Solution-cast Nafion/montmorillonite composite membrane with low methanol permeability

Perfluorosulfonate ionomer dispersions in dimethylformamide solvent were used to form solution-cast membranes. Modified composite membranes were prepared with the addition of 1 and 5 wt.% montmorillonite salts. Measurements of water uptake, solubility and methanol permeation of the cast membranes were conducted. Tangential conductivities were measured directly on membranes fully immersed in deionized water by means of impedance spectroscopy. Results show that the addition of a low quantity of silicate did not alter the conductivity (94-96 mS cm⁻¹ at 25 °C), but produced a marked decrease of methanol permeability (−6%). Also a simple model was proposed to explain the increase of tangential conductivity with decreasing thickness of Nafion^® and recast membranes.     

Characterisation of a re-cast composite Nafion 1100 series of proton exchange membranes incorporating inert inorganic oxide particles

A series of cation exchange membranes was produced by impregnating and coating both sides of a quartz web with a Nafion^® solution (1100 EW, 10%wt in water). Inert filler particles (SiO₂, ZrO₂ or TiO₂; 5-20%wt) were incorporated into the aqueous Nafion^® solution to produce robust, composite membranes. Ion-exchange capacity/equivalent weight, water take-up, thickness change on hydration and ionic and electrical conductivity were measured in 1 mol dm⁻³ sulfuric acid at 298 K. The TiO₂ filler significantly impacted on these properties, producing higher water take-up and increased conductivity. Such membranes may be beneficial for proton exchange membrane (PEM) fuel cell operation at low humidification. The PEM fuel cell performance of the composite membranes containing SiO₂ fillers was examined in a Ballard Mark 5E unit cell. While the use of composite membranes offers a cost reduction, the unit cell performance was reduced, in practice, due to drying of the ionomer at the cathode.     

Semi-interpenetrating polymer networks of Nafion and fluorine-containing polyimide with crosslinkable vinyl group

Fluorine-containing polyimide with crosslinkable vinyl group (FPI) was synthesized from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), and 4-amino styrene (AS). The reinforced composite membranes based on semi-interpenetrating polymer networks (semi-IPN) were prepared via solution casting of FPI and Nafion^®212, and crosslinking thereafter. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and oxidative stability of the composite membranes were investigated. Compared with the recast Nafion^® 212, the composite membrane shows better mechanical properties and improved dimensional stability. The tensile strength of the composite membranes ranges from 39.0 MPa to 80.0 MPa, which is higher than that of the recast Nafion^® 212 membrane (26.6 MPa). The dimensional stability of the composite membranes increases with increasing FPI content in the membranes, whereas the proton conductivity decreases. The composite membranes show considerable proton conductivity from 2.0 × 10⁻² S cm⁻¹ to 8.9 × 10⁻² S cm⁻¹ at a temperature from 30 °C to 100 °C, depending on the FPI contents. The composite membranes with semi-IPN from FPI and Nafion^®212 have considerable high proton conductivity, excellent mechanical properties, thermal and dimensional stabilities.     

Stabilized heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation

Organic/inorganic composite membranes with different inorganic heteropolyacid (HPA) additives maintain sufficient proton conductivities for atmospheric pressure elevated temperature (>100 °C) polymer electrolyte fuel cell (PEFC) operation. However, membrane and membrane electrode assembly (MEA) processing is severely curtailed because of the solubility of the HPA additives in aqueous media. Composite membranes with the HPA (phosphotungstic acid; PTA) additive rendered insoluble by ion exchanging protons with larger cations such as Cs⁺, NH₄⁺, Rb⁺ and Tl⁺ were fabricated. The additive loss in aqueous media was lowered from nearly 100% (unmodified HPA) to about 5% (modified HPA). The membranes were robust, and demonstrated low H₂ crossover currents of around 2 mA/cm² for a 28 μm thick membrane. All membranes were evaluated at high temperatures and low relative humidities in an operating fuel cell. The conductivities of the composite membranes at 120 °C and 35% relative humidity were on the order of 1.6 × 10⁻² S/cm.     

Crystallinity, morphology, mechanical properties and conductivity study of in situ formed PVdF/LiClO4/TiO2 nanocomposite polymer electrolytes

Nanocomposite polymer electrolytes composed of poly(vinylidene fluoride) (PVdF), lithium perchlorate (LiClO₄) and TiO₂ nanoparticles were prepared by a solution-cast method. The nanosized ceramic filler, TiO₂, was synthesized in situ by a sol-gel process. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) analysis revealed that the crystalline phase and crystallinity were slightly decreased with the addition of TiO₂ to the PVdF/LiClO₄ system. Scanning electron microscopy (SEM) micrographs showed that the PVdF/LiClO₄/TiO₂ solid polymer electrolyte (SPE) membranes had a porous structure to a certain extent, and that the pore size decreased with increasing TiO₂ content. The overfull nanoparticles tended to aggregate on the surface and inside the pores at TiO₂ content above 15 wt.% so that the porosity decreased. Regarding mechanical properties, the strength of the PVdF/LiClO₄/TiO₂ electrolytes decreased after the uptake of EC/PC solution. In contrast to the conductive behavior of wet PVdF/LiClO₄/TiO₂ membranes relative to the uptake of EC/PC solution, the conductive mechanism of the solid membranes, after the lithium ion of LiClO₄ had already been installed in the PVdF solid polymer network, was mainly influenced by the TiO₂ nanoparticles. At a TiO₂ content of 10 wt.%, the solid and wet PVdF/LiClO₄/TiO₂ systems had the maximum conductivity values of 7.1 × 10⁻⁴ and 1.8 × 10⁻³ S/cm, respectively.     

Proton conducting membranes based on semi-interpenetrating polymer network of Nafion and polybenzimidazole

A new strategy to prepare the reinforced composite membranes for polymer electrolyte membrane fuel cells (PEMFCs), which can work both in humidified and anhydrous state, was proposed via constructing semi-interpenetrating polymer network (semi-IPN) structure from polybenzimidazole (PBI) and Nafion^®212, with N-vinylimidazole as the crosslinker. The crosslinkable PBI was synthesized from poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole) and p-vinylbenzyl chloride. The semi-IPN structure was formed during the membrane preparation. The composite membranes exhibit excellent thermal stability, high-dimensional stability, and significantly improved mechanical properties compared with Nafion^®212. The proton transport in the hydrated composite membranes is mainly contributed by the vehicle mechanism, with proton conductivity from ∼10⁻² S/cm to ∼10⁻¹ S/cm. When the temperature exceeds 100 °C, the proton conductivity of the semi-IPN membranes decreases quickly due to the dehydration of the membranes. Under anhydrous condition, the proton conductivity of the membranes will drop to ∼10⁻⁴ S/cm, which is also useful for intermediate temperature (100-200 °C) PEMFCs. The benzimidazole structure of PBI and the acidic component of Nafion^® provide the possibility for the proton mobility via structure diffusion involving proton transfer between the heterocycles with a corresponding reorganization of the hydrogen bonded network.     

Nafion 115/zirconium phosphate composite membranes for operation of PEMFCs above 100 °C

Composite Nafion/zirconium phosphate membranes were investigated for high temperature operation of proton exchange membrane fuel cells (PEMFCs). The composite membranes were prepared via impregnation of Nafion films (either commercial Nafion 115 or recast Nafion) with zirconyl chloride and 1 M phosphoric acid at 80 °C. An MEA employing a composite membrane prepared starting from commercial Nafion 115 gave a H₂/O₂ PEMFC performance of about 1000 mA/cm² at 0.45 V at a temperature of 130 °C and a pressure of 3 bar; this result compares very favorably with the performance of an MEA based on commercial unmodified Nafion, which gave only 250 mA/cm² at 0.45 V when operated under the same conditions of temperature and pressure. Similar experiments performed with recast Nafion and recast Nafion/zirconium phosphate composites confirmed an analogous improvement of performance of the composite membranes over the unimpregnated ones. In this case, the composite recast Nafion/zirconium phosphate gave about 1500 mA/cm² at 0.45 V at a temperature of 130 °C and a pressure of 3 bar. The composite membranes showed stable behavior during time when maintained at 130 °C, while irreversible degradation affected Nafion under the same conditions.     

Effects of crosslinkers on semi-interpenetrating polymer networks of Nafion and fluorine-containing polyimide

A series of reinforced composite membranes were prepared from Nafion^®212 and crosslinkable fluorine-containing polyimide (FPI) with various crosslinkers. The crosslinkable FPI reacts with the crosslinkers and forms semi-interpenetrating polymer networks (semi-IPN) structure with Nafion^®212. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and chemical oxidation stability of the composite membranes are studied. The degree of crosslinking is characterized by gel fraction of the composite membranes. Compared to pure Nafion^®212, the composite membranes exhibit excellent thermal stability, improved mechanical properties and dimensional stability. The tensile strength of the composite membranes is in the range of 37.3-51.2 MPa. All the composite membranes exhibit high proton conductivity which ranges from 1.9 × 10⁻² to 9.9 × 10⁻² S cm⁻¹. The proton conductivity of the composite membrane with 2-propene-1-sulfonic acid sodium salt (SAS) as the crosslinker is 9.9 × 10⁻² S cm⁻¹ at 100 °C which is similar to that of Nafion^®212 under the same condition.     

Nafion/Analcime and Nafion/Faujasite composite membranes for polymer electrolyte membrane fuel cells

The Nafion/zeolite composite membranes were synthesized for polymer electrolyte fuel cells (PEMFCs) by adding zeolite in the matrix of Nafion polymer. Two kinds of zeolites, Analcime and Faujasite, having different Si/Al ratio were used. The physico-chemical properties of the composite membranes such as water uptake, ion-exchange capacity, hydrogen permeability, and proton conductivity were determined. The fabricated composite membranes showed the significant improvement of all tested properties compared to that of pure Nafion membrane. The maximum proton conductivity of 0.4373 S cm⁻¹ was obtained from Nafion/Analcime (15%) at 80 °C which was 6.8 times of pure Nafion (0.0642 S cm⁻¹ at 80 °C). Conclusively, Analcime exhibited higher improvement than Faujasite.     

Preparation and testing of Nafion/titanium dioxide nanocomposite membrane electrode assembly by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium dioxide (TiO₂) composite membranes were manufactured with a novel ultrasonic‐spray technique (UST) and tested in proton exchange membrane fuel cell (PEMFC). The structures of the membranes were investigated by scanning electron microscopy (SEM), X‐ray diffraction (XRD), and thermogravimetric analysis. The composite membranes gained good thermal resistance with insertion of TiO₂. The SEM and XRD techniques have proved the uniform and homogeneous distribution of TiO₂ and the consequent enhancement of crystalline character of these membranes. The existence of nanometer size TiO₂ has improved the thermal resistance, water uptake, and proton conductivity of composite membranes. Gas diffusion electrodes were fabricated by UST. Catalyst loading was 0.4 (mg Pt) cm^?2 for both anode and cathode sides. The membranes were tested in a single cell with a 5 cm² active area operating at the temperature range of 70°C to 110°C and in humidified under 50% relative humidity (RH) conditions. Single PEMFC tests performed at different operating temperatures indicated that Nafion/TiO₂ composite membrane is more stable and also performed better than Nafion membranes. The results show that Nafion/TiO₂ is a promising membrane material for possible use in PEMFC at higher temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40541.     

Hybrid proton-conducting membranes for polymer electrolyte fuel cells: Phosphomolybdic acid doped poly(2,5-benzimidazole)—(ABPBI-H3PMo12O40)

The synthesis and characterization of a novel hybrid organic-inorganic material formed by phosphomolybdic acid H₃PMo₁₂O₄₀ (PMo₁₂) and poly(2,5-benzimidazole) (ABPBI) is reported. This material, composed of two proton-conducting components, can be cast in the form of membranes from methanesulfonic acid (MSA) solutions. Upon impregnation with phosphoric acid, the hybrid membranes present higher conductivity than the best ABPBI polymer membranes impregnated in the same conditions. These electrolyte membranes are stable up to 200 °C, and have a proton conductivity of 3 × 10⁻² S cm⁻¹ at 185 °C without humidification. These properties make them very good candidates as membranes for polymer electrolyte membrane fuel cells (PEMFC) at temperatures of 100-200 °C.     

Chitosan/TiO2 nanocomposite pervaporation membranes for ethanol dehydration

Novel chitosan/titanium dioxide (CS/TiO₂) nanocomposite membranes were prepared using tetrabutyl titanate (TBT) as precursor and acetyl acetone as chelating agent by in situ sol-gel process, and characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetry (TG). The characterization results demonstrated that nano-sized TiO₂ particles dispersed homogeneously within the CS matrix, which could be assigned to the hydrogen and titanoxane bonds formed between CS and TiO₂. Moreover, the pervaporation performance of these membranes was investigated using the separation of ethanol-water mixture as model system. Compared with CS/TiO₂ hybrid membranes prepared by blending method, most of CS/TiO₂ nanocomposite membranes prepared by in situ sol-gel process exhibited higher permeation flux and separation factor under the identical conditions. Among all the prepared membranes, CS/TiO₂ nanocomposite membrane containing 6 wt% TiO₂ exhibited the best pervaporation performance, whose averaged permeation flux and separation factor were 0.340 kg m⁻² h⁻¹ and 196 for 90 wt% aqueous solution of ethanol at 80 °C, respectively.     

Piezoelectric properties and microstructures of ZnO-doped Bi0.5Na0.5TiO3 ceramics

This article studies the microstructure and piezoelectric properties of a ceramic lead-free NBT under different amount of ZnO doping. X-ray diffraction shows that Zn²⁺ diffuses into the lattice of (Bi_0.5Na_0.5)TiO₃ to form a solid solution with a pure perovskite structure. By modifying the zinc oxide content, the sintering behavior of (Bi_0.5Na_0.5)TiO₃ ceramics was significantly improved and the grain size was increased. The piezoelectric coefficient d₃₃ for the 1.0 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1050 °C was found to be 95 pC/N, and the electromechanical coupling factor k_p = 0.13. However, the piezoelectric coefficient d₃₃ for the 0.5 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1140 °C was found to be 110 pC/N, and the electromechanical coupling factor k_p = 0.17.     

Hybrid inorganic-organic proton conducting membranes based on Nafion, SiO2 and triethylammonium trifluoromethanesulfonate ionic liquid

New hybrid inorganic-organic nanocomposite proton conducting membranes based on Nafion, SiO₂ and triethylammonium trifluoromethanesulfonate (TEATF), a protic ionic liquid, were prepared in two steps. First, various [Nafion/(SiO₂)_ψ] membranes with a SiO₂ weight percentage ranging from 0 to 15 wt.% were prepared by a solvent-casting protocol from DMF solutions. Second, the resulting hybrid membranes were neutralized with triethylammine (TEA) and then impregnated with triethylammonium trifluoromethanesulfonate (TEATF). The structure of the bulk hybrid materials and the interactions occurring between their components were investigated by vibrational spectroscopy (FT-IR ATR and FT-Raman). These latter studies allowed to determine that: (a) the fluorocarbon domains of Nafion consist of a mixture of helical chains with 10₃ and 15₇ helical conformations; (b) interactions take place between the components of the hydrophilic domains, which give rise to micelle-like nanoparticles where a SiO₂ core is covered by a shell of solvating TEATF molecules. The thermal, mechanical and electrical properties of the proposed materials were investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements, by dynamical mechanical analyses (DMA) and by broadband dielectric spectroscopy (BDS), respectively. The electric measurements indicated that [Nafion/(SiO₂)_ψ(TEA)]_3.67/(TEATF)_1.2 exhibits a σ_dc of 4.7 × 10⁻³ S cm⁻¹ at T = 105 °C and a SRC of 5 < T < 135 °C. These results classified this membrane as a very promising material for application in fuel cells.     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

Synthesis and emission analysis of RE (Eu or Dy):Li2TiO3 ceramics

The development and photoluminescence analysis of Eu³⁺or Dy³⁺ ions in the matrix of lithium titanate (Li₂TiO₃) ceramics by using a solid state reaction method are reported. Emission spectra of Eu³⁺:Li₂TiO₃ ceramics have shown strong red emission at 611 nm (⁵D₀ → ⁷F₂) with λ_exci = 392 nm (⁷F₀ → ⁵L₆) and from the Dy³⁺:Li₂TiO₃, a blue emission at 493 nm (⁴F_9/2 → ⁶H_15/2) and also an yellow emission at 582 nm (⁴F_9/2 → ⁶H_13/2) have been observed with λ_exci = 366 nm (⁶H_15/2 → ⁶P_5/2). Both the rare-earth ions containing ceramics have displayed their brighter emission performance from their measured spectral results. In addition, X-ray diffraction (XRD), Fourier transform infra red (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) have been used to characterize the structural properties of (Eu³⁺ or Dy³⁺):Li₂TiO₃ ceramics.     

Comparatively electrochemical studies at different operational temperatures for the effect of nanoclay platelets on the anticorrosion efficiency of DBSA-doped polyaniline/Na-MMT clay nanocomposite coatings

In this paper, DBSA-doped polyaniline (PANI)/Na⁺-montmorillonite (MMT) clay nanocomposite (PCN) materials have been successfully prepared with dodecylbenzenesulfonic acid (DBSA) as emulsifier and dopant for the emulsion polymerization of aniline. The as-prepared DBSA-doped samples were subsequently characterized by FTIR spectroscopy, WAXRD patterns and TEM. It should be noted that the nanocomposite coating containing 1 wt.% of clay loading was found to exhibit an observable enhanced corrosion protection on cold-rolled steel (CRS) electrode at higher operational temperature of 50 °C, which was even better than that of uncoated and electrode-coated with PANI alone at room temperature of 30 °C based on the electrochemical parameter evaluations (e.g., E_corr, R_p, I_corr, R_corr and impedance). In this work, all electrochemical measurements were performed at a double-wall jacketed cell, covered with a glass plate, through which water was circulated from a thermostat to maintain a constant operational temperature of 30, 40 and 50 ± 0.5 °C. Moreover, a series of electrochemical parameters shown in Tafel, Nyquist and Bode plots were all used to evaluate PCN coatings at three different operational temperatures in 5 wt.% aqueous NaCl electrolyte. Effect of material composition on the molecular weight and optical properties of neat PANI and PCN materials, in the form of solution, were studied by gel permeation chromatography (GPC) and UV-vis spectra, respectively. Finally, electrical conductivity at three different operational temperatures of PANI and PCN powder-pressed pellets doped with different inorganic acids such as HCl, HNO₃ and H₂SO₄ was also investigated through the measurements of standard four-point-probe technique.     

Microwave dielectric properties of the (1 − x)Mg2TiO4-xCaTiO3-y wt.% ZnNb2O6 ceramics system

Composite ceramics based on (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12-0.16, y = 0-8) were prepared by a conventional mixed-oxide route. Zn²⁺ partially replaced Mg²⁺ in Mg₂TiO₄ and formed the spinel-structured (Mg_1−δZn_δ)₂TiO₄ phase. Nb²⁺, is known to be solid soluble in CaTiO₃, was found to change its shape from cubic to pliable. A bi-phase system (Mg_1−δZn_δ)₂TiO₄ and CaTiO₃ exhibited in all samples, where a small amount of second phase Mg_1−δZn_δTiO₃ was also detected. The microwave dielectric properties of specimens were strongly related to ZnNb₂O₆ and CaTiO₃ content. As y increased, ?_r and τ_f decreased, however, Q × f decreased to a minimum value and started to increase thereafter. It was also found that ?_r and τ_f increased and Q × f decreased with increasing x. The optimized microwave dielectric properties with ?_r = 18.37, Q × f = 31,027 GHz (at 6 GHz), and τ_f = 0.51 ppm/°C were achieved for (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12, y = 4) sintered at 1360 °C for 6 h.     

Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Novel graphite/TiO2 electrochemical cells as a safe electric energy storage system

A graphite/TiO₂ full cell has been developed as a new safety energy storage system using a highly safety process. The crystal structures of the anatase TiO₂ electrode have been investigated with respect to the performance of the electrodes. Due to the large anion intercalation into the graphite positive electrode, the possible charging potential can be raised to around 5.3 V against the Li/Li⁺ electrode, which is a higher charging voltage than lithium-ion batteries (maximum voltage is around 4.3 V vs. Li/Li⁺). In situ XRD measurements have been carried out on both the cathode and anode electrodes of the graphite/TiO₂ cell during the charge process to elucidate the intercalation mechanism.