直接甲醇燃料电池用PEMs的研究进展

本文介绍了用于直接甲醇燃料电池(DMFCs)的质子交换膜(PEMs)的工作原理与性能要求。讨论了影响DMFCs国PEMs的甲醇渗透性能的因素。综述了Nation、改性Nafion膜以及其它新品种膜的研究进展。     

Polymer blend for fuel cells based on SPEKK: Effect of cocontinuous morphology on water sorption and proton conductivity

Proton‐exchange membranes (PEM), suitable for micro and small sized fuel cells, were obtained by blending sulfonated poly(ether ketone ketone) (SPEKK) polymers with different ionic exchange capacity (IEC). This approach was used to limit the amount of swelling caused by water sorption without significantly decreasing the proton conductivity of the membrane. In particular a membrane with a cocontinuous biphasic morphology was obtained by blending two SPEKKs, with respectively, an IEC equal to 1.2 and 2.08 in the weight ratio 60/40, casted from 5% (w/v) solutions in dimethylacetamide. The effect of a cocontinuous morphology on water sorption and proton conductivity in comparison to neat SPEKK was investigated. In the range of temperatures between 40 and 70 °C, which is typical for small and micro fuel cells conditions, it was found that the ratio of proton conductivity to water sorption could be maximized. This has been attributed to the presence of percolative pathways for proton transport provided by the cocontinuous morphology along with the constraint effect of the less sulfonated component on the overall capacity of swelling of the membrane. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 395–404, 2007     

Composite polymer electrolyte membranes comprising triblock copolymer and heteropolyacid for fuel cell applications

Hybrid organic/inorganic composite polymer electrolyte membranes consisting of a triblock copolymer (tBC) and varying concentrations of heteropolyacid (HPA) were investigated for application in proton exchange membrane fuel cells (PEMFC). An ABC triblock copolymer, that is, polystyrene‐b‐poly(hydroxyethyl acrylate)‐b‐poly (styrene sulfonic acid), PS‐b‐PHEA‐b‐PSSA, at 28:21:51 wt % was synthesized via atom transfer radical polymerization (ATRP) and solution‐blended with a commercial HPA. Upon the incorporation of HPA into the tBC, the symmetric stretching bands of both the SO group (1187 cm^?1) and the ? OH group (3440 cm^?1) shifted to lower wavenumbers (1158 and 3370 cm^?1). The shift in these FTIR absorptions suggest that the HPA particles strongly interact with both the sulfonic acid groups in the PSSA domains and the hydroxyl groups in the PHEA domains. When the weight fraction of HPA was increased to 0.2, the room‐temperature proton conductivity of the composite membrane increased from 0.048 to 0.065 S/cm, presumably because of the intrinsic conductivity of the HPA particles and the enhanced acidity of the sulfonic acid in the tBC. The water uptake of the composite membranes decreased from 130 to 48% with an increase of the HPA weight fraction to 0.4. The decrease in water uptake is likely a result of the decrease in the number of available water absorption sites because of the hydrogen bonding interaction between the HPA particles and the tBC matrix. Scanning electron microscopy and transmission electron microscopy images showed that the HPA nanoparticles with a diameter of 200–300 nm were uniformly distributed throughout the tBC matrix up to an HPA weight fraction of 0.4. Thermal stability of the composite membranes (decomposition temperature > 400 °C) was enhanced as compared with the pristine tBC membrane, presumably because of the strong specific interaction of the HPA particles with the sulfonic acid and hydroxyl groups. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 691–701, 2008     

Water vapor sorption and diffusion in sulfonated aromatic polyamides

In this contribution polyamides with different sulfonation degrees were directly synthesized from a combination of sulfonated and nonsulfonated diamines and isophthaloyl chloride. They were then used as dense membranes to study water sorption and mass transport properties. The polymers were characterized by their inherent viscosity and by spectroscopic methods, and the water vapor unsteady sorption phenomena were studied using a gravimetric technique. The effect of sulfonation substitution concentration in these polymers produces very interesting and original results in a number of properties such as the ionic exchange capacity, water equilibrium sorption and diffusivity. Obtained results are discussed and explained in the light of existing theories. Sorption behavior for polymers with a low sulfonation degree, up to 30%, can be explained with Langmuir equation. With larger substitution degree (40 and 60%) an additional mechanism must be assumed to explain sorption data. Assuming the presence of two phases helps to explain the observed diffusivity results. The mass transport mechanism is assumed to be Fickian. When water activity is low diffusivity systematically decreases as the degree of sulfonation increases. However, as water activity increases less sulfonated and nonsulfonated (PA0, PA20 and PA30) behave completely different from PA40 and PA60. The first group of polymers shows a tendency to decrease the rate of diffusion as water activity increases while the second group shows the opposite behavior, with a maximum in diffusivity at intermediate water activities. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2007–2014, 2007     

Sorption and diffusion of methanol and water in PVDF‐g‐PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)‐graft‐poly(styrene sulfonic acid) (PVDF‐g‐PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in‐solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF‐g‐PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF‐g‐PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG‐NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF‐g‐PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3277–3284, 2000     

Composite Polymer Electrolytes for Fuel Cell Applications: Filler-Induced Effect on Water Sorption and Transport Properties

Nafion- and sulfonated polysulfone (SPS)- based composite membranes were prepared by incorporation of SnO₂ nanoparticles in a wide range of loading (0 35 wt. %). The composites were investigated by differential scanning calorimetry, dynamic vapor sorption and electrochemical impedance spectroscopy to study the filler effect on water sorption, water mobility, and proton conductivity. A detrimental effect of the filler was observed on water mobility and proton conductivity of Nafion-based membranes. An increase in water mobility and proton conductivity was instead observed in SPS-based samples, particularly at low hydration degree. Analysis of the water sorption isotherms and states of water revealed that the presence of SnO₂ in SPS enhances interconnectivity of hydrophilic domains, while not affecting the Nafion microstructure. These results enable the design of suitable electrolyte materials that operate in proton exchange membrane fuel cell conditions.     

An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes

Methanol diffusion in two polymer electrolyte membranes, Nafion 117 and BPSH 40 (a 40% disulfonated wholly aromatic polyarylene ether sulfone), was measured using a modified pulsed field gradient NMR method. This method allowed for the diffusion coefficient of methanol within the membrane to be determined while immersed in a methanol solution of known concentration. A second set of gradient pulses suppressed the signal from the solvent in solution, thus allowing the methanol within the membrane to be monitored unambiguously. Over a methanol concentration range of 0.5–8 M, methanol diffusion coefficients in Nafion 117 were found to increase from 2.9 × 10⁻⁶ to 4.0 × 10⁻⁶ cm² s⁻¹. For BPSH 40, the diffusion coefficient dropped significantly over the same concentration range, from 7.7 × 10⁻⁶ to 2.5 × 10⁻⁶cm² s⁻¹. The difference in diffusion behavior is largely related to the amount of solvent sorbed by the membranes. Increasing the methanol concentration results in an increase in solvent uptake for Nafion 117, while BPSH 40 actually excludes the solvent at higher concentrations. In contrast, diffusion of methanol measured via permeability measurements (assuming a partition coefficient of 1) was lower (1.3 × 10⁻⁶ and 6.4 × 10⁻⁷ cm² s⁻¹ for Nafion 117 and BPSH 40 respectively) and showed no concentration dependence. The differences observed between the two techniques are related to the length scale over which diffusion is monitored and the partition coefficient, or solubility, of methanol in the membranes as a function of concentration. For the permeability measurements, this length is equal to the thickness of the membrane (178 and 132 μm for Nafion 117 and BPSH 40 respectively) whereas the NMR method observes diffusion over a length of approximately 4–8 μm. Regardless of the measurement technique, BPSH 40 is a greater barrier to methanol permeability at high methanol concentrations.     

Water sorption and electrical/dielectric properties of organic-inorganic polymer blends

Organic-inorganic polymer blends (OIPB) were obtained by reaction of organic and inorganic oligomers. The organic oligomer was synthesized with 2,4-toluene diisocyanate (TDI) and oligooxypropylene glycols (OPG) with various molecular weights (MW). The inorganic component was a water solution of sodium silicate. The OIPB obtained are hydrophilic and have great water sorption ability (the relative weight of sorbed water reaches 2000 %). The kinetics of water sorption and the changes of electrical conductivity during sorption were studied. Sorption ability, and mechanical, electrical and dielectric properties of OIPB depend on molecular weight of OPG: conductivity increases with increasing MW, whereas the sorption ability correlates with the mechanical properties. The influence of the inorganic phase content on the electrical and dielectric properties was studied as well.     

Mobility-sensitive fluorescence probes for quantitative monitoring of water sorption and diffusion in polymer coatings

The fluorescent molecular rotor probes 4-tricyanovinyl-[N-(2-hydroxyethyl)-N-ethyl]-aniline, tricyano-4-(dimethylamino) benzylidene, and tricyanovinyljulolidene have been used as extrinsic fluorescence probes for quantitative monitoring of water uptake in polymeric coatings. The presence of water causes plasticization of the polymer, which results in increased local mobility within the film. The nonradiative decay pathways of the rotor probes are increased as local mobility increases, and the resulting decrease in fluorescence intensity of the probes is directly proportional to the amount of water sorbed. Beyond allowing for the characterization of sorbent content, this fluorescence technique can be used to determine the diffusion coefficient of water in a polymer film. The relative change in fluorescence fits well to a Fickian diffusion model, yielding a diffusion coefficient for water of 3 × 10^-8 cm²/s in poly(vinyl acetate), and a value of 6 × 10^-9 cm²/s in a room-temperature cured epoxypolyamide, in excellent agreement with diffusion coefficient values determined from gravimetric analysis. Preliminary studies also demonstrate the utility of molecular rotor probes to monitor water uptake in individual layers of multilayered polymer systems. © 1995 John Wiley & Sons, Inc.     

Crosslinked sulfonated polyimide networks as polymer electrolyte membranes in fuel cells

Sulfonated polyimides with tertiary nitrogen in the polymer backbone were synthesized with 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl 2,2′‐disulfonic acid, 2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and diaminoacrydine hemisulfate. They were crosslinked with a series of dibromo alkanes to improve the hydrolytic stability. The crosslinked sulfonated polyimide films were characterized for their thermal stability, ion‐exchange capacity (IEC), water uptake, hydrolytic stability, and proton conductivity. All the sulfonated polyimides had good thermal stability and exhibited a three‐step degradation pattern. With an increase in the alkyl chain length of the crosslinker, IEC decreased as 1.23 > 1.16 > 1.06 > 1.01, and the water uptake decreased as 7.29 > 6.70 > 6.55 > 5.63. The order of the proton conductivity of the crosslinked sulfonated polyimides at 90 °C was as follows: polyimide crosslinked with dibromo butane (0.070) > polyimide crosslinked with dibromo hexane (0.055) > polyimide crosslinked with dibromo decane (0.054). The crosslinked polyimides showed higher hydrolytic stability than the uncrosslinked polyimides. Between the crosslinked polyimides, the hydrolytic stability decreased with an increase in the alkyl chain length of the crosslinker. The crosslinked and uncrosslinked sulfonated polyimides exhibited almost the same proton conductivities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2370–2379, 2005     

Chitosan‐based composite membranes containing chitosan‐coated carbon nanotubes for polymer electrolyte membranes

Considering the poor dispersion and inert ionic conduction ability of carbon nanotubes (CNTs), functionalization of CNTs is a critical issue for their application in polymer electrolyte membranes. Herein, CNTs were functionalized by the polyelectrolyte, chitosan (CS), via a facile noncovalent surface‐deposition method. The obtained CS‐coated CNTs (CS@CNTs) were then incorporated into the CS matrix and fabricated composite membranes. The CS coating can enhance the compatibility between CNTs and the matrix, thus ensuring the homogenous dispersion of CS@CNTs and effectively improved the mechanical properties of the composites. Moreover, the CS coating can make CS@CNTs act as an additional proton‐conducting pathway through the membranes. The CS/CS@CNTs‐1 composite shows the highest proton conductivity of 3.46 × 10⁻² S cm⁻¹ at 80°C, which is about 1.5‐fold of the conductivity of pure CS membrane. Consequently, the single cell equipped with CS/CS@CNTs‐1 membrane exhibits a peak power density of 47.5 mW cm⁻², which is higher than that of pure CS (36.1 mW cm⁻²).     

Proton‐conductive membranes based on blends of polyimides

We prepared novel proton‐conductivity membranes based on blends of sulfonated polyimides. The blend membranes were prepared from a sulfonated homopolyimide and a sulfonated copolyimide with a solvent‐casting method. The proton conductivities of the blend membranes were measured as functions of the temperature with four‐point‐probe electrochemical impedance spectroscopy. The conductivity of the membranes strongly depended on the sulfonated homopolyimide content and increased with an increase in the content. The proton conductivity of all the blended membranes indicated a higher value than that determined in Nafion at 80 °C, and this may mean that the proton transfer in the blend membranes is responsible for the ionic channels induced by the hydrophobic and hydrophilic domains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1325–1332, 2007     

Current progress in membranes for fuel cells and reverse electrodialysis

The deterioration of the environmental situation has led to the need to restructure the world’s power industry, and clean renewable power sources are coming to the forefront. This review deals with recent advances in the development of promising ion-exchange membrane materials for two types of application that have been intensely developing recently, namely, hydrogen energy and reverse electrodialysis. Special attention is paid to the comparison of two properties of membranes, conductivity and selectivity, that are competing but fundamentally important in both areas. Perfluorinated sulfonic acid membranes now play a dominant role in hydrogen power engineering, as they provide not only high proton conductivity but also chemical stability and low gas permeability. The review also covers other types of membrane materials, including anion exchange membranes, polybenzimidazoles and hybrid membranes containing inorganic nanoparticles that have been actively developed in recent years. The milder operating conditions of membranes in reverse electrodialysis units allow one to use less expensive non-perfluorinated membranes, including grafted ones. It is of note that in devices of this type, the selectivity of membranes to the transfer of oppositely charged ions is a more important parameter.     

Proton conducting composite membranes from polysulfone and heteropolyacid for fuel cell applications

The viability of using composite membranes of heteropolyacid (HPA)/polysulfone (PSF), HPA/sulfonated polysulfone (SPSF) for use in proton exchange membrane fuel cells (PEMFC) was investigated. PSF and its sulfonated polymer, SPSF was solution‐blended with phosphotungstic acid, a commercially available HPA. Fourier transform infrared (FTIR) spectroscopy of the HPA–40/SPSF composite exhibited band shifts showing a possibility of intermolecular hydrogen bonding interaction between the HPA additive and the sulfonated polymer. The composite membranes exhibited improved mechanical strength and low water uptake. The conductivity of the composite membrane, HPA–40/SPSF, consisting of 40 wt % HPA and 60 wt % SPSF [with a degree of Sulfonation (DS) of 40%] exhibited a conductivity 0.089 S/cm at room temperature that linearly increased upto 0.14 S/cm at 120 °C, whereas the widely used commercial membrane Nafion 117, exhibited a room temperature conductivity of 0.1 S/cm that increased to only 0.12 S/cm at 120 °C. In contrast, the composite of HPA–40/PSF exhibited a proton conductivity of 0.02 S/cm at room temperature that increased only to 0.07 S/cm at a temperature of 100 °C. The incorporation of HPA into SPSF not only rendered the membranes suitable for elevated temperature operation of PEMFC but also provides an inexpensive alternative compared to Nafion. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1538–1547, 2005     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Composite polymer electrolyte membranes comprising P(VDF‐co‐CTFE)‐g‐PSSA graft copolymer and zeolite for fuel cell applications

Hybrid organic/inorganic composite polymer electrolyte membranes based on a poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) grafted membrane and varying concentrations of zeolite were investigated for application in proton exchange membrane fuel cells (PEMFC). A proton conducting comb copolymer consisting of poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) backbone and poly(styrene sulfonic acid) (PSSA) side chains, i.e. P(VDF‐co‐CTFE)‐g‐PSSA (graft copolymer) with 47 wt% of PSSA was synthesized using atom transfer radical polymerization (ATRP) and solution blended with zeolite. Upon incorporation of zeolite, the symmetric stretching band of both SO group (1169 cm^?1) and the ? OH group (3426 cm^?1) shifted to lower wavenumbers. The shift in these FT‐IR spectra suggests that the zeolite particles strongly interact with the sulfonic acid groups of PSSA chains. When the weight percent of zeolite 5A is above 7%, the proton conductivity at room temperature was reduced to 0.011 S/cm. The water uptake of the composite membranes decreased from 234 to 125% with an increase of the zeolite 5A weight percent to 10 wt%. The decrease in water uptake is likely a result of the decrease in the number of available water absorption sites because of the hydrogen bonding interactions between the zeolite particles and the graft copolymer matrix. This behavior is successfully investigated by scanning electron microscopy (SEM). The results of thermal gravimetric analysis (TGA) also showed that all the membranes were stable up to 300°C. Copyright © 2009 John Wiley & Sons, Ltd.     

A new sorption model with a dynamic correction for the determination of diffusion coefficients

This paper describes an improvement to the method used for the calculation of diffusion coefficients from data obtained by the measurement of vapor sorption kinetics in a flat, non-porous polymeric membrane. The advantage of our corrected model is that it can be applied to systems displaying both fast and slow sorption kinetics, as is demonstrated using cellulose myristate – hexane and cellulose acetovalerate – ethanol systems at a temperature of 298 K. Experiments were conducted on a specially developed sorption apparatus equipped with McBain’s spiral quartz balances. Sorption kinetics are generally described by the solution of Fick’s second law, the solution of which assumes relative pressure in the form of the unit step function. Our correction involves modifying this solution so that a more realistic relative pressure increase is assumed in terms of the Laplace transform.