Zeolite-Nafion composites as ion conducting membrane materials

Composite membranes formed of zeolitic fillers embedded in Nafion can be made by evaporating the solvents from a suspension of small zeolite crystals in a Nafion solution. Two natural zeolites were selected as fillers: chabazite and clinoptilolite. Membranes with various zeolite content were obtained. Composite membranes with zeolite content up to 40 vol.% exhibited uniform distribution of the zeolite fillers throughout the thickness, as ascertained by scanning electron microscopy (SEM), energy dispersive X-rays absorption (EDX) and ion conductivity measurements. Although more brittle than Nafion, these composite sheets retain—when hydrated—reasonable flexibility below 40 vol.% zeolite content. We have determined the hydrogen ion conductivity and the permeability of methanol molecules through these membranes (in the H⁺-form as well as in the Na⁺-form) in the temperature range 22-60 °C. We found that the presence of the zeolitic fillers in the membranes can bear notable changes of conductivity, permeability and selectivity with respect to membranes made out of sole Nafion.     

Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets

Polymer electrolyte membranes have been widely investigated for high performance fuel cells. Here, we report the synthesis of ionic conductive Nafion/graphene oxide (GO) composite membranes for application in direct methanol fuel cells. GOs interact with both the non-polar backbone and the polar ionic clusters of Nafion because of their amphiphilic characteristics attributable to hydrophobic conjugation and hydrophilic functional groups. Accordingly, GO sheets serve to modify the microstructures of two domains of Nafion. In particular, the transport properties of Nafion are favorably manipulated by the incorporation of GO. This modulated the ionic channels of Nafion and decrease methanol crossover while preserving ionic conductivity. Furthermore, strong interfacial interactions due to the insertion of GO nanofillers into the Nafion matrix improve the thermal and mechanical properties of the material. In particular, we exploit Nafion/GO composite membrane as electrolyte material for direct methanol fuel cell (DMFC) in order to resolve current issue of methanol crossover. This composite membrane-based DMFC compared to the Nafion 112-based DMFC remarkably enhanced cell performance, especially in severe operating conditions.     

Sulfonated polyether sulfone‐poly(vinylidene fluoride) blend membrane for DMFC applications

Direct methanol fuel cell (DMFC) proton exchange membranes were prepared by blending poly (vinylidene fluoride) (PVDF) with sulfonated poly(ether sulfone) (SPES). Using a diffusion cell and gas chromatographic technique, the effects of PVDF content on methanol permeability in the blended membranes were investigated. The thermal resistance and proton conductivity of the membranes were also determined by using a thermal gravimetric analysis (TGA) and an impedance analysis technique respectively. The presence of sulfonic acid groups in SPES was confirmed by Fourier transform infrared (FTIR). It was found that the methanol permeability in the blended membranes decreased with PVDF content at the expense of proton conductivity. Blended membranes show methanol permeability values much lower than that of Nafion 115, whereas the proton conductivities of the membranes are comparable with that of Nafion. The thermal stability of these blended membranes is above 250°C, which is sufficiently high for use in DMFC. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphology and properties of Nafion membranes prepared by solution casting

In this study, dilute Nafion solutions consisting of solvents with various dielectric constants ? and solubility parameters δ, i.e. N,N′-dimethyl acetamide, N,N′-dimethyl formamide, N-methyl formamide, methanol-water mixture (4/1 g/g), ethanol-water mixture (4/1 g/g), and isopropanol-water mixture (4/1 g/g), were freeze dried and the conformations of Nafion molecules in dilute solutions were observed using transmission electron microscope. The membranes were prepared by solution casting from these solutions and evaporating the solvents at temperatures below T_G of Nafion, then annealing the membranes at 150 °C which was ∼50 °C above T_G of Nafion. We show Nafion molecular conformations in dilute solutions are strongly influenced by δ and ? of solvents. And, thus the morphology, water uptake, proton conductivity, and methanol permeability of membranes prepared by solution casting are also influenced by δ and ? of solvents.     

Improvement of the Nafion–polytetrafluoroethylene membranes for potential direct methanol fuel cell use by reduction of the methanol crossover

The possibility of ultrathin Nafion/expanded polytetrafluoroethylene (ePTFE) membranes used as proton‐exchange membranes (PEMs) for direct methanol fuel cells (DMFCs) was investigated in this study. Nafion/ePTFE membranes with a thickness of ～ 14 μm were promoted by self‐assembling Pd nanoparticles on the surface to reduce the methanol crossover. The loading of the Pd nanoparticles assembled on the membranes was 1.6–1.8 μg/cm² and had little effect on the high conductivity of the Nafion membranes. With the self‐assembly of Pd nanoparticles, the methanol permeation noticeably decreased from 340 to 28 mA/cm². As a result, the open‐ circuit voltage of the Nafion/ePTFE membranes that were self‐assembled for 48 h had a more significant increase from 0.55 to 0.73 V. The reduction of methanol crossover significantly increased the DMFC voltage‐current performance, and this means that self‐assembled Nafion/polytetrafluoroethylene PEMs have promise in DMFCs. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Novel grafted nafion membranes for proton‐exchange membrane fuel cell applications

Novel poly(glycidyl methacrylate)‐grafted Nafion–phosphoric acid membranes for direct‐oxidation methanol fuel cells were prepared with a potassium persulfate chemical initiation system for the first time. The introduced epoxy groups were converted to amine groups through a reaction with ethylenediamine, which consequently doped with phosphoric acid ( PO₃H) groups. The latter significantly contributed to enhancing the ion‐exchange capacity, mechanical properties, and thermal stability. Factors affecting the modification steps were studied. Changes in the chemical and morphological structure were verified through Fourier transform infrared spectroscopy, TGA, and scanning electron microscopy characterization. Various grafting percentages (GP%'s) up to 32.31% were obtained. As a result, the thickness of the grafted membranes increased. Furthermore, the methanol permeability of the modified membranes was reduced with increasing grafted polymer content compared with that of the Nafion membrane. An 83.64% reduction in the methanol permeability was obtained with a polymer grafted content of 18.27%. Finally, the efficiency factor for all of the modified Nafion membranes was enhanced compared with that of Nafion. A fourfold improvement was obtained with membranes with a GP% of 18.27% as a maximum value. Such promising results nominate the used technique as a one for the improvement of Nafion membrane efficiency. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Analysis of Direct Methanol Fuel Cell (DMFC)‐Performance via FTIR Spectroscopy of Cathode Exhaust

Water and methanol flux through Nafion™ and polyaryl‐blend membranes prepared at ICVT were studied under DMFC operation. The water, methanol, and CO₂ content in the cathode exhaust were measured by FTIR spectroscopy. Both the water and methanol flux turned out to be strongly dependent on the operating temperature and thus on membrane swelling. Apart from this, water flux through the membrane is primarily affected by the gas volume flux on the cathode side. A coupling between water flux and methanol flux was observed, which leads to the conclusion that methanol is transported both by diffusion and by convection caused by the superimposed water flux. Polyaryl‐blend membranes showed a reduced diffusive methanol transport when compared to Nafion™ due to their different internal microstructure. The impact of methanol cross‐over on cathode losses at high current density needs further clarification with respect to the prevailing mechanism of methanol oxidation at the cathode.     

Influence of ligands of palladium complexes on palladium/Nafion composite membranes for direct methanol fuel cells by supercritical CO2 impregnation method

Palladium/Nafion composite membranes were synthesized by supercritical impregnation method to reduce methanol crossover in direct methanol fuel cells. The palladium complexes used in this study were palladium(II) acetylacetonate, palladium(II) hexafluoroacetylacetonate, and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato). The palladium complexes with various loading amounts from 0.010 to 0.050 g in a high-pressure vessel were dissolved in supercritical CO₂, and impregnated into Nafion membranes.The SEM images indicated that the palladium complexes were successfully deposited into Nafion membrane, and there were no problems such as cracking and pinhole. The EDX analysis showed that the palladium particles were distributed both at the membrane surface and also extended deeper into the membrane. The TEM images indicated that thin dense band of agglomerated Pd particles can be observed near the membrane surface, and a significant number of isolated Pd particles can be seen deeper into the membrane, when Pd(II) acetylacetonate was used as palladium complex. When palladium(II) hexafluoroacetylacetonate and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato) were used, dense band of agglomerated Pd particles cannot be observed near the membrane surface, and small Pd particles were observed inside the membranes.The XRD analysis indicated that the crystalline peak of Nafion membrane at 2θ = 17° increased with the supercritical CO₂ treatment. It means that the degree of crystallinity for Nafion membrane increased by supercritical CO₂. The metal Pd peak at 2θ = 40° was observed for the Pd/Nafion membranes.The methanol crossover was reduced and the DMFC performance was improved for the Pd/Nafion membranes compared with Nafion membrane at 40 °C. The successful preparation of Pd/Nafion membranes by supercritical CO₂ demonstrated an effective alternative way for modifying membranes and for depositing electrode catalytic nanoparticles onto electrolyte.     

Modification of Nafion membranes with ternary composite materials for direct methanol fuel cells

Composite membranes for direct methanol fuel cells (DMFCs) were prepared by using Nafion115 membrane modification with polyvinyl alcohol (PVA), polyimide (PI) and 8-trimethoxysilylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS). The performance of the composite membranes was evaluated in terms of water sorption, dimensional stability, thermal stability, proton conductivity, methanol permeability and cell performance. The proton conductivity was slightly decreased by 1-3% compared with Nafion115, which still kept the high proton conduction of Nafion115. The methanol permeability of Nafion/PI-PVA-TSPS composite membranes was remarkably reduced by 35-55% compared with Nafion115. The power density of DMFCs with Nafion/PI-PVA-TSPS composite membranes reached to 100 mW/cm², exceeding that with Nafion115 (68m W/cm²).     

Nano hexagonal boron nitride–Nafion composite membranes for proton exchange membrane fuel cells

This study investigates the proton conductivity and characterization of nano hexagonal boron nitride–Nafion composite membranes. Proton conducting composite membranes are prepared by mixing Nafion and nano hexagonal boron nitride (NhBN) particles with 3, 5, 10, and 15% weight ratios. Particle size distribution analysis is made for BN nanoparticles and the homogeneity of the membranes is proved by SEM and AFM. FTIR confirms the interaction between NhBN and Nafion. Methanol permeation and water absorption tests are made and it shows that hydrophobic NhBN particles decrease the swelling property and methanol retention of the membranes. X‐ray investigation of the composites supports semi‐crystalline nature of the composite materials. At dry conditions, the maximum proton conductivity was measured as 0.005 S/cm at 150°C for NafBN10, which is much higher than the pure Nafion. The results confirm that boron nitride nanoparticles have high contribution on the proton transfer, which may be explained with the formation of hydrogen bonds between amine and hydroxyl groups of boron nitride and sulfonic acid groups of Nafion. POLYM. COMPOS., 37:422–428, 2016. © 2014 Society of Plastics Engineers     

Methanol permeability and properties of DMFC membranes based on sulfonated PEEK/PVDF blends

Proton exchange membranes for a direct methanol fuel cell were prepared by blending poly(vinylidene fluoride) [PVDF] with sulfonated poly(etheretherketone) [SPEEK]. The effects of PVDF content on methanol permeability in the blend membranes were investigated by using a diffusion cell and gas chromatography technique. The thermal resistance and proton conductivity of the membranes were also determined by using a thermal gravimetric analysis (TGA) and an impedance analysis technique, respectively. It was found that methanol permeability in the blend membranes decreased with PVDF content at the expense of proton conductivity. The methanol permeability values of the blend membranes are much lower than that of Nafion 115, whereas proton conductivities of the membranes are comparable to that of Nafion. The thermal stability of these blend membranes are above 250°C which is sufficiently high for use in DMFC. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5941–5947, 2006     

聚吡咯修饰Nafion膜直接甲醇燃料电池的性能

用FeCl3化学氧化法制备了PPy/Nafion改性膜,采用浸渍-还原法在PPy/Nafion阴极侧上沉积Co金属,制得Co-PPy/Nafion电解质膜。采用TG、CV及交流阻抗谱测试了Nafion膜及改性膜的热稳定性,质子电导性和甲醇渗透性能,结果表明:PPy/Nafion及Co-PPy/Nafion改性膜具有更好的热稳定性和抗甲醇渗透性。分别以Co-PPy/Nafion改性膜、PPy/Nafion改性膜和纯Nafion膜为电解质膜,PtRu/C为阳极催化剂,Pt/C为阴极催化剂组装DMFC并考察其性能。实验结果表明:Co-PPy/Nafion改性膜组装单电池在高浓度甲醇及大电流密度的测试条件下,表现出更优异的电池性能。     

Cell performances of direct methanol fuel cells with grafted membranes

Cell performances were evaluated with grafted polymer membranes as an electrolyte for a direct methanol fuel cell (DMFC). The membranes were prepared using a poly(ethylene-tetrafluoroethylene), or ETFE, film. The base polymer film was added to sulfonic groups using γ-radiation activated grafting technique as ion-exchange groups. These membranes had more suitable properties for DMFCs, i.e. higher electric conductivity and lower methanol permeability than perfluorinated ionomer membrane (Nafion). Nevertheless, the cell performance with the grafted membrane was inferior to that with Nafion. The analysis of electrode potentials vs. reversible hydrogen electrode showed larger activation overpotential for both the electrodes on the grafted membranes. We concluded that this is due to poor bonding of the catalyst layers to the grafted membranes.     

Preparation and characterization of novel grafted cellophane‐phosphoric acid‐doped membranes for proton exchange membrane fuel‐cell applications

This work concerns preparation of acid‐base polyelectrolyte membranes for fuel‐cell applications from cellulosic backbones for the first time. Grafted cellophane‐phosphoric acid‐doped membranes for direct oxidation methanol fuel cells (DMFC) were prepared following three steps. The first two steps were conducted to have the basic polymers. The first step was introducing of epoxy groups to its chemical structure through grafting process with poly(glycidylmethacrylate) (PGMA). The second step was converting the introduced epoxy groups to imides groups followed by phosphoric acid (? PO₃H) doping as the last step. This step significantly contributes to induce ion exchange capacity (IEC) and ionic conductivity (IC). Chemical changes of the cellophane composition and morphology characters were followed using FTIR, TGA, and SEM analysis. Different factors affecting the membranes characters especially IEC, methanol permeability, and thermal stability were investigated and optimized to have the best preparation conditions. Compared to Nafion 117 membrane, cellophane‐modified membranes show a better IEC, less methanol permeability, and better mechanical and thermal stability. IEC in the range of 1–2.3 meq/g compared to 0.9 meq/g per Nafion was obtained, and methanol permeability has been reduced by one‐order magnitude. However, the maximum obtained IC for cellophane‐PGMA‐grafted membrane doped with phosphoric acid was found 2.33 × 10^?3 (S cm^?1) compared to 3.88 × 10^?2 (S cm^?1) for Nafion 117. The obtained results are very promising for conducting further investigations taking into consideration the very low price of cellophane compared to Nafion. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of electrolytes of Li salts,EMIMBF4, and mixed phases on electrochemical and physical properties of Nafion membrane

Electrolyte‐soaked Nafion is commonly used as an ionic polymer in soft actuators. Here, a multitechnique investigation was applied to correlate the electrochemical behavior of Nafion membranes with their microstructures and nanostructures as a function of electrolyte type. The influence of electrolytes of Li salts with different counteranions on the Nafion membranes was investigated in terms of hydration level, structure (using X‐ray diffraction and small angle X‐ray scattering), stress–strain characteristics, and electrochemical behavior (by cyclic voltammetery and electrochemical impedance spectroscopy). The effects of using ionic liquid (IL), as the electrolyte, addition of different supporting solvent and the addition of Li⁺ ions to water‐free IL‐soaked membranes on the structural and electrochemical properties of Nafion were examined. The nano‐ and microstructure of the Nafion changed considerably as a function of the identity of the electrolyte solution. The electrochemical behavior of the IL‐soaked samples was compared with that of the water‐soaked Li⁺‐exchanged Nafion. It was seen that the ionic conductivity of the Nafion membranes was reduced significantly when water was replaced by pure IL. Using the supporting solvents increased the conductivity of IL‐soaked Nafion membranes dramatically. The presence of a small amount of Li⁺ ions together with the IL ions caused a significant decrease in charge transfer resistance and increases in double layer capacitance and in ionic conductivity over that of the water‐free sample and also over water‐soaked Li⁺‐exchanged Nafion. These findings can be useful to improve the knowledge on Nafion's microstructure and also to improve the electromechanical behavior of Nafion‐based ionic polymer–metal composites actuators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45239.     

Solvent and pH resistance of surface crosslinked chitosan/poly(acrylonitrile) composite nanofiltration membranes

The resistance of novel surface crosslinked Chitosan/poly(acrylonitrile) (PAN) composite nanofiltration (NF) membranes to pH and organic solvents was studied with respect to the effects of crosslinking parameters, namely, glutaraldehyde concentration and crosslinking time. The pH resistance was determined by permeation of aqueous acidic (pH 2.5) and basic (pH 11) solutions as well as swelling studies in the pH range of 2.5–11. The solvent resistance was determined by swelling, immersion, and permeation studies with several industrially important organic solvents, namely methanol, ethanol, iso‐propanol, methyl ethyl ketone, ethyl acetate and hexane. It was observed that the crosslinked composite membranes maintain the permeate fluxes for test solvents for 2 h of continuous operation without any significant change in flux. SEM studies on membrane samples after immersion as well as permeation with the above‐mentioned solvents indicated that the membrane morphology was maintained. The results are explained in terms of solvent–membrane polar and hydrophobic interactions, using solubility parameters of membrane and solvents and dielectric constants of solvents. Pure water flux and polyethylene glycol transmission data indicated that at pH 2.5 and 11, the membrane stability increased with increasing glutaraldehyde concentration and was much better at pH 11 than at pH 2.5. All surface crosslinked membranes showed reduced swelling between pH 4–10. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1782–1793, 2000     

Intermolecular Interactions Between Methanol/Water Molecules and Nafion™ Membrane: An Infrared Spectroscopy Study

The intermolecular interactions between methanol/water and Nafion™ membranes have been investigated using IR spectroscopy. The evolution of IR spectra of the Nafion™ membranes, immersed in various concentrations of methanol solution depends strongly on the methanol concentration. The O–H bending vibration modes at 1,636, 1,660 and 1,672 cm^–1 associated with the hydrogen bonding of (H₃O⁺…SO₃^–) as well as (CH₃OH₂⁺…SO₃^–), and at 1,702, 1,717 and 1,711–1,736 cm^–1 associated with the hydrogen bonding of (CF₂…H–O–CH₃), (CF₂…H–OH), (CF₂…⁺H₃O) and (CF₂…H–OSO₂^–) were observed. The vibration mode of (CF₂…H–O) was found to be appearing obviously at 3,821–3,900 cm^–1 when the Nafion™ membrane was immersed in the methanol solution with concentration higher than 6 M. On the other hand, the wavenumber of the O–H stretching peak increases with an increase in the methanol concentration. Results of IR spectra revealed that the methanol molecules show better capability to penetrate into the hydrophobic domain of the Nafion™ membrane than water. The intermolecular interaction between the hydrophobic domains of Nafion™ and methanol molecules becomes more observable at a higher methanol concentration.     

Nafion modified with simple bases and amino acid derivatives: Survey of physical properties and search for effective pervaporation membranes

Nafion, a solid perfluorinated polymeric sulfonic acid, reacts readily with bases and can thus be converted to a tremendous number of individual modifications. A survey of spectral, mechanical, thermal, and pervaporation characteristics of Nafion modified with simple inorganic and organic bases and with the derivatives of amino acids is thus reported. The spectral characteristics, thermal stability, and T_g changed pronouncedly for Nafion modified with the stronger bases, such as ammonium or tris(hydroxymethyl)aminomethane, while minor changes were observed for Nafion modified with the derivatives of amino acids. Permeability and selectivity of Nafion observed for the pervaporation of methanol–dimethyl carbonate and methanol–methyl acetate mixtures were mainly influenced by the modification with amines bearing hydroxyl and, most pronouncedly, sulfo groups. Importantly, the pervaporation membrane from Nafion modified with taurine appeared similarly permeable and hygroscopic but more methanol-selective than that from pure Nafion.     

Preparation of chitosan-modified core–shell SiO2-acidic polymer multiple crosslinked membranes

This article prepares and studies a series of chitosan-modified core–shell SiO₂-acidic polymer multiple crosslinked membranes (modified membranes). The preparation process is simple, low cost, and environmentally friendly. The multiple crosslinking enhances the thermal stability and mechanical strength of membranes. TGA shows that the 10% weight loss temperatures of modified membranes are above 300 °C. Compared with water uptake, the methanol absorption of modified membranes is lower and decreases with increasing methanol concentration, which is very beneficial to their use in direct-methanol fuel cells. The lowest methanol diffusion coefficient of the modified membrane is 4.56 × 10⁻⁸ cm² s⁻¹, which is only about 1/50th of that of Nafion117. In addition, the membranes exhibit feasible proton conductivity and higher selectivity than Nafion117. These results indicate that the chitosan-modified core–shell SiO₂-acidic polymer multiple crosslinked membranes have potential applications in direct-methanol fuel cells. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48494.     

Proton‐conducting polymers with reduced methanol permeation

The permeability of Nafion®117 and some types of acid‐base and covalently crosslinked blend membranes to methanol was investigated. The methanol crossover was measured as a function of time using a gas chromatograph with a flame ionization detector. In comparison to Nafion, the investigated acid‐base and covalently crosslinked blend membranes show a significant lower permeation rate to methanol. Additionally, another method to reduce the methanol permeability is presented. In this concept a thin barrier layer is plasma polymerized on Nafion 117 membranes. It is shown that a plasma polymer layer with a thickness of 0.3 µm reduces the permeability to methanol by an order of magnitude. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 67–73, 1999