Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.     

Synthesis and characterization of novel sulfonated poly(arylene ether ketone) copolymers with pendant carboxylic acid groups for proton exchange membranes

A series of novel side-chain-type sulfonated poly(arylene ether ketone)s with pendant carboxylic acid groups copolymers (C-SPAEKs) were synthesized by direct copolymerization of sodium 5,5′-carbonyl-bis(2-fluorobenzenesulfonate), 4,4′-difluorobenzophenone and 4,4′-bis(4-hydroxyphenyl) valeric acid (DPA). The expected structure of the sulfonated copolymers was confirmed by FT-IR and ¹H NMR. Membranes with good thermal and mechanical stability could be obtained by solvent cast process. It should be noted that the proton conductivity of these copolymers with high sulfonatation degree (DS > 0.6) was higher than 0.03 S cm⁻¹ and increased with increasing temperature. At 80 °C, the conductivity of C-SPAEK-3 (DS = 0.6) and C-SPAEK-4 (DS = 0.8) reached up to 0.12 and 0.16 S cm⁻¹, respectively, which were higher than that of Nafion 117 (0.10 S cm⁻¹). Moreover, their methanol permeability was much lower than that of Nafion 117. These results showed that the synthesized materials might have potential applications as the proton exchange membranes for DMFCs.     

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.     

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.     

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.     

Surface chemistry and physical properties of Nafion/polypyrrole/Pt composite membrane prepared by chemical in situ polymerization for DMFC

We modified Nafion by means of chemical in situ polymerization of pyrrole monomers with platinum (Pt) precursors for an application into an electrolyte of direct methanol fuel cells (DMFCs). SEM and EPMA exhibited the presences of polypyrrole and Pt at the surface region of Nafion, after diffusing and polymerizing pyrrole monomers with Pt precursors. XPS and FT-IR spectra were used to characterize the surface of Naf–Ppy–Pt composite membranes, demonstrating that pyrrolinum groups of polypyrrole were interacted with sulfonic groups or Pt precursors (PtCl₆⁻ or PtCl₄⁻). After in situ polymerization of pyrrole monomers, the morphological reorganization of sulfonic groups in Naf–Ppy–Pt composite membranes occurred via electrostatic interaction. Thermal stability, proton conductivity, methanol permeability, and cell performance of composite membranes were analyzed by TGA, AC impedance, refractometer, and potentiostat. Naf–Ppy–Pt composite membranes had higher thermal stabilities of sulfonic groups and side chains than Nafion and Naf–Ppy as a result of the interaction between Nafion–SO₃⁻?polypyrrole–NH₂⁺ and the presence of thermally stable Pt. The cell performance of Naf–Ppy–Pt 0 0 2 was enhanced significantly compared to that of Nafion under the specific condition, due to more reduction of methanol crossover than that of proton conductivity. Therefore, this synthetic method offers a facile way to improve physical properties of polymer electrolyte for the fabrication of advanced composite membranes.     

Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs)

A series of cross-linked membranes based on SPEEK/Nafion have been prepared to improve methanol resistance and dimension stability of SPEEK membrane for the usage in the direct methanol fuel cells (DMFCs). Sulfonated diamine monomer is synthesized and used as cross-linker to improve the dispersion of Nafion in the composite membranes and decrease the negative effect of cross-linking on proton conductivity of membranes. FT-IR analysis shows that the cross-linking reaction is performed successfully. The effects of different contents of Nafion on the properties of cross-linked membranes are investigated in detail. All the cross-linked membranes show lower methanol permeability and better dimensional stability compared with the pristine SPEEK membrane. SPEEK-N30 with the 30 wt % Nafion shows a methanol permeability of 0.73 × 10⁻⁶ cm² s⁻¹ and a water uptake of 24.4% at 25 °C, which are lower than those of the pristine membrane. Meanwhile, the proton conductivity of SPEEK-N30 still remains at 0.041 S cm⁻¹ at 25 °C, which is comparable to that of the pristine SPEEK membrane. All the results indicate that these cross-linked membranes based on SPEEK/Nafion show good prospect for the use as proton exchange membranes.     

Composites membranes based on Nafion and PAMAM dendrimers for PEMFC applications

The aim of this work consists in to incorporate organic compounds as PAMAM (Poly-amidoamine) dendrimers into the polymeric Nafion matrix to prepare composite membranes as a possible alternative to reduce methanol crossover effect. Composite Nafion membranes were prepared using a 3 wt% of Generation-4 hydroxyl-terminated PAMAM (G4OH) dendrimers containing 64-terminal OH-functional groups. The influence of PAMAM-(G4OH) dendrimer on chemical–physical properties of the composite membrane was highlighted resulting in a reduction of the methanol permeability (1.05 × 10⁻⁶ cm² s⁻¹) if compared to a recast bare Nafion membrane (8.19 × 10−6 cm² s⁻¹), used as a reference. Good proton conductivity was also observed for PAMAM-(G4OH) composite membrane. The polarization curves carried out at 100 °C in the presence of 2 M methanol have revealed the higher performance of the PAMAM-(G4OH) membrane when compared to a recast Nafion membrane.     

Performance of composite Nafion/PVA membranes for direct methanol fuel cells

This work has been focused on the characterization of the methanol permeability and fuel cell performance of composite Nafion/PVA membranes in function of their thickness, which ranged from 19 to 97 μm. The composite membranes were made up of Nafion^® polymer deposited between polyvinyl alcohol (PVA) nanofibers. The resistance to methanol permeation of the Nafion/PVA membranes shows a linear variation with the thickness. The separation between apparent and true permeability permits to give an estimated value of 4.0 × 10⁻⁷ cm² s⁻¹ for the intrinsic or true permeability of the bulk phase at the composite membranes. The incorporation of PVA nanofibers causes a remarkable reduction of one order of magnitude in the methanol permeability as compared with pristine Nafion^® membranes. The DMFC performances of membrane-electrode assemblies prepared from Nafion/PVA and pristine Nafion^® membranes were tested at 45, 70 and 95 °C under various methanol concentrations, i.e., 1, 2 and 3 M. The nanocomposite membranes with thicknesses of 19 μm and 47 μm reached power densities of 211 mW cm⁻² and 184 mW cm⁻² at 95 °C and 2 M methanol concentration. These results are comparable to those found for Nafion^® membranes with similar thickness at the same conditions, which were 210 mW cm⁻² and 204 mW cm⁻² respectively. Due to the lower amount of Nafion^® polymer present within the composite membranes, it is suggested a high degree of utilization of Nafion^® as proton conductive material within the Nafion/PVA membranes, and therefore, significant savings in the consumed amount of Nafion^® are potentially able to be achieved. In addition, the reinforcement effect caused by the PVA nanofibers offers the possibility of preparing membranes with very low thickness and good mechanical properties, while on the other hand, pristine Nafion^® membranes are unpractical below a thickness of 50 μm.     

Polymer electrolytes based on sulfonated polysulfone for direct methanol fuel cells

This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SPSf-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30–40 °C) and high temperatures (100–120 °C). DMFC power densities were about 140 mW cm⁻² at 100 °C with the bare SPSf-60 membrane and 180 mW cm⁻² at 120 °C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm⁻² at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices.     

Characterization of direct methanol fuel cell (DMFC) applications with H2SO4 modified chitosan membrane

Chitosan (Chs) flakes were prepared from chitin materials that were extracted from the exoskeleton of Cape rock lobsters in South Africa. The Chs flakes were prepared into membranes and the Chs membranes were modified by cross-linking with H₂SO₄. The cross-linked Chs membranes were characterized for the application in direct methanol fuel cells. The Chs membrane characteristics such as water uptake, thermal stability, proton resistance and methanol permeability were compared to that of high performance conventional Nafion 117 membranes. Under the temperature range studied 20-60 °C, the membrane water uptake for Chs was found to be higher than that of Nafion. Thermal analysis revealed that Chs membranes could withstand temperature as high as 230 °C whereas Nafion 117 membranes were stable to 320 °C under nitrogen. Nafion 117 membranes were found to exhibit high proton resistance of 284 s cm⁻¹ than Chs membranes of 204 s cm⁻¹. The proton fluxes across the membranes were 2.73 mol cm⁻² s⁻¹ for Chs- and 1.12 mol cm⁻² s⁻¹ Nafion membranes. Methanol (MeOH) permeability through Chs membrane was less, 1.4 × 10⁻⁶ cm² s⁻¹ for Chs membranes and 3.9 × 10⁻⁶ cm² s⁻¹ for Nafion 117 membranes at 20 °C. Chs and Nafion membranes were fabricated into membrane electrode assemblies (MAE) and their performances measure in a free-breathing commercial single cell DMFC. The Nafion membranes showed a better performance as the power density determined for Nafion membranes of 0.0075 W cm⁻² was 2.7 times higher than in the case of Chs MEA.     

Poly(vinyl alcohol)/sulfated β-cyclodextrin for direct methanol fuel cell applications

We report a composite membrane based on poly(vinyl alcohol) and sulfated β-cyclodextrin in this paper. TGA and SEM tests provide direct evidence of the thermal stability and the uniform structure of the composite membranes. The performances of the composite membranes are investigated in terms of swelling behavior, methanol permeability and proton conductivity as function of sulfated β-cyclodextrin content. We find that the introduction of sulfated β-cyclodextrin can reduce water uptake. The temperature dependence of proton conductivity reveals that the proton conducting activation energy of the composite membranes is similar to that of Nafion 115, in other words, both the vehicle and Grotthus mechanisms are assumed to be responsible for the composite membranes’ proton transfer. Methanol permeability decreases as the methanol feed concentration increases from 2 M to 20 M. Both proton conductivity and methanol permeability increases with increasing sulfated β-cyclodextrin. The selectivity of the composite membranes defined as the ratio of proton conductivity to methanol permeability obtains the maximum of 1.710 × 10⁴ S s cm⁻³ at the composition of 17 wt.% sulfated β-cyclodextrin. The MEAs fabricate with these membranes are tested, no distinct change occurred to the composite membranes after the MEAs operating for 288 h. These data indicates the chemical and electrochemical stability of the membranes and their potential application in direct methanol fuel cells.     

Interpenetrating network-forming sulfonated poly(vinyl alcohol) proton exchange membranes for direct methanol fuel cell applications

Poly (vinyl alcohol) was sulfonated and subsequently cross-linked by a thermal curing reaction with dual cross-linkers to prepare membranes for direct methanol fuel cells. Sulfonated poly (vinyl) alcohol (SPVA) with a high degree of sulfonation was synthesized from 4-Formylbenzene-1,3-disulfonic acid disodium salt hydrate via an acetalization reaction with PVA. Various masses of the cross-linking agents 1,3-bis(3-glycidyloxypropyl) tetramethyldisiloxane and 4,4′-oxydiphthalic anhydride were polymerized with SPVA to facilitate manipulation of the properties of the membranes. Notably, the SPVA3 showed excellent proton conductivity (cf. σ  = 0.218 S cm⁻¹ at 70 °C and Nafion 117 = 0.127 S cm⁻¹), and low methanol permeability (around one half of that Nafion 117). These results suggest that the cross-linked SPVA membranes hold promise as potential proton exchange membranes and given their high proton conductivity and low methanol permeability they may offer advantages when used in direct methanol fuel cells (DMFCs) applications.     

Sol-gel based sulfonic acid-functionalized silica proton conductive membrane

Sulfonic acid-functionalized glass membranes have been synthesized via sol-gel reactions for low-power direct methanol fuel cells (DMFCs). Minimizing the fuel loss due to methanol crossover is the most important issue for creating long-life, low-power DMFC sources. The inorganic glass membrane is of interest due to its low methanol permeability compared to polymer membranes. Three different alkoxy silane reactants were investigated in the sol-gel reaction: 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-mercaptopropyl trimethoxysilane (3MPS), and tetraethoxy orthosilicate (TEOS). The effect of oxidation time of the thiol group on the 3MPS, the mole fraction within the sol, and the water ratio in the reactant mixture were investigated. The ionic conductivity and methanol permeability has been characterized and optimized. The goal in this study was to find a balance between the ionic conductivity and methanol permeability, which determines the fuel conversion efficiency and device lifetime. The optimum glass membrane had a conductivity of 3.71 mS cm⁻¹, and methanol permeability of 2.17 × 10⁻⁹ mol cm cm⁻² day⁻¹ Pa⁻¹, which was significantly better than Nafion or other previously reported membranes for this application.     

Sulfonated poly(ether ether ketone)/polybenzimidazole oligomer/epoxy resin composite membranes in situ polymerization for direct methanol fuel cell usages

A diamine-terminated polybenzimidazole oligomer (o-PBI) has been synthesized for introducing the benzimidazole groups (BI) into sulfonated poly(ether ether ketone) (SPEEK) membranes. SPEEK/o-PBI/4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) composite membranes in situ polymerization has been prepared for the purpose of improving the performance of SPEEK with high ion-exchange capacities (IEC) for the usage in the direct methanol fuel cells (DMFCs). The composite membranes with three-dimensional network structure are obtained through a cross-linking reaction between PBI oligomer and TMBP and the acid-base interaction between sulfonic acid groups and benzimidazole groups. Resulting membranes show a significantly increasing of all of the properties, such as high proton conductivity (0.14 S cm⁻¹ at 80 °C), low methanol permeability (2.38 × 10⁻⁸ cm² s⁻¹), low water uptake (25.66% at 80 °C) and swelling ratio (4.11% at 80 °C), strong thermal and oxidative stability, and mechanical properties. Higher selectivity has been found for the composite membranes in comparison with SPEEK. Therefore, the SPEEK/o-PBI/TMBP composite membranes show a good potential in DMFCs usages.     

A novel hybrid Nafion-PBI-ZP membrane for direct methanol fuel cells

Methanol crossover through proton conducting membranes represents one of the main drawbacks in DMFCs. This study presented a novel organic-inorganic hybrid membrane with several different compositions by casting mixtures of zirconium phosphate (ZP), polybenzimidazole (PBI) and Nafion dispersion in dimethylacetamide. The presence of PBI and ZP in the membranes was demonstrated with energy dispersive X-ray (EDX) analysis. From the scanning electron microscopy (SEM) analysis, it was observed that the hybrid Nafion-PBI-ZP membrane had the finest structure. This is because the synthesized films were homogeneous and therefore formed a dense membrane. The water content was higher in the hybrid membrane: 39.91% compared with 35.52% in Nafion117. The water content is important for the ion transportation in the membrane; therefore, a higher water uptake rate will contribute to a better fuel cell performance. It was determined that the proton conductivity of the hybrid membrane was 0.020 S cm⁻¹, which was comparable with Nafion117, which had a proton-conductivity of 0.022 S cm⁻¹. The methanol permeability of the hybrid membrane was 2.34 × 10⁻⁷ cm² s⁻¹, while the value for Nafion117 was 8.91 × 10⁻⁷ cm² s⁻¹. This showed that the methanol permeability of the hybrid membrane was almost 4 times lower than that of Nafion117. The selectivity factor for the Nafion-PBI 1%-ZP 1% membrane was 8.64 × 10⁴ Scm⁻³, while that of Nafion117 was 2.48 × 10⁴ S scm⁻³. From a thermogravimetry analysis (TGA), the addition of PBI and zirconium phosphate was shown to improve the thermal durability in the temperature range from room temperature to 450 °C over that of Nafion117. This study proofed that the Nafion-PBI 1%-ZP 1% performed better than commercial Nafion117 and other type of membranes. The membrane was tested on as single cell of DMFC. It gave the highest power density as compared to other type of membrane and proofed that it has potential to be used in DMFCs.     

Preparation and performance of nano silica/Nafion composite membrane for proton exchange membrane fuel cells

Composite membranes made from Nafion ionomer with nano phosphonic acid-functionalised silica and colloidal silica were prepared and evaluated for proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature and low relative humidity (RH). The phosphonic acid-functionalised silica additive obtained from a sol–gel process was well incorporated into Nafion membrane. The particle size determined using transmission electron microscope (TEM) had a narrow distribution with an average value of approximately 11 nm and a standard deviation of ±4 nm. The phosphonic acid-functionalised silica additive enhanced proton conductivity and water retention by introducing both acidic groups and porous silica. The proton conductivity of the composite membrane with the acid-functionalised silica was 0.026 S cm⁻¹, 24% higher than that of the unmodified Nafion membrane at 85 °C and 50% RH. Compared with the Nafion membrane, the phosphonic acid-functionalised silica (10% loading level) composite membrane exhibited 60 mV higher fuel cell performance at 1 A cm⁻², 95 °C and 35% RH, and 80 mV higher at 0.8 A cm⁻², 120 °C and 35% RH. The fuel cell performance of composite membrane made with 6% colloidal silica without acidic group was also higher than unmodified Nafion membrane, however, its performance was lower than the acid-functionalised silica additive composite membrane.     

Sol–gel derived sulfonated-silica/Nafion composite membrane for direct methanol fuel cell

Sulfonated-silica/Nafion^® composite membranes were prepared in a sol–gel reaction of (3-Mercaptopropyl)trimethoxysilane (SH-silane) followed by solution casting, and then oxidated using 10 wt% H₂O₂ solution. The chemical and physical properties of the composite membranes were characterized by using FT-IR, XPS, ²⁹Si NMR and SEM analyses. Experimental results indicated that the optimum oxidation condition was 60 °C for 1 h. The performance of the silica–SO₃H/Nafion^® composite membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The silica–SO₃H/Nafion^® composite membranes have a higher selectivity (C/P ratio = 26,653) than that of pristine Nafion^® (22,795), perhaps because of their higher proton conductivity and lower methanol permeability. The composite membrane with 0.6 wt% silica–SO₃H/Nafion^® performed better than pristine Nafion^®. The current densities were measured as 62.5 and 70 mA cm⁻² at a potential of 0.2 V with a composite membrane that contained 0 and 0.6 wt% silica–SO₃H, respectively. The cell performance of the DMFC was improved by introducing silica–SO₃H. The composite membrane with 0.6 wt% of silica–SO₃H yielded the maximum power density of 15.18 mW cm⁻². The composite membranes are suitable for DMFC applications with high selectivity.     

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.     

Nafion/sulfated β-cyclodextrin composite membranes for direct methanol fuel cells

Proton-conducting composite membranes based on H⁺-form sulfated β-cyclodextrin (sb-CD) in a Nafion matrix are prepared via the solution-casting method and their methanol permeabilities, proton conductivities, proton diffusion coefficients and cell performances are measured. The methanol permeabilities of the composite membranes increase very slightly with increases in their sb-CD content. As a result of adding sb-CD with its many sulfonic acid groups into the Nafion matrix, the proton conductivities of the composite membranes increase with increases in their sb-CD content. The methanol permeability and proton conductivity results are used to show that the best selectivity of the membranes is that of the NC5 membrane (‘NCx’ denotes a Nafion/sb-CD composite membrane containing x wt.% sb-CD). The proton diffusion coefficients are measured with ¹H pulsed field gradient nuclear magnetic resonance (PFG-NMR) and found to increase with increase in the sb-CD content in the order NC5 > NC3 > NC1 > NC0. Thus the presence of sb-CD in the Nafion membranes increases the proton diffusion coefficients as well as the proton conductivities, ionic cluster size, water uptakes and the ion-exchange capacities (IECs). A maximum power density of 58 mW cm⁻² is obtained for the NC5 membrane. The combination of these effects should lead to an improvement in the performance of direct methanol fuel cells prepared with Nafion/sb-CD composite membranes.