Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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MnO2 Nanoflowers Deposited on Graphene Paper as Electrode Materials for Supercapacitors Omer Sadak,† Weizheng Wang,‡ Jiehao Guan,‡ Ashok K. Sundramoorthy,§ and Sundaram Gunasekaran*,†,‡
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Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue Madison, Wisconsin 53706-1595, United States ‡ Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, Wisconsin 53706, United States § Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, India S Supporting Information *
ABSTRACT: We recently reported a simple and costeffective green method to produce free-standing, flexible, and highly conductive electrochemically exfoliated graphene paper (GrP) for a supercapacitor application. To improve the capacitance behavior of GrP, manganese dioxide (MnO2) was electrochemically deposited on GrP with different number of MnO2 cycles. After the electrochemical deposition process, MnO2 nanoflowers were formed, which provide a fast transfer of electrolyte ions. After 10 cycles of electrodeposition, MnO2-coated GrP (GrP/10-MnO2, which is the optimal composition) exhibited an excellent capacitive performance with a high specific capacitance of 385.2 F·g−1 at 1 mV·s−1 in 0.1 M Na2SO4 electrolyte and outstanding capacitance retention after 5000 consecutive cycles. Taking advantage of both superior mechanical and capacitance behavior of GrP and GrP/10-MnO2 electrodes, a flexible solid-state asymmetric supercapacitor (SASc) device was assembled using GrP/10-MnO2 and GrP as positive and negative electrode, respectively. The fabricated SASc device exhibited not only high areal capacitances of 76.8 mF cm−2 at a current density of 0.05 mA cm−2 but also excellent cycling stability of 82.2% after 5000 consecutive galvanostatic charge/discharge cycles. This flexible supercapacitor can also deliver a high energy density of 6.14 mWh·cm−2 with a power density of 36 mW·cm−2. This research represents a new direction for exploring the potential of free-standing GrP and its nanocomposites in flexible energy-storage systems. KEYWORDS: electrochemical exfoliation, electrodeposition, graphene paper, green method, MnO2 nanoflowers
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INTRODUCTION Electrochemical energy-storage devices such as batteries and supercapacitors have unique properties. Among them, supercapacitors (SC) have attracted extensive investigations because of their high power density, quick charge/discharge times, long cycle life (>100 000 cycles), and low cost of fabrication.1−3 However, they suffer from low energy density, which limits their extensive commercial applications.4−6 Much research has been directed at improving the energy density of supercapacitor electrode materials, primarily using carbonaceous composites made of graphene foam/paper, carbon nanotube (CNT) film, and carbon fiber cloth with transition-metal oxides or conjugated polymers.7,8 A wide range of metal oxides (nickel, ruthenium, and manganese oxides, etc.) are used as SC electrode materials.9 Among them, manganese can exist in different valence states (+2, +3, +4). It can form various stable oxides (MnO, Mn3O4, Mn2O3, MnO2) with a variety of electrochemical properties, crystal structures, morphology, porosity, and textures.10 Among © XXXX American Chemical Society
them, MnO2 has the greatest potential because of its stability and different structural forms, α-, β-, γ-, and λ-type, among others.11 MnO2 is also naturally abundant, inexpensive, and environmentally benign.12 However, low electrical conductivity (10−5−10−6 S·cm−1), low cyclic stability, and low rate capacity have limited the applications of MnO2.13,14 To overcome these limitations, an effective strategy is to combine carbonaceous material with manganese nanostructures.15 Graphene, an outstanding two-dimensional (2D) carbon structure, has attracted strong considerations as a result of its exceptional mechanical, electrical, thermal, and optical properties.16−18 Graphene is also a standout among most materials for flexible and portable devices19 because it can be integrated into flexible, interconnected, and free-standing three-dimensional (3D) networks such as paper,1,20 foam,21 film,22 aerogels,23 Received: April 30, 2019 Accepted: June 11, 2019 Published: June 11, 2019 A
DOI: 10.1021/acsanm.9b00797 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials Scheme 1. Fabrication of GrP/x-MnO2 Electrode Using GrP via Electrochemical Deposition
hydrogels,24 or other substrates.25 Thus, 3D porous graphene structures are suitable platforms to fabricate flexible supercapacitors.26 However, cost-effective and environmentally friendly methods are required to afford large-scale manufacturing of graphene and broaden its applications.27 In this vein, the graphene paper (GrP) we have fabricated is an easy-to-prepare, green, and low-cost substrate.20 Herein, we report a green, rapid, and cost-effective method to produce flexible, free-standing, and highly conductive SC electrodes made of GrP decorated with MnO2 nanoflowers for enhanced charge storage. GrP was prepared via electrochemical exfoliation of graphite sheets, which were then selfassembled using vacuum filtration. MnO2 nanoflowers were electrochemically deposited on GrP. The amount of MnO2 nanoflowers used was optimized by varying the number of electrodeposition cycles (1, 3, 5, 7, 10, 15, and 20). The electrode material obtained after 10 cycles of MnO2 deposition on GrP (GrP/10-MnO2, which is the optimal composition) exhibited high specific capacitance (Cs) with excellent capacitance retention. Then, a flexible solid-state asymmetric supercapacitor (SASc) device was assembled using GrP/10MnO2 and GrP as positive and negative electrode, respectively. The fabricated SASc device exhibited high areal capacitance of 76.8 mF cm−2 and long-term cycling stability of 82.2% after 5000 consecutive galvanostatic charge/discharge (GCD) cycles, and it delivered a high energy density of 6.14 mWh· cm−2 with a power density of 36 mW·cm−2.
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reference, and working electrode, respectively. After the deposition of MnO2, the electrodes were thoroughly washed with 5% HCl and DI water, followed by air drying at 60 °C for 30 min. The electrodes were labeled as GrP/x-MnO2 where x is the number of MnO2 deposition cycles used in the electrode preparation. The amount of MnO2 deposited on 0.2 mg of GrP ranged from 0 to 0.19 mg (Table S1). Electrochemical Measurements. An electrochemical workstation (CH-660D, CH Instruments Inc., Austin, TX, U.S.A.) was used for the electrochemical analysis. A three-electrode configuration with Ag/AgCl (1 M KCl), Pt wire, and GrP/x-MnO2 as reference, counter, and working electrode, respectively, was used for the analysis of as-prepared electrodes. GCD and cyclic voltammetry (CV) tests were performed in 50 mM Na2SO4 as electrolyte at room temperature. The electrodes were also characterized by electrochemical impedance spectroscopy (EIS) using 0.1 M KCl with 5 mM ferrocyanide/ ferricyanide ([Fe(CN)6]3−/4−) over 0.1 Hz to 100 kHz frequency range at open circuit potential. First, CV and GCD cycles were always discarded.
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RESULTS AND DISCUSSION Recently, our group reported a novel synthesis method for GrP, which possesses low sheet resistance, high thermal and mechanical stability, and high capacitance compared with other reported flexible and free-standing 3D graphene network structures.20 Exfoliated graphene sheets, which form GrP by self-assembly, were further characterized by AFM. The height profile across the graphene sheet (Figure S1) reveals that it is
