Core-shell structured Ni6MnO8@carbon nanotube hybrid as high-performance pseudocapacitive electrode material

Electrochimica Acta 320 (2019) 134627

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Core-shell structured Ni6MnO8@carbon nanotube hybrid as highperformance pseudocapacitive electrode material Juan Liu a, b, Tingjiao Xiong b, Tao Liu b, Chao Yang b, Honghui Jiang b, Xiaocheng Li b, * a Jiangxi Province Key Laboratory of Mining Engineering, School of Resources and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China b Jiangxi Province Key Laboratory of Power Battery and Materials, School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2018 Received in revised form 22 July 2019 Accepted 30 July 2019 Available online 31 July 2019

Binary metal oxides have been proved to possess better pseudocapacitive performance than single metal oxide analogies. Substitution of toxic and high-cost ions (Co and Mo) in binary metal oxides with lowcost and environmentally-friendly ions (Ni and Mn) is of great economic and environmental importance. In this paper, murdochite-type binary metal oxides (i.e., Ni6MnO8) and binary metal oxides@carbon nanotubes (i.e., Ni6MnO8@CNTs) core-shell structure were synthesized via a facile hydrothermal approach followed by a calcination process. Electrochemical measurement results indicated that the synthesized Ni6MnO8@CNT core-shell structure showed a high specific capacitance of 1213 F g
1 at 1 A g
1 with two times capacitance value of Ni6MnO8 at same current density. Even at the high current density of 20 A g
1, the Ni6MnO8@CNT core-shell structure can still deliver a high capacitance value of 711 F g
1, which value is also much higher than that of Ni6MnO8 at the current density of 1 A g
1. With the prepared Ni6MnO8@CNT hybrid as positive electrode and activated-polyanilinederived-carbon as negative electrode, an asymmetric supercapacitor cell (ASC) was successfully assembled. The assembled ASC device exhibited excellent pseudocapacitive performance with a high specific capacitance of 154 F g
1 at 1 A g
1 and a high energy density of 58.2 Wh kg
1 at a power density of 831.4 W kg
1. Even at a high power density of 16.6 kW kg
1, the ASC device can still deliver a high energy density of 23.1 Wh kg
1, suggesting the promising applications of Ni6MnO8@CNT hybrid in supercapacitors. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Ni6MnO8 Carbon nanotube Specific capacitance Asymmetric supercapacitor Energy density

1. Introduction In recent years, construction of energy storage and conversion systems has attracted tremendous attention due to the overconsumption of fossil fuel and thus induced environmental pollution [1e6]. Supercapacitors are considered as attractive energy storage and conversion devices owing to their fast charge/discharge rates, long life cycles, and low maintenance cost [7e9]. Based on the storage mechanism, supercapacitors can be classified into two different groups: (a) electric double layer capacitors (EDLC) based on adsorption/desorption of electrolyte ions on electrode/electrolyte interface, and (b) pseudocapacitors related to the faradaic redox reactions that occurring not only on the surface but also in

* Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.electacta.2019.134627 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

the near-surface of the electroactive materials. Pseudocapacitivetype electroactive materials can provide higher energy density as compared to the EDLC-based carbonaceous materials. Transition metal oxide and hydroxide are rising stars of recent pseudocapacitive materials owing to their high theoretic specific capacitance value, robust chemical/thermal stability, environment-benign nature, low-cost and easy preparation process. Up to now, pseudocapacitive performance of transition metal oxides, such as NiO [10], Co3O4 [11], Ni(OH)2 [12], Co(OH)2 [13], Fe3O4 [14] and MnO2 [15], has been widely investigated. These transition metal oxides do demonstrate much better capacitive performance than EDLC-based carbonaceous materials. Recently, binary metal oxides have been reported to exhibit better performance than single oxide analogies because of their high electrical conductivities and multiple oxidation states during the faradaic reactions. Triggered by these intriguing features, several binary metal oxides, including NiCo2O4 [16,17], NiMoO4 [18], MnCo2O4 [19] and CoMoO4 [20], have been

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successfully developed and their pseudocapacitive performances are systematically explored. Despite the remarkable progress, the current reserves of cobalt and molybdenum are inadequate to meet the growing global demand from various fields, which greatly limits the applications of cobalt-/molybdenum-based binary metal oxides in pseudocapacitors. Substitution of toxic and high-cost ions (Co and Mo) with cheap, abundant and environmentally-friendly ions (Ni and Mn) is thereby of great economic and environmental importance. Unfortunately, limited information is available for the electrochemical performance of Ni-Mn-based binary metal oxides. Up to now, only the electrochemical performance of NiMn2O4 and NiMnO3 material is preliminarily investigated [21e23]. These two Ni-Mn-O compounds delivers extremely low capacitance value of ~300 F g-1, can not reach the critical requirement of supercapacitors for electrode material. In fact, besides NiMn2O4 and NiMnO3, Ni6MnO8 also is a promising alternative pseudocapacitive material due to its good chemical stability and special crystal structure. Firstly, the face-centered cubic structure of Ni6MnO8 contains the octahedrally coordinated Ni2þ, Mn4þ and cation vacancies in cubic structure. One-eighth of Ni2þ ions is replaced by Mn4þ ions and cation vacancies, two of which occupy the alternating (111) lattice structure of Ni6MnO8 and thus is favorable for the stabilization of the crystal structure of Ni6MnO8 [24]. Secondly, Ni6MnO8 can provide high redox active sites owing to the variability of the Ni and Mn lattice positions. These intriguing features make Ni6MnO8 the promising pseudocapacitive material. Besides the crystal structural stability, the electrical conductivity of the host material also influences its electrochemical performance. Similar as most of transition metal oxides and binary metal oxides, the intrinsic electrical conductivity of Ni6MnO8 is relatively low. Therefore, it is still needed to explore a highly conductive matrix to support Ni6MnO8 so that its excellent pseudocapacitive performance can be fully exhibited. Carbon nanotube (CNT) is a kind of well-ordered, hollow graphic nanomaterials made of cylinders of sp2-hybridized carbon atoms. Numerous results suggest that CNT possesses high specific surface area, desirable electronic conductivity, high corrosion resistance and excellent mechanical stability. These features make CNT the leading building blocks for nanoelectronics [4,6]. Also thanks for its high specific surface area and good conductivity, in recent few years, CNT is used as skeleton to support psudocapacitive materials and boost the capacitive performance of host materials. The synthesized CNT@metal oxides and CNT@binary metal oxides exhibited outstanding pseudocapacitive performance due to the advantage of core-shell structure in shortening the electron pathway and enhancing the electrical conductivity of host materials [25e27]. This inspires us to deposit the electroactive Ni6MnO8 on CNT skeleton and construct the core-shell structured Ni6MnO8@CNT hybrid so that the electrochemical performance of Ni6MnO8 can be significantly improved. Herein, for the first time, we synthesize the Ni6MnO8 and coreshell structured Ni6MnO8@CNT hybrid through a simple hydrothermal reaction and explore its possible applications in supercapacitors. Thanks for the excellent intrinsic properties of CNTs and structuraladvantage of core-shell configuration in boosting the electrochemical performance of host material, the developed Ni6MnO8@CNT hybrid delivers high specific capacitance, good rate capability, as well as good cycling stability. Coupled with our recent developed high-performance activated-polyaniline-derived-carbon (APDC) as negative electrode, the assembled asymmetric supercapacitor cell (ASC) shows a maximum energy density of 58.2 Wh kg
1 and good cycling stability at high current density. The electrochemical performance of the Ni6MnO8@CNTs//APDC ASC device is much better than the other aqueous asymmetric supercapacitor devices based on Ni-Mn-O composites.

2. Experimental section 2.1. Preparation of Ni6MnO8@CNTs/NF positive electrode CNTs were firstly dispersed in concentrated HNO3 acid and then reflux at 70  C for 30 min to introduce oxygen-containing functional groups on CNT surface. The acidulated CNTs were collected by centrifuging at 10,000 rpm for 30 min, followed by drying overnight in a freeze-dryer. The Ni6MnO8@CNT hybrid was prepared through a simple hydrothermal method followed by a calcination treatment process. Typically, 1 mmol NiCl.26H2O, 3 mmol MnCl.24H2O and 5 mmol hexamethylenetetramine (HMTA) were dissolved in 15 ml deionized water and stirred for 15 min to form a homogeneous solution, denoted as Solution A. 20 mg of functionalized CNTs were dispersed in 15 ml de-ionized water to form a uniform suspension, denoted as Suspension B. To improve the stability and dispersity of CNTs in water, 40 mg of sodium dodecyl sulfonate (SDS) was also added into Suspension B during stirring. After stirring for 15 min, Suspension B was added into Solution A and stirred for another 15 min. Subsequently, the obtained homogeneous solution was transferred into Teflon-lined stainless autoclaves and heated at 90  C for 6 h. After the autoclave was naturally cooled down to room temperature, the resultant was collected by centrifugation, washed with de-ionized water and dried at 80  C for 12 h. Finally, the product was annealed at 300  C for 2 h with a ramp rate of 2  C min
1 to obtain Ni6MnO8@CNT hybrid. The Ni6MnO8 was synthesized through the same procedure but without the addition of CNTs. The Ni6MnO8@CNT positive electrode was prepared by using a slurry-casting technique. The slurry was obtained by grinding the electroactive material (85 wt%), acetylene black (10 wt%) and polytetrafluoroethylene (5 wt%). The resultant slurry was uniformly casted on nickel foam (NF) with size of 1*1 cm2 followed by drying in an oven at 80  C for 12 h and finally pressing under the pressure of 8 MPa for 10 min. The mass of electroactive material (Ni6MnO8@CNT hybrid or Ni6MnO8) is ~2.1 mg. 2.2. Preparation of APDC/NF negative electrode The negative APDC powder was synthesized through polymerization of aniline, carbonization of polyaniline and KOH activation, as described elsewhere [28]. The detailed synthetic process of APDC powder was described in supporting information. APDC/NF negative electrode was prepared in the same manner for preparation of Ni6MnO8@CNT/NF positive electrode. 2.3. Material characterization The as-prepared electroactive materials were characterized by using a filed-emission scanning electron microscopy (FESEM, JOEL, JSM-6701F, Japan), a high-resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2 TF20, America), a scanning electron microscopy (SEM, JEOL, JSM-5601LV, Japan) equipped with energydispersive X-ray spectroscopy (EDS) and an X-ray diffractrometer (XRD, PANalytical, Empyrean, Netherlands). 2.4. Electrochemical measurements Electrochemical performances of the as-prepared electrodes were studied by using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrical impedance spectroscopy (EIS) techniques. The electrochemical measurements were performed on a commercial CHI 660E electrochemical working station within a three-electrode system with the prepared electrode as

J. Liu et al. / Electrochimica Acta 320 (2019) 134627

working electrode, Pt foil as counter electrode, SCE as reference electrode, and 1 M KOH aqueous solution as electrolyte. EIS measurement was carried out within frequency ranging from 100 kHz to 0.01 Hz at open circuit potential with a perturbation of 5 mV. 2.5. Assembly of asymmetric supercapacitor cell The ASC device was assembled by using the as-prepared Ni6MnO8@CNT/NF as positive electrode, APDC/NF as negative electrode, commercial cellulose paper as separator and 1 M KOH aqueous solution as electrolyte. According to the well-known charge balance theory (qþ ¼ q-), the mass ratio of Ni6MnO8@CNT þ and APDC on current collector is determined by the formula m m
¼ C
U
Cþ Uþ ,

where m, C and U were the mass, specific capacitance and

potential window of electroactive material, respectively. The “þ“/”-” represents the positive and negative electrode, respectively. 3. Results and discussion 3.1. Structural and morphological characterization of Ni6MnO8@CNTs The crystal structure of the prepared compound was investigated by XRD technique. As can be seen in Fig. 1a, three typical characteristic peaks are well defined at about 43.5 , 37.2 and 63.1, which can be indexed to the (400), (222) and (440) planes of Ni6MnO8 (JCPDS 42-0479), respectively, suggesting the cubic murdochite-type structure of the prepared Ni6MnO8 compound [29,30]. The typical atomic arrangement in Ni6MnO8 is demonstrated in Fig. 1b. According to XRD patterns, no diffraction peaks of

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other Ni-Mn oxide impurities, such as NiMn2O4 [21] and NiMnO3 [22], can be detected in the synthesized compound, implying the high purity of the synthesized Ni6MnO8. The diffraction peak of CNTs at about 26 is too weak to be identified, certifying that the CNTs should be sufficiently enwrapped by Ni6MnO8, similar to that in Ni-Co LDH@CNT composite [27]. The chemical composition of the samples is further detected by EDS spectrum (Fig. S1), in which the signal of nickel, manganese, oxygen and carbon elements are clearly observed. Based on the EDS results, Ni/Mn atomic ratio is calculated to be 6, further verifying the stoichiometric ratio of Ni/ Mn in the prepared Ni6MnO8 hybrid. Surface morphology of the synthesized Ni6MnO8 and Ni6MnO8@CNT hybrid was investigated by using scanning electron microscopy. Obviously, the synthesized Ni6MnO8 material through hydrothermal method demonstrates the irregular nanosheet morphology with planar size of ~300 nm and thickness of 70e100 nm (Fig. 2a and b). These nanosheets stack together and form serious agglomerations. When CNTs were introduced during the hydrothermal process, the resultant product shows typical linear shape (Fig. 3a), similar to that of CNTs. High-magnification FESEM image and HRTEM image, as demonstrated in Fig. 3b and c, indicate that the Ni6MnO8 nanoparticle firmly anchored on the surface of CNTs, giving a typical core-shell structure with CNTs as skeleton and Ni6MnO8 as shell layer. The formation of the Ni6MnO8@CNT core-shell structure is closely related to the strong electrostatic adsorption between the oxygen-containing functional groups with negative charges on CNT surface and the metal ions with positive charges [31]. This strong adsorption firmly anchors the Ni/Mn ions on the surface of CNTs at the initial stage of the hydrothermal process and thus forms a typical core-shell structure during the subsequent calcination process. Due to the reasonable

Fig. 1. (a) XRD pattern of Ni6MnO8@CNT hybrid and (b) crystal structure of Ni6MnO8.

Fig. 2. FESEM images of Ni6MnO8 nanosheets with different magnifications.

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Fig. 3. (a, b) FESEM images and (c) HRTEM image of Ni6MnO8@CNTs at various magnifications.

shorten the electron transportation pathway of Ni6MnO8@CNTbased electrode during the energy conversion process. By and large, the typical core-shell structure of Ni6MnO8@CNT composite, together with the large numbers of voids between the Ni6MnO8@CNTs, would significantly promote the rate capability of Ni6MnO8@CNT-based electrode. All these will be verified by their electrochemical performances in the following sections.

3.2. Electrochemical performance of Ni6MnO8@CNTs

Fig. 4. N2 adsorption/desorption isotherms of the Ni6MnO8 and Ni6MnO8@CNT composite.

weight ratio of CNTs and Ni/Mn binary metal salts, all Ni6MnO8 nanoparticles are uniformly deposited on CNT surface and no aggregation of Ni6MnO8 nanoparticles/nanosheets was observed. Obviously, the introduce of CNTs and formation of core-shell structure in Ni6MnO8@CNT composite would greatly increase the specific surface area of the composite. This speculation is supported by the nitrogen adsorption/desorption isotherms of two samples. Fig. 4 shows the nitrogen adsorption/desorption isotherms of Ni6MnO8 and Ni6MnO8@CNT hybrid. Although both the isotherms of two samples exhibit the type IV isotherm and H3 hysteresis loop, a noticeable increase of N2 adsorption in Ni6MnO8@CNT composite can still be easily observed. Based on the BET method, the specific surface area of Ni6MnO8 and Ni6MnO8@CNT composite is calculated to be 84.2 m2 g
1 and 97.6 m2 g
1, respectively, confirming the increases of the specific surface area of Ni6MnO8@CNT composite due to the introduction of CNTs. The large specific surface area of Ni6MnO8@CNTs can provide more electroactive sites to participate into the redox reaction during the energy storage process and thus greatly improves specific capacitance of Ni6MnO8@CNT composite. Besides, due to the strong mechanical properties of CNTs, large numbers of voids are clearly observed between the Ni6MnO8@CNTs. These large voids can act as large reservoirs for electrolyte ions and thus greatly shorten the transportation pathway of electrolyte ions during the faradaic reaction process [31]. Additionally, the core-shell structure of Ni6MnO8@CNT hybrid would greatly improve the usage efficiency of electroactive Ni6MnO8 and

Fig. 5a and b shows the CV curves of Ni6MnO8 and Ni6MnO8@CNT hybrid at various scan rates in a potential range of 0e0.6 V. As observed, both the CV loops of the two electrodes show a pair of redox peaks, which can be assigned to the redox reaction of M
O or M-OOH with the OH
in alkaline electrolyte (M represents Ni and Mn ions) [32], suggesting a typical faradaic reaction process. Moreover, all redox peaks demonstrate the good symmetry, implying the high reversibility of the involved faradaic redox reactions of two electrodes at various scan rates. Additionally, with the increases of the scan rate, the redox current peak of the CV loops of the two electrodes keep increasing, indicating the good I-V response of the two Ni6MnO8-based electrodes. Figs. S2a and b shows the galvanostatic charge/discharge curves of Ni6MnO8 and Ni6MnO8@CNTs composite at different current densities within potential window of 0e0.45 V. The discharge branch of the GCD curves of two electrodes shows a potential plateau at ~0.2 V, which agrees well with the reduction peaks in their CV curves. According to the defined equation (Eqn S(1)) for calculating the specific capacitance of pseudocapacitive materials, the synthesized Ni6MnO8 material shows relatively low specific capacitances of 602, 510, 482, 444, 403 and 400 F g
1 at current densities of 1, 3, 5, 10, 15 and 20 A g
1, respectively. Encouragingly, the core-shell structured Ni6MnO8@CNT hybrid can deliver high specific capacitances of 1213, 1007, 944, 867,800 F g
1 at the same current density (see Fig. 5c). Obviously, the Ni6MnO8@CNT hybrid demonstrates approximate two times capacitance values of Ni6MnO8 electrode. Considering the high gravimetric fraction and low capacitance contribution of CNTs whose capacitance value is normally low than 100 F g
1 (see Figs. S4a and b), the capacitance value of Ni6MnO8 on CNT skeleton can reach more than three times those of sole Ni6MnO8 compound. Further increase the current density up to 20 A g
1, the Ni6MnO8@CNT electrode can still deliver high capacitance of 711 F g
1, which value is even much higher that of Ni6MnO8 at 1 A g
1. To further demonstrate the electrochemical performance of the prepared electrodes, we employ the cycling test to examine the stability of the Ni6MnO8 electrode and Ni6MnO8@CNT electrode at a high current density of 10 A g
1. As can be seen from Fig. 6, the Ni6MnO8 electrode shows a continuous decay during the whole charge/discharge measurement period and gives an extremely low

J. Liu et al. / Electrochimica Acta 320 (2019) 134627

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Fig. 5. Electrochemical performance of Ni6MnO8 and Ni6MnO8@CNT hybrid: (a, b) CV curves of two electrodes at various scan rates, (c) specific capacitance at different current density and (d) EIS spectra of two electrodes.

Fig. 6. Cycling stability of the Ni6MnO8@CNTs and Ni6MnO8 electrode at a current density of 10 A g
1.

capacitance value of 110 F g
1 after 5000 cycles. While for Ni6MnO8@CNT electrode, at the high current density of 10 A g
1, it experiences a perceptible decrease at initial 900 cycles and then tends to be relatively stable. After 5000 cycles, the Ni6MnO8@CNT electrode delivers a high specific capacitance of 711 F g
1 with a high capacitance retention of 82%. The superior cycling stability of Ni6MnO8@CNT electrode is mainly attributed to the CNTs which anchored the metal ions during the hydrothermal process and act as the skeleton to stabilize the geometry configuration of Ni6MnO8 during the faradaic reaction process. Based on the above results, we can conclude that the CNTs play a

significant role in improving the electrochemical performance of Ni6MnO8. This can be accounted for by considering the following reasons. Firstly, the highly-conductive CNT can effectively enhance the transportation rate of electrons and boost the faradaic reaction kinetics of the composite during the energy storage process. Secondly, the open spaces between Ni6MnO8@CNTs provide reservoirs for electrolyte and thus allow the easy access of the electrolyte ions to the electrode/electrolyte interface during faradaic reactions, especially at high current density. Thirdly, the unique core-shell structure of Ni6MnO8@CNT hybrid greatly shortens the electron transportation pathway during the faradaic reactions. Fourthly, the presence of CNTs in Ni6MnO8@CNT hybrid remarkably increases the specific surface area of Ni6MnO8 and provides more electroactive sites during the faradaic reactions. Based on aforementioned reasons, the Ni6MnO8@CNT electrode shows superior electrochemical performance over Ni6MnO8 electrode. To validate this viewpoint, we perform the EIS measurement of the Ni6MnO8@CNTs and Ni6MnO8 electrode. As observed in Fig. 5d, both the Nyquist plot of the Ni6MnO8@CNTs and Ni6MnO8 electrode contain a depressed semi-circle in the high-frequency region correlating to the charge-transfer resistance at electrode/electrolyte interface and a sloped line in the low-frequency region associating with the diffusion resistance of the electrolyte ions. In the low-frequency region, the slope of EIS spectrum of Ni6MnO8@CNT electrode is larger than that of Ni6MnO8 electrode, implying the advantage of electrolyte reservoirs between Ni6MnO8@CNT coreshell structures in shortening the diffusion distance of the electrolyte ions during the faradaic reactions. In the high-/medianfrequency region, the diameter of the depressed semi-circle of Ni6MnO8@CNT electrode is much small than that of Ni6MnO8 electrode, suggesting the lower charge-transfer resistance of Ni6MnO8@CNT electrode due to the introducing of CNTs. The reduced charge-transfer resistance is closely related to the core-

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shell structure in shortening the electron transportation pathway. 3.3. Electrochemical performance of Ni6MnO8@CNTs//APDC asymmetric supercapacitor device Considering that the potential window of Ni6MnO8@CNTs composite is narrow (0e0.6 V), the symmetric device with Ni6MnO8@CNTs as component can only output the relatively low working voltage. Instead, the asymmetric device with Ni6MnO8@CNT hybrid as positive electrode and carbon-based materials as negative electrode can fully utilize the potential window of two electrodes and maximizingly extends the working voltage of the device, and thus notably increases the energy density of the device. With this in mind, in this study, we assemble the ASC device with the prepared Ni6MnO8@CNTs hybrid as positive electrode and recent developed APDC as negative electrode, and evaluate its practical performance. Before assemble the ASC device, the electrochemical performance of negative APDC was systematically measured and the optimized mass ratio of positive/negative electroactive material was determined. As shown in Fig. S3a, the APDC electrode displays typical EDLC characteristics within a potential window of
1.0 to 0 V and can deliver the specific capacitances of 410, 339, 320, 300, 285 and 280 F g
1 at current densities of 1, 3, 5, 10, 15 and 20 A g
1, respectively (see Fig. S3b). According to the charge balance principle as well as the electrochemical performances of APDC and Ni6MnO8@CNT hybrid, the mass ratio of positive to negative material on NF current collector was set as 0.73 in the assembled ASC device. Fig. 7a shows the CV curves of the assembled ASC device at a scan rate of 10 mV s
1 within different potential windows. It indicates that the potential window of the ASC device can extend up to 1.65 V without obvious polarization. As the potential window approaches 1.7 V, an obvious undesirable oxygen revolution reaction-induced peak is clearly observed. This implies that the assembled ASC device can steadily work at the voltage window of

0e1.65 V. Fig. 7b shows the CV curves of the ASC device within 0e1.65 V at various scan rates. All CV curves demonstrate the complex characteristics of EDLC and pseudocapacitance with a quasi-rectangular shaped loop plus a broad faradaic redox peak. Even at the scan rate of 50 mV s
1, the CV curve of the ASC device still demonstrates its original shape as that at low scan rate. The current of the CV curve also keeps increasing with the increases of the scan rate, implying a quick I-V response of the ASC device. GCD curves of the ASC device at different current densities are shown in Fig. 7c. The discharge branch of GCD curves demonstrates an obvious inflexion point at ~0.55 V which agrees well with the reduction peak in their CV curves. Based on the total mass of electroactive materials on both electrodes, the specific capacitance of the ASC device is calculated to be 154, 125, 109, 85, 73 and 61 F g
1 at the current densities of 1, 3, 5, 10, 15 and 20 A g
1, respectively. Based on Eqns. S2 and S3, the energy density and power density are also calculated. As shown in Fig. 7d, the assembled ASC device can deliver a high energy density of 58.2 Wh kg
1 at a power density of 831.4 W kg
1, which value is much higher than other aqueous pseudocapacitive devices with Ni-Mn based oxides as positive electrode and carbonaceous materials as negative electrode, such as NiWO4//AC [33], NiMnO3/rGO//NiMnO3/rGO [22], NiCo2O4/rGO//AC [34], NiCo2O4-MnO2/GF//CNT/GF [35], NiCo2S4@NiO//AC [36], and Ni6MnO8//graphene [32]. Even at a high power density of 16.6 kW kg
1, the ASC device can still deliver a high energy density of 23.1 Wh kg
1, which value is also comparable to the highest energy density that achieved in some of NiObased and MnO2-based ASC devices as well as activated carbonbased symmetric devices [10,15,37e39]. Long-term cycling stability is another important parameter for practical application of supercapacitors device. Fig. 8 shows the cycling performance of ASC device at a high current density of 5 A g
1. It can be observed that the ASC device undergoes a fast performance fading at initial few hundreds cycles, in accordance with that of Ni6MnO8@CNT hybrid at initial stage. After 1000 cycles,

Fig. 7. Electrochemical performance of the Ni6MnO8@CNTs//APDC ASC device: (a) CV curves within different potential windows. (b) CV curves at different scan rates within potential window of 0e1.65 V. (c) GCD curves at various current densities and (d) Ragone plot of the ASC device.

J. Liu et al. / Electrochimica Acta 320 (2019) 134627

[3]

[4]

[5] [6]

[7]

[8]

[9]

[10] Fig. 8. Cycling stability of Ni6MnO8@CNT//APDC ASC device at a current density of 5 A g
1. [11]

the performance of the ASC device keeps stable and presents 70.6% of the initial capacitance at 5000th cycles. The high energy density and good cycling stability of the ASC device suggest the promising applications of Ni6MnO8@CNT hybrid in energy storage and conversion system.

[12]

[13]

4. Conclusion

[14]

We have successfully prepared the Ni6MnO8 and core-shell structured Ni6MnO8@CNT hybrid by a simple hydrothermal approach followed by calcination treatment process. The core-shell structured Ni6MnO8@CNT hybrid demonstrates enhanced electrochemical performance over that of Ni6MnO8, which makes it more suitable for assembling ASC device and exploring its practical applications. The assembled ASC device with Ni6MnO8@CNTs as positive electrode and APDC as negative electrode delivers a high energy density of 58.2 Wh kg
1 at a power density of 831.4 W kg
1, and retains 23.1 Wh kg
1 even at a high power density of 16.6 kW kg
1. The assembled ASC device also demonstrates the good cycling stability with a high capacitance retention of 70.6% after 5000 cycles. More importantly, the Ni6MnO8@CNT hybrid only contains cost-effective Ni and Mn ions and without high-cost ions such as Co and Mo. These prominent advantages as well as the outstanding electrochemical performance of the Ni6MnO8@CNT hybrid make it more competitive with other pseudocapacitive materials in the coming future.

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51862013 and 51704042) and Ganzhou Science & Technology Innovation Talent Program. Appendix A. Supplementary data

[23]

[24]

[25]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134627. [26]

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