrm{e}}^{-}$$

(1)

$$\mathrm{CoOOH}+{\mathrm{OH}}^{-} \rightleftharpoons {\mathrm{CoO}}_2 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{e}}^{-}$$

(2)

As the scan rate increased from 5 to 50 mV s⁻¹, the area of the CV curves and redox peak current were increased, demonstrating the ideal capacity performance of the ZnCo₂O₄ electrode [23]. Moreover, redox peak shifts can be seen with the increase in scan rate. This effect may be due to the internal resistance of the ZnCo₂O₄ nanowires electrode during the Faradaic reactions [23,30]. Even at a high scan rate (50 mV s⁻¹), the redox peaks were still detected with a similar shape, showing that the ZnCo₂O₄ nanowires electrode has excellent rate capability. Figure 6b shows a plot of the anodic and cathodic peak currents of the necklace-type ZnCo₂O₄ nanowires electrode with respect to the square root of the scan rate. A linear relationship can be observed, indicating that the Faradaic redox reactions of the necklace-type ZnCo₂O₄ nanowires electrode are diffusion-controlled processes [40].

The charge/discharge measurement was performed to determine the specific capacity, rate capability, and durability of the ZnCo₂O₄ nanowires electrode. Figure 6c displays the GCD curves of the ZnCo₂O₄ nanowires electrode at different current densities from 1 to 20 A g⁻¹ within the potential window of 0~0.4 V. The GCD curves were almost similar to the CV profile, which fairly expressed the battery-type behavior of our ZnCo₂O₄ nanowires electrode. The specific capacity values (Cs) were calculated from the discharge time according to the following equation:

Cs = (I Δt)/(m)

(3)

where C_s, I, Δt, and m are the specific capacity (C g⁻¹), the discharging current (A), the discharging time (s), and the mass of the electrode material (g), respectively.

The calculated specific capacity values were high as 439.6, 396.8, 373.2, 354.4, 336.0, 286.0, and 208.0 C g⁻¹ at a current density of 1, 2, 3, 4, 5, 10, and 20 A g⁻¹, respectively. For comparison with the available literature data for ZnCo₂O₄ nanomaterials, we calculated the specific capacitance (F g⁻¹) values according to the Equation (S1) given in the Supplementary Material. The calculated specific capacitance value of necklace-type ZnCo₂O₄ nanowires at 1 A g⁻¹ was high as 1099 F g⁻¹. The specific capacitance values of our necklace-type ZnCo₂O₄ nanowires electrode is comparable to or higher than that of the reported ZnCo₂O₄ nanomaterials (Table S1). This comparative result suggests that our mesoporous necklace-type ZnCo₂O₄ nanowires electrode has rapid ion transport during the charge/discharge processes, which ultimately improves the electrochemical supercapacitor performance. The rate performance of the necklace-type ZnCo₂O₄ nanowires electrode was displayed in Figure 6d. It shows that the specific capacity of the ZnCo₂O₄ nanowires electrode decreased with increasing current densities from 1 to 20 A g⁻¹. At higher current densities, the diffusion-controlled Faradaic reactions decreased, and surface-controlled Faradaic reactions increased, resulting in a lower specific capacity [40]. The specific capacity retention of the necklace-type ZnCo₂O₄ nanowires electrode was 47.31%, demonstrating good rate capability at high current densities.

Figure 7 shows the long-term cycling stability of the ZnCo₂O₄ nanowires electrode at a higher current density of 40 A g⁻¹. After 5000 continuous charge/discharge cycles, the final specific capacity retention value was 84.82 %, suggesting that the ZnCo₂O₄ nanowires electrode has excellent long-term stability due to the significant number of electroactive sites for the facile transport of electrolyte ions. The Coulombic efficiency (η) of the ZnCo₂O₄ nanowires electrode was calculated using the following equation [21]:

η = td/tc × 100%

(4)

where td and tc are the discharge and charge time, respectively. The calculated Coulombic efficiency of the ZnCo₂O₄ nanowires electrode over the entirety of the cycles was 100%. This result demonstrates the excellent kinetic reversibility of the Faradaic reactions in the ZnCo₂O₄ nanowires electrode [23]. The CV, GCD, and stability data showed that the as-prepared necklace-type ZnCo₂O₄ nanowires electrode has outstanding electrochemical properties and can be considered a potential electrode for battery-type supercapacitors.

To further demonstrate the potential application of the necklace-type ZnCo₂O₄ nanowires electrode, a HSC device was fabricated with ZnCo₂O₄ nanowires (positive electrode) and activated carbon (negative electrode), which is represented as ZnCo₂O₄//AC. Before evaluating the electrochemical properties of the ZnCo₂O₄//AC HSC device in 2M KOH aqueous solution, the CV and GCD curves of the negative electrode were studied in a three-electrode configuration. The CV curves (Figure S2a) of AC at different scan rates from 10 to 50 mV s^–1 clearly show a rectangular shape, indicating an EDLC-type negative electrode [41]. Figure S2b shows the linear and symmetrical GCD curves of the negative electrode at various current densities. The specific capacitances of the negative electrode Cs (F g⁻¹) were calculated using the Equation (S1). The calculated specific capacitance values were 152.3, 131.0, 117.6, 106.8, and 97.5 F g^–1 under current densities of 1, 2, 3, 4, and 5 A g^–1, respectively.

The comparative CV curves of the positive (ZnCo₂O₄) and negative (AC) electrodes at a scan rate of 50 mV s⁻¹ in a three-electrode system are presented in Figure 8a. From this CV data, a maximum potential window of 1.6 V was fixed for the ZnCo₂O₄//AC HSC device. The CV curves of the ZnCo₂O₄//AC HSC device were obtained at different scanning rates from 10 to 50 mV s⁻¹ and displayed partial redox peaks (Figure 8b), indicating that battery-type and EDLC mechanisms were involved in our HSC device. Furthermore, the shape of the CV curves was maintained, even at a high scan rate, signifying excellent rate performance of the ZnCo₂O₄//AC HSC device. Figure 8c displays charge/discharge profiles of the ZnCo₂O₄//AC HSC device evaluated at various current densities from 1 to 5 A g⁻¹. The shape of the GCD curves was consistent with the CV profile and the specific capacitance values estimated according to the Equation (S1). The calculated specific capacitance values were 117.7, 100.8, 91.1, 83.2, and 75.6 F g⁻¹ at current densities of 1, 2, 3, 4, and 5 A g⁻¹, respectively. Remarkably, the ZnCo₂O₄//AC HSC device delivered a high specific capacitance of 117.7 F g⁻¹ at a current density of 1 A g⁻¹, and retained 64.23 % even at a high current density of 5 A g⁻¹ (75.6 F g⁻¹), displaying outstanding rate performance (Figure 8d). Furthermore, the ZnCo₂O₄//AC HSC device showed exceptional cycling stability over the 10000 cycles at 20 A g⁻¹ (~73.53% capacitance retention), demonstrating impressive durability (Figure 8e). Moreover, remarkable Coulombic efficiency of more than 97 % during the entire stability test was observed (Figure 8e, blue color), which revealed the excellent reversibility of our HSC device.

Based on the specific capacitance values, the energy density (E, Wh kg⁻¹) and the power density (P, W kg⁻¹) of the ZnCo₂O₄//AC device were calculated using the following equations [41]:

E = Cs ΔV²/7.2

(5)

P = 3600 E/Δt

(6)

where Cs (F g⁻¹) is the specific capacitance, Δt (s) is the discharge time, and ΔV (V) is the potential window. Figure 8f shows the Ragone plot of the ZnCo₂O₄//AC HSC device. The Ragone plot showed a maximum energy density of 41.87 Wh kg⁻¹ for the HSC device at a power density of 800.00 W kg⁻¹. The ZnCo₂O₄//AC HSC device retained 64.2% of the energy density, even at a power density of 4000.00 W kg⁻¹. These values were comparable, or even superior, to those of the previously reported data for the other HSC devices, such as ZnCo₂O₄ nanoparticles//AC (26.28 Wh kg⁻¹ at 716 W kg⁻¹) [26], ZnCo₂O₄ ultra-thin curved sheets//AC (20.31 Wh kg⁻¹ at 855 W kg⁻¹) [42], ZnCo₂O₄/H:ZnO//AC (3.75677 Wh kg⁻¹ at 653.34 W kg⁻¹) [43], NiCo₂O₄–MnO₂//activated graphene (9.4 Wh kg⁻¹ at 175 W kg⁻¹) [44], NiCo₂O₄ nanosheets@hollow microrod arrays//AC (15.42 Wh kg⁻¹ at 780 W kg⁻¹ [45], NiCo₂O₄–rGO//AC (23.32 Wh kg⁻¹ at 324.9 W kg⁻¹) [46], NiCo₂O₄//RGO (23.9 Wh kg⁻¹ at 650 W kg⁻¹) [47], Ni–Co–oxide//AC (12.0 Wh kg⁻¹ at 100 W kg⁻¹) [48], CuCo₂O₄@CuCo₂O₄//AC (18 Wh kg⁻¹ at 125 W kg⁻¹) [49], and FS–CoFe₂O₄/GS//graphene sheet (28.4 Wh kg⁻¹ at 900 W kg⁻¹) [50]. Overall, the necklace-type ZnCo₂O₄ nanowires electrode is a promising candidate for electrochemical supercapacitor applications due to their unique morphology, mesoporous structure, high surface area, synergistic effect between Zn and Co metals, and higher electrical conductivity.

3. Experimental

Analytical grade chemicals, such as zinc acetate dihydrate (Zn(CH₃COO)₂.2H₂O), cobalt acetate tetrahydrate (Co(CH₃COO)₂.4H₂O), urea, ammonium fluoride, EG, potassium hydroxide, polyvinylidene difluoride, and N-methyl-2-pyrrolidinone were purchased from Sigma Aldrich Chemicals. Deionized water (DI H₂O) was used throughout this work. Absolute ethanol (C₂H₅OH, 99.9%) was procured from DUKSAN Pure Chemicals, South Korea. Commercial NF (mass density: 346 g/m², thickness: 1.6 mm) was obtained from MTI Korea.

Necklace-type ZnCo₂O₄ nanowires were synthesized by a simple hydrothermal method using water/EG mixed solvent and subsequent calcination treatment. First, ~0.219 g of Zn(CH₃COO)₂.2H₂O, ~0.498 g of Co(CH₃COO)₂.4H₂O, ~0.30 g of urea, and ~0.074 g of NH₄F were dissolved in a mixed solution of 20 mL water and 20 mL EG with magnetic stirring for 30 min. After that, the clear pink-color solution was transferred to a 50 mL autoclave, and a hydrothermal treatment was carried out at 140 °C for 12 h in an oven. When the hydrothermal process was finished, the as-formed pink-color precipitates were washed with DI H₂O and C₂H₅OH. Then, the cleaned precipitate was dried at 70 °C, and the necklace-type ZnCo₂O₄ nanowires precursor was successfully collected. Finally, the as-grown precursor was calcined to form necklace-type ZnCo₂O₄ nanowires at 400 °C for 2 h in a muffle furnace at a heating rate of 5 °C min⁻¹. The detailed experimental photographs, physicochemical characterization, and electrochemical supercapacitor studies were shown in Supplementary Material (SM).

4. Conclusions

Mesoporous necklace-type ZnCo₂O₄ nanowires were synthesized by a simple one-pot hydrothermal method and subsequent annealing process in air. Crystalline phase, morphology, and surface area analyses revealed that the as-synthesized ZnCo₂O₄ nanowires possessed AB₂O₄-type cubic spinel structure and assembled by numerous mesoporous tiny building blocks of nanoparticles. The evaluated supercapacitor properties in a three-electrode system suggested that the necklace-type ZnCo₂O₄ nanowires had a high specific capacity (439.6 C g⁻¹ at 1 A g⁻¹), decent rate performance (47.31 % capacity retention at 20 A g⁻¹), and long-term durability (84.82 % capacity retention after 5000 cycles). The superior electrochemical properties of the ZnCo₂O₄ nanowires electrode were attributed to the unique morphology, mesoporous structure, high surface area, and synergistic effect between the two metals. Successively, the HSC device fabricated with the ZnCo₂O₄ nanowires and AC electrodes delivered a reasonable specific capacitance (117.7 F g⁻¹ at a current density of 1 A g⁻¹), high energy density of 41.87 Wh kg⁻¹, and power density of 800.0 W kg⁻¹. This study on ZnCo₂O₄ nanowires highlights their great potential as an electrode material for supercapacitor applications due to their high electrochemical properties.