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.     

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.     

Study on the conductivity of recast Nafion/montmorillonite and Nafion/TiO2 composite membranes

Nafion^® composite membranes were formed from a recast procedure already applied by the authors. Montmorillonite (MMT) and titanium dioxide (TiO₂) were used separately as fillers in the recast process and dimethylformamide (DMF) was used as the casting solvent. Addition of 1 wt.% MMT or 1 wt.% TiO₂ to the ionomer dispersions prior to the heat treatment demonstrated that there is an increase in water content of the recast membranes. For both the samples, it was verified that the tangential conductivity increases with increasing relative humidity (RH) of the environment. Fuel cell tests carried out with recast membranes showed that the best performances are seen when the anode and cathode humidification temperatures are low. With a ΔT of −35 °C between cell temperature and anode humidification, the conductivity of additive-containing samples is 10% higher than that of additive-free membranes.     

The effect of Nafion sulfonate surface concentration on cationic polyacrylamide adsorption

Perfluorosulfonate ionomer membranes (PFSIs) were cast from Nafion^® propanol-water solutions of varying equivalent weights (950, 1100, and 1200 Da) and all annealed at 100 °C under vacuum. Sulfonate group surface concentration on the various membranes and its effect on the adsorption of a cationic polyacrylamide were investigated utilizing X-ray photoelectron spectroscopy (XPS). The cationic polyacrylamide is employed in the microelectronics industry for electroless printed circuit fabrication and may be used to pattern electrodes directly onto the PFSI surfaces for fuel cell miniaturization. Sulfonate surface concentration was found to be well correlated to the sulfonate bulk concentration obtained from the known equivalent weights of the Nafion^® solutions. Adsorbed cationic polyacrylamide surface coverage was found to be independent of the immersion time in the adsorbate solution. A wide variation in adsorbate coverage (from 0 to 60% of the surface) occurred over a comparatively small variation in the sulfonate repeat unit surface concentration (from 0 to 20% of the total Nafion^® repeat units). Adsorbate coverage goes through a maximum as the sulfonate concentration increases and is consistent with a random, enthalpy-driven adsorption process. Greater sulfonate concentration and lower associated adsorbate coverage was detected on cast membranes than previously found with commercially prepared membranes.     

Solution-cast Nafion/montmorillonite composite membrane with low methanol permeability

Perfluorosulfonate ionomer dispersions in dimethylformamide solvent were used to form solution-cast membranes. Modified composite membranes were prepared with the addition of 1 and 5 wt.% montmorillonite salts. Measurements of water uptake, solubility and methanol permeation of the cast membranes were conducted. Tangential conductivities were measured directly on membranes fully immersed in deionized water by means of impedance spectroscopy. Results show that the addition of a low quantity of silicate did not alter the conductivity (94-96 mS cm⁻¹ at 25 °C), but produced a marked decrease of methanol permeability (−6%). Also a simple model was proposed to explain the increase of tangential conductivity with decreasing thickness of Nafion^® and recast membranes.     

Optimisation of polypyrrole/Nafion composite membranes for direct methanol fuel cells

Acidic and neutral Nafion^® 115 perfluorosulphonate membranes have been modified by in situ polymerization of pyrrole using Fe(III) and H₂O₂ as oxidizing agents, in order to decrease methanol crossover in direct methanol fuel cells. Improved selectivities for proton over methanol transport and improved fuel cell performances were only obtained with membranes that were modified while in the acid form. Use of Fe(III) as the oxidizing agent can produce a large decrease in methanol crossover, but causes polypyrrole deposition on the surface of the membrane. This increases the resistance of the membrane, and leads to poor fuel cell performances due to poor bonding with the electrodes. Surface polypyrrole deposition can be minimized, and surface polypyrrole can be removed, by using H₂O₂. The use of Nafion in its tetrabutylammonium form leads to very low methanol permeabilities, and appears to offer potential for manipulating the location of polypyrrole within the Nafion structure.     

Preparation of a Novel Composite Material Based on a Nafion® Membrane and Polypyrrole for Potential Application in a PEM Fuel Cell

The process of embedding polypyrrole (PPy) on the surface of a Nafion^® membrane was studied. Three methods of PPy synthesis directly on the membrane surface were compared. The diffusion method based on the separation of monomer and oxidant (peroxidisulphate) solutions by the membrane to be modified is proposed as the most promising one. The monomer diffuses through the membrane to the oxidant side, where it is polymerized. In this case sulphate is incorporated into the film as a counter-ion. PPy film prepared in this way adheres well to the Nafion^® surface and shows promising electrochemical activity. The permeability of the composite for monomer in comparison to self-standing Nafion^® film is reduced significantly. This may be important for the potential application of this composite, especially in a direct methanol fuel cell, as an alternative membrane-electrode assembly (MEA), particularly with regard to the currently used MEA’s permeability for fuel.     

Influence of Nafion film on oxygen reduction reaction and hydrogen peroxide formation on Pt electrode for proton exchange membrane fuel cell

The influence of Nafion^® film on ORR kinetics and H₂O₂ formation on a Pt electrode was investigated using RRDE in 0.1 M HClO₄. It was found that the Nafion^®-coated Pt system showed lower apparent ORR activity and more H₂O₂ production than the bare Pt electrode system. From the temperature sensitivity, it was revealed that the apparent activation energies of ORR in the Nafion^®-coated Pt system were lower than the bare Pt electrode system, and the H₂O₂ formation was suppressed with the increase of the temperature. In order to analyze the results furthermore, other systems (0.1/1.0 M, HClO₄/CF₃SO₃H) with the bare Pt electrodes were also examined as references. It was exhibited that the ORR kinetic current, the H₂O₂ formation, and the apparent activation energies of 1.0 M CF₃SO₃H system were close to those of the Nafion^®-coated Pt system. We concluded that the orientation of anion species of Nafion^® and CF₃SO₃H to the Pt surface via water molecules, as well as a fluorocarbon polymer network of Nafion^®, might block O₂ adsorption, resulting in the smaller effective surface area of the Pt electrode for ORR, the smaller ORR kinetic current, and the more H₂O₂ production.     

Cost effective cation exchange membranes: A review

This paper will look at developments of new polymer electrolyte membranes to replace high cost ion exchange membranes such as Nafion^®, Flemion^® and Aciplex^®. These perfluorinated polymer electrolytes are currently the most commercially utilized electrolyte membranes for polymer electrolyte fuel cells, with high chemical stability, proton conductivity and strong mechanical properties. While perfluorinated polymer electrolytes have satisfactory properties for fuel cell applications, they limit commercial use due to significant high costs as well as reduced performance at high temperatures and low humidity. A promising alternative to obtain high performance proton-conducting polymer electrolyte membranes is through the use of hydrocarbon polymers. The need for inexpensive and efficient materials with high thermal and chemical stability, high ionic conductivity, miscibility with other polymers, and good mechanical strength is reviewed in this paper. Though it is difficult to evaluate the true cost of a product based on preliminary research, this paper will examine several of the more promising materials available as low cost alternatives to ion exchange membranes. These alternative membranes represent a new generation of cost effective electrolytes that can be used in various ion exchange systems. This review will cover recent and significant patents regarding low cost polymer electrolytes suitable for ion exchange membrane applications. Promising candidates for commercial applications will be discussed and the future prospects of cost effective membranes will be presented.     

Recycling and regeneration of used perfluorosulfonic membranes for polymer electrolyte fuel cells

This paper describes a method for the recycling and regeneration of used perfluorosulfonic Nafion^® (Dupont) membranes by dissolution and recasting. The dissolution of the used Nafion^® membranes from polymer electrolyte fuel cells is realized using dimethyl sulfoxide as a solvent under atmospheric pressure and 190 °C. A mechanically robust membrane can be reproduced by a recast process of the dissolved Nafion^® solution at 170 °C. The recycled membrane has shown a good crystalline structure and high mechanical strength. Membrane properties, including water uptake, exchange capacity and resistance are similar to that of the as-received Nafion^® 115 membrane. Fuel cells prepared by the recycled membrane demonstrate a comparable performance to that of the fresh fuel cell.     

FTIR and electrochemical observation of water content reduction in a thin Nafion film induced by an impregnation of metal complex cations

A thin Nafion^® ion exchange membrane was coated on a graphite electrode, and then impregnated by a metal complex couple of Os(bpy)₃^2+/3+, which served as a typical system to investigate the water content change induced by ion exchange. Cyclic voltammetry and FTIR reflection-adsorption spectroscopic methods were employed to characterize the complex loading, electrochemical behavior of the impregnates inside the film and the water content change. A direct observation of the water content reduction induced by the cation impregnation in the Nafion^® film was achieved. A monotonic relationship between the water reduction percentage and the Os(bpy)₃^2+/3+ loading was also obtained, that is, the heavier the complex loading, the more the water content will be reduced. The FTIR spectroscopic results suggested that the structure of the Nafion^® film could also be changed with water content reduction.     

Nafion/ORMOSIL nanocomposites via polymer-in situ sol-gel reactions. 1. Probe of ORMOSIL phase nanostructures by Si solid-state NMR spectroscopy

A series of Nafion^®/ORMOSIL hybrids, generated by in situ sol-gel co-polymerizations of tetraethylorthosilicate (TEOS) and semi-organic R′_nSi(OR)_4−n co-monomers (SOC), were developed to generate a spectrum of nanoscale chemical environments within the Nafion^® morphological template. The molecular structures of the ORMOSIL phases were analyzed by means of ²⁹Si solid-state NMR (SSNMR) spectroscopy. A high average degree of Si atom coordination about SiO₄ molecular sub-units can be achieved, but a significant number of unreacted SiOH groups on reacted Q=Si(O_1/2)₄ units is always present. The shifting, with relative ORMOSIL composition, of D=RR′Si(O_1/2)₂ or T=R″Si(O_1/2)₃ (R, R′ and R″ are organic moeities) peak envelopes for difunctional or trifunctional SOCs is suggested to reflect random co-condensation rather than distinct Q and D (or T) block formation. The numbers of membrane-incorporated Q and D (or T) units per fixed sulfonate group were calculated from the ²⁹Si SSNMR spectra for those particular hybrids that had a reasonably low noise/signal aspect. Spectra for hybrids based on in situ sol-gel reactions for TEOS (no SOC present) suggest that inserted hydroquinone molecules interfere with condensation reactions between (RO)_4−xSi(OH)_x molecules and silanol oligomers to yield silicate structures with lower average coordination. While earlier small angle X-ray scattering studies indicated that ORMOSIL structures can be grown within the polar regions of Nafion^®, the results reported here address the specific compositions of these nanoscale structures.     

The effect of corrugated membranes on salt splitting

An investigation of the electrohydrolysis of sodium sulfate using a corrugated Nafion^® 117 membrane is reported. A comparison of the performance of a flat and corrugated Nafion^® 117 in a two-compartment membrane electrolysis cell is made. Corrugating the membrane increased the active membrane area by 57% compared to the projected area. The effect of flow rate, current density and salt concentration on current efficiencies, transport properties and achievable product concentrations are presented. The results show a large improvement on transport properties, current efficiencies and product formation using corrugated membranes. Corrugated membranes gave an improvement of up to 77% on achievable base concentration and an increase of approximately 22% in current efficiency.     

Synthesis and characterization of PMMA/PEG-TPE semi-interpenetrating polymer networks

Two-component semi-interpenetrating polymer networks (Semi-IPNs) of thermoplastic urethane elastomer based on poly(ethylene glycol) and polymethyl methyacrylate were synthesized by the sequential technique. The Semi-IPNs obtained were characterized with respect to their mechanical properties such as tensile strength, elongation. Glass transition temperatures were carried out using dynamic mechanical thermal analysis. Phase morphology was studied by scanning electron microscopy. The effect of the compositional variation on the above-mentioned properties was examined. The results demonstrate that the components are immiscible, phase separation at the micrometer scale is observed, the extent of interpenetrating is dependent on variations in composition.     

Stabilized heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation

Organic/inorganic composite membranes with different inorganic heteropolyacid (HPA) additives maintain sufficient proton conductivities for atmospheric pressure elevated temperature (>100 °C) polymer electrolyte fuel cell (PEFC) operation. However, membrane and membrane electrode assembly (MEA) processing is severely curtailed because of the solubility of the HPA additives in aqueous media. Composite membranes with the HPA (phosphotungstic acid; PTA) additive rendered insoluble by ion exchanging protons with larger cations such as Cs⁺, NH₄⁺, Rb⁺ and Tl⁺ were fabricated. The additive loss in aqueous media was lowered from nearly 100% (unmodified HPA) to about 5% (modified HPA). The membranes were robust, and demonstrated low H₂ crossover currents of around 2 mA/cm² for a 28 μm thick membrane. All membranes were evaluated at high temperatures and low relative humidities in an operating fuel cell. The conductivities of the composite membranes at 120 °C and 35% relative humidity were on the order of 1.6 × 10⁻² S/cm.     

Influence of counter-ions on the permeability of polypyrrole films to hydrogen

The influence of the size and nature of counter-ions on the permeability of polypyrrole films to hydrogen was tested. The permeabilities were determined using the method of limiting currents on a Pt rotating disc electrode with subsequent Koutecky-Levich analysis. The work has focused mainly on the influence of the anion size, but the effect of polymeric anion hydrophilicity was also considered. Attention was especially paid to Nafion^® as a counter-ion. It was found that Nafion^® markedly increases polypyrrole permeability. The film growth was followed by means of EQCN, which allowed determination of the polypyrrole film densities. ^†ISE member     

Kevlar-functionalized graphene nanoribbon for polymer reinforcement

Multiwalled carbon nanotubes (MWCNTs) have been widely used as reinforcement fillers in past decades. However, the reinforcement effect has been greatly hindered by the limited available interface area (AIA) with polymer matrices for polymer composites. Successively, the method of oxidative unzipping MWCNTs into graphene nanoribbons (GNRs) was demonstrated to be the effective way for addressing the inherent drawback of MWCNTs. However, the GNRs are easy to agglomerate in polymer matrix even at relatively low loading amount. In this paper, we found that the functionalization of GNRs with Kevlar^® can significantly improve the dispersion state of GNRs in polymer matrix. Consequently, Kevlar^®-functionalized graphene nanoribbons (KGNRs) were successfully prepared through non-covalent functionalization of π–π stacking interaction between the aromatic area of Kevlar^® and the graphitic surface of GNRs. As-prepared KGNRs were characterized by FT-IR, TGA, XRD and TEM measurements. Poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) were selected as model polymers to investigate the reinforcement effect of KGNRs. The KGNRs could be well dispersed in PVC and PMMA matrices at relatively high loading level. Meantime, the ultimate tensile strengths and Young's modulus of KGNRs/PVC and KGNRs/PMMA composite films were significantly improved. Based on the observations above, KGNRs hold great promise in many potential applications in the future.     

Electrochemical synthesis and anticorrosive properties of Nafion-poly(aniline-co-o-aminophenol) coatings on stainless steel

The objective of this work was to compare the electrochemical behavior and possible anticorrosive properties of composite with Nafion^®, poly(aniline-co-o-aminophenol) (P(An-co-OAP)) and polyaniline (PAn) films with those of corresponding simple films. The electrochemical synthesis of polymer films was carried out on stainless steel AISI 304 (SS) surfaces by using the cyclic potential sweep (CPS) deposition. Scanning electron microscopy (SEM) was used for the characterization of the structure and morphology of deposited films. Evaluation of anticorrosive properties of films in 0.5 M H₂SO₄ without and with chlorides was achieved by monitoring the open circuit potential (E_OC) of coated SS electrodes as well as by tracing the anodic current-potential polarization curves. These studies have shown that the SS remains in its passive state in the presence of polymer coatings. Composite with Nafion^®, P(An-co-OAP) and PAn films, keep their redox activity in chloride-containing acid solutions providing almost a complete protection of the SS substrate against pitting corrosion. These films prevent chloride exchange with solution because of the cation permselectivity of the Nafion^® membrane. The charge compensation during redox reactions occurs mainly by protons since sulfonate groups of Nafion^® act as dopants in composite films. The redox behavior of the Nafion^®-P(An-co-OAP) film is improved as compared with that of the Nafion^®-PAn film in both Cl⁻-free and Cl⁻-containing solutions. This behavior may be ascribed to the functional group -OH that facilitates charge compensation through proton during redox reactions.     

Performance comparison of portable direct methanol fuel cell mini-stacks based on a low-cost fluorine-free polymer electrolyte and Nafion membrane

A low-cost fluorine-free proton conducting polymer electrolyte was investigated for application in direct methanol fuel cell (DMFC) mini-stacks. The membrane consisted of a sulfonated polystyrene grafted onto a polyethylene backbone. DMFC operating conditions specifically addressing portable applications, i.e. passive mode, air breathing, high methanol concentration, room temperature, were selected. The device consisted of a passive DMFC monopolar three-cell stack. Two designs for flow-fields/current collectors based on open-flow or grid-like geometry were investigated. An optimization of the mini-stack structure was necessary to improve utilization of the fluorine-free membrane. Titanium-grid current collectors with proper mechanical stiffness allowed a significant increase of the performance by reducing contact resistance even in the case of significant swelling. A single cell maximum power density of about 18 mW cm⁻² was achieved with the fluorine-free membrane at room temperature under passive mode. As a comparison, the performance obtained with Nafion 117 membrane and Ti grids was 31 mW cm⁻². Despite the lower performance, the fluorine-free membrane showed good characteristics for application in portable DMFCs especially with regard to the perspectives of significant cost reduction.     

Chlorate Transport through Ion-Exchange Membranes

A laboratory scale chlor-alkali membrane cell was used to measure the chlorate concentration in the outlet NaOH as a function of current density, temperature, film thickness, brine strength and various membrane properties. The chlorate concentration in the NaOH increased with increasing anolyte chlorate spiking level and temperature and decreasing current density and carboxylate film thickness and was strongly dependent on the type of ion-exchange membrane used. In addition, the presence or absence of sacrificial fibers in the membrane did not measurably influence the resultant chlorate concentration. Chlorate ions were transported to the catholyte side by diffusion and electroosmotic convection and transported toward the anolyte side by migration. This balance between the three modes of transport dictates the chlorate concentration present in the NaOH product.