Chemically stable conducting polyaniline composite coatings

Aniline monomer was dispersed in polymethyl methacrylate (PMMA) and polyvinyl carbazole (PVK) solutions. The resulting mixtures were coated on substrates, dried in toluene atmosphere and enclosed later on in a chamber containing an oxidized atmosphere of HCl and (NH₄)₂S₂O₈. Electrical and optical characterizations indicate that polyaniline (PANI) was formed in the PMMA and PVK matrixes. Sheet resistance as a function of the pH value demonstrates that both PANI–PMMA and PANI–PVK composite coatings keep their R_□ almost unchanged when they are immersed in a moderate basic solution (pH9). Especially, PANI–PVK coatings are more stable than PANI–PMMA samples for longer time of immersion and also in solutions with a higher pH value. Scanning electron micrographs show different surface morphologies of these two composite materials suggesting a possible explanation of their chemical stability.     

Fourier transform infrared spectroscopy studies of polypyrrole composite coatings

Pyrrole monomer was dispersed in polyvinyl alcohol (PVA) and polyvinyl acetate (PVAc) solutions, which were previously mixed with an iron chloride solution. The resulting mixtures were coated on substrates, and as the solution solvents were evaporated pyrrole was polymerized in the composite coatings. The very low percolation threshold values and the continuous percolation phenomena of the electrical conductivity of these coatings suggest a chain structure of the conductive PPy in the composites and a good miscibility between the conductive phase and the insulator hosts as well. FT-IR studies of the coatings suggest molecular interactions between the functional groups of the polymer matrices and PPy through the iron salt molecules, which could be the reasons for the good miscibility between the semiconductor and insulator components of the composite coatings.     

Development of ZnO-based transparent conductive coatings

We have developed a range of single and multilayer transparent conducting coatings consisting of three to five alternating layers of Al-doped ZnO (AZO) and metals using E-beam evaporation method. The prepared optimized coatings show excellent optical and electrical properties, improved thermal and long-term stability. Optimum thickness of metal and AZO layers was determined for high optical transmittance and good electrical conductivity. X-ray diffraction, spectrophotometer, atomic-force microscopy, scanning electron microscopy and four-point probe were used to explore the possible changes in electrical and optical properties. It was found that the multilayer coatings consisting of Al-doped ZnO and Ag metal show satisfactory properties of low resistance of 5 Ω/sq, high transmittance of 90% and thermal stability up to 500 °C.     

Morphology and electrical properties of conductive carbon coatings for cathode materials

Many interesting cathode materials, such as LiFePO₄, LiMnPO₄, LiFeBO₃ or the recently discovered Li₂FeSiO₄ and Li₂MnSiO₄, exhibit extremely low electronic conductivity (<10⁻⁹ S cm⁻¹). A very efficient way for improving the electronic transport in such materials is supposed to be the preparation of carbon coatings around individual active particles. Despite the increasing number of reports on preparation of various carbon coatings, neither the formation mechanism nor the detailed coating properties have been explained satisfactorily. The present paper is an attempt to find a clear correlation between the synthesis parameters, the resulting coating morphology and, finally, its electrical properties. As a substrate material for deposition of coatings, more or less monodisperse TiO₂ particles in various sizes were used. As a carbon precursor, citrate was used because it had given excellent results in our previous investigation of the LiFePO₄ system. It is shown that citrate precursor delivers pretty good conductivity (ca. 30 S cm⁻¹) after a 10 h heat treatment at 700 °C or higher. The conductivity percolation threshold can be reached already at 1.5 vol.% of carbon, while the plateau conductivity of the whole composite is about 0.1 S cm⁻¹. At that level, the carbon phase is supposed to form a well-distributed 3D electrical network within the composite.     

Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.     

Expanded graphite-based electrically conductive composites as bipolar plate for PEM fuel cell

In this study, expanded graphite-based composite bipolar plates are developed from expanded graphite (EG), which is synthesized by chemical intercalation of natural graphite and rapid expansion at high temperature. The expanded graphite synthesized in this study has an expansion ratio between 75–100 cc/gm. The composite bipolar plate with varying weight percentage of EG gives different bulk density, electrical conductivity, mechanical properties and air tightness. The critical weight percentage of filler content is 50 to achieve the desired electrical conductivity and mechanical properties of bipolar plate as per U.S. DOE targets. The composite bipolar plate with 50 wt% of EG gives bulk density of 1.50 g/cm³, electrical conductivity >120 S/cm, bending strength 54 MPa, modulus 6 GPa and shore hardness 50. I–V characteristic of a cell assembly with EG-based composite plates are similar with the performance of a cell with commercial composite plates. These lightweight bipolar plates reduced the volume and weight of ultimate fuel cell stack and helped in improving the fuel cell performance.     

High performance polyvinylidene fluoride/graphite/multi-walled carbon nanotubes composite bipolar plate for PEMFC with segregated conductive networks

Composite bipolar plates (BPs) are preferred to graphite BPs and metal BPs, in proton exchange membrane fuel cells (PEMFC), due to their pronounced advantages. However, facile and high-efficiency fabrication of high performance composite BPs, remains a challenge. In this study, high performance polyvinylidene fluoride (PVDF)/graphite/multi-walled carbon nanotubes (MWCNTs) composite BPs with segregated conductive network are prepared by structural design and compression molding. Due to the “brick-mud” structure formed in composite BPs by structural manipulation, its conductivity of low filler content is greatly improved. In addition, segregated synergistic conductive networks are observed in composite BPs after adding MWCNTs. The composite BP (5 wt% MWCNTs and 35 wt% graphite) exhibited electrical conductivity of 161.57 S/cm and area specific resistances of 7.5 mΩ cm². Moreover, the composite BPs have good flexural strength, excellent hydrophobicity and corrosion resistance. In summary, our work provides a simple and feasible strategy for manufacturing high performance composite BPs with low fillers.     

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

The role of aluminum particle size in electrostatic ignition sensitivity of composite energetic materials

Very often unintentional ignition of composite energetic materials (CEM) occurs when static electricity is discharged into the CEM. An interesting finding recently reported showed that for micron particle CEM formulations, only aluminum (Al) combined with copper oxide (CuO) was electrostatic discharge (ESD) ignition sensitive, while commonly used Al combined with molybdenum trioxide (MoO3) was deemed not ESD ignition sensitive. In practice however, nano Al-MoO3 results in frequent unintentional ESD ignition events. This study examines the role of Al particle size on ESD ignition sensitivity and measures electrical conductance for each CEM. Results show that as Al particle size is reduced, electrical conductance increases dramatically as does ESD ignition sensitivity. Overall, electrical conductance is shown to increase linearly with increasing Al surface area to volume ratio and the alumina passivation shell surrounding Al core particles plays a significant role in enhancing ignition sensitivity.     

Blackening of aluminium-zinc (galvalume) coatings for solar energy utilization

The authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such coatings have a solar absorptance () of 0.90–0.92 and thermal emittance (ε) of 0.25–0.40. The coating has moderate corrosion resistance. In order to improve this, a post-treatment is necessary. The post-treated coatings in dichromate solution offer good corrosion resistance. Thermal cycling tests show that the coatings are stable to 220°C. Tape tests show that the coating is strongly adherent.     

Synthesis of silica immobilized phosphotungstic acid (Si-PWA)-poly(vinyl alcohol) (PVA) composite ion-exchange membrane for direct methanol fuel cell

A 80 μm thick composite ion-exchange membrane was synthesized by uniformly dispersing sub-micron to nano sized silica immobilized phosphotungstic acid (Si-PWA) inorganic ion exchanger into cross-linked poly(vinyl alcohol) (PVA) matrix. ATR-IR spectrum confirmed the PVA cross-linking and presence of Si-PWA in membrane. Amorphous behavior of the membrane indicated uniform blending of crystalline Si-PWA particles with cross-linked PVA. Membrane's tensile strength (93 MPa) was much higher than Nafion 117 (34 MPa). Ion exchange capacity of the membrane (0.90 meqg⁻¹) was higher than the values reported for the other PVA based membranes. Na⁺ transport number was 0.92, indicating good ion-selectivity of the membrane. Membrane showed a high water uptake of 35% while its methanol uptake was low (8.4%) and thereby reduced methanol permeability (1.6 × 10⁻⁷ cm² s⁻¹) compared to Nafion-117 was observed, a highly desirable property for DMFC application. Proton conductivity increased from 7.04 mS cm⁻¹ to 10.5 mS cm⁻¹ with increase in temperature from 30 °C to 50 °C. At 35 °C, the single cell DMFC with membrane showed higher OCV (0.8 V) and comparable peak power density to Nafion-117.     

Electrocatalytic properties of porous Ni-Co-WC composite electrode toward hydrogen evolution reaction in acid medium

Porous Ni-Co-(WC)_x ternary composite electrodes were fabricated by means of electrodeposition on a foam Ni substrate. The surface morphology and microstructure of the electrodes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrocatalytic properties of porous Ni-Co-(WC)_x electrodes for hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ solution at temperatures from 25 to 50 °C were conducted by means of cathodic polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry and chronoamperometry (CA). These Ni-Co-WC electrodes are efficient electrocatalysts for HER. Compared with the porous Ni-Co electrode, the porous Ni-Co-(WC)_x electrode exhibited a lower HER overpotential, a lower electrochemical impedance, a lower apparent activation energy and a higher exchange current density. The apparent exchange current density of porous Ni-Co-(WC)_x (x = 10, 20, 30 and 40 g/l) is 2.01, 3.01, 7.8 and 19.91 times of porous Ni-Co electrode, respectively. With the increase of WC concentration and temperature, the apparent exchange current density of HER was enhanced. With the increase of WC concentration and potential, the HER resistance and the activation energy decreased. The Ni-Co-(WC_)x electrode exhibited superior corrosion resistance and stability for HER.     

Development of Ag-(BaO)0.11(Bi2O3)0.89 composite cathodes for intermediate temperature solid oxide fuel cells

The electrochemical performances of Ag-(BaO)_0.11(Bi₂O₃)_0.89 (BSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 electrolytes have been investigated for intermediate temperature solid oxide fuel cells (ITSOFCs) using ac impedance spectroscopy from 500 to 700 °C. Results indicate that the electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the cathode. The optimum BSB addition (50% by volume) to Ag resulted in about 20 times lower area specific resistance (ASR) at 650 °C. The ASR values for the Ag50-BSB and Ag cathodes were 0.32 and 6.5 Ω cm² at 650 °C, respectively. The high performances of Ag-BSB cathodes are determined by the high catalytic activity for oxygen dissociation and ionic conductivity of BSB, and by the excellent catalytic activity for oxygen reduction of silver. The maximum power density of the Ag50-BSB cathode was 224 mWcm⁻² at 650 °C, which classify this composite as a promising material for ITSOFC.     

Photothermal radiometry on nickel (pigmented aluminium oxide) selective solar absorbing surface coatings

Photothermal radiometry (PTR) is applied to characterize nickel-pigmented aluminium oxide solar absorbing coatings. A modulated laser beam is used to heat the solar samples. The subsequent emission of thermal radiation is measured as a function of modulated frequency in the range of 10 Hz to 10 kHz. A simple one-dimensional model is used to fit the experimental PTR results, allowing for the extraction of some thermal parameters for the solar absorbing coatings. Finally, comparison of the emissivity measured by traditional technique and the photothermal radiometry is made.     

Electrospun capric acid/polyethylene terephthalate composite nanofibres for storage and retrieval of thermal energy

Abstract

Composite nanofibres based on capric acid (CA) and polyethylene terephthalate (PET) with different mass ratios of CA/PET ranging from 0·5∶1, 1∶1, 1·5∶1, 2∶1 to 2·5∶1 were fabricated by electrospinning as innovative form-stable phase change materials for storage and retrieval of thermal energy. The morphological structures, thermal energy storage properties and thermal stability of electrospun CA/PET composite nanofibres were characterised by field emission scanning electron microscopy (FE-SEM), transmission electronic microscopy (TEM), differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) respectively. The FE-SEM images revealed that the electrospun CA/PET composite nanofibres had a cylindrical morphology with average fibre diameters in the range of about 145–192 nm. Additionally, the FE-SEM and TEM images indicated that the CA distributed on the surface and within the core of the composite nanofibres. The results acquired from DSC analyses indicated that the mass ratio of CA versus PET played an important role on the enthalpy values of melting and crystallisation of the composite nanofibres, while it had no appreciable effect on the temperatures of phase transitions. Moreover, the results of DSC thermal cycling suggested that the thermal energy storage properties of the CA in the composite nanofibres had hardly been influenced during thermal cycling, indicating that the electrospun CA/PET composite nanofibres had good thermal reliability. The TGA results showed that both the onset thermal degradation temperature and the charred residue at 700°C of the composite nanofibres were lower than those of pure PET nanofibres as a result of the thermal instability of the CA molecular chains.     

Effect of conducting composite polypyrrole/polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO₄²⁻ groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.     

Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage

This paper deals with the preparation, characterization, and determination of thermal energy storage properties of polyethylene glycol (PEG)/diatomite composite as a novel form-stable composite phase change material (PCM). The composite PCM was prepared by incorporating PEG in the pores of diatomite. The PEG could be retained by 50 wt% into pores of the diatomite without the leakage of melted PEG from the composite. The composite PCM was characterized by using SEM and FT-IR analysis technique. Thermal properties of the composite PCM were determined by DSC analysis. DSC results showed that the melting temperature and latent heat of the composite PCM are 27.70 °C and 87.09 J/g, respectively. Thermal cycling test was conducted to determine the thermal reliability of the composite PCM and the results showed that the composite PCM had good thermal reliability and chemical stability. TG analysis showed that the impregnated PEG into the diatomite had good thermal stability. Thermal conductivity of the composite PCM was improved by adding expanded graphite in different mass fractions. Thermal energy storage performance of the composite PCM was also tested.     

Study on the dehydrogenation properties and reversibility of Mg(BH4)2AlH3 composite under moderate conditions

Mg(BH₄)₂ has been considered as one of the promising light metal complex hydrides due to its high hydrogen capacity and low cost. But its higher thermal stability (dehydrogenation at above 300 °C) needs to be improved for the practical application. In this study, the aluminum hydride AlH₃ was introduced into complex borohydride Mg(BH₄)₂ to synthesize a new Mg(BH₄)₂AlH₃ composite by ball milling method. It is found that the active Al¹ formed from the self-decomposition of AlH₃ can effectively improve the dehydrogenation properties of Mg(BH₄)₂, the Mg(BH₄)₂AlH₃ composite starts to release hydrogen at 130.8 °C with a total hydrogen capacity of 11.9 wt.%. The dehydrogenated products of the composite is composed of Mg₂Al₃ and B at 350 °C, resulting in the improved hydrogen desorption properties of Mg(BH₄)₂AlH₃ composite. The Mg₂Al₃ and B products would be further transformed into MgAlB₄ and Al at 500 °C. Moreover, the Mg₂Al₃ and B dehydrogenated products show better reversible hydrogen storage property than that of the MgAlB₄ and Al products. This research shows a way to alter hydrogen de/hydrogenation route and reversibility of Mg(BH₄)₂ complex hydride by compositing with AlH₃ and controlling the dehydrogenation temperature.     

High conductive (LiNaK)2CO3Ce0.85Sm0.15O2 electrolyte compositions for IT-SOFC applications

Composite electrolytes of lithium, sodium, and potassium carbonate ((LiNaK)₂CO₃), and samarium doped ceria (SDC) have been synthesized and the carbonate content optimized to study conductivity and its performance in intermediate-temperature solid oxide fuel cell (IT-SOFC). Electrolyte compositions of 20, 25, 30, 35, 45 wt% (LiNaK)₂CO₃–SDC are fabricated and the physical and electrochemical characterization is carried out using X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscope, and current–voltage measurements. The ionic conductivity of (LiNaK)₂CO₃–SDC electrolytes increases with increasing carbonate content. The best ionic conductivity is obtained for 45 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.72 S cm^?1 at 600 °C) followed by the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.55 S cm^?1 at 600 °C). The symmetrical cell of the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte with lanthanum strontium cobalt ferrite (LSCF) electrode in air gives an area specific resistance of 0.155 Ω cm² at 500 °C. The maximum power density of the fuel cell using 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte, composite NiO anode and composite LSCF cathode is found to be 801 mW cm^?2 at 550 °C.