Synthesis and characterization of proton conducting inorganic-organic hybrid nanocomposite membranes based on tetraethoxysilane/trimethylphosphate/3-glycidoxypropyltrimethoxysilane/heteropoly acids

A series of novel proton conductive inorganic-organic nanocomposite hybrid membranes doped with phosphotungstic acid (PWA)/phosphomolybdic acid (PMA) and trimethylphosphate PO(OCH₃)₃ have been prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), and tetraethoxysilane (TEOS) as precursors. The hybrid membranes were studied with respect to their structural and thermal properties, elastic moduli and proton conductivity. Thermal analysis including TG and DTA confirmed that the membranes were thermally stable up to 200 °C. Thermal stability of membranes was significantly enhanced by the presence of SiO₂ framework. Proton conductivity of 1.59 × 10⁻² S/cm with composition of 50TEOS-5PO(OCH₃)₃-35GPTMS-10PWA was obtained (1.15 × 10⁻² S/cm for 10 mol% PMA) at 90 °C under 90% relative humidity. The proton conductivity of the nanocomposite membranes is due to the proton-conducting path through the GPTMS-derived “pseudopolyethylene oxide (pseudo-PEO)” networks in which the trapped solid acid (PWA/PMA) as a proton donor is contained. The molecular water absorbed in the polymer matrix is also presumed to provide high proton mobility, resulting in an increase of proton conductivity with increasing relative humidity.     

Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells

Highly conductive and hydration retentive organic-inorganic hybrid proton exchange membranes for direct methanol fuel cells were synthesized by in situ sol-gel generation of mesoporous silica (mSiO₂) in sulfonated polyimide (SPI) via self-assembly route of organic surfactant templates for the tuning of the architecture of silica. The microstructure and properties of the resulting hybrid membranes were extensively characterized. The mesopores of about 3 nm in silica dispersion phase were formed in the SPI matrix. The existence of the mesoporous structure of silica improved the thermal stability, water-uptake and proton conductivity as well as methanol resistance of the hybrid membranes. The hybrid membrane with 30 wt.% mSiO₂ exhibited the water-uptake of 44.8% at 25 °C, and proton conductivity of 0.214 S cm⁻¹ at 80 °C at RH = 100%, while pure SPI exhibited the values of 40.6% and 0.179 S cm⁻¹ in the same test conditions, respectively. The results suggested that the highly hydrophilic character of Si-OH groups and the large surface area of mSiO₂ should contribute to the improvement of the water-uptake, meanwhile the mesoporous channels may supply the continuous proton conductive pathway in the hybrid membranes.     

Proton conductivity and morphology of new composite membranes based on zirconium phosphates,phosphotungstic acid,and silicic acid for direct hydrocarbon fuel cells applications

The effect of Phosphotungstic acid (PWA) on the proton conductivity and morphology of zirconium phosphate (ZrP), porous polytetrafluoethylene (PTFE), glycerol (GLY) composite membrane was investigated in this work. The composite membranes were synthesized using two approaches: (1) Phosphotungstic acid (PWA) added to phosphoric acid and, (2) PWA + silicic acid were added to phosphoric acid. ZrP was formed inside the pores of PTFE via the in situ precipitation. The membranes were evaluated for their morphology and proton conductivity. The proton conductivity of PWA–ZrP/PTFE/GLY membrane was 0.003 S cm^?1. When PWA was combined with silicic acid, the proton conductivity increased from 0.003 to 0.059 S cm^?1 (became about 60% of Nafion’s). This conductivity is higher than the proton conductivity of Nafion–silica–PWA membranes reported in the literature. The SEM results showed a porous structure for the modified membranes. The porous structure combined with this reasonable proton conductivity would make these membranes suitable as the electrolyte component in the catalyst layer for direct hydrocarbon fuel cell applications.     

Proton conducting polydimethylsiloxane/zirconium oxide hybrid membranes added with phosphotungstic acid

Inorganic polymer based hybrid membranes consisting of zirconium oxide and polydimethylsiloxane (PDMS) have been synthesized by sol-gel processes. The organic/inorganic polymeric hybrid membranes showed thermal stability and flexibility up to 300 °C. The membrane becomes proton conducting polymer electrolyte when added with 12-phosphotungstic acid (PWA). The conductivity of the membranes was measured in the temperature range from room temperature to 150 °C under saturated humidity and a maximum conductivity of 5×10⁻⁵ S cm⁻¹ was obtained at 150 °C.     

PWA/silica doped sulfonated poly(ether sulfone) composite membranes for direct methanol fuel cells

A novel sulfonated poly(ether sulfone) (SPES)/phosphotungstic acid (PWA)/silica composite membranes for direct methanol fuel cells (DMFCs) application were prepared. The structure and performance of the obtained membranes were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), water uptake, proton conductivity, and methanol permeability. Compared to a pure SPES membrane, PWA and SiO₂ doped membranes had a higher thermal stability and glass transition temperature (T_g) as revealed by TGA‐FTIR and DSC. The morphology of the composite membranes indicated that SiO₂ and PWA were uniformly distributed throughout the SPES matrix. Proper PWA and silica loadings in the composite membranes showed high proton conductivity and sufficient methanol permeability. The selectivity (the ratio of proton conductivity to methanol permeability) of the SPES‐P‐S 15% composite membrane was almost five times than that of Nafion 112 membrane. This excellent selectivity of SPES/PWA/silica composite membranes indicate a potential feasibility as a promising electrolyte for DMFC. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Organic-inorganic composite membranes as addition of SiO2 for high temperature-operation in polymer electrolyte membrane fuel cells (PEMFCs)

Organic-inorganic composite membranes for operation above 100 °C in polymer electrolyte membrane fuel cells (PEMFCs) were prepared, characterized and cell-tested. Composite membranes were obtained by mixing organic polymers, which have a SO₃H group as a proton conductor with inorganic material, SiO₂, using the sol-gel process. Electron probe micro analyser (EPMA) was used to show the homogeneous and uniform distribution of SiO₂. The physico-chemical properties of all membranes were investigated regarding their tensile strength, water uptake and thermogravimetric analyzer (TGA). Due to a higher water uptake and thermal stability of composite membranes, the cell performances at high temperatures above 100 °C, were improved. In addition, the SiOH group in the composite membrane was shown to play a major role in capturing water strongly and maintaining proton conductivity even at high temperature. Furthermore, the fuel cell performance of organic-inorganic composite membranes was superior to that of the Nafion membrane at high current density over all ranges of temperature.     

Proton conducting polydimethylsiloxane/metal oxide hybrid membranes added with phosphotungstic acid(II)

Flexible polymer electrolyte membranes consisting of zirconia (titania) and polydimethylsiloxane (PDMS) with the different molecular mass of 4500 and 600 have been synthesized by sol-gel processes. The polymeric membranes showed thermal stability and flexibility up to 300 °C due to the presence of cross-linkable inorganic nano-phase in the hybrid macromolecular matrix. The membrane becomes proton conducting polymer electrolyte by addition of 12-phosphotungstic acid (PWA). The conductivity increased up to 7.7×10⁻² S/cm through ultrasonic treatment on the membrane, which is in the application level to polymer electrolyte fuel cells. The hybrid materials can be recognized as new, low cost and environment friendly electrolyte membranes.     

Blend membranes based on sulfonated poly(ether ether ketone) and poly(vinylidene fluoride) for high performance direct methanol fuel cell

Various polymeric blends based on sulfonated poly(ether ether ketone) (sPEEK)/poly(vinylidene fluoride) (PVdF) were prepared for the membranes of direct methanol fuel cell. The blend membranes showed good compatibility within a limited composition range of less than around 10 wt% of PVdF. The blend membrane containing 2.5 wt% exhibited highest proton conductivity at room temperature among the tested blends. The dimensional stability was enhanced with introducing PVdF into the blend membrane. These could contribute to high performance of the cell based on the blend membrane.     

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.     

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.     

Fast proton conductor under anhydrous condition synthesized from 12-phosphotungstic acid and ionic liquid

In this study, we synthesized a molecular hybrid conductor electrolyte using PWA ([H₃PW₁₂O₄₀·nH₂O]) and [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl) amide] ([BMIM][TFSI]) ionic liquid. The [BMIM][TFSI] ionic liquid can interact with the strongly acidic PWA. The hybrids were hydrophilic, and included some water molecules in the structure of the hybrids. The water molecules remained up to 80 °C, contributing to improve conductivity under an anhydrous N₂ atmosphere. The conductivity of PWA-[BMIM][TFSI] hybrid under anhydrous conditions increased from 10⁻⁴ S/cm to 0.04 S/cm at 60 °C. The conductivity of the hybrids at each temperature was higher than that of pure PWA and [BMIM][TFSI] under anhydrous condition. It can be due to the protonic carriers.     

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.     

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.     

Studies on anhydrous proton conducting membranes based on imidazole derivatives and sulfonated polyimide

Anhydrous proton conducting membranes based on sulfonated polyimide (sPI) and imidazole derivatives were prepared. The acid-base composite membranes show a good chemical oxidation stability and high thermal stability. The addition of imidazole derivatives in sPIs can improve the chemical oxidation stability of the composite membranes enormously, and even much better than that of pure sPI. The proton conductivity of a typical sPI/xUI(2-undecylimidazole) composite membrane can reach 10⁻³ S cm⁻¹ at 180 °C under the anhydrous condition. The proton conductivity of the acid-base composite membranes increases significantly with increasing content of UI. Moreover, UI in sPI/xUI composite membrane is difficult to be brought out by the vapor due to the existence of long hydrophobic moiety, which will improve the stability and lifetime of the membranes in the fuel cells.     

New organic-inorganic crystalline electrolytes synthesized from 12-phosphotungstic acid and the ionic liquid [BMIM][TFSI]

New organic-inorganic hybrid crystalline electrolytes comprised of 12-phosphotungstic acid (PWA) and the ionic liquid [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl)amide] ([BMIM][TFSI]) with high thermal stability and high ion conductivity at high temperatures were obtained. In the new hybrids, there was a strong interaction between [BMIM]⁺ of the ionic liquid and PWA. The hybrids were very stable up to about 400 °C and showed a high ion jump during heating and cooling processes. Based on results from TG-DTA, DSC, and NMR spectroscopy, the ion jump was due to melting and solidification of the hybrid. The structures of powder and single-crystal samples of the hybrids were also studied. The chemical formula of the hybrid in the single crystal was determined to be PW₁₃C₃₂H₅₆O₅₄N₈S_0.16F_0.26. This is nearly the same as those determined from the powder samples. By analyzing single-crystal X-ray diffraction (XRD) data, the hybrid was determined to crystallize in the space group Pca2₁ with the lattice constants a = 18.316(3), b = 18.327(3), and c = 16.657(3) Å. The powder XRD data of the hybrid were assigned.     

Synthesis of sulfonated poly(arylene-co-naphthalimide)s as novel polymers for proton exchange membranes

A series of novel sulfonated poly(arylene-co-binaphthalimide)s (SPPIs) were successfully synthesized via Ni(0) catalytic coupling of sodium 3-(2,5-dichlorobenzoyl)benzenesulfonate and bis(chloronaphthalimide)s. Bis(chloronaphthalimide)s were conveniently prepared from 5-chloro-1,8-naphthalic anhydride and various diamines. Tough and transparent SPPI membranes were prepared and the electrolyte properties of the copolymers were intensively investigated as were the effects of different diamine structures on the copolymer characterisitics. The copolymer membrane Ia-80, with an ion exchange capacity (IEC) of 2.50 meq g⁻¹, displayed a higher proton conductivity, i.e. 0.135 S cm⁻¹ at 20 °C, as compared to Nafion 117 (0.09 S cm⁻¹, 20 °C). The copolymer membrane Id-70, containing 3,3′-dimethyl-4,4′-methylenedianiline (DMMDA) units, exhibited excellent stability toward water and oxidation due to the introduction of hydrophobic methyl groups on the ortho-position of the imido bond in the copolymer. The mechanical property of Id-70 remained virtually unchanged after immersing membrane in pressured water at 140 °C for 24 h. Furthermore, the introduction of aliphatic segment a hexane-1,6-diamine (HDA) in copolymer led to a significant increase in proton conductivity and water uptake with increasing temperature; the proton conductivity of the Ic-70 membrane reached 0.212 S cm⁻¹ at 80 °C, which was higher than Nafion 117 as well as of the membranes based on aromatic diamines at equivalent IEC values. Consequently, these materials proved to be promising as proton exchange membranes.     

PWA-13X-CS复合质子交换膜的性能研究

用13X分子筛负载无机质子导体-磷钨酸,然后加入壳聚糖(CS)中制备得到PWA-13X-CS复合质子交换膜,对其进行扫描电镜表征,测试了其吸水率、溶胀度、质子导电率、甲醇渗透系数等性质。结果表明PWA-13X-CS复合质子交换膜溶胀度较小,机械性能较好,质子导电率明显高于壳聚糖空白膜,且随温度升高呈上升趋势,其质子导电活化能低于壳聚糖空白膜,甲醇渗透系数小于Nafion117膜。将其与同样添加负载磷钨酸的13X分子筛的聚酰亚胺复合膜及聚乙烯醇复合膜性能进行对比,结果表明PWA-13X-CS复合质子交换膜综合性能较优,在直接甲醇燃料电池中具有较好的应用潜力。     

Improvement in silicon-containing sulfonated polystyrene/acrylate membranes by blending and crosslinking

Silicon-containing sulfonated polystyrene/acrylate-poly(vinyl alcohol) (Si-sPS/A-PVA) and Si-sPS/A-PVA-phosphotungstic acid (Si-sPS/A-PVA-PWA) composite membranes were fabricated by solution blending and physical and chemical crosslinking methods to improve the properties of silicon-containing sulfonated polystyrene/acrylate (Si-sPS/A) membranes. FTIR spectra clearly show the existence of various interactions and a crosslinked silica network in composite membranes. The potential of the composites to act as proton exchange membranes in direct methanol fuel cells (DMFCs) was assessed by studying their thermal and hydrolytic stability, swelling, methanol diffusion coefficient, proton conductivity and selectivity. TGA measurements show that the composite membranes possess good thermal stability up to 190 °C, satisfying the requirement for fuel cell operation. Compared to the unmodified membrane, the composites exhibit less swelling and a superior methanol barrier. Most importantly, all of the composite membranes have significantly lower methanol diffusion coefficients and significantly higher selectivity than those of Nafion^® 117. The Si-sPS/A-20PVA-20PWA membrane is the best applicant for use in DMFCs because it exhibits an optimized selectivity value (5.93 × 10⁵ Ss cm⁻³) that is approximately 7.8 times of that of the unmodified membrane and is 27.8 times higher than that of Nafion^® 117.     

New highly proton-conducting membrane poly(vinylpyrrolidone)(PVP) modified poly(vinyl alcohol)/2-acrylamido-2-methyl-1-propanesulfonic acid (PVA-PAMPS) for low temperature direct methanol fuel cells (DMFCs)

A new type of chemically cross-linked polymer blend membranes consisting of poly(vinyl alcohol) (PVA), 2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) and poly(vinylpyrrolidone) (PVP) have been prepared and evaluated as proton conducting polymer electrolytes. The proton conductivity (σ) of the membranes was investigated as a function of cross-linking time, blending composition, water content and ion exchange capacity (IEC). Membranes were also characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), and the differential scanning calorimetry (DSC). Membrane swelling decreased with cross-linking time, accompanied by an improvement in mechanical properties and a small decrease in proton conductivity due to the reduced water absorption. The membranes attained 0.088 S cm⁻¹ of the proton conductivity and 1.63 mequiv g⁻¹ of IEC at 25±2 °C for a polymer composition PVA-PAMPS-PVP being 1:1:0.5 in mass, and a methanol permeability of 6.1×10⁻⁷ cm² s⁻¹, which showed a comparable proton conductivity to Nafion 117, but only one third of Nafion 117 methanol permeability under the same measuring conditions. The membranes displayed a relatively high oxidative durability without weight loss of the membranes (e.g. 100 h in 3% H₂O₂ solution and 20 h in 10% H₂O₂ solution at 60 °C). PVP, as a modifier, was found to play a crucial role in improving the above membrane performances.     

Proton exchange membranes based on sulfonated crosslinked polystyrene micro particles dispersed in poly(dimethyl siloxane)

Proton exchange membranes of sulfonated crosslinked polystyrene (SXLPS) particles dispersed in crosslinked poly(dimethyl siloxane) matrix were investigated. Three different sizes of particles—25, 8 and 0.08 μm—were used at loadings from 0 to 50 wt% and the influence of these variables on the water and methanol uptake and proton conductivity were observed. With the reduction in particle size in the composite membrane, more water or methanol uptake was observed. Three different states of water were revealed in the composite membranes by differential scanning calorimetry (DSC). The number of bound water molecules per SO₃H group was 11-15 in membranes with 8- and 25-μm SXLPS. The ratio of bound to unbound water molecules was more than one in these membranes, whereas it was less than one in membranes with 0.08-μm SXLPS. The proton conductivities of the membranes increased with the increase in particle loading. At particle loadings above 35 wt%, membranes containing 8-μm SXLPS had higher conductivity compared to 25-μm SXLPS at room temperature. The conductivity of membranes containing 0.08-μm SXLPS was restricted to 10⁻³ S/cm because of the inherently low IEC of the particles. Increasing the temperature from 30 to 80 °C drastically enhanced the conductivity of the composite membranes compared to Nafion^® 112. At 80 °C, conductivities as high as 0.11±0.04 S/cm were observed for membranes containing more than 30 wt% of 25-μm SXLPS particles.