Organic/inorganic composite membranes based on polybenzimidazole and nano-SiO2

Organic/inorganic composite membranes based on polybenzimidazole (PBI) and nano-SiO₂ were prepared in this work. However, the preparation of PBI/SiO₂ composite membrane is not easy since PBI is insoluble in water, while nano-SiO₂ is hydrophilic due to the hydrophilicity of nano-SiO₂ and water-insolubility of PBI. Thus, a solvent-exchange method was employed to prepare the composite membrane. The morphology of the composite membranes was studied by scanning electron microscopy (SEM). It was revealed that inorganic particles were dispersed homogenously in the PBI matrix. The thermal stability of the composite membrane is higher than that of pure PBI, both for doped and undoped membranes. PBI/SiO₂ composite membranes with up to 15 wt% SiO₂ exhibited improved mechanical properties compared with PBI membranes. The proton conductivity of the composite membranes containing phosphoric acid was studied. The nano-SiO₂ in the composite membranes enhanced the ability to trap phosphoric acid, which improved the proton conductivity of the composite membranes. The membrane with 15 wt% of inorganic material is oxidatively stable and has a proton conductivity of 3.9 × 10⁻³ S/cm at 180 °C.     

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.     

Influence of solvent on ion conductivity of polybenzimidazole proton exchange membranes for vanadium redox flow batteries

Polybenzimidazole(PBI) is a kind of proton transport membrane material, and its ion conductivity is a key factor affecting its application in vanadium redox flow batteries(VRFBs). The casting solvent of PBI has a significant influence on the acid doping level of PBI membranes which is closely related to ionic conductivity. In this paper, 3,3′-diaminobenzidine(DABz) and 4,4′-Dicarboxydiphenylether(DCDPE) were used as raw materials by solution condensation to prepare the PBI with ether bond groups. The chemical structure of PBI was determined by1~H NMR and FT-IR, and the prepared PBI had good solubility which can be dissolved in a variety of solvents. The PBI proton exchange membranes were prepared by solution coating with 5 different solvents of N,N-dimethylformamide(DMF), N,N-dimethylacetamide(DMAc), dimethyl sulfoxide(DMSO), 1-methyl-2-pyrrolidone(NMP), methane sulfonic acid(MSA). The effects of different solvents on the ion conductivity and physicochemical properties were discussed in detail. The results showed that the PBI membrane prepared by using MSA as solvent(the PBI + MSA membrane) exhibits high water uptake, acid doping level and low vanadium ion permeability. The VRFB assembled with the PBI + MSA membrane exhibited higher coulombic efficiency(CE) 99.87% and voltage efficiency(VE) 84.50% than that of the commercial Nafion115 membrane at100 m A·cm~(-2), and after 480 cycles, the EE value can still be maintained at 83.73%. The self-discharge time of a single battery was recorded to be as long as 1000 h. All experimental data indicated that MSA is the best solvent for casting PBI membrane.     

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.     

Anhydrous proton conductor based on composites of PEO and ATMP

A new type anhydrous PEM material based on Poly (ethylene oxide) (PEO)/Amino Trimethylene Phosphonic Acid (ATMP) composite was prepared. In this study, PEO assumed to “grab” protons via hydrogen bond between PEO and ATMP. Based on this point, the PEO/ATMP composites were prepared firstly as the preliminary study to verify this proton conducting system. Then, PVDF was added to enhance the membrane's stability. The PVDF/PEO/ATMP composite membranes were thermally stable up to 200 °C in the studied composition ranges. The membrane had relatively compact structure by SEM images. Proton conductivity of 59% PVDF/29% PEO/12% ATMP was up to 6.71 × 10 ⁻³ S cm⁻¹ at 86 °C after doping with 7.9 wt% phosphoric acid without extra humidification.     

Effect of composition on the properties of PEM based on polybenzimidazole and poly(vinylidene fluoride) blends

Inadequate performance, short term durability and high cost of polymer electrolyte membrane (PEM) are the major roadblocks that need to be resolved for successful commercialization of high temperature PEM fuel cell. In this report, we investigated the viability of previously developed miscible blend membranes of polybenzimidazole and poly (vinylidene fluoride) (PBI/PVDF), as potential PEMs. In addition, we have carried out several advanced analytical techniques such as dynamic mechanical analysis (DMA), ¹³C CP-MAS solid state NMR (SS-NMR) and wide-angle X-Ray diffraction (WAXD) to prove the miscible behavior of the polymer pair. Sub-ambient temperature DMA studies confirmed the miscible behavior of PBI/PVDF blends at different compositions based on single T_g criterion. SS-NMR and WAXD showed the presence of interactions between the functional groups of the polymers and their dependence on blend composition. Thermogravimetric analysis of phosphoric acid (PA) doped and undoped blend membranes confirmed the improved thermal stability of the membranes compared to neat PBI. The membranes exhibited excellent oxidative stability than pristine PBI membrane. The swelling ratio and volume after dipping in PA was found to be significantly low in the blend membranes owing to the hydrophobic nature of PVDF. Among the blends prepared, 90/10 and 75/25 membranes showed higher proton conductivity than PBI, attributed in part, to electronegativity of fluorine and crystallinity of PBI in PA that activate proton transport. The results demonstrated the potential usefulness of the blend membranes as PEM in fuel cell.     

Alginic acid-imidazole composite material as anhydrous proton conducting membrane

Recently, membranes with high anhydrous proton conducting have been attracted remarkable interest for the application to the polymer electrolyte membrane fuel cell (PEFC). In this paper, we have investigated the anhydrous proton conductor consisting of alginic acid (AA), one of the acidic biopolymers, and imidazole (Im). This AA-Im composite material showed the proton conductivity of 2×10⁻³ S cm⁻¹ at 130 °C under anhydrous conditions. Additionally, these AA-Im composite materials have the highly mechanical property and thermal stability. Furthermore, the biological products, such as biopolymer, are cheap, non-hazardous, and environmentally benign. The proton conductive biopolymer composite material may have the potential for its superior ion conducting properties, in particular, under anhydrous (water-free) or extremely low humidity conditions.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Novel triazole functional sol-gel derived inorganic-organic hybrid networks as anhydrous proton conducting membranes

In this work, novel inorganic-organic hybrid networks were prepared to obtain anhydrous proton conducting membranes for fuel cells. 3-glycidoxypropyl trimethoxy silane (GPTMS) was functionalized with 1H-1,2,4-triazole (Tri) and 3-aminotriazole (ATri) via ring opening of the epoxide ring and then sol-gel polymerization was performed to produce triazole containing silane networks abbreviated as Si-Tri and Si-ATri. In addition during sol-gel process trifluoromethane sulfonic acid (TA) was introduced into the matrix with several stoichometric ratios. Fourier transform infrared spectroscopy (FT-IR) confirmed the tethering of the Tri and ATri into the silane compound and the sol-gel reaction. Thermogravimetry analysis (TGA) showed that the membranes are thermally stable up to 200 °C. Differential scanning calorimeter (DSC) verified the softening effect of the dopant. The morphology of the membranes was analyzed with SEM images. The proton conductivity of these novel silane networks were studied by dielectric-impedance spectroscopy. Although proton conductivity of these membrane electrolytes depends on the acid ratio, the membrane without dopant produced a proton conductivity of 8.7 × 10⁻⁵ S/cm at 150 °C in dry state. The conductivity isotherms show Vogel-Tamman-Fulcher (VTF) behavior which implies the coupling of the charge carriers with the segmental motion of the polymer chains. A maximum proton conductivity of 8.9 × 10⁻⁴ S/cm was obtained for the sample Si-TriTA1 in the anhydrous condition.     

Methanol permeability and proton conductivity of polybenzimidazole and sulfonated polybenzimidazole

The preparation of sulfonated polybenzimidazole (sPBI) by the grafting of (4‐bromomethyl) benzenesulfonate onto polybenzimidazole (PBI) has been investigated. The methanol permeability and proton conductivity of PBI and sPBI have been studied, and the effects of methanol concentration and temperature on the methanol permeability of PBI and sPBI membranes are discussed. The results showed that the PBI membrane is a good methanol barrier. Methanol permeability in this membrane decreases with increasing methanol concentration and increases with increasing temperature. The temperature‐dependence of methanol permeability of PBI and sPBI membranes is of the ‘Arrhenius type’. Methanol permeation of sPBI is less sensitive to temperature than that of PBI. However, sPBI is a poorer methanol barrier when compared to PBI. Methanol permeability in sPBI membranes increases with increasing methanol concentration and temperature. The proton conductivity of sPBI is 4.69 × 10^?4 S cm^?1 at room temperature in the hydrated state. The DC conductivity of sPBI–H₃PO₄ increases with increasing temperature. Proton transport in sPBI–H₃PO₄ is less sensitive to temperature than that in PBI–H₃PO₄. Copyright © 2004 Society of Chemical Industry     

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.     

Physicochemical properties of new amide-based protic ionic liquids and their use as materials for anhydrous proton conductors

We prepared 3 protic ionic liquids based on trifluoromethanesulfonic acid and an amide, namely isobutyramide (ITSA), n-butyramide(NTSA), and benzamide(BTSA). All of the protic ionic liquids exhibit excellent thermal stability (above 200 °C). ITSA has the highest ionic conductivity, which is 32.6 mS/cm at 150 °C. ITSA was used to prepare anhydrous, conducting composite membranes based on polymers of polyvinylidene-fluoride (PVDF) to serve as intermediate temperature proton exchange membrane fuel cells. This type of composite membrane possesses good thermal stability, high ionic conductivity and good mechanical properties. Increasing the polymer content leads to the improvement of mechanical properties, but is accompanied by a reduction in ionic conductivity. We made efforts to eliminate the trade-off between strength and conductivity of the ITSA/PVDF composite membrane by adding polyamide imide, which resulted in a simultaneous increase in strength and conductivity. A conductivity of 7.5 mS/cm is achieved in a membrane containing 60 wt.% ITSA and 5 wt.% PAI in PVDF at 150 °C.     

A “proton pump” concept for the investigation of proton transport and anode kinetics in proton exchange membrane fuel cells

A novel concept for the measurement of proton transport properties and electrode kinetics in proton exchange membrane fuel cells (PEMFC) is presented. The “proton pump” is essentially a fuel cell operated with pure nitrogen or very low hydrogen partial pressure instead of oxygen-containing gas on the cathode side, avoiding the complicated electrode kinetics of oxygen reduction. In this first study using this concept, we investigated the proton transport in high temperature PEMFC based on polybenzimidazole (PBI)/phosphoric acid membranes. The impedance spectra of the proton pump allow the clear distinction between anode and cathode kinetics and proton transport in the membrane. Identifying and analyzing the contribution of the anodic processes in the impedance spectra enabled the quantitative investigation of anode kinetics based on the Butler-Volmer equation. The proton transport was investigated in more detail in the current saturation region, where proton transport turned out to be the limiting process in case of sufficient H₂ supply at the anode. The maximum proton transport capacity of the PBI/phosphoric acid membrane was found to be comparable to those of Nafion^® membranes.     

Solid-state internal reference electrode based on quinhydrone for hydrogen sensor with acid-doped polybenzimidazole

An electrode consisting of a mixture of quinhydrone (QH) and polybenzimidazole (PBI) with carbon was investigated as a solid reference electrode for hydrogen sensors. The vibrational behaviour of the polybenzimidazole/quinhydrone blend was investigated by infrared spectroscopy. The results suggest the existence of hydroquinone in the blend. The quinone/hydroquinone redox couple was evidenced by cyclic voltammetry. The potential of the solid-state reference electrode was found to be very stable over a period of 700 h. The drift was less than 0.1 mV per day. This electrode was successfully implemented in a potentiometric sensor. The standard potential was of the order of −550 mV at room temperature and increased as a function of the temperature with a slope of 1 mV/°C. The standard potential was also insensitive to changes in relative humidity (rh) in the sample gas over a period of 1 day.     

Preparation and electrical conductivity of KTaO3-based proton conductor

KTaO₃ and KTa_0.9M_0.1O_3-α (M = Ti, Hf, Zr) were prepared by solid state reaction at 1330 °C for 2 h and characterized by x-ray diffraction. The AC impedance technique was used to analyze the sintered solid electrolytes in 1%H₂/Ar and dry air atmosphere. Among KTa_0.9M_0.1O_3-α (M = Ti, Hf, Zr), KTa_0.9Zr_0.1O_3-α displays the highest conductivity in 1%H₂/Ar atmosphere. The carriers transport numbers of solid electrolytes were measured by concentration cell method. The results show KTa_0.9Zr_0.1O_3-α is a pure proton conductor below 525 °C. Stability tests show that KTa_0.9Zr_0.1O_3-α has good chemical stability against CO₂ and H₂O.     

Variation in acid moiety of polybenzimidazoles: Investigation of physico-chemical properties towards their applicability as proton exchange and gas separation membrane materials

A series of polybenzimidazoles (PBIs) were prepared from 3,3′-diaminobenzidine (DAB) and substituted aromatic dicarboxylic acids. Effects of added polarity, bulk and isomerism in the dicarboxylic acid moiety on the properties of formed aromatic polybenzimidazoles were investigated. Solution polycondensation procedure was optimized for individual case of PBI synthesis in order to obtain inherent viscosity of ≥1 dL/g. Analysis of physical properties, water uptake, acid doping (H₃PO₄ and H₂SO₄) and gas permeability was performed. All these PBIs exhibited high thermal stability, good solvent solubility and amorphous nature. The uptake of H₃PO₄ varied from 9 to 20.1 moles per repeat unit (mol/RU), H₂SO₄ uptake varied from 3.39 to 3.81 mol/RU, while water uptake varied from 1.8 to 3.6 mol/RU of PBI. The dibromoterephthalic acid and tert-butylisophthalic acid based PBI showed the highest H₃PO₄ uptake in the series, while tert-butylisophthalic acid based PBI exhibited the highest water uptake. Acid uptake was correlated with swelling of the PBI matrix, while density estimation of H₃PO₄-doped PBI by He gas expansion method could be correlated to the physical state of PBI. 5-tert-Butylisophthalic acid and 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) based PBI exhibited higher H₂ and O₂ permeability than other PBIs. The ideal gas selectivity for O₂/H₂ was considerably higher for most of the PBIs than conventional gas separation membrane materials. These analyses suggested that some of these PBIs have a potential to be used as a PEM or gas separation membrane material.     

磷酸氢二钾和磷酸三钾混合溶液的电导率测定

测定了磷酸三钾和磷酸氢二钾不同摩尔比的混合溶液在不同温度下的电导率。研究结果表明:电导率随温度的升高而升高,且在其他条件相同的情况下,PO43-浓度较低时电导率随n(K3PO4)/n(K2HPO4)的升高而升高;PO43-浓度较高时,在较低温度范围n(K3PO4)/n(K2HPO4)越低,电导率越高,而在较高温度范围n(K3PO4)/n(K2HPO4)越高,电导率越高。     

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.     

Structure and transport properties of the spark plasma sintered barium cerate based proton conductor

BaCe_0.7Y_0.2In_0.1O_3–δ (BCYI) compositions were prepared by a modified Pechini method, following this the ceramic samples were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) at 1250–1500 °C for 3–10 min. The structural and microstructural characteristics of the samples were determined using XRD, SEM and TEM. The total, bulk and grain boundary ionic conductivities were evaluated using the AC impedance method in dry air, wet air and dry Ar. It was shown that application of SPS in case of nanocrystalline BCYI allows to reduce the sintering time, and in case of microcrystalline BCYI application of SPS after CS allows to improve hardness and total conductivity through reduction of grain boundary resistance.     

Effects of solvent on microstructure and proton conductivity of organic-inorganic hybrid membranes

Effects of solvent on the microscopic structure and proton conductivity of organic-inorganic hybrids containing heteropolyacid are reported. The organic-inorganic hybrids were prepared by sol-gel synthesis of 1,8-bis(triethoxysilyl)octane (TES-Oct) in the presence of phosphotungstic acid (PWA). The proton conductivity of the membranes prepared under different solvents exhibited greater values in the order methanol < 1-butanol < 2-propanol < ethanol < 1-propanol < 2-butanol. The conductivity strongly correlated to the amount of incorporated PWA in the membranes, which was determined by titration of the solutions containing PWA leaked from the membrane. Small-angle X-ray scattering (SAXS), atomic force microscopy (AFM) and thermogravimetric analysis (TGA) were employed to reveal the relationship between the proton conductivity and microscopic structure of the TES-Oct/PWA membranes. It was found that there is an optimal condition for the formation of the condensed domain structure, indicating the important role of the structures in the membrane conductivity.