Polyelectrolyte Nanocomposite Membranes with Imidazole-Functionalized Multi-Walled Carbon Nanotubes for Use in Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® and imidazole modified multi-walled carbon nanotubes (MWCNT-Im), for direct methanol fuel cell (DMFC) applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of multi-walled carbon nanotubes (MWCNT), and the negatively charged sulfonic acid groups of Nafion®, new electrostatic interactions can be formed in the interface of the Nafion® and MWCNT-Im which result in both lower methanol permeability and also higher proton conductivity. The physical characteristics of these manufactured nanocomposite membranes were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/MWCNT-Im membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter in comparison to neat Nafion® or Nafion® containing –OH functionalized multi-walled carbon nanotubes (MWCNT-OH) membranes. The obtained results indicated that the Nafion®/MWCNT-Im membranes could be utilized as efficient polyelectrolyte membranes for direct methanol fuel cell applications.     

Polyelectrolyte Nanocomposite Membranes Using Surface Modified Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® (E. I. du Pont de Nemours and Co., Ltd., for its copolymer of tetrafluoroethylene and perfluorinated vinyl ether) and sulfonic acid (-SO₃H) or phosphotungstic acid (PWA) modified nanosilica (Si-SO₃H or Si-PWA, respectively), for direct methanol fuel cell (DMFC) applications are described. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy (SEM), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter, in comparison to the neat Nafion® or Nafion®/pristine nanosilica membranes. The obtained results indicated that both the Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

Polyelectrolyte Nanocomposite Membranes,Based on Chitosan-phosphotungstic Acid Complex and Montmorillonite for Fuel Cells Applications

A polyelectrolyte complex (PEC) of chitosan and phosphotungstic acid (PWA) was prepared and characterized as a proton-conducting membrane for direct methanol fuel cell (DMFC) applications. Fourier transform infrared spectroscopy showed the presence of stable PWA in PEC. To reduce the methanol permeability, several amounts of montmorilonite (MMT) nanoclays (trade name: Cloisite Na) were introduced to the system. The X-ray diffraction patterns of nanocomposite membranes proved the nanoclay layers were exfoliated in the membranes at loading weights of MMT lower than 3 wt%. Proton conductivity and methanol permeability were measured. According to the selectivity parameter—ratio of proton conductivity to methanol permeability—PEC containing 2 wt% MMT (PEC/2 wt% MMT) was identified as the optimum composition. Finally, DMFC performance tests were investigated at 70°C and 5 M methanol feed and the optimum membrane showed higher maximum power density in comparison with Nafion 117. The results indicated the optimum nanocomposite membrane is a promising polyelectrolyte membrane (PEM) for DMFC applications.     

Sulfonated Aromatic Polymers and Organically Modified Montmorillonite Nanocomposite Membranes for Fuel Cells Applications

Aromatic polymers, such as sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO), sulfonated poly(ether ether ketone) (SPEEK), and sulfonated poly(ether sulfone) (SPES), at the optimum degrees of sulfonation (DS), are suggested and evaluated as alternatives to Nafion for direct methanol fuel cells (DMFCs) applications. To reduce the methanol cross-over, which decreases the efficiency of the cell, organically modified montmorillonite nanoclays (OMMT) were added at 1 wt% to the sulfonated matrices with the optimum DS. The X-ray diffraction (XRD) patterns of nanocomposite membranes proved that the nanoclay layers were exfoliated. The proton conductivity and methanol permeability of the membranes, as well as the ion-exchange capacity (IEC), were measured. The selectivity parameter, ratio of proton conductivity to methanol permeability, was identified at 25°C for the nanocomposite membranes and the results were compared with Nafion117. Finally, the DMFC performance tests were investigated at 70°C and 5 M methanol feed for the manufactured nanocomposite polyelectrolyte membranes (PEMs). The SPEEK-based nanocomposite membrane showed the highest maximum power density in comparison with Nafion 117 and SPES and SPPO nanocomposite membranes. The results indicated that the nanocomposite membranes were promising PEMs for DMFC applications.     

Preparation and Characterization of Nafion/Sulfonated SiO2 Molecular Sieve Composite Membranes

A Nafion/sulfonated SiO₂ molecular sieve composite membrane was prepared by solution casting with sulfonated SiO₂ molecular sieve as the modifier. The ATR/FT-IR results showed that sulfonated SiO₂ molecular sieve did not change the structure of the membrane. The SEM and XRD results showed that the molecular sieve was distributed uniformly in the membrane. The proton conductivity, methanol permeability, water content, and swelling degree were measured. Compared with Nafion membrane, the composite membrane had higher water content and proton conductivity and lower methanol permeability. The overall performance was the best when the content of sulfonated SiO₂ molecular sieve was 5 wt%. These results indicated that Nafion/sulfonated SiO₂ molecular sieve composite membranes would be excellent candidate membrane materials for direct methanol fuel cell (DMFC) applications.     

Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method

Zeolite/Nafion composite membranes with high proton selectivity were successfully fabricated using the solution-casting method. The types of zeolites are nano-sized and large sized Na-ZSM-5, H-ZSM-5, and their ball-milled ones. Two different schemes of experiments were conducted depending on the type of solvent. In case of using as-received Nafion® ionomer dispersions, the experimental results clearly show that the proton conductivity of zeolite composite membrane using either H-type or Na-type ZSM-5 depends on the type of solvent. It is thought that when propanol and water as the solvents were used, more hydrophilic H-type ZSM-5 seems to have been more randomly dispersed into hydrophobic region rather than hydrophilic ionic clustered channels within Nafion. Therefore, H-type ZSM-5 existing near hydrophobic region seems to provide additional path for proton migration but weakening the mechanical strength. These composite membranes show higher water uptake than commercial Nafion® 115, strongly suggesting better water retention ability of zeolite. The most interesting result is that the methanol permeability has decreased with increasing zeolite contents even when the proton conductivity increased, and the proton selectivities of these composite membranes expressed as characteristic factor were higher than that of Nafion® 115. In case of using a mixture of high boiling point DMF and ethanol as the solvent, unlike the previous case where no DMF was used, the proton conductivity slightly dropped with increasing zeolite contents. These results should have been attributed to a blocking effect of zeolite particles surrounded by inversely oriented hydrophilic micelles of Nafion. However, the values of proton conductivity of most composite membranes were significantly higher than that of Nafion® 115, and methanol permeability also decreased with increasing zeolite contents. The significantly lower methanol permeability of the composite membrane fabricated with DMF as the solvent is probably due to the more effective blocking effect of H-ZSM-5 for ionic clustered channels as well as difficult transport of methanol through zeolite pores.In case of the composite membranes containing ZSM-5 with large crystal size, it is found that the methanol permeability has increased considerably with the increasing of zeolite contents due to void fractions between polymer phases and zeolite particles. In case of using ball-milled ZSM-5 with small crystal size, however, the value of characteristic factor tends to increase with increasing zeolite contents. Consequently, it is seen that the characteristic factor of Zeolite/Nafion composite membranes was much higher than Nafion® 115. The results obtained throughout this study strongly suggest that zeolites with small crystal size and high hydrophilicity are very prospective for composite membrane for direct methanol fuel cells in the future.     

Improving the Performance of Nafion®-Based Fuel Cell Membranes by Introducing Histidine Functionalized Carbon Nanotubes

A new polymer nanocomposite membrane based on Nafion and functionalized carbon nanotubes (CNTs) was developed for proton exchange membrane fuel cell (PEMFC) applications. Histidine, an imidazole-based amino acid, was used for modifying the surface of CNTs. The modification of CNTs was characterized by means of Fourier transform infrared spectroscopy (FTIR) and their Zeta potential. The imidazole groups, due to forming and breaking of hydrogen bonding, can facilitate proton transport across the polymer matrix by the Grotthuss mechanism. The final structure of the Nafion/CNT nanocomposites was investigated by small angle X-ray scattering (SAXS). The results confirm that the transport properties of the fabricated new membranes were significantly improved in comparison with unmodified and conventional Nafion® membranes. The power density of the imidazole-CNT (Im-CNT) Nafion® composite membranes was about three times more than Nafion® membranes. Also, the experimental results showed that the proton conductivity for the conventional Nafion® membranes decreased over 100°C but the conductivity for the Nafion®/Im-CNT remained at a nearly constant value above 100°C up to 120°C. Thus, the nanocomposite based on Nafion/imidazole functionalized CNT can be considered as an anhydrous PEMFC membrane for high-temperature applications.     

Design of novel SPEEK-based proton exchange membranes by self-assembly method for fuel cells

Fuel cell represents a new energy conversion device, which promises to provide clean source of power. Fuel cell [particularly proton exchange membrane fuel cell and direct methanol fuel cell (DMFC)] is a promising candidate for transportation and portable power source applications. In DMFC, there is a problem of methanol crossover. In order to reduce such a problem, there has been an intensive research activity in the modification of Nafion. In the present investigation, self-assembled membranes were fabricated with sulfonated polyether ether ketone as the core part of the membrane. Aminated polysulfone and sulfonated polysulfone were used as the layers in order to prevent the crossover of methanol. The assembled membranes were characterized by ion exchange capacity, water and methanol absorption, and durability. The methanol permeability and selectivity ratio proved a strong influence on DMFC application. Scanning electron microscopy proved smooth surface, which established strong cohesive force for the polymer chains. Among the synthesized self-assembled membranes, the membrane with two bilayers was the best in terms of power density in DMFC. The membrane electrode assembly with two bilayers showed higher performance (~61.05 mW/cm²) than sulfonated poly(ether ether ketone) and Nafion in DMFC.     

Surface-modified Nafion membrane by trioctylphosphine-stabilized palladium nanoparticles for DMFC applications

Trioctylphosphine (TOP)/Pd composites have been synthesized and used as a methanol-barrier material to modify the surface of Nafion 115. The TOP/Pd composites have been applied to the surface of Nafion instead of being incorporated into the Nafion matrix, to provide the best chance of maintaining the inherent proton conductivity of Nafion. The properties of the TOP/Pd-modified membrane, in terms of its conductivity and methanol permeability, as well as the performance of the membrane electrode assembly (MEA) in direct methanol fuel cell (DMFC), have been analyzed and compared with those of bare Nafion. The DMFC performance of the TOP/Pd-modified membrane is somewhat better than that of the bare Nafion one at methanol concentration of 2 M and significantly better at a high concentration of 5 M. The TOP/Pd-modified membrane is able to operate the DMFC using a high concentration of methanol, which can satisfy the requirement to reduce the reactant volumes for portable applications as well as to achieve high performance. In contrast to bare Nafion, the TOP/Pd-modified membrane with its well-adhering and crack-free modified surface shows effect on reducing the methanol loss.     

Reduced methanol crossover and enhanced proton transport in nanocomposite membranes based on clay−CNTs hybrid materials for direct methanol fuel cells

Hybrid nanomaterial based on the combination between a 2D silicate structure of a smectic clay (SWy) and 1D structures of carbon nanotubes has been synthesized and used as additive in the polymer matrix of Nafion for the preparation of electrolyte nanocomposite membranes. The CNTs anchored on the clay’s lamellae were subsequently oxidized and organo-functionalized by sulphonic groups. The hybrid membranes have been tested in direct methanol fuel cells (DMFCs) and studied by NMR spectroscopy (pulse field gradient technique and relaxation times), electrochemical impedance spectroscopy and SEM microscopy. The study of the molecular dynamics of methanol and protons, as well as the tests in the DMFC, shows the effectiveness of these “branched particles” for the reduction of the methanol crossover, whilst ensuring appropriate proton conductivity, especially in conditions of low humidity and high temperature (>100 °C).     

Novel cross-linked membrane for direct methanol fuel cell application: sulfonated poly(ether ether nitrile)s

In order to reduce water uptake, swelling ratio, and methanol permeability in sulfonated proton exchange membranes (PEM), novel-sulfonated aromatic poly(ether ether nitrile)s-bearing pendant propenyl groups had been synthesized by direct copolymerization method. All the results showed that the propenyl groups were suitable cross-linkable groups, and that this method was an effective way to overcome the drawbacks of sulfonated polymers at high ion exchange capacity (IEC) values. By cross-linking, the water uptake, swelling ratio, and methanol diffusion could be restricted owing to the formation of compact network structure. For example, CSPEN-60 membranes showed the proton conductivity of 0.072 S cm^?1 at 80 °C, while the swelling ratios and water uptake (17.9 and 60.7 %) were much lower than that of the SPEN-60 membrane (60.8 and 295.2 %). Meanwhile, a 1.1 × 10^?7 cm² s^?1 of methanol diffusion was obtained which was much lower than that of Nafion 117 (14.1 × 10^?7 cm² s^?1). Although the proton conductivity of the CSPEN-60 membranes is lower than that of the SPEN-60 membrane, the selectivity is much higher. The CSPEN-60 membrane exhibited the highest selectivity among the tested membranes, about 5.8 times higher compared with that of Nafion117.     

Influence of the carboxylic acid groups on the structure and properties of sulfonated poly(arylene ether nitrile) copolymer

A series of novel sulfonated poly(arylene ether nitrile) (SPEN) containing carboxylic acid group was successfully synthesized by direct aromatic nucleophilic substitution polycondensation of 2,6-difluorobenzonitrile (DFBN), potassium 2,5-dihydroxybenzenesulfonate (SHQ), phenolphthalin (PPL), and 4,4′-biphenol (BP). The expected chemical structure of copolymers was confirmed by using FTIR and ¹H NMR. To balance the performance for PEM applications, the proportion of four different components were controlled. The influences of the carboxylic acid groups on the structure and properties of SPEN, including thermal and mechanical properties, oxidative stability, water uptake, swelling, proton conductivity, and methanol permeability, were investigated in detail. The results revealed that SPEN membranes containing nitrile and carboxylic acid groups could lead low water absorption, swelling, and methanol penetration. In such a way, efficient proton transport channels were constructed by the formation of the hydrogen bonds. The proton conductivity of SPEN with high sulfonation degree (DS >?0.6) was higher than 0.05 S/cm and increased with increasing temperature. Especially, the conductivity of SPEN-0.6 and SPEN-0.7 reached up to 0.157 and 0.267 S/cm at 80 °C, respectively. Meanwhile, SPEN membranes exhibit low methanol permeability (0.13 ×?10^-6– 0.52 ×?10^-6 cm²·s^?1). Consequently, the highest selectivity of SPEN-0.6 reaches 2.02 ×?10⁵ S·cm^?3·s, which is about 4.5 times higher than that of Nafion 117 (0.45 ×?10⁵ S·cm^?3·s). All the data prove that this series of membranes exhibits excellent comprehensive performance and might have potential applications in direct methanol fuel cells.     

Identification of ionic aggregates in PVDF-g-PSSA membrane by tapping mode AFM and HADDF STEM

Previous literature has shown that poly(vinylidene fluoride)-graft-poly(styrene sulfonated acid) (PVDF-g-PSSA) exhibits a lower methanol permeability than commercial Nafion and so is better suited to use as a proton exchange membrane (PEM) in direct methanol fuel cells (DMFCs). A number of studies have suggested that the microstructures of ionic aggregates explain their lower methanol permeability, but few direct morphological observations have been reported. In this study, the use of a tapping mode atomic force microscope (AFM) and a high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) has identified the phase separation of PVDF and sulfonated PS, and ionic sulfonic aggregates, 3-5 nm, in sulfonated PS regions. An experiment to elucidate the microstructural changes in the membrane with and without methanol immersion shows that PVDF-g-PSSA has ionic aggregates with a more stable microstructure than Nafion.     

Strengthen the performance of sulfonated poly(ether ether ketone) as proton exchange membranes with phosphonic acid functionalized carbon nanotubes

Nanocomposite polymers based on phosphonic acid functionalized carbon nanotubes (CNT-POH) and sulfonated poly(ether ether ketone) (SPEEK) have been fabricated and employed as highly efficient proton exchange membranes. CNT-POH were synthesized through the grafting of carbon nanotubes (CNT) with diethylphosphatoethyl triethoxysilane and subsequent acidification of phosphate to phosphonic acid ligands. Incorporating CNT-POH into SPEEK matrix improves the proton conductivity at different temperatures and relative humidity, which can be attributed to the homogeneous dispersion of highly hydrophilic phosphonic acid groups and the formation of proton transport channels in the membrane. The methanol permeability of the composite membranes is also decreased, owing to the increased tortuosity of the methanol transport channel. The CNT in SPEEK matrix also enhance the dimensional stability and mechanical property remarkably. Consequently, this phosphonic acid functionalized CNT/SPEEK composite membrane (SPEEK-POH) is a potential candidate for application in direct methanol fuel cells (DMFC).     

Preparation and properties of crosslinked hybrid proton exchange membrane based on sulfonated poly (arylene ether nitrile) with improved selectivity for fuel cell application

Novel sulfonated poly (arylene ether nitrile) with pendant carboxylic group copolymers have been prepared as proton exchange membranes which were applied in direct methanol fuel cells (DMFCs). Compared with others, this work shows two main advantages: the crosslinked method is uncomplicated and the membranes were prepared via the hydroquinonesulfonic acid potassium salt (SHQ) as crosslinker mingled in sulfonated poly (arylene ether nitrile) (SPEN) to avoid the decrease of proton conductivity. The obtained crosslinked membranes exhibited improved dimensional stability; larger tensile strength than that of pure SPEN; and good thermal, mechanical properties. Furthermore, after crosslinking, the membranes had low methanol permeability values (0.78–3.4 × 10^?7 cm² s^?1) and displayed good proton conductivities in the range of 0.0328–0.0385 S·cm^?1 at room temperature. The sample of SPEN-SHQ-5 % showed highest selectivity value of 4.205 × 10⁵ S·s cm^?3, which was 11.9 times higher than that of Nafion 117. All of these results indicated that these membranes would be the potential candidates as proton exchange membranes (PEMs) in DMFCs.     

SAXS cluster structure and properties of sPEEK/PEI composite membranes for DMFC applications

Sulfonated poly(ether ether ketone)/polyethyleneimine (sPEEK/PEI) composite membranes were prepared to reduce the water uptake and methanol permeability of highly sulfonated PEEK membranes (> 65%). Incorporation of small amounts of PEI reduced ionic cluster size via electrostatic complex formation between anionic sulfonic groups of the sPEEK and the cationic amine groups of the PEI, and thus affected membrane properties considerably. Ion cluster size decreased with increasing PEI concentration by small angle X-ray scattering pattern. Addition of 1 wt.% of PEI resulted in reduction of water uptake and methanol permeability by 30% at 60 °C and 85% at room temperature, respectively. The thermal and mechanical stabilities were also enhanced by formation of physical cross-linking induced by electrostatic interactions between acid/base polymers. Although proton conductivity was also reduced by PEI incorporation as a part of the sulfonic acid groups involved in ionic complex formation, its effect on proton conductivity was not as strong as on methanol permeability.     

Improved dimensional stability of Nafion membrane modified using a layer by layer self-assembly of biophilic polymers

The modification of perfluorinated proton exchange membranes (e.g. Nafion) utilizing a layer by layer (LbL) electrostatic assembly of oppositely charged biophilic polymers such as poly-l-lysine as positive layer and dsDNA as a negative layer is developed to protect the interface between the catalyst layer and the membrane in a low temperature fuel cell. Various physicochemical measurements including water uptake, proton conductivity and surface tension were investigated for both the as-received Nafion and the biopolymeric LbL modified Nafion. The smaller water contact angle and the less swelling characteristics of the biopolymer modified Nafion membrane was founded compared to that of as-received Nafion. This clearly indicates that the more hydrophilic surface of biopolymeric layers on Nafion plays an important role in the enhanced dimensional stability. In addition, a potential hypothesis explaining the higher proton conductivity from the LbL biopolymeric film coated Nafion is suggested.     

Transport properties of modified Nafion membranes: Effect of zeolite and precursors

Fe-silicalite/Nafion composite membranes with high relative selectivity (as defined by the proton conductivity to methanol permeability ratio) of 5.4 and proton conductivity of 11 mS cm^− 1 were prepared by in situ hydrothermal synthesis of the zeolite within the pores of Nafion membranes. The effects of the zeolite structure and precursor structure were evaluated in terms of transport properties and acidity levels for a series of Nafion membranes modified with silica and tetrapropylammonium (TPA) and tetrabutylammonium (TBA) cations. Introduction of up to 40% (w/w) of silica vs. pure Nafion shows little effect on the transport and acidity properties of the composite membranes. Introduction of tetraalkylammonium (TAA) cations reduces water uptake of the membranes, and results in the appearance of protons that are inaccessible for titration in water media. The selectivity of the composite membranes increases in the order: SiO₂/Nafion < TAA/Nafion < Fe-silicalite/Nafion.     

Preparation and characterization of proton exchange membranes with through-membrane proton conducting channels

The primary goal of this study is to develop a novel PEMs with unique surface structure utilizing the high viscosity of the impregnation solution. SiO₂ nanofiber mats were prepared via the electrospinning method and introduced into sulfonated poly(ether sulfone) (SPES) matrix to prepare hybrid membrane. The effect of concentration of impregnation solution on the morphology and properties of the proton exchange membranes (PEMs), including thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeability were investigated. SEM results showed that a unique surface structure was prepared due to the high solution concentration. Moreover, the hydrophilic nanofibers on the surface constructed continuous proton pathways, which can enhance the proton conductivity of the membranes, a maximum proton conductivity of 0.125 S/cm was obtained when the SPES concentration was 40 wt% at 80 °C, and the conductivity was improved about 1.95 times compared to that of pure SPES membrane. The SiO₂ nanofiber mat-supported hybrid membrane could be used as PEMs for fuel cell applications.     

Characterization of nanohybrid membranes for direct methanol fuel cell applications

A series of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-PPO) and sulfonated poly(ether ether ketone) (S-PEEK) at various sulfonation degrees were prepared and characterized for their degree of sulfonation, water uptake, ion exchange capacity, proton conductivity and methanol permeability. Based on the obtained results, the optimum samples were determined and subsequently blended together at different compositions. A single glass transition temperature (T_g) was determined for all blend samples, which was attributed to the presence of sulfonate groups on polymer backbones resulting in the formation of electrostatic cross-linking besides phenyl–phenyl interactions. Moreover, the molecular level of mixing in blends was verified through WAXS patterns. According to the membrane selectivity and hydrolytic stability measurements, 75 wt.% of S-PPO and 25 wt.% of S-PEEK was selected as the optimum composition. Afterwards, different amounts of an organically modified montmorillonite (MMT) were incorporated into the predetermined optimum composition matrices to reduce the methanol permeability of the resulted nanocomposite proton exchange membranes. The XRD patterns of nanocomposites revealed the exfoliated microstructure of the clay nanolayers in the polymeric matrices. Transport property measurements of nanohybrid membranes showed that the maximum selectivity parameter of 75 wt.% S-PPO/25 wt.% S-PEEK composition appeared in the presence of 1.5 wt.% of MMT, which is 1.53 times higher than the corresponding value for Nafion^® 117. The DMFC single cell test of the optimum nanohybrids membrane at 5 M methanol feed showed an open circuit voltage of 0.77 V and maximum power density of 135 mW cm^? 2 in comparison with 0.67 V and 108 mW cm^? 2 for Nafion^® 117, respectively. Fabricated nanohybrid membranes, thanks to their high selectivity, desirable transport properties and tenability, could be considered as promising polyelectrolytes for direct methanol fuel cell applications.