Preparation of stable carbon nanotube aerogels with high electrical conductivity and porosity
Introduction
Aerogels are highly porous, low-density materials comprising a solid, three-dimensional (3D) nanoscale network completely accessible to ions and molecules [1], [2], [3], [4], [5]. Aerogels have already demonstrated orders of magnitude faster response for sensing, energy storage, and energy conversion than other pore-solid architectures [6], [7], [8]. Carbon nanotubes (CNTs) represent a rare material that exhibits a number of outstanding properties in a single material system, such as high aspect ratio, small diameter, light weight, high mechanical strength, high electrical and thermal conductivities, and unique optical and optoelectronic properties. By combining extraordinary properties of CNTs with those of aerogels, a new class of materials becomes accessible with unique multifunctional material properties, which may find applications in fuel cells, super capacitors, 3D batteries, advanced catalyst supports, energy absorption materials, multifunctional composites, chemical and biological sensors, etc.
Bryning and coworkers created CNT aerogels from wet CNT-surfactant gel precursors, and they showed that polyvinyl alcohol-reinforced CNT aerogels (typical CNT loadings range from 25 to 33 wt.%) are strong and electrically conductive (∼10−2 S/cm) [9]. Worsley and coworkers fabricated carbon-reinforced single-walled CNT(SWCNT) aerogels (SWCNT loading up to 55 wt.%) by pyrolysis of a dried gel mixture of SWCNTs, resorcinol, and formaldehyde at 1050 °C under nitrogen [10], [11], [12]. These carbon-reinforced SWCNT aerogels are mechanically robust and highly electrically conductive (up to 1.12 S/cm) and show specific surface area up to 184 m2/g, which are excellent fillers for high-performance polymer composites [12]. Worsley and coworkers were also able to use the similar approach to incorporate double-walled CNTs (DWCNTs) into a carbon aerogel, which was, however, limited in the amount of DWCNTs (up to 8 wt.%) that could be incorporated into the carbon aerogel framework and in its ability to achieve monolithic densities below 70 mg/cm3 [13]. Kwon and coworkers fabricated multi-walled CNT (MWCNT)-based aerogels with aligned porous structures using an ice-templating process [14]. These anisotropic MWCNT aerogels are electrically conductive (up to 1.9 × 10−2 S/cm) and have specific surface area up to 181 m2/g. Very recently, Gui and coworkers synthesized highly porous CNT sponges containing large-diameter MWCNTs (30–50 nm) by a chemical vapor deposition method [15]. These MWCNT sponges display exceptional structural flexibility, excellent electrical conductivity (∼1.7 S/cm), and good specific surface area (300–400 m2/g). While this paper was in preparation, Zou and coworkers reported the synthesis of an ultralight MWCNT aerogel, which shows large specific surface area (580 m2/g) and has an electrical conductivity of 3.2 × 10−2 S/cm that can be further increased to 0.67 S/cm by a high-current pulse method [16].
In this article we report a new approach to the synthesis of stable CNT aerogels. Our method involves following two distinctive aspects: (1) 3D chemical assembly of CNTs in solution to form a stable gel using a chemical cross-linker such as ferrocene-grafted poly(p-phenyleneethynylene) (Fc-PPE, Fig. 1) [17], followed by a CO2 supercritical drying to create stable aerogels; (2) thermal annealing of these aerogels in air to significantly improve their electrical and mechanical properties and enhance their surface area and porosity. We have demonstrated the preparation of thermally annealed CNT aerogels containing small-diameter CNTs such as SWCNTs and DWCNTs, which are mechanically stable and stiff, highly porous (∼99%), and exhibit excellent electrical conductivity (∼1–2 S/cm) and large specific surface area (∼590–680 m2/g).
Section snippets
Experimental section
Two chemical cross-linkers (Fc-PPE and Fc-PPETE, Fig. 1) were synthesized and characterized according to literature methods [18], [19]. Purified SWCNTsHiPco and DWCNTs were purchased from Carbon Nanotechnologies Inc. and were used without further purification. Fc-PPE–CNT and Fc-PPETE–CNT gels were prepared in chlorobenzene according to our previous procedure [17]. The mass ratio of CNT:chemical cross-linker (CCL) was kept at 1, 2, and 4, respectively (Table 1). The freestanding monolithic gel
CNT organogels
Stable CNT gels are critical precursors to stable, highly porous, 3D interconnected CNT aerogels [14], [17], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Pristine SWCNTs and DWCNTs are not soluble in most solvents and do not form stable, freestanding monolithic gels because of weak physical interactions among CNTs.
We recently developed a versatile and nondamaging chemistry platform that enabled us to engineer specific CNT surface properties, while preserving CNT’s intrinsic
Summary
We have developed a new approach to the synthesis of stable CNT aerogels. The approach involves three steps: (1) 3D chemical assembly of CNTs in solution to form a stable gel using a chemical cross-linker; (2) CO2 supercritical drying of CNT gels to create stable aerogels; (3) thermal annealing of these aerogels in air to significantly enhance their electrical and mechanical properties, and enhance their surface area and porosity. We have demonstrated the preparation of thermally annealed CNT
Acknowledgements
J.C. thanks the financial support from the National Science Foundation (DMI-06200338), UWM start-up fund, UWM Research Growth Initiative award, and the Lynde and Harry Bradley Foundation. We thank Magda Salama in the Materials Research Institute at the Pennsylvania State University for collecting all of the surface area and porosity data and invaluable discussions. We thank Prof. Marija Gajdardziska-Josifovska for TEM access at the HRTEM Laboratory of University of Wisconsin-Milwaukee and
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