Elsevier

Carbon

Volume 184, 30 October 2021, Pages 811-820
Carbon

Research Paper
The shape tunable gelatin/carbon nanotube wet-gels for complex three-dimensional cellular structures with high elasticity

https://doi.org/10.1016/j.carbon.2021.08.086Get rights and content

Abstract

Porous three-dimensional (3D) assemblies of nanocarbons with large surface areas and tunable physical properties are attractive for a wide range of applications, such as energy storage electrodes, pollution adsorption, and thermal insulators. However, the full utilization of these materials is impeded by two challenges: forming 3D robust porous network structures that retain the intrinsic properties of the nanocarbon and achieving high shape-tunability to create structures with desired shapes. To address these challenges, gelatins with zwitterionic and hydrogen-bonded helical structure were considered a bio-derived binder to modulate the structural integrity of carbon nanotube (CNT) network which consists of overlapping nanotubes held by weak van der Waals attraction. The resulting CNT/gelatin wet-gels have favorable rheological properties, enabling 3D printing into complex porous architectures. Moreover, the 3D-printed complex wet-gel structure was successfully transformed into elastic and hierarchical porous structures consisting of micro-, meso-, and macro-scale pores with large specific surface areas of ∼988 m2 g−1 and excellent mechanical stability without permanent deformation even at 85% compressive strain. The resulting high shape-tunability, high elasticity, and large surface area are attributed to the reinforcement of the existing nanotube cross-linking points strengthened by the gelatin-derived graphitic layer coating without deteriorating the intrinsic properties of CNTs.

Graphical abstract

Thermosensitive gelatin association into carbon nanotube (CNT) dispersions strengthens the structural integrity of the nanotube network and achieves favorable rheological properties for transforming the wet-gels into complex porous architectures using 3D printing. Furthermore, the gelatin-derived graphene-coating around nanotube junctions transforms mechanically fragile carbon nanotube networks into ultra-compressible cellular structure.

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Introduction

Highly porous ultralight carbon aerogels have unique properties such as low density, high surface area-to-volume ratio, and high strength-to-weight ratio. They have been studied for a wide range of applications, such as energy storage [1], pollution adsorption [2], electromagnetic shielding systems [3], sensors [4] and supercapacitors [5]. Among various carbon allotropes, carbon nanotubes (CNTs) have been widely utilized due to their large aspect ratio, excellent electrical conductivity, high specific surface area (SSA), and exceptional mechanical stiffness [6,7]. High-quality and multifunctional CNT aerogels have been developed [[8], [9], [10], [11], [12], [13], [14]]. Typically, porous networks are fabricated using a combination of chemically functionalized nanotubes and a chemical cross-linking agent [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. However, this method creates defect sites on the CNTs, causing reduced electrical and mechanical performance [16]. This method also has a high manufacturing cost and potential environmental concerns. Another technique involves the fabrication of CNT aerogels using isolated CNTs in water with a dispersant. This method enables shape and size tuning without the degradation of intrinsic properties [13]. Specifically, individually dispersed single-walled carbon nanotube (SWCNT) rods can readily form rigid networks through van der Waals attractions due to their small diameters (dcnt ≈ 1 nm) and large length (lcnt ≈ 1000 nm) [24]. Hough et al. [25] investigated the internanotube interaction energy per bond (Ebond) of an associating rigid rod in SWCNT network (Ebond∼40KBT). However, in the case of typical CNTs having relatively larger diameters (dcnt > 1.2 nm) and lengths of several micrometers, rigid wet-gels cannot be readily produced because the curly nature of long CNTs and weak inter-tube bonding (Ebond∼30KBT, see details in supporting information) prevent the formation of a rigid network. Gelatin is an environmental friendly material with a hydrogen-bonded helical structure that can be easily altered by temperature change, that is, the sol-gel transition [[26], [27], [28]]. Moreover, the zwitterionic structure of gelatin enhances its interfacial affinity with the CNT surfaces. There have been many researches for building porous nano-carbon structures using gelatin as the porous carbon precursor [12,[29], [30], [31], [32], [33]]. However, most results show limited physical enhancement due to the inevitable deterioration of intrinsic properties for nano-carbons during the process.

Recently, extrusion-based 3D printing has been widely used to construct specific architectures, including hierarchical structures. Polymers [34], metals [35], ceramics [36] and nano-carbons [5,37,38] have been used as printing ink for additive manufacturing. Gelatin/CNT (Gel-CNT) wet-gels have a high potential as an ink material because they are scalable and easily processable. Herein, the well-structured Gel-CNT ink was smoothly extruded through a nozzle while maintaining the CNT network to acquire the desired shape of the resultant aerogels. Thereafter, the extruded Gel-CNT wet-gels were dried by critical point drying (CPD) and then pyrolyzed to obtain graphene-coated CNT (Gr-CNT) aerogels. The Gr-CNT aerogels has excellent mechanical properties, such as compressibility and resilience. Carbonized gelatin also forms micropores during pyrolysis, resulting in an increased SSA and high porosity [32].

This study proposes a simple method for (i) the synthesis of shape-tunable and scalable Gel-CNT wet-gels by using thermo-reversible gelatin and (ii) the formation of graphene-coated CNT aerogels that have various tunable properties such as shape, density, SSA, Young's modulus, and compressibility. First, we investigate the percolation concentration of gelatin using rheological analysis. Thereafter, we determine the optimal gelatin concentration and temperature to enable the use of Gel-CNT wet-gels as inks for 3D printing. Finally, we test the mechanical compressibility of the Gr-CNT aerogels for bulk and 3D-printed aerogels.

Section snippets

Fabrication of Gr-CNT aerogels

Single-walled carbon nanotubes were used to prepare nanotube wet-gels, hydrogels, and aerogels. The CNTs were purchased from OCSiAl (Tuball™) and had diameters of ∼1.6 nm and lengths of ∼5 μm, resulting in an aspect ratio ∼3125. The CNTs were individually dispersed in water using a sodium dodecybenzenesulfonate surfactant (Sigma-Aldrich) [66,67]. A gelatin (Kanto Chemical) aqueous solution was added to the suspended CNT solution at gelatin:CNT weight ratios of 0.5:1, 1:1, 2:1, 5:1, and 10:1

Fabrication of gelatin-derived CNT aerogels

As depicted in Fig. 1, we fabricated thermo-reversible Gel-CNT wet-gels by adding gelatin to CNT networks. We used large SWCNTs with diameters of 1.6 ± 0.4 nm and lengths of ∼5 μm obtained from OCSiAl (Tuball™) [39]. Gelatin (Kanto Chemical Co., Inc., MW = 15–250 kDa) was used as a binder material to strengthen the CNT network junctions and to form stable Gel-CNT wet-gels (Fig. 1a). Gelatin can reversely undergo sol-gel transitions, which can be readily controlled by tuning the gelatin

Conclusions

We propose a simple and eco-friendly method for the fabrication of thermo-reversible porous structures by incorporating gelatin networks into a SWCNT dispersion. Gelatins with zwitterionic and hydrogen-bonded helical structure were considered a bio-derived binder to modulate the structural integrity of CNT network. Thermally reversible Gel-CNT wet-gels form either stable or rigid networks and can be formed into complex porous structures using 3D printing.

Gr-CNT aerogels can be fabricated by the

Author contributions

Y.O. developed and designed the project. M.J., H.J. and H.C. carried out experiments as well as collected and analyzed data. Y.O. and D.S. gave technical and conceptual advice. B.P. was involved in the fabrication of Gr-CNT aerogels. J.K. performed the experimental setup of 3D-printing and technical support. J.Y., T.P., J.Y. and D.S. contributed to the discussion. M.J. and H.J. contributed equally to this work and wrote the manuscript and Y.O. have given approval to the final version of the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Research Fund of the Korea Institute of Materials Science (KIMS) (No. PNK7330 and PNK7670), the National Research Foundation of Korea (NRF) grants funded by the Korea government (Ministry of Science and ICT) (No. NRF-2019R1A2C100375312 and No. NRF-2020M3D1A208162521), and National Research Council of Science & Technology- Korea Institute of Materials Science Postdoctoral Research Fellowship for Young Scientists at KIMS in South KOREA.

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