Modification of sulfonated poly(ether ether ketone) proton exchange membrane for reducing methanol crossover

A drawback of sulfonated aromatic main-chain polymers such as sulfonated poly(ether ether ketone)s (SPEEKs) is their high methanol crossover when the proton conductivity is sufficient for direct methanol fuel cell (DMFC) applications. To overcome this disadvantage, in this paper, the SPEEK substrate was coated with the crosslinked chitosan (CS) barrier layer to form the two-layer composite membranes. Scanning electron microscope (SEM) micrographs showed that the CS layer was tightly adhered on the SPEEK substrate and the thickness of CS layer could be adjusted by varying the concentration of CS solution. It was noticed that with the increment of thickness of CS layer, the methanol diffusion coefficient of the composite membranes significantly dropped from 3.15 × 10⁻⁶ to 2.81 × 10⁻⁷ cm² s⁻¹ at 25 °C which was about one order of magnitude lower than those of the pure SPEEK and Nafion^® 117 membranes. In addition to the effective methanol barrier, the composite membranes possessed adequate thermal stability (the 5% weight lose temperature exceeded 240 °C) and good proton conductivity. The proton conductivity of all composite membranes was in the order of 10⁻² S cm⁻¹ and increased with the elevation of temperature. Furthermore, the composite membranes exhibited much higher selectivity (conductivity/methanol diffusion coefficient) compared with the pure SPEEK and Nafion^® 117 membranes. These results indicated that introducing the crosslinked CS layer onto the SPEEK surface was an effective method for improving the performance of the SPEEK membrane, especially for reducing the methanol crossover.     

Preparation and characterization of self-crosslinked organic/inorganic proton exchange membranes

A series of silicon-containing sulfonated polystyrene/acrylate (Si-sPS/A) nanoparticles are successfully synthesized via simple emulsion polymerization method. The Si-sPS/A latexes show good film-forming capability and the self-crosslinked organic/inorganic proton exchange membranes are prepared by pouring the Si-sPS/A nanoparticle latexes into glass plates and drying at 60 °C for 10 h and 120 °C for 2 h. The potential of the membranes in direct methanol fuel cells (DMFCs) is characterized preliminarily by studying their thermal stability, ion-exchange capacity, water uptake, methanol diffusion coefficient, proton conductivity and selectivity (proton conductivity/methanol diffusion coefficient). The results indicate that these membranes possess excellent thermal stability and methanol barrier due to the existence of self-crosslinked silica network. In addition, the proton conductivity of the membranes is in the range of 10⁻³-10⁻² S cm⁻¹ and all the membranes show much higher selectivity in comparison with Nafion^® 117. These results suggest that the self-crosslinked organic/inorganic proton exchange membranes are particularly promising in DMFC applications.     

Blended Nafion/SPEEK direct methanol fuel cell membranes for reduced methanol permeability

Sulfonated poly(ether ether ketone)s (SPEEKs) were substituted on a polymer main chain that had previously been prepared by sulfonation of poly(ether ether ketone)s in concentrated sulfuric acid for a specified time. The product was then blended with Nafion^® to create composite membranes. The blended SPEEK-containing membranes featured flaky domains dispersed in the Nafion^® matrix. These blends possessed a high thermal decomposition temperature. Additionally, owing to the more crystalline, the blended membranes had a lower water uptake compared to recast Nafion^®, the methanol permeability was reduced to 1.70 × 10⁻⁶ to 9.09 × 10⁻⁷ cm² s⁻¹ for various SPEEK concentrations, and a maximum proton conductivity of ∼0.050 S cm⁻¹ was observed at 30 °C. The single-cell performances of the Nafion^®/SPEEK membranes, with various SPEEK concentrations and a certain degree of sulfonation, were 15–25 mW cm⁻² for SPEEK53 and 19–27 mW cm⁻² for SPEEK63, at 80 °C. The power density and open circuit voltage were higher than those of Nafion^® 115 (power density = 22 mW cm⁻²). The blended membranes satisfy the requirements of proton exchange membranes for direct methanol fuel cell (DMFC) applications.     

Performance of passive direct methanol fuel cell with poly(vinyl alcohol)-based polymer electrolyte membranes

Polymer electrolyte membranes (PEMs) were prepared from poly(vinyl alcohol) (PVA) and a modified PVA polyanion containing 2 mol% of 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer. The effect of the AMPS content and the crosslinking conditions on the properties of the membranes was investigated in PEMs with various AMPS contents prepared under various crosslinking conditions. The proton conductivity and the permeability of methanol through the PEMs increased with increasing AMPS content, C_AMPS, and with decreasing annealing temperature, T_a, because of the increase in the degree of swelling. The permeability coefficient of methanol through the PEM prepared under the conditions of C_AMPS = 2.0 mol% and T_a 190 °C was approximately 30 times lower than that of Nafion^® 117 under the same measurement conditions. A maximum proton permselectivity of 96 × 10³ S cm⁻³ s, which is defined as the ratio of the proton conductivity to the permeability of methanol, was obtained for this PEM. The permselectivity value is about three times higher than that of Nafion^® 117. A passive air-breathing-type DMFC test cell constructed using the PEM delivered 2.4 mW cm⁻² of maximum power density, P_max, at 2 M methanol concentration, which is smaller than the value obtained with Nafion^® 117. However, at high methanol concentrations (>10 M), the P_max of the PEM decreases slightly to 1.6 mW cm⁻² (at a methanol concentration of 20 M), whereas the P_max of Nafion^® 117 falls to almost zero.     

Preparation and properties of crosslinked sulphonated poly(arylene ether sulphone) blend s for direct methanol fuel cell applications

HMS-based sulphonated poly(arylene ether sulphone) (HMSSH) is synthesised using 4,4′-dihydroxy-α-methylstilbene (HMS) monomer to introduce an interesting stilbene core as crosslinkable group. Crosslinked blend membranes are obtained by blending the BPA-based sulphonated poly(arylene ether sulphone) (BPASH) with crosslinkable HMS-based sulphonated poly(arylene ether sulphone) by UV irradiation of the blend membrane. Compared to the native BPASH with crosslinked BPASH/HMSSH blend membranes, the crosslinked blend membranes greatly reduce the water uptake and methanol permeability with only a slight reduction in proton conductivity. The crosslinked blend membrane, which has a 6% HMSSH content, has a water uptake of 59%, methanol permeability of 0.75 × 10⁻⁶ cm² s⁻¹, and proton conductivity of 0.08 S cm⁻¹. A membrane-electrode assembly is used to investigate single-cell performance and durability test for DMFC applications. Both the power density and open circuit voltage are higher than those of Nafion^® 117. A maximum power density of 32 mW cm⁻² at 0.2 V is obtained at 80 °C, which is higher than that of Nafion^® 117 (25 mW cm⁻²).     

Proton conducting membranes prepared by incorporation of organophosphorus acids into alcohol barrier polymers for direct methanol fuel cells

A novel type of DMFC membrane was developed via incorporation of organophosphorus acids (OPAs) into alcohol barrier materials (polyvinyl alcohol/chitosan, PVA/CS) to simultaneously acquire high proton conductivity and low methanol permeability. Three kinds of OPAs including amino trimethylene phosphonic acid (ATMP), ethylene diamine tetra(methylene phosphonic acid) (EDTMP) and hexamethylene diamine tetra(methylene phosphonic acid) (HDTMP), with different molecular structure and phosphonic acid groups content were added into PVA/CS blends and served the dual functions as proton conductor as well as crosslinker. The as-prepared OPA-doped PVA/CS membranes exhibited remarkably enhanced proton conducting ability, 2–4 times higher than that of the pristine PVA/CS membrane, comparable with that for Nafion^®117 membrane (5.04 × 10⁻² S cm⁻¹). The highest proton conductivities 3.58 × 10⁻², 3.51 × 10⁻² and 2.61 × 10⁻² S cm⁻¹ for ATMP-, EDTMP- and HDTMP-doped membranes, respectively were all achieved at highest initial OPA doping content (23.1 wt.%) at room temperature. The EDTMP-doped PVA/CS membrane with an acid content of 13.9 wt.% showed the lowest methanol permeability of 2.32 × 10⁻⁷ cm² s⁻¹ which was 16 times lower than that of Nafion^®117 membrane. In addition, the thermal stability and oxidative durability were both significantly improved by the incorporation of OPAs in comparison with pristine PVA/CS membranes.     

Nafion/nitrated sulfonated poly(ether ether ketone) membranes for direct methanol fuel cells

Sulfonated poly(ether ether ketone)s (SPEEKs) are substituted on the main chain of the polymer by nitro groups and blended with Nafion^® to attain composite membranes. The sulfonation, nitration and blending are achieved with a simple, inexpensive process, and the blended membranes containing the nitrated SPEEKs reveal a liquid-liquid phase separation. The blended membranes have a lower water uptake compared to recast Nafion^®, and the methanol permeability is reduced significantly to 4.29 × 10⁻⁷-5.34 × 10⁻⁷ cm² s⁻¹ for various contents of nitrated SPEEK for S63N17, and 4.72 × 10⁻⁷-7.11 × 10⁻⁷ cm² s⁻¹ for S63N38, with a maximum proton conductivity of ∼0.085 S cm⁻¹. This study examines the single-cell performance at 80 °C of Nafion^®/nitrated SPEEK membranes with various contents of nitrated SPEEK and a degree of nitration of 23-25 mW cm⁻² for S63N17 and 24-29 mW cm⁻² for S63N38. Both the power density and open circuit voltage are higher than those of Nafion^® 115 and recast Nafion^®.     

Proton-conducting membranes with high selectivity from cross-linked poly(vinyl alcohol) and poly(vinyl pyrrolidone) for direct methanol fuel cell applications

A series of hydrocarbon membranes consisting of poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA) and poly(vinyl pyrrolidone) (PVP) were synthesized and characterized for direct methanol fuel cell (DMFC) applications. Fourier transform infrared (FT-IR) spectra confirm a semi-interpenetrating (SIPN) structure based on a cross-linked PVA/SSA network and penetrating PVP molecular chains. A SIPN membrane with 20% PVP (SIPN-20) exhibits a proton conductivity value comparable to Nafion^® 115 (1.0 × 10⁻² S cm⁻¹ for SIPN-20 and 1.4 × 10⁻² S cm⁻¹ for Nafion^® 115). Specifically, SIPN membranes reveal excellent methanol resistance for both sorption and transport properties. The methanol self-diffusion coefficient through a SIPN-20 membrane conducted by pulsed field-gradient nuclear magnetic resonance (PFG-NMR) technology measures 7.67 × 10⁻⁷ cm² s⁻¹, which is about one order of magnitude lower than that of Nafion^® 115. The methanol permeability of SIPN-20 membrane is 5.57 × 10⁻⁸ cm² s⁻¹, which is about one and a half order of magnitude lower than Nafion^® 115. The methanol transport behaviors of SIPN-20 and Nafion^® 115 membranes correlate well with their sorption characteristics. Methanol uptake in a SIPN-20 membrane is only half that of Nafion^® 115. An extended study shows that a membrane-electrode assembly (MEA) made of SIPN-20 membrane exhibits a power density comparable to Nafion^® 115 with a significantly higher open current voltage. Accordingly, SIPN membranes with a suitable PVP content are considered good methanol barriers, and suitable for DMFC applications.     

Direct methanol fuel cell performance of sulfonated poly (2,6-dimethyl-1,4-phenylene oxide)-polybenzimidazole blend proton exchange membranes

Various sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polybenzimidazole (PBI) blend membranes were prepared and investigated as proton exchange membranes (PEMs) for direct methanol fuel cell (DMFC) applications. With increasing PBI content water swelling, ion exchange capacity, proton conductivity and methanol permeability of SPPO-PBI membranes were found to be decreased due to acid-base interactions between sulfonate and the amine groups of the blended components. Among various SPPO-PBI blend membranes, 80:20 wt% was found as the optimum composition, which showed the highest membrane selectivity parameter. Direct methanol-air single fuel cell tests revealed a higher cell efficiency of 11.6% for SPPO80-PBI20 than 10.9% for Nafion^®117 at 5 M methanol feed, and also a higher power density of 57.6 mW.cm⁻² compared to 39.4 mW.cm⁻² for Nafion^®117. Transport properties as well as DMFC performance results of SPPO-PBI blend PEMs converge to indicate their potential for DMFC applications.     

Effects and properties of various molecular weights of poly(propylene oxide) oligomers/Nafion acid–base blend membranes for direct methanol fuel cells

Various molecular weights of poly(propylene oxide) diamines oligomers/Nafion^® acid–base blend membranes were prepared to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The acid–base interactions were studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity was slightly reduced by acid–base interaction. The methanol permeability of the blend D2000/Nafion^® was 8.61 × 10⁻⁷ cm² S⁻¹, which was reduced 60% compared to that of pristine Nafion^®. The cell performance of D2000/Nafion^® blend membranes was enhanced significantly compared to pristine Nafion^®. The current densities that were measured with Nafion^® and 3.5 wt% D2000/Nafion^® blend membranes were 62.5 and 103.5 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend poly(propylene oxide) diamines oligomers/Nafion^® membranes critically improved the single-cell performance of DMFC.     

Covalent-ionically cross-linked polyetheretherketone proton exchange membrane for direct methanol fuel cell

In this paper, the proton exchange membrane prepared by covalent-ionically cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC) is reported. Compared with covalent cross-linked PEEK-WC membrane, this covalent-ionically cross-linked PEEK-WC membrane exhibits extremely reduced water uptake and methanol permeability, but just slightly sacrificed proton conductivity. The proton conductivity of the covalent-ionically cross-linked PEEK-WC membrane reaches to 2.1 × 10⁻² S cm⁻¹ at room temperature and 4.1 × 10⁻² S cm⁻¹ at 80 °C. The methanol permeability is 1.3 × 10⁻⁷ cm² s⁻¹, 10 times lower than that of Nafion^® 117 membrane. The results suggest that the covalent-ionically cross-linked PEEK-WC membrane is a promising candidate for direct methanol fuel cell because of low methanol permeability and adequate proton conductivity.     

Proton exchange membranes based on semi-interpenetrating polymer networks of fluorine-containing polyimide and Nafion

A series of reinforced composite membranes as proton exchange membranes were prepared from Nafion^®212 and crosslinkable fluorine-containing polyimides (FPI). FPI was prepared from the polymerization of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB), and 3,5-diaminobenzoic acid (DABA). Then FPI was thermally crosslinked during the membrane preparation and formed the semi-interpenetrating polymer networks (semi-IPN) structure in the composite membranes. The thermal properties of the composite membranes were characterized by thermogravimetric analysis. The crosslinking density of FPI in the composite membranes was evaluated by the gel fraction. These membranes showed excellent thermal stabilities and good oxidative stabilities. Compared with Nafion^®212, the obtained composite membranes displayed much improved mechanical properties and dimensional stabilities. The tensile strength of the composite membranes was more than twice that of Nafion^®212. The composite membranes exhibited high proton conductivity, which ranged from 2.3 × 10⁻² S cm⁻¹ to 9.1 × 10⁻² S cm⁻¹. All membranes showed an increase in proton conductivity with temperature elevation.     

Proton conducting CS/P(AA-AMPS) membrane with reduced methanol permeability for DMFCs

A novel polyelectrolyte complex (PEC) membrane for direct methanol fuel cells (DMFCs) was prepared by blending a cationic polyelectrolyte, chitosan (CS), with an anionic polyelectrolyte, acrylic acid-2-acrylamido-2-methylpropane sulfonic acid copolymer (P(AA-AMPS)). The presence of –NH₃⁺ species detected by X-ray photoelectron spectroscopy (XPS) revealed that an ionic cross-linked interpenetrating polymer network (IPN) was formed between the two polyelectrolyte polymers. Methanol permeability and proton conductivity were measured and compared with the Nafion^®117 membrane. The dual function of P(AA-AMPS) as both an ionic crosslinker and a proton conductor led to not only a notable reduction in methanol permeability but also an increase in proton conductivity. The CS/P(AA-AMPS) membrane with a P(AA-AMPS) content of 41 wt.% exhibited a methanol permeability (P) of 2.41 × 10⁻⁷ cm² s⁻¹ which was fifteen times lower than that of the Nafion^®117 membrane, whereas its proton conductivity (σ) was comparatively high (3.59 × 10⁻² S cm⁻¹). In terms of the overall selectivity index (β = σ/P), the PEC membrane showed a remarkably higher selectivity than the Nafion^®117 membrane, and, furthermore, the overall selectivity index increased with the increase of P(AA-AMPS) content. The mechanism of proton transfer was tentatively discussed based on the activation energy of conductivity.     

Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

Semi-interpenetrating polymer network electrolyte membranes composed of sulfonated poly(ether ether ketone) and organosiloxane-based hybrid network

A semi-interpenetrating polymer network (semi-IPN) proton exchange membrane is prepared from the sulfonated poly(ether ether ketone) (sPEEK) and organosiloxane-based organic/inorganic hybrid network (organosiloxane network). The organosiloxane network is synthesized from 3-glycidyloxypropyltrimethoxysiane and 1-hydroxyethane-1,1-diphosphonic acid. The semi-IPN membranes prepared were stable up to 300 °C without any degradation. The methanol permeability is much lower than Nafion^® 117 under addition of the organosiloxane network. The proton conductivity of semi-IPN membranes increases with an increase the organosiloxane network content; the membrane containing the 20-24 wt% organosiloxane network shows higher conductivity than Nafion^® 117. The power density of the MEA fabricated with the semi-IPN membrane with 24 wt% organosiloxane network is 135 mW cm⁻², much better than that of the pristine sPEEK membrane, 85 mW cm⁻². Chemical synthesis of the semi-IPN membranes is identified using FTIR, and its ion cluster dimension examined using SAXS. The dimensional stability associated with water swelling and dissolution is investigated at different temperatures, and the semi IPN membranes dimensionally stable in water at elevated temperature.     

Investigation of the effects of AMPS-modified nanoclay on fuel cell performance of sulfonated aromatic proton exchange membranes

Here we describe preparation and characterization of a series of nanocomposite polyelectrolytes based on partially sulfonated poly (ether ether ketone) (SPEEK) and organically modified montmorillonite (MMT). Optimum degree of sulfonation for SPEEK is selected based on its transport properties. MMT is modified via ion exchange reaction using a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as a functional modifier. AMPS-MMT at different loadings is introduced into the SPEEK matrices via the solution intercalation technique. Also, the nanocomposite membranes are fabricated using SPEEK and commercially available nanoclays like Cloisite Na (Na-MMT) and Cloisite 15A. Transport properties, proton conductivity and methanol permeability of the fabricated composite membranes are evaluated. Presence of AMPS-MMT significantly decreases the activation energy needed for proton conductivity. A membrane based on SPEEK/AMPS-MMT-3 wt% is selected as an optimum formulation which exhibits a high selectivity and power density at the elevated methanol concentrations. Moreover, it is found that the optimum nanocomposite membrane not only provides higher power output compared to the neat SPEEK and Nafion^®117 membranes, but also exhibits a higher open circuit voltage (OCV) in comparison with pristine SPEEK and commercial Nafion^® 117 membranes. Owing to the desirable transport and electrochemical properties SPEEK/AMPS-MMT nanocomposite can be considered as an alternative membrane for direct methanol fuel cell applications.     

Fabrication of novel proton exchange membranes for DMFC via UV curing

The radiation hardening of various UV curable resins provides a simple but powerful method to fabricate thin films or membranes with desirable physical and chemical properties. In this study, we proposed to use this method to fabricate a novel proton exchange membrane (PEM) for direct methanol fuel cells (DMFC) with good mechanical, transport and stability properties. The PEM was prepared by crosslinking a mixture of a photoinitiator, a bifunctional aliphatic urethane acrylate resin (UAR), a trifunctional triallyl isocyanate (TAIC) crosslinker and tertrabutylammonium styrenesulfonate (SSTBA) to form a uniform network structure for proton transport. Key PEM parameters such as ion exchange capacity (IEC), water uptake, proton conductivity, and methanol permeability were controlled by adjusting the chemical composition of the membranes. The IEC value of the membrane was found to be an important parameter in affecting water uptake, conductivity as well as the permeability of the resulting membrane. Plots of the water uptake, conductivity, and methanol permeability vs. IEC of the membranes show a distinct change in the slope of their curves at roughly the same IEC value which suggests a transition of structural changes in the network. It is demonstrated that below the critical IEC value, the membrane exhibits a closed structure where hydrophilic segments form isolated domains while above the critical IEC value, it shows an open structure where hydrophilic segments are interconnected and form channels in the membrane. The transition from a closed to an open proton conduction network was verified by the measurement of the activation energy of membrane conductivity. The activation energy in the closed structure regime was found to be around 16.5 kJ mol⁻¹ which is higher than that of the open structure region of 9.6 kJ mol⁻¹. The membranes also display an excellent oxidative stability, which suggests a good lifetime usage of the membranes. The proton conductivities and the methanol permeabilities of all membranes are in the range of 10⁻⁴ to 10⁻² S cm⁻¹ and 10⁻⁸ to 10⁻⁷ cm² s⁻¹, respectively, depending on their crosslinking density. The membranes show great selectivity compared with those of Nafion^®. The possibility of using this PEM for DMFC devices is suggested.     

Sulfonated poly(propylene oxide) oligomers/Nafion acid-base blend membranes for DMFC

A novel functional poly(propylene oxide)-backboned diamine of M_w 400 (abbreviated as D400) was grafted with sulfonic acid (abbreviated as D400-PS) to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The interaction of the D400-PS with Nafion^® was studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend Nafion^®/D400-PS membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity of the blend membrane was slightly reduced by rendering proton conductivity to D400 by functionalized with an organic sulfonic acid. The methanol permeability of the blend membrane decreased with increasing of D400-PS content. The methanol permeability of the blend Nafion^®/D400-PS with the composition 3/1 (–SO₃H/–NH₂) was 1.02 × 10⁻⁶ cm² S⁻¹, which was reduced 50% compared to that of pristine Nafion^®. The current densities that were measured with Nafion^®/D400-PS blend membranes in the ratio 1/0 and 5/1 (–SO₃H/–NH₂), were 51 and 72 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend Nafion^®/D400-PS membranes critically improved the single-cell performance of DMFC.     

Preparation of nitrated sulfonated poly(ether ether ketone) membranes for reducing methanol permeability in direct methanol fuel cell applications

Sulfonated poly(ether ether ketone)s (SPEEKs) were further substituted on the polymer main chain by nitration. All sulfonation and nitration were achieved with an inexpensive and simple post substitute reaction. The nitrated SPEEKs have a high glass transition temperature and thermal decomposition temperature, and a lower water uptake than SPEEK, which provides sufficient mechanical strength without swelling in the direct methanol fuel cell (DMFC) application. The methanol permeability of nitrated SPEEKS is reduced to 1.76 × 10⁻⁷ cm² s⁻¹ for S53N22 and 1.86 × 10⁻⁷ cm² s⁻¹ for S63N17 with no loss of conductivity in the DMFC application, and a proton conductivity that reached 0.026 S cm⁻¹. The nitrated SPEEK membranes satisfy the requirements of proton-exchange membranes for the DMFC.