Continuous intercalation compound fibers of bromine wires and aligned CNTs for high-performance conductors
Graphical abstract
Introduction
Carbon nanotube fibers (CNTFs) are amongst the most promising lightweight conductors for the electrification of mobility. Continuous improvements in CNT synthesis and assembly over the last decade have led to macroscopic CNTFs with greater alignment and better packing of long CNTs [[1], [2], [3], [4], [5]] resulting in bulk tensile mechanical properties above the best synthetic fibers and longitudinal electrical conductivity approaching the theoretical limit for CNT bundles 1–3 MS/m [6]. Introduction of small molecules that produce a transfer of free charge carriers to the CNT fibers can further increase electrical conductivity even by over an order of magnitude and surpass most metallic materials on a mass basis [6].
Intercalation is a particularly appealing non-covalent doping strategy [7,8], where small molecules/ions are hosted in nanotube bundles, and a charge transfer process takes place between the two, which increases electrical conductivity. Despite a wealth of background on graphite intercalation compounds (GICs) [7], including examples of conductivity as high as 65 MS/m [9], other forms of CNTF doping such as surface adsorption and substitutional doping have received more attention [10]. This is largely because of limited availability of CNTFs with a suitable structure for intercalation, i.e., combining high internal organization as ordered bundles and high conjugation of the CNTs. A notable exception is aligned fibers of high purity CNTs spun from liquid crystal solutions in superacids, which present high internal order over macroscopic lengths. These fibers contain acid molecules trapped in bundles during wet-spinning, and which remain stable under ambient conditions [11]. The acid acts as a p-type dopant and fortuitously increases fiber conductivity by up to an order of magnitude without adversely affecting fiber tensile properties [12]. After removal of residual acid, these liquid crystal-spun CNT fibers are also prone to doping [13,14]. Intercalation has also been studied on single-walled carbon nanotubes (SWCNTs) in powder form [8,15] and on porous fibers of collapsed CNTs [16].
Despite an increasing number of reports on doping of CNTFs, there is limited understanding of intercalated CNTs and the methods to improve the electrical properties of CNTFs beyond those of Cu. This requires, for example, identifying structural features leading to intercalation over other doping morphologies, determination of the different dopant configurations (e.g., in neutral or ionic form) and the crystal structure of the intercalated compounds, together with reliable methods for quantification of charge transfer, and separation of contributions to the bulk transport properties.
This work sets out to address the aforementioned issues. We prepare CNTF intercalated compounds with long-range order and study most of these aspects. The main objective is to shed light into the structure of intercalated CNTF and the effects of intercalation on their bulk properties. Through direct HRTEM observation and 2D WAXS measurements, we show the formation of ordered bromine domains intercalated as “wires” along the interstitial channels between closed-packed double wall carbon nanotubes (DWCNTs) across the whole section of the fiber. We determine the longitudinal and transverse transport properties of the fibers and their bulk tensile mechanical properties and rationalize them based on the observed intercalation structure with preserved inter-tube separation. These Br-intercalated CNTFs have 45 times the specific strength of Cu, and 70% of its mass-normalized electrical conductivity (4560 S m2/kg, or equivalent to 4.56 MS/m over specific gravity (SG)) at ambient temperature.
Section snippets
Materials
The CNTF and CNT tapes used in this study were purchased from DEXMAT. They are composed of highly aligned DWCNTs and spun as reported elsewhere [1]. The samples were annealed at 1400 °C in an inert atmosphere to remove residual acid. The annealing procedure and evidence of acid removal are reported in a previous paper [17]. Fibres of DWCNTs were chosen due to availability from suppliers, and because they have been shown to produce superior bulk properties to those based on SWCNTs, for example [
Structure of the intercalation compound
In this study we use continuous macroscopic fibers of DWCNTs wet-spun from liquid crystalline solutions in superacids. These fibers are characterized by a very low polydispersity in terms of CNT diameters and number of layers, and an extremely high degree of alignment. The constituent DWCNTs are thus closely packed into essentially a semi-continuous bundle. After acid removal, these fibers were intercalated with bromine using a vapor process.
Following exposure to Br2 vapor, Br is introduced in
Conclusions
This work presents a new form of intercalation compound consisting of long-range ordered bromine species extended along the interstitial channels between hexagonally-closed packed DWCNTs, with a stoichiometry close to C17Br. The tribromide intercalate forms supramolecular chains, analogous to “wires”, that can be resolved by WAXS. Simulations show that the formation of anions occur over a bromine concentration threshold, and that charge transfer is maximized for a 1:1 ratio of Br2/
CRediT authorship contribution statement
Cristina Madrona: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. Seungki Hong: Data curation, Formal analysis, Investigation, Methodology. Dongju Lee: Data curation, Formal analysis, Investigation, Methodology. Julia García-Pérez: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. José Manuel Guevara-Vela: Conceptualization, Software, Data
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.
Acknowledgments
The authors are grateful for generous financial support provided by the European Union Seventh Framework Program under grant agreement 101045394 (ERC-UNIYARNS), by the Air Force Office of Scientific Research of the US (NANOYARN FA9550-18-1-7016), by The Carbon Hub, and by "Comunidad de Madrid" FotoArt-CM project (S2018/NMT-4367). This work was partially supported by grants from the Korea Institute of Science and Technology institutional program (2Z06694, 2E31902). Additional support from the
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