Hierarchical NiMn₂O₄@CNT nanocomposites for high-performance asymmetric supercapacitors

Abstract

Miniaturized energy storage devices have attracted considerable research attention due to their promising applications in various smart electronic devices. In this work, a high performance asymmetric supercapacitor (ASC) device was designed and fabricated wherein a novel nanocomposite consisting of manganese oxide (NiMn₂O₄) nanosheets with carbon nanotubes (CNTs) was used as the active material. High capacitance of 151 F g⁻¹ and energy density of 60.69 W h kg⁻¹ were achieved for the CNT@NiMn₂O₄ nanocomposites ASC at a current density of 1 A g⁻¹, which attributing to the widen operation voltage window ranging from 0 to 1.7 V. Moreover, the CNT@NiMn₂O₄ nanocomposites ASC also showed remarkable cycling stability with 96.3% energy density retention after 5000 cycles. As a result, the CNT@NiMn₂O₄ nanocomposite is a possible contender materials for next generation supercapacitors in high energy density storage systems.

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1. Introduction

When asked about the most crucial problem facing society today, many people say that it is the serious energy crisis, caused by over usage of fossil-fuel resources and environmental pollution. Considering the increasing consumption of fossil fuels and continuous worsening climate, renewable energy resources have started to take place in our daily lives. In the meanwhile, energy storage systems with high power density and/or high energy density have being a pressing need. On the other hand, with the tremendous development of the portable and wearable electronic devices, there has been an increasing demand for lightweight, environmentally friendly and high performance energy storage/conversion systems.^1–5 As the most promising strategies for energy storage, supercapacitors (also known as electrochemical capacitors) have attracted intensive attention because of they not only can instantaneously provide higher power density than batteries and higher energy density than conventional dielectric capacitors which benefit from their outstanding properties: superior power density, high charging and discharging rates, and long cycle life,^6–8 but also can be used in foldable and rollup devices.^9–12 In recent years, the increasing worldwide interest in carbon materials (CNTs, activated carbon, and graphene) as electrodes for supercapacitors is based on the fact that carbon-based supercapacitors will ultimately serve as a safety and low cost alternative to current commercial electrical double layer supercapacitors (EDLCs). These EDLCs offer high power density and excellent cycling performance but suffer from relatively low capacitance and energy density.^13,14 On the other hand, the pseudo-capacitors supercapacitors, with transition metal oxides and/or conductive polymers as active materials, have high energy density with a storage mechanism based on surface electrochemical reaction.^15,16 However, due to the grand challenges in device fabrication and limited selections of raw materials, the research of pseudo-capacitors is still a titsinfancy.^15,17–23

As an important cubic, spinel binary metal oxide, NiMn₂O₄ is promising electrode materials due to their exceptional chemical stability and outstanding electrochemical capacitance.²⁴ Firstly, the nickel cations almost exclusively occupy the octahedral B-sites in preference to the tetrahedral A-sites of the cubic close packed oxygen sublattice (an inverse spinel).^25,26 Secondly, NiMn₂O₄ can provide high redox active sites owning to the variability of the Ni and Mn lattice positions. These intriguing features make nickel manganese oxide to be investigated widely and extensively in many fields, such as catalysis, negative temperature coefficient thermistors, magnetism, sensors, supercapacitors, etc. However, the utilization of NiMn₂O₄ in energy storage devices is often hindered by its poor electronic conductivity.^27–32 For instance, porous structured NiMn₂O₄ material has been successfully synthesized by calcining oxalate precursors.³³ Spinel nickel manganese oxide with large specific surface area and suitable pore size is also prepared from an peroxide-driven sol–gel process.³⁴ Nano-sized and well dispersed manganese oxide and nickel manganese oxide (Ni–Mn–O) powders are synthesized via the hydrothermal route.³⁵ In order to improve the electrical properties, various approaches have been proposed.^36–44 For example, porous electroconductive NiMn₂O₄/C hierarchical tremella-like nanostructures have been used as anode and showed high lithium storage properties.⁴⁵ However, despite all these recent progresses have been demonstrated efficiently, they still suffered from complicated method, lower specific capacitance and easily coalesce into group after several cycles, etc. Therefore, it is still an urge to exploit new electrode material with high electroconductive, larger specific surface area and good dispersity.

CNTs are molecular wires that have become the leading building blocks for nanomaterials and have shown great potential in supercapacitors due to their excellent conductivity, very high strength, chemical stability, high specific surface area, and low density. Due to the outstanding characters esp. the electron transfer capability and easy availability at present, the CNTs welled up inside our mind to work as electrode firstly. Although several studies focusing on CNT-supported nanomaterials have been reported, there is still the challenge of the direct growth of NiMn₂O₄ on CNTs by a facile method.

In this work, we designed and synthesized hierarchical CNT@NiMn₂O₄ nanocomposites from NiMn₂O₄ nanosheets and CNTs, where in the CNTs matrix provided electron conductive channels and NiMn₂O₄ nanosheets provided large surface area for electrochemical reaction. From the perspective of device fabrication and design, we took CNT@NiMn₂O₄ nanocomposites as a model system to fabricate asymmetric supercapacitors. It is worth mentioning that the CNT@NiMn₂O₄//Active Carbon (AC) asymmetric supercapacitor shows high energy density of 60.69 W h kg⁻¹ at a power density of 874 W kg⁻¹ as well as long-term stability, outperforming most currently available ASCs.

2. Experimental section

2.1. Chemicals

All the reagents used in the experiment are analytical grade and used as received without further purification. Multiwalled carbon nanotubes (MWNTs-2040, purity of 95%, average diameter of 20–40 nm, and length range 5–15 μm) were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). Acetylene black, active carbon and polytetrafluoroethylene (PTFE) were achieved from Guangzhou Songbai Chemical Industry Co., Ltd. (Guangdong, China). The nickel(II) chloride hexahydrate (NiCl₂·6H₂O) and potassium permanganate (KMnO₄) were purchased from Tianjin Chemical Co., Ltd. (Tianjin, China) All aqueous solutions were freshly prepared with high purity water (18 MΩ resistance).

2.2. Multiwalled carbon nanotubes (CNT) immobilized NiMn₂O₄ nanosheet arrays (CNT@NiMn₂O₄)

CNT@NiMn₂O₄ nanocomposites were synthesized by a facile hydrothermal method. The synthetic route is illustrated in Scheme 1. Firstly, the reaction solution was obtained by mixing 1 mg CNT, 0.5 mmol nickel(II) chloride hexahydrate, 1 mmol potassium permanganate in 20 mL of distilled water under magnetic stirring for 1 h in air. Then the resulting solution was transferred into a 60 mL Teflon-lined stainless steel autoclave. After keeping 6 h at 100 °C, the autoclave was then naturally cooled down to room temperature. Subsequently, in order to remove the free debris and residual reactant, the mixture was centrifuged and washed several times with deionized water and absolute alcohol. Finally, the as-synthesized CNT@NiMn₂O₄ core–shell nanosheet arrays was dried at 80 °C for 6 h in an air oven to obtained the products.

Scheme 1 Schematic image of the formation of CNT@NiMn₂O₄ nanocomposites.

2.3. Materials characterization

The crystal structure of CNT@NiMn₂O₄ nanocomposites was measured with X-ray diffraction (XRD). XRD patterns were taken from 10° to 80° with a continuous scan mode to collect 2θ data using Cu Kα radiation. The chemical composition of the product was examined by means of energy dispersive X-ray spectroscopy (EDX, S-4800, Hitachi Co., Ltd., Tokyo, Japan). The interfacial property of the nanosheet arrays was examined with a scanning electron microscope (SEM, S-4800, Hitachi Co., Ltd., Tokyo, Japan). The morphologies of the carbon nanotubes immobilized NiMn₂O₄ nanoparticles were characterized with a JEM-1200 EX/S transmission electron microscope (TEM). Powders were dispersed in ethyl alcohol in an ultrasonic bath for 5 min and then deposited on a copper grid covered with a perforated carbon film. X-ray photoelectron spectroscopy (XPS) analysis was performed in a VG ESCALAB 220i-XL UHV surface analysis system with a monochromatic Al K_α X-ray source (1486.6 eV). The BET specific surface area and the pore size distribution of the sample were investigated by the Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–Halenda (BJH) methods.

2.4. Preparation of working electrode

The working electrodes were prepared by coating mixtures of the CNT@NiMn₂O₄ nanocomposites, acetylene black, and polytetrafluoroethylene binder (weight ratio of 8 : 1 : 1) on a slice of nickel foil current collector (1 mm thick). Working electrode with a geometric surface area of about 1 cm² contained about 2 mg of CNT@NiMn₂O₄ nanocomposites. The as-prepared electrode was dried at 80 °C for 12 h in vacuum.

2.5. Electrochemical measurements

The electrochemical measurements of the CNT@NiMn₂O₄ nanocomposites electrode were carried out in a three-electrode electrochemical cell containing 6 M KOH aqueous solution as the working electrolyte with a platinum wire and Ag/AgCl electrode as the counter and reference electrode, respectively. The cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were performed on a CHI 760D electrochemical workstation (Chenhua, Shanghai, China) in the potential range of −0.2–0.5 V.

To further evaluate the CNT@NiMn₂O₄ nanocomposite for real device applications, we fabricated asymmetric capacitors (ACS) in two-electrode test system with 6 M KOH as the electrolyte. The ASC was assembled using CNT@NiMn₂O₄ nanocomposites and commercially available activated carbon (AC) as positive and negative electrodes, respectively. Particularly, the negative electrode was prepared as follows: first, 90 wt% activated carbon and 10 wt% polyvinylidene fluoride binder dispersed in 1-methylpyrrolidone solvent were mechanically mixed to produce a homogeneous paste. Then the mixture was coated onto the nickel foil substrate to form the electrode layer. Finally, the fabricated electrode was pressed and then dried at 60 °C for 12 h. The electrochemical performances of the ASC device were measured by the same procedure as mentioned above. All the electrochemical measurements were conducted at room temperature.

3. Results and discussion

3.1. Characterizations

Morphology of the synthesized CNT@NiMn₂O₄ nanocomposites was investigated by SEM. The low-magnification micrograph (see Fig. 1a) reveals that numerous NiMn₂O₄ nanosheet were found well-grown around individual CNTs to form a conformal coating on the surface. From the enlarged view (see Fig. 1b), the nanosheet are in mutual contact, which might give better mechanical strength and form a better conductive network not easily detached from the CNT core. Such a nanostructure may bring significant benefit by an increase in its specie area while maintaining a conductive core, which is of great importance to the electrochemical performance of electrode materials.⁴⁶ Further insight into the detailed nanostructure is elucidated by TEM. In Fig. 1c and d, the hybrid oxide nanoparticles consisting of nanocrystallites were uniformly grown on the surface of the CNTs backbone. The above mentioned results show that CNTs are highly suitable substrates for spinel nickel manganese's nucleation and crystal growth. Furthermore, the unique features mentioned above can make CNT@NiMn₂O₄ nanocomposites accessible to electrolytes to a larger extent, which makes the nanostructures potential candidates as electrodes for supercapacitors with high capacitance and excellent cycling stability.⁴⁷ The XRD pattern (see Fig. 2a) exhibits several distinct peaks corresponding to the (111), (222), (400) and (422) planes in the standard NiMn₂O₄ spectrum (JCPDS 44-0141). The additional peaks at 25.6°, 43.9°, 63.5° and 77.5° can be attributed to the (002), (100), (440) and (006) reflections of CNTs, confirming the successful integration of NiMn₂O₄ with CNTs. It is worth mentioning that no other peaks are observed, which effectively confirms the purity of the obtained NiMn₂O₄ phase, which was supported on one-dimensional CNT. The EDS measurement (see Fig. S1†) shows that the molar ratio of Ni to Mn is almost 1 : 2, which is in accordance with the result of XRD. Oxidation state of the corresponding transition metal ions in the obtained sample is further investigated with X-ray photoelectron spectroscopy (XPS). A survey spectrum (Fig. 2b) shows the presence of Ni, Mn, and O as well as C and there is no other impurity. A Mn 2p core level spectrum (Fig. 2c) shows two major peaks with binding energies of 641.7 and 653.2 eV, assigned to the Mn 2p_3/2 and Mn 2p_1/2 peaks, respectively.⁴⁸ After refined fitting, the spectrum composes of four peaks. Those with binding energies of 641.8 eV and 653.3 eV are ascribed to Mn³⁺. Another two peaks at 640.7 eV and 652.3 eV are ascribed to Mn²⁺. Similarly, the Ni 2p spectrum (Fig. 2d) shows two spin–orbit doublets characteristics of Ni²⁺ and Ni³⁺ states and shake-up peaks at around 861.2 and 879.7 eV at the high binding energy side of the Ni 2p_3/2 and Ni 2p_1/2 edge,^49,50 The fitted peaks at 854.5 and 872.1 eV are attributed to Ni²⁺, while the other peaks at 855.8 and 874.0 eV are related to Ni³⁺. In the C 1s spectrum (Fig. 2e) and O 1s spectrum (Fig. 2f) the peaks are consistent with the reported values.⁵¹ According to the XPS analyses, the couples of Mn³⁺/Mn²⁺ and Ni³⁺/Ni²⁺ are coexisting in the spinel NiMn₂O₄ nanostructures. The atomic ratio of Ni and Mn is ∼1 : 2 based on the areas of their corresponding XPS peaks.

Fig. 1 Characterization of the CNT@NiMn₂O₄ hybrid nanoarrays by SEM: (a) low-magnification; (b) high-magnification; (c) TEM image; (d) TEM magnified images. The insets show the corresponding SAED pattern.

Fig. 2 (a) XRD patterns of CNT@NiMn₂O₄ nanocomposites; XPS spectra for the as-prepared CNT@NiMn₂O₄ nanocomposites: (b) a survey spectrum, (c) Mn 2p, (d) Ni 2p, (e) C 1s and (f) O 1s.

In addition, the porous characteristics of the above mentioned sample were further studied by nitrogen adsorption and desorption measurement, as presented in Fig. 3. Fig. 3a shows the adsorption–desorption isotherm of CNT@NiMn₂O₄ nano-composites. It is a typical type III isotherm, which could be attributed to N₂ adsorption–desorption in metallic oxide structures. The calculated specific surface area of the CNT@NiMn₂O₄ nanocomposites is 237.8 m² g⁻¹, according to the BET test. The large specific area could greatly increase the utilization of NiMn₂O₄ as an electrochemically active material in electrodes.⁵² Fig. 3b shows the corresponding pore size distribution calculated by the BJH model. As can be seen, the product possesses a wide pore size distribution, which is attributed to the mesoporous structure of NiMn₂O₄ nanosheets and the large pores between these nanosheets. This hierarchically porous structure facilitates the mass transportation of electrolytes within the bulk of electrode materials, promoting electrolyte accessibility, kinetic reversibility, and electrochemical reaction homogeneity.^53–55

Fig. 3 (a) N₂ adsorption–desorption isotherm of CNT@NiMn₂O₄ nanocomposites; (b) BJH measurement of CNT@NiMn₂O₄ nanostructure.

3.2. Electrochemical studies

3.2.1. Electrode performances. The electrochemical performance of CNT@NiMn₂O₄ nanocomposites was investigated in three-electrode systems. Fig. 4a shows representative cyclic voltammetry curves of a CNT@NiMn₂O₄ nanocomposites electrode at different scan rates ranging from 5 mV s⁻¹ to 100 mV s⁻¹ in the potential range of −0.2–0.5 V. Clearly, a pair of redox peaks can be observed in these CV curves, which should be attributed to the Faradaic redox reactions related to M–O/M–O–O–H (M represents Ni and Mn ions).^56,57 Fig. 4b shows the galvanostatic charge–discharge plots of the sample within a potential range of −0.2 to 0.5 V at various current densities ranging from 0.5 A g⁻¹ to 10 A g⁻¹. It can be noted that the potential–time curves at all current densities are nearly symmetrical, in other words, the electrode exhibits outstanding charge–discharge with a stable coulombic efficiency and a low polarization.⁵⁸ Based on the charge–discharge curves, the specific capacitance can be calculated by the formula:
where i is the current applied, ΔV/Δt is the slope of the discharge curve after the iR drop, and m is the mass of CNT@NiMn₂O₄ nanocomposites.

Fig. 4 (a) CV curves of CNT@NiMn₂O₄ at different scan rates (vs. Ag/AgCl); (b) charge–discharge profiles of CNT@NiMn₂O₄ at various current densities; (c) current density dependence of the specific capacitance; (d) Nyquist plots of CNT @NiMn₂O₄, bare nickel foil, CNT and NiMn₂O₄.

The calculated rate performance of the material is shown in Fig. 4c, it can be observed that the specific capacitance can attain a maximum of 836 F g⁻¹ at a current density of 0.5 A g⁻¹. Even at current density as high as 10 A g⁻¹, 471 F g⁻¹ can still be retained, which suggests that about 44.6% of the specific capacitance is lost when the current density increases from 0.5 A g⁻¹ to 10 A g⁻¹. For comparison, the pure NiMn₂O₄ nanosheet shows lower capacitance and poorer rate performance at the same conditions (as shown in Fig. 4c), the capacitance of the powder decreases from 368 F g⁻¹ (at 0.5 A g⁻¹) to 181 F g⁻¹ (at 10 A g⁻¹), with a reduction of 50.9%. It is easy to conclude that the rate capacitance of CNT@NiMn₂O₄ is superior to that of pure NiMn₂O₄. The superior electrochemical performance of CNT@NiMn₂O₄ nanocomposites should be attributed to the large surface area of NiMn₂O₄ nanosheet shell and highly conductive CNT core.

EIS measurements were also carried out to evaluate the charge transfer and electrolyte diffusion in the electrode/electrolyte interface, as shown in Fig. 4d. Obviously, the EIS of the CNT@NiMn₂O₄ nanocomposites and bare NiMn₂O₄ electrode was composed of a semicircle at the high-frequency region and a straight line at the low-frequency region. Obviously, the CNT@NiMn₂O₄ nanocomposites electrode exhibits slightly lower diffusive resistance (Warburg impedance) than bare NiMn₂O₄ electrode at the low-frequency region. However, compared to CNT and Ni foil electrode materials, the CNT@NiMn₂O₄ nanocomposites electrode had relatively larger diffusive resistance. Since Warburg resistance is related to the ion diffusion/transport in the electrolyte, the lower Warburg resistance will facilitate the diffusion of electrolyte ions (OH⁻) into the electrodematerials,⁵⁹ accordingly, a facile and reversible Faradaic redox would be taken,⁶⁰ and a good electrochemical property could be realized for the CNT@NiMn₂O₄ nanocomposites electrode.

3.2.2. Asymmetric supercapacitor performances (ACS). To obtain a high energy density, an asymmetric supercapacitor was assembled by use of CNT@NiMn₂O₄ nanocomposites and active carbon as positive and negative electrodes, respectively. As for an asymmetric supercapacitor, it is necessary to balance the charges stored at the positive and negative, namely, q⁺ = q⁻.^61,62 where q⁺ means the charges stored at the positive electrode, q⁻ means the charges stored at the negative electrode. The charge stored by each electrode usually depends on the specific capacitance (C_m), the potential window for the charge–discharge process (ΔE), and the mass of the electrode (m), which is following the equation:⁶³ q = C × ΔE × m, the mass ratio between the positive and negative electrodes needs to follow:

Subsequently, to ensure the charge balance of the hybrid cell, the optimal weight ratio between the positive and negative electrodes is 0.89 : 1, and the active mass of the ASC includes materials of both the anode and cathode electrodes. To estimate the stable potential window of CNT@NiMn₂O₄//AC ASC, CV measurements were performed on the two electrode materials in 6 M KOH aqueous solution before evaluating the asymmetric cell, using a three-electrode system with a platinum wire as counter electrode and Ag/AgCl electrode as reference electrode.

As shown in Fig. 5a, the CNT@NiMn₂O₄ nanocomposites electrode is measured within a potential window of −0.2 to 0.5 V, while the activated carbon (AC) electrode is measured within a potential window of −1 to 0 V at a scan rate of 100 mV s⁻¹. The CV curve of the activated carbon electrode exhibits a nearly rectangular shape, which is a characteristic of an electric double layer capacitor. As for the CNT@NiMn₂O₄ nanocomposites, two redox peaks are easily observed, which have already been analyzed above. On the basis of these results, it can be seen that the potential window of CNT@NiMn₂O₄ nanocomposites electrode is from −0.2 to 0.5 V and the active carbon is from −1 to 0 V with capacitive behavior. Furthermore, it can be observed that the two materials are stable in a different range of potentials. Therefore, the overall capacitance of the CNT@NiMn₂O₄//AC ASC device derives from the combined contribution of electrical double layer capacitance and redox pseudocapacitance. The total operating cell voltage can be expressed as the sum of the potential range for the two electrodes. It is possible to conclude that the cell potential can be extended up to 1.7 V in 6 M KOH aqueous solution, if the two electrodes are assembled into ASC devices. Therefore, we construct an ASC device by using CNT@NiMn₂O₄ as the positive electrode and activated carbon (AC) as the negative electrode in a 6 M KOH electrolyte. Fig. 5b shows the CV curves of our ASC device in a potential window of 0–1.7 V in 6 M KOH electrolyte at different scan rates of 5, 10, 20, 50, and 100 mV s⁻¹. Compared to that of pure CNT@NiMn₂O₄ and activated carbon electrode, the ASC device shows relatively rectangular CV curves without obvious redox peaks, even at the potential up to 1.7 V, indicating the ideal capacitive behavior. In addition, CV profiles still retain a relatively rectangular shape without obvious distortion with increasing potential scan rates, indicating the desirable fast charge–discharge property for power devices. Rate capability is an important factor for the electrochemical capacitors in power applications. So we perform galvanostatic charge–discharge tests at various current densities in 6 M KOH electrolyte. A good electrochemical energy storage device is required to provide its high specific capacitance and high energy density at a high charge–discharge rate. Plots of voltage versus time for the ASC at various current densities are shown in Fig. 5c. Fig. 5d presents the relationship between the specific capacitance of the ASC device and current density. As we can seen, the ASC device delivers a maximum specific capacitance of 151 F g⁻¹, 147 F g⁻¹, 113.5 F g⁻¹, 94.7 F g⁻¹ and 72.8 F g⁻¹ at the current density of 1 A g⁻¹, 2 A g⁻¹, 5 A g⁻¹, 10 A g⁻¹ and 20 A g⁻¹, respectively. The specific capacitance drops 66.9% when the current density increases from 1 to 20 A g⁻¹. The specific capacitance gradually decreases with the increase of current density, since diffusion most likely limits the movement of ions and electrons due to the time constraint. Fig. 5e shows the Ragone plot of the ASC device measured in the voltage window of 0–1.7 V. The energy and power densities are derived from the discharge curves at different current densities. For comparison, the asymmetric supercapacitor devices on CNT@NiMn₂O₄//AC, NiCo₂O₄@MnO₂//AC,⁶⁴ Co₃O₄@MnO₂//AC,⁶⁵ Ni–Co sulfide//AC⁶⁶ and NiO//carbon⁶⁷ are assembled and characterized. Specific capacitance of these ASC devices have lower energy and power densities than the CNT@NiMn₂O₄//AC ASC devices. The cycling stability of the asymmetric supercapacitor was further investigated by galvanostatic charge–discharge between 0 and 1.7 V at a current density of 5 A g⁻¹ (Fig. 5f). After galvanostatic charge–discharge for 5000 cycles, the ASC device presents a capacitance of 109.3 F g⁻¹ with 96.3% of the maximum value, indicating that the high stability of our ASC device is suitable for high-performance supercapacitor applications.

Fig. 5 (a) CV curves of CNT@NiMn₂O₄ electrode and activated carbon (AC) electrode performed in a three-electrode cell in 6 M KOH electrolyte at a scan rate of 100 mV s⁻¹; (b) CV curves of the CNT@NiMn₂O₄//AC-ASC device measured at a potential window of 0–1.7 V at different scan rates; (c) galvanostatic charge–discharge curves of the ASC device at different current densities; (d) current density dependence of the specific capacitance; (e) Ragone plots of the as-fabricated CNT@NiMn₂O₄//AC-ASC device. The values reported for others devices are given here for a comparison; (f) cycling performance of the CNT@NiMn₂O₄//AC-ASC device.

Lightweight and flexible supercapacitors that have excellent electrochemical performance even under mechanical deformation are highly desired to meet the demands of future development of multifunctional flexible electronics. The schematic diagram of the asymmetric ASC is illustrated in Fig. 6a. In order to explore the potential application of the CNT@NiMn₂O₄//AC ASC as a power source, the ASC could power many colors LEDs after charging for only 30 s (Fig. 6b and c).

Fig. 6 (a) Schematic diagram of a solid-state CNT@NiMn₂O₄//AC ACS; LED powered before (b) and after (c) charged tandem ASC devices.

Several contributing factors are considered for the high specific capacitance and high energy density of the ASC device: firstly, the energy density of electrochemical capacitors is proportional to the square of operating voltage. The voltage range of our ASC devise is extended to 1.7 V, consequently resulting in a significantly increase in energy density. Secondly, the CNT@NiMn₂O₄ nanocomposites with large specific area ensures the high utilization of active materials, fast ion and electron transfer, and good structural stability, further decreasing the kinetic limitations of the pseudocapacitive electrode and hence resulting in the high specific capacitance and energy density of our ASC device. Thirdly, the carbon material in the cathode electrode, which is widely used as a conductive additive in electrochemical systems, possesses excellent electric conductivity as an electric double layer capacitor electrode. Apart from, the synergistic effect of the two electrodes is also an important contributing factor of the excellent electrochemical performances of the ASC device.

4. Conclusions

A highly crystalline CNT@NiMn₂O₄ nanocomposites was synthesized by a facile hydrothermal method. The CNT@NiMn₂O₄ nanocomposites electrode delivers noticeable pseudocapacitive performance with a high capacitance of 836 F g⁻¹ at 0.5 A g⁻¹. For more practical application, a novel and durable ASC on CNT@NiMn₂O₄//AC is fabricated, the device shows electrochemical capacitance performance within a voltage range of 0–1.7 V and exhibits high specific capacitance from 151 to 72.8 F g⁻¹ as the current density increases from 1 to 20 A g⁻¹, even specific capacitance retention of 96.3% after 5000 cycles at 5 A g⁻¹, as well as high energy density and good electrochemical stability, the ASC could power many colors LEDs after charging for only 30 s. The CNT@NiMn₂O₄ nanocomposites is expected to be a promising candidate for application in high-performance electrochemical capacitors, and the as-fabricated ASC also shows great potential in the development of energy storage devices with high energy and power densities.

Acknowledgements

This work was financially supported by the projects (no. 21371007) from National Natural Science Foundation of China, Anhui Provincial Natural Science Foundation (1208085QB28), Anhui Provincial Natural Science Foundation for Distinguished Youth (1408085J03), Natural Science Foundation of Anhui (KJ2012A139) and the Program for Innovative Research Team at Anhui Normal University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00979k

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