Nafion–polyfurfuryl alcohol nanocomposite membranes for direct methanol fuel cells

Commercial Nafion 115 membranes were successfully modified by in situ acid-catalyzed polymerization of furfuryl alcohol (PFA) within Nafion structures. FT-IR and AFM were used to characterize the chemical and morphological structures of the Nafion–PFA nanocomposite membrane obtained. The methanol permeation experiments showed that the methanol flux through the Nafion–PFA nanocomposite membranes dropped by a factor of 2.2–2.7 when PFA loading was 3.9–8.0 wt.%. Importantly, the proton conductivity of the membranes decreased only slightly at a low PFA loading (<8 wt.%). The nanocomposite membranes with higher selectivity (e.g., proton conductivity/methanol crossover) achieved a much higher DMFC performance at both room temperature and 60 °C.     

聚乙烯醇/聚乙烯吡咯烷酮碱性复合膜的制备及其性能

通过在不同浓度KOH溶液中进行掺杂,制备出了聚乙烯醇/聚乙烯吡咯烷酮(PVA/PVP)碱性聚合物电解质膜.详尽考察了膜的组成、微观结构、热稳定性、离子电导率和甲醇吸收率.结果表明,PVA与PVP两者具有较好的相容性,当m(PVA)∶m(PVP)=1∶0.5时,膜断面致密、均匀,未发生大尺度相分离.PVP的混入可以极大提高复合膜的电导率和热稳定性.当m(PVA)∶m(PVP)=1∶1时,复合膜的电导率可达2.01×10-3 S.cm-1.PVA/PVP/KOH膜的甲醇吸收率随温度的升高没有明显变化,100℃时其甲醇吸收率仅为同条件下Nafion 115膜的1/4.这表明该复合膜有望作为一种新型的碱性直接甲醇燃料电池用固体电解质膜且可提高膜的使用温度.     

Eco-friendly composite resins based on renewable biomass resources: Polyfurfuryl alcohol/lignin thermosets

The properties of an innovative polyfurfuryl alcohol (PFA)/lignin combined matrix have been investigated. Furfuryl alcohol (FA) and lignin are, respectively, monomeric and polymeric precursors issued from biomass feedstock. In the present work, a plasticized lignin (PL) has been blended during polymerization of FA into PFA. Two kinds of samples were prepared at different FA/lignin ratio. Structural investigations were made on resins by ¹³C NMR while the thermo-mechanical performances of the combined materials were studied using thermogravimetric (TGA) and dynamic mechanical analysis (DMA). TGA results have permitted us to determine the thermal stability and the composition of the cured material on the basis of the ash content. According with these results, it was found that the lignin ratio in the cured material is more important than in the initial threshold. The TGA reveals that the PFA/PL thermo-oxidative degradation occurs at higher temperature compared to the natural (PL) component system, together with a lower rate of decomposition. This underlines a good interpenetration of lignin within the furanic matrix. The morphologies of the combined PFA/lignin systems, controlled by scanning electron microscopy (SEM), reveal a monophasic structure. These observations are in good agreement with the presence of a unique relaxation peak as shown in the DMA results.     

New insights on the thermal degradation pathways of neat poly(furfuryl alcohol) and poly(furfuryl alcohol)/SiO2 hybrid materials

Poly(furfuryl alcohol)/SiO₂ hybrid material was prepared and properties characterized in comparison with those of neat poly(furfuryl alcohol) (PFA) thermoset. A morphological study conducted by transmission electron microscopy (TEM) on the hybrid material reveals well-dispersed silica nanoclusters. Combination of micro- and submicro-structural organizations between the organic and inorganic networks generates new thermo-mechanical performances. A study of relaxation process by dynamic mechanical analysis (DMA) gives some evidence of the nanoconfinement effect on the hybrid network. These restrictions on the molecular mobility lead to an increase of thermal stability of the PFA/SiO₂ material compared to the unfilled matrix. Thermogravimetric analyses (TGAs) coupled with GC-MS have permitted us to highlight a multi-step degradation pathway and chain scission reactions are proposed based on identified VOCs.     

A novel composite Nafion membrane for direct alcohol fuel cells

Trifluoromethanesulfonic acid or triflate acid, chemical formula CF₃SO₃H, is regarded as one of the strongest acids and resembles Nafion^® in structure. Erbium triflate, a lanthanum salt of triflate, is thermally stable. This paper reports data on the formation of membranes by the fixation of erbium triflate salts (ErTfO) into the Nafion structure. Five different loadings of ErTfO were used to fabricate ErTfO/Nafion composite membranes and these were characterized, extensively for possible use in direct alcohol fuel cells. The membranes were characterized using XRD, TGA, FTIR, and for mechanical strength, water uptake, ion exchange capacity, alcohol uptake, swelling, proton conductivity, alcohol permeability and oxygen stability. The ErTfO/Nafion composite membranes reduced alcohol permeability by 77–80%. The proton conductivity of 3% ErTfO/Nafion composite membranes was 38% higher than that of a pure cast Nafion membrane. The oxygen stability of the ErTfO/Nafion composite membranes was higher than pure cast Nafion. However, the mechanical strength of 7% and 9% ErTfO/Nafion was lower than that of pure cast Nafion. The composite membrane was chemically stable and has potential for use in direct alcohol fuel cells.     

Characterization of PVdF-HFP/Nafion/AlO[OH]n composite membranes for direct methanol fuel cell (DMFC)

Poly-(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)/Nafion ionomer/aluminum oxy hydroxide nanocomposite membranes were prepared by phase inversion technique. The resultant membranes were subjected to protonic conductivity, methanol permeability, infra-red and thermogravimmetric analysis. The infra-red spectroscopic measurements revealed the presence of sulfonic acid groups in the composite membranes. The thermal stability and ionic conductivity of the polymer membranes have been greatly varied upon the addition of AlO[OH]_n. Although the PVDF-HFP/Nafion/AlO[OH]_n composite membranes have moderate protonic conductivity it has lower methanol permeability and may be considered as a candidate for DMFC applications.     

Preparation and characterization of inorganic–organic hybrid proton exchange membranes based on phosphorylated PVA and PEG-grafted silica particles

We reported proton-conducting membranes with novel microstructure based on partially phosphorylated poly(vinyl alcohol) (P-PVA) and polyethylene glycol (PEG) grafted silica (PEG-SiO₂) particles. The PEG-SiO₂ particles were synthesized through acid catalyzed hydrolysis and condensation reactions. The membranes were characterized for their mechanical, structural, morphological, and electrical properties by employing tensile test, Fourier transform infrared (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), impedance analyzer, respectively. In these membranes, P-PVA acts as the proton source and PEG act as the proton solvent. The PEG-riched phases in the hybrid membrane form continuous ionic conducting pathways and subsequently give high ionic conductivity. The results suggest that the obtained membrane shows good thermal stability, excellent mechanical property and high ionic conductivity, and the low-cost hybrid membrane can be a promising candidate for intermediate temperature fuel cell systems.     

燃料电池用聚乙烯醇基碱性聚合物电解质膜的制备及其性能

通过在不同浓度KOH溶液中进行掺杂,制备出了聚乙烯醇(PVA)、聚乙烯醇/聚乙烯吡咯烷酮(PVA/PVP)和聚乙烯醇/聚乙二醇二甲醚(PVA/PEGDE)碱性聚合物电解质膜详细考察了膜的外观形貌、微观结构、热稳定性、离子电导率和化学稳定性等.结果表明,PVA与PVP以及PEGDE具有很好的相容性,所制备的复合膜断面致密...     

SPEEK-zirconium hydrogen phosphate composite membranes with low methanol permeability prepared by electro-migration and in situ precipitation

Sulfonated poly(ether ether ketone) (SPK)-zirconium hydrogen phosphate (ZrP) composite membranes were prepared by electro-driven migration of Zr(4+) and simultaneous in situ precipitation of ZrP using phosphoric acid under different electrical gradient, in order to avoid loss in its mechanical stability. Degree of sulfonation was estimated from (1)H NMR and ion-exchange capacity study that was found to be 61% and 57%, respectively. In this method Zr(4+) and HPO(4)(2-) were allowed to diffuse within the pores/channels of the preformed SPK membrane under given electrical gradient and ZrP was precipitated within the membrane matrix. ZrP loading density was measured as a function of applied electrical gradient for a definite reaction time (4 h) and electrolytic environment. Membranes with varied ZrP loading densities were characterized for their thermal and mechanical stabilities, physicochemical and electrochemical properties using thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), water content, proton conductivity and methanol permeability. No loss in thermal and mechanical stability of membranes was observed due to incorporation of inorganic component (ZrP) in the membrane matrix. Although the composite membranes exhibited low proton conductivity in comparison to SPK membrane at room temperature, but the presence of the inorganic particles led to an improvement in high temperature conductivity. Selectivity parameter of these composite membranes was estimated at two temperatures namely 30 and 70 degrees C, in latter case it was found significantly higher than that for Nafion membrane (0.79 x 10(5) S s cm(-3)) under similar experimental conditions.     

Hybridization of Nafion membranes with an acid functionalised polysiloxane: Effect of morphology on water sorption and proton conductivity

Polysiloxane-modified hybrid Nafion membranes were prepared by casting a mixture of Nafion solution and a precursor of acid functionalised polysiloxane based on tetraethoxysilane and a mercaptan-organoalkoxysilane.Scanning Electron Microscopy (SEM) and Atomic Force Microscopy analysis revealed that the functionalised polysiloxane was dispersed either as finely nanosized inclusions or as coarse domains depending on the rate of the solvent evaporation during the casting procedure. In particular the slower is the rate of solvent evaporation the more interpenetrated and homogenously dispersed at nanosized level is the polysiloxane inside the Nafion membrane.The hybridization process increases the thermal stability of the membranes of about 50 °C relatively to the unmodified Nafion. Small angle X-ray scattering (SAXS) analysis reveals that the hybrid membranes exhibited the typical morphology of Nafion consisting of distinct hydrophilic and hydrophobic domains.Water vapor sorption and proton conductivity were measured varying the temperature (up to 120 °C) and the water activity conditions (from 0.1 to 0.8). The polysiloxane network always increases the water vapor uptake of the membranes and increases significantly the proton conductivity at higher temperature depending on the type of morphology developed by the manufacturing method. In particular hybrid membranes exhibiting nanosized polysiloxane dispersion show a proton conductivity which is up to one-and-half time higher than Nafion recast membrane at high temperature and low water content.     

Composite Polymer Electrolytes for Fuel Cell Applications: Filler-Induced Effect on Water Sorption and Transport Properties

Nafion- and sulfonated polysulfone (SPS)- based composite membranes were prepared by incorporation of SnO₂ nanoparticles in a wide range of loading (0 35 wt. %). The composites were investigated by differential scanning calorimetry, dynamic vapor sorption and electrochemical impedance spectroscopy to study the filler effect on water sorption, water mobility, and proton conductivity. A detrimental effect of the filler was observed on water mobility and proton conductivity of Nafion-based membranes. An increase in water mobility and proton conductivity was instead observed in SPS-based samples, particularly at low hydration degree. Analysis of the water sorption isotherms and states of water revealed that the presence of SnO₂ in SPS enhances interconnectivity of hydrophilic domains, while not affecting the Nafion microstructure. These results enable the design of suitable electrolyte materials that operate in proton exchange membrane fuel cell conditions.     

Thermal characterization and electrical properties of Fe-modified cellulose long fibers and micro crystalline cellulose

Thermal properties of polylactic acid (PLA) filled with Fe-modified cellulose long fibers (CLF) and microcrystalline cellulose (MCC) were studied using thermo gravimetric analysis (TG), differential scanning calorimetry, and dynamic mechanical analysis (DMA). The Fe-modified CLFs and MCCs were compared with unmodified samples to study the effect of modification with Fe on electrical conductivity. Results from TG showed that the degradation temperature was higher for all composites when compared to the pure PLA and that the PLA composites filled with unmodified celluloses resulted in the best thermal stability. No comparable difference was found in glass transition temperature (T _g) and melting temperature (T _m) between pure PLA and Fe-modified and unmodified CLF- and MCC-based PLA biocomposites. DMA results showed that the storage modulus in glassy state was increased for the biocomposites when compared to pure PLA. The results obtained from a femtostat showed that electrical conductivity of Fe-modified CLF and MCC samples were higher than that of unmodified samples, thus indicating that the prepared biocomposites have potential uses where conductive biopolymers are needed. These modified fibers can also be tailored for fiber orientation in a matrix when subjected to a magnetic field.     

Synthesis and Ex-situ Characterization of Nafion/TiO2 Composite Membranes for Direct Ethanol Fuel Cell

Nafion/TiO₂ composite membranes for different loadings of TiO₂ were prepared by casting method for the possible application in direct ethanol fuel cell (DEFC). The properties of the composite membranes were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), thermogravimetric analyser (TGA), ion exchange capacity, water and alcohol uptake, swelling ratio, proton conductivity, and ethanol crossover. The observed characteristics of the membranes were evaluated for DEFC and compared with the direct methanol fuel cell (DMFC) membrane. The analysis reveales a significant influence on the TiO₂ surface characteristics, water and alcohol uptake, and swelling of the membrane. The TiO₂ composite membranes exhibited a sharp decrease in methanol and ethanol crossover for 5% TiO₂ and the proton conductivity was heighest for 1% TiO₂ loading. The best compromise between proton conductivity and crossover has been found out with the help of the characteristic factor ϕ. The optimum loading of 5% TiO₂ composite membrane has shown the maximum characteristic factor.     

Performance assessment of hybrid multiblock copolymers included with ionic liquid for fuel cell applications

To improve the proton conductivity and thermal stability of proton exchange membrane, hybrid poly (arylene ether) multiblock copolymers were synthesized by using 6F-bisphenol A monomer. The hydrophobic oligomers poly (arylene ether sulfone) containing 6F-bisphenol A with varying molecular weight were copolymerised with hydrophilic oligomer disulfonated poly (arylene ether ketone) containing pendant carboxylic acid group to prepare multiblock copolymers. For further enhancing the proton conductivity, ionic liquid is embedded into the synthesized multiblock copolymers to fabricate the hybrid multiblock membranes. The ¹H NMR studies confirmed the synthesis of oligomers and multiblock copolymers whereas the FT-IR spectra revealed the interaction of ionic liquid with the multiblock copolymers. The proton conductivity of the membranes has also been examined at different temperatures and the activation energy required for the proton transport was calculated by using Arrhenius equation. At 30 °C, the maximum proton conductivity of 0.14 S/cm were shown by hybrid membrane (with 50% ionic liquid, 6FB1/I.L-50%), which is of 3.5 times greater than that of pristine 6FB1 membrane. Compared with pristine membranes, the hybrid membranes exhibit improved oxidative, thermal and mechanical stability. Moreover, the scanning electron microscopy (SEM) investigation depicts better phase separation in hybrid membranes than pristine membranes by forming ionic clusters. The membranes have been tested in H₂/O₂ fuel cell and their performance is compared with the state-of-art Nafion 117 membrane.     

Radiation synthesis of acrylic acid onto poly(tetraflouroethylene-perflourovinyl ether) film: Chemical modifications and electrical conductivity

Graft-polymerization of acrylic acid (AAc) monomer onto poly(tetraflouroethylene-perflouro vinyl ether) (PFA) copolymer film was carried out using gamma irradiation technique to synthesize grafted copolymer film PFA-g-PAAc (PFA-COOH). The effect of the dose on the degree of grafting of AAc onto PFA film was investigated. The results showed that the degree of grafting increases with increasing the irradiation dose. The grafted [PFA-COOH] film was chemically modified by reaction with aniline to produce modified [PFA-CO-NH-ph] film, followed by sulphonation reaction to introduce sulfonic acid (SO₃H) groups to get other modified [PFA-CO-NH-ph-SO₃H] film. The chemical structures of the grafted and modified films were identified by FT-IR, XRD, and SEM. It is of particular interest to measure the electrical conductivity of grafted and modified membranes as a function of degree of grafting. It was found that the conductivity of the grafted films increases with increasing the degree of grafting, however a slightly increase in conductivity was observed in [PFA-CO-NH-ph-SO₃H] sample. The electrical conductivity property of the modified PFA membranes suggests their possible use for fuel cell applications.     

Transport properties of composite membranes containing silicon dioxide and Nafion

Silicon dioxide (SiO₂) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO₂ membranes were fabricated with SiO₂ content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO₂ content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO₂ content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO₂ content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO₂ nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.     

Effect of SiO2 on relaxation phenomena and mechanism of ion conductivity of [Nafion/(SiO2)x] composite membranes

This report describes a study of the effect of SiO2 nanopowders on the mechanism of ionic motion and interactions taking place in hybrid inorganic-organic membranes based on Nafion. Five nanocomposite membranes of the formula [Nafion/(SiO2)x] with SiO2 ranging from 0 to 15 wt % were prepared by a solvent casting procedure. TG measurements demonstrated that the membranes are thermally stable up to 170 degrees C but with the loss water it changes the cluster environments and changes the conductivity properties. MDSC investigations in the 90-300 degrees C temperature range revealed the presence of three intense overlapping endothermal peaks indicated as I, II, and III. Peak I measures the order-disorder molecular rearrangement in hydrophilic polar clusters, II corresponds to the endothermic decomposition of -SO3 groups, and III describes the melting process in microcrystalline regions of hydrophobic fluorocarbon domains of the Nafion moiety. ESEM with EDAX measurements revealed that the membranes are homogeneous materials with smooth surfaces. DMA studies allowed us to measure two relaxation modes. The mechanical relaxation detected at ca. 100 degrees C is attributed to the motion of cluster aggregates of side chains and is diagnostic for R-SO3H...SiO2 nanocluster interactions. DMA disclosed that at SiO2/-SO3H (psi) molar ratios lower than 1.9, the oxoclusters act to restrict chain mobility of hydrophobic domains of Nafion and the dynamics inside polar cages of [Nafion/(SiO2)x] systems; at psi higher than 1.9, the oxoclusters reduce the cohesiveness of hydrophilic polar domains owing to a reduction in the density of cross-links. FT-IR and FT-Raman studies of the [Nafion/(SiO2)x] membranes indicated that the fluorocarbon chains of Nafion hydrophobic domains assume the typical helical conformation structure with a D(14pi/15) symmetry. These analyses revealed four different species of water domains embedded inside polar cages and their interconnecting channels: (a) bulk water [(H2O)n]; (b) water solvating the oxonium ions directly interacting with sulfonic acid groups [H3O+...SO3(-)-].(H2O)n; (c) water aggregates associated with H3O+ ions [H3O+.(H2O)n]; and (d) low associated water species in dimer form [(H2O)2]. The conductivity mechanism and relaxation events were investigated by broadband dielectric spectroscopy (BDS). [Nafion/(SiO2)x] nanocomposite membranes were found to possess two different molecular relaxation phenomena which are associated with the alpha-relaxation mode of PTFE-like fluorocarbon domains and the beta-relaxation mode of acid side groups of the Nafion component. Owing to their strong coupling, both these relaxation modes are diagnostic for the interactions between the polar groups of the Nafion host polymer and the (SiO2)x oxoclusters and play a determining role in the conductivity mechanism of the membranes. The studies support the proposal that long-range proton charge transfer in [Nafion/(SiO2)x] composites takes place due to a mechanism involving exchange of the proton between the four water domains. This latter proton transfer occurs owing to a subsequent combination of domain intersections resulting from the water domain fluctuations induced by the molecular relaxation events of host Nafion polymer.     

Optimal method for preparing sulfonated polyaryletherketones with high ion exchange capacity by acid-catalyzed crosslinking for proton exchange membrane fuel cells

Sulfonated polyaryletherketones (SPAEK) bearing four sulfonic acid groups on the phenyl side groups were synthesized. The benzophenone moiety of polymer backbone was further reduced to benzydrol group with sodium borohydride. The membranes were crosslinked by acid-catalyzed Friedel-Crafts reaction without sacrifice of sulfonic acid groups and ion exchange capacity (IEC) values. Crosslinked membranes with the same IEC value but different water uptake could be prepared. The optimal crosslinking condition was investigated to achieve lower water uptake, better chemical stability (Fenton's test), and higher proton conductivity. In addition, the hydrophilic ionic channels from originally course and disordered could be modified to be narrow and continuous by this crosslinking method. The crosslinked membranes, CS4PH-40-PEKOH (IEC = 2.4 meq./g), reduced water uptake from 200 to 88% and the weight loss was reduced from 11 to 5% during the Fenton test compared to uncrosslinked one (S4PH-40-PEK). The membrane showed comparable proton conductivity (0.01–0.19 S/cm) to Nafion 212 at 80°C from low to high relative humidity (RH). Single H₂/O₂ fuel cell based on the crosslinked SPAEK with catalyst loading of 0.25 mg/cm² (Pd/C) exhibited a peak power density of 220.3 mW/cm², which was close to that of Nafion 212 (214.0 mW/cm²) at 80°C under 53% RH. These membranes provide a good option as proton exchange membrane with high ion exchange capacity for fuel cells.     

Electrical and structural analysis of conductive polyaniline/polyacrylonitrile composites

Conducting composites of polyacrylonitrile (PAN) copolymer containing 10% mass ratio methylacrylate and dodecylbenzene sulfonic acid doped polyaniline (PANI-DBSA) were prepared by solution blending. Electrical properties of the blends were characterized by means of electrical conductivity measurements and the phase structures were investigated via scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR spectroscopy, differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA). It was found that the electrical conductivity of the composites increased with the increase of PANI-DBSA content and the percolation threshold lay around 3.2 wt%. DSC and DMA measurements showed that there was only one T_g for each blend and the values of T_g varied with the PANI-DBSA content, implying that the PANI-DBSA/PAN blend was at least partially compatible. The formation of the hydrogen bonding between the carbonyl groups in PAN copolymer and the imine groups in PANI-DBSA was identified by the FT-IR spectra. XRD demonstrated that the intrinsic layered arrangement of PANI-DBSA was disaggregated in the blends. Nanosize network structure of PANI-DBSA dispersing in PAN matrix and the so-called phase reverse occurring in the skin layer of the film samples at low PANI-DBSA loading were observed by SEM.     

Synthesis and characterization of SPE membrane based on sulfonated FEP‐g‐acrylic acid by radiation induced graft copolymerization for PEM fuel cell

A Novel solid polymer electrolyte (SPE) membrane containing both ? COOH and ? SO₃H group has been prepared by simultaneous method of radiation grafting of acrylic acid onto FEP followed by sulfonation. The presence of weakly acidic acrylic acid controls the swelling in water while ? SO₃H group provides conductivity due to its strongly ionic characteristic. FEP‐g‐acrylic acid and its sulfonated derivatives were characterized by their properties. While the mechanical properties decreased, other properties such as ion exchange capacity (IEC), water uptake and ionic conductivity increased with increase in graft content. These properties further changed on sulfonation. Acrylic acid being weakly acidic in nature, conductivity values of the grafted membrane were quite low. However, introduction of strong ? SO₃H group resulted in conductivity closer to Nafion 117. Few sulfonated membranes have been tested with respect to H₂/O₂ fuel cell performance. Short‐term fuel cell test for 100 hr gave a stable performance. These membranes are less expensive compared to Nafion. Copyright © 2004 John Wiley & Sons, Ltd.