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.     

Enhanced performance of supercritical CO2 treated Nafion 212 membranes for direct methanol fuel cells

Supercritical carbon dioxide (Sc-CO₂) thermal treatment to enhance performances of both Nafion 212 (NR212) commercial membranes with H-form and Na-form for direct methanol fuel cells (DMFCs) is described. XRD measurements show that the crystallinity of H-form NR212 membranes increases with increasing the treated temperature in the Sc-CO₂ system, however, the crystallinity of Na-form NR212 membranes decreases with increasing the treated temperature. Since the bigger crystallites formed after the Sc-CO₂ treatments, it improves the mechanical strength and dimensional stability of the Sc-CO₂ treated NR212 membranes with H-form and Na-form. Compared with the as-received NR212 membranes, all the Sc-CO₂ treated NR212 membranes show higher proton conductivity and better capacity of barrier to methanol crossover. From Fenton test, it can be found that the Sc-CO₂ treated NR212 membranes have better chemical stability than that of NR212 membranes. Therefore, NR212 membranes treated by the Sc-CO₂ method may be promising candidate electrolytes for DMFC applications.     

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.     

Sulfonated graphene oxide/Nafion composite membranes for high-performance direct methanol fuel cells

An easy and effective method for producing low methanol-crossover membranes is developed by dispersing sulfonated graphene oxide (SGO) into a Nafion matrix. A SGO/Nafion mixture with low SGO content exhibits unique viscosity behavior and allows for better SGO dispersion within the Nafion. After film casting, the composite membranes show lower methanol and water uptakes, a reduced swelling ratio, improved proton conductivity in low relative humidity, and extremely high methanol selectivity, which can be implemented in direct methanol fuel cells (DMFCs). The regular backbone of the composite membrane shows a higher storage modulus, increased α-relaxation (transition temperature), and improved tolerance to pressure during membrane electrode assembly (MEA). The small angle X-ray spectra indicate the shrinkage of the ionic clusters in the composite membranes, which thus reduce methanol crossover. The hybrid membranes applied to DMFCs demonstrate performances superior to that of the commercial Nafion 115 in 1 M and 5 M methanol solutions.     

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.     

Preparation and properties of hybrid direct methanol fuel cell membranes by embedding organophosphorylated titania submicrospheres into a chitosan polymer matrix

Organophosphorylated titania submicrospheres (OPTi) are prepared and incorporated into a chitosan (CS) matrix to fabricate hybrid membranes with enhanced methanol resistance and proton conductivity for application in direct methanol fuel cells (DMFC). The pristine monodispersed titania submicrospheres (TiO₂) of controllable particle size are synthesized through a modified sol-gel method and then phosphorylated by amino trimethylene phosphonic acid (ATMP) via chemical adsorption, which is confirmed by XPS, FTIR and TGA. The morphology and thermal property of the hybrid membranes are explored by SEM and TGA. The ionic cross-linking between the -PO₃H₂ groups on OPTi and the -NH₂ groups on CS lead to better compatibility between the inorganic fillers and the polymer matrix, as well as a decreased fractional free volume (FFV), which is verified by positron annihilation lifetime spectroscopy (PALS). The effects of particle size and content on the methanol permeability, proton conductivity, swelling and FFV of the membranes are investigated. Compared to pure CS membrane, the hybrid membranes exhibit an increased proton conductivity to an acceptable level of 0.01 S cm⁻¹ for DMFC application and a reduced methanol permeability of 5 × 10⁻⁷ cm² s⁻¹ at a 2 M methanol feed.     

Noble metal nanowires incorporated Nafion membranes for reduction of methanol crossover in direct methanol fuel cells

We electrodeposited noble metal (palladium, platinum) nanowires into the hydrophilic pores of Nafion membrane for mitigating the problem of methanol crossover in direct methanol fuel cells (DMFCs). The DMFC performance result shows that the composite membranes yield lower rate of methanol crossover and better cell performance than the pure Nafion^® membrane. At low current densities, the Pd nanowire incorporated Nafion membrane shows the best performance. In comparison, the highest performance is achieved at higher current densities with the Pt nanowire modified Nafion membrane. Based on the above findings, we suggest that for the Pd nanowire incorporated Nafion membrane, the mechanism for the suppression of the methanol crossover is mainly the blocking effect due to the ‘narrowed’ hydrophilic channels in Nafion membrane. For the Pt nanowire modified Nafion membrane, the mechanism includes both increasing the membrane tortuosity and so-called ‘on-way consumption’ of methanol on the Pt nanowires deposited into the Nafion membrane when the fuel cell is discharging.     

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/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^®.     

Use of in situ polymerized phenol-formaldehyde resin to modify a Nafion membrane for the direct methanol fuel cell

Commercial Nafion^®-115 (trademark registered to DuPont) membranes were modified by in situ polymerized phenol formaldehyde resin (PFR) to suppress methanol crossover, and SO₃⁻ groups were introduced to PFR by post-sulfonatation. A series of membranes with different sulfonated phenol formaldehyde resin (sPFR) loadings have been fabricated and investigated. SEM-EDX characterization shows that the PFR was well dispersed throughout the Nafion^® membrane. The composite membranes have a similar or slightly lower proton conductivity compared with a native Nafion^® membrane, but show a significant reduction in methanol crossover (the methanol permeability of sPFR/Nafion^® composite membrane with 2.3 wt.% sPFR loading was 1.5 × 10⁻⁶ cm² s⁻¹, compared with the 2.5 × 10⁻⁶ cm² s⁻¹ for the native Nafion^® membrane). In direct methanol fuel cell (DMFC) evaluation, the membrane electrode assembly (MEA) using a composite membrane with a 2.3 wt.% sPFR loading shows a higher performance than that of a native Nafion^® membrane with 1 M methanol feed, and at higher methanol concentrations (5 M), the composite membrane achieved a 114 mW cm⁻² maximum power density, while the maximum power density of the native Nafion^® was only 78 mW cm⁻².     

Zeolite beta-filled chitosan membrane with low methanol permeability for direct methanol fuel cell

Zeolite beta particles with different sizes and narrow size distribution were hydrothermally synthesized and incorporated into chitosan (CS) matrix to prepare CS/zeolite beta hybrid membranes for direct methanol fuel cell (DMFC). It was found that the chitosan membrane filled by zeolite beta particles about 800 nm in size exhibited the lowest methanol permeability, which can be ascribed to their optimum free volume and methanol diffusion characteristics. To further improve the performances of CS/zeolite beta hybrid membranes, zeolite beta particles about 800 nm in size were sulfonated via three different approaches. The results indicated that the introduction of sulfonic groups could reduce the methanol permeability further as a result of the enhanced interfacial interaction between zeolite beta and chitosan matrix. Furthermore, in terms of the overall selectivity index, CS/zeolite beta hybrid membranes were comparable to Nafion^® 117 membrane at low methanol concentration (2 mol L⁻¹) and much better at high methanol concentration (12 mol L⁻¹).     

Effect of zeolites on chitosan/zeolite hybrid membranes for direct methanol fuel cell

Zeolites including 3A, 4A, 5A, 13X, mordenite, and HZSM-5 were incorporated into chitosan (CS) matrix to fabricate the hybrid membranes for direct methanol fuel cell (DMFC). Due to the presence of hydrogen bonds between CS and zeolite, the hybrid membranes displayed desirable thermal and mechanical stabilities. Through free volume characteristics analysis by positron annihilation lifetime spectroscopy (PALS) technique, it was found that incorporation of hydrophilic zeolites would increase the free volume cavity size whereas incorporation of hydrophobic zeolites would decrease the free volume cavity size. Through the investigations on water/methanol uptake, swelling, and methanol permeability, it was found that the membrane performance was highly dependent on the zeolite particle and pore size, content, and hydrophilic/hydrophobic nature. Based on the solution–diffusion mechanism, it was found that incorporation of hydrophobic zeolites increased the diffusion resistance of methanol and consequently decreased the methanol permeability, whereas incorporation of hydrophilic zeolites decreased the diffusion resistance of methanol and consequently increased the methanol permeability. Moreover, under the identical conditions, all the as-prepared membranes exhibited much lower methanol permeability than Nafion^® 117 while the proton conductivity of the membranes remained high enough for DMFC applications.     

Nafion/bio-functionalized montmorillonite nanohybrids as novel polyelectrolyte membranes for direct methanol fuel cells

Polyelectrolyte membranes based on Nafion^® and bio-functionalized montmorillonite (BMMT) with chitosan biopolymer, as polycationic intercalant were fabricated by solvent casting method. X-ray diffraction analysis confirmed the exfoliated structure of clay. Methanol permeability results revealed that the presence of 10 wt% BMMT in synthesized nanohybrid membranes can reduce the permeability to 5.72 × 10⁻⁸ cm² s⁻¹ in comparison with 2.00 × 10⁻⁶ for that of Nafion^® 117. However proton conductivity of nanohybrids was decreasing with increasing BMMT loading, but obtained values were indicating the lower sacrificing of conductivity in comparison with membranes based on unmodified MMT. According to selectivity parameter, membranes containing 2 wt% of BMMT showed optimum properties. It was suggested that improvement of transportation properties could be due to the electrostatic interaction between amino groups of chitosan and Nafion^® sulfone groups. Considering the suitable thermal stability, low methanol crossover and appropriate proton conductivity properties, Nafion^®/BMMT nanohybrid membranes, could be proposed as novel polyelectrolytes for direct methanol fuel cell application.     

Improving the performance of direct methanol fuel cells by implementing multilayer membranes blended with cellulose nanocrystals

Methanol crossover through the proton exchange membranes of a direct methanol fuel cell (DMFC) significantly affects its performance and efficiency. Low methanol permeability and high proton conductivity of the membrane is desired for optimum performance. In this work, a multilayer (ML) membrane configuration prepared by a simple pressing technique is employed with and without the incorporation of sprayed cellulose nanocrystals (CNC) to achieve enhanced membrane properties. Assembled multilayer electrolytes show 19% enhanced maximum power density, while the addition of 1.5 wt% CNC (wt % of total final membrane) further improves the performance, giving a 38% better performance compared to standard Nafion N115. Methanol flux density and electrochemical impedance measurements attribute these improvement to the ~20% enhancement in the proton conductivity for the multilayer membrane which is enhanced further by an 11% reduction in methanol crossover when the cellulose nanocrystals are added.     

Nafion/polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene composite membranes with electric field-aligned domains for improved direct methanol fuel cell performance

Composite membranes were fabricated consisting of aligned domains of Nafion^® 1100 surrounded by a supporting matrix of polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS). The structure of the composite was controlled via the application of an electric field during solvent casting. Nafion^® domains were aligned across the membrane thickness to provide paths for proton conduction. The surrounding SEBS domain limits methanol permeability. Compared to randomly structured Nafion^®/SEBS composites, the membranes with field-aligned domains display significantly enhanced performance in direct methanol fuel cells (DMFCs). The membranes with field-aligned domains display DMFC performance better than commercial Nafion^® 117 membranes under high methanol fuel concentration.     

Synthesis and characterization of proton exchange membranes based on sulfonated poly(fluorenyl ether nitrile oxynaphthalate) for direct methanol fuel cells

A series of sulfonated poly(fluorenyl ether nitrile oxynaphthalate) (SPFENO) copolymers with different degree of sulfonation (DS) are synthesized via nucleophilic polycondensation reactions with commercially available monomers. Incorporation of the naphthalanesulfonate group into the copolymers and their copolymer structures are confirmed by ¹H NMR spectroscopy. Thermal stability, mechanical properties, water uptake, swelling behavior, proton conductivity and methanol permeability of the SPFENO membranes are investigated with respect to their structures. The electrochemical performance of a direct methanol fuel cell (DMFC) assembled with the SPFENO membrane was evaluated and compared to a DMFC with a Nafion 117 membrane. The DMFC assembled with the SPFENO membrane of proper DS exhibits better electrochemical performance compared to the Nafion 117-based cell.     

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.     

Fabrication and performances of solid superacid embedded chitosan hybrid membranes for direct methanol fuel cell

This study reports the fabrication and performances of hybrid proton-conducting membranes by dispersing nanosized solid superacid inorganic fillers, TiO₂-SO₄²⁻ (STiO₂), into chitosan (CS) matrix. Fourier transform infrared spectra demonstrate intermolecular interactions between STiO₂ and chitosan segmental chains. High resolution scanning electron microscope characterization reveals an essentially homogeneous dispersion of the solid superacid fillers within chitosan matrix. The incorporation of the superacid fillers leads to a reduced fractional free volume (FFV) of the hybrid membranes as confirmed by positron annihilation lifetime spectroscopy (PALS) analysis. This reduced FFV and more tortuous pathway significantly enhance the methanol diffusion resistance through the membranes, resulting in a decreased methanol crossover. Under identical conditions, compared with TiO₂ embedded membranes, the STiO₂-filled hybrid membranes exhibit simultaneously improved methanol barrier and proton transport properties due to the enhanced interfacial interaction and proton conductive ability. Moreover, compared with Nafion 117 membrane, all the STiO₂-filled hybrid membranes display much lower methanol crossover whereas the proton conductivity of the membranes remains high enough for DMFC applications. Meanwhile, due to the interfacial interactions between STiO₂ and chitosan chains, the hybrid membranes exhibit an enhanced mechanical strength and adequate thermal stability as verified by mechanical strength characterization and thermogravimetric analysis.     

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).     

Synthesis and characterization of polyvinyl alcohol copolymer/phosphomolybdic acid-based crosslinked composite polymer electrolyte membranes

Polymer electrolyte membrane fuel cells (PEMFCs) are very promising as future energy source due to their high-energy conversion efficiency and will help to solve the environmental concerns of energy production. Polymer electrolyte membrane (PEM) is recognised as the key element for an efficient PEMFC. Chemically crosslinked composite membranes consisting of a poly(vinyl alcohol-co-vinyl acetate-co-itaconic acid) (PVACO) and phosphomolybdic acid (PMA) have been prepared by solution casting and evaluated as proton conducting polymer electrolytes. The proton conductivity of the membranes is investigated as a function of PMA composition, crosslinking density and temperature. The membranes have also been characterized by FTIR spectroscopy, TGA, AFM and TEM. The proton conductivity of the composite membranes is of the order of 10⁻³ S cm⁻¹ and shows better resistance to methanol permeability than Nafion 117 under similar measurement conditions.