Covalent organic/inorganic hybrid proton-conductive membrane with semi-interpenetrating polymer network: Preparation and characterizations

A series of new covalent organic/inorganic hybrid proton-conductive membranes, each with a semi-interpenetrating polymer network (semi-IPN), for direct methanol fuel cell (DMFC) applications is prepared through the following sequence: (i) copolymerization of impregnated styrene (St), p-vinylbenzyl chloride (VBC) and divinylbenzene (DVB) within a supporting polyvinyl chloride (PVC) film; (ii) reaction of the chloromethyl group with 3-(methylamine)propyl-trimethoxysilane (MAPTMS); (ii) a sol–gel process under acidic conditions; (iv) a sulfonation reaction. The developed membranes are characterized in terms of Fourier transform infrared/attenuated total reflectance (FTIR/ATR), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDXA), elemental analysis (EA) and thermogravimetric analysis (TGA), which confirm the formation of the target membranes. The developed copolymer chains are interpenetrating with the PVC matrix to form the semi-IPN structure, and the inorganic silica is covalently bound to the copolymers. These features provide the membranes with high mechanical strength. The effect of silica content is investigated. As the silica content increases, proton conductivity and water content decrease, whereas oxidative stability is improved. In particular, methanol permeability and methanol uptake are reduced largely by the silica. The ratio of proton conductivity to methanol permeability for the hybrid membranes is higher than that of Nafion 117. All these properties make the hybrid membranes a potential candidate for DMFC applications.     

Surface-modified Nafion membranes with mesoporous SiO2 layers via a facile dip-coating approach for direct methanol fuel cells

In this study, Nafion^® 117 membrane is surface-modified with mesoporous silica layers through in situ surfactant-templated sol–gel reaction. The reaction makes use of tetraethyl orthosilicate (TEOS) under acidic condition via dip-coating technique on both sides. Scanning electron microscopy (SEM), Fourier transformation infrared (FTIR), and thermogravimetric analysis (TGA) are employed to characterize the resultant membranes. Proton conductivity and methanol permeability of the membranes are also studied. It is determined that the aging time, along with the number of the silicon dioxide (SiO₂) layer, influence both proton conductivity and methanol permeability. Specifically, double-side modified membrane with 5 min interval of the second layer (S (5)) exhibits optimal properties on the combined criterion of conductivity and permeability. However, the application of mesoporous silica layer in modifying commercial Nafion membranes through dip-coating is proven to be a facile route in improving the said criteria simultaneously.     

Layer-by-layer self-assembly of polyaniline on sulfonated poly(arylene ether ketone) membrane with high proton conductivity and low methanol crossover

Sulfonated poly(arylene ether ketone) bearing carboxyl groups (SPAEK-C) membranes were first modified by alternating deposition of oppositely charged polyaniline (PANI) and phosphotungstic acid (PWA) via the layer-by-layer method in order to prevent the crossover of methanol in a direct methanol fuel cell. The methanol permeability of SPAEK-C–(PANI/PWA)₅ is 2 orders of magnitude less than those of Nafion 117 and pristine SPAEK-C. Furthermore, the modified membrane shows a proton conductivity of 0.093 Scm⁻¹ at 25 °C and 0.24 Scm⁻¹ at 80 °C, which are superior to those of Nafion 117 and pristine SPAEK-C. Fourier transform infrared spectroscopy confirms that PANI and PWA are assembled in the multilayers. The SEM images show the presence of thin PANI/PWA layers coated on the SPAEK-C membrane. Thermal stability, water uptake, water swelling, proton and electron conductivity at different temperature of the SPAEK-C and SPAEK-C-(PANI/PWA)_n membranes are also investigated.     

Chemical modification of Nafion membrane with 3,4-ethylenedioxythiophene for direct methanol fuel cell application

Nafion 117 membranes were modified by in situ chemical polymerization of 3,4-ethylenedioxythiophene using H₂O₂ as oxidant for direct methanol fuel cell application. Methanol permeability and proton conductivity of the poly(3,4-ethylenedioxythiophene)-modified Nafion membranes as a function of temperature were investigated. An Arrhenius-type dependency of methanol permeability and proton conductivity on temperature exists for all the modified membranes. Compared with Nafion 117 membrane at 60 °C, the methanol permeability of these modified membranes is reduced from 30% to 72%, while the proton conductivity is decreased from 4% to 58%, respectively. Because of low methanol permeability and adequate proton conductivity, the DMFC performances of these modified membranes were better than that of Nafion 117 membrane. A maximum power density of 48.4 mW cm⁻² was obtained for the modified membrane, while under same condition Nafion 117 membrane got 37 mW cm⁻².     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

Methanol-blocking Nafion composite membranes fabricated by layer-by-layer self-assembly for direct methanol fuel cells

In this paper, a series of Nafion composite membranes with improved methanol barrier properties were fabricated from chitosan (CS) and silicotungstic acid (SiWA) by Layer-by-layer (LbL) self-assembly technique. The Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra and scanning electron microscope (SEM) were employed to prove the successful deposition of CS/SiWA layers. The effect of the methanol-blocking layer on proton conductivity, methanol permeability and direct methanol fuel cell (DMFC) performance was studied. Both proton conductivity and methanol permeability of Nafion composite membranes decreased with the increase of CS/SiWA bi-layer number. The Nafion composite membrane with a reduced proton conductivity of 22% showed a reduced methanol permeability of 47%. Approximate 18% improvement of the selectivity was achieved as compared with plain Nafion membrane. But it did not result in better DMFC performance than that of plain Nafion membrane.     

Direct methanol fuel cell membranes from Nafion–polybenzimidazole blends

Proton conducting membranes for a direct methanol fuel cell (DMFC) were fabricated from blends of Nafion^® and polybenzimidazole (PBI) by solution casting. Prior to dissolution in the casting solvent, the sulfonic acid groups of the Nafion component of the blend were partially exchanged with sodium ions. The dependence of membrane proton conductivity and methanol permeability on the extent of proton substitution of Nafion during blending and on the PBI content of the final membrane was studied. It was found that membrane selectivity (the ratio of proton conductivity to methanol permeability) was the highest (four times that of Nafion 117) when fully protonated Nafion was used during blending and when the PBI content was 8%. DMFC performance of Nafion–PBI membranes (approximately 60 μm in thickness) was found to be superior to that of Nafion 117 at 1.0 and 5.0 M methanol feeds.     

Incorporating graphene oxide to improve the performance of Nafion-mordenite composite membranes for a direct methanol fuel cell

The proton exchange membrane is one of the critical parts of a direct methanol fuel cell. High proton conductivity and low methanol permeability are required. To enhance the performance of a direct methanol fuel cell, graphene oxide was incorporated to Nafion-mordenite composite membranes to enhance the compatibility and to decrease methanol permeability. It was found that the membrane with silane grafted on graphene oxide-treated mordenite with a graphene oxide content of 0.05% presented the highest proton conductivity (0.0560 S·cm⁻¹, 0.0738 S·cm⁻¹ and 0.08645 S·cm⁻¹ at 30, 50, and 70 °C, respectively). This was about 1.6-fold of the recast Nafion and commercial Nafion 117 and was about 1.5-fold of that without graphene oxide incorporation. Finally, the operating condition was optimized using response surface methodology and the maximum power density was investigated. Power density of about 4-fold higher than that of Nafion 117 was obtained in this work at 1.84 M and 72 °C with a %Error between the model prediction and the fuel cell experiment of 0.082%.     

Novel sulfonated poly(ether ether ketone)s containing nitrile groups and their composite membranes for fuel cells

A series of novel sulfonated poly(ether ether ketone)s containing a cyanophenyl group (SPEEKCNxx) are prepared based on (4-cyano)phenylhydroquinone via nucleophilic substitution polycondensation reactions. To further improve their properties, novel composite membranes composed of sulfonated poly(ether ether ketone)s containing cyanophenyl group as an acidic component and aminated poly(aryl ether ketone) as a basic component are successfully prepared. Most of the membranes exhibit excellent thermal, oxidative and dimensional stability, low-swelling ratio, high proton conductivity, low methanol permeability and high selectivity. The proton conductivities of the membranes are close to Nafion 117 at room temperature. And especially, the values of SPEEKCN40 and its composite membranes are higher than Nafion 117 at 80 °C (0.17 S cm⁻¹ of Nafion, 0.26 S cm⁻¹ of SPEEKCN40, 0.20 S cm⁻¹ of SPEEKCN40-1, and 0.18 S cm⁻¹ of SPEEKCN40-2). Moreover, the methanol permeability is one order magnitude lower than that of Nafion 117. All the data prove that both copolymers and their composite membranes may be potential proton exchange membrane for fuel cells applications.     

Nafion-assisted cross-linking of sulfonated poly(arylene ether ketone) bearing carboxylic acid groups and their composite membranes for fuel cells

In this study, a new type of cross-linked composite membrane is prepared and considered for its potential applications in direct methanol fuel cell. Nafion and sulfonated poly(arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) are blended and subsequently cross-linked by a Friedel-Craft reaction using the carboxylic acid groups in the SPAEK-C to achieve lower methanol permeability. The perfluoroalkyl sulfonic acid groups of Nafion act as a benign solid catalyst, which assist the cross-linking of SPAEK-C. The physical and chemical characterizations of the cross-linked composite membranes are performed by varying the contents of SPAEK-C. The c-Nafion-15% membrane exhibits appropriate water uptake (10.49-25.22%), low methanol permeability (2.57 × 10⁻⁷ cm² s⁻¹), and high proton conductivity (0.179 S cm⁻¹ at 80 °C). DSC and FTIR analyze suggest the cross-linking reaction. These results show that the self-cross-linking of SPAEK-C in the Nafion membrane can effectively reduce methanol permeability while maintaining high proton conductivity.     

Novel modification method to prepare crosslinked sulfonated poly(ether ether ketone)/silica hybrid membranes for fuel cells

Crosslinked organic-inorganic hybrid membranes are prepared from hydroxyl-functionalized sulfonated poly(ether ether ketone) (SPEEK) and various amounts of silica with the aims to improve dimensional stability and methanol resistance. The partially hydroxyl-functionalized SPEEK is prepared by the reduction of some benzophenone moieties of SPEEK into the corresponding benzhydrol moieties which is then reacted with (3-isocyanatopropyl)triethoxysilane (ICPTES) to get a side chained polymer bearing triethoxysilyl groups. These groups are subsequently co-hydrolyzed with tetraethoxysilane (TEOS) and allow the membrane to form a crosslinked network via a sol-gel process. The obtained hybrid membranes with covalent bonds between organic and inorganic phases exhibit much lower methanol swelling ratio and water uptake. With the increase of silica content, the methanol permeability coefficient of the hybrid membranes decreases at first and then increased. At silica content of about 6 wt.%, the methanol permeability coefficient reaches a minimum of 7.15 × 10⁻⁷ cm² s⁻¹, a 5-fold decrease compared with that of the pristine SPEEK. Despite the fact that the proton conductivity is decreased to some extent as a result of introduction of the silica, the hybrid membranes with silica content of 4-8 wt.% shows higher selectivity than Nafion117.     

Nafion/Pd–SiO2 nanofiber composite membranes for direct methanol fuel cell applications

One of the major challenges for direct methanol fuel cells is the problem of methanol crossover. With the aim of solving this problem without adverse effects on the membrane conductivity, Nafion/Palladium–silica nanofiber (N/Pd–SiO₂) composite membranes with various fiber loadings were prepared by a solution casting method. The silica-supported palladium nanofibers had diameters ranging from 100 nm to 200 nm and were synthesized by a facile electro-spinning method. The thermal properties, ionic exchange capacities, water uptake, proton conductivities, methanol permeabilities, chemical structures, and micro-structural morphologies were determined for the prepared membranes. It was found that the transport properties of the membranes were affected by the fiber loading. All of the composite membranes showed higher water uptake and ion exchange capacities compared to commercial Nafion 117 and proved to be thermally stable for use as proton exchange membranes. The composite membranes with optimum fiber content (3 wt%) showed an improved proton conductivity of 0.1292 S cm⁻¹ and a reduced methanol permeability of 8.36 × 10⁻⁷ cm² s⁻¹. In single cell tests, it was observed that, the maximum power density measured with composite membrane is higher than those of commercial Nafion 117.     

Nanocomposite membranes of surface-sulfonated titanate and Nafion® for direct methanol fuel cells

Various thiol and sultone groups were grafted onto the surface of titanate nanosheets to render organic sulfonic acid (HSO₃–) functionality. The nanocomposite membranes were cast together with Nafion^® using these materials as inorganic fillers. Nanocomposite membranes containing surface-sulfonated titanates showed higher proton conductivity than composite membranes containing untreated TiO₂ P25 particles. They showed better mechanical and thermal stability than Nafion alone. The methanol permeability of nanocomposite membranes decreased with increasing the content of the sulfonated titanate in the nanocomposite membranes. The relative permeability of methanol through these composite membranes with 2 and 5 M methanol solutions was reduced by up to 38 and 26%, respectively, relative to pristine Nafion 115 membranes. The membrane electrode assembly using Nafion/sulfonated titanate nanocomposite membranes exhibited up to 57% higher power density than the assembly containing a pristine Nafion membrane under typical operating conditions of direct methanol fuel cells.     

Surface-modified Y zeolite-filled chitosan membrane for direct methanol fuel cell

Hybrid membranes composed of chitosan (CS) as organic matrix and surface-modified Y zeolite as inorganic filler are prepared and their applicability for DMFC is demonstrated by methanol permeability, proton conductivity and swelling property. Y zeolite is modified using silane coupling agents, 3-aminopropyl-triethoxysilane (APTES) and 3-mercaptopropyl-trimethoxysilane (MPTMS), to improve the organic–inorganic interfacial morphology. The mercapto group on MPTMS-modified Y zeolite is further oxidized into sulfonic group. Then, the resultant surface-modified Y zeolites with either aminopropyl groups or sulfonicpropyl groups are mixed with chitosan in acetic acid solution and cast into membranes. The transitional phase generated between chitosan matrix and zeolite filler reduces or even eliminates the nonselective voids commonly exist at the interface. The hybrid membranes exhibit a significant reduction in methanol permeability compared with pure chitosan and Nafion117 membranes, and this reduction extent becomes more pronounced with the increase of methanol concentration. By introducing –SO₃H groups onto zeolite surface, the conductivity of hybrid membranes is increased up to 2.58 × 10⁻² S cm⁻¹. In terms of the overall selectivity index (β = σ/P), the hybrid membrane is comparable with Nafion117 at low methanol concentration (2 mol L⁻¹) and much better (three times) at high methanol concentration (12 mol L⁻¹).     

Novel hybrid polymer electrolyte membranes with high proton conductivity prepared by a silane-crosslinking technique for direct methanol fuel cells

In order to prepare a hybrid proton exchange membrane with low methanol permeability and high proton conductivity, two silane monomers, namely 3-glycidoxypropyl-trimethoxysilane (GPTMS) and 3-mercaptopropyl-trimethoxysilane (MPTMS) are first blended with a sulfonated poly(arylene ether ketone) (SPAEK). Then the blended membrane is heated to induce the grafting of GPTMS onto SPAEK. Finally, a hydrolysis-condensation is performed on the grafted membrane to induce cross-linking. The -SH groups of MPTMS are oxidized to sulfonic acid groups, which are attributed to enhance the proton conductivity of hybrid membranes. Fourier transform infrared spectroscopy is used to characterize and confirm the structures of SPAEK and these cross-linked hybrid membranes. The proton conductivity of a cross-linked hybrid membrane G50M50 reaches up to 0.20 S cm⁻¹ at 80 °C, which is comparable to that of SPAEK and much higher than that of Nafion. Meanwhile, the methanol permeability is nearly three times lower than that of Nafion and two times lower than that of SPAEK. The ion-exchange capacity, water uptake, membrane swelling and thermal stability are also investigated to confirm their applicability in fuel cells.     

Synthesis and characterization of polysulfone-containing sulfonated side chains for direct methanol fuel cells

Two series of novel polysulfone-based ionomers containing pendant sulfonic acid groups (designated as PSf-sph-y and PSf-sna-z) have been prepared using commercially available polysulfone and sulfoaryl monomers. The synthesized ionomers have been characterized by nuclear magnetic resonance, differential scanning calorimetry, and thermogravimetric analysis. The thermal characterization data show that these polymers are stable up to 210 °C in acid form and 270 °C in salt form in air. Ion exchange capacity, water uptake, swelling, proton conductivity, and methanol permeability of the membranes have been investigated and compared with those of Nafion 115. The PSf-sna-z membranes bearing highly ionic groups on the flexible side chains show lower water uptake, swelling, and methanol permeability compared to the PSf-sph-y and Nafion 115 membranes. The optimized PSf-sna-58 membrane shows better performance in direct methanol fuel cells (DMFC) than Nafion 115 membranes.     

Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers

Novel Nafion composite proton exchange membranes are prepared using mesoporous MCM-41 silica nanospheres as inorganic fillers. The novelty of this study lies in the structural design of inorganic silica fillers: the nanosized and monodisperse spherical morphology of fillers facilitates the preparation of homogenous composite membranes, whilst the superior water adsorption of the mesostructure in fillers consigns enhanced water retention properties to the polymer membranes. Scanning electron microscopy images of the composite membranes indicate that well-dispersed silica nanospheres are embedded in the Nafion matrix, but a large amount of added fillers (3 wt.%) causes some agglomeration of the nanospheres. Compared with the Nafion cast membrane, the composite membranes offer improved thermal stability, enhanced water retention properties, and reduced methanol crossover. Despite the enhancement of water retention, the composite membranes still exhibit a proton conductivity reduction of 10–40% compared with pristine Nafion. This is likely due to the incorporation of much less conductive silica fillers than Nafion. The composite membrane containing 1 wt.% of fillers displays the best cell performance in direct methanol fuel cell tests; it gives a maximum power density of 21.8 mW cm⁻², i.e., ∼20% higher than the Nafion cast membrane. This is attributed to its similar conductivity to Nafion, and its markedly reduced methanol crossover, namely, ∼1.2 times lower.     

Nafion/polyaniline/silica composite membranes for direct methanol fuel cell application

This work investigates the characterization and performance of polyaniline and silica modified Nafion membranes. The aniline monomers are synthesized in situ to form a polyaniline film, whilst silica is embedded into the Nafion matrix by the polycondensation of tetraethylorthosilicate. The physicochemical properties are studied by means of X-ray diffraction and Fourier transform infrared techniques and show that the polyaniline layer is formed on the Nafion surface and improves the structural properties of Nafion in methanol solution. Nafion loses its crystallinity once exposed to water and ethanol, whilst the polyaniline modification allows crystallinity to be maintained under similar conditions. By contrast, the proton conductivities of polyaniline modified membranes are 3–5-fold lower than that of Nafion. On a positive note, methanol crossover is reduced by over two orders of magnitude, as verified by crossover limiting current analysis. The polyaniline modification allows the membrane to become less hydrophilic, which explains the lower proton conductivity. No major advantages are observed by embedding silica into the Nafion matrix. The performance of a membrane electrode assembly (MEA) using commercial catalysts and polyaniline modified membranes in a cell gives a peak power of 8 mW cm⁻² at 20 °C with 2 M methanol and air feeding. This performance correlates to half that of MEAs using Nafion, though the membrane modification leads to a robust material that may allow operation at high methanol concentration.     

Novel cross-linked sulfonated poly (arylene ether ketone) membranes for direct methanol fuel cell

To prepare a cross-linked proton exchange membrane with low methanol permeability and high proton conductivity, poly (vinyl alcohol) is first blended with sulfonated poly (arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) and then heated to induce a cross-linking reaction between the carboxyl groups in SPAEK-C and the hydroxyl groups in PVA. Fourier transform infrared spectroscopy is used to characterize and confirm the structure of SPAEK-C and the cross-linked membranes. The proton conductivity of the cross-linked membrane with 15% PVA in weight reaches up to 0.18 S cm⁻¹ at 80 °C (100% relative humidity), which is higher than that of Nafion membrane, while the methanol permeability is nearly five times lower than Nafion. The ion-exchange capacity, water uptake and thermal stability are investigated to confirm their applicability in fuel cells.     

Anhydrous proton conducting membranes for PEM fuel cells based on Nafion/Azole composites

Proton conducting membranes are the most crucial part of energy generating electrochemical systems such as polymer electrolyte membrane fuel cells (PEMFCs). In this work, Nafion based proton conducting anhydrous composite membranes were prepared via two different approaches. In the first, commercial Nafion115 and Nafion112 were swelled in the concentrated solution of azoles such as 1H-1,2,4-triazole (Tri), 3-amino-1,2,4-triazole (ATri) and 5-aminotetrazole (ATet) as heterocyclic protogenic solvents. In the second, the proton conducting films were cast from the Nafion/Azole solutions. The partial protonation of azoles in the anhydrous membranes were studied by Fourier transform infrared (FT-IR) spectroscopy. Thermal properties were investigated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA results showed that Nafion/ATri and Nafion/ATet electrolytes are thermally stable at least up to 200 °C. Methanol permeability measurements showed that the composite membranes have lower methanol permeability compared to Nafion112. Nafion115/ATri system has better conductivity at 180 °C, exceeding 10⁻³ S/cm compared to other Nafion/heterocycle systems under anhydrous conditions.