Enhanced electrical conductivity of nylon 6 composite using polyaniline-coated multi-walled carbon nanotubes as additives

Aggregation in polymer composites is one of the major obstacles in the carbon nanotubes (CNTs) applications. Authentic CNTs are known to have very good electrical conductivity and mechanical strengths. Surface functionalization can avoid aggregation and help dispersion of CNTs, but reduces CNT’s electrical conductivities and mechanical strengths dramatically. It needs a good way to resolve the above dilemma situation; i.e., poor dispersion-good conductivity vs. good dispersion-poor conductivity. Herein, we demonstrate that in-situ polymerized polyaniline (PANI)-coated CNTs have good polymer matrix compatibility, and are superior electrically conductive fillers to nylon 6 composites. In this report, multi-walled CNTs (MWCNTs) were surface-modified with poly(acrylic acids) (PAA), followed by further coating with PANI. The electrical conductivity of (PANI-MWCNTs)-nylon 6 composite thin film was increased from 10⁻¹² to 7.3 × 10⁻⁵ S/cm in the presence of 1 wt% PANI-coated MWCNTs prepared by physical mixing of PANI and PAA-grafted MWCNTs. When in-situ polymerized PANI-coated MWCNTs were added, the electrical conductivity of MWCNTs-nylon 6 composite was further enhanced by 3 orders to be 3.4 × 10⁻² S/cm at the same 1 wt% loading of MWCNTs. Both Fourier-transformed infrared and uv-visible absorption spectra indicate that there exist very strong site-specific charge transfer interactions between the quinoid rings of PANI and MWCNTs, which results in the superior electrical conductivity of MWCNT-nylon 6 composite.     

Fabrication and electrochemical capacitance of hierarchical graphene/polyaniline/carbon nanotube ternary composite film

A film composed of graphene (GN) sheets, polyaniline (PANI) and carbon nanotubes (CNTs) has been fabricated by reducing a graphite oxide (GO)/PANI/CNT precursor prepared by flow-directed assembly from a complex dispersion of GO and PANI/CNT, followed by reoxidation and redoping of the reduced PANI in the composite to restore the conducting PANI structure. Scanning electron microscope images indicate that the ternary composite film is a layered structure with coaxial PANI/CNT nanocables uniformly sandwiched between the GN sheets. Such novel hierarchical structure with high electrical conductivity perfectly facilitates contact between electrolyte ions and PANI for faradaic energy storage and efficiently utilizes the double-layer capacitance at the electrode–electrolyte interfaces. The specific capacitance of the GN/PANI/CNT estimated by galvanostatic charge/discharge measurement is 569 F g⁻¹ (or 188 F cm⁻³ for volumetric capacitance) at a current density of 0.1 A g⁻¹. In addition, the GN/PANI/CNT exhibits good rate capability (60% capacity retention at 10 A g⁻¹) and superior cycling stability (4% fade after 5000 continuous charge/discharge cycles).     

Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance

A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g⁻¹ (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s⁻¹ compared to 115 F g⁻¹ for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg⁻¹ at a power density of 70 kW kg⁻¹.     

Preparation and electrochemical performance of polyaniline-based carbon nanotubes as electrode material for supercapacitor

Nitrogen-containing carbon nanotubes (CNTs) with open end and low specific surface area were prepared via the carbonization of polyaniline (PANI) nanotubes synthesized by a rapidly mixed reaction. On the basis of analyzing the morphologies and structures of the original and carbonized PANI nanotubes, the electrochemical properties of PANI-based CNTs obtained at different temperatures as electrode materials for supercapacitors using 30 wt.% aqueous solution of KOH as electrolyte were investigated by galvanostatic charge/discharge and cyclic voltammetry. It was found that the carbonized PANI nanotubes at 700 °C exhibit high specific capacitance of 163 F g⁻¹ at a current density of 0.1 A g⁻¹ and excellent rate capability in KOH solution. Using X-ray photoelectron spectroscopy measurement the nitrogen state and content in PANI-CNTs were analysed, which could play important roles for the enhancement of electrochemical performance. When the appropriate content of nitrogen is present, the presence of pyrrole or pyridone and quaternary nitrogen is beneficial for the improvement of electron mobility and the wettability of electrode.     

High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate

Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. The effect of substrate is studied by comparing the results obtained using platinum, stainless steel and porous carbon substrates. PANI deposited at 100 mV s⁻¹ sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Experimental variables, namely, concentrations of aniline monomer and H₂SO₄ supporting electrolyte are varied and arrived at the optimum concentrations to obtain a maximum capacitance of PANI. Low concentrations of both aniline and H₂SO₄, which produce PANI at low rates, are desirable. The PANI deposits prepared under these conditions possess network morphology of nanofibrils. Capacitance values as high as 1600 F g⁻¹ are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as high as 45 mA cm⁻² (19.8 A g⁻¹). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling.     

Dielectric spectroscopy on melt processed polycarbonate—multiwalled carbon nanotube composites

Complex permittivity and related AC conductivity measurements in the frequency range between 10⁻⁴ and 10⁷ Hz are presented for composites of polycarbonate (PC) filled with different amounts of multiwalled carbon nanotubes (MWNT) varying in the range between 0.5 and 5 wt%. The composites were obtained by diluting a PC based masterbatch containing 15 wt% MWNT by melt mixing using a Micro Compounder. From DC conductivity measurements it was found that for samples processed at a mixing screw speed of 150 rpm for 5 min, the percolation occurs at a threshold concentration (p_c) between 1.0 and 1.5 wt% MWNT. For concentrations of MWNT near the percolation threshold, the processing conditions (screw speed and mixing time) were varied. The differences in the dispersion of the MWNT in the PC matrix could be detected in the complex permittivity and AC conductivity spectra, and have been explained by changes in p_c. The AC conductivity and permittivity spectra are discussed in terms of charge carrier diffusion on percolation clusters and resistor-capacitor composites.     

Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications

The novel composites of sulfonated multi-walled carbon nanotubes (sMWCNTs) modified polyaniline (PANI) nanorods (PANI/sMWCNTs) were synthesized successfully by in situ oxidative polymerization method in the HClO₄ solution. FTIR and Raman spectra revealed the presence of π–π interaction between the PANI and the sulfonated carbon nanotubes and the formation of charge transfer composites. It was found that the specific capacitance of the PANI/sMWCNT composites was markedly influenced by their morphological structure and the content of PANI which was coated onto the sMWCNT. The specific capacitance of the PANI/sMWCNT composite exhibited a maximum value of 515.2 F g⁻¹ at the 76.4 wt% PANI. The charge–discharge tests showed the PANI/sMWCNT composites possessed a good cycling stability (below 10% capacity loss after 1000 cycles) compared to PANI nanorods.     

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.     

Synthesis and characterization of hybrid composites based on carbon nanotubes

Multi-wall carbon nanotubes (CNTs) were coated with protonated polyaniline (PAni) in situ during the chemical polymerization of aniline. Uniform coating of CNT with PAni was observed by scanning electronic microscopy. An improvement in the covering of CNT composites was found by the association of poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT). The conductivity of composites has been compared with the conductivity of the PAni and CNT. A maximum conductivity of 96.8 S cm⁻¹ has been found for a PAni/PDMcT/CNT composite. High capacitance value (289.4 F g⁻¹) was also determined for this composite, indicating that all materials, PAni, PDMcT and CNT, remain active during the charge–discharge cycling. The reduction in the capacitance after 100 cycles was found to be less than 25%. The capacitive behavior of all materials was confirmed by impedance analysis.     

Latex technology as a simple route to improve the thermal conductivity of a carbon nanotube/polymer composite

A novel route was revealed to reduce the interfacial phonon scattering that was considered as the bottleneck for carbon nanotubes (CNTs) to highly improve the thermal conductivity of CNT/polymer composites. Semicrystalline polyurethane (PU) dispersions were used as latex host to accommodate multi-walled carbon nanotubes (MWCNTs) following the colloidal physics method. The thermal conductivity increased from 0.15 W m⁻¹ K⁻¹ to 0.47 W m⁻¹ K⁻¹, by ∼210%, as the addition of the MWCNTs increases to 3 wt%. The morphology of the composites that was characterized by optical microscopy, scanning electron microscopy and differential scanning calorimeter suggested that the continuous nanotube-rich phase existing in the interstitial space among the latex particles and the crystallites nucleated at the nanotube–polymer interface were the main factors for the effective reduction of interfacial phonon scattering.     

The reaction of polyaniline with iodine

Polyaniline (PANI) (emeraldine) base has been exposed to iodine in an ethanol-water suspension. The conductivity of PANI increased from 10⁻⁹ S cm⁻¹ to 10⁻⁴ S cm⁻¹ already at the molar ratio [I₂]/[PANI] = 1, and a higher content of iodine had only a marginal effect. This is the result of the protonation of PANI base with hydriodic acid, which is a by-product of the oxidation of the emeraldine form of PANI to pernigraniline constitutional units. The reaction is discussed on the basis of FTIR spectra. An alternative reaction, a ring-iodination of PANI, is marginal. Only one iodine atom substitutes a hydrogen atom in about 12 aniline units, even at high iodine concentration, [I₂]/[PANI] = 8. The film of polyaniline base can be used in sensing iodine; after exposure to the iodine vapor, the conductivity of the polyaniline film increased.     

Rheology and properties of melt-processed poly(ether ether ketone)/multi-wall carbon nanotube composites

Poly(ether ether ketone) (PEEK)/multi-wall carbon nanotube (MWNT) composites containing up to 17 wt% filler were prepared using a twin screw extruder. Transmission electron microscopy (TEM) images reveal that the MWNTs were homogeneously dispersed in the PEEK matrix. Linear viscoelastic measurements show that both complex viscosity and moduli increase with increasing MWNT concentration. The storage modulus, G^′ exhibits a dramatic seven order increase in magnitude around 1 wt%, leading to a solid-like low-frequency behaviour at higher loadings; the effect can be attributed to network formation at a rheological percolation threshold. Rheotens measurements show that the melt strength also increases significantly on addition of nanotubes, however, the drawability decreases. An analytical Wagner model was used to calculate the apparent elongational viscosity over a wide range of elongational rates, and to reveal significant increases on addition of MWNTs, with a similar threshold behaviour. The electrical response is also dominated by percolation effects, increasing by nearly 10 orders of magnitude from 10⁻¹¹ to 10⁻¹ S/cm, on the addition of only 2 wt% MWNTs. In contrast, the thermal conductivity and tensile elastic modulus of the composites increased linearly with nanotube content, rising by 130% and 50%, at 17 wt% MWNTs, respectively.     

Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites

Epoxy composites based on aligned CVD-grown multi-wall carbon nanotubes with weight fractions ranging from as low as 0.001 up to 1 wt% were produced. The resulting electrical properties were analysed by AC impedance spectroscopy. The composite conductivity σ follows a percolation scaling law of the form σ∝(p−p_c)^t with the critical mean concentration p_c to form a conductive network of approximately 0.0025 wt% and an exponent, t, of 1.2. The results are compared to previous studies investigating the percolation behaviour of entangled carbon nanotubes and spherical carbon black particles in the same matrix processed under similar conditions. The experimental percolation threshold for the aligned nanotubes used in this study represents the lowest threshold observed for carbon-nanotube-based polymer composites yet reported.     

Electrostatic heterocoagulation of carbon nanotubes and LiCoO2 particles for a high-performance Li-ion cell

Nanotubes were coated on the surface of active LiCoO₂ particles using electrostatic heterocoagulation to enhance the electrochemical properties of a Li-ion battery. Only 0.5 wt% of multiwalled carbon nanotubes (MWCNTs) was added as a conducting agent into the LiCoO₂ cathode, which had a density of 4.0 g cm⁻³. We found that our electrode that was prepared using heterocoagulation with 0.5 wt% of thin MWCNTs maintained a volumetric capacitance of 403 mAh cm⁻¹ after 40 cycles from the initial 624 mAh cm⁻¹, compared with previous result of 310 mAh cm⁻¹ obtained from simple mixing with 3 wt% MWCNTs. The high volumetric capacity with smaller swelling using less amount of MWCNTs was attributed to the self-assembled nanotube network formed between active particles during coagulation, which was maintained with volume expansion during cycle testing.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

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.     

3D porous polymeric conductive material prepared using LbL deposition

In this work a 3D porous polymeric conducting material is derived from a multi-percolated polymer blend system. The work has focused on the preparation of low surface area porous substrates from polymer blends followed by the deposition of polyaniline conductive polymer (PANI) on the internal porous surface using a layer-by-layer (LbL) technique. The approach reported here allows for the percolation threshold concentration of polyaniline conductive polymer (PANI) to be reduced to values of no more than 0.19%. Furthermore, depending on the amount of PANI deposited, the conductivity of the porous substrate can be controlled from 10⁻¹⁵ S cm⁻¹ to 10⁻³ S cm⁻¹.Ternary and quaternary multi-percolated systems comprised of high-density polyethylene (HDPE), polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) are prepared by melt mixing and subsequently annealed in order to obtain large interconnected phases. Selective extraction of PS, PMMA and PVDF result in a fully interconnected porous HDPE substrate of ultra-low surface area and highly uniform sized channels. This provides an ideal substrate for subsequent polyaniline (PANI) addition. Using a layer-by-layer (LbL) approach, alternating poly(styrene sulfonate) (PSS)/PANI layers are deposited on the internal surface of the 3-dimensional porous polymer substrate. The PANI and sodium poly(styrene sulfonate) (PSS) both adopt an inter-diffused network conformation on the surface. The sequential deposition of PSS and PANI has been studied in detail and the mass deposition profile demonstrates oscillatory behavior following a zigzag-type pattern. The presence of salt in the deposition solution results in a more uniform deposition and more thickly deposited PSS/PANI layers. The conductivity of these samples was measured and the conductivity can be controlled from 10⁻¹⁵ S cm⁻¹ to 10⁻⁵ S cm⁻¹ depending on the number of deposited layers. In the case of a porous sample which can be crushed, applying a load to the substrate can be used as an additional control parameter. In that sample a high load results in higher conductivity with values as high as 10⁻³ S cm⁻¹ obtained. The work described above has focused on very low surface area porous substrates in order to generate a conductive device with the lowest possible concentration values of polyaniline, but high surface area substrates can also be readily prepared using this approach.     

Ru oxide/carbon nanotube composites for supercapacitors prepared by spontaneous reduction of Ru(VI) and Ru(VII)

A novel method based on spontaneous reduction of Ru(VI) and Ru(VII) is reported for the deposition of Ru oxide on multi-walled carbon nanotubes (MWCNT). Both purified and acid functionalized nanotubes (p-MWCNT and a-MWCNT) have been used to produce composite materials for use in high power aqueous supercapacitors. Specific capacitances of 213 ± 16 F g⁻¹ and 184 ± 11 F g⁻¹ were obtained for Ru oxide/p-MWCNT and Ru oxide/a-MWCNT composites, respectively. Specific capacitances for the Ru oxide component were 704 ± 62 F g⁻¹ and 803 ± 72 F g⁻¹, respectively. Current vs. potential curves exhibited capacitance peaks at ca. +0.5 V vs. Ag/AgCl. The Ru oxide/p-MWCNT composite was shown to be stable over 20,000 charge/discharge cycles. An advantage of the method is that no pre-treatment of the MWCNT is required for optimum performance.     

CVD synthesis of carbon nanotubes using a finely dispersed cobalt catalyst and their use in double layer electrochemical capacitors

Carbon nanotubes (CNT) were obtained by chemical vapour deposition (CVD), decomposing turpentine oil over finely dispersed Co metal as a catalyst at 675 °C. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images reveal that the nanotubes are densely packed and of 10-50 nm in diameter. The XRD pattern of purified CNT shows that they are graphitic in nature. Resistivity measurements of these CNT indicate that they are highly conducting. Hall measurements of CNT reveal that electrons are the majority carriers with a carrier concentration of 1.35×10²⁰ cm⁻³. Cyclic voltammetry (CV) and constant current charging/discharging was used to characterise the behaviour of electrochemical double layer capacitors of purified CNT with H₂SO₄. For CNT/2 M H₂SO₄/CNT, a capacitance of 12 F g⁻¹ (based on the weight of the active material) was obtained.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.