Research Article www.acsami.org
Robust and Flexible Aramid Nanofiber/Graphene Layer-by-Layer Electrodes Se Ra Kwon,† Meagan B. Elinski,‡ James D. Batteas,‡,§ and Jodie L. Lutkenhaus*,†,§ †
Department of Chemical Engineering, ‡Department of Chemistry, and §Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *
ABSTRACT: Aramid nanofibers (ANFs), or nanoscale Kevlar fibers, are of interest for their high mechanical performance and functional nanostructure. The dispersible nature of ANFs opens up processing opportunities for creating mechanically robust and flexible nanocomposites, particularly for energy and power applications. The challenge is to manipulate ANFs into an electrode structure that balances mechanical and electrochemical performance to yield a robust and flexible electrode. Here, ANFs and graphene oxide (GO) sheets are blended using layer-by-layer (LbL) assembly to achieve mechanically flexible supercapacitor electrodes. After reduction, the resulting electrodes exhibit an ANF-rich structure where ANFs act as a polymer matrix that interfacially interacts with reduced graphene oxide sheets. It is shown that ANF/GO deposition proceeds by hydrogen bonding and π−π interactions, leading to linear growth (1.2 nm/layer pairs) and a composition of 75 wt % ANFs and 25 wt % GO sheets. Chemical reduction leads to a high areal capacitance of 221 μF/cm2, corresponding to 78 F/cm3. Nanomechanical testing shows that the electrodes have a modulus intermediate between those of the two native materials. No cracks or defects are observed upon flexing ANF/GO films 1000 times at a radius of 5 mm, whereas a GO control shows extensive cracking. These results demonstrate that electrodes containing ANFs and reduced GO sheets are promising for flexible, mechanically robust energy and power. KEYWORDS: aramid nanofiber, graphene, layer-by-layer, supercapacitor, structural energy and power
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ing.13−15 However, the closely packed structure renders PPTA relatively inert, which has limited its processability and incorporation into composite materials.14,16 Significant efforts to improve the interactions between PPTA and other materials, such as hydrolysis and surface coating, have been undertaken.14,17−21 A recently developed method that produces nanoscale PPTA fibers (aramid nanofibers, ANFs) provides a simpler alternative to such processing issues. In this method, macroscale bulk Kevlar thread is dissolved into nanoscale aramid fibers in dimethyl sulfoxide (DMSO) with potassium hydroxide (KOH).13 During the process, ANFs form by deprotonation of the amide groups, leading to the dissociation of hydrogen bonds between polymer chains.13 The end product is a dispersion of polymeric nanofibers 20−40 nm in diameter and 5−10 μm in length with a negative surface charge.13,14 As a liquid dispersion, the ANFs can be utilized in various processing techniques such as layer-by-layer (LbL) assembly and vacuum-assisted filtration, amenable for a variety of applications where the mechanical properties are of interest.22−25 Graphene, a two-dimensional material consisting of atomically thin sheets of sp2-hybridized carbon, has attracted great
INTRODUCTION With an increasing demand for flexible energy and power systems, considerable research efforts have focused on designing battery and capacitor electrodes that can withstand repeated mechanical deformation without loss of performance.1−5 To meet both mechanical and electrochemical needs, multifunctional hybrid materials are required. Graphene and graphene-based composites have been widely studied for use as electrode materials in energy storage devices such as batteries and supercapacitors.6−9 For supercapacitors, graphene stores charge by an electric double layer mechanism and by a pseudocapacitive mechanism with oxygen-containing functional groups at the surface.10−12 In this regard, the combination of mechanically strong aramid nanofibers (ANFs) and graphene sheets is an attractive solution to develop flexible electrodes with high electrochemical performance. However, there remain questions as to how aramid nanofibers might affect the energy storage and mechanical properties of the graphene-based electrode. Kevlar, a para-aramid polymer synthesized from poly(pphenylene terephthalamide) (PPTA), is well-known for its superior mechanical properties and is a promising candidate for reinforcing materials.13 The impressive mechanical properties of PPTA, a modulus of 103 GPa and a tensile strength of 3.8 GPa, arise from interactions between PPTA chains, such as π−π stacking, van der Waals forces, and hydrogen bond© 2017 American Chemical Society
Received: March 9, 2017 Accepted: April 28, 2017 Published: April 28, 2017 17125
DOI: 10.1021/acsami.7b03449 ACS Appl. Mater. Interfaces 2017, 9, 17125−17135
Research Article
ACS Applied Materials & Interfaces
propylene carbonate, lithium perchlorate, potassium permanganate, and sodium nitrate were purchased from Sigma-Aldrich. Graphite (SP1) was purchased from Bay Carbon. Potassium hydroxide was purchased from Amresco. Lithium foil and hydrazine monohydrate were purchased from Alfa Aesar. Indium tin oxide (ITO)-coated glass (resistance
