The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions

Carbon nanotubes (CNTs) are prone to aggregation and precipitation in water due to their high hydrophobicity and aspect ratio. However, the addition of dissolved organic matter (DOM) has been reported to disperse and stabilize certain CNTs, suggesting the potential transport and bioavailability of CNTs in natural aqueous environments. For a better understanding of the CNT-DOM interaction, five multiwalled CNTs with outer diameters of <10 (MWCNT10), 10-20 (MWCNT20), 20-40 (MWCNT40), 40-60 (MWCNT60), and 60-100 nm (MWCNT100) were used to investigate their sorptive and suspension behavior in tannic acid (TA, as a DOM surrogate) solution; the effects of ionic strength and pH on the TA-CNT interaction were examined. Suspension of MWCNTs sharply improved with increasing TA concentration and leveled off at an initial TA concentration ca. 20 mg/L. Suspension of MWCNTs was in the order of MWCNT40 > MWCNT60 > MWCNT20 > MWCNT100 > MWCNT10. The TA-stabilized CNTs were stable within pH 5-11, while they quickly precipitated at pH < 5. Different ions (Na⁺, Mg²⁺, Ca²⁺, or La³⁺) aggregated the stabilized CNTs, with a critical coagulation concentration exponentially correlated to ionic valence. Changes of steric repulsion and electrostatic interaction with the added TA could account for the variation of CNT stability.     

Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites

A homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in syndiotactic polystyrene (sPS) is obtained by a simple solution dispersion procedure. MWCNTs were dispersed in N-methyl-2-pyrrolidinone (NMP), and sPS/MWCNT composites are prepared by mixing sPS/NMP solution with MWCNT/NMP dispersion. The composite structure is characterized by scanning electron microscopy and transmission electron microscopy. The effect of MWCNTs on sPS crystallization and the composite properties are studied. The presence of MWCNTs increases the sPS crystallization temperature, broadens the crystallite size distribution and favors the formation of the thermodynamically stable β phase, whereas it has little effect on the sPS γ to α phase transition during heating. By adding only 1.0 wt.% pristine MWCNTs, the increase in the onset degradation temperature of the composite can reach 20 °C. The electrical conductivity is increased from 10^{−10∼−16} (neat sPS) to 0.135 S m⁻¹ (sPS/MWCNT composite with 3.0 wt.% MWCNT content). Our findings provide a simple and effective method for carbon nanotube dispersion in polymer matrix with dramatically increased electrical conductivity and thermal stability.     

Development of electrical conductivity with minimum possible percolation threshold in multi-wall carbon nanotube/polystyrene composites

A method is reported that involves the bulk polymerization of styrene monomer in the presence of multi-wall carbon nanotubes (MWCNTs) and polystyrene (PS) beads, for the preparation of MWCNT/PS conducting composites with a significantly lower (0.08 wt.% MWCNT) percolation threshold than previously reported. Thus, the conductivities of 7.62 × 10⁻⁵ and 1.48 × 10⁻³ S cm⁻¹ were achieved in the MWCNT/PS composites through homogeneous dispersion of 0.08 and 0.26 wt.% CNTs, respectively in the in situ polymerized PS region by using 70 wt.% PS beads during the polymerization. The extent of dispersion and location of the MWCNTs in the PS matrix has been investigated with a scanning and transmission electron microscopy. The conductivity of the composites was increased with increasing wt.% of the PS beads at a constant CNT loading, indicating the formation of a more continuous network structure of the CNTs in PS matrix.     

Ruthenium electrodeposition on silicon from a room-temperature ionic liquid

Electrochemical deposition of ruthenium on n-type silicon from an ionic liquid is reported for the first time. The study was performed by dissolving ruthenium(III) chloride in a 1-butyl-3-methyl imidazolium hexafluorophosphate (BMIPF₆) room-temperature ionic liquid (RTIL). Cyclic voltammetry (CV) studies demonstrate reduction and stripping peaks at −2.1 and 0.2 V vs. Pt quasi-reference, corresponding to the deposition and dissolution of ruthenium, respectively. Metallic Ru films of ∼100 nm thickness have been deposited and were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).     

Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids

Various ionic liquids (ILs) were prepared via metathesis reaction from two kinds of 1-(2-hydroxyethyl)-3-methyl imidazolium ([HEMIm]⁺) and N-(2-hydroxyethyl)-N-methyl morphorinium ([HEMMor]⁺) cations and three kinds of tetrafluoroborate ([BF₄]⁻), bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) and hexafluorophosphate ([PF₆]⁻) anions. All the [HEMIm]⁺ derivatives were in a liquid state at room temperature. In particular, [HEMIm][BF₄] and [HEMIm][TFSI] showed no possible melting point from −150 °C to 200 °C by DSC analysis, and their high thermal stability until 380-400 °C was verified by TGA analysis. Also, their stable electrochemical property (electrochemical window of more than 6.0 V) and high ionic conductivity (0.002-0.004 S cm⁻¹) further confirm that the suggested ILs are potential electrolytes for use in electrochemical devices. Simultaneously, the [HEMMor]⁺ derivatives have practical value in electrolyte applications because of their easy synthesis procedures, cheap morpholinium cation sources and possibilities of high Li⁺ mobility by oxygen group in the morpholinium cation. However, [HEMMor]⁺ derivatives showing high viscosity usually had lower ionic conductivities than [HEMIm]⁺ derivatives.     

Electrochemical behavior of glassy carbon electrodes modified by multi-walled carbon nanotube/surfactant films in a buffer solution and an ionic liquid

The electrochemical behavior of glassy carbon (GC) electrodes coated with multi-walled carbon nanotube (MWCNT)/surfactant films was studied in an ionic liquid and a phosphate buffer solution (pH = 6.86), using cyclic voltammetry. The dispersion of MWCNTs in different media was investigated by scanning and transmission electron microscopy. Cast films of MWCNT/zwitterionic dodecyldimethylamine oxide on a GC electrode show a typical redox couple in phosphate buffer solution, which is better than that of MWCNT/anionic sodium dodecyl sulfate and cationic alkyltrimethylammonium bromide. However in the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), the GC electrode modified by MWCNT/cationic surfactant films shows a well-defined irreversible reduction of MWCNTs. The cyclic voltammograms clearly show that the surfactant hydrophilic group plays an important role in the electrochemical behavior of the MWCNTs. The electrolytes also have an important effect. In an ionic liquid, the strong binding of the ionic liquid cations with the MWCNTs may change the structure of the modified films and lead to changes of electrochemical behavior.     

Modified carbon nanotube composites with high dielectric constant, low dielectric loss and large energy density

Flexible dielectric polystyrene based composites containing multi-walled carbon nanotubes (MWCNTs) were reported. The MWCNTs were coated with polypyrrole (PPy) by an inverse microemulsion polymerization. Transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy indicated that the MWCNTs were coated with PPy. Our composites presented a stable high dielectric constant (∼44), rather low loss (<0.07), and large energy density (up to 4.95 J cm⁻³). The largely-enhanced dielectric performance originates from the organic shell PPy, which not only ensure good dispersion of MWCNTs in the polymer matrix but also screen charge movement to shut off leakage current. Such MWCNT composites can be used to store charge and electrical energy and play a key role in modern electronics and electric power systems.     

The cutting of multi-walled carbon nanotubes and their strong interfacial interaction with polyamide 6 in the solid state

The cutting of multi-walled carbon nanotubes (MWCNTs) using solid state shear milling (S³M) method and their strong interfacial interaction with polyamide 6 (PA6) in the solid state were studied. Transmission electron microscopy showed that after milling, the CNTs were greatly reduced in length, and disentangled, being straighter with open ends. Fourier transform infrared spectra and differential scanning calorimeter analysis indicated the existence of strong interfacial interactions between MWCNTs and PA6 of the pan-milled PA6/CNTs powder. It was further quantified by thermogravimetric analysis that about 30 wt.% of PA6 formed a strong combining force with CNTs after pan-milling. The mechanism of cutting CNTs and the reason for their strong interfacial interactions with PA6 in the solid state were discussed. A fine and homogeneous dispersion of CNTs throughout PA6 matrix was observed by scanning electron microscopy. The tensile properties of the composites prepared by the S³M method were significantly improved compared to those of pure PA6 and composites prepared by conventional melt mixing. Upon incorporation of only 1.5 wt.% MWCNTs, the tensile modulus of PA6 was enhanced from 2448 MPa to 4439 MPa, by about 80%, and the tensile strength was increased by about 23%.     

Ionic liquid mediated magnesium ion conduction in poly(ethylene oxide) based polymer electrolyte

Magnesium ion conduction in poly(ethylene oxide) (PEO) based polymer electrolyte incorporated with room temperature ionic liquid (RTIL) is reported. The electrolyte films comprise the PEO complexed with magnesium trifluoromethanesulfonate (or magnesium triflate) added with different amount of ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf). The polymer electrolyte with ∼50 wt.% of ionic liquid offers a maximum electrical conductivity of ∼5.6 × 10⁻⁴ S cm⁻¹ at room temperature (∼25 °C) with improved thermal and electrochemical stabilities. The Mg²⁺ ion conduction in the PEO-complex is confirmed from cyclic voltammetry, impedance and transport number measurements. A significant increase in the Mg²⁺ ion transport number (tMg2+) is observed with increasing content of the ionic liquid in PEO–Mg salt complex and the maximum value is obtained to be ∼0.45 for ∼50 wt.% of ionic liquid. The interaction of imidazolium cations with ether oxygen of PEO, as evidenced from FTIR and Raman studies, play an important role in the substantial enhancement in the tMg2+ value.     

Effect of zwitterionic salt on the electrochemical properties of a solid polymer electrolyte with high temperature stability for lithium ion batteries

In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N′-methyl imidazolium tetrafluoroborate (EMIBF₄), LiBF₄ and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 °C and high room temperature ionic conductivity near to 10⁻³ S cm⁻¹. Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10⁻³ S cm⁻¹ at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol⁻¹. The charge-discharge performance of a Li₄Ti₅O₁₂/SPE/LiCoO₂ cell indicates the potential application of the as-prepared SPE in lithium ion batteries.     

Enhanced conductivity in polybenzoxazoles doped with carboxylated multi-walled carbon nanotubes

Two representative polybenzazoles, poly(p-phenylene benzobisoxazole) (PBO) and poly(2,5-benzoxazole) (ABPBO), have been used as matrix materials for fabricating electrically conducting nanocomposite films. In this strategy, pristine multi-walled carbon nanotubes (MWCNTs) were first treated with nitric acid to form carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). Subsequently, MWCNTs-COOH were dispersed efficiently in the methanesulfonic acid (MSA) solution of polybenzazole, sonicated, and then processed into thin films. MWCNTs-COOH in MSA formed an isotropic regime at the concentration of ∼0.1 wt.%. Nanotubes could form net like structures and conductive channels in the polymer matrix to improve electrical conductivity, mechanical properties, and thermal stability. At the MWCNT-COOH composition of 5 wt.%, polybenzazole/MWCNT-COOH composite films exhibited a dramatic enhancement in electrical conductivity by 8 orders of magnitude from ∼10⁻¹² to 1.6 × 10⁻⁴ S cm⁻¹ without significantly sacrificing optical transparency.     

Improved ionic conductivity of nitrile rubber/ionic liquid composites

Polymer electrolytes with high ionic conductivity and good elasticity were prepared by mixing nitrile rubber (poly(acrylonitrile-co-butadiene) rubber; NBR) with ionic liquid, N-ethylimidazolium bis(trifluoromethanesulfonyl)imide (EImTFSI). The NBR/EImTFSI composites were obtained as homogeneous and transparent films when the ionic liquid content was less than 60 wt%. Raman spectroscopy suggested the interaction between nitrile group of NBR and TFSI anion. Sample with ionic liquid content of 50 wt% showed the ionic conductivity of 1.2×10⁻⁵ S cm⁻¹ at 30 °C. Addition of lithium salt to this NBR/EImTFSI composite further enhanced the ionic conductivity to about 10⁻⁴ S cm⁻¹ without spoiling mechanical properties. DSC studies showed two glass transition temperatures for composites indicating microphase separation.     

Enhanced optical absorption cross-section characteristics of multi-wall carbon nanotubes

The optical absorption anisotropy of multi-walled carbon nanotubes (MWCNTs) has been quantitatively characterized through the determination of the absorbance and the degree of linear polarization. A model considering the orientation of the MWCNTs and the sensitivity to both co-polarized and cross-polarized radiation, through electric field depolarization effects, was used to understand the experimental results. The MWCNT optical absorption cross-sections for both the co-polarized radiation (∼0.1 Å²/atom) and the cross-polarized radiation (∼0.05 Å²/atom) were found to be much larger than for single-walled carbon nanotubes. Our results indicate the promise of MWCNTs for applications involving radiation absorption.     

Polyethylene multiwalled carbon nanotube composites

Polyethylene (PE) multiwalled carbon nanotubes (MWCNTs) with weight fractions ranging from 0.1 to 10 wt% were prepared by melt blending using a mini-twin screw extruder. The morphology and degree of dispersion of the MWCNTs in the PE matrix at different length scales was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). Both individual and agglomerations of MWCNTs were evident. An up-shift of 17 cm⁻¹ for the G band and the evolution of a shoulder to this peak were obtained in the Raman spectra of the nanocomposites, probably due to compressive forces exerted on the MWCNTs by PE chains and indicating intercalation of PE into the MWCNT bundles. The electrical conductivity and linear viscoelastic behaviour of these nanocomposites were investigated. A percolation threshold of about 7.5 wt% was obtained and the electrical conductivity of PE was increased significantly, by 16 orders of magnitude, from 10⁻²⁰ to 10⁻⁴ S/cm. The storage modulus (G′) versus frequency curves approached a plateau above the percolation threshold with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behaviour. The ultimate tensile strength and elongation at break of the nanocomposites decreased with addition of MWCNTs. The diminution of mechanical properties of the nanocomposites, though concomitant with a significant increase in electrical conductivity, implies the mechanism for mechanical reinforcement for PE/MWCNT composites is filler-matrix interfacial interactions and not filler percolation. The temperature of crystallisation (T_c) and fraction of PE that was crystalline (F_c) were modified by incorporating MWCNTs. The thermal decomposition temperature of PE was enhanced by 20 K on addition of 10 wt% MWCNT.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

Gel polymer electrolyte based on poly(acrylonitrile-co-styrene) and a novel organic iodide salt for quasi-solid state dye-sensitized solar cell

A gel polymer electrolyte based on poly(acrylonitrile-co-styrene) as polymer matrix and N-methyl pyridine iodide salt as I⁻ source was prepared. Controlling the concentration of polymer matrix of poly(acrylonitrile-co-styrene) at 17.5 wt.%, mixing the binary organic solvents mixture ethylene carbonate and propylene carbonate with 6:4 (w/w), and the concentration of N-methyl pyridine iodide and iodine with 0.5 and 0.05 M, respectively, the gel polymer electrolyte attains the maximum ionic conductivity (at 30 °C) of 4.63 mS cm⁻¹. Based on the gel polymer electrolyte, a quasi-solid state dye-sensitized solar cell was fabricated and its overall energy conversion efficiency of light-to-electricity of 3.10% was achieved under irradiation of 100 mW cm⁻².     

Microwave-polyol synthesis of nanocrystalline ruthenium oxide nanoparticles on carbon nanotubes for electrochemical capacitors

The ruthenium oxide nanoparticles dispersed on multi-wall carbon nanotubes (CNTs) were successfully synthesized via microwave-polyol process combined with forced hydrolysis without additional thermal oxidation or electrochemical oxidation treatment. The HRTEM, Raman spectra and TGA curve indicate that CNTs were uniformly coated with crystalline and partially hydrous RuO₂·0.64H₂O nanoparticles of 2 nm diameter and the loading amount of ruthenium oxide in the composite could be controlled up to 70 wt.%. The specific capacitance was 450 Fg⁻¹ of ruthenium oxide/CNT composite electrode with 70 wt.% ruthenium oxide at the potential scan rate of 10 mV s⁻¹ and it decreased to 362 Fg⁻¹ by 18% at 500 mV s⁻¹. The specific capacitance of ruthenium oxide in the composite was 620 Fg⁻¹ of ruthenium oxide at 10 mV s⁻¹. The ruthenium oxide nanoparticles in ruthenium oxide/CNT nanocomposite electrode had a high ratio of outer charge to total charge of 0.81, which confirmed its high-rate capability of the composite through the preparation of the nano-sized ruthenium oxide particles on the external surface of CNTs.     

Direct electrochemistry of guanosine on multi-walled carbon nanotubes modified carbon ionic liquid electrode

A multi-walled carbon nanotubes (MWCNTs) modified carbon ionic liquid electrode (CILE) was fabricated and used to investigate the electrochemical behavior of guanosine. CILE was prepared by mixing hydrophilic ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄), graphite powder and liquid paraffin together. The fabricated MWCNTs/CILE showed great electrocatalytic ability to the oxidation of guanosine and an irreversible oxidation peak appeared at 1.067 V (vs. SCE) with improved peak current. The electrochemical behavior of guanosine on the MWCNTs/CILE was carefully studied by cyclic voltammetry and the electrochemical parameters such as the charge transfer coefficient (α) and the electrode reaction standard rate constant (k_s) were calculated with the result as 0.66 and 2.94 × 10⁻⁴ s⁻¹, respectively. By using differential pulse voltammetry (DPV) as the detection method, a linear relationship was obtained between the oxidation peak current and the guanosine concentration in the range from 1.0 × 10⁻⁷ to 4.0 × 10⁻⁵ mol/L with the detection limit as 7.8 × 10⁻⁸ mol/L (3σ). The common coexisting substances showed no interferences to the guanosine detection and the modified electrode showed good ability to distinguish the electrochemical response of guanosine and adenosine.     

Preparation of low-platinum-loading electrocatalysts using electroless deposition method for proton exchange membrane fuel cell systems

One promising preparative method that offers the potential for improved platinum (Pt) dispersion of electrocatalysts is electroless deposition (ED). In this study, the effects of multiwalled carbon nanotubes (MWCNTs) pretreatment and synthesis procedure on properties of the four catalysts, synthesized by ED method, have been considered. The results of energy-dispersive X-ray spectroscopy (EDS), X-ray dot-mapping, X-ray fluorescence (XRF) and cyclic voltammetry (CV) analyses showed that using palladium (Pd) precursor during two-step sensitization-activation coating procedure gives uniform Pt particles distribution on MWCNTs with low aggregation and high specific surface area (∼80 m² g⁻¹). In addition, to investigate the performance of the synthesized catalysts in experimental fuel cell system, thin-film method was used to fabricate the membrane electrode assemblies (MEAs). Obtaining the polarization curves for the fabricated MEAs (Pt loading ∼0.4 mg cm⁻²) and a commercial MEA (ElectroChem, Pt loading ∼1 mg cm⁻²) demonstrated that the catalyst prepared by two-step sensitization-activation coating procedure possesses a good performance despite of its lower Pt content.     

A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors

A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flow-directed assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g⁻¹ at 0.1 A g⁻¹ and a good rate capability (49% capacity retention at 50 A g⁻¹), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices.