Fast mass and charge transport through electrically aligned CNT/polymer nanocomposite membranes

This work represents the properties of electrically aligned carbon nanotubes (CNT)/polycarbonate (PC) nanocomposites towards the development of hydrogen gas separation membranes. A fraction (0.1 weight %) of CNTs synthesized by chemical vapour deposition method have been dispersed homogeneously throughout PC matrix by ultrasonication. The alignment of CNT in PC matrix has been accomplished by applying an external electric field of 1250 V/cm during solution casting. These nanocomposites have been studied by gas permeation, electrical, and dielectric constant measurements. Gas permeability measurements obtained here that electrically aligned nanocomposite membranes can be used as good hydrogen separating media. I–V characteristics and dielectric constant shows the enhancement in conductivity and permittivity of these nanocomposites. Overall experimental results exhibit here that alignment of CNTs in polymer matrix shows the dramatic improvement in mass and charge transport properties. Copyright © 2016 John Wiley & Sons, Ltd.     

Structure and performance of porous polymer electrolytes based on P(VDF-HFP) for lithium ion batteries

In order to develop polymer electrolyte for lithium ion batteries, highly porous P(VDF-HFP) membranes were prepared by using phase inversion method, then they were immerged in 1 mol kg⁻¹ solution of LiClO₄-EC/PC(1:1) to form porous polymer electrolytes. Conductivity of the polymer electrolytes was found to be as high as 10⁻³ S cm⁻¹. Structures of the porous membranes were observed with SEM. Porous membranes with different structure, porosity and pore diameter were prepared by changing the processing conditions. There are two kinds of typical structure, one is honeycomb-like (type I), and the other is network-like (type II). Membrane structures were found to be important to the performance of the porous polymer electrolytes. Small pore diameter with narrow distribution is needed to prevent solution leakage and high porosity is needed to achieve high conductivity. The type II membranes can meet the requirements. The model lithium ion batteries made of the resulting porous polymer electrolytes have good cycleablity.     

Performance of direct methanol fuel cell using carbon nanotube-supported Pt–Ru anode catalyst with controlled composition

The performance of a single-cell direct methanol fuel cell (DMFC) using carbon nanotube-supported Pt–Ru (Pt–Ru/CNT) as an anode catalyst has been investigated. In this study, the Pt–Ru/CNT electrocatalyst was successfully synthesized using a modified polyol approach with a controlled composition very close to 20 wt.%Pt–10 wt.%Ru, and the anode was prepared by coating Pt–Ru/CNT electrocatalyst on a wet-proof carbon cloth substrate with a metal loading of about 4 mg cm⁻². A commercial gas diffusion electrode (GDE) with a platinum black loading of 4 mg cm⁻² obtained from E-TEK was employed as the cathode. The membrane electrode assembly (MEA) was fabricated using Nafion^® 117 membrane and the single-cell DMFC was assembled with graphite endplates as current collectors. Experiments were carried out at moderate low temperatures using 1 M CH₃OH aqueous solution and pure oxygen as reactants. Excellent cell performance was observed. The tested cell significantly outperformed a comparison cell using a commercial anode coated with carbon-supported Pt–Ru (Pt–Ru/C) electrocatalyst of similar composition and loading. High conductivity of carbon nanotube, good catalyst morphology and suitable catalyst composition of the prepared Pt–Ru/CNT electrocatalyst are considered to be some of the key factors leading to enhanced cell performance.     

Comparison of the small angle X-ray scattering study of sulfonated poly(etheretherketone) and Nafion membranes for direct methanol fuel cells

The microstructural evolution and swelling behaviors of sulfonated poly(etheretherketone) (SPEEK) and Nafion polymer membranes have been investigated by small angle X-ray scattering (SAXS) after equilibrating them in 2 M methanol solution at various temperatures, which is relevant for their use in direct methanol fuel cells (DMFC). The relationships among Bragg distance, sulfonation levels of the membrane, equilibrating temperature and transport properties are discussed. The proton conduction properties of the SPEEK and Nafion membranes have been investigated by electrochemical impedance spectroscopy. The network cluster model is employed to retrieve the structural information from the scattering and proton conductivity data. While the SPEEK membranes have narrower pathways for methanol/water permeation at T < 70 °C, the Nafion membranes have a wider channel even at lower temperatures, resulting in a higher methanol permeability in the latter. Based on the differences in the structural/cluster evolutions, the advantages and limitations of the two polymer membranes for use in DMFC are discussed.     

Electrochemical capacitance of nanocomposite films formed by loading carbon nanotubes with ruthenium oxide

This work reports the supercapacitive properties of composite films of multiwalled carbon nanotubes (MWNT) and ruthenium oxide (RuO₂). Transmission and scanning electron microscopy, cyclic voltammetry, and electrochemical studies revealed that the nanoporous three-dimensional arrangement of RuO₂-coated MWNT in these films facilitated the improvement of electron and ion transfer relative to MWNT films. The capacitance was measured for films of different RuO₂ loading, revealing specific capacitances per mass as high as 628 F g⁻¹. The energy storage density of the electrode has increased about three times as compared to MWNT treated with piranha solution.     

Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell

Electrochemical surface oxidation of carbon black Vulcan XC-72 and multiwalled carbon nanotube (MWNT) has been compared following potentiostatic treatments up to 168 h under condition simulating PEMFC cathode environment (60 °C, N₂ purged 0.5 M H₂SO₄, and a constant potential of 0.9 V). The subsequent electrochemical characterization at different treatment time intervals suggests that MWNT is electrochemically more stable than Vulcan XC-72 with less surface oxide formation and 30% lower corrosion current under the investigated condition. As a result of high corrosion resistance, MWNT shows lower loss of Pt surface area and oxygen reduction reaction activity when used as fuel cell catalyst support.     

Vibrational,ac impedance and dielectric spectroscopic studies of poly(vinylacetate)–N,N–dimethylformamide–LiClO4 polymer gel electrolytes

Polymer electrolyte membranes that consist of poly(vinyl acetate) (PVAc) and LiClO₄ with different concentrations of plasticizer (N,N-dimethylformamide (DME) have been prepared by a solution-casting technique. The formation of polymer-salt complexes has been confirmed by FT-IR spectral studies. Conductivity studies have been carried out using ac impedance spectroscopy in the frequency range 42 Hz–5 MHz. The influence of the addition of plasticizer (DMF) on the ionic conductivity of the PVAc–LiClO₄ polymer electrolyte complex has been discussed. The maximum value of bulk conductivity for PVAc(70)–DMF(20)–LiClO₄(10) system is found to be 4.2×10⁻⁴ S cm⁻¹ at 303 K. The temperature dependence of the conductivity of the polymer electrolytes follows the Vogel–Tamman–Fulcher relationship. Transport properties, such as activation energy and charge carrier concentration, have been calculated from the VTF formalism. The ionic transference number of the mobile ions has been estimated by Wagner’s polarization method and is found to be ≥0.96 for all the samples.     

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180 W/m K. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm² over a 2 mm × 2 mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ～140 W/cm² over a 10 × 10 mm² heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.     

Influence of carbon nanotube suspension on the thermal performance of a miniature thermosyphon

An experimental study was carried out to understand the heat transfer performance of a miniature thermosyphon using water-based carbon nanotube (CNT) suspensions as the working fluid. The suspensions consisted of deionized water and multi-wall carbon nanotubes with an average diameter of 15 nm and a length range of 5–15 μm. Experiments were performed under three steady operation pressures of 7.4 kPa, 13.2 kPa and 20 kPa, respectively. Effects of the CNT mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the thermosyphon were investigated and discussed. Experimental results show that CNT suspensions can apparently improve the thermal performance of the thermosyphon and there is an optimal CNT mass concentration (about 2.0%) to achieve the maximum heat transfer enhancement. The operation pressure has a significant influence on the enhancement of the evaporation heat transfer coefficient, and slight influences on the enhancement of the critical heat flux. The enhanced heat transfer effect is weak at low heat fluxes while it is increased gradually with increasing the heat flux. The present experiment confirms that the thermal performance of a miniature thermosyphon can be strengthened evidently by using CNT suspensions.     

Using silica nanoparticles for modifying sulfonated poly(phthalazinone ether ketone) membrane for direct methanol fuel cell: A significant improvement on cell performance

Sulfonated poly(phthalazinone ether ketone) (sPPEK) with a degree of sulfonation of 1.23 was mixed with silica nanoparticles to form hybrid materials for using as proton exchange membranes. The nanoparticles were found homogeneously dispersed in the polymer matrix and a high 30 phr (parts per hundred resin) loading of silica nanoparticles can be achieved. The hybrid membranes exhibited improved swelling behavior, thermal stability, and mechanical properties. The methanol crossover behavior of the membrane was also depressed such that these membranes are suitable for a high methanol concentration in feed (3 M) in cell test. The membrane with 5 phr silica nanoparticles showed an open cell potential of 0.6 V and an optimum power density of 52.9 mW cm⁻² at a current density of 264.6 mA cm⁻², which is better than the performance of the pristine sPPEK membrane and Nafion^® 117.     

Proton exchange membranes based on PVDF/SEBS blends

Proton-conductive polymer membranes are used as an electrolyte in the so-called proton exchange membrane fuel cells. Current commercially available membranes are perfluorosulfonic acid polymers, a class of high-cost ionomers. This paper examines the potential of polymer blends, namely those of styrene–(ethylene-butylene)–styrene block copolymer (SEBS) and polyvinylidene fluoride (PVDF), in the proton exchange membrane application. SEBS/PVDF blends were prepared by twin-screw extrusion and the membranes were formed by calendering. SEBS is a phase-segregated material where the polystyrene blocks can be selectively functionalized offering high ionic conductivity, while PVDF insures good dimensional stability and chemical resistance to the films. Proton conductivity of the films was obtained by solid-state grafting of sulfonic acid moieties. The obtained membranes were characterized in terms of conductivity, ionic exchange capacity and water uptake. In addition, the membranes were characterized in terms of morphology, microstructure and thermo-mechanical properties to establish the blends morphology–property relationships. Modification of interfacial properties between SEBS and PVDF was found to be a key to optimize the blends performance. Addition of a methyl methacrylate–butyl acrylate–methyl methacrylate block copolymer (MMA–BA–MMA) was found to compatibilize the blend by reducing the segregation scale and improving the blend homogeneity. Mechanical resistance of the membranes was also improved through the addition of this compatibilizer. As little as 2 wt.% compatibilizer was sufficient for complete interfacial coverage and lead to improved mechanical properties. Compatibilized blend membranes also showed higher conductivities, 1.9 × 10⁻² to 5.5 × 10⁻³ S cm⁻¹, and improved water management.     

High capacity Si/C nanocomposite anodes for Li-ion batteries

Nanocomposites of Si/C were synthesized from Si and polystyrene (PS) resin using high-energy mechanical milling (HEMM) followed by subsequent heat-treatment. The resultant nanocomposites are comprised of amorphous carbon and nanocrystalline silicon as verified by X-ray diffraction (XRD). The XRD results also indicate the presence of iron silicide (FeSi) arising as a contaminant during HEMM. The Si/C nanocomposite corresponding to Si:C = 1:2 composition obtained after milling in two stages of 12 h each for a total time period of 24 h shows a capacity as high as ∼850 mAh/g with reasonable capacity retention (∼1.1% loss/cycle). The increase in either heat-treatment temperature or milling time renders the nanocomposites more stable at the expense of capacity. Transmission electron microscopy (TEM) analysis shows that the HEMM derived Si nanocrystallites <50 nm in size are distributed homogeneously within the amorphous carbon matrix.     

Preparation and characterization of sulfonated PEEK-WC membranes for fuel cell applications: A comparison between polymeric and composite membranes

Sulfonated poly(etheretherketone) with a cardo group (SPEEK-WC) exhibiting a wide range of degree of sulfonation (DS) was used to prepare polymeric membranes and composite membranes obtained by incorporation of an amorphous zirconium phosphate sulfophenylenphosphonate (Zr(HPO₄)(O₃PC₆H₄SO₃H), hereafter Zr(SPP)) in a SPEEK-WC matrix. The nominal composition of the composite membranes was fixed at 20 wt% of Zr(SPP). Both types of membrane were characterized for their proton conductivity, methanol permeability, water and/or methanol uptake, morphology by SEM and mechanical properties. For comparison, a commercial Nafion 117 membrane was characterized under the same operative conditions. The composite membranes exhibited a reduced water uptake in comparison with the polymeric membranes especially at high DS values and temperature higher than 50 °C. As a result, the water uptake into composite membranes remained about constant in the range 20–70 °C. The methanol permeability (P) of both polymeric and composite membranes was always lower than that of a commercial Nafion 117 membrane. At 22 °C and 100% relative humidity (RH), the proton conductivities (σ) of the polymeric membranes increased from 6 × 10⁻⁴ to 1 × 10⁻² S cm⁻¹ with the increase of DS from 0.1 to 1.04. The higher conductivity value was comparable with that of Nafion 117 membrane (3 × 10⁻² S cm⁻¹) measured under the same operative conditions. The conductivities of the composite membranes are close to that of the corresponding polymeric membranes, but they are affected to a lesser extent by the polymer DS. The maximum value of the σ/P ratio (about 7 × 10⁴ at 25 °C) was found for the composite membrane with DS = 0.2 and was 2.5 times higher than the corresponding value of the Nafion membrane.     

Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10⁵–10⁸) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart.     

Development of materials for mini DMFC working at room temperature for portable applications

Methanol permeability measurements and direct methanol fuel cell tests were performed at room temperature with different commercially available or recast Nafion^® membranes and sulfonated polyimide (SPI) membranes. Power densities as high as 20 mW cm⁻² could be obtained with Nafion^® 115. However, in order to meet the technological requirements for portable applications, thinner membranes have to be considered. As the MeOH crossover increases greatly (from (7 to 20) × 10⁻⁸ mol s⁻¹ cm⁻²) while Nafion^® membranes thickness decreases, non-perfluorinated polymers having high IEC are promising candidates for DMFC working at room temperature. The development catalysts tolerant to methanol is also relevant for this application. In spite of the low permeability to MeOH of SPI membranes, the obtained electrical performance with E-TEK electrodes based MEAs was lower than that obtained with Nafion^® membranes. No significant increase of performances was neither evidenced by using homemade PtCr(7:3)/C and PtRu(4:1)/C catalysts instead of E-TEK electrodes with recast Nafion^® based MEAs. However, MEAs composed with thin SPI membranes (50 μm) and homemade PtCr/C catalysts gave very promising results (18 mW cm⁻²). Based on experimental observations, a speculative explanation of this result is given.     

Development of PVA based micro-porous polymer electrolyte by a novel preferential polymer dissolution process

A micro-porous polymer electrolyte based on PVA was obtained from PVA–PVC based polymer blend film by a novel preferential polymer dissolution technique. The ionic conductivity of micro-porous polymer electrolyte increases with increase in the removal of PVC content. Finally, the effect of variation of lithium salt concentration is studied for micro-porous polymer electrolyte of high ionic conductivity composition. The ionic conductivity of the micro-porous polymer electrolyte is measured in the temperature range of 301–351 K. It is observed that a 2 M LiClO₄ solution of micro-porous polymer electrolyte has high ionic conductivity of 1.5055 × 10⁻³ S cm⁻¹ at ambient temperature. Complexation and surface morphology of the micro-porous polymer electrolytes are studied by X-ray diffraction and SEM analysis. TG/DTA analysis informs that the micro-porous polymer electrolyte is thermally stable upto 277.9 °C. Chronoamperommetry and linear sweep voltammetry studies were made to find out lithium transference number and stability of micro-porous polymer electrolyte membrane, respectively. Cyclic voltammetry study was performed for carbon/micro-porous polymer electrolyte/LiMn₂O₄ cell to reveal the compatibility and electrochemical stability between electrode materials.     

A novel PTFE-based proton-conductive membrane

The demand for a solid polymer electrolyte membrane (SPEM) for fuel-cell systems, capable of withstanding temperatures above 130 °C, decreasing the electrode-catalyst loadings and reducing poisoning by carbon monoxide, has prompted this study. A novel, low-cost, highly conductive, nanoporous proton-conducting membrane (NP-PCM) based on a polytetrafluoroethylene (PTFE) backbone has been developed. It comprises non-conductive nano-size ceramic powder, PTFE binder and an aqueous acid. The preparation procedures were studied and the membrane was characterized with the use of: SEM, EDS, pore-size-distribution measurements (PSD), TGA–DTA and electrochemical methods. The ionic conductivity of a membrane doped with 3 M sulfuric acid increases with the ceramic powder content and reaches 0.22 S cm⁻¹ at 50% (v/v) silica. A non-optimized direct-methanol fuel cell (DMFC) with a 250 μm thick membrane has been assembled. It demonstrated 50 and 130 mW cm⁻² at 80 and 130 °C, respectively. Future study will be directed to improving the membrane-preparation process, getting thinner membranes and using this membrane in a hydrogen-fed fuel cell.     

Development of a biodegradable polymer electrolyte for rechargeable batteries

The possibility of producing a biodegradable polymer electrolyte based on poly-ɛ-caprolactone (PCL) with different concentrations of LiClO₄ has been investigated. The maximum ionic conductivity obtained at room temperature was 1.2 × 10⁻⁶ S cm⁻¹ for PCL complexed with 10 wt.% LiClO₄. In this mixture, complete biodegradation occurred after 110 days and was attributed to the presence of ester groups in the polymer matrix. The large electrochemical stability window of approximately 5 V showed that the PCL/LiClO₄ electrolyte had important electrochemical properties that would make it useful in the production of rechargeable batteries with a lower environmental impact.     

Effects of carbon nanotube arrays on nucleate pool boiling

Experiments were performed to assess the impact coating silicon and copper substrates with nanotubes (CNTs) have on pool boiling performance. Different CNT array densities and area coverages were tested on 1.27 × 1.27 mm² samples in FC-72. The CNT preparation techniques used provided strong adherence of CNTs to both substrate materials. Very small contact angle enabled deep penetration of FC-72 liquid inside surface cavities of smooth uncoated silicon surfaces, requiring unusually high surface superheat to initiate boiling. Fully coating the substrate surface with CNTs was highly effective at reducing the incipience superheat and greatly enhancing both the nucleate boiling heat transfer coefficient and critical heat flux (CHF). Efforts to further improve boiling performance by manipulating CNT area coverage of the substrate surface proved ineffective; best results were consistently realized with full surface coverage. Greater enhancement was achieved on silicon than on copper since, compared to uncoated copper surfaces, the uncoated silicon surfaces were very smooth and void of any sizeable nucleation sites to start with. This study is concluded with detailed metrics to assess the enhancement potential of the different CNT array densities and area coverages tested.     

Physical properties of the CNT:TiO2 thin films prepared by sol–gel dip coating

Uniform and highly adherent thin films of CNT:TiO₂ were synthesized by sol–gel dip coating method. Both TiO₂ and CNT:TiO₂ films showed very identical structural characteristics and no significant changes in the lattice values were observed. The crystalline size decreased from 20 nm for TiO₂ film to 17 nm for the 4%CNT:TiO₂ film. The film surface was very smooth and compact, as indicated by the roughness data obtained from AFM measurements; the root mean square (rms) average of the roughness was as low as 3 nm. The HRTEM showed that the CNTs are embedded in the matrix of TiO₂ indicating the formation of a composite. In Raman spectra the characteristic vibrations of the TiO₂ are identified, the increase in the FWHM of main anatase peak (144 cm^?1) in the case of the 4%CNT:TiO₂ film is interpreted as due to the incorporation of CNTs in the film. At the wavelength of 600 nm the refractive index of pure TiO₂ was 2.07 and the 4%CNT:TiO₂ showed a value of 2.29. The photoresponse curves showed typical features of charge trapping centers in the band gap of the films.