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Gas barrier property and microstructure of reduced graphene oxide/polyethyleneimine multilayer films createdviarecast and layer-by-layer deposition processes.

Nafion composite membranes with various amounts of sulfonated multi-walled carbon nanotubes (Su-CNTs) were prepared either in or without a strong magnetic field. Interestingly, it is found that being recast in a strong magnetic field, the tensile strength at break and proton conductivity of Su-CNTs/Nafion composite membranes parallel to the magnetic field direction were higher than those perpendicular to magnetic field. Remarkably, such anisotropic mechanical property and proton conductivity of Su-CNTs/Nafion composite membranes strongly depend on water uptake and temperature, in particular the content of Su-CNTs. Quite a different dependence of the proton conductivity of the composite membranes on Su-CNT content, relative humidities and temperatures is well explained for the anisotropic proton conductivity of the Su-CNTs/Nafion membrane recast in a strong magnetic field, which is attributed to the preferential proton transporting along ionic water channels mainly coalesced by the orientated Su-CNTs at a specific water uptake. For Nafion composite membranes with 5 wt.% Su-CNTs, the proton conductivity along the direction of treating magnetic field approaches to 0.216 S/cm, which is $$\sim$$ 46% higher than that perpendicular to the magnetic field at 95 $$^{\circ }$$ C. The present work provides a deep insight of the humidity and temperature dependence of proton conductivity of Su-CNTs/Nafion composite membranes and a new strategy for fabrication proton exchange membranes with high proton conductivity as well as good mechanical property.

Nowadays, graphene/polymer composite films with multilayer structure have attracted significant attention for gas barrier application. In this study, a series of reduced graphene oxide/polyethyleneimine (RGO/PEI) composite films were created

A series of MIL-101-SO3H/Nafion composite membranes was synthesized. They show an improved proton conductivity, due to the abundance of SO3H groups, which fosters proton conduction by binding the water molecules and enabling a larger number of conducting sites. Gas (including water vapor, hydrogen, and oxygen) permeability, crystallinity, and free volumes of the MIL-101-SO3H/Nafion composite membranes were investigated, as well as their correlation. By increasing the MIL-101-SO3H content, the gas permeability of the membranes significantly decreases, since the crystalline region is larger and the water-bearing MIL-101-SO3H particles are efficient barriers for the gas molecules. The gas permeation in the composite membranes is a very complex process and the results indicate no simple linear relation between the gas permeability and the free volume size (VFV), or between the gas permeability and the crystallinity. Moreover, it is very interesting to observe that the influence of VFV on the gas permeability is closely related to the size of the particular gas molecules: the larger the size of the gas molecules, the larger the free volume needed to achieve their rapid diffusion in the membrane. The results suggest the presence of a threshold value for VFV, which depends on the size of the gas molecules: when VFV is lower than this value, the gas molecules cannot easily jump through neighboring free volumes to a neighboring site, and, as a result, the permeability drops quickly.

How to convert heat energy into other forms of usable energy more efficiently is always crucial for our human society. In traditional heat engines, such as the steam engine and the internal combustion engine, high-grade heat energy can be easily converted into mechanical energy, while a large amount of low-grade heat energy is usually wasted owing to its disadvantage in the temperature level. In this work, for the first time, the generation of mechanical energy from both high- and low-temperature steam is implemented by a hydrophilic polymer membrane. When exposed to water vapor with a temperature ranging from 50 to 100 °C, the membrane repeats rolling from one side to another. In nature, this continuously rolling of membrane is powered by the steam, like a miniaturized “steam engine”. The differential concentration of water vapor (steam) on the two sides of the membrane generates the asymmetric swelling, the curve, and the rolling of the membrane. In particular, results suggest that this membrane based “steam engine” can be powered by the steam with a relatively very low temperature of 50 °C, which indicates a new approach to make use of both the high- and low-temperature heat energy.

Free volumes, backbone mobility, crystallinity and proton conductivity of Nafion membranes upon thermal annealing were intensively studied. The development of size and distribution of free volumes in Nafion membranes as a function of environmental humidity was found to be closely associated with the adsorbed water molecules and the Nafion backbone mobility. Results suggested a gradual decrement in Nafion backbone mobility as a function of annealing temperature, due to the thermal induced recrystallization. This was further supported by a linear decrement in the o-Ps intensity as a function of membrane crystallinity, because o-Ps atoms mostly annihilated in the amorphous phase in Nafion. Particularly, a significant increment in crystallinity was found in Nafion membranes annealed at temperatures higher than 140 °C. The present results showed that annealing at ∼ 140 °C of Nafion membranes is favourable for their proton conductivity because of the low crystallinity.

Polyamide thin film composite reverse osmosis membranes on porous polysulfone support were prepared using m-phenylenediamine, trimesoyl chloride and dimethyl sulfoxide (DMSO) with various contents as precursors via an interfacial polymerization. The membrane with heterogeneous multilayer structure was depth-profiled by positron annihilation lifetime spectroscopy based on a pulsed slow positron beam and the thickness of the polyamide active layer can be estimated through fitting the results of depth dependence of o-Ps intensity. The free volume characteristics in the polyamide selective layer of the membranes were evaluated and were correlated with the permeation properties. With increasing DMSO content, mean free volume size (VFV) was enlarged, which was responsible for the enhanced water and sodium chloride permeation. However, the salt rejection was significantly decreased with a slight increase in the VFV around a critical value of 88.0 A3, which was the average size of hydrated sodium and chloride ions. It was well interpreted for that the broadened free volume distribution enables an amount of larger free volumes to be involved in the salt permeation, although little change in VFV was found.

Compared with most cathodes for lithium-ion batteries (LIBs), α-MoO3 exhibits high specific capacity and therefore receives widespread attention. However, due to its irreversible structural transformation, the capacity of α-MoO3 declines rapidly upon discharge-charge process. Herein, via H2 plasma etching, oxygen-deficient α-MoO3-x with improved electrochemical performance is prepared on the basis of pristine α-MoO3 (MoO3 (I)). The obtained MoO3 (II) (10 min etching) and MoO3 (III) (20 min etching) have reduced bandgap and expanded van der Waals gap (vdW gap). Electrochemical test results demonstrate MoO3(II) obtains the largest Li+ diffusion coefficient, the lowest charge transfer resistance and the slightest polarization. In situ X-ray diffraction characterization further reveals that during cycling, vdW gap of MoO3 (II) changes periodically within a small range, indicating a stabilized crystal structure. This work shows that moderate oxygen vacancies can enhance the capacity and stabilize the structure of molybdenum trioxide cathode for LIBs, which has great potential in other cathode materials.

Silicone rubber is widely used as the housing material of composite insulator for outdoor high-voltage insulation. However, corona-induced aging in silicone rubber contributes to the ingress of water or moisture, which finally leads to fracture of a composite insulator. To investigate water transportation in silicone rubber helps to shed light upon the insulator fracture mechanism. In this work, water transportation behaviors in silicone rubber treated by different applied voltages of corona discharge were systematically studied using electrochemical impedance spectroscopy (EIS). Low voltages (3, 4 kV) of corona treatments result in growing value of $$f_{\theta \mathrm{min}}$$ in Bode phase angle, indicative of severer degree of surface delamination. After 5 kV of corona treatment, water can finally diffuse through the silicone rubber, and then induces double-layer capacitance ( $$C_{\mathrm{dl}}$$ ) and the charge transfer resistance ( $$R_{\mathrm{ct}}$$ ) in EIS spectra. For high-voltage ( $$\ge 6\ \hbox {kV}$$ ) corona-exposed samples, great number of surface cracks are observed, and then Warburg impedance appears in EIS spectra on initial period of immersion, suggesting a direct water transportation process. The polar groups generated in silicone rubber during corona exposure also facilitate this water diffusion process. Calculated water diffusion coefficients show a sharp increase between 5 and 6 kV, which serves as an indicator for percolation of cracks or defects in bulk of silicone rubber.

Nafion has been extensively used as a proton exchange membrane in the promising energy source of polymer electrolyte fuel cells. However, the phase morphology in Nafion, which basically governs its...

Proton exchange membrane fuel cell (PEMFC) is one of the most promising green power sources, in which perfluorinated sulfonic acid ionomer-based membranes (e.g., Nafion) are widely used. However, the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions. In this work, the high-performance sulfonated carbon nanotubes/Nafion composite membranes (Su-CNTs/Nafion) for the PEMFCs were prepared and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied. Microstructure evolution in Nafion membranes during water uptake was investigated by positron annihilation lifetime spectroscopy, and results strongly showed that the Su-CNTs or CNTs in Nafion composite membranes significantly reinforced Nafion matrices, which influenced the development of ionic-water clusters in them. Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the mass formation of proton-conducting pathways (water channels) along the Su-CNTs. In particular, these pathways along Su-CNTs in Su-CNTs/Nafion membranes interconnected the isolated ionic-water clusters at low humidity and resulted in less tortuosity of the water channel network for proton transportation at high humidity. At a high temperature of 135 °C, Su-CNTs/Nafion membranes maintained high proton conductivity because the reinforcement of Su-CNTs on Nafion matrices reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.

Transparent GaN doped SnO2 thin films were deposited on glass substrates by e-beam evaporation with GaN:SnO2 targets of various GaN weight ratios. The effects of doping level and annealing temperature on the optoelectronic properties of GaN codoped SnO2 thin films were investigated. A conversion from n-type conduction to p-type was observed for GaN doped thin films upon annealing at 440 °C regardless doping level. However, it converted back to n-type conduction at various higher temperatures depending on GaN doping levels. Hole concentration for p-type GaN:SnO2 thin films could be achieved as high as 1.797 × 1019 cm−3 because of the codoping of Ga and N. Hall measurements showed that upon proper thermal treatments, Ga3+-Sn4+ and N3−-O2− substitution reactions occurred in the thin films, which regulated the polarity of conduction and carrier concentration. The formation of No substitutions in the GaN:SnO2 thin films and decomposition of them at certain higher temperature were mainly responsible for the n-p-n conduction transition.

Fluorine-doped tin oxide (FTO) thin films were prepared by a radio frequency magnetron sputtering technique. The defects and charge carrier transport behavior in FTO thin films were evaluated during the transition process from amorphous to nanocrystalline structures. The stable lattice structure in FTO thin films was obtained as the annealing temperature reached 400 °C. Positron annihilation results indicated that defect evolution in the FTO thin films was shown in two stages, formation and reduction of vacancies/vacancy clusters. The carrier mobility of the FTO thin films annealed at 600 °C was enhanced twice the amount than that of the unannealed samples. The correlation between the results obtained from positron annihilation and the Hall effect revealed the importance of defect scattering in deciding the charge carrier mobility. A defect scattering mechanism was proposed to interpret the noticeable increment of carrier mobility in FTO thin films after thermal treatment.

In a proton-exchange membrane fuel cell, the proton-exchange membrane is located between the oxidant and the fuel, which facilitates proton conduction and prevents the penetration of gas molecules. In this study, sulfonated graphene oxides (SG) were synthesized, and a series of SG/Nafion composite multilayer membranes were prepared using layer-by-layer assembling. This multilayer membrane-structure has an efficient orientation effect on the SG sheets. The thinner the layer is, the more significant is the orientation effect. A statistical angle between the SG sheets and the membrane surface ( Θ ) is used to represent the orientation of SG sheets, and a correlation between the thickness of the layers and Θ is found. Under various conditions, these multilayer composite membranes improve the gas-barrier properties (H 2 , O 2 , and water vapor) and proton conductivity. The effect of Θ on the gas-barrier properties and proton conductivity of the membranes is discussed. Our results reveal that the SG sheets have excellent gas-barrier properties in the longitudinal direction and good proton conductivity in the lateral direction. Furthermore, based on positron-annihilation lifetime spectroscopy, it is found that the diffusion coefficients of gas molecules in all the SG/Nafion composite membranes are almost identical. In other words, the orientation of the SG sheets changes the gas-barrier properties of the membranes by altering the morphology of the diffusion paths for the gas molecules during the permeation.

Proton exchange membranes, such as the widely used Nafion membranes, are essential components in industrial proton-exchange membrane fuel cells. In this study, multilayer sulfonated carbon-nanotubes/Nafion composite membranes with high performances have been prepared using layer-by-layer assembly. The membranes consist of a certain number of equally thick sulfonated carbon-nanotubes/Nafion layers. The thickness of the layers varies from 40 μm to 0.5 μm. This study indicates that the multilayered structure in composite membranes, especially in membranes with ultrathin layers ( 1 μm), encourages the alignment of the sulfonated carbon-nanotubes and the proton-conducting pathways in the lateral direction (parallel to the membrane interface). The multilayered structure also improves the dispersion of sulfonated carbon-nanotubes, which leads to a uniformly-distributed proton-conducting network. Therefore, the multilayer composite membranes are excellent proton conductors in the lateral direction at different temperatures. In addition, due to the formation of a hydrophilic interface between the layers, the composite membranes show a higher water-retention capacity. Furthermore, the lateral-aligned sulfonated carbon-nanotubes lead to significantly improved mechanical properties of the multilayer composite membranes, which results in a better electrochemical stability and a long-term performance stability of membranes.

Temperature dependent conductivities, surface morphologies, defects and antimony valence state of antimony doped tin oxide (ATO) films were investigated. Much higher conductivity is found for the film calcined at 600 °C than 400 °C due to enhancement of Sb 5+ ratio and removal of grain boundaries, vacancies and interstitial dopants; the conductivity increases slightly for the film calcined at 800 °C because of further elimination of vacancy clusters and grain boundaries. A strong temperature dependent conductivity is merely observed in the film calcined at 400 °C Positron annihilation parameter W is well correlated with the conductivity for ATO films.

The free volumes and proton conductivities of Nafion membranes were investigated at different humidities by positron annihilation lifetime spectroscopy (PALS) and using an electrochemical workstation, respectively. The results showed that the variation in o-Ps lifetime τo-Ps was closely associated with the microstructure evolution and the development of hydrophilic ion clusters in Nafion membranes as a function of water uptake, regardless of metal oxide additives. In particular, with increasing relative humidity, the maximum value of τo-Ps in the Nafion membranes corresponded to the formation of numerous water channels for proton transportation. Numerous well-connected water channels in Nafion–TiO2 hybrid membranes could be formed at a much lower relative humidity (∼40% RH) than in the pristine one (∼75% RH), due to the better water retention ability of the Nafion–TiO2 membranes. Further, a percolation behavior of proton conductivity at high water uptake in Nafion membranes was observed, which showed that the percolation of ionic-water clusters occurred at the water uptake of ∼4.5 wt%, and ∼6 wt% was basically enough for the formation of a well-connected water channel network.

Fluorine doped tin oxide (FTO) thin films were deposited on glass substrates by e-beam evaporation. Much higher carrier concentration, broader optical band gap, and average transmittance over 80% were obtained with SnF2 doped SnO2 thin films. Positron annihilation results showed that there are two kinds of vacancy clusters with different sizes existing in the annealed FTO thin films, and the concentration of the larger vacancy clusters of VSnO in the thin films increases with increasing SnF2 contents. Meanwhile, photoluminescence spectra results indicated that the better electrical and optical properties of the FTO thin films are attributed to FO substitutions and oxygen vacancies with higher concentration, which are supported by positron annihilation Doppler broadening results and confirmed by X-ray photoelectron spectroscopy. The results showed that widening of the optical band gap of the FTO thin films strongly depends on the carrier concentration, which is interpreted for the Burstein-Moss effect and is associated with the formation of FO and oxygen vacancies with increasing SnF2 content.Fluorine doped tin oxide (FTO) thin films were deposited on glass substrates by e-beam evaporation. Much higher carrier concentration, broader optical band gap, and average transmittance over 80% were obtained with SnF2 doped SnO2 thin films. Positron annihilation results showed that there are two kinds of vacancy clusters with different sizes existing in the annealed FTO thin films, and the concentration of the larger vacancy clusters of VSnO in the thin films increases with increasing SnF2 contents. Meanwhile, photoluminescence spectra results indicated that the better electrical and optical properties of the FTO thin films are attributed to FO substitutions and oxygen vacancies with higher concentration, which are supported by positron annihilation Doppler broadening results and confirmed by X-ray photoelectron spectroscopy. The results showed that widening of the optical band gap of the FTO thin films strongly depends on the carrier concentration, which is interpreted for the Burstein-Moss effect and ...

This paper investigates the effect of aluminum hydroxide (ATH) content on the free volume and surface recovery property of polydimethylsiloxane (PDMS)–based silicone rubber containing low-molecular-weight siloxanes. With increasing ratio of ATH up to 43.1 wt %, the concentration of cyclic siloxanes (Dn = [(CH3)2SiO]n, n = 4–12) in the PDMS matrix increases remarkably, indicative of a spacing effect of ATH particles on the crosslinking of PDMS chains. When more ATH is added, the concentration of D4–D12 began to decrease. PDMS network variation is verified by free volume size corresponding to τ3 in positron annihilation lifetime spectroscopy. The o-Ps intensity decreases linearly with ATH content. Data obtained from X-ray photoelectron spectroscopy suggest the surface recovery property is weakened by ATH. This process is dominated by the amount of free volume holes in the sample. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45803.