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.     

Crosslinked SPEEK/AMPS blend membranes with high proton conductivity and low methanol diffusion coefficient for DMFC applications

The crosslinked sulfonated poly (ether ether ketone)/2-acrylamido-2-methyl-1-propanesulfonic acid (SPEEK/AMPS) blend membranres were prepared and evaluated as proton exchange membranes for direct methanol fuel cell (DMFC) applications. The structure and morphology of SPEEK/AMPS membranes were characterized by FTIR and SEM, respectively. The effects of crosslinking and AMPS content on the performance of membranes were studied and discussed in detail. The proton conductivity and methanol diffusion coefficient of SPEEK/AMPS membranes increased gradually with the increase of AMPS content. Most SPEEK/AMPS membranes exhibited higher proton conductivity than Nafion^® 117 (0.05 S cm⁻¹ at 25 °C). However, all the membranes possessed much lower methanol diffusion coefficient compared with Nafion^® 117 (2.38 × 10⁻⁶ cm² s⁻¹) under the same measuring conditions. Even the methanol diffusion coefficient (8.89 × 10⁻⁷ cm² s⁻¹) of SPEEK/AMPS 30 sample with the highest proton conductivity (0.084 S cm⁻¹ at 25 °C) was only about one third of that of Nafion^® 117. The selectivity of all the SPEEK/AMPS membranes was much higher in comparison with Nafion^® 117 (2.8 × 10⁴ S s cm⁻³). In addition, the SPEEK/AMPS membranes possessed relatively good thermal and hydrolytic stability. These results suggested that the SPEEK/AMPS membranes were particularly promising to be used as proton exchange membranes in DMFCs, and the high proton conductivity, low methanol diffusion coefficient and high selectivity were their primary advantages for DMFC applications.     

Nafion/SiO2 hybrid membrane for vanadium redox flow battery

Sol–gel derived Nafion/SiO₂ hybrid membrane is prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. Nafion/SiO₂ hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO₂ hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO₂ hybrid membrane presents a higher coulombic and energy efficiencies over the entire range of current densities (10–80 mA cm⁻²), especially at relative lower current densities (<30 mA cm⁻²), and a lower self-discharge rate compared with the Nafion system. The performance of VRB with Nafion/SiO₂ hybrid membrane can be maintained after more than 100 cycles at a charge–discharge current density of 60 mA cm⁻². The experimental results suggest that the Nafion/SiO₂ hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB.     

Tuning transport properties by manipulating the phase segregation of tetramethyldisiloxane segments in modified polyimide electrolytes

A series of copolymer electrolytes containing 4,4′-oxydianiline (ODA)-based sulfonated polyimide and siloxane segments, in various ratios, are prepared and characterized for direct methanol fuel cell applications. The chemical Structure of the sulfonated copolymers is confirmed by FT-IR and NMR. The prepared composite membranes are found to be flexible and show good thermal stability as well as good proton conductivity. A maximum proton conductivity of 5.78 × 10⁻² S cm⁻¹ (cf. Nafion117 = 8.31 × 10⁻² S cm⁻¹) is obtained for the sulfonated polyimide blended with sulfonated polyimide with a grafted tetramethyldisiloxane segment (cf. SPI_DSX75 membrane) at 90 °C. The membranes showed low methanol crossover below 10⁻⁷ cm² s⁻¹ (cf. Nafion117 = 10⁻⁶ cm² s⁻¹). The transport properties of the membranes are found to be strongly influenced by water uptake and by the number and nature of the ionic clusters in the hydrophilic domains. When the number of siloxane segments is increased, the relationship between the methanol self-diffusion coefficient (D_M) and water molecules per sulfonic acid group (λ) indicate that the water molecules are interacting with channels inside the membrane. In addition, the segregated nanophase also affects the ion transport and sometimes enhances the corresponding ionic conductivity. TEM and SAXS analyses shows evidence for phase segregation in the membranes and reveal the influence of flexible siloxane segments in ionic clustering.     

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.     

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).     

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.     

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.     

Direct methanol fuel cell (DMFC) based on PVA/MMT composite polymer membranes

A novel composite polymer electrolyte membrane composed of a PVA polymer host and montmorillonite (MMT) ceramic fillers (2–20 wt.%), was prepared by a solution casting method. The characteristic properties of the PVA/MMT composite polymer membrane were investigated using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and micro-Raman spectroscopy, and the AC impedance method. The PVA/MMT composite polymer membrane showed good thermal and mechanical properties and high ionic conductivity. The highest ionic conductivity of the PVA/10 wt.%MMT composite polymer membrane was 0.0368 S cm⁻¹ at 30 °C. The methanol permeability (P) values were 3–4 × 10⁻⁶ cm² s⁻¹, which was lower than that of Nafion 117 membrane of 5.8 × 10⁻⁶ cm² s⁻¹. It was revealed that the addition of MMT fillers into the PVA matrix could markedly improve the electrochemical properties of the PVA/MMT composite membranes; which can be accomplished by a simple blend method. The maximum peak power density of the DMFC with the PtRu anode based on Ti-mesh in a 2 M H₂SO₄ + 2 M CH₃OH solution was 6.77 mW cm⁻² at ambient pressure and temperature. As a result, the PVA/MMT composite polymer appears to be a good candidate for the DMFC applications.     

Role of post-sulfonation of poly(ether ether sulfone) in proton conductivity and chemical stability of its proton exchange membranes for fuel cell

Commercially available poly(ether ether sulfone), PEES, was directly sulfonated using concentrated sulfuric acid at low temperatures by minimizing degradation during sulfonation. The sulfonation reaction was performed in the temperature range of 5–25 °C. Sulfonated polymers were characterized by FTIR, ¹H NMR spectroscopy and ion exchange capacity (IEC) measurements. Degradation during sulfonation was investigated by measuring intrinsic viscosity, glass transition temperature and thermal decomposition temperature of sulfonated polymers. Sulfonated PEES, SPEES, membranes were prepared by solvent casting method and characterized in terms of IEC, proton conductivity and water uptake. The effect of sulfonation conditions on chemical stability of membranes was also investigated via Fenton test. Optimum sulfonation condition was determined to be 10 °C with conc. H₂SO₄ based on the characteristics of sulfonated polymers and also the chemical stability of their membranes. SPEES membranes exhibited proton conductivity up to 185.8 mS cm⁻¹ which is higher than that of Nafion 117 (133.3 mS cm⁻¹) measured at 80 °C and relative humidity 100%.     

Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.     

Proton conduction in Nafion composite membranes filled with mesoporous silica

Studies of proton-conductive polymer membranes are vital for the future development of high-performance polymer electrolyte membrane fuel cells (PEM-FC). In particular, a method for inhibiting the volatility of water in the polymer matrix at high temperatures is a crucial issue, directly related to the operation of PEM-FC system. In this study, we focus on polymer composite membranes, which consist of commercial Nafion and mesoporous silica (MPSi) as novel inorganic additives, and investigate an improvement in the total proton conductivities and the good electrochemical stability at high temperatures. MPSi, which can be synthesized with pore sizes from 1 to 10 nm, has a wide range of potential applications because of its extraordinary properties, such as extremely large surface area, flawless surface condition and well-regulated porous structure. We found that the Nafion composites filled with MPSi have approximately 1.5 times higher proton conductivities (more than 0.1 S cm⁻¹ at 80 °C and 95%RH) than pure Nafion and can display good temperature performance relative to pure Nafion and the particle SiO₂ composite. Moreover, the conductivity of Nafion/sulfonated MPSi was the highest (0.094 S cm⁻¹) at 40 °C and 95%RH. These are probably due to the large surface area of MPSi, which can increase the water adsorption in Nafion matrix.     

Composite membranes based on micro and mesostructured silica: A comparison of physicochemical and transport properties

The aim of this work is to incorporate inorganic compounds into the Nafion matrix (composite membranes) for high temperature of a polymer electrolyte fuel cell (T_cell > 100 °C). Three silicon oxides having a different morphology were synthesized starting with a tetraethyl orthosilicate as a precursor via sol-gel method: SBA15, SBA15-SH and SiO₂. Successively, composite Nafion membranes were prepared using a 3% (w/w) of each powder through a standardized casting method. The influence of SiO₂ morphology on chemical-physical properties of the membranes was highlighted resulting in a reduction of the swelling parameters of the composite membranes if compared at T ≥ 80 °C to a recast bare Nafion membrane, used as a reference. Good proton conductivity was also observed for all composite membranes with values of 0.144 S cm⁻¹, 0.136 S cm⁻¹, 0.090 S cm⁻¹and 0.078 S cm⁻¹ recorded at 80 °C (100% RH) for Nrecast, NSBA15, NSBA15-SH and NSiO₂, respectively. The polarisation curves carried out at 120 °C (75% RH, 1.5 abs. bar) have revealed a higher stability for NSBA15 membrane after a short time-test, probably because the silica morphology is able to retain water within the polymer matrix and, in accordance to the swelling data.     

Preparation of main-chain-type and side-chain-type sulfonated poly(ether ether ketone) membranes for direct methanol fuel cell applications

Novel main-chain-type and side-chain-type sulphonated poly(ether ether ketone)s (MS-SPEEKs) are synthesised by reacting the sulphonic acid groups of pristine SPEEKs with 2-aminoethanesulphonic acid to improve the nano-phase separated morphology of the material. ¹H NMR and FT-IR spectroscopy are employed to determine the structure and composition of main-chain-type and side-chain-type sulphonated polymers. Flexible and tough membranes with reasonable thermal properties are obtained. The MS-SPEEKs show good hydrolytic stability, and water uptake values ranging from 15% to 30% are observed. Compared to Nafion 117^®, the methanol permeability of the MS-SPEEKs is dramatically reduced to 8.83 × 10⁻⁸ cm² s⁻¹ to 3.31 × 10⁻⁷ cm² s⁻¹. The proton conductivity increases with increasing temperature, reaching 0.013-0.182 S cm⁻¹. A maximum power density and open circuit voltage of 115 mW cm⁻² and 0.830 V are obtained at 80 °C, respectively, which is significantly greater than the values generated with Nafion 117^®. The introduction of pendent side-chain-type sulphonic acid groups increases the single-cell performance by more than approximately 20%; thus, the lower water diffusivity, methanol permeability, electro-osmotic drag coefficient and high cell performance indicated that MS-SPEEK is a promising candidate for DMFC applications.     

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.     

Synthesis and characterization of new sulfonated polytriazole proton exchange membrane by click reaction for direct methanol fuel cells (DMFCs)

Sulfonated polytriazole (SPTA) in which the acidic sulfonic acid and basic triazole groups act as physical crosslinking sites within a polymer backbone has been successfully prepared, for use as a proton exchange membrane, using the click reaction. The acid-base interactions of the SPTA membranes leads to the formation of well-dispersed ionic clusters and the random distribution of ion channels with good connectivity resulting in lower methanol permeabilities at ambient temperatures and similar or higher proton conductivities than Nafion 117 at 80 °C in conditions of near zero relative humidity. Proton conductivities (σ) of 0.149 S cm⁻¹ at 80 °C and 9 × 10⁻⁵ S cm⁻¹ in anhydrous conditions together with low methanol permeability (P) at 0.1 × 10⁻⁶ cm² s⁻¹ that are comparable or superior to Nafion 117 (σ_T=80 : 0.151 S cm⁻¹; σ_RH=0 : 3 × 10⁻⁵ S cm⁻¹; P_T=30: 1.31 × 10⁻⁶ cm² s⁻¹) were achieved. Additionally, the selectivity of SPTA is approximately four times higher than that of Nafion 117, thus it may have potential for use in direct methanol fuel cells (DMFCs).     

A facile surface modification of Nafion membrane by the formation of self-polymerized dopamine nano-layer to enhance the methanol barrier property

By immersing Nafion membrane into dopamine aqueous solution under mild conditions, a series of modified Nafion membranes for the application in direct methanol fuel cell (DMFC) were fabricated. High resolution scanning electron microscope and Fourier transform infrared spectra characterization revealed that a dense nano-layer around 50 nm was formed and adhered tightly to Nafion surface. Small-angle X-ray scattering, wide X-ray diffractometer and positron annihilation lifetime spectroscopy analysis implied that the microstructure such as phase-separated structure and ion-cluster channel of Nafion layer was slightly changed after surface modification. The influence of modification conditions including pH value, dopamine concentration and immersing time upon membrane performance was investigated. Due to the effective reduction of methanol dissolution and enhancement of methanol diffusion resistance, the methanol crossover of the modified membranes was dramatically suppressed by about 79% from 3.14 × 10⁻⁶ to about 0.65 × 10⁻⁶ cm² s⁻¹. Meanwhile, the proton conductivity of the modified membranes was slightly decreased to be around 0.06 S cm⁻¹. Consequently, the comprehensive performance of the modified membranes was improved by about five times. These results hinted the application promises of such modified Nafion membranes in DMFC.     

SPEEK/epoxy resin composite membranes in situ polymerization for direct methanol fell cell usages

Sulfonated poly(ether ether ketone) (SPEEK)/4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) composite membranes in situ polymerization were prepared for the purpose of improving the methanol resistance and mechanical properties of SPEEK membranes with high ion-exchange capacities (IEC) for the usage in the direct methanol fuel cells (DMFCs). The effects of introduction of TMBP content on the properties of the composite membranes were investigated in detail. The composite membranes have good mechanical, thermal properties, lower swelling ratio, lower water diffusion coefficient (0.87 × 10⁻⁵ cm² s⁻¹ at 80 °C) and better methanol resistance (5.26 × 10⁻⁷ cm² s⁻¹ at 25 °C) than SPEEK membranes. The methanol diffusion coefficients of the composite membranes are much lower than that of SPEEK membrane (17.5 × 10⁻⁷ cm² s⁻¹ at 25 °C). Higher selectivity was been found for the composite membranes in comparison with SPEEK. Therefore, the SPEEK/TMBP composite membranes show a good potential in DMFCs usages.     

Synthesis of poly(arylene ether sulfone)s with locally and densely sulfonated pentiptycene pendants as highly conductive polymer electrolyte membranes

Poly(aryl ether sulfone)s containing sulfonated pentiptycene groups SPES-x-PPD are firstly synthesized through nucleophilic aromatic substitution polycondensation by using pentiptycene-6,13-diol, bis(4-hydroxyphenyl) sulfone and 4,4′-difluorodiphenyl sulfone, followed by postsulfonation with concentrated sulfuric acid at room temperature. The structures of SPES-x-PPD are characterized by IR, ¹H NMR and ¹³C NMR spectra. These ionomers generally show high thermal stability. Transmission electron microscopic observations reveal that SPES-x-PPD membranes form well-defined microphase separated structures. SPES-40-PPD with the IEC value 2.36 mmol g⁻¹ shows conductivity of 2.6 × 10⁻¹ S cm⁻¹ which is much higher than that of perfluorinated Nafion 117 membrane (1.1 × 10⁻¹ S cm⁻¹) at 80 °C and 94% RH. At 80 °C and 34% RH, SPES-40-PPD membrane displays the conductivity of 2.7 × 10⁻³ S cm⁻¹ which is comparable with that of Nafion 117 membrane (3.0 × 10⁻³ S cm⁻¹).     

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.