High mass loading flower-like MnO₂ on NiCo₂O₄ deposited graphene/nickel foam as high-performance electrodes for asymmetric supercapacitors

Abstract

The implementation of high mass loading MnO₂ on electrochemical electrodes of supercapacitors is currently challenging due to the poor electrical conductivity and elongated electron/ion transport distance. In this paper, a NiCo₂O₄/MnO₂ heterostructure was built on the surface of three-dimensional graphene/nickel foam (GNF) by a hydrothermal method. The petal structured NiCo₂O₄ loaded on graphene played a wonderful role as a supporting framework, which provided more space for the growth of high mass loading MnO₂ microflowers, thereby increasing the utilization rate of the active material MnO₂. The GNF@NiCo₂O₄/MnO₂ composite was used as a positive electrode and achieved a high areal capacitance of 1630.5 mF cm⁻² at 2 mA cm⁻² in the neutral Na₂SO₄ solution. The asymmetric supercapacitor assembled with the GNF@NiCo₂O₄/MnO₂ positive electrode and activated carbon negative electrode possessed a wide voltage window (2.1 V) and splendid energy density (45.9 Wh kg⁻¹), which was attributed to the satisfactory electroactive area, low resistance, quick mass diffusion and ion transport caused by high mass loading MnO₂.

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Introduction

The research on portable supercapacitors has been given great attention owing to the high power density, fast charging and discharging, excellent cycling performance, indefinite lifespan, nonvulnerability to temperature change, nontoxicity in nature, and their being more reliable than batteries. According to the different charge-storage mechanisms, supercapacitors are divided into electric double-layer capacitors (EDLCs) and pseudocapacitors.^1–3 Compared with the carbon-based active materials for EDLCs, the pseudocapacitive electrode materials can provide higher specific capacitance and energy density which is related to specific capacitance and voltage window. Therefore, there are usually two strategies for increasing energy density, namely increasing specific capacitance and expanding the voltage window. Rational design of the nanostructure of the composites to increase the content and utilization of high-capacitance active materials per unit area of the electrode material is a feasible method.^4,5 Many composites of different pseudocapacitive materials and carbon materials with high specific capacitances have been researched, such as MnO₂/graphene/NF (NF, nickel foam),⁶ RuO₂/graphene,⁷ RuO₂/graphene/CNT (CNT, carbon nanotubes),⁸ and V₂O₅-CNTs/GF (GF, graphene form)⁹ with different morphologies.^10,11 Numerous studies have shown that combining MnO₂ with a highly conductive material to form a composite material can significantly enhance its electrical conductivity and electrochemical performance.^12–14 For example, Garakani et al. used GF substrates to grow NiCo₂O₄/MnO₂ arrays as a freestanding supercapacitor electrode with great enhancement in both gravimetric and areal capacitances.¹⁵ Lu et al. obtained supercapacitors with good cycling performance using WO_3−x@Au@MnO₂ core–shell structure nanocomposites as electrodes.¹⁶ Wang's group prepared composites of hierarchical NiCo₂O₄/MnO₂ nanosheets on carbo cloth which used as positive electrodes and achieved excellent electrochemical performance.¹⁷ However, the specific capacitance mostly reported so far could be obtained just by loading a low mass MnO₂ (<1 mg cm⁻²). Therefore, it's still urgent to load higher mass MnO₂ to acquire higher specific capacitance, higher energy density and wide voltage window in practical application.^18–22 In this paper, we designed heterogeneous NiCo₂O₄/MnO₂ materials on a graphene modified NF (GNF) substrate. The conductive petal-like NiCo₂O₄ arrays constructed on NF provided additional space to allow higher mass loading of MnO₂ micro-flowers. With GNF@NiCo₂O₄/MnO₂ composite material as the positive electrode and GNF@AC (AC, activated carbon) as the negative electrode, an asymmetric supercapacitor (ASC) was fabricated in a neutral Na₂SO₄ aqueous electrolyte. The electrode with high mass loading provides outstanding specific capacitance of 1630.5 mF cm⁻² at 2 mA cm⁻². The assembled ASC shows excellent cycling performance, with its voltage window of 2.1 V, and the energy density of 505.2 μWh cm⁻² (45.9 Wh kg⁻¹, 5.1 mWh cm⁻³), demonstrating its application potential in the field of energy storage.

Experimental section

Growth of graphene on Ni foam (GNF)

Fig. 1 shows the synthesis process of GNF@NiCo₂O₄/MnO₂ electrode. First, graphene layer was deposited on the surface of NF by radio-frequency plasma enhanced chemical vapor deposition system (RF-PECVD).^23–27 Three pieces of NF with the area of 1 × 2 cm² and the thickness of 0.5 mm were ultrasonically cleaned in acetone, 1 M HCL and ethanol, and blown dried with moisture-free air. The quartz boat with NF substrates was then placed in the heating zone of RF-PECVD. The chamber was evacuated to a pressure less than 300 mTorr and then hydrogen gas was introduced at a rate of 6 sccm. Plasma was generated for 10 min with the RF power of 300 W to further remove contaminants on the substrates. When heating the chamber to 750 °C, graphene layer was grown by introducing CH₄, Ar and H₂ at flow rates of 10 sccm for 30 min under a plasma power of 250 W and a total pressure of ∼400 mTorr. After the temperature was lowered to room temperature at a frequency of 10 °C min⁻¹, the GNF was obtained.

Fig. 1 Schematic of the synthesis process of GNF@NiCo₂O₄/MnO₂.

Preparation of GNF@NiCo₂O₄ and GNF@NiCo₂O₄/MnO₂

Petal shaped NiCo₂O₄ nanoarrays were deposited on GNF by using hydrothermal method. Typically, Co(NO₃)₂·6H₂O (1 mmol), Ni(NO₃)₂·6H₂O (2 mmol) and urea (12 mmol) were added into 100 mL deionized (D.I.) water and stirred for 30 minutes to obtain a homogenous mixed solution. GNF was treated with plasma for 100 s to make it hydrophilic and ready for use. Subsequently, a piece of treated GNF and 10 mL of the mixed solution were put into a Teflon-lined stainless-steel autoclave. The autoclave was heated to 160 °C in the oven and kept it for 10 h, 15 h, 20 h and 25 h, respectively, followed by naturally cooled to room temperature. The obtained precursors were washed repeatedly with D.I. water and ethanol, thereafter desiccated in the oven at 60 °C for 12 h. Lastly, the precursors were annealed in atmosphere at 300 °C for two hours to obtain petal-like GNF@NiCo₂O₄. By weighing the GNF before and after NiCo₂O₄ deposition, the mass loading of NiCo₂O₄ obtained under different reaction time was obtained. Furthermore, GNF@NiCo₂O₄/MnO₂ was prepared by the second hydrothermal treatment. The KMnO₄ (0.45 g) was added into D.I. water (10 mL), and was stirred for 30 minutes to obtain purple solution. The solution was placed in an autoclave simultaneously with as-synthesized GNF@NiCo₂O₄, heated to 160 °C and maintained for 10 hours, then freely cooled to ambient temperature. After that, the resulting compound was washed repeatedly to remove the residue and desiccated in oven at 60 °C for 12 hours, then GNF@NiCo₂O₄/MnO₂ was obtained. As a control, we also synthesized GNF@MnO₂ (without NiCo₂O₄) and NF@NiCo₂O₄/MnO₂ (without graphene) under the same conditions.

Fabrication of GNF@NiCo₂O₄/MnO₂//AC asymmetric pseudocapacitor

The formula for balancing the charge between the positive and negative electrodes is as follows:^28,29

q = C × ΔV × m (1)

q⁺ = q⁻ (2)

where C and ΔV are the specific capacitance and charge/discharge potential range of each electrode, respectively. AC, acetylene black and polyvinylidene fluoride (PVDF) binder were mixed evenly into a slurry according to the weight ratio of 8 : 1 : 1, and then pressed on NF to construct the negative electrode. Next, it was dried in an oven at 60 °C for 10 hours. The positive electrode, negative electrode and separator were cut into 1 × 1 cm² size and 1 M Na₂SO₄ was used as the electrolyte to assemble the CR2018 coin cell.

Materials characterization

To determine the morphology and microstructural characteristic of the products, Hitachi S-4800 field emission scanning electron microscope (FE-SEM) was employed. The crystalline structures of samples were examined by a Bruker D8 Discover X-ray powder diffractometer (XRD, Cu K_α, λ = 0.15418 nm). The valence state of the composite was measured by X-ray photoelectron spectrometer (XPS, Thermo Scientific ESCALAB 250Xi). Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore-size distribution analysis were carried out by a surface area analyzer (Autosorb-iQ). Lab RAM HR-Evolution (Horiba, 532 nm laser excitation) Raman spectrometer was utilized to investigate the Raman spectra of graphene. The mass loading was obtained by measuring the mass change of the sample before and after the reaction with a microbalance. Specially, the mass loading of graphene was 0.1 mg cm⁻² in the obtained GNF; it was determined to be about 1.4 mg cm⁻² of NiCo₂O₄ for the obtained GNF@NiCo₂O₄ (hydrothermal reaction time was 20 hours); the mass loadings of MnO₂ for GNF@MnO₂ and GNF@NiCo₂O₄/MnO₂ were 3.0 mg cm⁻² and 3.5 mg cm⁻², respectively. Hence, the total mass loading of active materials for GNF@NiCo₂O₄/MnO₂ achieved 5.0 mg cm⁻². The mass loading of negative electrode was 6.0 mg cm⁻².

Electrochemical measurements

The electrochemical test was performed using 760E electrochemical workstation (CH instrument, China). The electrochemical property of single electrode was examined under there-electrode configuration in 1 M Na₂SO₄ solution. A Pt foil was applied as counter electrode and saturated calomel electrode (SCE) electrode was employed as reference electrode. The electrochemical impedance spectroscopy (EIS) was analyzed at the open circuit potential in frequency range from 0.01 Hz to 100 kHz. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were performed in the operating potential range of 0–0.8 V. The specific capacitance (C_s, C_m) is calculated by the following formulas (3) and (4) using GCD data.^30–32where I (mA), Δt (s), s (cm²), m (g), and ΔV (V) represent the discharge current, discharge time, area, loading mass, and voltage window, respectively. The cycling capacity retention rate was the ratio between the final capacity and the initial one. The energy density E of the ASC was calculated by eqn (5), and (6) was used to calculate the power density:^33–35where C represented the specific capacitance obtained from above equations, and V and t were the voltage window and discharge time, respectively.

Results and discussion

Morphologies and structures of GNF@NiCo₂O₄/MnO₂

High-quality graphene was obtained by adjusting the gas flow rate, the reaction gas ratio, the reaction time, and the radio frequency power during the reaction via RF-PECVD system. Fig. 2a and b show the morphology of the bare NF surface before and after the deposition of graphene. Clear graphene wrinkles are uniformly covered on the 3D NF network. From the SEM images of GNF@NiCo₂O₄ in Fig. 2c–f, it is found that the GNF is covered by NiCo₂O₄ sheet arrays through hydrothermal treatment. As the reaction time increases, the surface morphology of GNF@NiCo₂O₄ changes significantly. When the reaction is carried out for 10 hours, some scattered flaky networks are overlaid on GNF. After 15 hours, large sheets begin to grow between the flaky networks, and form new network arrays. When the time reaches 20 hours, the large petal-like micro-sheets form a 3D layered framework, which is beneficial for further deposition of new materials. After 25 hours of reaction, the larger sheets are scattered on the top of the network formed by the smaller sheets and do not form an orderly network. When MnO₂ is deposited on the surface of GNF@NiCo₂O₄ (reaction time of 20 hours), the flower-like MnO₂ microspheres are evenly dispersed on the NiCo₂O₄ nanosheet arrays, as shown in Fig. 2g and h. The microspheres are also formed by many sheets, which provides a 3D structure and is beneficial for electron transmission and ion diffusion. The microstructures were also characterized by the Raman spectrum and XRD pattern. From the Raman spectrum of GNF in Fig. 3a, it shows the characteristic D, G, and 2D peaks located at 1353 cm⁻¹, 1580 cm⁻¹, and 2702 cm⁻¹, respectively. The position of Raman peak and the intensity ratio of graphene will be affected by defects and the number of graphene layers.³⁶ The D band is disordered sp² C in graphene.³⁷ The intensity ratio of the D band to the G band (I_D/I_G = 0.27) indicates the graphene with few defects.³⁸ The peak at 1580 cm⁻¹ (G band) stands for sp² C in graphene, and the I_G/I_2D ratio of 2.05 represents its multi-layer structure.^39,40 The XRD pattern of GNF (Fig. 3b blue line) presents a sharp peak at 2θ ≈ 26.4°, which corresponds to (002) reflection of graphene (JCPDS 75-1621).⁴¹ For GNF@NiCo₂O₄-20 h samples, in addition to the three typical peaks of Ni, another six well-defined diffraction peaks are shown at 2θ equalling to ∼19.0° (111), 32.9° (220), 38.4° (311), 59.1° (511), 62.7° (440) and 69.4° (531), which represent the spinel crystalline structure of NiCo₂O₄ (JCPDS file no. 20-0781).⁴² The XRD pattern of GNF@MnO₂ (black line in Fig. 3b) shows that the diffraction peaks near to 12.3°, 24.8°, 37.3°, and 65.9° can well match (001), (002), (−111), and (310) planes of monoclinic birnessite-type MnO₂ (JCPDS 42-1317).^43,44 The XRD pattern of the GNF@NiCo₂O₄/MnO₂ (the pink line in Fig. 3b) shows a small peak at 2θ ≈ 26.4° corresponding to the graphene and other characteristic peaks related to crystallographic planes of NiCo₂O₄ (JCPDS 020-0781) and MnO₂ (JCPDS 42-1317). The surface areas and pore size of the samples were measured by nitrogen adsorption/desorption tests. The specific surface area of GNF@NiCo₂O₄ is 13.02 m² g⁻¹ calculated by Brunauer–Emmett–Teller (BET) method, which is two times higher than that of GNF (Fig. 3c). It indicates that the presence of NiCo₂O₄ effectively increases the specific surface area of the composite. Barrett–Joyner–Halenda (BJH) pore size distribution shows the microporous and mesoporous of GNF@NiCo₂O₄ are distributed in 1.7–8.0 nm range (Fig. 3d).^45–47 The 3D structure of GNF@NiCo₂O₄ increases the distribution space of MnO₂, thereby increasing the surface area of MnO₂ in contact with the electrolyte, providing more electron transport channels and shortening the electron/ion transport distance.

Fig. 2 SEM images of (a) bare Ni foam, (b) GNF, (c) GNF@NiCo₂O₄-10 h, (d) GNF@NiCo₂O₄-15 h, (e) GNF@NiCo₂O₄-20 h, (f) GNF@NiCo₂O₄-25 h, (g and h) GNF@NiCo₂O₄/MnO₂.

Fig. 3 (a) Raman spectra of GNF, (b) XRD patterns of GNF (blue line), GNF@MnO₂ (black line), GNF@NiCo₂O₄ (red line), and GNF@NiCo₂O₄/MnO₂ (pink line). (c and d) The nitrogen adsorption/desorption isotherms and the corresponding BJH pore size distribution of Ni foam (black line), GNF (red line), and GNF@NiCo₂O₄ (blue line).

The chemical composition of GNF@NiCo₂O₄/MnO₂ was also investigated by XPS measurement. Fig. 4a shows the survey spectrum, indicating the presence of C, Mn, O, Ni, and Co elements. Fig. 4b shows that the O 1s core level spectrum of the prepared GNF@NiCo₂O₄/MnO₂ and can be divided into three characteristic peaks, namely O₁, O₂ and O₃ located at 529.5 eV, 530.3 eV and 531.4 eV, respectively. The fitted peak O₁ can be attributed to the lattice O²⁻, which corresponds to a typical metal–oxygen bond.⁴⁸ The peak of O₂ is associated with the surface oxygen in OH⁻.⁴⁹ The O₃ peak is related to oxygen ions with low coordination on the surface of metal oxides, and formally these oxide ions are O⁻ species with lower electron density than the classical O²⁻ ions.⁵⁰ Fig. 4c is the scanning spectrum of Mn 2p.

Fig. 4 XPS spectra of GNF@NiCo₂O₄/MnO₂. (a) Survey scan, (b) O 1s, (c) Mn 2p, (d) Mn 3s, (e) Ni 2p, and (f) Co 2p core level spectra.

Two distinct characteristic peaks located at 642.4 eV and 654.0 eV correspond to Mn 2p_3/2 and Mn 2p_1/2, respectively. The distance between the two peaks is about 11.6 eV, which confirms the formation of MnO₂.⁵¹ In the Mn 3s spectrum (Fig. 4d), the peaks are observed near 84.1 eV and 88.8 eV, the energy separation of 4.7 eV further indicates that the Mn element in the sample mainly exists in the form of +4 valence.⁵² Fig. 4e shows the scanning spectrum of the Ni 2p energy level, showing two distinct main peaks corresponding to Ni 2p_3/2 and Ni 2p_1/2. The two fitted peaks placed at 854.0 eV and 871.8 V correspond to Ni²⁺, and the two peaks at 855.9 eV and 873 eV correspond to Ni³⁺. The satellite peaks located at 860.5 eV and 878.5 eV are indicators of the two shake-type peaks of Ni.^53,54 The Co 2p spectrum (Fig. 4f) similarly shows two spin–orbit double peaks and two satellite peaks. The two main peaks are attributed to Co 2p_3/2 and Co 2p_1/2. Among them, the two fitted peaks at 781.7 V and 797 eV are attributed to Co²⁺, while the other two at 780.3 eV and 797.0 eV belong to Co.³ The two satellite peaks are located at 785.7 eV and 801.0 eV.⁵⁵

Supercapacitive performance of electrodes

To investigate the electrochemical properties of the materials, the composites were immersed in a three-electrode configuration by adopting it as the working electrode. Fig. S1† provides the CV curves of pure NF at different scan rates. The areas of the CV curves are very small, which indicates that pure NF has a low capacity. The effect of hydrothermal reaction time on the electrochemical performance of GNF@NiCo₂O₄ was explored at the same scanning frequency. As shown in Fig. 5a, the current response of GNF@NiCo₂O₄ with 20 hours hydrothermal reaction is the largest, showing that the sample may possess the highest specific capacitance. This is verified by the GCD curves which shows the relationship between the discharge time of different GNF@NiCo₂O₄ samples and hydrothermal reaction time (Fig. 5b). The longest discharge time of the GNF@NiCo₂O₄-20 h proves its highest area specific capacitance. But the CV curve of GNF@NiCo₂O₄ does not show a clear redox peak, and the GCD curve has no sloping plateau, indicating that its redox-active is suppressed in salt solutions, and its short charge and discharge time also prove this result.

Fig. 5 (a) The CV curves of GNF@NiCo₂O₄ electrodes with different reaction time at 5 mV s⁻¹, (b) the GCD curves of GNF@NiCo₂O₄ electrodes with different reaction time at 2 mA cm⁻², (c) the CV curves of different GNF@NiCo₂O₄/MnO₂ electrodes at 10 mV s⁻¹, (d) the GCD curves of different GNF@NiCo₂O₄/MnO₂ electrodes at 2 mA cm⁻².

When MnO₂ was deposited on the GNF@NiCo₂O₄ with different reaction time for 10 hours to obtain different GNF@NiCo₂O₄/MnO₂ composites, the CV curve (Fig. 5c) and GCD curve (Fig. 5d) of the sample with GNF@NiCo₂O₄-20 h are more symmetrical rectangular and triangular than other samples, indicating the pseudocapacitive properties of the electrode are primarily attributed to the two electron-transferred charge storage mechanism of MnO₂ in the mild Na₂SO₄ aqueous electrolyte. This involves surface adsorption/desorption and the reversible redox transition, the exchange of protons and/or cations with the electrolyte and different oxidation states, and can be expressed as follows

(MnO₂)_surface + Na⁺ + e⁻ ↔ (MnO^–Na⁺)_surface, MnO₂ + Na⁺ + e⁻ ↔ MnOONa (7)

The composite material with GNF@NiCo₂O₄-20 h has the largest CV curve area and the longest discharge time among all composites, revealing that it has the highest specific capacitance. Therefore, it is used in the following study.

The electrochemical properties of the GNF@NiCo₂O₄/MnO₂, GNF@NiCo₂O₄, GNF@MnO₂ and NF@NiCo₂O₄/MnO₂ were studied in detail. Fig. 6 shows the CV and GCD curves of these electrodes. A largest integrated area in CV and longest discharging time in GCD curves of the GNF@NiCo₂O₄/MnO₂ electrode demonstrate its best electrochemical performance in comparison with GNF@NiCo₂O₄, GNF@MnO₂ and NF@NiCo₂O₄/MnO₂. Rectangular-like CV shapes of GNF@NiCo₂O₄/MnO₂ reveal good capacitive reversibility, and the triangular symmetry of GCD curves hints its excellent pseudocapacitive properties. Meanwhile, its nearly symmetric GCD profiles and the lower IR drop than other samples suggest favourable conductivity. Fig. 7a illustrates the specific capacitances of these samples obtained by the GCD data. It can be observed that the GNF@NiCo₂O₄/MnO₂ delivers higher specific capacitance of 1630.5, 1402.5, 1150.0, 832.5, and 387.5 mF cm⁻², compared to only 94.5, 63.75, 30.0, 12.5 and 10.0 mF cm⁻² for the GNF@NiCo₂O₄, 619.3, 366.3, 172.5, 25.0 and 11.1 mF cm⁻² for the GNF@MnO₂, and 1422.3, 1067.5, 690.0, 292.5, and 18.8 mF cm⁻² for the NF@NiCo₂O₄/MnO₂ at current densities of 2, 5, 10, 20 and 50 mA cm⁻², respectively. Note that the areal capacitance of GNF@NiCo₂O₄ is almost negligible. The areal specific capacitance of GNF@NiCo₂O₄/MnO₂ is 17.3 and 2.6 times higher than those of GNF@NiCo₂O₄ and GNF@MnO₂ at 2 mA cm⁻², respectively. The mass loading of MnO₂ has increased from 3 mg cm⁻² of GNF@MnO₂ to 3.5 mg cm⁻² of GNF@NiCo₂O₄/MnO₂. The increase in mass loading is very small, but the areal capacitance of the composite has increased by 2.6 times than GNF@MnO₂. It can be seen that the increase in capacitance is not caused by the increase in the mass loading of MnO₂, but the structure of GNF@NiCo₂O₄/MnO₂ effectively improves the utilization of the active MnO₂. The petal structured NiCo₂O₄ provides large surface area for the high mass loading of MnO₂ and more electron transmission channels to MnO₂, and reduces the accumulation of MnO₂. As a result, the capacitance utilization of MnO₂ is improved and the areal capacitance of the composite material increases.⁵⁶ In addition, it is observed that when the current density increases from 2 mA cm⁻² to 20 mA cm⁻², the original surface capacitance of the GNF@NiCo₂O₄/MnO₂ loses 48.9%, demonstrating its outstanding rate capability. The specific capacitance of NF@NiCo₂O₄/MnO₂ is only 208.3 mF cm⁻² lower than that of GNF@NiCo₂O₄/MnO₂ at 2 mA cm⁻². However, its areal capacitance drops by nearly 80% from 2 to 20 mA cm⁻², illustrating its poor rate performance. This demonstrates that the graphene effectively improves the rate performance of the electrodes. The EIS test was conducted to estimate charge transfer kinetics of the hybrid materials and the obtained Nyquist plots are given in Fig. 7b. In general, the equivalent series resistance (ESR, R_s) of an electrode can be determined by observing the EIS spectrum in the high frequency region. It includes the inherent resistance of the electrode material (both current collector and active material), the contact resistance of the electrode/electrolyte interface and the bulk resistance of the electrolyte, and its value depends on the intercept of the spectrum on the transverse axis. That is, the R_s of the electrode can be seen at the intersection of the transverse axis.⁵⁷ The semicircle in the Nyquist plot can be used to determine the charge transfer resistance R_ct, the larger the diameter of the semicircle, the larger R_ct of the electrode.⁵⁸ The GNF@NiCo₂O₄/MnO₂ electrode reveals a smaller equivalent series resistance R_s (5.3 Ω) and charge transfer resistance R_ct (10.4 Ω) compared to that of GNF@MnO₂ (R_s 5.7 Ω, R_ct 55.7), suggesting its better electrical conductivity and rapid charge mobility. Low frequency range is related to the Warburg impedance which reflects the ion diffusion/transport in the electrolyte.⁵⁹ The slope of GNF@NiCo₂O₄/MnO₂ is higher than that of GNF@MnO₂, indicating its lower diffusion resistance. These results further convincingly prove that the existence of petal structured NiCo₂O₄ as skeleton for the depositing of MnO₂ can effectively decrease the internal resistance and facilitate the electron transfer, which is beneficial to promote the areal capacitance of the hybrid GNF@NiCo₂O₄/MnO₂.⁶⁰

Fig. 6 (a, c, e and g) CV curves of GNF@NiCo₂O₄/MnO₂, GNF@MnO₂, GNF@NiCo₂O₄ and NF@NiCo₂O₄/MnO₂ samples at different scan rates, (b, d, f and h) GCD curves of GNF@NiCo₂O₄/MnO₂, GNF@MnO₂, GNF@NiCo₂O₄ and NF@NiCo₂O₄/MnO₂ samples at different current densities.

Fig. 7 (a) Areal capacitances of different electrodes at different current densities, (b) Nyquist plots of different samples over a frequency range of 0.01 Hz to 100 kHz.

Electrochemical performance of the asymmetric supercapacitor

In order to further evaluate the potential of the GNF@NiCo₂O₄/MnO₂ electrode in practical applications, ASC was fabricated with GNF@NiCo₂O₄/MnO₂ composite and GNF@AC composite as the positive and negative, respectively. The electrochemical test of the assembled supercapacitor shows that its large voltage window is 2.1 V, which is beneficial for the promotion of energy density. Fig. 8a shows the CV curve with a voltage window of 1.8 V–2.4 V. Fig. 8b exhibits the CV curves of supercapacitors at different scan rates. The CV curves maintain a good symmetry and are almost rectangular, showing its typical capacitive performance and stable electrochemical performance. The GCD curves of the device at various current densities are depicted in Fig. 8c. Its triangular-like shapes manifest a reversible capacitive behavior. The areal capacitance based on GCD test is calculated to be 824.8 mF cm⁻² at 2 mA cm⁻² (Fig. 8d). The reason for the excellent capacitance of the device is mainly due to the heterostructure of the GNF@NiCo₂O₄/MnO₂ composite material. The MnO₂ is a petal-like nanosheet with a small thickness. This microstructure ensures high electrochemical utilization of active materials and provides a large surface area and more channels for the enhancement of the electrolyte permeation. NiCo₂O₄ is used as a conductive scaffold and its large surface area and rich pore structure provide more growth space for MnO₂, increasing the number of active sites for pseudo-active MnO₂ species, and providing more electron transmission path, thereby realizing a high utilization rate of MnO₂.^61,62 At a high current density of 20 mA cm⁻², the fabricated ASC achieves an excellent capacitance retention rate of 80.7% even after 5000 cycles (Fig. 8e). In addition, the capacitance of the supercapacitor shows an increasing trend in the first 500 cycles due to an activation process of the electrode.⁶³

Fig. 8 (a) The CV curves of ASC with different voltage windows, (b) the CV curves of ASC at different scan rates, (c) the GCD curves of ASC at different current densities, (d) GCD profiles of the composite electrodes at different current densities, (e) cyclic stability of the ASC for 2000 cycles at 20 mA cm⁻², (f) Nyquist plots of the ASC over a frequency range of 0.01 Hz to 100 kHz.

EIS was performed to test the electrochemical properties of ASC, and the Nyquist plots of the ASC are summarized in Fig. 8f. The Nyquist plots show the impedance R_s of the device is 3.2 Ω and the charge transfer resistance R_ct of the device is 13.6 Ω. The high slope in the low frequency region suggests that the supercapacitor possesses low diffusion resistance and excellent capacitive behavior.⁶⁴ When its power density is 2100.0 μW cm⁻² (190.9 W kg⁻¹, 21.0 mW cm⁻³), the ASC delivers a good specific energy density of 2100.0 μW cm⁻² (190.9 W kg⁻¹, 21.0 mW cm⁻³). It possesses better performance than numerous energy storage devices previously reported (Table 1), such as MnO₂/graphene//graphene (30.4 Wh kg⁻¹ at 100 W kg⁻¹),⁶⁵ NF/NiCo₂O₄/MnO₂//AC (35 Wh kg⁻¹ at 163 W kg⁻¹),⁶⁶ MnO₂/C//N-doped carbon (39.5 Wh kg⁻¹ at 200 W kg⁻¹),²⁸ et al.

Table 1 Performance comparison between our ASC and other similar supercapacitors

Positive electrode	Electrolyte	Voltage window	Energy density	Power density	Ref.
GNF/NiCo2O4/MnO2	KOH	1.6 V	27.8 Wh kg^−1	400.3 W kg^−1	69
MnO2/G	Na2SO4	2.0 V	30.4 Wh kg^−1	100 W kg^−1	65
NF/NiCo2O4/MnO2	NaOH	1.5 V	35 Wh kg^−1	163 W kg^−1	66
MnO2/C	Na2SO4	2.0 V	39.5 Wh kg^−1	200 W kg^−1	28
G/MnO2	Na2SO4	1.8 V	51.1 Wh kg^−1	102.2 W kg^−1	43
δ-MnO2	Na2SO4	2.0 V	52.22 Wh kg^−1	100 W kg^−1	61
NiCo2O4/MnO2/G	KOH	1.5 V	55.1 Wh kg^−1	187.5 W kg^−1	15
Na–MnO2/CNTF//MoS2/CNTF	Na2SO4	2.2 V	178.4 μWh cm^−2	1100.9 μW cm^−2	70
MnO2/NiCo2O4	KOH	1.5 V	0.715 mWh cm^−3	353.7 mW cm^−3	71
NiCo2O4/MnO2	KOH	1.6 V	2.55 mWh cm^−3	22 mW cm^−3	17
CNT/MnO2/G/CC	Na2SO4	2.2 V	10.18 mWh cm^−3	20 mW cm^−3	18
GNF/NiCo2O4/MnO2	Na2SO4	2.1 V	45.9 Wh kg^−1, 505.2 μWh cm^−2, 5.1 mWh cm^−3	190.9 W kg^−1, 2100.0 μW cm^−2, 21.0 mW cm^−3	This work

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Accelerated aging tests were also performed under floating conditions after 5000 charge–discharge cycles. The leakage current was recorded during each 2 hours period of constant voltages (1 V–2.1 V) and then the device was permitted to self-discharge for 2 h. As shown in the Fig. S2,† during the leakage current measurements, once the device is fully charged, the leakage current decreases rapidly within seconds and quickly reaches equilibrium. At higher voltages (2 V and 2.1 V), it takes a little longer to reach a plateau. In all cases, however, the leakage current drops sharply to values as low as 250 μA cm⁻² within a period of 5 minutes. This illustrates the good stability of the cell electrodes. It can be found from Table S1† that the values of leakage currents at different voltages are very small, only 0.007 μA cm⁻² at 1 V. And as the voltage decreases, the value of leakage current also becomes smaller, which shows that the rate of leakage current depends on the voltage.^67,68 Self-discharge is a spontaneous voltage drop of the energy storage device for a certain storage time, which is detrimental to the voltage and energy retention of the supercapacitor. The self-discharge characteristics can be straightforwardly captured by the time-related voltage retention. Fig. S3† shows the decay of the voltage within 2 h at different starting voltages, and it can be found that the voltage of the supercapacitor decreases gradually with time. It quickly stabilizes at low voltages, and as the voltage increases, it takes longer to reach a stable value. However, at high voltages (1.8 V–2.1 V), the voltage value over 2 h is higher than 1.5 V, which is a generally suitable voltage for supplying power to electronic devices.⁷² This proves the potential of the supercapacitor for practical applications. Floating test is a dependable technique for stability analysis of supercapacitor electrode. After the leakage current and self-discharge tests, a float test was performed for 100 hours to further evaluate the stability of the ASC. The voltage was kept at 2.1 V and five GCD cycles were performed at 4 mA cm⁻² at 10 h intervals. As shown in Fig. 9, the capacity retention value after 100 h of voltage hold is 82.4%, which indicates that the ASC voltage is stable over this voltage window and time. The outstanding performance of the ASC indicates its potential in practical application.

Fig. 9 Floating test of ASC at a constant voltage of 2.1 V for 100 h.

Conclusions

In summary, heterogeneous structured NiCo₂O₄/MnO₂ was designed on GNF via a convenient two-step hydrothermal method. The petal structured NiCo₂O₄ provided large surface area for the growth of high mass loading MnO₂ micro-flowers, which effectively avoided the excessive stacking of high massing loading MnO₂ and increased the electron transmission channel with MnO₂. The addition of graphene buffer layer also effectively improved the rate performance of the electrode materials, presenting a high areal capacitance of 1630.5 mF cm⁻² at 2 mA cm⁻². The asymmetric supercapacitor was fabricated by using GNF@NiCo₂O₄/MnO₂ as the positive electrode, GNF@AC as the negative electrode, and an environmentally friendly Na₂SO₄ solution as the electrolyte. The ASC reached a wide voltage window of 2.1 V and a high specific energy density of 505.2 μWh cm⁻² (45.9 Wh kg⁻¹, 5.1 mWh cm⁻³) at a specific power of 2100.0 μW cm⁻² (190.9 W kg⁻¹, 21.0 mW cm⁻³). The superior performance of the asymmetric supercapacitor indicates its practical application for the design of high mass loading supercapacitors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51775510, 51875525, 11672269, 11972323, 51572242 and 51675485), the Zhejiang Outstanding Youth Fund (No. LR19E020004), the Zhejiang Provincial Natural Science Foundation of China (Grant no. LR20A020002 and LR18E050002), the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-B2019004), the 111 Project (No.: D16004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra10948g

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