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.     

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).     

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.     

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.     

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 exchange membranes based on poly(vinylidene fluoride) and sulfonated poly(ether ether ketone)

Blend membranes were obtained by solution casting from poly(vinylidene fluoride) (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) in N,N-dimethylacetamide (DMAc). DSC and XRD were used to characterize the structure of the blend membranes. The effect of PVDF content on the membrane properties was investigated. The methanol permeability, water uptake and the swelling ratio of blend membranes decreased with the increase of PVDF content. Though the proton conductivity decreased upon the addition of PVDF, they were still comparable to that of Nafion^® 117 membrane. Higher selectivities were also found for most blend membranes in comparison with Nafion^® 117 membrane. The effect of methanol concentration on solution uptake, swelling ratio and methanol permeability of the blend membranes was also studied.     

Modified Sulfonated Poly(ether ether ketone) Based Mixed Matrix Membranes for Direct Methanol Fuel Cells

Mixed matrix membranes based on zeolite 4A‐methane sulfonic acid (MSA)‐sulfonated poly(ether ether ketone) (SPEEK) are evaluated as a potential polymer electrolyte membrane (PEM) for direct methanol fuel cells (DMFCs). Ion‐exchange capacity, sorption of water, and water–methanol mixture, proton conductivity, and methanol permeability for the mixed‐matrix membranes have been extensively investigated. The mixed‐matrix membranes are also characterized for their cross‐sectional morphology, mechanical, and thermal properties. DMFCs employing SPEEK‐MSA (20 wt.%) blend, zeolite 4A (4 wt.%)‐SPEEK‐MSA (20 wt.%) mixed matrix membranes deliver peak power densities of 130 and 159 mW cm^–2, respectively; while a peak power density of only 95 mW cm^–2 is obtained for the DMFC employing pristine SPEEK membrane at 70 °C. The results showed that these SPEEK based mixed matrix membranes exhibit higher DMFC performance and lower methanol permeability in comparison to Nafion‐117 membrane.     

Sulfonated poly(ether ether ketone) membranes crosslinked with sulfonic acid containing benzoxazine monomer as proton exchange membranes

The incorporation of benzoxazine (Ba) or sulfonic acid containing benzoxazine (SBa) as a crosslinking agent in SPEEK proton exchange membrane (PEM) can substantially improve the SPEEK membrane performance. The SPEEK-SBa membranes give higher effective selectivity than corresponding SPEEK-Ba membranes under close crosslinker loading and thus are more suitable to be used in direct methanol fuel cells. The best achieved SPEEK-SBa composition (SBa40) gives reasonable proton conductivity (0.91 × 10⁻² S cm⁻¹) but significantly lower methanol permeability (6.5 × 10⁻⁸ S² cm⁻¹). The achieved effective selectivity (Φ = SPEEK-SBa40: 14.0 × 10⁴ S s cm⁻³) is substantially higher than the plain SPEEK (Φ = 7.24 × 10⁴ S s cm⁻³) which has great potential for practical applications in DMFCs.     

Preparation and investigation of acid–base composite membranes with modified graphitic carbon nanosheets for direct methanol fuel cells

The ethylenediamine-modified graphite oxide (EGO)-doped sulfonated poly (arylene ether ketone) (SPEEK) composite membranes have been prepared and developed for fuel cell applications in the present work. The base-modified EGO improves the dispersion of inorganic nanosheet in the polymer matrix and enhances proton conductivity by creating continuous conduction pathways. Furthermore, the methanol barrier property also be enhanced due to the nanosheet block the methanol-transport channels. EGO-filled membranes display improved dimensional stability, proton conductivity, and ethanol permeability than those using SPEEK control and graphite oxide (GO)-filled membranes. In the direct methanol fuel cells (DMFCs), the SPEEK/EGO-1.5 membrane displays the highest current density of 395.9 mA/cm² at 60°C, which is 1.6- and 1.4-fold higher than that of SPEEK (254.0 mA/cm²) and SPEEK/GO membrane (292.6 mA/cm²).     

A novel heteropolyacid-doped carbon nanotubes/Nafion nanocomposite membrane for high performance proton-exchange methanol fuel cell applications

A novel proton-exchange polymer composite membrane was synthesized using Nafion^®, tetraethoxysilane-modified carbon nanotubes (CNTs) and phosphotungstic acid-modified carbon nanotubes with the aim of using direct methanol fuel cells (DMFCs). Physicochemical properties of the modified CNTs and fabricated composite membranes were investigated by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, water uptake, thermogravimetric analysis, ion exchange capacity, proton conductivity and methanol permeability tests. It was demonstrated that chemical surface modification of CNTs and introduction of the phosphotungstic acid (PWA) groups effectively improved the performance of DMFC. It was found that the presence of PWA groups on the surface of CNTs led to the formation of strong electrostatic interactions between the PWA groups and clusters of sulfonic acid in Nafion^® macromolecules. Hence, the incorporation of inorganic phosphotungstic super-acid-doped silicon oxide-covered carbon nanotubes (CNT@SiO₂-PWA) into Nafion^® matrices enhanced the proton conductivity of the prepared membranes. Moreover, the methanol permeability was reduced to 2.63 × 10^?7 cm² s^?1 in comparison with the recast Nafion^® membrane (2.25 × 10^?6 cm² s^?1). Enhancing the proton conductivity and reducing the methanol permeability, the selectivity of the prepared nanocomposite membranes was enhanced to a greater value of 330,700 S s cm^?3 as compared to the value of 38,222 S s cm^?3 for recast Nafion^®.     

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 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.     

Study of basic biopolymer as proton membrane for fuel cell systems

Up to now, many research groups work to improve the electrical and mechanical properties of membranes with a low cost of production. The biopolymers could be an answer to produce proton membranes at low cost. This work demonstrates that the intrinsic membrane polymer and clays properties can help to develop a novel proton exchange membranes. Biopolymer composites (chitosan-oxide compounds) present conductivity between 10⁻³ and 10⁻² S cm⁻¹. The measurements were calculated by EIS (1 MHz-0.05 Hz) using the two-electrode configuration. Different oxides were used: MgO, CaO, SiO₂, Al₂O₃. The ionic conductivities were compared with Nafion^®'s in the same conditions of P and T. The catalyst layer/membrane ensemble was made during the design with the subsequent demonstration as membrane electrode assemblies and finally the fuel cell was built. Our focus was to increase the compatibility between the proton basic polymer exchange membrane and basic clays as CaO and test a new kind of fuel cell.     

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.     

Do not forget the electrochemical characteristics of the membrane electrode assembly when designing a Proton Exchange Membrane Fuel Cell stack

The membrane electrode assembly (MEA) is the key component of a PEMFC stack. Conventional MEAs are composed of catalyzed electrodes loaded with 0.1–0.4 mg_Pt cm⁻² pressed against a Nafion^® membrane, leading to cell performance close to 0.8 W cm⁻² at 0.6 V. Due to their limited stability at high temperatures, the cost of platinum catalysts and that of proton exchange membranes, the recycling problems and material availability, the MEA components do not match the requirements for large scale development of PEMCFs at a low cost, particularly for automotive applications.Novel approaches to medium and high temperature membranes are described in this work, and a composite polybenzimidazole–poly(vinylphosphonic) acid membrane, stable up to 190 °C, led to a power density of 0.5 W cm⁻² at 160 °C under 3 bar abs with hydrogen and air. Concerning the preparation of efficient electrocatalysts supported on a Vulcan XC72 carbon powder, the Bönnemann colloidal method and above all plasma sputtering allowed preparing bimetallic platinum-based electrocatalysts with a low Pt loading. In the case of plasma deposition of Pt nanoclusters, Pt loadings as low as 10 μg cm⁻² were achieved, leading to a very high mass power density of ca. . Finally characterization of the MEA electrical properties by Electrochemical Impedance Spectroscopy (EIS) based on a theoretical model of mass and charge transport inside the active and gas diffusion layers, together with the optimization of the operating parameters (cell temperature, humidity, flow rate and pressure) allowed obtaining electrical performance greater than 1.2 W cm⁻² using an homemade MEA with a rather low Pt loading.     

Poly(vinyl alcohol)-based polymer electrolyte membranes for direct methanol fuel cells

We have prepared polymer electrolyte membranes (PEMs) from poly(vinyl alcohol) (PVA) and modified PVA polyanion containing 2 or 4 mol% of 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer. The PEMs of various AMPS content and cross-linking conditions were prepared to determine the effect of AMPS content and cross-linking conditions on PEM properties. Proton conductivity and permeability of methanol through the PEMs increased with increasing AMPS content, C_AMPS, and with decreasing cross-linker concentration, C_GA, because of the increase in the water content. The permeability coefficient of methanol through the PEM prepared under the conditions of C_AMPS = 2.7 mol% and C_GA = 0.35 vol% was about 30 times lower than that of Nafion^®117 under the same measurement conditions. The proton permselectivity of the PEM, which is defined as the ratio of the proton conductivity to the permeability coefficient of methanol, gave a maximum value of 66 × 10³ S cm⁻³ s. The value is about three times higher than that of Nafion^®117.     

Chitosan membranes filled by GPTMS-modified zeolite beta particles with low methanol permeability for DMFC

Uniform zeolite beta particles about 800 nm in diameter were synthesized by a hydrothermal method, and functionalized by γ-glycidoxypropyltrimethoxysilane (GPTMS). Subsequently, chitosan (CS) membranes filled by GPTMS-modified zeolite beta particles were prepared, and characterized by SEM, FT-IR, XRD and TGA. Compared with the pure CS and Nafion^®117 membrane, these CS/zeolite beta hybrid membranes show apparently the lower methanol permeability, which could be assigned to the better interfacial morphology and compatibility between the GPTMS-modified zeolite beta particles and chitosan matrix. In all the prepared CS/zeolite beta hybrid membranes, the CS membrane filled by 10 wt.% GPTMS-modified zeolite beta particles exhibits the lowest methanol permeability, which is 4.4 × 10⁻⁷ and 2.2 × 10⁻⁷ cm² s⁻¹ at 2 and 12 M methanol concentration, respectively. The proton conductivity of this hybrid membrane is 1.31 × 10⁻² S cm⁻¹, which is slightly lower than that of the pure CS membrane. The selectivity of CS/GPTMS-zeolite beta membranes is comparable with Nafion^® 117 at 2 M methanol concentration, and much higher at 12 M methanol concentration.     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

This review summarizes efforts in developing sulfonated hydrocarbon proton exchange membranes (PEMs) with excellent long-term electrochemical fuel cell performance in medium-temperature and/or low-humidity proton exchange membrane fuel cell (PEMFC) applications. Sulfonated hydrocarbon PEMs are alternatives to commercially available perfluorosulfonic acid ionomers (PFSA, e.g., Nafion^®) that inevitably lose proton conductivity when exposed to harsh operating conditions. Over the past few decades, a variety of approaches have been suggested to optimize polymer architectures and define post-synthesis treatments in order to further improve the properties of a specific material. Strategies for copolymer syntheses are summarized and future challenges are identified. Research pertaining to the sulfonation process, which is carried out in the initial hydrocarbon PEM fabrication stages, is first introduced. Recent synthetic approaches are then presented, focusing on the polymer design to enhance PEM performance, such as high proton conductivity even with a low ion exchange capacity (IEC) and high dimensional stability. Polymer chemistry methods for the physico-chemical tuning of sulfonated PEMs are also discussed within the framework of maximizing the electrochemical performance of copolymers in membrane-electrode assemblies (MEAs). The discussion will cover crosslinking, surface fluorination, thermal annealing, and organic–inorganic nanocomposite approaches.     

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.