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.     

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.     

A new anhydrous proton conductor based on polybenzimidazole and tridecyl phosphate

Most of the anhydrous proton conducting membranes are based on inorganic or partially inorganic materials, like SrCeO₃ membranes or polybenzimidazole (PBI)/H₃PO₄ composite membranes. In present work, a new kind of anhydrous proton conducting membrane based on fully organic components of PBI and tridecyl phosphate (TP) was prepared. The interaction between PBI and TP is discussed. The temperature dependence of the proton conductivity of the composite membranes can be modeled by an Arrhenius relation. Thermogravimetric analysis (TGA) illustrates that these composite membranes are chemically stable up to 145 °C. The weight loss appearing at 145 °C is attributed to the selfcondensation of phosphate, which results in the proton conductivity drop of the membranes occurring at the same temperature. The DC conductivity of the composite membranes can reach ∼10⁻⁴ S/cm for PBI/1.8TP at 140 °C and increases with increasing TP content. The proton conductivity of PBI/TP and PBI/H₃PO₄ composite membranes is compared. The former have higher proton conductivity, however, the proton conductivity of the PBI/H₃PO₄ membranes increases with temperature more significantly. Compared with PBI/H₃PO₄ membranes, the migration stability of TP in PBI/TP membranes is improved significantly.     

A polymer electrolyte membrane for high temperature fuel cells to fit vehicle applications

Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H₃PO₄ were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35.19 MPa. After doping with the phosphoric acid, the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0.3 S cm⁻¹ at a relative humidity 8.4% and temperature of 180 °C with a 300% H₃PO₄ doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1.2 W cm⁻² at cell voltages >0.4 V and current densities of 3.0 A cm⁻². The PTFE/PBI/H₃PO₄ composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 °C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs.     

Anhydrous proton conductive membrane consisting of chitosan

We have investigated a low production cost anhydrous proton conductor consisting of a composite of chitosan, one of the world's discarded materials, and methanediphosphonic acid (MP) having a high proton exchange capacity. This chitosan-200 wt.% MP composite material showed the high proton conductivity of 5 × 10⁻³ S cm⁻¹ at 150 °C under anhydrous conditions. Additionally, the proton conducting mechanism of the chitosan-MP composite material was due to proton transfer to the proton defect site without the assistance of diffusible vehicle molecules. The utilization of a biopolymer, such as chitosan, for PEMFC technologies is novel and challenging where biological products are usually considered as waste, non-hazardous, and environmentally benign. Especially, the low production cost of the biopolymer is an attractive feature. Anhydrous proton conducting biopolymer composite membranes may have potential not only for PEMFCs operated under anhydrous conditions, but also for bio-electrochemical devices including an implantable battery, bio-sensors, etc.     

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.     

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^®.     

Characterisation of a re-cast composite Nafion 1100 series of proton exchange membranes incorporating inert inorganic oxide particles

A series of cation exchange membranes was produced by impregnating and coating both sides of a quartz web with a Nafion^® solution (1100 EW, 10%wt in water). Inert filler particles (SiO₂, ZrO₂ or TiO₂; 5-20%wt) were incorporated into the aqueous Nafion^® solution to produce robust, composite membranes. Ion-exchange capacity/equivalent weight, water take-up, thickness change on hydration and ionic and electrical conductivity were measured in 1 mol dm⁻³ sulfuric acid at 298 K. The TiO₂ filler significantly impacted on these properties, producing higher water take-up and increased conductivity. Such membranes may be beneficial for proton exchange membrane (PEM) fuel cell operation at low humidification. The PEM fuel cell performance of the composite membranes containing SiO₂ fillers was examined in a Ballard Mark 5E unit cell. While the use of composite membranes offers a cost reduction, the unit cell performance was reduced, in practice, due to drying of the ionomer at the cathode.     

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.     

Proton exchange membranes based on sulfonated crosslinked polystyrene micro particles dispersed in poly(dimethyl siloxane)

Proton exchange membranes of sulfonated crosslinked polystyrene (SXLPS) particles dispersed in crosslinked poly(dimethyl siloxane) matrix were investigated. Three different sizes of particles—25, 8 and 0.08 μm—were used at loadings from 0 to 50 wt% and the influence of these variables on the water and methanol uptake and proton conductivity were observed. With the reduction in particle size in the composite membrane, more water or methanol uptake was observed. Three different states of water were revealed in the composite membranes by differential scanning calorimetry (DSC). The number of bound water molecules per SO₃H group was 11-15 in membranes with 8- and 25-μm SXLPS. The ratio of bound to unbound water molecules was more than one in these membranes, whereas it was less than one in membranes with 0.08-μm SXLPS. The proton conductivities of the membranes increased with the increase in particle loading. At particle loadings above 35 wt%, membranes containing 8-μm SXLPS had higher conductivity compared to 25-μm SXLPS at room temperature. The conductivity of membranes containing 0.08-μm SXLPS was restricted to 10⁻³ S/cm because of the inherently low IEC of the particles. Increasing the temperature from 30 to 80 °C drastically enhanced the conductivity of the composite membranes compared to Nafion^® 112. At 80 °C, conductivities as high as 0.11±0.04 S/cm were observed for membranes containing more than 30 wt% of 25-μm SXLPS particles.     

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.     

Studies on anhydrous proton conducting membranes based on imidazole derivatives and sulfonated polyimide

Anhydrous proton conducting membranes based on sulfonated polyimide (sPI) and imidazole derivatives were prepared. The acid-base composite membranes show a good chemical oxidation stability and high thermal stability. The addition of imidazole derivatives in sPIs can improve the chemical oxidation stability of the composite membranes enormously, and even much better than that of pure sPI. The proton conductivity of a typical sPI/xUI(2-undecylimidazole) composite membrane can reach 10⁻³ S cm⁻¹ at 180 °C under the anhydrous condition. The proton conductivity of the acid-base composite membranes increases significantly with increasing content of UI. Moreover, UI in sPI/xUI composite membrane is difficult to be brought out by the vapor due to the existence of long hydrophobic moiety, which will improve the stability and lifetime of the membranes in the fuel cells.     

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.     

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.     

Synthesis and properties of sulfonated copoly(phthalazinone ether imides) as electrolyte membranes in fuel cells

A new series of six-member sulfonated copolyimides (SPIs) were prepared by one-step solution copolycondensation from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 1,2-dihydro-2-(4-amino-2-sulfophenyl)-4-[4-(4-amino-2-sulfonphenoxy)-phenyl] (2H)phthalazin-1-one (S-DHPZDA), 4,4′-bis(4-aminophenoxy) biphenyl (BAPB) and 1,2-dihydro-2-(4-aminophenyl)-4-[4-(4-(aminophenoxyl)phenyl)](2H)phthalazin-1-one (DHPZDA). The sulfonation degree (DS) of the SPIs was controlled by the mol ratio of the sulfonated diamine and non-sulfonated diamine. The obtained SPI membranes had excellent thermal stability, high mechanical property and proton conductivity as well as low methanol permeability. The tensile strength of the SPI membranes was ranging from 54.7 to 98.1 MPa, which was much higher than that of Nafion^®. The SPI membranes exhibited high proton conductivity (σ) and low methanol permeability ranged from 10⁻³ to 10⁻² S/cm and 10⁻⁸ to 10⁻⁷ cm²/s depending on the DS of the polymers, respectively.     

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.     

Polybenzimidazole containing benzimidazole side groups for high-temperature fuel cell applications

A polybenzimidazole (PBI) containing bulky basic benzimidazole side groups, poly[2,2′-(2-benzimidazole-p-phenylene)-5,5′-bibenzimidazole] (BIpPBI), was prepared via the condensation polymerization of 3,3′-diaminobenzidine tetrahydrochloride dihydrate with 2-benzimidazole terephthalic acid in PPA. BIpPBI was found to be soluble in aprotic polar solvents without the addition of inorganic salts, such as lithium chloride, and the BIpPBI film also showed very good acid retention capability as well as very high proton conductivity. The maximum acid content of the BIpPBI film was approximately 81 wt.% and the proton conductivity value of the acid-doped BIpPBI membrane was 0.16 S cm⁻¹ at 180 °C and a 0% relative humidity. For comparison, the maximum proton conductivity of the most commonly used polymer for the high-temperature fuel cell membrane, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (mPBI) membrane, is approximately 0.06 S·cm⁻¹ at 180 °C under anhydrous conditions at a 65 wt.% acid content, which is the maximum acid content that a mPBI membrane can have.     

Intrinsically proton-conducting poly(1-vinyl-1,2,4-triazole)/triflic acid blends

In the present work, proton conductivity in a polymer blend comprising proton solvating heterocycles was examined. Poly(1-vinyl-1,2,4-triazole), PVTri was produced by free radical polymerization of 1-vinyl-1,2,4-triazole and then proton-conducting polymer electrolytes were obtained by blending of PVTri with trifluoromethanesulfonic acid, triflic acid (TA). To promote the intrinsic proton conductivity the percent blending ratio was changed from 25% to 150% with respect to polymer repeat unit. The protonation of aromatic heterocyclic rings was proved with Fourier-transform infrared spectroscopy (FT-IR). Thermogravimetry (TG) analysis showed that the samples are thermally stable up to approximately 300 °C. Differential scanning calorimetry (DSC) results illustrated that the samples are homogeneous and their glass transition temperatures are located within 130-160 °C. The surface morphology of the materials were characterized by scanning electron microscopy (SEM). The proton conductivity of the blends increased with triflic acid concentration and the temperature. In the anhydrous state, the proton conductivity of PVTriTA100 is 2.2 × 10⁻⁴ S/cm at 150 °C and that of PVTriTA150 is approximately 0.012 S/cm at 80 °C which is similar to that of hydrated Nafion^®.     

Nafion/Analcime and Nafion/Faujasite composite membranes for polymer electrolyte membrane fuel cells

The Nafion/zeolite composite membranes were synthesized for polymer electrolyte fuel cells (PEMFCs) by adding zeolite in the matrix of Nafion polymer. Two kinds of zeolites, Analcime and Faujasite, having different Si/Al ratio were used. The physico-chemical properties of the composite membranes such as water uptake, ion-exchange capacity, hydrogen permeability, and proton conductivity were determined. The fabricated composite membranes showed the significant improvement of all tested properties compared to that of pure Nafion membrane. The maximum proton conductivity of 0.4373 S cm⁻¹ was obtained from Nafion/Analcime (15%) at 80 °C which was 6.8 times of pure Nafion (0.0642 S cm⁻¹ at 80 °C). Conclusively, Analcime exhibited higher improvement than Faujasite.     

Blend membranes based on disulfonated poly(aryl ether ether ketone)s (SPEEK) and poly(amide imide) (PAI) for direct methanol fuel cell usages

Highly disulfonated poly(aryl ether ether ketone)s (SPEEK-70) copolymer was synthesized via direct polymerization to precisely control the degree of sulfonation (D_s = 1.40), which was confirmed and estimated by ¹H NMR. As expected, the proton conductivity of SPEEK-70 membrane is 0.084 S/cm at 25 °C and increases to 0.167 S/cm at 80 °C, surpassing that of Nafion^® 117. However, the relatively high methanol crossover and excessively swelling properties limited its usage in DMFC. Poly(amide imide) was blended with SPEEK-70 to improve the methanol resistance and mechanical properties. These blend membranes were characterized as a function of weight fraction of PAI in terms of ion exchange capacity (IEC), water uptake, water desorption, proton conductivity and methanol permeability in detail. Although the proton conductivities decreased upon the addition of PAI, higher selectivity values defined as the ratio of proton conductivity to methanol permeability were found for the blend membranes. Therefore, the SPEEK/PAI blend membranes are promising for usage in DMFC.