MnO₂/ZnCo₂O₄ with binder-free arrays on nickel foam loaded with graphene as a high performance electrode for advanced asymmetric supercapacitors

Abstract

ZnCo₂O₄ nanosheets were successfully arrayed on a Ni foam surface with graphene using a hydrothermal method followed by annealing treatment; then MnO₂ nanoparticles were electrodeposited on the ZnCo₂O₄ nanosheets to obtain a synthesized composite binder-free electrode named MnO₂/ZnCo₂O₄/graphene/Ni foam (denoted as MnO₂/ZnCo₂O₄/G/NF). After testing the binder-free composite electrode of MnO₂/ZnCo₂O₄/G/NF via cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy testing, we found that it exhibited ultrahigh electrochemical properties, with a high specific areal capacitance of 3405.21 F g⁻¹ under a current density of 2 A g⁻¹, and wonderful cycling stability, with 91.2% retention after 5000 cycles. Moreover, an asymmetric supercapacitor (ASC) based on MnO₂/ZnCo₂O₄/G/NF//G/NF was successfully designed. When tested, the as-designed ASC can achieve a maximum energy density of 46.85 W h kg⁻¹ at a power density of 166.67 W kg⁻¹. Finally, the ASC we assembled can power a commercial red LED lamp successfully for more than 5 min, which proves its practicability. All these impressive performances indicate that the MnO₂/ZnCo₂O₄/graphene composite material is an outstanding electrode material for electrochemical capacitors.

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1. Introduction

With societal development, and science and technology progress, the electronics industry urgently needs energy storage devices that are high-performance, lightweight and environmentally friendly to achieve sustainable and renewable energy.¹ For utilizing renewable sources, supercapacitors are one of the most promising candidates on account of their fast charging capabilities, high power densities and good cycling performances.² Although they have these excellent features, the relatively low energy densities of supercapacitors seriously limit their large-scale practical application.³ Thus, it is necessary to improve the energy densities of supercapacitors to meet future energy demand. In recent years, it has been found that seeking the best electrode material is key to the development of supercapacitor research.⁴ However, traditional electrode materials have been unable to meet the higher energy density requirements of supercapacitors. Therefore, developing new electrode materials with excellent properties is critical.

Recently, ternary ZnCo₂O₄ has attracted a great amount of research attention because its structure, a normal spinel structure with Zn²⁺ occupying the tetrahedral sites in the cubic spinel and Co³⁺ occupying the octahedral sites, renders ZnCo₂O₄ nanomaterials with high theoretical specific capacitances, excellent electrical conductivities, and rich active sites for redox reactions.^5–7 Meanwhile, it has the advantages of abundant availability, low cost and environmental friendliness.⁸ However, it also presents problems, such as a low surface area and a large decrease in performance following long-term utilization. Adhering flexible materials to a conductive substrate to form a porous composite with channels conducive to the efficient diffusion of ions and the obtaining of high surface area, and the direct growth of hetero-structures on conductive substrates to make adequate use of the merits of different kinds of materials are viable ways to solve these issues. For instance, Gao et al. reported that the facile hydrothermal deposition of ZnCo₂O₄ nanoflakes on Ni foam with reduced graphene oxide could delivered a high capacitance of 860 F g⁻¹ and high rate capabilities.⁹ Li et al. successfully synthesized MnO₂/ZnCo₂O₄ nanosheet arrays with a specific capacitance of 286 F g⁻¹ in a three-electrode system via a facile solvothermal method, and assembled supercapacitors exhibited a high energy density of 16.94 W h kg⁻¹ at 750 W kg⁻¹.¹⁰

As is well known, graphene can serve as a skeleton for a variety of composite materials with enhanced supercapacitor performance due to it being a conductive 2D flexible carbon material.¹¹ MnO₂ has advantages of having a high theoretical capacitance of up to 1370 F g⁻¹, being low cost and nontoxic, and having abundant resources, and it has been widely used in supercapacitors.¹² Therefore, considering the merits of these materials, combining ZnCo₂O₄ with graphene and MnO₂ would be propitious for further improving the energy densities of supercapacitors.

Based on the above discussion, in this study, we demonstrate a composite electrode comprising MnO₂ nanoparticles loaded on a porous ZnCo₂O₄ nanosheet material supported on graphene coated on a Ni foam substrate, which is shown in Fig. 1. Graphene was first dropped onto the Ni foam, and then the ZnCo₂O₄ material was grown on the graphene substrate via a simple hydrothermal strategy, followed by annealing treatment; finally, MnO₂ was deposited on the ZnCo₂O₄ substrate to form the MnO₂/ZnCo₂O₄/graphene/Ni foam (denoted as MnO₂/ZnCo₂O₄/G/NF) electrode. The MnO₂/ZnCo₂O₄/G/NF electrode can offer a maximum capacity of 3405.21 F g⁻¹ at a current density of 2 A g⁻¹ with good cycling stability, showing 91.2% retention after 5000 cycles at a consistent current density of 20 A g⁻¹. Furthermore, using MnO₂/ZnCo₂O₄/G/NF as a positive electrode and G/NF as a negative electrode to assemble an asymmetric supercapacitor (ASC) results in excellent performance, with an energy density of 46.85 W h kg⁻¹ at a power density of 166.67 W kg⁻¹.

Fig. 1 A schematic illustration of the formation process of the MnO₂/ZnCo₂O₄/G/NF composite electrode.

2. Experimental details

2.1 Materials and reagents

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) and urea (CO(NH₂)₂) were purchased from Shanghai Hansi Chemical Industry Co. Ltd. Absolute ethanol, manganese acetate (Mn(CH₃COO)₂), sodium sulphate (Na₂SO₄) and hydrochloric acid (HCl) were commercially available from Aladdin Chemical Co. All materials were analytical grade reagents, commercially available and used without any purification. Ni foam (thickness: 1.5 mm, pore density: 100 PPI, areal density: 380 g m⁻²) was obtained from Changsha Lyrun New Materials Corporation. An oily slurry of graphene (5 wt%) was purchased from Suzhou Graphene Nanotechnology Co. Ltd. All solutions were prepared with deionized water throughout the experiments.

2.2 Preparation of the G/NF substrate

The Ni foam was cleaned in DI water, hydrochloric acid, DI water, absolute ethanol and DI water, respectively, for 5 min under ultrasonication. The Ni foam was then removed and put in a drying oven at 60 °C for 30 min. 0.5 g of graphene slurry was dispersed into 30 mL of absolute ethanol using mild water bath ultrasound technology. Then the solution was repeatedly dropped on the Ni foam. Finally, the Ni foam was dried at 60 °C for 1 h to acquire the graphene/Ni foam (G/NF) substrate.

2.3 Synthesis of ZnCo₂O₄ nanosheets on the G/NF substrate

To grow ZnCo₂O₄ nanosheets on the G/NF substrate, 0.44 g of Zn(NO₃)₂·H₂O, 0.86 g of Co(NO₃)₂·H₂O, and 0.424 g urea were dissolved in a mixed solution of 35 mL of H₂O and 5 mL of ethanol under magnetic stirring for 30 min to gain a homogeneous solution. The transparent solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The prepared G/NF was dipped into the solution before sealing the autoclave. Subsequently, the autoclave was heated at 130 °C for 5 h to hydrothermally deposit the formed ZnCo precursor onto the G/NF substrate. After the hydrothermal process was finished, the autoclave was removed from the electric oven and cooled to room temperature, and repeat rinses with deionized water were used to wash the sample to eliminate redundant activated material before drying at 60 °C for 3 h. Thereafter, the sample was annealed at 300 °C for 4 h in order to obtain thin layered nanosheets of ZnCo₂O₄ on the G/NF substrate. Under the same hydrothermal conditions but changing the annealing time to 2, 3 and 5 h, different electrodes were also prepared for comparison. In addition, a single ZnCo₂O₄ electrode without graphene was prepared to evaluate the effects of graphene on the capacitance performance.

2.4 Synthesis of MnO₂ nanoparticles on the ZnCo₂O₄/G/NF substrate

0.22 g of Mn(CH₃COO)₂ and 0.43 g of Na₂SO₄ were dissolved in 30 mL of DI water under ultrasonication with stirring for 10 min to obtain a homogeneous solution for electrodepositing MnO₂. Electrodeposition was performed at room temperature with Pt as the negative electrode and ZnCo₂O₄/G/NF as the positive electrode through adopting chronopotentiometry mode at a fixed current density of 1 mA cm⁻² for 45 min to obtain the MnO₂/ZnCo₂O₄/G/NF electrode. In order to compare the performances of electrodes obtained after different plating times, the plating time was changed to 5, 15, 30 and 60 min to acquire different electrodes.

2.5 Apparatus and measurement procedures

The morphologies and microstructures of the electrode materials were analyzed via scanning electron microscopy (SEM, JSM-6360LA), field-emission transmission electron microscopy (TEM/HRTEM, JEM 2100F), and selected area electron diffraction (SAED) coupled with energy dispersive spectroscopy (EDS). The phases and crystal structures of the electrode materials were evaluated via X-ray powder diffraction (XRD, Rigaku, RINT2000, Japan). The chemical elements and their states in the electrode materials were examined via X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, UK).

The electrochemical properties of the as-fabricated electrodes were measured using different electrochemical techniques, such as cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests on a CHI604E electrochemical workstation and electrochemical impedance spectroscopy (EIS) tests on a Zahner Ennium electrochemical workstation. The measurements were performed using a standard three-electrode configuration, with a prepared electrode, a platinum plate and a saturated calomel electrode (SCE) as the working electrode, counter electrode and reference electrode, respectively; 1 M Na₂SO₄ aqueous solution was used as the electrolyte.

3. Results and discussion

3.1 Morphology and structure of MnO₂/ZnCO₂O₄/G/NF electrode

The crystal structures of graphene, graphene coated with ZnCo₂O₄ nanosheets, and ZnCo₂O₄ nanosheets with MnO₂ nanoparticles loaded on graphene were confirmed via XRD measurements, as shown in Fig. 2. The XRD peaks at 16.6°, 26.5° and 36.449° were indexed to the (002), (004) and (210) crystal planes of graphene. The XRD peaks at 31.3°, 36.9°, 59.7° and 65.2° were indexed to the (220), (311), (511) and (440) crystal planes of ZnCo₂O₄ (JCPDS No. 23-1390), respectively. In the XRD pattern of MnO₂/ZnCo₂O₄/G/NF, ZnCo₂O₄ and graphene peaks could be observed, while the presence of MnO₂ was not confirmed. The existence of MnO₂ can be verified from SEM studies, as shown in Fig. 3d and e, and EDS data, as shown in Fig. 4a. The two sharp peaks marked with “♣” corresponded to the Ni foam substrate.¹³

Fig. 2 XRD patterns of ZnCo₂O₄, graphene coated with ZnCo₂O₄, and ZnCo₂O₄ with MnO₂ loaded on graphene.

Fig. 3 (a)–(c) SEM images of ZnCo₂O₄ nanosheets; (d) and (e) SEM images of ZnCo₂O₄ nanosheets with MnO₂ nanoparticles; (f and g) TEM images at different magnifications of ZnCo₂O₄ nanosheets with MnO₂ nanoparticles; (h) a HR-TEM image of ZnCo₂O₄ nanosheets with MnO₂ nanoparticles; and (i) a SAED image of ZnCo₂O₄ nanosheets with MnO₂ nanoparticles.

Fig. 4 (a) The EDS spectrum of the MnO₂/ZnCo₂O₄/G/NF electrode; (b) the XPS survey spectrum of the MnO₂/ZnCo₂O₄/G composite; (c) the core-level spectrum of Zn 2p; (d) the core-level spectrum of Co 2p; (e) the core-level spectrum of Mn 2p; and (f) the core-level spectrum of O 1s.

The structure of graphene on a Ni foam substrate has been presented in our previous paper.¹⁴ Fig. 3a shows a low magnification SEM image of the sample grown on the G/NF substrate, indicating the formation of ZnCo₂O₄ nanosheets. In Fig. 3b, further observations show that the ZnCo₂O₄ nanosheets are densely packed and highly uniform, like a forest grown on the G/NF substrate. The nanosheets are interlinked with each other, showing a more stable structure, which enhances the surface area of the material significantly. In addition, the holes between nanosheets can be used as operating transmission channels for the electrolyte during the charging/discharging process. In Fig. 3c, a high magnification SEM image of the sample grown on the G/NF substrate is shown, revealing that the ZnCo₂O₄ nanosheets are connected to each other with adequate space between each individual nanosheet, and the nanosheets possess smooth surfaces with an average length of 1.08 μm. This structure provides large spaces for the growth of MnO₂ nanoparticles and is extremely beneficial for the formation of heterostructures. Compared with Fig. 3c, d and e reveal different magnification SEM images of the composite materials, indicating that the MnO₂ nanoparticles are uniformly deposited on the ZnCo₂O₄ nanosheets. In Fig. 3e, it can be seen more clearly that the core ZnCo₂O₄ nanosheets are wrapped in thousands of small MnO₂ nanoparticles, which make the composites more intense and thicker. The uniformly deposited MnO₂ on the ZnCo₂O₄ nanosheets can promote ion/electron transport between the electrode and electrolyte, which is essential for improving the electrochemical performance. More detailed information about the morphological characteristics of the composite materials was obtained via TEM, HR-TEM and SAED, as shown in Fig. 3f–i. Fig. 3f and g shows TEM images at different magnifications of the MnO₂ nanoparticles coated on one of the ZnCo₂O₄ nanoflakes; it can be seen clearly that the MnO₂ nanoparticles have grown on the surface of ZnCo₂O₄. Therefore, this heterostructure significantly increases the effective number of electrochemical active sites on the electrode. Fig. 3h displays a HR-TEM image; this shows a lattice fringe of 0.352 nm, which is consistent with the MnO₂ (002) plane, and one of 0.244 nm, which matches well with the spacing of the (311) plane of ZnCo₂O₄. The selected area electron diffraction (SAED) pattern shows a clear polycrystalline structure, as shown in Fig. 3i. In addition, EDS mapping analysis of the elemental composition of MnO₂/ZnCo₂O₄/G/NF affirms that ZnCO₂O₄, MnO₂ and a graphene composite are attached to the nickel foam substrate, as shown in Fig. 4a.

The formation of the G/NF-substrate-supported MnO₂/ZnCo₂O₄ composite was further analyzed via XPS. Fig. 4b shows the XPS survey spectrum, indicating the presence of five distinct peaks, which are Zn 2p, Co 2p, Mn 2p, O 1s, and C1s, confirming the presence of Zn, Co, Mn, O and C. The core level spectrum of the Zn 2p region is revealed in Fig. 4c; the two strong peaks with binding energies of 1020.98 and 1043.78 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively, thus indicating the Zn²⁺ oxidation state of ZnCo₂O₄ in MnO₂/ZnCo₂O₄/G/NF.¹⁵ The Co 2p spectrum is shown in Fig. 4d; two major peaks at 779.98 and 794.88 eV are visible, which are assigned to the spin-orbits of Co 2p3/2 and Co 2p1/2, with a splitting value above 14.9 eV. Both peaks also can be separated into two peaks, suggesting the presence of Co²⁺ and Co³⁺ in the form of cobaltate.¹⁶ Fig. 4e shows the presence of Mn ions in the oxide form of MnO₂ with binding energies of 642.18 eV and 653.98 eV, relating to Mn 2p3/2 and Mn 2p1/2, respectively. The separation of the peak energy is 11.7 eV, which agrees well with previously reported data by Montree Sawangphruk et al., strongly suggesting that the oxidation state of Mn is +4.¹⁷ The detailed O 1s spectrum is displayed in Fig. 4f; one peak at 529.68 eV points to the formation of oxides with Zn–Co and Mn. The other peak at 531.38 eV can be thought of as resulting from a multiplicity of physically and chemically bonded oxygen on and within the surface.¹⁸ The SEM, TEM, XRD and XPS results show that the MnO₂/ZnCo₂O₄/G/NF composite material electrode has been successfully fabricated.

3.2 Characterization of the MnO₂/ZnCo₂O₄/G/NF electrode

In order to establish the best annealing time for the ZnCo₂O₄ electrode material and the optimal length of time for the electrodeposition of MnO₂ on the ZnCo₂O₄ electrode, the electrochemical properties of a number of samples were measured in a typical three electrode system, using 1 M Na₂SO₄ as the electrolyte, via CV, GCD and EIS measurements. Fig. 5a shows the CV curves from different ZnCo₂O₄ annealing times on the G/NF substrate (the annealing time is 4 h for the electrodes with pure ZnCo₂O₄ on the Ni substrate and graphene on the Ni substrate), which were obtained in the potential window of 0–0.6 V at a scan rate of 50 mV s⁻¹. It can be seen clearly that the CV of G/NF is a straight line in the picture and the CV area of ZnCo₂O₄/G/NF is larger than the CV area of ZnCo₂O₄/NF, indicating that graphene provides more attachment sites for ZnCo₂O₄. Among the samples, the ZnCo₂O₄/G/NF electrode annealed for 4 h is better than the other electrodes. On this basis, different MnO₂ electrodeposition times were studied for loading on the ZnCo₂O₄/G/NF electrode and the CV curves are shown in Fig. 5b. With an increase in the MnO₂ deposition time, the capacitive current and CV area increase gradually, indicating that MnO₂ indeed increased the capacitance of the ZnCo₂O₄/G/NF electrode. There was a decrease when the deposition time was more than 45 min, which might be due to thicker layers of MnO₂ preventing the transportation of electrolyte ions to some extent. MnO₂/ZnCo₂O₄/G/NF shows better performance than the other electrodes. Fig. 5c shows CV curves from MnO₂/ZnCo₂O₄/G/NF at different scan rates ranging from 10 to 100 mV s⁻¹. With an increase in the scan rate, the CV curve becomes steeper and no redox peaks are observed, which may have been because of the overlapping of peaks resulting from multiple redox transitions, lattice defects creating active sites with slightly varying redox potentials, and ZnCo₂O₄ and MnO₂ surface reactions on the electrodes;¹⁹ however, the electrodes still show pseudo-capacitive symmetry behavior.

Fig. 5 (a) CV curves recorded at 20 mV s⁻¹ of ZnCo₂O₄/G/NF composite electrodes synthesized with different annealing times and pure ZnCo₂O₄; (b) CV curves recorded at 20 mV s⁻¹ of MnO₂/ZnCo₂O₄/G/NF composite electrodes synthesized with different electrodeposition times; and (c) CV curves of MnO₂/ZnCo₂O₄/G/NF recorded at different scan rates.

To further prove the electrochemical behavior of the MnO₂/ZnCo₂O₄/G/NF electrode, Fig. 6 depicts GCD curves from all electrodes in a potential window ranging from 0 to 0.5 V. The GCD curves from the pure ZnCo₂O₄ electrode and electrodes with different ZnCo₂O₄ annealing times on the G/NF substrate at a current density of 2 A g⁻¹ are displayed in Fig. 6a. Evidently, the GCD curves are almost symmetrical in morphology, indicating the presence of ZnCo₂O₄ thin films with good pseudo-capacitive properties. Meanwhile, a ZnCo₂O₄ annealing time of 4 h on the G/NF substrate leads to the longest charge–discharge time, which means that it has better performance than the other electrodes. Then GCD curves from different MnO₂ electrodeposition times on the ZnCo₂O₄/G/NF electrodes are shown in Fig. 6b. The GCD curves still remain basically symmetrical and the corresponding discharge times are longer than those of the ZnCo₂O₄/G/NF electrodes at the same current density. Fig. 6c shows the GCD curves from MnO₂/ZnCo₂O₄/G/NF electrodes at different current densities from 2 to 20 A g⁻¹. Specific capacitance values were calculated according to the equation:

C = IΔt/mΔV (1)

where C is the specific capacitance, I is the discharge current, Δt is the discharge time, m is the mass of electrode material, and ΔV is the potential window of the discharge process. In experiments, the quantities of graphene, ZnCo₂O₄ and MnO₂ on one piece of Ni foam were about 0.68 mg, 2.11 mg, and 1.05 mg, respectively. So, the mass loading on the electrode was about 3.84 mg. Through calculations, the MnO₂/ZnCo₂O₄/G/NF electrode exhibits a rather high specific capacitance of 3405.21 F g⁻¹ at a current density of 2 A g⁻¹ and a specific capacitance of 2208.3 F g⁻¹ even at a high current density of 20 A g⁻¹. Fig. 6d displays the specific capacitance curves for electrodes made with different MnO₂ electrodeposition times on ZnCo₂O₄/G/NF at different current densities. The results are consistent with CV measurements. This electrochemical behavior of the enhanced MnO₂/ZnCo₂O₄/G/NF electrode can be attributed to its three-dimensional nanostructures and intrinsically good electro-conductivity.²⁰ The cycling stability of the MnO₂/ZnCo₂O₄/G/NF electrode is checked through GCD tests at a constant current density of 20 A g⁻¹ for 5000 cycles, as depicted in Fig. 6e. In the first 900 cycles, the specific capacitance decreases first and then increases to 1441.67 F g⁻¹, which can be attributed to a wetting process involving electrolyte penetration into the material. After 1400 cycles, the specific capacitance decreases slightly to 1375 F g⁻¹, then tends to stay stable, keeping 91.2% of the initial capacitance. The insets (a) and (b) in Fig. 6e show a comparison of the last three cycles with the first three cycles, and the morphological changes in the MnO₂/ZnCo₂O₄/G/NF electrode after 5000 cycles at a current density of 20 A g⁻¹, respectively. From inset (a) of Fig. 6e, it can be seen that the shapes of the last three cycles are almost the same as the first three cycles, which proves the outstanding cycling behavior and that the electrode materials have good stability. From inset (b) of Fig. 6e, the ZnCo₂O₄ sheets with MnO₂ nanoparticles have become thinner due to ion transport to the inner and outer surfaces of the MnO₂/ZnCo₂O₄/G/NF electrode during the process of repeating charge–discharge. However, the electrode still maintains the ZnCo₂O₄ framework without collapse. The results indicate that the electrode materials have good corrosion resistance and further prove that the electrode materials have good stability.

Fig. 6 (a) GCD curves recorded at 2 A g⁻¹ of ZnCo₂O₄/G/NF composite electrodes synthesized with different annealing times and pure ZnCo₂O₄; (b) GCD curves recorded at 2 A g⁻¹ of MnO₂/ZnCo₂O₄/G/NF composite electrodes synthesized with different electrodeposition times; (c) GCD curves of MnO₂/ZnCo₂O₄/G/NF electrodes recorded at different current densities; (d) the specific capacitances of ZnCo₂O₄/G/NF electrodes with different MnO₂ electrodeposition times at various current densities; (e) the cycling stability of the MnO₂/ZnCo₂O₄/G/NF electrode at 20 A g⁻¹ (inset (a) is a comparison of the shapes of the first three cycles and last three cycles; inset (b) is a SEM image taken after 5000 cycles); and (f) Nyquist plots of ZnCo₂O₄/G/NF and ZnCo₂O₄/G/NF electrodes with different MnO₂ electrodeposition times.

As well as CV and GCD measurements, the impedance spectra of ZnCo₂O₄/G/NF electrodes with different MnO₂ electrodeposition times from 5 to 60 min with an annealing time of 4 h were obtained in the frequency range from 100 kHz to 0.01 Hz with a perturbation of 5 mV. The corresponding Nyquist plots in Fig. 6f can be fitted with an equivalent circuit (inset of Fig. 6f). It can be seen from the figure that when the electrodeposition time is less than 45 min, in the low frequency region, the slope is far higher than 45°; as the electrodeposition time increases, the lines become more vertical, which means a higher transmission efficiency of electrolyte. In the high frequency region, as the electrodeposition time increases, the intercepts with the real axis decrease gradually, which means the equivalent series resistance becomes lower.^21,22 When the electrodeposition time is 45 min, the equivalent resistance of the electrode is 0.66 Ω lower than those of the other electrodes. When the electrodeposition time exceeds 45 min, it can be seen from the low-frequency and high-frequency regions that excess MnO₂ aggregation prevents the transfer of ions/electrons, which reduces the electrochemical performance of the electrode.

3.3 Preparation of an asymmetric supercapacitor

To further test the potential of the MnO₂/ZnCo₂O₄/G/NF electrode for actual applications, a MnO₂/ZnCo₂O₄/G/NF electrode as the positive electrode, a G/NF electrode as the negative electrode and a piece of cellulose paper as the separator were used to fabricate an asymmetric supercapacitor. Fig. 7 shows the electrochemical performance testing of the as prepared ASC in 1 M Na₂SO₄ electrolyte. Fig. 7a displays the CV curves of the as prepared ASC device recorded at a scan rate of 20 mV s⁻¹ in different voltage windows. Obviously, the redox current increases with an increase in the voltage window, showing more significant pseudo-capacitance characteristics in a larger voltage range. When the voltage window reached 1.8 V, the CV curves became distorted because of some side reactions. Meanwhile, the GCD curves of the ASC device in different voltage windows at a current density of 1 A g⁻¹ are shown in Fig. 7b, which indicate that the ideal voltage window of the ASC is 1.6 V. Hence, 1.6 V is selected as the maximized voltage window of the ASC to further evaluate the overall electrochemical performance. CV curves from the ASC device at different scan rates from 10 to 100 mV s⁻¹ are shown in Fig. 7c. The CV curves still retain a fairly symmetrical shape upon increasing the scan rate, which indicates the good rate capabilities and good reversibility of the ASC. As presented in Fig. 7d, GCD curves at different current densities are almost symmetric, affirming that the as prepared ASC device has ASC-type capacitor behavior, consistent with the CV results. It was discovered in Fig. 7e that the specific capacitance of the ASC device is 131.77 F g⁻¹ at 0.5 A g⁻¹ and it even remains at 48.18 F g⁻¹ upon increasing the current density to 5 A g⁻¹, which displays the fine rate capabilities. Moreover, the cycling stability is an important indicator for estimating the properties of an ASC. According to the data, Fig. 7f implies that the specific capacitance decreases slowly with increasing cycle number; after 800 cycles, it tends to be stable, and even after 5000 cycles, 90.5% of the initial capacitance is retained. Additionally, the energy density and power density are the key indexes of an ASC, which can be calculated based on the GCD data and these equations:

Fig. 7 (a) CV curves of ASC devices within different voltage windows at a scan rate of 20 mV s; (b) GCD curves of ASC devices within different potential windows at a current density of 3 A g; (c) CV curves at different scan rates from the ASC device; (d) GCD curves at different current densities from the ASC device; (e) the specific capacities of the ASC device at different current densities; and (f) the cycling stability of the ASC device.

The resulting values are drawn up in the form of a Ragone plot, as presented in Fig. 8. The ASC device that we prepared showed an energy density of 46.85 W h kg⁻¹ at a power density of 166.67 W kg⁻¹, and this still remained at 17.13 W h kg⁻¹ at a maximum power density of 1666.67 W kg⁻¹. These values are better than others reported, as shown in Table 1. In the end, the ASC we assembled can power a commercial red LED lamp successfully for more than 5 minutes, which indicates its practicability, as shown in the inset of Fig. 8.

Fig. 8 A Ragone plot of the ASC device (the inset shows an image of a commercial red LED at different times after lighting).

Table 1 A comparison of various ZnCo₂O₄ nanostructures as electrode materials for SCs

Electroactive material	Maximum SC (F g^−1 (mF cm^−2))	Current density (A g^−1 (mA cm^−2))	Energy density (W h kg^−1)	Power density (W kg^−1)	Cycling stability (%)	Reference
ZnCo2O4	183.54	1.72	42.83	425	92.5 (10 000 cycles)	6
ZnCo2O4/MnO2	161	2.5	37.8	648	91 (5000 cycles)	8
ZnCo2O4/MnO2	54.2	1	16.94	7500	98.5 (1500 cycles)	10
ZnCo2O4/MnCo2O4	60	1	19.5	750	98.5 (2000 cycles)	19
ZnCo2O4/rGO	101	0.5	31.25	375	97.3 (2000 cycles)	21
MnO2/ZnCo2O4/graphene	131.78	0.5	46.85	1666.67	90.5 (5000 cycles)	Our work

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4. Conclusions

In summary, mesoporous ZnCo₂O₄ nanosheets and uniformly distributed MnO₂ nanoparticle composites were successfully synthesized on a G/NF substrate via a one-step hydrothermal process followed by annealing and a one-step electrodeposition process. In addition, we found that, under the same hydrothermal conditions, the composite electrode with an annealing time of 4 h and a MnO₂ electroplating time of 45 min has better electrochemical properties. The synergistic effects of the MnO₂ nanoparticles, ZnCO₂O₄ nanosheets and graphene arrays on the Ni foam substrate enable the binder-free electrode to exhibit a specific capacitance of 3405.21 F g⁻¹ at a current density of 2 A g⁻¹ and it maintains capacitance retention of about 64.85% at 20 A g⁻¹. Furthermore, the capacitance remained at 91.2% at a current density of 20 A g⁻¹ after 5000 cycles, and the electrode exhibits outstanding stability. An ASC was made with MnO₂/ZnCO₂O₄/G/NF as the anode and G/NF as the cathode. The device delivers a high specific capacitance of 131.78 F g⁻¹ at 0.5 A g⁻¹ and cycling stability, with 90.5% retention after 5000 cycles, working in a large potential range of 0–1.6 V. It also exhibits a high energy density of 46.85 W h kg⁻¹ and a maximum power density of 1666.67 W kg⁻¹. These excellent results confirm that combining porous ZnCo₂O₄ nanosheets with MnO₂ nanoparticles in a composite material is a promising approach in the field of energy storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to thank the Engineering Research Center of Agricultural Multi-Dimensional Sensor Information Perception, Heilongjiang Province, and the Heilongjiang Provincial Key Laboratory of Micro-nano Sensor Components. This work was jointly supported by the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (grant no. UNPYSCT-2016087), the Scientific Research Foundation for the Returned Overseas Chinese Scholars in Heilongjiang Province, the Project of Plant Food Processing Technology-Heilongjiang Province Superiority and Characteristic Discipline (Grant No. YSTSXK201873), the Fundamental Research Funds in Heilongjiang Provincial Universities (No. 135109244, 135309115, 135309211), and the Heilongjiang Science Foundation Project (JQ2019F003, ZD2019F004).

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Footnote

† Rui Miao and Wanjuan Cong are common first authors.

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