INVITED REVIEW Carbon Aerogel Evolution: Allotrope, Graphene-Inspired, and 3D-Printed Aerogels
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INVITED REVIEW Carbon aerogel evolution: Allotrope, graphene-inspired, and 3D-printed aerogels Swetha Chandrasekaran, Patrick G. Campbell, Theodore F. Baumann, and Marcus A. Worsleya) Lawrence Livermore National Laboratory, Livermore, California 94551, USA (Received 11 July 2017; accepted 26 September 2017) https://doi.org/10.1557/jmr.2017.411 – . Carbon aerogels (CAs) are a unique class of high surface area materials derived by sol gel chemistry. Their high mass-specific surface area and electrical conductivity, environmental compatibility, and chemical inertness make them very promising materials for many applications, such as energy storage, catalysis, sorbents, and desalination. Since the first CAs were made via pyrolysis of resorcinol–formaldehyde (RF)-based organic aerogels in the late 1980s, the field has really grown. Recently, in addition to RF-derived amorphous CAs, several other carbon allotropes have been realized in aerogel form: carbon nanotubes (CNTs), graphene, graphite, and diamond. Furthermore, the popularity of graphene aerogels has inspired research into aerogels made of a host of graphene analog materials (e.g., boron nitride, transition metal dichalcogenides, etc.), with potential for an even wider array of applications. Finally, the development of three- https://www.cambridge.org/core/terms dimensional-printed aerogels provides the potential for CAs to have an even broader impact on energy-related technologies. Here, we will present recent work covering the novel synthesis of RF-derived, CNT, graphene, graphite, diamond, and graphene analog aerogels. – I. INTRODUCTION chalcogenides,12 metals,13 15 and various two-dimensional 16–18 Aerogels cover a class of solid materials distinguished by (2D) materials. A major driver for realizing aerogels their extreme low density and ultrafine, open pore structure. from a wider materials set is the potential to unlock novel Initially synthesized as a wet gel, aerogels are created by properties that these materials only exhibit as aerogels. replacing the liquid phase of the wet gel with gas, which Carbon aerogels (CAs), in particular, possess a unique results in a dry porous solid. As the pore structure is combination of ultralow density, large surface area, high minimally perturbed during this process, it is not uncom- electrical conductivity, thermal and chemical robustness, , subject to the Cambridge Core terms of use, available at mon for aerogels to consist of greater than 95% porosity, and good mechanical properties, not available in other aerogel materials. These properties arise directly from with pores that average less than 100 nm. These features 2 alone give aerogels in general some very unique properties, assembling amorphous sp carbon nanoparticles into such as large accessible surface areas and extremely low a highly porous, low-density aerogel. Consequently, fi CAs have enjoyed steady growth in research interest thermal conductance. In fact, the rst metal-oxide aerogels 7 prepared by Kistler et al. targeted applications in catalysis1 since their invention in the early 1990s [Fig. S1(a)] and 30 Sep 2021 at 12:00:47 and thermal insulation2 to take advantage of these novel have been actively pursued for applications touching , on a wide variety of fields including energy storage, catal- properties. Aerogel research has continued to grow since fi Kistler et al. prepared the first aerogels in the 1930s. For the ysis, ltration, chemical sensors, energy generation, fi sorbents, and electronics. The discovery of new carbon rst few decades, though new synthesis routes were 19–21 170.106.34.90 reported,3,4 the composition of aerogels was limited to allotropes (e.g., nanotubes, graphene, and fullerene) metal oxides. However, in the past 30 years, there has been has fueled pursuits to synthesize aerogels on the basis of a collective push to not only develop new methods5 to these novel nanomaterials. Graphene aerogels (GAs) . IP address: produce traditional metal oxides but also to expand the have been an exceptionally popular topic, showing a very variety of materials that aerogels cover. Some notable rapid rise in published works and citations since its examples of these new aerogels include reports of invention [Fig. S1(b)]. The intense interest in graphene organic aerogels,6 carbons,7–10 conducting oxides,11 is in part largely due to the advantages that the graphene allotrope exhibits compared with amorphous or nano- crystalline carbon. Furthermore, graphene has inspired Contributing Editor: Paolo Colombo a)Address all correspondence to this author. interest in other 2D materials, such as boron nitride (BN), e-mail: [email protected] transition metal dichalcogenides, and black phosphorus https://www.cambridge.org/core DOI: 10.1557/jmr.2017.411 (BP). This interest in graphene-inspired materials has also 4166 J. Mater. Res., Vol. 32, No. 22, Nov 28, 2017 Ó Materials Research Society 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from S. Chandrasekaran et al.: Carbon aerogel evolution: Allotrope, graphene-inspired, and 3D-printed aerogels led to novel synthesis routes to make aerogels based on method is directly utilized to assist in building the matrix these 2D materials. And finally, CAs have been playing of other carbon allotrope aerogels. a prominent role in the emerging body of literature devoted to three-dimensional (3D)-printed aerogels. This B. CNT aerogel has led to the development of novel 3D printing schemes CNTs possess exceptional mechanical, thermal, and to make aerogel synthesis compatible with current 3D electrical properties.23 Therefore, structures fabricated from printing technologies. Conventional CAs have been these materials hold technological promise for a variety of covered in previous reviews, but a broad look at how applications,8,28–34 including structural composites, thermal the CA synthesis has evolved and expanded the aerogel interfaces, energy storage, actuators, and artificial mem- https://doi.org/10.1557/jmr.2017.411 . landscape is lacking.22–26 Therefore, this review seeks to branes. Synthesizing CNT aerogels that retain the intrinsic provide the reader with a survey of the CA synthesis properties of individual tubes, however, is extremely methods with a focus on novel carbon allotropes, challenging. In general, CNT aerogel synthesis begins with graphene-inspired or 2D materials, and 3D printing, as a dispersion of CNTs. Although there are a few examples well as some perspective with regard to the future outlook where CNTs are grown within an aerogel matrix,35–38 they for aerogel synthesis. are the exception. Pristine CNTs, being hydrophobic, require a surfactant if dispersal in aqueous media is II. CARBON ALLOTROPES required. Otherwise, organic solvents or highly oxidized CNTs must be used. Next, gelation is induced via a physical Allotropes refer to the different forms of a chemical https://www.cambridge.org/core/terms or chemical cross-linking mechanism, followed by drying element, which may possess very different properties. As [typically freeze drying or critical point drying (CPD)], and such, aerogels of the various allotropes would also be any desired post-processing. expected to reflect these different properties. In the case Bryning et al. reported the first CNT aerogel in 2007. of carbon, aerogels based on four different carbon Bryning et al. used a surfactant to successfully disperse allotropes have been reported: (i) amorphous carbon, a low concentration of single-walled CNTs (SWCNT) in (ii) carbon nanotubes (CNTs), (iii) diamond, (iv) graph- water. The suspension would form a physical gel over- ite, and (v) graphene.27 Following is a discussion of the night and then be washed in water or an aqueous synthesis methods, properties, and applications of each of polyvinylalcohol (PVA) solution to remove surfactant these important CAs. and, in the case of the PVA solution, mechanically A. Amorphous carbon reinforce the aerogel. Both freeze-drying and CPD were used to achieve the final aerogel (Fig. 2). The final Traditional CAs consist of a 3D network of intercon- properties of the CNT aerogel (e.g., electrical conductiv- nected amorphous carbon nanoparticles. Because of their ity, mechanical robustness, and density) were highly , subject to the Cambridge Core terms of use, available at high surface area, mechanical robustness, and electrical dependent on the CNT concentration, PVA concentra- conductivity, they have been widely pursued for elec- tion, and drying method. CPD was shown to give trical energy storage, hydrogen storage, desalination, and consistently better results overall than freeze-drying, catalysis. CAs were originally derived from organic and increasing CNT concentration gave electrical con- aerogels synthesized via the polycondensation of resor- ductivities as high as 1 S/cm when no PVA was used. cinol with formaldehyde.6 These resorcinol–formaldehyde 30 Sep 2021 at 12:00:47 This is quite remarkable given that a traditional CA of (RF) aerogels were then fired under inert gas to produce , on – equal density would be expected to exhibit an order of RF-derived CAs (Fig. 1). This RF sol gel route to magnitude lower conductivity. However, the pure CNT sp2-hybridized carbon gels was a key development in fi aerogels were fragile because they rely solely on van der the evolutionary