Hybrid Reduced Graphene Oxide Nanosheet Supported Mn–Ni–Co

May 18, 2017 - Growth and Characterization of 3D Flower-Like β-NiS on Carbon Cloth: A Dexterous and Flexible Multifunctional Electrode for Supercapat...
0 downloads 0 Views 5MB Size
Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

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

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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.

3 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

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

4 ACS Paragon Plus Environment

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

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

6 ACS Paragon Plus Environment

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

identical to the published works.

30,31

Page 8 of 30

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+.

8 ACS Paragon Plus Environment

37

The high-resolution for

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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.

9 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

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

10 ACS Paragon Plus Environment

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

11 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

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)

12 ACS Paragon Plus Environment

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

13 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

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

14 ACS Paragon Plus Environment

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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-

15 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

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

16 ACS Paragon Plus Environment

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

17 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

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

18 ACS Paragon Plus Environment

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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.

19 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

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).

References

20 ACS Paragon Plus Environment

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(1) Vlad A., Singh N., Galande C., Ajayan P. M. Design Considerations for Unconventional Electrochemical Energy Storage Architectures. Adv. Energy Mater. 2015, 5, 14021151402168. (2) Kou L., Huang T., Zheng B., Han Y., Zhao X., Gopalsamy K., Sun H., Gao C. Coaxial WetSpun Yarn Supercapacitors for High-energy Density and Safe Wearable Electronics. Nature Commun. 2014, 5, 3754-3764. (3) Wu C., Cai J., Zhu Y., Zhang K. Nanoforest of Hierarchical Core/shell CuO@NiCo2O4 Nanowire Heterostructure Arrays on Nickel Foam for High-performance Supercapacitors. RSC Adv. 2016, 6, 63905-63914. (4) Byeon A., Glushenkov A. M., Anasori B., Urbankowski P., Li J., Byles B. W., Blake B., Van Aken K. L., Kota S., Pomerantseva E., Lee J. W. Chen Y., Gogotsi Y. Lithium-ion capacitors with 2D Nb2CTx (MXene)-carbon nanotube electrodes. J. Power Sources 2016, 326, 686-694. (5) Krishnamoorthy K., Pazhamalai P., Veerasubramani G. K., Kim S. J. Mechanically delaminated few layered MoS2 nanosheets based high performance wire type solid-state symmetric supercapacitors. J. Power Sources 2016, 321, 112-119. (6) Jäckel N., Krüner B., Van Aken K. L., Alhabeb M., Anasori B., Kaasik F., Gogotsi Y., Presser V. Electrochemical in situ tracking of volumetric changes in two-dimensional metal carbides (MXenes) in ionic liquids. ACS Appl. Mater. Interfaces 2016, 8, 32089-32093. (7) Du H., Yang H., Huang C., He J., Liu H., Li Y. Graphdiyne applied for lithium-ion capacitors displaying high power and energy densities. Nano Energy 2016, 22, 615-622.

21 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

(8) Tang Q., Chen M., Wang L., Wang G. A Novel Asymmetric Supercapacitors Based on Binder-free Carbon Fiber Paper@Nickel Cobaltite Nanowires and Graphene Foam Electrodes. J. Power Sources 2015, 273, 654-662. (9) Kong D., Ren W., Cheng C., Wang Y., Huang Z., Yang H. Y. Three-dimensional NiCo2O4@polypyrrole Coaxial Nanowire Arrays on Carbon Textiles for High-performance Flexible Asymmetric Solid-state Supercapacitor. ACS Appl. Mater. Interfaces 2015, 7, 21334-21346. (10) Zhu B., Tang S., Vongehr S., Xie H., Meng X. Hierarchically MnO2-Nanosheet Covered Submicrometer-FeCo2O4-Tube Forest as Binder-Free Electrodes for High Energy Density All-Solid-State Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 4762-4770. (11) Wu C., Cai J., Zhang Q., Zhou X., Zhu Y., Shen P. K., Zhang K. Hierarchical Mesoporous Zinc-Nickel-Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High-Performance Electrodes for Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 26512-26521. (12) Yin C., Yang C., Jiang M., Deng C., Yang L., Li J., Qian D. A Novel and Facile One-Pot Solvothermal Synthesis of PEDOT-PSS/Ni-Mn-Co-O Hybrid as an Advanced Supercapacitor Electrode Material. ACS Appl. Mater. Interfaces 2016, 8, 2741-2752. (13) Yin B. S., Wang Z. B., Zhang S. W., Chang L., Ren Q. Q., Ke K. In Situ Growth of FreeStanding All Metal Oxide Asymmetric Supercapacitor. ACS Appl. Mater. Interfaces 2016, 8, 26019-26029.

22 ACS Paragon Plus Environment

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(14) Xiong G., He P., Liu L., Chen T., Fisher T. S. Plasma-grown Graphene Petals Templating Ni-Co-Mn Hydroxide Nanoneedles for High-rate and Long-cycle-life Pseudocapacitive Electrodes. J. Mater. Chem. A 2015, 3, 22940-22948. (15) Yin C., Yang C., Jiang M., Deng C., Yang L., Li J., Qian D. A Novel and Facile One-Pot Solvothermal Synthesis of PEDOT-PSS/Ni-Mn-Co-O Hybrid as an Advanced Supercapacitor Electrode Material. ACS Appl. Mater. Interfaces 2016, 8, 2741-2752. (16) Yuan C., Wu H. B., Xie Y., Lou X.W.D. Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications. Angew. Chem. Int. Ed. 2014, 53, 1488-1504. (17) Zhang Q., Zhao B., Wang J., Chong Q., Sun H., Zhang K., Liu M. High-performance Hybrid Supercapacitors Based on Self-supported 3D Ultrathin Porous Quaternary Zn-Ni-AlCo Oxide Nanosheets. Nano Energy 2016, 28, 475-485. (18) Peng Z., Lin J., Ye R., Samuel E. L. G., Tour J. M. Flexible and Stackable Laser-induced Graphene Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 3414-3419. (19) Kumar N. A., Baek J. B. Doped Graphene Supercapacitors. Nanotechnology 2015, 26, 492001-492017. (20) Wei B., Wang L., Miao Q., Yuan Y., Dong P., Vajtai R., Fei W. Fabrication of Manganese oxide/three-dimensional Reduced Graphene Oxide Composites as the Supercapacitors by a Reverse Microemulsion Method. Carbon 2015, 85, 249-260. (21) Zhu J., Wang T., Fan F., Mei L., Lu B. Atomic-Scale Control of Silicon Expansion Space as Ultrastable Battery Anodes. ACS Nano 2016, 10, 8243-8251.

23 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

(22) Yu X., Wang B., Gong D., Xu Z., Lu B. Graphene Nanoribbons on Highly Porous 3D Graphene for High-Capacity and Ultrastable Al-Ion Batteries. Adv. Mater. 2016, DOI: 10.1002/adma.201604118. (23) Zheng Y., Zhou T., Zhang C., Mao J., Liu H., Guo Z. Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 3408-3413. (24) Kumar R., Singh R. K., Dubey P. K., Singh D. P., Yadav R. M. Self-Assembled Hierarchical Formation of Conjugated 3D Cobalt Oxide Nanobead-CNT-graphene Nanostructure Using Microwaves for High-performance Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2015, 7, 15042-15051. (25) Zhang C., Kuila T., Kim N. H., Lee S. H., Lee J. H. Facile Preparation of Flower-like NiCo2O4/three dimensional Graphene Foam Hybrid for High Performance Supercapacitor Electrodes. Carbon 2015, 89, 328-339. (26) Qu L., Zhao Y., Khan A. M., Han C., Hercule K. M., Yan M., Liu X., Chen W., Wang D., Cai Z., Xu W., Zhao K., Zheng X., Mai L. Interwoven Three-Dimensional Architecture of Cobalt Oxide Nanobrush-Graphene@NixCo2x(OH)6x for High-Performance Supercapacitors. Nano Letters 2015, 15, 2037-2044. (27) Lu H. B., Wang S. M., Zhao L., Li J. C., Dong B. H., Xu Z. X. Hierarchical ZnO Microarchitectures Assembled by Ultrathin Nanosheets: Hydrothermal Synthesis and Enhanced Photocatalytic Activity. J. Mater. Chem. 2011, 21, 4228-4234.

24 ACS Paragon Plus Environment

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(28) Gao G., Lu S., Xiang Y., Dong B., Yan W., Ding S. Free-standing Ultrathin CoMn2O4 Nanosheets Anchored on Reduced Graphene Oxide for High-performance Supercapacitors. Dalton Trans. 2015, 44, 18737-18742. (29) Zhang Q., Chen H., Han X., Cai J., Yong Y., Liu M., Zhang K. Graphene-Encapsulated Nanosheet-Assembled Zinc-Nickel-Cobalt Oxide Microspheres for Enhanced Lithium Storage. ChemSusChem 2016, 9, 186-196. (30) Li D., B. Muller M., Gilje S., Kaner R. B., Wallace G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101-105. (31) Weng B., Yang M. Q., Zhang N., Xu Y. J. Toward the Enhanced Photoactivity and Photostability of ZnO Nanospheres via Intimate Surface Coating with Reduced Graphene Oxide. J. Mater. Chem. A 2014, 2, 9380-9389. (32) Wang D. W., Li Y. Q., Wang Q. H., Wang T. M. Facile Synthesis of Porous Mn3O4 Nanocrystal-graphene Nanocomposites for Electrochemical Supercapacitors. Eur. J. Inorg. Chem. 2012, 4, 628-635. (33) Wang L., Li Y. H., Han Z. D., Chen L., Qian B., Jiang X. F., Pinto J., Yang G. Composite Structure and Properties of Mn3O4/graphene Oxide and Mn3O4/graphene. J. Mater. Chem. A 2013, 1, 8385-8397. (34) Chigane M., Ishikawa M. Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc. 2000, 147, 2246-2251.

25 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

(35) Qian L., Gu L., Yang L., Yuan H., Xiao D. Direct Growth of NiCo2O4 Nanostructures on Conductive Substrates with Enhanced Electrocatalytic Activity and Stability for Methanol Oxidation. Nanoscale 2013, 5, 7388-7396. (36) Song X., Ru Q., Mo Y., Guo L., Hu S., An B. A Novel Porous Coral-like Zn0.5Ni0.5Co2O4 as an Anode Material for Lithium Ion Batteries with Excellent Rate Performance. J. Power Sources 2014, 269, 795-803. (37) Kim J. G., Pugmire D. L., Battaglia D., Langell M. Analysis of the NiCo2O4 Spinel Surface with Auger and X-ray Photoelectron Spectroscopy. A. Appl. Surf. Sci. 2000, 165, 70-84. (38) Zhou G., Wang D. W., Yin L. C., Li N., Li F., Cheng H. Oxygen Bridges Between NiO Nanosheets and Graphene for Improvement of Lithium Storage. ACS Nano 2012, 6, 32143223. (39) Wu Z., Ren W., Wen L., Gao L., Zhao J., Chen Z., Zhou G., Li F., Cheng H. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS Nano 2010, 4, 3187-3194. (40) Hu J., Ramadan A., Luo F., Qi B., Deng X., Chen J. One-Step Molybdate Ion Assisted Electrochemical Synthesis of R-MoO3-Decorated Graphene Sheets and Its Potential Applications. J. Mater. Chem. 2011, 21, 15009-15014. (41) Bou M., Martin J. M., Mogne T. L. Chemistry of the Interface Between Aluminium and Polyethyleneterephthalate by XPS. Applied Surf. Sci. 1991, 47, 149-161.

26 ACS Paragon Plus Environment

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(42) Yang X., Fan K., Zhu Y., Shen J., Jiang X., Zhao P., Li C. Tailored Graphene-encapsulated Mesoporous Co3O4 Composite Microspheres for High-performance Lithium Ion Batteries. J. Mater. Chem. 2012, 22, 17278-17283. (43) Zhang X., Wang X., Su J., Wang X., Jiang L., Wu H., Wu C. The Effects of Surfactant Template Concentration on the Supercapacitive Behaviors of Hierarchically Porous Carbons. J. Power Sources 2012, 199, 402-408. (44) Li L., Zhang Y. Q., Liu X. Y., Shi S. J., Zhao X. Y., Zhang H., Ge X., Cai G. F., Gu C. D., Wang X. L., Tu J. P. One-dimension MnCo2O4 Nanowire Arrays for Electrochemical Energy Storage. Electrochim. Acta 2014, 116, 467-474. (45) Liu X. Y., Shi S. J., Xiong Q. Q., Li L., Zhang Y. J., Tang H., Gu C. D., Wang X. L., Tu J. P. O4@NiCo2O4 Core/shell Nanoflake Arrays as High-performance Supercapacitor Materials. ACS Appl. Mater. Interfaces 2013, 5, 8790-8795. (46) Wu C., Wang X., Ju B., Zhang X. Y., Jiang L. L., Wu H. Supercapacitive Behaviors of Activated Mesocarbon Microbeads Coated with Polyaniline. Int. J. Hydrogen Energy 2012, 37, 14365-14372. (47) Qiu K., Lu Y., Zhang D., Cheng J., Yan H., Xu J., Liu X., Kim J. K., Luo Y. Mesoporous, Hierarchical Core/shell Structured ZnCo2O4/MnO2 Nanocone Forests for High-performance Supercapacitors. Nano Energy 2015, 11, 687-696. (48) Qu D., Studies of the Activated Carbons Used in Double-layer Supercapacitors. J. Power Sources 2002, 109, 403-411.

27 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

(49) Zhang X., Wang X., Jiang L., Wu H., Wu C., Su J. Effect of Aqueous Electrolytes on the Electrochemical Behaviors of Supercapacitors Based on Hierarchically Porous Carbons. J. Power Sources 2012, 216, 290-296. (50) Burke A. Ultracapacitors: Why, How, and Where Is the Technology. J. Power Sources 2000, 91, 37-50. (51) Wang L., Li Y., Han Z., Chen L., Qian B., Jiang X., Pinto J., Yang G. Composite Structure and Properties of Mn3O4/Graphene Oxide and Mn3O4/Graphene. J. Mater. Chem. A 2013, 1, 8385-8397. (52) Xu K., Li W., Liu Q., Li B., Liu X., An L., Chen Z., Zou R., Hu J. Hierarchical Mesoporous NiCo2O4@MnO2 Core-shell Nanowire Arrays on Nickel Foam for Aqueous Asymmetric Supercapacitors. J. Mater. Chem. A 2014, 2, 4795-4802 (53) Lu X. F., Wu D. J., Li R. Z., Li Q., Ye S. H., Tong Y. X., Li G. R. Hierarchical NiCo2O4 Nanosheets@hollow Microrod Arrays for High-performance Asymmetric Supercapacitors. J. Mater. Chem. A, 2014, 2, 4706-4713. (54) Wang Y., Lei Y., Li J., Gu L., Yuan H., Xiao D. Synthesis of 3D-nanonet Hollow Structured Co3O4 for High Capacity Supercapacitor. ACS Appl. Mater. Interfaces 2014, 6, 6739-6747. (55) Lu Q., Chen Y., Li W., Chen J. G., Xiao J. Q., Jiao F. Ordered Mesoporous Nickel Cobaltite Spinel with Ultra-high Supercapacitance. J. Mater. Chem. A 2013, 1, 2331-2336. (56) Tang C., Tang Z., Gong H. Hierarchically porous Ni-Co oxide for high reversibility asymmetric full-cell supercapacitors. J. Electrochem. Soc. 2012, 159, A651-A656. 28 ACS Paragon Plus Environment

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

(57) Huang J., Xu P., Cao D., Zhou X., Yang S. Li Y., Wang G. Asymmetric Supercapacitors Based on β-Ni(OH)2 Nanosheets and Activated Carbon with High Energy Density. J. Power Sources 2014, 246, 371-376. (58) Yuan C., Zhang L., Hou L., Pang G., Oh W. C. One-step Hydrothermal Fabrication of Strongly Coupled Co3O4 Nanosheets-reduced Graphene Oxide for Electrochemical Capacitors. Rsc Adv. 2014, 4, 14408-14413. (59) Liu Y., He D., Wu H. Duan J., Zhang Y. Hydrothermal Self-assembly of Manganese dioxide/manganese

Carbonate/reduced

Graphene

Oxide

Aerogel

for

Asymmetric

Supercapacitors. Electrochimi. Acta 2015, 164, 154-162. (60) Jing M., Hou H., Yang Y., Zhu Y., Wu Z. Ji X. Electrochemically Alternating Voltage Tuned Co2MnO4/Co Hydroxide Chloride for an Asymmetric Supercapacitor. Electrochimi. Acta 2015, 165, 198-205. (61) Kong L. B., Lu C., Liu M. C., Luo Y.C., Kang L., Li, X., Walsh F. C. The Specific Capacitance of Sol-gel Synthesised Spinel MnCo2O4 in an Alkaline Electrolyte. Electrochim. Acta, 2014, 115, 22-27. (62) Zhang X., Zhang J., Wang K. Codoping-Induced, Rhombus-Shaped Co3O4 Nanosheets as an Active Electrode Material for Oxygen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 21745-21750.

29 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 30 of 30