Article pubs.acs.org/Langmuir
MnO2 Nanowire/Biomass-Derived Carbon from Hemp Stem for HighPerformance Supercapacitors MinHo Yang,†,∥ Dong Seok Kim,‡,∥ Seok Bok Hong,‡ Jae-Wook Sim,‡ Jinsoo Kim,§ Seung-Soo Kim,*,‡ and Bong Gill Choi*,‡ †
Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡ Department of Chemical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, Republic of Korea § Department of Chemical Engineering, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do 17104, Republic of Korea S Supporting Information *
ABSTRACT: Hierarchical 3D nanostructures based on waste biomass are being offered as promising materials for energy storage due to their processabilities, multifunctionalities, environmental benignities, and low cost. Here we report a facile, inexpensive, and scalable strategy for the fabrication of hierarchical porous 3D structure as electrode materials for supercapacitors based on MnO2 nanowires and hemp-derived activated carbon (HC). Vertical MnO2 wires are uniformly deposited onto the surface of HC using a one-step hydrothermal method to produce hierarchical porous structures with conductive interconnected 3D networks. HC acts as a near-ideal 3D current collector and anchors electroactive materials, and this confers a specific capacitance of 340 F g−1 at 1 A g−1 with a high rate capability (88% retention) of the 3D MnO2/HC composite because of its open-pore system, which facilitates ion and electron transports and synergistic contribution of two energy-storage materials. Moreover, asymmetric supercapacitors fabricated using 3D HC as the anode and 3D MnO2/HC as the cathode are able to store 33.3 Wh kg−1 of energy and have a power delivery of 14.8 kW kg−1.
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INTRODUCTION Three-dimensional (3D) nanostructures are promising for a variety of applications in photonics, electronics, sensors, and energy conversion/storage devices due to their superior intrinsic properties, which include high surface areas, short diffusion pathways, and excellent interconnectivities.1−6 In particular, conductive 3D nanostructures, such as carbon-based aerogels and hydrogels, Ni foams, and bicontinuous metals are highly attractive as major parts (i.e., current collector and active materials) of energy-storage devices to increase the device performances.7−13 For example, Ni foams have been extensively explored as 3D current collectors for depositing active materials or hard templates for preparing 3D nanostructures.14−16 However, the lack of electrochemical activity and mechanical rigidity of Ni foams limits their applications. Because graphene materials exhibit energy storage ability with improved electrochemical performances in supercapacitors (SCs) and lithiumion batteries, a number of synthetic methods, such as selfassembly, template-assisted method, or direct deposition, have been developed to prepare 3D graphene structures.3,4,8,10,17 Despite these efforts, their large-scale and reproducible production at low cost remains to be resolved before practically © 2017 American Chemical Society
utilized. Therefore, a simple, cost-effective, and industrial-scale production method is highly required for the manufacture of electrode materials. Porous carbon materials, such as activated carbons, have received considerable research attention in the energy-storage field, particularly for SCs and lithium-ion batteries, because the manufacturing process is a straightforward, inexpensive, and suitable for industrial production.18−24 As prospective carbon sources, natural biomass (e.g., woods, sea weeds, coconut shell, and agricultural wastes) has been recently attracted because of abundance, low cost, environmental benignity, and well-defined pore structures and topologies.26,27 In addition, manufacturing processes based on activated carbon from biomass could significantly impact the production of conductive 3D nanostructures. Dangbegnon et al. reported 3D porous carbon materials derived from renewable pine cone biomass that exhibited high-energy densities of ∼19 Wh kg−1.28 Although porous carbon electrode-based electrical double-layer capacitors Received: February 20, 2017 Revised: May 3, 2017 Published: May 8, 2017 5140
DOI: 10.1021/acs.langmuir.7b00589 Langmuir 2017, 33, 5140−5147
Article
Langmuir
Figure 1. Schematic for preparation of 3D MnO2/HCs. on consistent N2 flow (300 cm3 STP min−1). As-prepared 3D HC was added into the mixture solution containing KMnO4 and MnSO4 (KMnO4/MnSO4 = 1:2.2, molar ratio). The resulting solution was gently stirred for 30 min at room temperature and transferred to a Teflon-lined stainless-steel autoclave. The hydrothermal growth of MnO2 was performed in autoclave at 140 °C for 24 h. After hydrothermal reaction, the autoclave was left to cool to room temperature naturally. The resultant HCs was collected by filtration, washed several times with deionized water and ethanol, and dried at 65 °C in a vacuum oven. Sample Characterizations. Scanning electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (S-4800). High-resolution transmission electron microscopy (TEM) images were obtained using a field-emission TEM (JEM2100F, JEOL) operated at 200 kV. X-ray diffraction (XRD) data were obtained on a Rigaku D/MAX-2500 using Cu Kα radiation generator. Brunauer−Emmett−Teller analysis was performed on Micrometrics ASAP 2020 surface analyzer to obtain isotherm and specific surface area of different samples. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo MultiLab 2000 system. An Al Mgα X-ray source at 200 W was used with pass energy of 20 eV and a 45° takeoff angle in a 10−7 Torr vacuum analysis chamber. Electrochemical Tests. Cyclic voltammetry, galvanostatic charge/ discharge measurements, electrochemical impedance spectroscopy (EIS, 105 to 10−2 Hz, 10 mV amplitude), and cycling performance were evaluated using a VersaSTAT 4 (Princeton Applied Research). For electrochemical tests, the working electrode was first prepared by coating a slurry (3D MnO2/HC, carbon black, PVDF (8:1:1)) onto Ti-foil current collector and dried at 65 °C under vacuum. The areal mass loading of 3D MnO2/HC was 0.76 mg cm−2. On the basis of the cross-section SEM image (Figure S1), the density of 3D MnO2/HC was 0.13 g cm−3. Electrochemical properties of individual electrodes were evaluated using a three-electrode cell with Pt wire as the counter electrode and Ag/AgCl as the reference electrode with a neutral electrolyte (1 M Na2SO4). The capacitive performance test of asymmetric supercapacitors (ASCs) assembled using 3D HC as a negative electrode and 3D MnO2/HC as a positive electrode were conducted in full-cell configuration. The specific capacitance of the assembled ASCs was obtained from discharge curves of galvanostatic charge/discharge measurements as the following equation: C = (IΔt)/ (ΔV), where I is the current density (A g−1), Δt is the discharge time (s), and ΔV is voltage window after iR drop. On the basis of galvanic discharge curves, the energy (E, Wh kg−1) and power (P, W kg−1) densities of ASCs were calculated as follows, E = 1/2C(ΔV)2 and P = (ΔV)2/4RM, where C represents specific capacitance, ΔV is the operating voltage window, R (= Vdrop/2I) is the internal resistance from iR drop, and M is the total weight of both electrodes.
(EDLCs) are available commercially, their energy densities are limited. Much attention has been paid to the preparation of hybrid electrodes that combine a pseudocapacitive electrode and a carbon electrode.29−34 The incorporation of pseudocapacitive materials into 3D conductive carbon supports facilitates mass transport and provides electrochemical benefits due to synergism between electrode materials.1−4 Herein we report the incorporation of MnO2 into 3D honeycomb-like activated carbon (HC) derived from hemp (Cannabis Stiva L.) biowaste for high-performance SCs. Hemp is composed of natural lignocellulosic fibers ranging from 5 to 10 nm in diameter, which benefits its carbonized counterparts because of their well-defined pore systems, high levels of crystalline cellulose, fine fibrous networks, and low cost.35,36 3D HCs were produced by pretreating hemp in a carbonization process, followed by activation in steam. The 3D MnO2/HC composites were obtained from in situ growth of MnO2 nanowires in 3D porous structure of HCs through a hydrothermal method. The MnO2 wires were selected as pseudocapacitive materials due to their high theoretical capacitance (∼1370 F g−1), cost effectiveness, and environmental benignity.37,38 In particular, 1D nanostructure will provide short ionic diffusion pathway and highly accessible surface area, leading to improved performance of SCs.12 Although various methods, such as electrodeposition and hydrothermal method, have been adopted for deposition of MnO2 on porous carbon materials,38 control of morphology and crystallinity is still a challenging task. In particular, 3D hierarchical architectures based on MnO2 nanowires deposited on 3D macroporous carbon materials have rarely been reported for electrochemical energy-storage applications. The resultant MnO2/HC composites showed 3D hierarchical architectures with favorable features, such as a high surface area, a high electrical conductivity, and an efficient ion diffusion characteristic. These unique properties of 3D MnO2/HC composite electrodes resulted in a high specific surface area, a high rate capability, and an excellent cycling characteristic for SC applications.
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EXPERIMENTAL SECTION
Preparation of 3D HC and 3D MnO2/HC. Hemp stems as biomass materials were ground with knife mill to small particles (
