Article pubs.acs.org/cm
In Situ Synthesis of Hybrid Aerogels from Single-Walled Carbon Nanotubes and Polyaniline Nanoribbons as Free-Standing, Flexible Energy Storage Electrodes Dengteng Ge,† Lili Yang,*,‡,† Apiradee Honglawan,† Jie Li,† and Shu Yang*,† †
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, 150090, P.R. China
‡
S Supporting Information *
ABSTRACT: Hybrid aerogels consisting of interpenetrating single-walled carbon nanotubes and polyaniline (SWCNT/ PANI) nanoribbons were prepared as free-standing, flexible lithium ion battery (LIB) electrodes. Assisted by camphorsulfonic acid, the anilinium cations formed complexation with micelles of dodecylbenzene sulfonate anions within the wet SWCNT network. Very thin PANI nanoribbons (thickness of 10−100 nm, width of 50−1000 nm, and length of 10−20 μm) were formed within the network after polymerization of aniline. By varying the concentration of aniline, we were able to fine-tune the morphologies of final PANI nanostructures, including nanoribbons, porous nanofibers, and nanoparticles. Specifically, SWCNT/PANI nanoribbon aerogels showed high capacity (185 mAh/g) and good cycle performance (up to 200 times), which could be attributed to synergistic effects of efficient ion/electron transport within the 3D carbon nanotubes network, shortened ion diffusion distance and optimized strain relaxation from nanoribbons and nanotubes, and effective penetration of electrolyte within interconnected nanopores in the network.
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INTRODUCTION Increasing interests in wearable electronic devices, roll-up displays, portable gadgets, and biomedical devices have propelled the advancement of flexible, lightweight, and inexpensive energy storage devices, including lithium ion batteries (LIBs) and supercapacitors.1−6 Among them, various carbon nanotube (CNT)-based electrodes have been prepared, including free-standing entangled CNT films, aligned CNTs infiltrated with active materials (e.g., polyaniline, iron oxide), and composites coated with silicon and tin dioxide,7−11 due to the excellent conductivity, large specific surface area, and high mechanical strength of CNTs. However, challenges remain, such as the cost of scaling-up the production of pure CNT films,12 requirement of a supporting substrate, and possible blockage of active sites on CNTs by the inorganic coatings. Conducting polymers, storing charge from reversible redox doping/dedoping process, offer attractive alternatives because they are flexible, highly conductive, and easily processable.13−15 However, the performance of conducting polymers alone as electrodes is often disappointing. For example, they have shown poor cycling stability, high self-discharge rate, mass transport limitation within thick layers due to the swelling and volume change upon redox switching, and insufficient doping levels.13,16−18 In the past decade, polyaniline (PANI) has been extensively studied as a promising electrode material because of the good redox reversibility, high stability in air or © 2014 American Chemical Society
aqueous solutions, and facile synthesis with low production cost.19 There have been efforts to combine the advantages of CNTs and PANI using CNT/PANI nanocomposites as electrodes, e.g., by coating PANI nanoparticles on entangled SWCNT films20,21 and aligned CNTs7 via electrochemical deposition and layer-by-layer synthesis of PANI nanofibers on CNTs.22 The electrochemical performance has been improved with volumetric capacity up to 210 mAh/cm.3,22 Nevertheless, all the composites reported so far have to be confined on a conductive substrate. Aerogels are three-dimensional (3D) networks consisting of interpenetrating micropores and mesopores. They offer ultralow densities, large specific areas, and disordered open pores.23−26 For applications in electrodes, aerogels potentially can allow for a high percentage of electrolyte infiltration, a small ion-transport resistance, and a short solid-state ion diffusion path length, as well as efficient dissipation of the stress associated with electrode expansion and contraction.24−27 Recently, CNTs, cellulose fibers, and graphene have been used as building blocks for the assembly of carbon-based aerogels.28−30 A few groups have reported fabrication of carbon-based hybrid aerogels for batteries or supercapacitors, Received: December 7, 2013 Revised: January 27, 2014 Published: February 5, 2014 1678
dx.doi.org/10.1021/cm404025g | Chem. Mater. 2014, 26, 1678−1685
Chemistry of Materials
Article
Figure 1. (a) Illustration of the procedure to prepare the single-walled carbon nanotube (SWCNT)/polyaniline (PANI) nanoribbon hybrid aerogel. An, aniline. SDBS, sodium dodecylbenzene sulfonate. CSA, camphorsulfonic acid. APS, ammonium peroxydisulfate. (b) SEM image of pure SWCNT aerogels. (c) SEM image of the interpenetrated PANI nanoribbons within the composite consisting of 25 wt % SWCNTs. (d) High resolution SEM image of thin PANI nanoribbons in the composite. Inset: cross-sectional view of CNTs embedded in the nanoribbons. carbonate and diethyl carbonate (1:1 v/v), from Sigma-Aldrich), was used to assemble the coin cells. The Li metal and coin cell cases were purchased from MTI Corporation, USA. Preparation of SWCNT Aerogels. SWCNT aerogels were prepared according to the literature.28 The concentration of SWCNT and SDBS was 8 mg/mL and 64 mg/mL, respectively. The as-formed gel was then soaked into the mixture of deionized (DI) water and ethanol (1:1, v/v), and the bath solution was changed at least three times over the course of 2−3 days. To remove SDBS completely, the gel was immersed into 20 mL of nitric acid (1 M) aq. solution at 50 °C for 4 h. SWCNT aerogels were obtained after the critical point drying (SAMDRI-PVT-3D), followed by 400 °C for 12 h. In Situ Synthesis of SWCNT/PANI Nanoribbon Aerogels. The as-formed SWCNT gel was soaked into the 20 mL CSA/An aq. solution ([CSA] = [An] = 0.05 M) for 24 h. Then the infiltrated gel was carefully immersed into the 20 mL of CSA/APS solution ([CSA] = 0.05 M and [An]/[APS] = 2:1) in ice water for 3−72 h. The resulting gel was washed by water/ethanol (1:1, v/v) and anhydrous ethanol for several times, followed by critical point drying to obtain the aerogel nanocomposites. The weight of SWCNT and PANI in the composite was determined by the weight of SWCNT used in the SWCNT gels and the aerogel composites after the infiltration. Characterization. The morphologies of SWCNT aerogels, SWCNT/PANI composites, and pure PANI were imaged by fieldemission scanning electron microscope (FESEM, JEOL 7500F SEM) at 20.0 kV. Their surface chemistry was analyzed using a Fourier Transform Infrared (FT-IR) spectrometer (Nicolet 8700) and UV− vis−NIR spectrophotometer (Cary 5000, Varian). Fabrication and Testing of Two-Electrode Cells. Coin-type (CR 2016) half-cells were assembled in an argon-filled glovebox with H2O and O2 contents < 1 ppm. Li metal was used as the negative electrode, and the composites were assembled as the positive electrode. The galvanostatic charge−discharge of the electrode was
for example, the graphene/PANI and carbon aerogel/Fe2O3 composites by sol−gel chemistry or hydrothermal synthesis,31−35 using aerogels as templates to infiltrate active materials. However, few have paid attention to the assembly behaviors of the nanomaterials within the aerogel network or the resulting battery performance. Here we report preparation of SWCNT/PANI nanoribbon hybrid aerogels as free-standing, flexible LIB electrodes by micelle-induced self-assembly of aniline monomers, followed by in situ polymerization of PANI within the wet SWCNT gels. Assisted by camphorsulfonic acid, the anilinium cations formed complexation with micelles of dodecylbenzene sulfonate anions within the wet SWCNT network. We also observed morphological evolution of PANI nanostructures from nanoribbons to porous nanofibers and nanoparticles when decreasing the aniline concentration. Because of the intrinsic flexibility of nanotubes and nanoribbons and the double interpenetrating network, the freestanding nanocomposite film could be bent up to 180°. High capacity (185 mAh/g) and good cycling performance of nanocomposites (200 times) were also observed.
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MATERIALS AND METHODS
Materials. All reagents were used as received. SWCNTs were purchased from Cheap Tubes Inc. (purity >90% with ashes
