Three-Dimensional Self-Standing and Conductive MnCO3

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Three-Dimensional Self-Standing and Conductive MnCO3@Graphene/ CNT Networks for Flexible Asymmetric Supercapacitors Shuxing Wu, Canbin Liu, Duc Anh Dinh, Kwan San Hui, Kwun Nam Hui, Je Moon Yun, and Kwang Ho Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05935 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Sustainable Chemistry & Engineering

Three-Dimensional Self-Standing and Conductive MnCO3@Graphene/CNT Networks for Flexible Asymmetric Supercapacitors

Shuxing Wu,† Canbin Liu,† Duc Anh Dinh,‡ Kwan San Hui,*, § Kwun Nam Hui,*, ‖ Je Moon Yun, ┴ an d Kwang Ho Kim*, ┴,

†School

┳

of Chemical Engineering and Light Industry, Guangdong University of Technology,

Guangzhou 510006, PR China ‡NTT

Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam

§Engineering,

‖Institute

University of East Anglia, Norwich, NR4 7TJ, United Kingdom

of Applied Physics and Materials Engineering, University of Macau, Avenida da

Universidade, Taipa, Macau, China ┴Global

Frontier R&D Center for Hybrid Interface Materials, Pusan National University, 30 J

angjeon-dong, Geumjung-gu, Busan 609-735, South Korea ┳ School

of Materials Science and Engineering, Pusan National University, San 30 Jangjeon-

dong, Geumjeong-gu, Busan 609-735, Republic of Korea

*Corresponding

author:

E-mail: [email protected] (Kwun Nam Hui) E-mail: [email protected] (Kwan San Hui) E-mail: [email protected] (Kwang Ho Kim)

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ABSTRACT The practical applications of flexible supercapacitor depend strongly on the successful fabrication of advanced electrode materials with high electrochemical performance. Herein, three-dimensional conductive network-based self-standing MnCO3@graphene/CNT hybrid film fabricated through a combination of hydrothermal method and vacuum filtration for flexible solid-state supercapacitors is reported. Ingenious MnCO3@graphene structure is embedded in a CNT network, in which monodispersed MnCO3 nanorod is well confined in graphene nanosheets. This hierarchical structure provides rapid electron/electrolyte ion transport pathways and exhibits excellent structural stability, resulting in rapid kinetics and long lifecycle. The MnCO3@graphene/CNT electrode delivers high specific capacity (467.2 F g−1 at 1 A g−1). Asymmetric supercapacitor (ASC) devices are assembled with the MnCO3@graphene/CNT film as positive electrode and activated carbon/carbon cloth as negative electrode, which exhibits a high energy density of 27 W h kg−1. Remarkably, 93% capacitance retention is obtained for the ASC devices after 6000 cycles.

KEYWORDS: MnCO3, CNT, Graphene, Flexible Asymmetric Supercapacitor


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INTRODUCTION In recent years, flexible supercapacitor has attracted considerable attention in wearable and lightweight electronic devices, such as smartphones, e-readers, and bendable electronic gadgets because of its fast charging/discharging capacity, high power density, long lifespans, and remarkable flexibility.1, 2 Besides, flexible supercapacitors offer distinctive advantage of the stability and safety because of the use of gel-like solid-state electrolyte, which eliminates the safety issue aroused from the leakage of electrolyte and short circuit issues.3 However, flexible supercapacitors suffer from low energy density that hinders their practical applications. Development of electroactive materials possessing high specific capacitance, superior conductivity, good mechanical properties, and high stability is one of the keys for fabricating flexible all-solid-state supercapacitor.4 Electrochemical active MnCO3 is regarded as a potential cathode material for supercapacitors because of its redox-richness and low-manganese valence (+2).5, 6 Meanwhile, MnCO3 is earth abundant, environmentally friendly, and often utilized as a sacrificial template for Mn-based materials synthesis. When applied as a supercapacitor electrode, stable rhodochrosite structure of MnCO3 could stabilize the [MnO6] octahedral structure via [CO3] planes during chargedischarge cycles.5 Zhang et al.7 prepared MnCO3 nanospheres by precipitation method and exhibited a specific capacitance of 129 F g−1. Tang et al.5 reported peanut-like MnCO3 microcrystals, which exhibited a capacitance of 293.7 F g−1 and 71.5% retained specific capacitance after 6000 cycles. Nevertheless, critical drawbacks including low specific capacitance, limited conductivity, and poor cycling performance, hinder the application of MnCO3 as electrode material for supercapacitors.8 Building nanohybrids with carbonaceous materials that serve as structural buffer and electroactive material is an effective approach to overcome the abovementioned 3 ACS Paragon Plus Environment


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drawbacks.9, 10 In particular, graphene is well-regarded a promising carbon matrix owing to its remarkable properties, such as high conductivity11, superb mechanical flexibility12, and large specific surface area13. Thus, many reports regarding the enhancement of electrochemical performance of MnCO3/graphene composites have been reported.14-18 However, the reported MnCO3/graphene composites still suffer from unsatisfactory rate capability and short cycling performance because of the non-intimate contact between MnCO3 and graphene. Herein, the encapsulation of MnCO3 nanorod within graphene nanosheet (MnCO3@graphene) is realized via a cost-effective, one-pot hydrothermal method. Each MnCO3 nanorod is well wrapped by graphene rather than being deposited on the surface of graphene or aggregated within

the

graphene

sheets.

Finally,

a

lightweight,

thin,

and

self-standing

MnCO3@graphene/CNT film is fabricated via the vacuum filtration of mixed MnCO3@graphene and CNT dispersion. The MnCO3@graphene/CNT electrodes deliver a specific capacitance of 467.2 F g−1, which is about 1.5 times higher than MnCO3@graphene electrodes (322.1 F g−1). A flexible asymmetric solid-state supercapacitor (ASC) with an output cell voltage of 1.8 V is fabricated utilizing such MnCO3@graphene/CNT as a positive electrode, activated carbon/carbon cloth (AC/CC) as a negative electrode, and Na2SO4/poly(vinyl alcohol) (PVA) as an electrolyte. The ASC delivers an energy density of 27 W h kg−1 and excellent cyclability (capacity retention of 93% after 6000 cycles). EXPERIMENTAL SECTION Preparation of MnCO3@Graphene. The present amount of KMnO4 with KMnO4/graphene oxide (GO; for synthetic details,19, 20 see Supporting Information) weight ratio of 10:3 was dispersed into the GO solution, and then the mixture (30 mL) was transferred into 50 mL Teflon-lined stainless-steel autoclave. The autoclave was held at 180 °C in an oven for 12 h before cooling down to RT naturally.17 After hydrothermal treatment, the sample was washed 4 ACS Paragon Plus Environment

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with DI water. The elemental composition of the MnCO3@Graphene was probed using inductively coupled plasma mass spectrometry. Moreover, the mass ratio of MnCO3 in the composite was calculated to be 80.0%. Preparation of MnCO3@Graphene/CNT. The self-standing MnCO3@graphene/CNT film was obtained simply by using a vacuum filtration method. Single-walled CNT (50 mg, Iljin Nanotech) was added in DI water (100 mL), and then probe sonicated to ensure the sufficient mix. After that, the CNT dispersion (15 mL) was mixed with the well-dispersed MnCO3@graphene (72 mg) in 30 mL of water under stirring for 1 h. The as-obtained solution was obtained by vacuum filtration by a PTFE membrane (SciLab; pore size (0.2 µm), diameter (47 mm)), and a self-standing film was carefully removed off from the membrane. The areal density of the hybrid film is 4.04 mg cm−2. Characterization. The morphologies and structures of the as-obtained samples were investigated by scanning electron microscope (SEM; Hitachi), high-resolution TEM (HRTEM; FEI Talos, USA), X-ray powder diffraction (XRD; Rigaku MPA-2000), X-ray photoelectron spectroscopy (XPS; AXIS SUPRA), and Raman spectrometer (XperRam 200). Electrochemical

Measurements.

Electrochemical

performances

were

tested

with

electrochemical station (IVIUM Nstat) in three-electrode test for single electrodes and in a twoelectrode system for flexible ASC devices. In the three-electrode test, platinum foil, saturated calomel electrode (Hg/Hg2Cl2), and 1 M NaSO4 solution were utilized as the counter electrode, reference electrode, and the electrolyte, respectively.21, 22 Paper-type samples (1 cm2) were utilized as the working electrode. The capacitance (Cs, F g−1) of a single electrode was obtained according to the equation, 𝐼 × ∆𝑡

𝐶𝑠 = 𝑚

× ∆𝑉

,

(1)

where I (A), Δt (s), m (g), and ΔV (V) are the applied specific discharge current, the time for 5 ACS Paragon Plus Environment


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discharge, the mass of active materials, and the voltage window for one scanning segment, respectively. The flexible ASC was fabricated with MnCO3@graphene/CNT as positive electrode, AC/CC as negative electrode, and Na2SO4/PVA as electrolyte. The AC/CC electrode was fabricated similarly as the aforementioned preparation of an individual electrode, except that the current collector was CC (2 cm × 2 cm) (AvCarb 1071 HCB). The CC was sonicated in acetone, ethanol, and DI water for 0.5 h before use. The Na2SO4/PVA was fabricated as follows: Na2SO4 (6 g) and PVA (6 g) were dissolved in 60 mL of water with vigorous and continuous stirring at 90 °C for 1 h.23 Gold-coated PET membrane was utilized as the conductive substrate. Prior to assembling, MnCO3@graphene/CNT and AC/CC electrodes were soaked into Na2SO4/PVA for 2 h, and then held at 80 °C for 12 h to remove excess water. Finally, the MnCO3@graphene/CNT and AC/CC electrodes were assembled together under pressing. To obtain charge balance, the AC (m₋) to MnCO3@graphene/CNT (m₊) mass ratio (2.75) was obtained according to the equation, 𝑚+ 𝑚―

=

𝐶 ― ∆𝑉 ― 𝐶 + ∆𝑉 +

,

(2)

For the flexible ASC, specific capacitances (Casy, F g−1), energy densities (E, Wh kg−1), and power densities (P, W kg−1) were obtained according to the equations: 𝐼 × ∆𝑡

𝐶𝑎𝑠𝑦 = 𝑀

𝐸 =

𝑃 =

× ∆𝑉

,

(3)

11000𝐶𝑎𝑠𝑦∆𝑉2 2

3600

,

(4)

3600𝐸 ∆𝑡

.

(5)

RESULTS AND DISCUSSION


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Figure 1. (a) Proposed MnCO3@graphene evolution process. (b and c) SEM images and (d) XRD pattern of MnCO3@graphene, (e) crystal structure of MnCO3. (f) Raman spectrum of MnCO3@graphene. (g) XPS spectrum of Mn 2p.

Figure 1a shows the preparation of the MnCO3@graphene composites. The formation of Mn2+ is easier than that of MnO2 in the current hydrothermal process by comparing the electrode potential according to the Nernst equation.17 And the rod-like shape is generated through the rolling mechanism.24 A possible mechanism is proposed as follows: Permanganate is reduced into Mn2+ ions and further electrostatically bond with the oxygen atoms of the oxygen functional groups on graphene sheets as anchor sites. At the same time, graphene was reduced from graphene oxide. Under the hydrothermal conditions, MnCO3 nuclei appear first in the solution, and then MnCO3 nanosheets formed through a condensation reaction. Subsequently, 7 ACS Paragon Plus Environment


the sheet-like structure of MnCO3 curls into nano-rod. MnCO3 was formed along the graphene nanosheet framework during the hydrothermal process, leading to the wrapping of graphene nanosheets on the surface of MnCO3 nanorods. SEM images of as-synthesized MnCO3@graphene reveals all particles are rod-like and monodispersed with a length approximately

Three-Dimensional Self-Standing and Conductive MnCO3

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