Introduction

With the development of science and technology and the needs for human production and life, the problems of environmental pollution and energy scarcity have gradually intensified. Currently, storing energy mainly relies on batteries. But the batteries usually contain environmentally unfriendly materials (lithium, lead-acid, etc.). There are urgent needs for efficient storage performance, low-cost, and green energy storage devices. With high power density, fast charging and discharging, green environment protection, and long service life, supercapacitors are considered one of the most promising energy storage devices among the new energy storage technologies in the energy fields [1,2,3,4,5]. However, the energy density of supercapacitor is relatively low, which greatly affects their application in the market [6,7,8]. According to the energy density equation E = 1/2CV2, energy density is directly related to the specific capacitance (CS) and voltage window (V) [9,10,11]. By designing and developing electrode materials with excellent properties, the specific capacitance can be improved [12]. In addition, by using two different electrode materials and assembling asymmetric supercapacitors, the voltage window can be expanded and thus the energy density can be improved [13]. Therefore, the exploration of high-performance electrode materials has become a research hotspot worldwide. Compared to the conventional carbon materials, multi-metal oxides are not only environmentally friendly and stable in nature, but also have significant synergistic effects and rich microstructures. These advantages are useful to increase the electrical conductivity, specific surface area, surface active sites, and other favorable factors, all of which effectively enhance the electrochemical properties of the materials [14,15,16]. Wang et al. [17] reported multilayer CoFe2O4 hollow microspheres with a high specific capacitance value of 1231 F g−1. Han et al. [18] reported a specific capacitance of 810.93 F g−1 for SrBiO3 at a current density of 1 A g−1. Guan et al. [19] reported that the specific capacitance of NiCo2O4 nanocomposite materials was 1055.3 F g−1. Among these bimetallic oxides, ZnCo2O4 [20] has a unique pore size structure, large specific surface area, good electrical conductivity, and large theoretical specific capacity, which makes it a very promising electrode active material for supercapacitors. Zhu et al. [21] successfully prepared the porous ZnCo2O4 nanosheets. The capacitance maintenance rate reached 96.3% after 5000 cycles of charge and discharge, indicating an excellent cycling stability performance. Shang et al. [22] prepared hollow ZnCo2O4 nano-microspheres. The material had a high specific capacitance of 776.2 F g−1. Xu et al. [23] prepared porous ZnCo2O4 nanostructured materials. The ZnCo2O4//RGO asymmetric devices exhibited a high energy density of 84.48 Wh kg−1 at a wide potential window of 1.6 V. Therefore, the design and development of electrode materials with excellent electrochemical properties are an important strategy to drive the commercialization of supercapacitors.

Among the many preparation methods, the nanostructured materials synthesized by the water bath method can be grown directly onto the conductive substrate, the active material can be uniformly distributed on its surface, and the substrate structure can remain stable with good bonding between the active material. Calcination treatment has many advantages. Firstly, it can keep the matrix structure stable and combine well with the active material. Secondly, it can also eliminate any bad impurities produced in the early preparation to ensure uniform and stable metal dispersion on the carrier. Therefore, in this paper, the precursors were first prepared in a water bath, and then the electrochemical properties of the materials were improved by calcination treatment.

In this paper, ZnCo2O4 NWAs were successfully prepared using a simple one-step water bath method and calcination treatment. These nanowires grow interlaced with each other and formed large number of pores structures. These characteristic advantages facilitate the transfer of ions and electrons between the material interface and surface. The ZnCo2O4 electrode has a high specific capacitance of 2300 F g−1 at a current density of 1 A g−1 and a capacitance retention rate of 96.84% with good cycling stability. At last, we assembled the asymmetric supercapacitor device using the as-prepared ZnCo2O4 NWAs as the positive electrode and the AC as the negative electrode. This asymmetric device shows a wide potential window of 1.6 V, a high energy density of 88.18 Wh kg−1 (the power density of 800 W kg−1), and excellent cycling performance (97.42% after 10,000 charge/discharge cycles).

Experiment

As a conductive substrate, CC has good electrical conductivity and a loose fiber braided structure, which is conducive to the diffusion of electrolytes into the electrode material. At the same time, the specific capacity of CC is low and its properties are stable. Therefore, its electrochemical performance analysis of materials can be almost ignored. In the experiments, analytical grade reagents were used without further purification.

Preparation of ZnCo2O4 NWAs electrodes

CC as a conductive substrate has good electrical conductivity and a loose fiber weave structure, and this structure facilitates the full diffusion of electrolytes into the electrode material. In this paper, ZnCo2O4 NWAs were prepared on the surface of CC as a conductive substrate using a one-step water bath thermal method followed by heat treatment. The ZnCo2O4 NWAs were prepared directly grown on CC by one-pot low-temperature water bath method with the reaction temperature of 80 ℃ for 2 h. Firstly, 1 mmol Zn(NO3)2•6H2O, 3 mmol Co(NO3)2•6H2O, 3.5 mmol CO(NH2)2, and 2 mmol NH4F were mixed in 40 ml of ultrapure water with sonication for 3.5 h. The mixtures were placed in a water bath with a piece of cleaned CC and kept at 80 ℃ for 3 h. Then, the samples were cooled to room temperature and dried for 8 h in an oven at 50 ℃. Then, the samples were calcined at 550 ℃ for 5 h. Finally, ZnCo2O4 NWAs electrode was obtained.

Preparation of AC electrode

AC material was used as the negative electrode material for the asymmetric device. The preparation process of this negative electrode material is as follows: the corresponding proportions of activated carbon, carbon black, and polyvinylidene fluoride (PVDF) (mass ratio R = 85:10:5) are firstly weighed, and the weighed powder is ground in a mortar. Then, the well-ground powder is added to an appropriate amount of N-methyl-pyrrolidone (NMP) organic solvent. Finally, the above mixture was stirred on a magnetic stirrer for a certain time until a homogeneous and viscous slurry was formed. The slurry is evenly applied to a CC conductive substrate with an area of 1 × 1 cm2. The treated activated carbon negative electrode material was put into a vacuum drying oven and dried at 60 ℃ for 6 h. The AC electrode was successfully prepared.

Preparation of gel state asymmetric supercapacitor

An asymmetric supercapacitor device was assembled using the ZnCo2O4 electrode as the positive electrode and the activated carbon electrode as the negative electrode. The PVA/KOH gel electrolyte was synthesized by adding 2.8 g KOH and 3 g polyvinyl alcohol (PVA) to 25 mL deionized water. The electrode materials and separators were immersed in the PVA/KOH gel electrolyte for a few minutes by heating and stirring at 80 °C for 5 h. The electrode materials were then removed from the gel and the separators were immersed in the PVA/KOH gel electrolyte. Then, they were removed from the gel electrolyte and assembled together. The device was placed in the air for 12 h and became solid.

Electrochemical performance test

The electrochemical performance tests in this thesis were carried out using the Shanghai C&H CHI660C electrochemical workstation. The tests under three-electrode conditions were carried out with a 1 × 4 cm2 platinum sheet as the counter electrode and the test electrolyte was an alkaline 2 M KOH solution, so a saturated glycerol electrode was chosen as the reference electrode for the tests in the experiment, and the prepared material was used directly as the working electrode to test the capacitive performance. The device’s electrochemical properties were tested under two-electrode conditions and the rest of the conditions were identical. Cyclic voltammetry (CV), constant current charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS, 0.01 ~ 100 kHz with an amplitude of 5 mV) were tested, and the calculated capacitance was as follows:

$${C}_{s}=It / mV$$
(1)

CS (F g−1) is the specific capacitance of a single electrode. I (A) represents the discharge current. ∆t (s) is the discharge time. m (g) is the mass of the active material, and ∆V (V) is the voltage window. The electrochemical measurements of the energy density (E) and power density (P) of the device are calculated by the following equations. Herein, the mass of positive ZnCo2O4 is 2.5 g, and the mass of activated carbon AC is 0.32 g calculated according to Eq. 3.

$$q=C\times \Delta V\times m$$
(2)
$$m_+/m_-=\left(C_-\times\Delta V-\right)/\left(C_+\times{\Delta V}_+\right)$$
(3)
$$E=O.5{C}_{S}{V}^{2}$$
(4)
$$P=3600E/t$$
(5)

Material characterization

The morphology of the prepared samples was observed with a scanning electron microscope (SEM, JEOL JSM-7500F) and a transmission electron microscope (TEM, JEOL JEM-2100F). The crystal structure of the prepared materials was observed by X-ray diffraction (XRD, RigakuD/max-2600 PC, radiation source Cu Kα, λ = 1.5406 Å). Electrochemical tests were carried out on a Shanghai Chenhua CHI660E electrochemical workstation.

Results and discussion

The morphological tests of the as-prepared samples are shown in Fig. 1. Figure 1 a shows the SEM image of the CC, which can be found to be composed of many carbon fibers with a single carbon-fiber diameter of 10 µm. Figure 1b  shows the SEM images of ZnCo2O4 nanowires on the CC conductive substrate. It can be clearly seen that a large number of ZnCo2O4 NWAs are uniformly distributed on the CC conductive substrate skeleton. Figure 1c shows a high magnification SEM image of ZnCo2O4 NWAs and it can be found that the nanowires are of uniform size. ZnCo2O4 NWAs are interconnected to form a structure and this structure increases the specific surface area of the material, which can make the electrons and ions fully react with the active material more adequately and thus store more charges. In order to further analyze the elemental composition of the products, we performed SEM mapping tests on the products, as shown in Fig. 1d. The results show the presence of three elements, Zn, Co, and O, and no other elements, indicating that the prepared product is pure ZnCo2O4.

Fig. 1
figure 1

a–c SEM images of ZnCo2O4 nanowires at different magnifications; d–f elemental mapping by EDS of Co, O, and Zn elements, respectively]

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To further investigate the nanostructure of ZnCo2O4, we performed TEM tests on the products. As can be seen from Fig. 2a, the diameter of a single nanowire is 20 nm. Figure 2b is an HRTEM image of the product. It can be found that the lattice spacing of the prepared ZnCo2O4 is 0.23 nm and 0.24 nm, respectively. The results were corresponding to the (222) and (311) lattice planes of ZnCo2O4. TEM mapping test is carried out in the yellow box in Fig. 2a in the illustration. It can be found that there are only three elements of Zn, Co, and O. Figure 2c shows the EDS test of the product with CC as the conductive substrate, the results show the presence of Zn, Co, O, and C elements. This was consistent with the EDS and TEM results, indicating that the prepared samples were ZnCo2O4. Figure 2d shows the XRD pattern of the material, showing ZnCo2O4 as a crystalline structure, which is confirmed as ZnCo2O4 (JCPDS Card No. 23–1390) and the peak intensity is also high without the presence of other impurity peaks. The above multiple results indicate that the prepared sample is ZnCo2O4 with high purity.

Fig. 2
figure 2

a TEM image of ZnCo2O4 NWAs structure. The area in the orange box is used for the HRTEM test of ZnCo2O4 NWAs structure; b HRTEM images of ZnCo2O4 NWAs structure; c EDS measurement of ZnCo2O4 NWAs; d XRD patterns of ZnCo2O4 NWAs materials

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We also further studied the surface chemical composition and elemental composition of the obtained samples. XPS test method was used to characterize and analyze the samples, as shown in Fig. 3a. Through XPS analysis of the composite, we can find that peaks of four elements, O, C, Zn, and Co, exist simultaneously in the total spectrum. Among them, C is from CC, while Zn, Co, and O are from zinc cobaltate, and no peaks of other elements are found, which is consistent with the XRD test results. In the peak spectrum of C, the peak of carbon is located at 248.8 eV, which is the standard peak spectrum of carbon [24, 25]. Figure 3b shows the XPS diffraction peak spectrum of Zn 2p, where Zn 2p1/2 is located at the peak position of 1044.76 eV, while Zn 2p3/2 is located at the peak position of 1021.5 eV, and the difference between the two peaks is about 23.23 eV, representing the Zn 2p1/2 and Zn 2p3/2 peaks of Zn2+, respectively [26]. Figure 3c shows the energy spectrum of Co 2p, with two sharp peaks observed at 795.3 eV and 780.4 eV, corresponding to Co 2p1/2 and Co 2p3/2, respectively, with a difference of about 14.9 eV between the two peak positions [27]. The peak spectrum of O 1 s is shown in Fig. 3d, in which the peak at 532.07 eV is a typical metal–oxygen bond, while the peak at 530 eV is an oxygen ion bond with low coordination between metal and surface. The difference between the two peaks is about 2.07 eV, which is consistent with the values reported in previous literature [28]. The above peak values are consistent with the theoretical values of zinc ions occupying tetrahedral positions in spinel structures. The above analysis shows that the oxidation valence states of Zn (II) and Co (III) are consistent with the corresponding valence states of ZnCo2O4. XRD and XPS results show that the samples are high-purity ZnCo2O4.

Fig. 3
figure 3

XPS of ZnCo2O4: a survey scan, b Zn 2p, c Co 2p, and d O 1 s

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In the paper, we also analyzed the formation mechanism of ZnCo2O4 nanowires [29]. In the initial stage of the water bath thermal reaction, the Zn2+ ions and Co2O42− ions dissolved in the solution combine to form small ZnCo2O4 nuclei particles, which are aggregated on the CC. These nanoparticles are unstable due to their large surface energy. As the reaction proceeds, these small nanoparticles will continue to aggregate and become larger thus reducing the surface energy. By controlling the reaction time and temperature of the water bath thermal reaction, the ZnCo2O4 nanowire structure can be formed by directional growth.

In the experiment, the electrochemical performances of ZnCo2O4 NWAs were tested in a three-electrode system using 2 M KOH as an electrolyte. At a sweep rate of 5 mV s−1, we plotted the cyclic voltammetric curves of CC and ZnCo2O4 NWAs, as shown in Fig. 4a. The results show that the specific capacity of the ZnCo2O4 NWAs electrode is much higher than that of the pure CC, indicating that the contribution of pure CC to the capacitance of the ZnCo2O4 NWAs electrode is negligible. Figure 4b  shows the constant current charge–discharge curves of the CC and ZnCo2O4 NWAs electrodes. At a current density of 2 A g−1, the discharge time of the ZnCo2O4 NWAs electrode is much longer than that of the CC electrode. It also shows that the specific capacity of the ZnCo2O4 NWAs electrode is much larger than that of the CC. From Eq. 1, we calculated the specific capacitance of the CC and ZnCo2O4 NWAs electrodes at a current density of 2 A g−1, as shown in Fig. 4c. The specific capacitances of the CC and ZnCo2O4 NWAs were 30 F g−1 and 2192 F g−1, respectively. These results further confirm that the CC has little effect on the total capacitance of the ZnCo2O4 NWAs electrode. Figure 4d shows the CV curves of the ZnCo2O4 NWAs electrode at different sweep rates. The CV curves show obvious redox peaks, which indicate that ZnCo2O4 NWAs are pseudocapacitive energy storage properties. As the scan rate increased, the peak current increased. The curve increases proportionally and the shape remains basically unchanged. It indicates that it has good electrochemical reversibility and the prepared ZnCo2O4 NWAs electrode has fast electron and ion transport capability. In addition, the charge storage mechanism of ZnCo2O4 NWs has also been investigated in this paper. Under alkaline conditions, Zn ions readily react with OH− in solution to form Zn(OH)42−. When the temperature gradually increases to a suitable temperature, Co2+ and Zn2+ react with hydroxyl ions to form ZnCo2(OH)4 particles. By contacting with air, the ZnCo2O4 microstructure is eventually formed [30]. The chemical reactions involved are as follows.

Fig. 4
figure 4

a Cyclic voltammetry curves of CC, ZnCo2O4 NWAs; b charge and discharge of CC and ZnCo2O4 NWAs at the current density of 2 A g−1; c comparison of specific capacity between CC and ZnCo2O4 NWAs at 2 A g−1; d cyclic voltammetry of ZnCo2O4 NWAs at different sweep rates; e charge and discharge tests of ZnCo2O4 NWAs under different current intensities; f specific capacitance diagram of ZnCo2O4 NWAs under different current densities

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$${Zn}^{2+}+4{OH}^{-}\to Zn{\left(OH\right)}_{4}^{2-}$$
(6)
$$\mathrm{Zn}\left(\mathrm{OH}\right)_4^{2-}\rightarrow\mathrm{Zn}O+{\mathrm H}_2\mathrm O+{2\mathrm{OH}}^-$$
(7)
$$2\mathrm{Co}^{2+}+\mathrm{Zn}^{2+}+6O^{\mathrm H^-}\rightarrow\mathrm{Zn}{\mathrm{Co}}_2{\left(\mathrm{OH}\right)}_4$$
(8)
$$\mathrm{Zn}{{\mathrm{Co}}_2\left(\mathrm{OH}\right)}_4+\frac160_2\rightarrow\mathrm{Zn}{\mathrm{Co}}_2{\mathrm O}_4+{3\mathrm H}_2\mathrm O$$
(9)

Figure 4e shows the constant current charge and discharge curves at different current densities. These charge/discharge curves have good symmetry and show good electrochemical reversibility. However, there is still a phenomenon of shorter discharge time, which is mainly due to the chemical charge storage in the whole process, and the ions and electrons will have certain influence in the embedding or ejection process [31]. The specific capacitance values of the ZnCo2O4 NWAs electrodes were calculated to be 2300, 2130, 1940, 1792, 1620, and 1440 F g−1 at current densities of 1, 3, 5, 8, 10, and 15 A g−1, respectively, as shown in Fig. 4f. The theoretical specific capacity of ZnCo2O4 is 2604 F g−1. At the current densities of 1, 3, 5, 8, 10, and 15 A g−1, the specific capacity reaches 2300, 2130, 1940, 1792, 1620, and 1440 F g−1, respectively. The ratio and capacity retention rates reached 88.3%, 81.8%, 74.6%, 68.8%, 62.2%, and 55.3%, respectively. Through the search of relevant literature, the multiplier performance is still very excellent [32,33,34,35]. The results indicate that the prepared ZnCo2O4 NWAs electrodes have excellent electrochemical properties.

In order to further investigate the diffusion effect of ZnCo2O4 nanomaterial, we tested the cyclic voltammetry curve of ZnCo2O4 nanomaterial under different scanning speeds and different conditions, and studied the kinetic behavior of ZnCo2O4 nanomaterial in detail. Figure 5a shows the CV curves of ZnCo2O4 NWAs materials at different sweeping speeds. It can be found that with the increase of sweeping speed, the CV curves of materials also gradually increase, which is due to the increase of the response current of materials due to the large sweeping speed. In addition, the oxidation peak shifts to the high voltage while the reduction peak shifts to the low voltage with the gradual increase of the sweeping speed, and the difference of polarization voltage increases [36]. According to the current research theory, the electrochemical properties of materials are mainly affected by the pseudocapacitance behavior controlled by diffusion and surface control. According to literature reports [37, 38], the electrochemical behavior of ZnCo2O4 NWAs can be analyzed by using the formula (i = avb, log(i) = blog(v) + log(a), i = k1v + k2v1/2). For a value of b close to 0.5, the performance is limited by the diffusion control behavior, while a value of b close to 1.0 indicates pseudocapacitance behavior, which is determined by capacitance. Figure 5b  shows the contribution rate of pseudocapacitance capacity at different sweeping speeds. It can be found that the capacitance proportion gradually increases with the increase of sweeping speed. According to the phase of gradual reaction represented by the highest redox peak, the b values of cathode and anode processes are 0.68 and 0.58, respectively, as shown in Fig. 5c. It shows that the reaction of ZnCo2O4 NWAs is a mixed behavior [39]. According to the above formula, the capacitance contribution of ZnCo2O4 NWAs is 54.6% at 10 mV s−1, and the ratio increases to 82.2% at 100 mV s−1. These results indicate that ZnCo2O4 has superior electrochemical capability at high rates.

Fig. 5
figure 5

a CV curves of ZnCo2O4 NWAs at the scan rate from 10 to 100 mV s.−1. b Contribution rate of pseudocapacitance at different sweeping speeds. c Linear diagram of log (i) and log (v)

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The cycling stability is also an important factor in evaluating the performance of the material. In the experiments, we tested the capacity retention of the material after 10,000 cycles. As shown in Fig. 6a, the specific capacitance of the ZnCo2O4 NWAs electrode decreased from 1930 F g−1 to 1869 F g−1 at a current density of 5 A g−1, maintaining a capacitance retention of 96.84%. We have compared our research work with the electrochemical properties of other materials, as shown in Table 1. Figure 6b shows the impedance plots for the ZnCo2O4 NWAs electrode after the 1st and 10,000th cycles. There is no significant difference in the arc increment in the high-frequency region, indicating that this holds up well over the 10,000th cycle. After 10,000 cycles, the linear slope in the low-frequency region decreases, which indicates a slight increase in the diffusion resistance of the material, which is due to the corrosion of the nanomaterials by dissolved oxygen in the electrolyte, resulting in the loss of some active substances [40]. The extremely small impedance and excellent cycling stability of the material may be mainly attributed to the interweaving of nanowires, which creates a large surface area and thus improves the utilization of the electrode material.

Fig. 6
figure 6

a 10,000 cycles of stability test at current density 5 A g−1; b Nyquist diagrams of the first and 10,000th cycles of ZnCo2O4 nanostructures are illustrated as equivalent circuit diagrams

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Table 1 Comparison of properties of electrode materials with references
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The rate performance of the ZnCo2O4 NWAs electrode at different current densities is shown in Fig. 7. At a current density of 5 A g−1, the ZnCo2O4 NWAs electrode showed a stable specific capacity of 1947 F g−1 for the first 100 cycles. Over the next 600 cycles, varying the current density, the ZnCo2O4 NWAs electrode always showed a stable capacitance. When returning to the initial current, its specific capacity was 1938 F g−1. The specific capacity remained almost unchanged, indicating that the ZnCo2O4 NWAs electrode has excellent cycling stability performance. According to the above performance research, it is concluded that the reason why this material has these excellent properties is mainly due to the special structure of CC and NWAs. CC has better conductivity; the material grows directly on the conductive substrate and has better binding force with the material, which can effectively prevent the active material from falling off from the conductive substrate. This ZnCo2O4 NWAs structure is conducive to the transmission of electrolyte ions. Also, ZnCo2O4 NWAs grown on CC with nanostructures provide a relatively high specific surface area and abundant reactive active sites for the material, which improves the electrochemical reaction efficiency.

Fig. 7
figure 7

Rate and cycle stability tests under different current densities

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Synthesizing the above studies related to the performance, we conclude that the reasons for the excellent performance are as follows: firstly, the water bath method makes ZnCo2O4 grow uniformly on the conductive substrate, and the two have good bonding force. Then, after calcination treatment, the structure of the substrate is stabilized to eliminate the undesirable impurities generated in the early stage and ensure the uniform and stable dispersion of the metal on the carrier. Secondly, carbon cloth has the advantages of good electrical conductivity and large specific surface area. As the growth skeleton supporting the active material, the carbon cloth not only prevents the re-accumulation of ZnCo2O4 NWAs, but also enhances the connection between the carbon fiber skeleton and ZnCo2O4 NWAs, which constructs a high-speed channel for the charge transfer and storage. Finally, the special structure of nanowire arrays, which provides a very large specific surface area, shortens the diffusion distance of ions and improves the electrochemical properties of the material.

To further investigate the practical applications of the synthesized material, it was assembled into devices and its device performance was investigated. An asymmetric supercapacitor (ASC) device was assembled using ZnCo2O4 NWAs as the positive electrode and AC as the negative electrode. The device was tested in a two-electrode system with a 2 M KOH solution as the electrolyte.

Figure 8a  shows the charging and discharging curves of the AC electrode at current densities of 1, 3, 5, 8, 10, and 15 A g−1, respectively. The charging and discharging curves are nearly triangular with good symmetry, which indicates that the AC electrode has a good charge and discharge reversibility. The specific capacitance values of the AC electrode at different current densities were calculated according to Eq. 1, as shown in Fig. 8b. The specific capacitance of the device is 227, 208, 190, 181, 171, and 131 F g−1 with the current densities of 1, 3, 5, 8, 10, and 15 A g−1, respectively. The materials were further assembled into ZnCo2O4//AC asymmetric devices and the capacitive performance of the asymmetric devices was investigated. Under the condition of scanning speed of 8 mV s−1 and 2 M KOH electrolyte, ZnCo2O4 and AC are the cyclic voltammetry curves of positive and negative electrodes, respectively, as shown in Fig. 9a. The voltage window of the device is different between the potential window of the positive and negative electrodes. The theoretical potential window of ZnCo2O4 NWAs//AC reaches 1.6 V. Figure 9b shows the CV curves of the ZnCo2O4 NWAs//AC device for different potential windows. It is clear that the shape of the CV curves at different voltage windows is broadly similar and the area increases with increasing potential window, which indicates that the device is stable over a potential window range of 0 to 1.6 V. It is consistent with the theoretical potential window. Figure 9c shows the CV curves of the ASC device at different scan rates. The overall shape of the CV curves hardly changes as the scan rates increase and the area of the curve increases with the scan rate. It indicated that the electrochemical reaction speeds up and the amount of charge stored increases with the increase of the scanning rates. Charge and discharge tests of the ACS devices were carried out at current densities of 1, 3, 5, 8, 10, and 15 A g−1, as shown in Fig. 9d. The charge–discharge curves are basically symmetrical, indicating good electrochemical stability and performance. The specific capacitance of the device was calculated according to Eq. 1 and the results are shown in Fig. 9e. The specific capacitance of the device was 248, 222, 194, 184, 176, and 151 F g−1 at current densities of 1, 3, 5, 8, 10, and 15 A g−1. The device was then tested for cycling stability, as shown in Fig. 9f. The device was charged and discharged for 10,000 cycles at a current density of 5 A g−1 and the specific capacitance value decreased from 194 F g−1 at the beginning to 189 F g−1 with a retention rate of 97.42%, showing excellent cycling stability.

Fig. 8
figure 8

a Charge and discharge tests of AC under different current intensities; b specific capacitance diagram of AC under different current densities

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Fig. 9
figure 9

a Cyclic voltammetry of ZnCo2O4 and AC electrode materials in the electrolyte with sweep speed of 8 mV s−1 and 2 M KOH under the condition of three electrodes; b cyclic voltammetry curve of ZnCo2O4//AC asymmetric device when the voltage window changes from 0.8 to 1.6 V at a sweep speed of 10 mV s−1; c cyclic voltammetry test diagram of ZnCo2O4//AC asymmetric device at different sweep speeds; d charge–discharge curve test diagram of ZnCo2O4//AC at different current density; e specific capacitance diagram of ZnCo2O4//AC asymmetric devices at different current densities; f stability test diagram of ZnCo2O4//AC asymmetric device with 10,000 cycles at the current density of 5 A g.−1

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Experimentally, the assembly of the device involves a mass relationship between the positive and negative active substances which we can calculate by means of the charge balance relationship: q +  = q− [50]. q + and q− represent the charge stored at the positive and negative electrodes respectively. The value of the charge q at each electrode is related to the specific capacity of the electrode material (Cs), the voltage of the discharge section of the electrode material (ΔV), and the mass of the electrode material, as shown in Eq. 2. Rectifying the equations, the equation for the relationship between the positive (m+) and negative (m−) electrode masses can be derived as follows: Eq. 3. Bringing in the relevant data respectively, the mass ratio of the positive and negative electrode materials for the asymmetric device is calculated as m+/m− ≈ 1/8.

Finally, we tested the morphology and structure of the nanomaterials before and after cycling, as shown in Fig. 10a,b . From the figures, we can find that the structure of the materials before and after cycling is basically unchanged, and the linear structure is still well maintained. This indicates that the ZnCo2O4 NWAs nanomaterials prepared by this method have excellent structural stability. In order to observe the physicochemical properties of the materials after cycling, we also measured the XRD and XPS analyses, as shown in Fig. 10c,d. It can be seen from the figures that no significant changes were found in the ZnCo2O4 NWAs material after cycling, but there was a slight trend of weakening of the XRD or XPS characteristic peak intensity. This indicates that the experimentally prepared ZnCo2O4 NWAs have good cycling stability [51, 52], which is consistent with the SEM test.

Fig. 10
figure 10

SEM image of ZnCo2O4 NWAs: a cycle before; b after circulation; c XRD patterns of ZnCo2O4 NWAs materials after cycling; d XPS of ZnCo2O4 NWAs after cycling

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We compared our work with other energy storage devices [53,54,55,56], as shown in Fig. 11. The energy density and power density of the ZnCo2O4 NWAs//AC asymmetric device were calculated according to Eqs. 4 and 5, respectively. At an operating voltage of current density of 1 A g−1, the maximum energy density is 88.18 Wh kg−1 and the corresponding power density is 800 W kg−1. At a current density of 15 A g−1, the device has a maximum power density of 12,000 W kg−1 and an energy density of 53.69 Wh kg−1. The device we have assembled can be comparable to the lithium batteries.

Fig. 11
figure 11

Energy density and power density of ZnCo2O4 NWAs//AC compared to other energy storage devices

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Conclusion

In summary, a binary metal oxide ZnCo2O4 NWAs with excellent electrochemical performances were prepared by a pot water bath method and calcination treatment. The prepared ZnCo2O4 NWAs electrode achieved a high specific capacitance of 2300 F g−1 at a current density of 1 A g−1 and maintained an initial specific capacitance of 96.84% after 10,000 charge/discharge cycles at a current density of 5 A g−1. The NWAs structure gives the ZnCo2O4 material a good specific capacitance and excellent cycling stability. In addition, we have assembled a ZnCo2O4 NWAs//AC asymmetric device. The ZnCo2O4 NWAs//AC asymmetric device with good specific capacity (248 F g−1 at 1 A g−1) and excellent capacity retention rate (97.42% after 10,000 cycles at 5 A g−1). The device has a maximum energy density of 88.18 Wh kg−1 (specific power 800 W kg−1) and a maximum power density of 12,000 W kg−1 (specific energy 53.69 Wh kg−1). All these results confirm that the prepared electrodes and devices are very promising for the development of energy storage devices.