Enhanced Thermal Conductivity of Individual Polymeric Nanofiber Incorporated with Boron Nitride Nanotubes

Thermal conductivity of individual polyvinylpyrrolidone (PVP) nanofibers embedding boron nitride nanotube (BNNT) fillers has been measured. The PVP nanofibers were electrospun on suspended microdevices in order to better understand the effect of BNNT fillers on the thermal conductivity of polymeric nanofibers. Various material characterization methods provided evidence that ketone group in the PVP interacted with the surface of BNNTs via strong intermolecular forces, thereby resulting in an effective heat transfer between the polymer matrix and BNNTs. The individual PVP nanofiber containing 30 wt % of BNNTs exhibited approximately 2-fold higher thermal conductivity than that of the bulk PVP

Role of Intertube Interactions in Double- and Triple-Walled Carbon Nanotubes

Resonant Raman spectroscopy studies are performed to access information about the intertube interactions and wall-to-wall distances in double- and triple-walled carbon nanotubes. Here, we explain how the surroundings of the nanotubes in a multiwalled system influence their radial breathing modes. Of particular interest, the innermost tubes in double- and triple-walled carbon nanotube systems are shown to be significantly shielded from environmental interactions, except for those coming from the intertube interaction with their own respective host tubes. From a comparison of the Raman results for bundled as well as individual fullerene-peapod-derived double- and triple-walled carbon nanotubes, we observe that metallic innermost tubes, when compared to their semiconducting counterparts, clearly show weaker intertube interactions. Additionally, we discuss a correlation between the wall-to-wall distances and the frequency upshifts of the radial breathing modes observed for the innermost tubes in individual double- and triple-walled carbon nanotubes. All results allow us to contemplate fundamental properties related to DWNTs and TWNTs, as for example diameter- and chirality-dependent intertube interactions. We also discuss differences in fullerene-peapod-derived and chemical vapor deposition grown double- and triple-walled systems with the focus on mechanical coupling and interference effects

Thermal-Treatment-Induced Enhancement in Effective Surface Area of Single-Walled Carbon Nanohorns for Supercapacitor Application

We investigated the importance of the specific effective surface area through a detailed study on the relationship between electrical conductivity of single-walled carbon nanohorns (SWCNHs) and accessibility of the electrolyte ions in the SWCNH-based supercapacitor. After heat treatment of the SWCNHs, the ratio of sp²/sp³ carbons dramatically increased, suggesting an enhanced electrical conductivity of the SWCNHs. Even though the specific surface area (SSA) slightly decreased by 16% as a result of heat treatment, the specific capacitance per SSA of the SWCNH electrode remarkably increased from 22 to 47 μF cm^–2. Such a result indicates an explicit increase in accessible effective surface area by electrolyte ions. Our result clearly showed that a higher degree of utilization for the interstitial pore of SWCNHs by solvated ions is a key factor in achieving a high volumetric capacitance of SWCNH-based supercapacitors

Dual-Functional Additive for Solid Polymer Electrolytes for Enabling Highly Safe and Long-Life All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) are a promising alternative to carbonate-based liquid electrolytes for realizing flexible lithium batteries with high energy density and safety owing to their advantages such as lightweight, thinness, no leakage of electrolytes, excellent flexibility and processability, and good compatibility with Li-metal electrodes. However, SPEs present new challenges such as poor ionic conductivity and electrochemical stability as well as flammability, which are still a concern even though they are less flammable than liquid electrolytes. Herein, we demonstrate the dual functionalities of dimethyl methylphosphonate (DMMP) in facilitating Li ion migration and improving the flame retardancy of a poly(ethylene oxide) (PEO)-based polymer electrolyte. It acts as a plasticizer that aids the dissociation of Li salts and alleviates the binding energy between ethylene oxide (EO) groups and Li ions by counterbalancing the binding force between EOs and Li ions through the formation of binding interactions of DMMP molecules and Li ions. This significantly facilitates Li ion migration within the polymer electrolyte. Consequently, the prepared SPE exhibited improved ionic conductivity (1.29 × 10–5 S cm–1 at 25 °C), Li transference number (0.46), and oxidative stability (>4.3 V). The fabricated Li/Li symmetric cell maintained stable cycling performance over 500 cycles with low overpotential (41 mV) without short circuit. Importantly, the LiFePO4(LFP)/Li battery exhibited a high discharge capacity of 134.1 mAh g–1 with outstanding capacity retention of 95.4% after 400 cycles at 1C and excellent rate capability (123.3 mAh g–1 at 2C). Furthermore, stable cycling was confirmed to be possible at an extended voltage range (2.5–4.1 V) and low operating temperature (45 °C). Moreover, DMMP effectively suppressed combustion of the polymer electrolyte owing to its strong flame retardancy arising from the propensity to capture active radicals

Exposed Edge Planes of Cup-Stacked Carbon Nanotubes for an Electrochemical Capacitor

The end sites of graphitic planes and their catalytic, chemical, physical, and electrochemical roles have been a longstanding issue in the surface chemistry of carbon science. In this study, complete exposure of the active edge sites on the outer surface of catalytically grown cup-stacked carbon nanotubes is accomplished using a conventional exfoliation method, and its intrinsic contribution to the improvement of the electrochemical behavior in an electrochemical capacitor is demonstrated. The significant enhancement in the capacitance of the nanotubes after exfoliation, occurring without a distinctive change in pore structure, was confirmed with the exposure of the electrochemically active edge sites thus being able to accumulate more charge. Such active sites make nanotubes useful in the fabrication of high-performance electrochemical capacitors, catalysts, supporting materials for catalysts, and photocurrent generators in photochemical cells

Defect-Assisted Heavily and Substitutionally Boron-Doped Thin Multiwalled Carbon Nanotubes Using High-Temperature Thermal Diffusion

Carbon nanotubes have shown great potential as conductive fillers in various composites, macro-assembled fibers, and transparent conductive films due to their superior electrical conductivity. Here, we present an effective defect engineering strategy for improving the intrinsic electrical conductivity of nanotube assemblies by thermally incorporating a large number of boron atoms into substitutional positions within the hexagonal framework of the tubes. It was confirmed that the defects introduced after vacuum ultraviolet and nitrogen plasma treatments facilitate the incorporation of a large number of boron atoms (ca. 0.496 atomic %) occupying the trigonal sites on the tube sidewalls during the boron doping process, thus eventually increasing the electrical conductivity of the carbon nanotube film. Our approach provides a potential solution for the industrial use of macro-structured nanotube assemblies, where properties, such as high electrical conductance, high transparency, and lightweight, are extremely important

Formation of Nitrogen-Doped Graphene Nanoribbons <i>via</i> Chemical Unzipping

In this work, we carried out chemical oxidation studies of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. Reaction parameters such as oxidation reaction, reaction time, the oxidizer to nanotube mass ratio, and the temperature were varied, and their effect was carefully analyzed. The presence of nitrogen atoms makes CNx-MWCNTs more reactive toward oxidation when compared to undoped multiwalled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWCNTs results in the unzipping of large diameter nanotubes and the formation of a disordered oxidized carbon coating on small diameter nanotubes. The nitrogen content within unzipped CNx-MWCNTs decreased as a function of the oxidation time, temperature, and oxidizer concentration. By controlling the degree of oxidation, the N atomic % could be reduced from 1.56% in pristine CNx-MWCNTs down to 0.31 atom % in nitrogen-doped oxidized graphene nanoribbons. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWCNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded nitrogen-doped graphene nanoribbons (N-GNRs). The thermal reduction at relatively low temperature (300 °C) results in graphene nanoribbons with 0.37 atom % of nitrogen. This method represents a novel route to preparation of bulk quantities of nitrogen-doped unzipped carbon nanotubes, which is able to control the doping level in the resulting reduced GNR samples. Finally, the electrochemical properties of these materials were evaluated

Clean Nanotube Unzipping by Abrupt Thermal Expansion of Molecular Nitrogen: Graphene Nanoribbons with Atomically Smooth Edges

We report a novel physicochemical route to produce highly crystalline nitrogen-doped graphene nanoribbons. The technique consists of an abrupt N₂ gas expansion within the hollow core of nitrogen-doped multiwalled carbon nanotubes (CN_<i>x</i>-MWNTs) when exposed to a fast thermal shock. The multiwalled nanotube unzipping mechanism is rationalized using molecular dynamics and density functional theory simulations, which highlight the importance of open-ended nanotubes in promoting the efficient introduction of N₂ molecules by capillary action within tubes and surface defects, thus triggering an efficient and atomically smooth unzipping. The so-produced nanoribbons could be few-layered (from graphene bilayer onward) and could exhibit both crystalline zigzag and armchair edges. In contrast to methods developed previously, our technique presents various advantages: (1) the tubes are not heavily oxidized; (2) the method yields sharp atomic edges within the resulting nanoribbons; (3) the technique could be scaled up for the bulk production of crystalline nanoribbons from available MWNT sources; and (4) this route could eventually be used to unzip other types of carbon nanotubes or intercalated layered materials such as BN, MoS₂, WS₂, <i>etc.</i

Clean Nanotube Unzipping by Abrupt Thermal Expansion of Molecular Nitrogen: Graphene Nanoribbons with Atomically Smooth Edges

We report a novel physicochemical route to produce highly crystalline nitrogen-doped graphene nanoribbons. The technique consists of an abrupt N₂ gas expansion within the hollow core of nitrogen-doped multiwalled carbon nanotubes (CN_<i>x</i>-MWNTs) when exposed to a fast thermal shock. The multiwalled nanotube unzipping mechanism is rationalized using molecular dynamics and density functional theory simulations, which highlight the importance of open-ended nanotubes in promoting the efficient introduction of N₂ molecules by capillary action within tubes and surface defects, thus triggering an efficient and atomically smooth unzipping. The so-produced nanoribbons could be few-layered (from graphene bilayer onward) and could exhibit both crystalline zigzag and armchair edges. In contrast to methods developed previously, our technique presents various advantages: (1) the tubes are not heavily oxidized; (2) the method yields sharp atomic edges within the resulting nanoribbons; (3) the technique could be scaled up for the bulk production of crystalline nanoribbons from available MWNT sources; and (4) this route could eventually be used to unzip other types of carbon nanotubes or intercalated layered materials such as BN, MoS₂, WS₂, <i>etc.</i

Carbon Nanotube Core Graphitic Shell Hybrid Fibers

A carbon nanotube yarn core graphitic shell hybrid fiber was fabricated <i>via</i> facile heat treatment of epoxy-based negative photoresist (SU-8) on carbon nanotube yarn. The effective encapsulation of carbon nanotube yarn in carbon fiber and a glassy carbon outer shell determines their physical properties. The higher electrical conductivity (than carbon fiber) of the carbon nanotube yarn overcomes the drawbacks of carbon fiber/glassy carbon, and the better properties (than carbon nanotubes) of the carbon fiber/glassy carbon make up for the lower thermal and mechanical properties of the carbon nanotube yarn <i>via</i> synergistic hybridization without any chemical doping and additional processes