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Hybrid Reduced Graphene Oxide Nanosheet Supported Mn-NiCo Ternary Oxides for Aqueous Asymmetric Supercapacitors Chun Wu, Junjie Cai, Ying Zhu, and Kaili Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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ACS Applied Materials & Interfaces

Hybrid Reduced Graphene Oxide Nanosheet Supported Mn-Ni-Co Ternary Oxides for Aqueous Asymmetric Supercapacitors Chun Wu, Junjie Cai, Ying Zhu, Kaili Zhang* Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong * Corresponding author, E-mail address: [email protected] Abstract Hybrid reduced graphene oxide (RGO) nanosheet supported Mn-Ni-Co ternary oxides (MNCO) are prepared through a facile co-precipitation reaction with a subsequent calcination process as electrodes for supercapacitors. Electrochemical measurements prove that RGO can significantly improve the supercapacitive behaviors, compared with the pure MNCO electrode. A high specific capacity of 646.1 C g-1 at 1 A g-1 can be achieved and about 89.6 % of the capacity can be remained at 30 A g-1 relative to that of the low-current capacity, indicating attractive rate capability of the RGO-MNCO electrode. Moreover, an asymmetric supercapacitor (ASC) device is fabricated with nitrogen-enriched RGO as the negative electrode and the synthesized RGO-MNCO as the positive electrode. Electrochemical performances investigated at different potential range reveal that the ASC device presents excellent capacitive behavior and reversibility. A maximum energy density of 35.6 Wh kg-1 at power density of 699.9 W kg-1 can be delivered. Furthermore, stable cycle capability with 100% coulombic efficiency and 77.2% the capacitance retention is also achieved after 10000 cycles. The achieved outstanding electrochemical properties indicate that the obtained RGO-MNCO electrode materials are fairly ideal for progressive supercapacitors. Keywords: Mn-Ni-Co ternary oxides; Reduced graphene oxide nanosheet; Asymmetric supercapacitor; Electrode materials; Energy storage 1 ACS Paragon Plus Environment


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1. Introduction Owing to the wonderful cycling performance, high power density and outstanding reversible behaviors, supercapacitors have been identified as very promising electrochemical devices for portable electronics including electric vehicles, laptops and mobile phones.

1-3

Recently, to hunt for the high-performance electrode materials used for supercapacitors, tremendous effort has been made by researchers.

4-7

Nowadays, the nanostructured mixed

transition-metal oxides (MTMOs) exhibiting exceptionally high specific capacitance are emerging as promising supercapacitive electrodes and have been extensively investigated.

8-10

Notably, researches show that nanostructured MTMOs could provide richer redox reactions and exhibit better electronic conductivity than the single-component transition-metal oxides with nanostructure, and are much more beneficial for a significant improvement in supercapacitive behaviors utilized as electrode for supercapacitors. 11-15 Due to similar atom radii of the elements, Mn-Ni-Co ternary oxide (MNCO) without crystal structure change has been identified as one kind of the most attractive supercapacitive electrode materials. The improved supercapacitive property of the MNCO material can be ascribed to the synergetic effects originated from the complex chemical compositions and the different mixed valence states in the multi-metal centers. 16

In addition, it has been demonstrated that the mixed oxide electrodes of MNCO displays much

more excellent electrochemical behaviors than that of the binary-component Ni-Co, which can be attributed to the synergistic effects of the multi-metal components and the enhanced oxidation states in the electrodes.

17

Compared to Co3O4, MNCOs exhibit better safety and lower cost.

More significantly, these three metal elements can provide synergistic effects during the redox reaction process compared to the single component metal oxides. However, in consideration of the low intrinsic electronic conductivity and slow ion diffusion rates of the nanostructured

2 ACS Paragon Plus Environment

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MTMOs, most of them often show low specific capacitance and/or very poor cycling performance, which hinder their further practical applications. To address aforementioned problems, one effective way is to combine the nanostructured MTMOs with guest conducting materials, which can act as substrate to disperse, support or incorporate the MTMOs. Thanks to the excellent mechanical flexibility, large surface area, high thermal and chemical stability, and superior electrical conductivity, graphene has been considered as one of the most promising matrix for combining with nanostructured TMO to enhance the behaviors of electrode materials for supercapacitors.

18-23

As a result, extensive

investigations have been performed to study various structural models of graphene/TMO composites and their electrochemical behaviors when used as electrode materials for supercapacitors.

24-26

Nevertheless, to date, there is little investigation about the Mn-Ni-Co

ternary oxide nanoparticles encapsulated by reduced graphene oxide nanosheets. Here, we report the successful preparation of reduced graphene oxide (RGO) nanosheet supported Mn-Ni-Co ternary oxides (RGO-MNCO) by combining a facile co-precipitation reaction with a subsequent calcination process. In this hybrid nanostructure, the RGO nanosheets not only act as a 2D current substrate for the MNCOs, but also significantly increase the surface area and dramatically accelerate ion diffusion because of their low energy barriers. This hybrid nanostructured RGO-MNCO electrode exhibits attractive rate capability, high specific capacity and wonderful cycling property when applied as electrode materials for supercapacitors. To achieve high power density, an ASC device is fabricated with the RGO-MNCO positive electrode and nitrogen-enriched RGO negative electrode. Electrochemical performances investigated at different potential ranges reveal the excellent capacitive behavior and reversibility.



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The excellent electrochemical performances demonstrate that RGO-MNCO electrodes are fairly ideal for progressive supercapacitors

2. Results and Discussion 2.1. Structural Analysis The synthesis procedure about hybrid nanostructured RGO-MNCO electrode material is presented in Fig. 1. Firstly, the as-prepared graphene oxide (GO) solution is added into a mixture including Co2+, Ni2+, Mn2+, trisodium citrate dihydrate (TSC) and hexamethylenetetramine (HMT). In the mixture solution, the metal ions of Co2+, Ni2+, Mn2+ and the functional groups ( e.g., -OH, -COOH) are closely integrated with each other on the surface of the GO materials. When the reaction temperature arrives at 90 oC, the HMT would gradually decompose and result in the increase of the pH value, which would accelerate generation of the Mn-Ni-Co precursor with preferred nucleation on the as-prepared GO sheets surface. Meanwhile, abundant hydroxyl and carboxyl groups generated from the hydrolysis of TSC would further promote growth of the Mn-Ni-Co precursor with nanoparticle structure on the GO surface (GO-MNCO precursor).

27

Secondly, the hybrid nanostructure GO-MNCO precursor can be successfully transformed into RGO-MNCO with the calcination process at 450 °C. 28


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Figure 1 Scheme of the formation process of RGO-MNCO electrode materials

FESEM and TEM are utilized to investigate the morphology of the hybrid nanostructured RGO-MNCO electrode materials. As can be obviously observed in SEM images in Fig. 2(a-b), numerous nanoparticles with the diameter between 15 and 130 nm have been homogenously dispersed on highly flexible RGO, leading to the formation of the continuous nanosheets structure. These continuous flexible RGO in the hybrid nanostructure not only offers a 3D conductive support to the numerous interconnected nanoparticles, but also can effectively buffer volume changes during electrochemical measurements, which would lead to an improvement of supercapacitive performance. Meanwhile, the SEM figure of the pure MNCO sample is presented in Fig. S1(a), the porous microspheres with the diameter between 1 µm to 2 µm are severely agglomerated. More interestingly, through the facile, versatile and efficient method, some other RGO supported single component metal oxides (such as RGO-NiO, RGO-MnO2, RGO-Co3O4) can be successfully synthesized, which demonstrates the high efficiency and broad applications of this preparation method. The detailed microstructures and morphology of RGO5 ACS Paragon Plus Environment


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MNCO electrode materials are further characterized by TEM. The low-magnification image in Fig. 2(c) shows that the MNCO nanoparticles are anchored closely to the RGO sheets. From Fig. 2(d), the HRTEM image of the RGO-MNCO electrode materials presents the visible lattice fringes, which is measured to be 0.286 nm corresponding to the (220) facet of spinel MNCO.

(a)

(b)

500 nm

1 µm

(d)

(c)

d=0.286 nm (220)

200 nm

10 nm

Figure 2 (a and b) SEM images of RGO-MNCO electrode material, (c) and (d) TEM image and HRTEM image of the RGO-MNCO electrode material

Elemental mapping analysis in Fig. 3 evidently manifests the homogeneous distribution and coexistence of Ni, Co, Mn, O, and C in the RGO-MNCO electrode material. Such observations


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unambiguously confirm that the MNCO nanoparticles are successfully anchored closely to the RGO sheets to form the flexible RGO-MNCO electrode material.

Mn

Co

Ni

O

C

Figure 3 TEM elemental mapping of RGO-MNCO electrode material

The internal structures of the RGO-MNCO electrode materials are studied by XRD. The characteristic diffraction peaks shown in Fig. S2 reveal that the as-prepared MNCO and RGOMNCO electrode materials possess a similar spinel structure of space group Fd3m. All of the peaks can be indexed to Co3O4 (JCPDS 74-2120), which indicates that Mn and Ni ions partially replace the Co ions and the crystal structure of the MNCO materials would not be affected due to existence of the RGO materials.

29

Besides, no typical diffraction peaks are detected at 2θ=22-

27o belonging to RGO in the hybrid nanostructured RGO-MNCO materials. This is because of the relatively low diffraction intensity and ultrathin layer of the as-prepared RGO, which is 7 ACS Paragon Plus Environment


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identical to the published works.

30,31

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The TGA curve presented in Fig. S2(b) shows that the

weight loss about 5.1% below 200 oC can be ascribed to the evaporation of absorbed water in the electrode materials, while the weight fraction of RGO in the RGO-MNCO electrode materials is about 8.1%. The composition and surface electronic state of the RGO-MNCO electrode material is analyzed via XPS. Figure 4(a) presents a typical XPS spectrum survey of RGO-MNCO electrode material, revealing the existence characteristic peaks of nickel, cobalt manganese, oxygen and carbon, without other elements. The high resolution spectrum of Mn 2p in Fig. 4(b) exhibits two peaks, including the BE value at 641.6 eV for Mn 2p3/2 and the BE value at 653.3 eV for Mn 2p1/2, the energy separation of 11.7 eV between these two peaks is in good agreement with previously reported results corresponding to the presence of both Mn2+ and Mn3+.

32-33

Meanwhile, to distinguish the oxidation states of the Mn element, the high resolution spectrum of Mn 3s has been used and presented in Fig. S3. It can be noted that there are two multiplet split components existing with energy separation of 5.3 eV, which confirms the existence of both Mn2+ and Mn3+.

34

Figure 4(c) presents the spectrum of Ni 2p, in which the binding energy

around 872.8 and 855.4 eV can be ascribed to Ni 2p1/2 and Ni 2p3/2 levels, respectively. Two weak peaks at 879.6 and 861.4 eV are for Ni 2p1/2 satellites and Ni 2p3/2 satellites, respectively. In addition, the fitting peaks at 873.7 and 861.9 eV are indexed to Ni of Ni 2p at 872.5 and 854.9 eV are indexed to Ni

2+ 35

.

3+

, while the fitting peaks

As can be obviously noted in the high-

resolution of Co 2p spectrum in Fig. 4(d), two major peaks at 796.1 eV for Co 2p1/2 and 780.5 eV for Co 2p3/2 can be obtained, demonstrating the existence of Co3+ oxidation state.

36

Meanwhile, as indicated by the arrow in Fig. 4(d), a very intense, characteristic satellite at ~786.0 eV can be evidently noticed, indicating the presence of Co2+.


37

The high-resolution for

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the O 1s region (Fig. 4(e)) can be ascribed to different oxygen groups. The fitting peak about 529.5 eV is attributed to the metal-oxygen bonds. Another one located around 530.2 eV can be ascribed to the possible formation of a Ni-O-C bond, which may be beneficial for the improvement of the electrochemical behaviors. 38 While the peak at 531.1 eV can be attributed to the residual oxygen-containing groups bonded to carbon atoms in the RGO (such as -COOH and -OH). 39 As for the high-resolution spectrum of C 1s (Fig. 4(f)), it can be divided into three major functional groups. The binding energy at 284.3 eV can be ascribed to the nonoxygenated C-C bond, the binding energy at 285.3 eV is related to the carbon atoms in hydroxyl groups (COH/C-ONi), bond.

41

38, 40

and the binding energy at 288.3 eV can be attributed to carbon in the C=O

Meanwhile, the atom ratio of Co: Ni: Mn is 11.6: 9.3: 7.4, indicating that Ni and Mn

atoms partially replace the Co atoms. The measurement also shows that the oxygen content of the RGO (MNCO has been dissolved by HCl solution) in the RGO-MNCO composites obviously decreases when the value of 35.1 % is changed to 6.4 %. The results indicate that the GO-MNCO precursor has been successfully transformed into RGO-MNCO.



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s

Figure 4 (a) XPS survey and high-resolution spectra of Mn2p (b), Ni2p (c), Co2p (d), O1s (e) and C1s (f) of the RGO-MNCO electrode material


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The porous features about the as-obtained materials are characterized via N2 adsorption tests, and the curves of the measurement are presented in Fig. 5(a and b). Notably, the nitrogen adsorption-desorption isotherm of these three electrode materials are close to IV type with capillary condensation steps and distinct hysteresis loops according to the IUPAC nomenclature, 42,43

demonstrating that these three electrode materials possess a typical mesoporous structure.

The textural properties of the three samples are exhibited in Table 1. The average pore diameter, as indicated in Table 1, is 12.1 nm for pure MNCO and 6.3 nm for the hybrid nanostructured RGO-MNCO, which further confirms the existence of mesopores. More interestingly, the hybrid nanostructured RGO-MNCO material possesses a BET specific surface area of 87.7 m2 g-1, which is significantly higher than that of the pure MNCO (65.2 m2 g-1). The high specific surface area would lead to the increase of the reaction area and results in better penetration of the electrolyte during electrochemical measurements, which would lead to outstanding supercapacitive performance.

Figure 5 (a) N2 absorption-desorption isotherm and (b) the pore size distributions of RGO, MNCO and RGO-MNCO samples



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Table 1 Textural properties of the as-prepared electrode materials SBET

V

d

(m2 g-1)

(cm3 g-1)

(nm)

RGO

445.4

0.52

3.1

MNCO

65.2

0.14

12.1

RGO-MNCO

87.7

0.20

6.3

2.2. Electrochemical properties of the electrode materials Supercapacitive performance of the RGO-MNCO electrode is studied by cyclic voltammograms (CV). Comparisons of CV measurement at 2 mV s-1 between MNCO and RGOMNCO electrodes are presented in Fig. 6(a). Obviously, the area surrounded by the CV curve of RGO-MNCO electrodes is considerably larger than those of the MNCO electrodes and also exhibit the higher redox current, demonstrating much more advanced supercapacitive performance of the RGO-MNCO electrode. The detailed electrochemical performances of the MNCO electrode are shown in Figure S4. The CV curves of the RGO-MNCO electrode under various scan rates are shown in Fig. 6(b), which evidently displays the typical faradaic characteristics of the RGO-MNCO electrode. A pair of redox peaks can be clearly noted, demonstrating that the electrochemical behaviors of RGO-MNCO electrode result from their faradaic capacitance. The energy storage mechanism of the RGO-MNCO electrode can be assigned to the following equations: 44,45 MNCO + H 2O + OH − ↔ CoOOH + MnOOH + NiOOH + e −

(1)

CoOOH + MnOOH + OH − ↔ CoO2 + MnO2 + H 2O + e −

(2)


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In addition, the positions of the redox peaks observed from the CV curves progressively shift with the scan rate increase, which may be the existence of polarization with the increase of scan rate.

46

The specific capacities of the RGO-MNCO electrode are calculated to be about

630.1, 607.5, 586.1, 559, 503.1 and 432.4 C g-1 at 1, 2, 5, 10, 20 and 30 mV s-1, respectively. Figure 6(c) displays charge/discharge measurements of the RGO-MNCO sample. All of the test curves present symmetric shape during the charge/discharge process, demonstrating the high charge reversible redox reactions and storage efficiency of the RGO-MNCO electrode. The specific capacities of RGO-MNCO electrode are calculated to be about 646.1, 615.9, 614.5, 579 and 573 C g-1 at 1, 3, 5, 10, and 30 A g-1, respectively. The specific capacities obtained from various current densities of MNCO and RGO-MNCO electrode are displayed in Fig. 6(d). Specific capacities of the RGO-MNCO electrode are higher than those of the MNCO electrode at the same current density, and the capacity retention remains 89.6% at 30 A g-1, which indicates the outstanding rate performance of this as-prepared RGO-MNCO materials. The improved supercapacitive behaviors of the RGO-MNCO electrode can also be ascribed to the synergistic effect of the multi-component metal oxide and RGO. RGO in this hybrid nanostructure acts as the conductive support with large surface area and highly flexible property, fully exerting their supercapacitive performance of the multi-component metal oxide. EIS measurement is performed in order to exhaustively study the supercapacitive property of RGO-MNCO electrode based on the equivalent circuit in the inset of Fig. 6(e). Where W, Rs and Rct are Warburg impedance, internal and charge-transfer resistances, respectively. Cdl stands for the double layer capacitance. As recorded, the intersection of the curve in the real axis stands for Rs, including intrinsic resistance of active materials, ionic resistance of the KOH solution, and contact resistance between current collector and active electrode material. It can be obviously



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noted from the inset that the Rs is about 1.05 Ω, indicating its wonderful electronic conductivity of the nanostructured RGO-MNCO materials. Meanwhile, the typical semicircle of the measured curve with the diameter representing Rct can be ascribed to Faradic reactions during the electrochemical measurement.

47

The Rs and Rct values of the as-prepared RGO, MNCO and

RGO-MNCO electrodes are listed in Table S1. It can be found that the Rct and Rs values of the RGO-MNCO electrode is about 0.11 and 1.05 Ω. Such low resistances values guarantee the rapid redox reaction procedure under high rates for efficient energy storage systems. Furthermore, because of the existence of Warburg impedance, the curve presents a straight line during the low frequency region, which results from the ion diffusion to the interface of the electrode in KOH solution. 48-50 The rate capability is essential for practical viability of the supercapacitor in addition to low resistance value and high specific capacity. Thus, the hybrid nanostructured RGO-MNCO electrode is further subject to extensive charge/discharge cycle measurement under the progressively increasing current densities (1-50 A g-1) and recorded in Figure 6(f). An increasing high specific capacity up to 673.2 C g-1 after 100 cycles at 1 A g-1 is obtained by the hybrid nanostructured RGO-MNCO electrode. Subsequent reversible specific capacity of the hybrid nanostructured RGO-MNCO electrode after 100 cycles are 608, 543.5 and 421 C g-1 at rates of 5, 10, and 50 A g-1, respectively. Evidently, when the high current density is switched abruptly to the low current density of 1 A g-1, the original high specific capacity of the RGO-MNCO electrode can be reserved. Furthermore, the cycling behavior of the as-prepared electrode at 5 A g-1 shows 90% capacity retention after 10000 cycles (presented in Fig. S5), exhibiting the attractive electrochemical stability of the RGO-MNCO electrode. The above wonderful results further demonstrate that the synergistic effect between MNCO nanoparticles and RGO


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nanosheets in this hybrid nanostructure is apparent and strong with excellent cycle behavior and high rate capability. Attractive cycling stability, along with the superior rate capability, renders the hybrid nanostructured RGO-MNCO promising electrode materials for supercapacitors.

1 mV s-1 30 mV s-1

(f)

Figure 6 (a) CV curves of the MNCO and RGO-MNCO electrodes at 2 mV s-1 and (b) CV curves of RGO-MNCO electrode at different scan rates, (c) charge/discharge curves of the RGO-



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MNCO electrode at different current densities, (d) specific capacity of MNCO and RGO-MNCO electrodes at different current densities and (e) Nyquist plot and equivalent circuit of the RGOMNCO electrode, and (f) the rate capability of the RGO-MNCO electrode under various current densities

2.3. Supercapacitive behaviors of the RGO-MNCO//RGO ASC device In order to further measure the feasibility of the RGO-MNCO as positive electrode at the device level, an ASC device is fabricated by utilizing the RGO-MNCO material as positive electrode and nitrogen-enriched RGO as negative electrode. The photograph, SEM and TEM images, Raman spectra and the electrochemical measurements of the nitrogen-enriched RGO electrode materials are exhibited in Fig. S6, S7, S8 and S9, respectively. Observation from the spectrum suggests that the intensity ratio ID/IG of GO (0.89) is lower than that of RGO (0.97), demonstrating the reduction of GO and the defective structure of RGO due to N doping.51 Thanks to the superior electrical conductivity and excellent supercapacitive behaviors as electrode material, we believe that the nitrogen-enriched RGO could be a good choice as the negative electrode of the ASC device. Meanwhile, the two electrode materials applied for the ASC device are tested by CV via a three-electrode system and the CV plots at 2 mV s-1 are recorded in Fig. 7(a). As observed from Fig. 7(a), the CV plot of the nitrogen-enriched RGO electrode under -1 to 0 V potential window is nearly a rectangular shape, revealing the typical property of the double-layer capacitors. Besides, a pair of redox peaks can be evidently founded under 0 to 0.5 V, which exhibits the characteristic faradaic property of the RGO-MNCO electrode. Figure 7(b) shows the CV plots at 50 mV s-1 under various voltage windows for the RGO-MNCO//RGO ASC device, which shows the stable electrochemical voltage when extended


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to 1.5 V with all the CV plots presenting the similar and rectangular shape, revealing the excellent capacitive behavior and reversibility.

52,53

The CV and charge/discharge curves under

0-1.4 V of the RGO-MNCO//RGO ASC device at different scan rates and current densities are presented in Fig. 7(c and d). The CV plots in Fig. 7(c) present the rectangular-like shape from 1 to 200 mV s-1, exhibiting excellent supercapacitive properties of the ASC device. The charge/discharge plots of RGO-MNCO//RGO ASC device in Fig. 7(d) show that the potentials are nearly proportional to time, demonstrating the rapid I-V response as well as the ideal supercapacitive characteristics. Figure 7(e) shows the Ragone plot of the RGO-MNCO//RGO ASC device calculated from the charge/discharge measurements.

54,55

A maximum energy

density of 35.6 Wh kg-1 and power density of 5548.9 W kg-1 can be delivered for the ASC device, which shows relatively better behavior compared with various ASC devices, such as Ni-Co oxide//AC-ASC, 59

56

β-Ni(OH)2/Ni//AC-ASC,

57

Co3O4 NSs-rGO//AC-ASC,

58

MGA//GA-ASC,

CMO/CHC//AC-ASC. 60 The related values have been listed in Table S2. The electrochemical

reaction kinetics of the RGO-MNCO//RGO ASC device has been investigated via EIS test. Figure S10 represents a Rct value of 31.4 ohm, which can be attributed to charge storage mechanism difference between negative and positive electrodes in this RGO-MNCO//RGO ASC device.

57

Besides, the straight line representing Warburg resistance leans to the Y axis, which

results from ion diffusion/transport to the interface of the electrode in KOH solution. In addition, excellent long-term cycling stability is a vital parameter for ASC devices in their practical applications. Figure 7(f) records the cyclic stability of the RGO-MNCO//RGO ASC device evaluated at 10 A g-1 with the potential window 0 to 1.4 V. As indicated in the curves, capacitance retention about 77.2 % and the Coulombic efficiency of 100 % can be delivered after



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10000 cycles. The above behaviors demonstrate the promising potential of applying RGOMNCO as positive electrodes for high-rate and long-cycle-life ASC devices. (a)

(b)

(d)

(c)

(e)

(f)

Figure 7 (a) CV curves of RGO and RGO-MNCO electrodes at 2 mV s−1, (b and c) CV curves of the RGO-MNCO//RGO ASC device under different potential range and under different scan


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rates, (d) charge/discharge curves of the RGO-MNCO//RGO device under different current densities, (e) Ragone plots of the RGO-MNCO//RGO ASC device and (f) the capacitance retention and coulombic efficiency during the charge/discharge cycle of the RGO-MNCO//RGO ASC device at a current density of 10 A g-1 The significantly improved supercapacitive behaviors of the RGO-MNCO electrode in terms of high specific capacity, good cycle performance and attractive rate capability might be ascribed to our rational design about the special composition and unique architecture. Remarkably, in such a hybrid nanostructured electrode material, the highly flexible RGO nanosheets serve as a conductive support for the later MNCO growth, which can facilitate the penetration of the electrolyte and transportation of the electrons. Meanwhile, this flexible nanostructure is more beneficial for improving the structural integrity during the long cycle measurements. In addition, large specific surface area of this hybrid RGO-MNCO material can provide numerous electroactive sites, which is in favour of the improvement in the supercapacitive properties. More importantly, the synergic effects between different metal elements in the as-prepared electrode materials play a vital role for the excellent supercapacitive properties. Multiple oxidation states can be generated due to the element of Mn in this ternary metal oxide, which would bring about rich faradic reactions for faradic capacity generation. 61 In addition, more polarons could be produced by the incorporation of Ni element, leading to the significant improvement of the electrical conductivity. Besides, it also can offer rich faradic reactions, which can enable multiple oxidation states and high electrochemical activity can be generated (more active redox states).

62

All these will result in the enhanced electrochemical

performances.



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3. Conclusions Hybrid nanostructured reduced graphene oxide (RGO) nanosheet supported Mn-Ni-Co ternary oxides (MNCO) electrode material with enhanced supercapacitive behaviors is successfully fabricated via a facile co-precipitation reaction with a subsequent calcination process. RGO in this hybrid nanostructure acts as a conductive support with excellent mechanical flexibility and high surface area, fully exerting their supercapacitive performance of the multi-component metal oxide. As a result, a high specific capacity of 646.1 C g-1 at 1 A g-1 can be obtained and about 89.6 % capacity retention can be achieved at 30 A g-1 relative to that of the low-current capacity. When assembled as the asymmetric supercapacitor, a maximum energy density of 35.6 Wh kg-1 and power density of 5548.9 W kg-1 can be delivered for the RGO-MNCO//RGO ASC device. Furthermore, stable cycle capability with coulombic efficiency of 100 % and the capacitance retention of 77.2 % is also reached after 10000 cycles. The attractive supercapacitive behaviors indicate that the RGO-MNCO electrodes are fairly ideal for progressive supercapacitors. Moreover, we believe that this hybrid nanostrctured materials would be promising for other energy storage and conversion applications.

Acknowledgements This work was supported by the Hong Kong Research Grants Council (Project numbers CityU 11216815 and CityU 11338016) and Shenzhen Science and Technology Innovation Council (Project number JCYJ20160428154522334).

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