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.     

Synthesis and characterization of Nafion-MO2 (M = Zr, Si, Ti) nanocomposite membranes for higher temperature PEM fuel cells

Nafion^®-MO₂ (M = Zr, Si, Ti) nanocomposite membranes were synthesized with the goal of increasing the proton conductivity and water retention at higher temperatures and lower relative humidities (120 °C, 40% RHs) as well as to improve the thermo-mechanical properties. The sol-gel approach was utilized to incorporate inorganic oxide nanoparticles within the pores of Nafion^® membrane. The membranes synthesized by this approach were completely transparent and homogeneous as compared to membranes prepared by alternate casting methods which are cloudy due to the larger particle size. At 90 °C and 120 °C, all Nafion^®-MO₂ sol-gel composites exhibited higher water sorption than Nafion^® membrane. However, at 90 °C and 120 °C, the conductivity was enhanced in only Nafion^®-ZrO₂ sol-gel composite with a 10% enhancement at 40% RH over Nafion^®. This can be attributed to the increase in acidity of zirconia based sol-gel membranes shown by a decrease in equivalent weight in comparison to other nanocomposites based on Ti and Si. In addition, the TGA and DMA analyses showed improvement in degradation and glass transition temperature for nanocomposite membranes over Nafion^®.     

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.     

Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells

Proton-exchange membrane fuel cells have emerged as a promising emission free technology to fulfill the existing power requirements of the 21st century. Nafion^® is the most widely accepted and commercialized membrane to date and possesses excellent electrochemical properties below 80 °C, under highly humidified conditions. However, a decrease in the proton conductivity of Nafion^® above 80 °C and lower humidity along with high membrane cost has prompted the development of new membranes and techniques. Addition of inorganic fillers, especially silicate-based nanomaterials, to the polymer membrane was utilized to partially overcome the aforementioned limitations. This is because of the lower cost, easy availability, high hydrophilicity and higher thermal stability of the inorganic silicates. Addition of silicates to the polymer membrane has also improved the mechanical, thermal and barrier properties, along with water uptake of the composite membranes, resulting in superior performance at higher temperature compared to that of the virgin membrane. However, the degrees of dispersion and interaction between the organic polymer and inorganic silicates play vital roles in improving the key properties of the membranes. Hence, different techniques and solvent media were used to improve the degrees of nanofiller dispersion and the physico-chemical properties of the membranes. This review focuses mainly on the techniques of silicate-based nanocomposite fabrication and the resulting impact on the membrane properties.     

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.     

Effects of crosslinkers on semi-interpenetrating polymer networks of Nafion and fluorine-containing polyimide

A series of reinforced composite membranes were prepared from Nafion^®212 and crosslinkable fluorine-containing polyimide (FPI) with various crosslinkers. The crosslinkable FPI reacts with the crosslinkers and forms semi-interpenetrating polymer networks (semi-IPN) structure with Nafion^®212. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and chemical oxidation stability of the composite membranes are studied. The degree of crosslinking is characterized by gel fraction of the composite membranes. Compared to pure Nafion^®212, the composite membranes exhibit excellent thermal stability, improved mechanical properties and dimensional stability. The tensile strength of the composite membranes is in the range of 37.3-51.2 MPa. All the composite membranes exhibit high proton conductivity which ranges from 1.9 × 10⁻² to 9.9 × 10⁻² S cm⁻¹. The proton conductivity of the composite membrane with 2-propene-1-sulfonic acid sodium salt (SAS) as the crosslinker is 9.9 × 10⁻² S cm⁻¹ at 100 °C which is similar to that of Nafion^®212 under the same condition.     

Preparation and characterization of nanocomposite polyelectrolyte membranes based on Nafion ionomer and nanocrystalline hydroxyapatite

In this study nanocrystalline hydroxyapatite (nHA) was synthesized and characterized by means of FT-IR, XRD and TEM techniques and a series of proton exchange membranes based on Nafion^® and nHA were fabricated via solvent casting method. Thermogravimetric analysis confirmed thermal stability enhancement of the Nafion^® nanocomposite due to the presence of nHA nanopowder. SAXS and TEM analyses confirmed the incorporation of nHA into ionic phase of Nafion^®. Furthermore, the incorporation of elliptical nHA into the Nafion^® matrix improved proton conductivity of the resultant polyelectrolyte membrane up to 0.173 S cm⁻¹ at 2.0 wt% of nHA loading compared to that of 0.086 S cm⁻¹ for Nafion^® 117. Also, the inclusion of nHA nanoparticles into nanocomposite membranes resulted in a significant reduction of methanol permeability and crossover in comparison with pristine Nafion^® membranes. Membrane selectivity parameter of the nanocomposites at 2.0 wt% nHA was calculated and found to be 106,800 S s cm⁻³, which is more than two times than that of Nafion^® 117. Direct methanol fuel cell tests revealed that Nafion^®/nHA nanocomposite membranes were able to provide higher fuel cell efficiency and also better electrochemical performance in both low and high concentrations of methanol feed. Thus, the current study shows that nHA enhances the functionality of Nafion^® as fuel cell membranes.     

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

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

Solution-cast Nafion ionomer membranes: preparation and characterization

Perfluorosulfonate ionomer dispersions in three different solvents (ethylene glycol (EG), dimethyl sulfoxide (DMSO) and dimethylformamide (DMF)) were used to prepare solution-cast membranes. The dispersions were obtained by dissolution of Nafion^® 112 membranes in a reactor using a water-ethanol solution. Measurements of water uptake and solubility of the cast membranes were conducted. Conductivities were measured directly on carbon paper/membrane/carbon paper assemblies fully immersed in deionized water and were registered by means of impedance spectroscopy as a function of temperature. Results show that all cast samples have lower chemical stability than commercial membranes. Moreover, only membranes cast from dimethylformamide-based dispersions gave conductivity performance comparable to those of Nafion^® 112 and 115. Casting process from dispersion obtained using Nafion^® rejects gave better results than the process utilizing commercial dispersion.     

A Nafion®-bound platinized carbon electrode for oxygen reduction in solid polymer electrolyte cells

A full-factorial statistical method has been used to optimize the amount of binder, the compaction load and the compaction time in order to fabricate Nafion^®-bound carbon electrodes containing platinum and pyrolysed cobaltmeso-tetramethoxyphenylporphyrin (CoTMPP). These electrodes are designed for oxygen reduction in solid polymer electrolyte fuel cells, and it has been found that incorporation of Nafion^® gel with the carbon substrate facilitates its binding with the Nafion^® membrane of such a fuel cell. The polarization behaviour of a Nafion^®-bound carbon electrode containing platinum is compared to that containing pyrolysed CoTMPP.     

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.     

Proton conducting hydrocarbon membranes: Performance evaluation for room temperature direct methanol fuel cells

The methanol permeability, proton conductivity, water uptake and power densities of direct methanol fuel cells (DMFCs) at room temperature are reported for sulfonated hydrocarbon (sHC) and perfluorinated (PFSA) membranes from Fumatech^®, and compared to Nafion^® membranes. The sHC membranes exhibit lower proton conductivity (25–40 mS cm⁻¹ vs. ∼95–40 mS cm⁻¹ for Nafion^®) as well as lower methanol permeability (1.8–3.9 × 10⁻⁷ cm² s⁻¹ vs. 2.4–3.4 × 10⁻⁶ cm² s⁻¹ for Nafion^®). Water uptake was similar for all membranes (18–25 wt%), except for the PFSA membrane (14 wt%). Methanol uptake varied from 67 wt% for Nafion^® to 17 wt% for PFSA. The power density of Nafion^® in DMFCs at room temperature decreases with membrane thickness from 26 mW cm⁻² for Nafion^® 117 to 12.5 mW cm⁻² for Nafion^® 112. The maximum power density of the Fumatech^® membranes ranges from 4 to 13 mW cm⁻¹. Conventional transport parameters such as membrane selectivity fail to predict membrane performance in DMFCs. Reliable and easily interpretable results are obtained when the power density is plotted as a function of the transport factor (TF), which is the product of proton concentration in the swollen membrane and the methanol flux. At low TF values, cell performance is limited by low proton conductivity, whereas at high TF values it decreases due to methanol crossover. The highest maximum power density corresponds to intermediate values of TF.     

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.     

Effect of Hot Pressing on the Performance of Direct Methanol Fuel Cells

The effects of hot pressing of electrodes onto Nafion^® membranes in the preparation of membrane and electrode assemblies for direct methanol fuel cells have been investigated. Hot pressing does not significantly influence the cell resistance or methanol crossover, but it can decrease cell performance by restricting mass transport in the anode catalyst layer. It also increases the time required for the cell to reach optimum performance. Best performances were obtained with membrane and electrode assemblies that had not been pressed. It was also found that membranes that had not been subjected to hot pressing could easily be re-used, making recycling of membranes and catalysts more feasible.     

Use of novel permeable membrane and air cathodes in acetate microbial fuel cells

In the existing microbial fuel cells (MFCs), the use of platinized electrodes and Nafion^® as proton exchange membrane (PEM) leads to high costs leading to a burden for wastewater treatment. In the present study, two different novel electrode materials are reported which can replace conventional platinized electrodes and can be used as very efficient oxygen reducing cathodes. Further, a novel membrane which can be used as an ion permeable membrane (Zirfon^®) can replace Nafion^® as the membrane of choice in MFCs. The above mentioned gas porous electrodes were first tested in an electrochemical half cell configuration for their ability to reduce oxygen and later in a full MFC set up. It was observed that these non-platinized air electrodes perform very well in the presence of acetate under MFC conditions (pH 7, room temperature) for oxygen reduction. Current densities of −0.43 mA cm⁻² for a non-platinized graphite electrode and −0.6 mA cm⁻² for a non-platinized activated charcoal electrode at −200 mV vs. Ag/AgCl of applied potential were obtained. The proposed ion permeable membrane, Zirfon^® was tested for its oxygen mass transfer coefficient, K₀ which was compared with Nafion^®. The K₀ for Zirfon^® was calculated as 1.9 × 10⁻³ cm s⁻¹.     

Investigation of direct methanol fuel cell performance of sulfonated polyimide membrane

Sulfonated polyimide (SPI) membranes have been evaluated as electrolyte membranes in direct methanol fuel cells (DMFCs). The membrane-electrode assembly (MEA) was made by hot-pressing the membrane, an anode and a cathode, catalyzed with PtRu/CB (PtRu dispersed on carbon black) and Pt/CB bound with Nafion^® ionomer, respectively. The performance of the cell based on SPI was compared with that of Nafion^® 112 in various operation conditions such as cell temperature (T_cell), cathode relative humidity (RH), and methanol concentration (C_MeOH). The methanol crossover at the cell based on SPI was a half of Nafion^® 112, resulting in the improved cell efficiency. Advantage of the use of SPI became much distinctive from the conventional Nafion^® 112 when the DMFC was operated at a higher T_cell or a higher C_MeOH.     

Mass transport of direct methanol fuel cell species in sulfonated poly(ether ether ketone) membranes

Homogeneous membranes based on sulfonated poly(ether ether ketone) (sPEEK) with different sulfonation degrees (SD) were prepared and characterized. In order to perform a critical analysis of the SD effect on the polymer barrier and mass transport properties towards direct methanol fuel cell species, proton conductivity, water/methanol pervaporation and nitrogen/oxygen/carbon dioxide pressure rise method experiments are proposed. This procedure allows the evaluation of the individual permeability coefficients in hydrated sPEEK membranes with different sulfonation degrees. Nafion^® 112 was used as reference material. DMFC tests were also performed at 50 °C. It was observed that the proton conductivity and the permeability towards water, methanol, oxygen and carbon dioxide increase with the sPEEK sulfonation degree. In contrast, the SD seems to not affect the nitrogen permeability coefficient. In terms of selectivity, it was observed that the carbon dioxide/oxygen selectivity increases with the sPEEK SD. In contrast, the nitrogen/oxygen selectivity decreases. In terms of barrier properties for preventing the DMFC reactants loss, the polymer electrolyte membrane based on the sulfonated poly(ether ether ketone) with SD lower or equal to 71%, although having slightly lower proton conductivity, presented much better characteristics for fuel cell applications compared with the well known Nafion^® 112. In terms of the DMFC tests of the studied membranes at low temperature, the sPEEK membrane with SD = 71% showed to have similar performance, or even better, as that of Nafion^® 112. However, the highest DMFC overall efficiency was achieved using sPEEK membrane with SD = 52%.     

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.     

Analysis of proton exchange membrane fuel cell polarization losses at elevated temperature 120 °C and reduced relative humidity

Polarization losses of proton exchange membrane (PEM) fuel cells at 120 °C and reduced relative humidity (RH) were analyzed. Reduced RH affects membrane and electrode ionic resistance, catalytic activity and oxygen transport. For a cell made of Nafion^® 112 membrane and electrodes that have 35 wt.% Nafion^® and 0.3 mg/cm² platinum supported on carbon, membrane resistance at 20%RH was 0.407 Ω cm² and electrode resistance 0.203 Ω cm², significantly higher than 0.092 and 0.041 Ω cm² at 100%RH, respectively. In the kinetically controlled region, 20%RH resulted in 96 mV more cathode activation loss than 100%RH. Compared to 100%, 20%RH also produced significant oxygen transport loss across the ionomer film in the electrode, 105 mV at 600 mA/cm². The significant increase in polarization losses at elevated temperature and reduced RH indicates the extreme importance of designing electrodes for high temperature PEM fuel cells since membrane development has always taken most emphasis.