Molecular dynamics simulations of Krytox-Silica-Nafion composite for high temperature fuel cell electrolyte membranes

A five percent by weight of carboxylic acid terminated perfluoropolyether hybrid with silica (Krytox-Silica) in Nafion composite polymer was used in the modification of a polymer electrolyte fuel cell membrane in order to improve its efficiency at high operating temperatures. Molecular dynamics (MD) simulations were carried out in order to understand the microscopic properties of two systems, Krytox-Silica in Nafion and pure Nafion. A model of five percent Krytox-Silica in a Nafion composite polymer consisting of 15 Nafion side chains, 15 hydronium ions and one of Krytox-Silica was used. In another system, pure Nafion was modeled without Krytox-Silica. Models with various amounts of water molecules and temperatures were simulated to study the water content and temperature effects. The results were in good agreement with the experiments and could be used to describe the application of Krytox-Silica-Nafion composite at high temperatures. The effect of the amount of water molecules on the diffusion coefficient or proton conductivity showed more deviations between 5% wt of Krytox-Silica-Nafion composite and pure Nafion system at lower water content (or higher temperature) than at high water content (or low temperature). According to the diffusion coefficient results, the percentage of water molecules at each temperature corresponded to the known experimental trend. Silica, as the water absorbent in the hybrid polymer membrane, did not have a strong interaction with water molecules or H₃O⁺ ions; thus the proton conductivities will not be highly affected by adding Krytox-Silica to the Nafion.     

Hydrated proton self-diffusion study in ion-exchange membranes by MRI and impedance spectroscopy

Average self-diffusion coefficients of hydrated protons in sulfonated poly (ether sulfone) (SPES), Interpolymer and Nafion 117 membranes are measured by impedance spectroscopy and pulsed field gradient nuclear magnetic resonance (PFG NMR). Self-diffusion coefficient measured by impedance spectroscopy shows two patterns, one is at the low hydration states, i.e. λ = 2–4 and another at the high hydration states (starting from λ = 4). At lower hydration states the diffusion values increases slowly while steep increment is found at higher hydration level. PFG NMR shows similar trend throughout the measurements. The diffusion coefficients at higher temperatures are higher than at lower temperatures as a function of λ for SPES membrane. At the same value of λ, hydrated proton self-diffusion coefficient is found to be increased by 18% from 295.5 to 303 K. The thermo-mechanical properties of the membranes characterised by the means of DSC, TGA and DMA analysis show SPES, stable up to 450 °C and can be used for the high-temperature applications.     

Perfluorosulphonic acid (Nafion) membrane as a separator for an advanced alkaline water electrolyser

Nafion membranes of two different equivalent weights (eq. wt) were evaluated as a separator in an alkaline electrolyser with nickel screen electrodes in both KOH and NaOH electrolytes over the concentration range of 10–30 wt % and at temperatures from 25 to 160° C. For the same current densities, the cell voltage with 30% KOH electrolytes was more than twice that with 30% NaOH. This result correlates with the water content of the membrane which is almost twice as high in NaOH electrolytes. Thinner membranes and membranes of lower equivalent weight give lower cell voltages. Materials and performance considerations indicate that a membrane of 1000 eq. wt is the optimum separator for an alkaline electrolyser. Indications are that LiOH may be an even better electrolyte than NaOH for use with Nafion membranes. Further improvements in performance can be expected by membrane pretreatment such as exposing the membrane to elevated temperature in water. Nafion membranes have excellent physical and mechanical properties in alkaline electrolyte and can be used at temperatures up to 250° C.Work performed under the auspices of the US Department of Energy.     

Anisotropic membranes from electrospun mats of sulfonated/nonsulfonated poly (ether ketone)s containing ion-rich paths as proton exchange membranes

Anisotropic proton exchange membranes composed of five layers with different contents of ionic groups across the membrane were prepared by simultaneous electrospinning of sulfonated and nonsulfonated poly(ether ketone) (PEK)s. To prepare nonporous and defect- free membranes from electrospun mats, nonsulfonated fibers as hydrophobic part of the membrane were melted by hot-pressing so that covered sulfonated fibers (hydrophilic part). Prepared membranes showed better thermal and dimensional stability compared to Nafion 115. Proton conductivity of membranes was comparable with Nafion especially at higher temperatures. Water uptake of prepared membranes and mechanical strength of them were in an acceptable range. The results showed that the difference between sulfonated PEK fibers in surface and center of the membranes affect proton conductivity and mechanical properties of the membranes.     

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.     

PEO–PPO–PEO triblock copolymer/Nafion blend as membrane material for intermediate temperature DMFCs

This paper describes homogeneous triblock copolymer/Nafion blend membranes, which facilitate proton conduction in direct methanol fuel cells (DMFCs) at intermediate temperatures. The interaction between the two polymer components is investigated by FT-IR spectroscopy. The blend membranes show higher proton conductivity than recast Nafion under partially anhydrous conditions. Protons can be transported with the assistance of ether chain under such conditions at elevated temperature. In addition, the membranes exhibit more favourable methanol permeability and selectivity. This kind of blend membrane shows somewhat better performance in DMFC compared to bare recast Nafion at intermediate temperature (≥120 °C). This work is a first attempt in our group to design membrane materials with enhanced proton conductivity under conditions typical of intermediate temperature DMFCs.     

Novel Nafion–zirconium phosphate nanocomposite membranes with enhanced stability of proton conductivity at medium temperature and high relative humidity

In order to increase the stability of Nafion conductivity at temperatures higher than 100 °C, composite membranes made of recast Nafion filled with different percentages of zirconium phosphate (ZrP) were investigated. The membrane preparation was carried out by a simple synthetic procedure based on the use of solutions of ZrP precursors in dimethylformamide. The formation of insoluble -type ZrP nanoparticles within the Nafion matrix was proved by ³¹P-MAS NMR and X-ray diffractometry. The membranes were characterized by TEM microscopy, ion-exchange capacity determinations, static stress–strain mechanical tests and conductivity measurements as a function of filler loading, at controlled relative humidity (r.h.) and temperature. An increasing filler loading results in enhanced membrane stiffness and in lower conductivity compared with pure recast Nafion. At 90% r.h. and 100 °C, the conductivity decreases from ≈0.07 S cm⁻¹ for pure Nafion to ≈0.03 S cm⁻¹ for the composite membrane containing 25 wt.% ZrP. Systematic conductivity measurements as a function of r.h. and temperature were carried out to draw a stability map for the conductivity of pure recast Nafion and of a composite membrane filled with 10 wt.% ZrP. These maps provide for each r.h. value the maximum temperature at which the conductivity remains stable for at least 150 h. The effect of zirconium phosphate is to increase the stability of conductivity at high temperature, with a gain up to 20 °C. This stability enhancement has been ascribed to the higher stiffness of the composite membrane.     

硫酸水溶液中3-甲基吡啶透过Nafion膜的渗透

引　言Nafion膜是美国Dupont公司开发的一类全氟磺酸型阳离子交换膜, 由疏水性骨架与亲水性磺酸基团组成, 具有优良的离子选择透过性和化学稳定性, 广泛用于膜电解制碱工业. 在质子交换膜燃料电池、膜分离、有机电合成等领域的应用研究进展迅速[1~5]. 在直接燃料电池中, Nafio     

The study of PTFE/Nafion/Silicate membranes operating at low relative humidity and elevated temperature

The performance and stability of PTFE/Nafion/Silicate composite membranes (PNS membrane) were studied at low and medium operating temperatures with different humidity, and compared with the Nafion112 membrane at the same conditions. The PNS membrane was prepared by impregnation of PTFE/Nafion composite membrane via sol-gel process with TEOS (tetraethoxysilane). When operated cell at low temperature of 60 °C with 100% R.H. humidified H₂/O₂ gases, the PNS membrane performs better than Nafion112, with 1.0, and 0.4 W/cm², respectively. When operated cell at 60 °C with 37% R.H. humidified gases, the discharge stability of PNS membrane is stable than that of Nafion112, this is due to that silicate could hold more water in the PNS membrane at low relative humidity. While the inlet of cell gases temperature keeps at 80 °C, the cell temperature varied 90, 100, and 110 °C, with 20 psig back pressure, their relative humidities were 67, 48 and 33%, respectively. The stability of discharge current remains constant except in the case of cell temperature being as high as 110 °C. It is believed that silicate could hold water except in the case of cell temperature at 110 °C, which is resulted as the membrane dehydration. On the other hand, the Nafion112 cannot operate at low humidity with cell temperature higher than 80 °C owing to membrane dehydration. The silica modified PTFE/Nafion membrane shows the improving cell performance at lower relative humidity due to adsorbed water inside the membrane and catalyst layer.     

Thermal properties of alkali‐activated aluminosilicate composite with lightweight aggregates at elevated temperatures

Thermal diffusivity, specific heat capacity and linear thermal expansion coefficient of a lightweight composite with alkali‐activated aluminosilicate binder and heat‐resistant aggregate mixture of expanded vermiculite and electrical porcelain are measured in a wide temperature range. Experimental results show that the studied material has good prerequisites for high‐temperature applications. The apparent thermal diffusivity at one‐sided heating is significantly lower than at room temperature, and decreases for temperatures higher than 400^°C. The effective specific heat capacity at elevated temperatures increases with temperature in almost the whole analyzed temperature range. The linear thermal expansion coefficient is substantially lower compared with common cement‐based composites. Copyright © 2010 John Wiley & Sons, Ltd.     

Free volume and permeabilities of O2 and H2 in Nafion membranes for polymer electrolyte fuel cells

The mechanism of gas permeation in Nafion membranes for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Three different samples, a membrane with ionic exchange capacity (IEC) = 0.92 meq/g, and recast samples with IEC = 0.92 and 1.00 meq/g were used after drying. Free volume was quantified using the positron annihilation lifetime (PAL) technique and gas permeabilities were measured for O₂ and H₂ as functions of temperature and relative humidity. Good linear correlations between the logarithm of the permeabilities at different temperatures and reciprocal free volume indicate that gas permeation in dry Nafion is governed by the free volume. Nevertheless permeabilities are much smaller than the corresponding flexible chain polymer with a similar free volume size due to stiff chains of the perfluoroethylene backbone. In highly hydrated Nafion above 60% relative humidity, where the O₂ permeability varies oppositely to the free volume, gas permeation proved to be controlled by the gradual increase in overall flexibility of the Nafion–water system.     

Molecular modeling studies of polymer electrolytes for power sources

Density functional theory and classical molecular dynamics simulations permit us to elucidate details of ionic and molecular transport useful for the design of polymer electrolyte membranes. We consider two systems of current interest: (a) ionic transport in polyethylene-oxide compared to that in a polyphosphazene membrane targeted to be a good ionic carrier but a bad water carrier and (b) transport of oxygen and protons through hydrated nafion in the vicinity of a catalyst phase.It is shown that in polyphosphazene membranes, nitrogen atoms interact more strongly with lithium ions than ether oxygens do. As a result of the different complexation of Li⁺ with the polymer sites, Li⁺ has a much higher diffusion coefficient in polyphosphazene than in polyethylene oxide electrolyte membranes, with the consequent relevance to lithium-water battery technology.For the hydrated membrane/catalyst interface, our simulations show that the Nafion membrane used in low-temperature fuel cells interacts strongly with the catalytic metal nanoparticles directing the side chain towards the catalyst surface. Results at various degrees of hydration of the membrane illustrate the formation of water clusters surrounding the polymer hydrophilic sites, and reveal how the connectivity of these clusters may determine the transport mechanism of protons and molecular species.     

Processing induced morphological development in hydrated sulfonated poly(arylene ether sulfone) copolymer membranes

The development of morphological solid-state structures in sulfonated poly(arylene ether sulfone) copolymers (acid form) by hydrothermal treatment was investigated by water uptake, dynamic mechanical analysis (DMA), and tapping mode atomic force microscopy (TM-AFM). The water uptake and DMA studies suggested that the materials have three irreversible morphological regimes, whose intervals are controlled by copolymer composition and hydrothermal treatment temperature. Ambient temperature treatment of the membranes afforded a structure denoted as Regime1. When the copolymer membranes were exposed to a higher temperature, AFM revealed a morphology (Regime2) where the phase contrast and domain connectivity of the hydrophilic phase of the copolymers were greatly increased. A yet higher treatment temperature was defined which yielded a third regime, likely related to viscoelastic relaxations associated with the hydrated glass transition temperature (hydrated T_g). The required temperatures needed to produce transitions from Regime1 to Regime2 or Regime3 decreased with increasing degree of disulfonation. These temperatures correspond to the percolation and hydrogel temperatures, respectively. Poly(arylene ether sulfone) copolymer membranes with a 40% disulfonation in Regime2 under fully hydrated conditions showed similar proton conductivity (∼0.1 S/cm) to the well-known perfluorinated copolymer Nafion^® 1135 but exhibited higher modulus and water uptake. The proton conductivity and storage modulus are discussed in terms of each of the morphological regimes and compared with Nafion 1135. The results are of particular interest for either hydrogen or direct methanol fuel cells where conductivity and membrane permeability are critical issues.     

Transport parameters for the modelling of water transport in ionomer membranes for PEM-fuel cells

The water transport number (drag coefficient) and the hydraulic permeability were measured for Nafion. The results show a significant increase of both parameters with increasing water content indicating that they are strongly influenced by the membrane microstructure. Based on these experimental studies a new model approach to describe water transport in the H₂-PEFC membrane is presented. This approach considers water transport by electro-osmosis caused by the proton flux through the membrane and by osmosis caused by a gradient in the chemical potential of water. It is parametrized by the measured data for the water transport number and the hydraulic permeability of Nafion. First simulation results applying this approach to a one-dimensional model of the H₂-PEFC show good agreement with experimental data. Therefore, the developed model can be used for a new insight into the dominating mechanisms of water transport in the membrane.     

PBX9501热感度、热膨胀及力学性能的分子动力学模拟

采用分子动力学模拟研究了不同温度和压强条件下PBX9501炸药的热感度、热膨胀和力学性能。通过体系中各组分最大引发键键长的变化判断温度对其热感度的影响;预测了PBX9501体系在不同温度下的热膨胀系数;采用静态力学理论分析其力学性能随温度和压强的变化。结果表明,在295~450K,随温度的升高,PBX9501炸药的敏感性增大,且在375K时其引发键的最大键长显著增大;热膨胀系数随温度升高而减小;随温度升高其脆性越明显,随压强的增加其韧性越好。     

Nafion®—titania nanocomposite proton exchange membranes

Proton exchange membranes consisting of Nafion^® and crystallized titania nanoparticles have been developed to improve water‐retention and proton conductivity at elevated temperature and low relative humidity. The anatase‐type titania nanoparticles were synthesized in situ in Nafion solution through sol–gel process and the size of the formed titiania nanoparticles is in the range of 3–6 nm. The formed nanoparticles are well‐dispersed in Nafion solution at the titania concentration of 5 wt %. The glass transition temperature of the formed Nafion‐titania composite membrane is about 20^oC higher than that of plain Nafion membrane. At elevated temperature (above 100°C), the Nafion‐titania nanocomposite membrane shows higher water uptake ability and improved proton conductivity compared to pure Nafion membrane. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

氨的自扩散系数的分子动力学模拟研究

采用分子动力学模拟方法研究了氨在较宽温度和压力范围的分子自扩散系数。从常温到高温,自扩散系数的模拟值与实验值吻合得很好,这表明可以采用分子动力学模拟来代替实验,获得高温高压条件下实验难以测量的自扩散系数。     

Synthesis and performance of water‐retention PEMs with nafion‐intercalating‐montmorillonite hybrid

A proton exchange membrane modified with montmorillonite (MMT) was prepared by introducing perfluorosulfonylfluroride resin into the interlayer of MMT at high temperature and high pressure. The intercalating structure was analyzed by wide‐angle X‐ray diffraction (XRD), and the results showed that the polymer was indeed intercalated into the interlayer of the modified montmorillonite (m‐MMT) matrix without destroying the microstructure of the matrix by largely increasing the d‐spacings of m‐MMT. SEM analysis on the surface and cross‐section of these composite membranes shows that the MMT nanoparticles are dispersed in the membrane uniformly. This composite membrane reveals excellent water retention and conductance at high temperature. The composite membrane was evaluated in a single cell, and the results show that the performance of composite membrane is higher than that of pure Nafion membrane at hightemperature especially without external‐humidification. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Partially Sulfonated Poly(vinylidene fluoride) Induced Enhancements of Properties and DMFC Performance of Nafion Electrolyte Membrane

Partially sulfonated poly(vinylidene fluoride) (SPVdF) has been prepared by incorporation of sulfonic acid groups within poly(vinylidene fluoride), using chlorosulfonic acid as the sulfonating agent. The degree of sulfonation (DS) has been varied by modulating the duration of the sulfonation reaction. Blending of SPVdF (having DS = 36.78%) with Nafion at a constituent wt.% ratio of SPVdF:Nafion = 70:30 has resulted in the fabrication of polymer electrolyte membrane with superior properties compared to pristine Nafion‐117 membrane. This particular blend composition exhibited a proton conductivity value of 3.6 × 10⁻² S cm⁻¹ (i.e. ∼12.5% increase over Nafion‐117), a methanol permeability value of 6.81 × 10⁻⁷ cm² s⁻¹ at 6M methanol concentration (i.e. ∼99.31% decrease from Nafion‐117) and a corresponding membrane selectivity value of 5.29 × 10⁴ Ss cm⁻³ (i.e. an increase of approximately two‐orders of magnitude over Nafion‐117) at 20 °C. In addition, this blend composition has also exhibited (a) better heat stability at temperatures as high as 160 °C by virtue of it possessing higher glass transition temperature, (b) higher storage modulus, (c) higher stress relaxation at high angular frequency and (d) superior DMFC performance at high methanol feed concentration in presence of humidified, as well as, non‐humidified air as the catholyte, compared to Nafion‐117 membrane.     

DMFCs用磺化聚醚醚酮/磺化酚酞型聚醚砜共混质子交换膜

A sulfonated poly(ether ether ketone) (SPEEK) membrane with fairly high degree of sulfonation (DS) swells excessively and even dissolves at high temperature. To solve these problems, sulfonated phenolphthalein poly(ether sulfone) (SPES-C, DS 53.7%) is blended with the SPEEK matrix (DS 55.1%, 61.7%) to prepare SPEEK/SPES-C blend membrane. The decrease in swelling degree and methanol permeability of the membrane is dose-dependent. Pure SPEEK (DS 61.7%) membrane dissolves completely in water at 70ºC, whereas the swelling degree of the SPEEK (DS 61.7%)/SPES-C (40%, by mass) membrane is 29.7% at 80ºC. From room temperature to 80ºC, the methanol permeability of all SPEEK (DS 55.1%)/SPES-C blend membranes is about one order of magnitude lower than that of Nafion®115. At higher temperature, the addition of SPES-C polymer increases the dimensional stability and greater proton conductivity can be achieved. The SPEEK (DS 55.1%)/SPES-C (40%, by mass) membrane can withstand temperatures up to 150ºC. The proton conductivity of SPEEK (DS 55.1%)/SPES-C (30%, by mass) membrane approaches 0.16 S•cm－1, matching that of Nafion115 at 140ºC and 100% RH, while pure SPEEK (DS 55.1%) membrane dissolves at 90ºC. The SPEEK/SPES-C blend membranes are promising for use in direct methanol fuel cells because of their good dimensional stability, high proton conductivity, and low methanol permeability.