Surface-modified Nafion membrane by trioctylphosphine-stabilized palladium nanoparticles for DMFC applications

Trioctylphosphine (TOP)/Pd composites have been synthesized and used as a methanol-barrier material to modify the surface of Nafion 115. The TOP/Pd composites have been applied to the surface of Nafion instead of being incorporated into the Nafion matrix, to provide the best chance of maintaining the inherent proton conductivity of Nafion. The properties of the TOP/Pd-modified membrane, in terms of its conductivity and methanol permeability, as well as the performance of the membrane electrode assembly (MEA) in direct methanol fuel cell (DMFC), have been analyzed and compared with those of bare Nafion. The DMFC performance of the TOP/Pd-modified membrane is somewhat better than that of the bare Nafion one at methanol concentration of 2 M and significantly better at a high concentration of 5 M. The TOP/Pd-modified membrane is able to operate the DMFC using a high concentration of methanol, which can satisfy the requirement to reduce the reactant volumes for portable applications as well as to achieve high performance. In contrast to bare Nafion, the TOP/Pd-modified membrane with its well-adhering and crack-free modified surface shows effect on reducing the methanol loss.     

Polyelectrolyte Nanocomposite Membranes Using Imidazole-Functionalized Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane, based on DuPont Nafion/imidazole-modified nanosilica (Im-Si), for direct methanol fuel cell applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of nanosilica, and negatively charged sulfonic acid groups of Nafion, new electrostatic interactions can be formed in the interface of Nafion and Im-Si which result in both lower methanol permeability and also higher proton conductivity. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy, thermogravimetry analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, water uptake, methanol permeability, and ion-exchange capacity, as well as proton conductivity. The Nafion/Im-Si membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, higher selectivity parameter in comparison to the neat Nafion or Nafion/silica membranes. The obtained results indicated that the Nafion/Im-Si membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method

Zeolite/Nafion composite membranes with high proton selectivity were successfully fabricated using the solution-casting method. The types of zeolites are nano-sized and large sized Na-ZSM-5, H-ZSM-5, and their ball-milled ones. Two different schemes of experiments were conducted depending on the type of solvent. In case of using as-received Nafion® ionomer dispersions, the experimental results clearly show that the proton conductivity of zeolite composite membrane using either H-type or Na-type ZSM-5 depends on the type of solvent. It is thought that when propanol and water as the solvents were used, more hydrophilic H-type ZSM-5 seems to have been more randomly dispersed into hydrophobic region rather than hydrophilic ionic clustered channels within Nafion. Therefore, H-type ZSM-5 existing near hydrophobic region seems to provide additional path for proton migration but weakening the mechanical strength. These composite membranes show higher water uptake than commercial Nafion® 115, strongly suggesting better water retention ability of zeolite. The most interesting result is that the methanol permeability has decreased with increasing zeolite contents even when the proton conductivity increased, and the proton selectivities of these composite membranes expressed as characteristic factor were higher than that of Nafion® 115. In case of using a mixture of high boiling point DMF and ethanol as the solvent, unlike the previous case where no DMF was used, the proton conductivity slightly dropped with increasing zeolite contents. These results should have been attributed to a blocking effect of zeolite particles surrounded by inversely oriented hydrophilic micelles of Nafion. However, the values of proton conductivity of most composite membranes were significantly higher than that of Nafion® 115, and methanol permeability also decreased with increasing zeolite contents. The significantly lower methanol permeability of the composite membrane fabricated with DMF as the solvent is probably due to the more effective blocking effect of H-ZSM-5 for ionic clustered channels as well as difficult transport of methanol through zeolite pores.In case of the composite membranes containing ZSM-5 with large crystal size, it is found that the methanol permeability has increased considerably with the increasing of zeolite contents due to void fractions between polymer phases and zeolite particles. In case of using ball-milled ZSM-5 with small crystal size, however, the value of characteristic factor tends to increase with increasing zeolite contents. Consequently, it is seen that the characteristic factor of Zeolite/Nafion composite membranes was much higher than Nafion® 115. The results obtained throughout this study strongly suggest that zeolites with small crystal size and high hydrophilicity are very prospective for composite membrane for direct methanol fuel cells in the future.     

Nafion-sulfonated organosilica composite membrane for all vanadium redox flow battery

A novel Nafion-sulfonated diphenyldimethoxysilane (N-sDDS) composite membrane is prepared and employed in vanadium redox flow battery (VRB). Ion exchange capacity, proton conductivity, water transport behavior, and the cell performances are characterized. Fourier transform-infrared and X-ray diffraction analysis indicate that the sulfonated diphenyldimethoxysilane (sDDS) particles are successfully introduced into the Nafion matrix. In VRB single cell test, the VRB with N-sDDS membrane exhibits nearly the same coulombic efficiency as the unmodified Nafion membrane, but higher voltage efficiency than that of the VRB with unmodified Nafion membrane. The VRB with N-sDDS composite membrane keeps a stable performance after 60 times charge–discharge test. In the self-discharge test, the VRB with the N-sDDS membrane presented a lower self-discharge rate than that of the VRB with Nafion membrane. All results show that the addition of s-DDS is a simple and efficient way to improve the conductivity of Nafion, and the composite membrane shows good potential use for VRB.     

Preparation and Characterization of Nafion/Sulfonated SiO2 Molecular Sieve Composite Membranes

A Nafion/sulfonated SiO₂ molecular sieve composite membrane was prepared by solution casting with sulfonated SiO₂ molecular sieve as the modifier. The ATR/FT-IR results showed that sulfonated SiO₂ molecular sieve did not change the structure of the membrane. The SEM and XRD results showed that the molecular sieve was distributed uniformly in the membrane. The proton conductivity, methanol permeability, water content, and swelling degree were measured. Compared with Nafion membrane, the composite membrane had higher water content and proton conductivity and lower methanol permeability. The overall performance was the best when the content of sulfonated SiO₂ molecular sieve was 5 wt%. These results indicated that Nafion/sulfonated SiO₂ molecular sieve composite membranes would be excellent candidate membrane materials for direct methanol fuel cell (DMFC) applications.     

Hybrid Nafion–inorganic oxides membrane doped with heteropolyacids for high temperature operation of proton exchange membrane fuel cell

Performance of the proton exchange membrane fuel cell (PEMFC) with composite Nafion–inorganic additives such as silicon oxide (SiO₂), titanium dioxide (TiO₂), tungsten oxide (WO₃), and SiO₂/phosphotungstic acid (PWA) has been studied for the operation of temperature of above 100 °C. These composite membranes are prepared by the way of blending of the inorganic oxides with Nafion solution by the recasting procedure. The physico-chemical properties studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques have proved the uniform and homogeneous distribution of these oxides and the consequent enhancement of crystalline character of these membranes. The thermogravimetry analysis (TGA) results have indicated that the additives TiO₂ and WO₃ have accelerated decomposition of the membrane at an earlier temperature than that of the Nafion membrane. The modified membranes have shown higher uptake of water relative to that of the unmodified membranes. The proton conductivity of the modified membranes, except that of the Nafion/TiO₂, is found to be close to that of the native Nafion membrane at high temperature and at 100% relative humidity (RH), however, it was much higher at low RH. The performance of these modified membranes in the PEMFC operated at 110 °C and 70% RH is better than that of Nafion membrane and is found in the order of Nafion/SiO₂/PWA > Nafion/SiO₂ > Nafion/WO₃ > Nafion/TiO₂.     

Nafion/mordenite hybrid membrane for high-temperature operation of polymer electrolyte membrane fuel cell

Nafion/mordenite hybrid membranes for the operation of polymer electrolyte membrane fuel cells (PEMFCs) above 100 °C were prepared by mixing of H⁺-form mordenite powder and perfluorosulfonylfluoride copolymer resin. PEMFC operation above 100 °C reduces CO poisoning as well as passivation of the Pt anode electrocatalyst by other condensable species. The physico-chemical properties of hybrid membranes were investigated by tensile strength and proton conductivity measurements. As the mordenite content increases at the high temperature region, the proton conductivity of hybrid membranes increased due to the late dehydration rate of existent water in the mordenite. Also, from the results of current–voltage relationship for single cells under 130 °C of operation condition, the hybrid membrane cell with 10 wt.% mordenite showed better performance than that of the others over the entire current density range. This result indicated that the existent water in the hybrid membrane containing 10 wt.% mordenite was higher than that with the others, thereby maintaining its conductivity. The Nafion/mordenite hybrid membrane prepared by this present method is thought to be a satisfactory polymer electrolyte membrane for PEMFC operation above 100 °C.     

Improving the Performance of Nafion®-Based Fuel Cell Membranes by Introducing Histidine Functionalized Carbon Nanotubes

A new polymer nanocomposite membrane based on Nafion and functionalized carbon nanotubes (CNTs) was developed for proton exchange membrane fuel cell (PEMFC) applications. Histidine, an imidazole-based amino acid, was used for modifying the surface of CNTs. The modification of CNTs was characterized by means of Fourier transform infrared spectroscopy (FTIR) and their Zeta potential. The imidazole groups, due to forming and breaking of hydrogen bonding, can facilitate proton transport across the polymer matrix by the Grotthuss mechanism. The final structure of the Nafion/CNT nanocomposites was investigated by small angle X-ray scattering (SAXS). The results confirm that the transport properties of the fabricated new membranes were significantly improved in comparison with unmodified and conventional Nafion® membranes. The power density of the imidazole-CNT (Im-CNT) Nafion® composite membranes was about three times more than Nafion® membranes. Also, the experimental results showed that the proton conductivity for the conventional Nafion® membranes decreased over 100°C but the conductivity for the Nafion®/Im-CNT remained at a nearly constant value above 100°C up to 120°C. Thus, the nanocomposite based on Nafion/imidazole functionalized CNT can be considered as an anhydrous PEMFC membrane for high-temperature applications.     

Investigation of the proton transport in nafion® membranes as a function of direction,temperature and relative humidity

The conductivity of Nafion, especially as a function of water content, represents an important parameter for its applications as membrane in electrochemical devices. All Nafion types (105, 112, 115 and 117) exhibit a favourite swelling direction, most probably the strain direction. For this direction Nafion 115 and 117 exhibit an increment up to 30 % higher than the other planar dimension. Conductance (ohm⁻¹) data obtained by impedance measurements along the three dimensions of the Nafion membranes do not show any remarkable anisotropy. Along all the directions, an activation energy of about 130 meV for the proton transport is found for all types of Nafion, after swelling. In the longitudinal geometry, the activation energy of the conductance has been investigated in the temperature range 300–350 K as a function of relative humidity of the environment, which affects the membrane water content. The conductance and its activation energy has been found increasing by decreasing water content. This effect is, however, more pronounced for Nafion 117 than for Nafion 105. Paper presented partly at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1996     

Sulfonated Aromatic Polymers and Organically Modified Montmorillonite Nanocomposite Membranes for Fuel Cells Applications

Aromatic polymers, such as sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO), sulfonated poly(ether ether ketone) (SPEEK), and sulfonated poly(ether sulfone) (SPES), at the optimum degrees of sulfonation (DS), are suggested and evaluated as alternatives to Nafion for direct methanol fuel cells (DMFCs) applications. To reduce the methanol cross-over, which decreases the efficiency of the cell, organically modified montmorillonite nanoclays (OMMT) were added at 1 wt% to the sulfonated matrices with the optimum DS. The X-ray diffraction (XRD) patterns of nanocomposite membranes proved that the nanoclay layers were exfoliated. The proton conductivity and methanol permeability of the membranes, as well as the ion-exchange capacity (IEC), were measured. The selectivity parameter, ratio of proton conductivity to methanol permeability, was identified at 25°C for the nanocomposite membranes and the results were compared with Nafion117. Finally, the DMFC performance tests were investigated at 70°C and 5 M methanol feed for the manufactured nanocomposite polyelectrolyte membranes (PEMs). The SPEEK-based nanocomposite membrane showed the highest maximum power density in comparison with Nafion 117 and SPES and SPPO nanocomposite membranes. The results indicated that the nanocomposite membranes were promising PEMs for DMFC applications.     

Proton transport through aqueous Nafion membrane

We introduce a new model for proton transport through a single proton-conducting channel of an aqueous Nafion membrane based on a mechanism in which protons move under electrostatic effect provided by the sulfonate ( SO₃ ^-groups of the Nafion side chains, the spin effect of active components, the hydrogen bonding effect with water molecules, and the screening effect of water media. This model can describe the proton transport within various levels of humidification ranging from the low humidity to the high humidity as a function of operating temperature. At low humidity, this model approaches to the so-called surface mechanism, while at high humidity, it approaches the well-known Grotthuss one. Proton motion is considered as the transfer from cluster to cluster under a potential energy. A proton-proton interaction is comprised in the calculation. Using Green function method, we obtained the proton current as a function of the Nafion membrane temperature. We found that the lower the temperature, the higher the proton current transfer through the Nafion membrane in low temperatures compared to the critical point 10K, which separates magnetic regime from non-magnetic regime. The increasing of proton current at very low temperatures is attributed to the spin effect. As the membrane temperature is higher than 40 ^° C , the decreasing of proton current is attributed to the loss of water uptake and the polymer contraction. The results of this study are qualitatively in good agreement with experiments. The expression for the critical temperature is also presented as a function of structural and tunable parameters, and interpreted by experimental data.     

Potential-dependent electrolyte resistance and steady-state multiplicities of PEM fuel cells

The role of the non-linear conductivity of fully hydrated Nafion membranes is discussed on the performance of state-of-the-art PEM fuel cells. It is shown that the Nafion conductivity contains two components, one constant corresponding to proton transport in the aqueous phase of the membrane, the other exponentially dependent on potential, linearly increasing with membrane thickness and strongly increasing with hydrogen partial pressure at the anode.A simple macroscopic mathematical model, accounting for the observed exponential dependence of Nafion conductivity on potential is developed and shown to provide a semiquantitative fit to the experimental I–U curves.     

Transport properties of modified Nafion membranes: Effect of zeolite and precursors

Fe-silicalite/Nafion composite membranes with high relative selectivity (as defined by the proton conductivity to methanol permeability ratio) of 5.4 and proton conductivity of 11 mS cm^− 1 were prepared by in situ hydrothermal synthesis of the zeolite within the pores of Nafion membranes. The effects of the zeolite structure and precursor structure were evaluated in terms of transport properties and acidity levels for a series of Nafion membranes modified with silica and tetrapropylammonium (TPA) and tetrabutylammonium (TBA) cations. Introduction of up to 40% (w/w) of silica vs. pure Nafion shows little effect on the transport and acidity properties of the composite membranes. Introduction of tetraalkylammonium (TAA) cations reduces water uptake of the membranes, and results in the appearance of protons that are inaccessible for titration in water media. The selectivity of the composite membranes increases in the order: SiO₂/Nafion < TAA/Nafion < Fe-silicalite/Nafion.     

Incorporation of Ag nanoparticles into membrane mimetic systems composed by phospholipid layer‐by‐layer (LbL) films to achieve surface‐enhanced Raman scattering as a tool in drug interaction studies

The synergistic effect produced by nanoparticles when incorporated into different systems used as analytical tools represents a growing research field nowadays. On the other hand, the study of interactions involving pharmacological drugs and biological membranes using phospholipids as mimetic systems is a research field already well established. Here, we combine both the anionic phospholipid dipalmitoyl phosphatidyl glycerol (DPPG) and negative Ag nanoparticles (AgNP) to form layer‐by‐layer (LbL) multilayered films using the cationic polymer poly(allylamine hydrochloride) (PAH) as the supporting polyelectrolyte, which were further investigated in the presence of a phenothiazine compound (methylene blue—MB). The molecular architecture of the LbL films in terms of controlled growth, morphology with micro and nanometer spatial resolutions, and dispersion of both AgNP and MB within the DPPG matrix was determined combining spectroscopy [ultraviolet–visible (UV–Vis) absorption and micro‐Raman spectroscopy] and microscopy [scanning electron microscopy (SEM) and atomic force microscopy (AFM)]. The results showed that the LbL films can be grown in a controlled way at nanometer thickness scale with the surface morphology susceptible to the presence of both AgNP and MB. The surface‐enhanced phenomenon was applied to investigate the LbL films taking the advantage of the strong surface‐enhanced resonance Raman scattering (SERRS) signal presented by the MB molecules. Besides, as MB is a pharmacological drug of interest, its molecular arrangements when dispersed in LbL films containing DPPG, which is the biological membrane mimetic system here, were investigated. In this case, the AgNP played a key role in achieving the MB SERRS signal. Copyright © 2009 John Wiley & Sons, Ltd.     

Novel cross-linked membrane for direct methanol fuel cell application: sulfonated poly(ether ether nitrile)s

In order to reduce water uptake, swelling ratio, and methanol permeability in sulfonated proton exchange membranes (PEM), novel-sulfonated aromatic poly(ether ether nitrile)s-bearing pendant propenyl groups had been synthesized by direct copolymerization method. All the results showed that the propenyl groups were suitable cross-linkable groups, and that this method was an effective way to overcome the drawbacks of sulfonated polymers at high ion exchange capacity (IEC) values. By cross-linking, the water uptake, swelling ratio, and methanol diffusion could be restricted owing to the formation of compact network structure. For example, CSPEN-60 membranes showed the proton conductivity of 0.072 S cm^?1 at 80 °C, while the swelling ratios and water uptake (17.9 and 60.7 %) were much lower than that of the SPEN-60 membrane (60.8 and 295.2 %). Meanwhile, a 1.1 × 10^?7 cm² s^?1 of methanol diffusion was obtained which was much lower than that of Nafion 117 (14.1 × 10^?7 cm² s^?1). Although the proton conductivity of the CSPEN-60 membranes is lower than that of the SPEN-60 membrane, the selectivity is much higher. The CSPEN-60 membrane exhibited the highest selectivity among the tested membranes, about 5.8 times higher compared with that of Nafion117.     

Preparation and characterization of proton exchange membranes with through-membrane proton conducting channels

The primary goal of this study is to develop a novel PEMs with unique surface structure utilizing the high viscosity of the impregnation solution. SiO₂ nanofiber mats were prepared via the electrospinning method and introduced into sulfonated poly(ether sulfone) (SPES) matrix to prepare hybrid membrane. The effect of concentration of impregnation solution on the morphology and properties of the proton exchange membranes (PEMs), including thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeability were investigated. SEM results showed that a unique surface structure was prepared due to the high solution concentration. Moreover, the hydrophilic nanofibers on the surface constructed continuous proton pathways, which can enhance the proton conductivity of the membranes, a maximum proton conductivity of 0.125 S/cm was obtained when the SPES concentration was 40 wt% at 80 °C, and the conductivity was improved about 1.95 times compared to that of pure SPES membrane. The SiO₂ nanofiber mat-supported hybrid membrane could be used as PEMs for fuel cell applications.     

The surface modified composite layer formation with boron carbide particles on magnesium alloy surfaces through pulse gas tungsten arc treatment

A novel fabrication process of surface modified composite layer by pulse current gas tungsten arc (GTA) surface modification process was used to deposit B₄C particles on the surface of magnesium alloy AZ31. This method is an effective technique in producing a high performance surface modified composite layer. During the pulse current GTA surface modification process, considerable convection can exist in the molten pool due to various driving forces and the pulse current could cause violent stirring in the molten pool, and the large temperature gradient across the boundary between the GTA modified surface and matrix metal resulted in rapid resolidification with high cooling rates in the molten pool, so that the process result notable grain refinement in the GTA surface modified composite layer. The hardness and wear resistance of the GTA surface modified composite layer are superior to that of as-received magnesium alloy AZ31. The hardness values and wear resistance of GTA surface modified composite layer depend on the GTA process parameters and the B₄C particles powder concentration and distribution. The optimum processing parameters for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.     

Strong correlation between membrane effective fixed charge and proton conductivity in the sulfonated polysulfone cation-exchange membranes

The effectiveness of fixed charge in sulfonated polysulfone membranes and its correlation with proton conductivity and physicochemical properties have been investigated in this work. The membranes were prepared with various concentrations of sulfonating agent (6% to 10% v/v) and followed by the characterizations that include membrane potential measurements, proton conductivity, and physicochemical properties (contact angle, water uptake, and ion-exchange capacity). Here, the effective fixed-charge concentrations of the membranes were obtained based on the data of membrane potential measurements using the Teorell–Meyer–Siever equation. The analysis results exhibit that a strong correlation between effective charge concentration and proton conductivity, which is expressed by the linear increase of proton conductivity with QX. This correlation is also supported by the membranes physicochemical data, such as water uptake, ionic exchange capacity, surface contact angle against water and functional analysis using FTIR. Finally, it was also developed an ionic conductivity equation that describes the correlation between proton conductivity and QX values.     

Polyelectrolyte Nanocomposite Membranes with Imidazole-Functionalized Multi-Walled Carbon Nanotubes for Use in Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® and imidazole modified multi-walled carbon nanotubes (MWCNT-Im), for direct methanol fuel cell (DMFC) applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of multi-walled carbon nanotubes (MWCNT), and the negatively charged sulfonic acid groups of Nafion®, new electrostatic interactions can be formed in the interface of the Nafion® and MWCNT-Im which result in both lower methanol permeability and also higher proton conductivity. The physical characteristics of these manufactured nanocomposite membranes were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/MWCNT-Im membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter in comparison to neat Nafion® or Nafion® containing –OH functionalized multi-walled carbon nanotubes (MWCNT-OH) membranes. The obtained results indicated that the Nafion®/MWCNT-Im membranes could be utilized as efficient polyelectrolyte membranes for direct methanol fuel cell applications.     

Computer Simulation Study of Model Nafion Membrane in Water/Methanol Solvent

Using NPT and NVT molecular dynamics simulation techniques, we have simulated an atomistic model of solvated Nafion in the lithium salt form, with the following three main objectives: (i) to obtain details on the local environment of the lithium cations and to assess the solvent effect on their binding, (ii) to investigate the translational and rotational motion of solvent molecules (water and methanol) absorbed in the polymer matrix, and (iii) to elucidate details of the ionic transport though the hydrophilic regions of the membrane and to study the ionic conductivity as a function of solvent (water/methanol) composition. A property which is of central importance for understanding the functional features of Nafion materials, including direct methanol fuel cells, is the ionic conductivity and methanol crossover. We have found that conductivity parameter is strongly dependent on the solvent composition and determined by the solvation effects and the spatial distribution of polar sulfonate groups in ion-conductive channels.