Nafion Phase Research Articles

Morphology of Thin-Film Nafion on Carbon as an Analogue of Fuel Cell Catalyst Layers.

Species transport in thin-film Nafion heavily influences proton-exchange membrane (PEMFC) performance, particularly in low-platinum-loaded cells. Literature suggests that phase-segregated nanostructures in hydrated Nafion thin films can reduce species mobility and increase transport losses in cathode catalyst layers. However, these structures have primarily been observed at silicon-Nafion interfaces rather than at more relevant material (e.g., Pt and carbon black) interfaces. In this work, we use neutron reflectometry and X-ray photoelectron spectroscopy to investigate carbon-supported Nafion thin films. Measurements were taken in humidified environments for Nafion thin films (≈30-80 nm) on four different carbon substrates. Results show a variety of interfacial morphologies in carbon-supported Nafion. Differences in carbon samples' roughness, surface chemistry, and hydrophilicity suggest that thin-film Nafion phase segregation is impacted by multiple substrate characteristics. For instance, hydrophilic substrates with smooth surfaces correlate with a high likelihood of lamellar phase segregation parallel to the substrate. When present, the lamellar structures are less pronounced than those observed at silicon oxide interfaces. Local oscillations in water volume fraction for the lamellae were less severe, and the lamellae were thinner and were not observed when the water was removed, all in contrast to Nafion-silicon interfaces. For hydrophobic and rough samples, phase segregation was more isotropic rather than lamellar. Results suggest that Nafion in PEMFC catalyst layers is less influenced by the interface compared with thin films on silicon. Despite this, our results demonstrate that neutron reflectometry measurements of silicon-Nafion interfaces are valuable for PEMFC performance predictions, as water uptake in the majority Nafion layers (i.e., the uniformly hydrated region beyond the lamellar region) trends similarly with thickness, regardless of support material.

Study on the Nanostructure of Nafion Ultra-Thin Film at Pt/C Catalyst Layer in PEMFC: Molecular Dynamics Simulation Approaches

In this study, the hydrated Nafion film in the CL of the cathode for PEMFC is investigated using molecular dynamics simulation (MD) method, exhibiting different structural characteristics on the Pt and carbon surfaces. First, it is found that water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and Pt surface, whereas Nafion backbone chains are present in a high concentration at the interface between Nafion phase and carbon surface. Second, it is also found from pair correlation function analysis that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface, which is due to their strong attractive interactions with the Pt surface that makes the dimension of hydrated Nafion phase 4–7% thinner on the Pt surface. Third, it is observed from water-occupied volume analysis that water molecules on the carbon surface can form a large-size water phase between the Nafion phase and carbon surface because the Nafion-carbon interface is not tightly integrated due to their weak interaction. In these structural characteristics, it is demonstrated that the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in water phase as well as polymer phase in addition to the strong molecular interaction with the Pt surface, whereas the proton hopping diffusion is enhanced due to the well-developed organized water phase via hydrogen bonding network.

Pore-Scale Modeling of the Fuel Cell Catalyst Layer: The Role of Nafion Thin Films

In this work, a multiphysics pore-scale model of electrochemical reactive transport inside a fuel cell catalyst layer (CL) is presented and is used to elucidate the role of Nafion thin films on the overall performance of the CL. One of the main challenges of modeling transport in CL is the hierarchy of length scales and the multiphysics nature of transport. Consequently, traditional continuum methods, in which volume-averaged quantities are used to encapsulate the microstructural features, cannot give a clear picture of the multiphysics transport processes inside the CL. Alternatively, pore-scale methods, which take into account the true geometry down to the pore level, are suited to capture these processes for such systems. Previously, researchers have mainly employed direct numerical simulation (DNS) techniques for pore-scale modeling of the CL. While these techniques are highly accurate, they are very computationally expensive. This limits the domain size that can be simulated in a reasonable time. In this work, pore network modeling, a computationally cheap technique to simulate pore-scale transport, is used to study transport in the CL. Using this approach, we were able to speed up the simulation runtime up to 20-50x compared to DNS. Consequently, simulation of much larger domains of the CL was made possible, leading to more representative results. Finally, the developed multiphysics pore-scale model was used to study the role of Nafion thin films on the overall performance of the CL. It is notable that the network approach was only applied to the void phase, and the Nafion region was resolved using DNS. Therefore, yet further speed up can potentially be obtained by applying the network approach to the Nafion phase.

Structure and dynamics of microbial fuel cell catalyst layer

Molecular dynamics method is used to investigate the mass transfer rules of reactants H3O+ and O2 in the cathode catalyst layer in the presence of five kinds of cations (Na+, K+, NH4+, Mg2+, Ca2+) in the microbial fuel cell solution environment. Structural characteristics of the catalyst layer Pt/C substrate/Nafion/solution three-phase interface are also analyzed. The results show that the transport mechanism and pathway of H3O+ in the catalyst layer are similar to those of monovalent cations (Na+, K+, NH4+). The minimum diffusion coefficient of H3O+ appears in the presence of K+. On the other hand, H3O+ mainly transport inside the water clusters in the presence of divalent cations (Mg2+, Ca2+). The diffusion law of metal cations (K+> Na+> Mg2+> Ca2+) is still applicable in the catalyst layer containing Nafion ionomer and unaffected by cation concentration. In general, the higher concentration of O2 molecules in the main part of the Nafion phase and the farther distribution from the carbon substrate cause a larger O2 diffusion coefficient. Moreover, the O2 transport pathways in the main part of the Nafion phase are along the Nafion hydrophilic/hydrophobic phase interface and inside the hydrophobic phase. In the presence of Ca2+, the concentration of O2 near the Pt particles is the highest. These O2 molecules tend to adsorb on the surface of Pt particles with small size. Thus, the catalyst utilization rate is high. In addition, Ca2+ has a strong cross-linking effect on the sulfonic acid groups from different Nafion molecules, which helps to enhance the stability of the catalyst layer.

Development of a Semi-Empirical Model for Description of PEMFC Polarization Curve Based on Effect of Fuel Crossover and Internal Current Through a PEM

The manuscript presents a new semi-empirical cell voltage model of performance of PEMFC based on Srinivasan et al’ s model with addition of an extra term to take account the effect of fuel crossover and internal current through membrane using numerical simulations developed under CFD software. This semi empirical model describes the effect of charge and mass transport (O2, H2 and e-) through a PEM such as permeability, solubility, and internal current on polarization curves. The permeability within hydrated Nafion were obtained based on the assumption that the gas solubility and gas diffusivity were determined as the volume averaged value based on the solubility, diffusivity and volume fraction of both water and Nafion phase. The effect of current exchange density on polarization curves was studied. The obtained results show that overpoential can be reduced by using thicker membrane; in addition temperature and water content involve over potential. Indeed, internal currents and fuel crossover increase as temperature, water content increase while OCV decreases with decreasing in current exchange density.

Hidden Thermoreversible Actuation Behavior of Nafion and Its Morphological Origin

Recent morphological characterization of Nafion as correlated to its proton transport properties has led to the conclusion that the structural ordering in the ionic phase of Nafion may be quite similar to liquid crystalline polymers. Liquid crystalline polymers, on the other hand, are well-known for their thermoreversible actuation (or two-way shape memory) behavior. This has motivated us to design and conduct thermomechanical experiments to probe Nafion’s potential liquid crystalline characteristics from the standpoint of thermoreversible actuation. Our experimental results surprisingly revealed the thermoreversible actuation behavior of Nafion, buried alongside the irreversible creep behavior commonly observed for some polymers. As such, the current study provides evidence supporting Nafion’s structural similarity to liquid crystalline polymers.

In Situ Fabrication of Nafion–Titanate Hybrid Electrolytes for High-Temperature Direct Ethanol Fuel Cell

The synthesis and characterization of a novel Nafion-matrix nanocomposite by the in situ conversion of titania particles into titanate nanofilaments such as nanotubes and nanorods are reported. Titania nanoparticles grown inside Nafion hydrophilic domains were converted by a microwave-assisted hydrothermal reaction into the proton conducting titanate nanotubes and nanorods. Detailed characterization by Raman spectroscopy, X-ray diffraction, small-angle X-ray scattering, and transmission electron microscopy evidenced an intimate interaction between titanate nanostructures and the ionomer hydrophilic phase. The favored localization of such high aspect ratio nanofilaments in the ionic phase of Nafion has a marked impact on the properties of the composites. The experimental data showed enhanced mechanical stability at high temperature (∼130 °C) that was correlated to a strong temperature dependence of the proton conductivity in the same temperature range. Such properties contributed to a significant increase ...

Influence of LbL surface modification on oxygen cross-over in self-assembled thin composite membranes

The paper is concerned with the investigation of oxygen transport in layer-by-layer (LbL) self assembly of polystyrene sulfonic acid sodium salt (PSS) and polyallylamine hydrochloride (PAH) on Nafion membrane depending on the number of deposited bilayers, ion type within the multilayers and temperature. It is observed from SEM analysis that the polyelectrolyte layers growth on each side of Nafion membrane regularly. The oxygen permeability (PO2) of (PAH–PSS)20 is 691cm3/(m2daybar) at 25°C while PO2 of pristine Nafion is 2329cm3/(m2daybar) at the same temperature. This significant reduction (70.3%) in gas permeability can be explained by the formation of LbL multilayers which both restrict the mobility of gas through the polymer matrix and adjust gas solubility properties in conjunction with ion–dipole interaction between the polar phase of Nafion and the secondary ammonium groups of PAH. (PAH/PSS)20–Na+ and (PAH/PSS)20–H+ exhibit 64.4% and 52.3% reduction in oxygen permittivity at 25°C in comparison with the pristine Nafion®117, respectively, while the proton conductivities of these membranes are 106.9 and 136.9mS/cm. Promisingly, it is found that the membrane selectivity values (Φ) of all multilayered membranes in both H+ and Na+ form are much higher than that of perfluorosulfonated ionomer. The thickness of deposited bilayers dominates the diffusion and solubility properties of oxygen through the composite membrane while the major effect of surface hydrophilicity is not observed on oxygen permeability.

Bulk modification of Nafion ® with purple membrane for direct methanol fuel cell applications

Novel methanol-blocking polymer electrolyte composite membranes were prepared by inserting purple membrane (PM) to the bulk phase of Nafion ® resin. Results of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) analyses show that PM was well-dispersed in Nafion ® membrane. Compared with recast Nafion ®, blending PM changed the microstructure of Nafion ® membrane and significantly improved the methanol-blocking efficiency by 31.6% in maximum. With PM content of 0.3 wt% in the composite, the highest membrane selectivity factor and single cell power density of 62.1 mW cm −2 were achieved. This satisfactory cell performance has given a promising prospect of Nafion ®/PM composite membranes in the application in DMFC.

Modeling an ordered nanostructured cathode catalyst layer for proton exchange membrane fuel cells

A 3D mathematical model of an ordered nanostructured cathode catalyst layer (CCL) has been developed for proton exchange membrane (PEM) fuel cells. In an ordered nanostructured CCL, carbon nanotubes (CNTs) are used as support material for Pt catalyst, upon which a thin layer of proton-conducting polymer (Nafion) is deposited, which are then aligned along the main transport direction (perpendicular to the membrane) of various species. The model considers all the relevant processes in different phases of an ordered nanostructured CCL. In addition, the effect of Knudsen diffusion is accounted in the model. The model can predict not only the performance of an ordered nanostructured CCL at various operating and design conditions but also can predict the distributions of various fields in different phases of an ordered nanostructured CCL. The predicted nanostructured CCL performance with estimated membrane overpotential is validated with measured data found in the literature, and a good agreement is obtained between the model prediction and measured result. Moreover, a parametric study is conducted to investigate the effect of key design parameters on the performance of an ordered nanostructured CCL. In the absence of liquid water, it is found that oxygen diffusion in the pore phase is not the limiting factor for the performance of an ordered nanostructured CCL, owing to its parallel gas pores and high porosity. However, the transport of dissolved oxygen through the Nafion phase has a significant effect on the performance of an ordered nanostructured CCL. Further, it is found that increasing the spacing between CNTs results in a considerable drop in the performance of an ordered nanostructured CCL at the base case conditions considered in the simulation.

Preparation and characterization of nanocomposite polyelectrolyte membranes based on Nafion® ionomer and nanocrystalline hydroxyapatite

In this study nanocrystalline hydroxyapatite (nHA) was synthesized and characterized by means of FT-IR, XRD and TEM techniques and a series of proton exchange membranes based on Nafion® and nHA were fabricated via solvent casting method. Thermogravimetric analysis confirmed thermal stability enhancement of the Nafion® nanocomposite due to the presence of nHA nanopowder. SAXS and TEM analyses confirmed the incorporation of nHA into ionic phase of Nafion®. Furthermore, the incorporation of elliptical nHA into the Nafion® matrix improved proton conductivity of the resultant polyelectrolyte membrane up to 0.173 S cm−1 at 2.0 wt% of nHA loading compared to that of 0.086 S cm−1 for Nafion® 117. Also, the inclusion of nHA nanoparticles into nanocomposite membranes resulted in a significant reduction of methanol permeability and crossover in comparison with pristine Nafion® membranes. Membrane selectivity parameter of the nanocomposites at 2.0 wt% nHA was calculated and found to be 106,800 S s cm−3, which is more than two times than that of Nafion® 117. Direct methanol fuel cell tests revealed that Nafion®/nHA nanocomposite membranes were able to provide higher fuel cell efficiency and also better electrochemical performance in both low and high concentrations of methanol feed. Thus, the current study shows that nHA enhances the functionality of Nafion® as fuel cell membranes.

Phosphotungstic acid (HPW) molecules anchored in the bulk of Nafion as methanol-blocking membrane for direct methanol fuel cells

In the present study, a novel composite methanol-blocking polymer electrolyte membrane was prepared by anchoring water-soluble phosphotungstic acid (HPW) in bulk phase of Nafion through interactions between HPW and Cs+ ions. Results of morphology and elemental mapping analysis indicated that this composite membrane had a dense structure with uniformly distributed Cs–HPW clusters. The composite membrane exhibited a rather low methanol diffusion coefficient (P, 0.97×10−6cm2s−1) and also the comparable conductivity (σ, 3.6×10−2Scm−1) contrast with pristine Nafion (σ, 5.1×10−2Scm−1). The selective factor (σ/P) of the composite membrane was six times higher than that of pristine Nafion. Single cell performance results showed that the maximum power density increased by 26% over the cell performance of pristine Nafion under the same conditions.

The effect of self-assembled multilayer formation via LbL technique on thermomechanical and transport properties of Nafion ®112 based composite membranes for PEM fuel cells

Highly proton conductive and mechanically stable Nafion ®112 based composite membranes with suppressed methanol permeability were prepared via Layer-by-Layer (LbL) technique for fuel cell applications. It was observed from UV–vis results that the polyelectrolyte layers growth on Nafion membrane surface regularly. (PAH/PSS) 20–H + with 0.01 M NaCl exhibited 39.9% reduction in methanol permittivity and nearly 2 times higher proton conductivity (0.152 S cm −1) than that of untreated Nafion ®112. Promisingly, it was found that the membrane selectivity ( Φ) values of all multilayered composite membranes in proton form prepared from polyelectrolytes comprising NaCl were higher than that of both untreated and chemically treated Nafion ®112. Furthermore, the mechanical and thermal properties of the composite membrane obtained from polyelectrolytes free salt were enhanced due to the interaction between the polar phase of Nafion and anionic/cationic charges of polyions.

Solvent effects during multicomponent ion uptake into a Nafion cation-exchange membrane

A multicomponent ion partition coefficient model has been used to predict competitive alkali metal ion uptake into a Nafion ® cation-exchange membrane from solvents composed of methanol, methanol/water, and acetonitrile/water. The analysis was similar to that used previously for monovalent and divalent uptake and transport in Nafion, where the membrane was modeled as an array of parallel cylindrical pores of constant radius with a continuous distribution of fixed charges along the pore wall. Ion–ion electrostatic interactions, electric-field induced solvent dipole alignment, and ion solvation free energy effects were taken into account. In the present study, the model was modified to account for a different solvent composition in the membrane, as compared to that in the external solution (solvent uptake data were collected experimentally). The model accurately predicted competitive K +/Na + uptake from methanol and methanol/water mixed solvents and Cs +/K + uptake selectivities for acetonitrile/water solvents. In the latter case, model calculations suggested that some acetonitrile absorbed into the amorphous PTFE phase of Nafion, when the external acetonitrile concentration was greater than 33 vol%. The addition of water to methanol caused a drop in the potassium/sodium selectivity (to its pure water value), whereas the addition of acetonitrile to water caused the Cs +/K + selectivity to decrease.

Domain Size Manipulation of Perflouorinated Polymer Electrolytes by Sulfonic Acid-Functionalized MWCNTs To Enhance Fuel Cell Performance

The application of sulfonic acid-functionalized multiwalled (s-MWNT) carbon nanotubes to manipulate the hydrophilic domain size of Nafion membranes is explored here as an option for tuning the proton conductivity of polymer electrolyte membranes for hydrogen-oxygen fuel cells. The electrochemical impedance experiments provide preliminary evidence of increased proton conductivity, while small-angle X-ray scattering measurements line out enhanced ionic cluster domain size in these composite membranes as the central reason for higher conductivity (70 A for the optimum composite membrane vs 50 A for Nafion 115) values. Scanning electrochemical microscopy indicates synergistic interaction between the sulfonic acid functional groups present in the Nafion membrane and those on the nanotube surface. More interestingly, the nanotube-tailored Nafion membranes ameliorate the performance of fuel cells as confirmed by measurements at a single-cell level, which reveal a maximum power density of 380 mW cm(-2), higher than those of Nafion 115 (250 mW cm(-2)) and recast Nafion (230 mW cm(-2)) membranes. Thus, in addition to providing an elegant means of controlling the ionic cluster size, the strategic approach of using CNT both as an anchoring backbone for -SO(3)H groups to enrich proton conductivity and as a blending agent to improve the mechanical characteristics of the Nafion phase might be helpful in alleviating many critical problems associated with the use of commercial Nafion membranes.

Synthesis of platinum nanoparticles using electrostatic stabilization and cluster duplication of perfluorinated ionomer

Platinum (Pt) nanoparticles were prepared by the liquid-phase reduction of tetraammineplatinum (II) chloride ([Pt(NH3)4]Cl2) using Nafion as a stabilizer under various conditions of the Nafion phase. This method is novel in its use of electrostatic interactions between the Pt complex ions and sulfonic groups in the hydrated Nafion molecules. The synthesized Pt nanoparticles of the recast film system had a cubic shape. In the case of the Nafion solution system, the Pt nanoparticles mainly had a spherical shape. The shapes and sizes of the Pt nanoparticles were strongly influenced by the Nafion phase.

Water Permeation through Nafion Membranes: The Role of Water Activity

The permeation of water through 1100 equivalent weight Nation membranes has been measured for film thicknesses of 51-254 microm, temperatures of 30-80 degrees C, and water activities (a(w)) from 0.3 to 1 (liquid water). Water permeation coefficients increased with water content in Nafion. For feed side water activity in the range 0 < a(w) < 0.8, permeation coefficients increased linearly with water activity and scaled inversely with membrane thickness. The permeation coefficients were independent of membrane thickness when the feed side of the membrane was in contact with liquid water (a(w) = 1). The permeation coefficient for a 127 microm thick membrane increased by a factor of 10 between contacting the feed side of the membrane to water vapor (a(w) = 0.9) compared to liquid water (a(w) = 1). Water permeation couples interfacial transport across the fluid membrane interface with water transport through the hydrophilic phase of Nafion. At low water activity the hydrophilic volume fraction is small and permeation is limited by water diffusion. The volume fraction of the hydrophilic phase increases with water activity, increasing water transport. As a(w) --> 1, the effective transport rate increased by almost an order of magnitude, resulting in a change of the limiting transport resistance from water permeation across the membrane to interfacial mass transport at the gas/membrane interface.

Analysis of a cathode catalyst layer model for a polymer electrolyte fuel cell

A macroscopic model for a non-isothermal cathode catalyst layer (CL) in a proton exchange membrane (PEM) fuel cell is presented, in which liquid water in the CL pores is neglected. The model couples three phases: an electrically conductive carbon/platinum phase, gas pores, and a proton-conducting Nafion phase. The reaction–diffusion dynamics are described by a nonlinear system of differential equations which, due to the wide range of physical parameters, is very stiff. To reduce the stiffness of the system, an appropriate scaled model is introduced. A numerical algorithm consisting of two embedded Newton loops, specifically developed to handle the stiffness, is presented. Several numerical results are used to investigate the dynamics of the layer and the validity of the interface reduction, or zero CL thickness limit, which is often used in large fuel cell computations. In the limit of infinite water adsorption rate between ionomer phase and pore, a boundary layer emerges at either side of the CL, in which the water sorption equilibrium [Zawodzinski, T., Derouin, C., Radzinski, S., Sherman, R., Smith, V., Springer T., Gottesfeld, S., 1993. A comparative study of water uptake by and transport through ionomeric fuel cell membranes. Journal of the Electrochemical Society 140, 1981–1985] between ionomer and pore is violated.

Physical and electrochemical properties of Nafion/polypyrrole composite membrane for DMFC

Nafion/polypyrrole composite membrane was prepared by chemical in situ polymerization for a direct methanol fuel cell. The composite membrane was characterized by quantitative analysis, DMA, TGA, EPMA, water and methanol uptake tests, conductivity and permeability measurements, and unit cell operation. The mechanical and thermal properties of the composite membrane were enhanced due to the interaction between the polar phase of Nafion and secondary ammonium groups of polypyrrole. In addition, the sorption properties of the composite membrane influenced two transport properties, proton conductivity and methanol crossover. The optimization of cell performance was related to the distribution of polypyrrole particles over the cross section of the membrane. In particular when the polypyrrole particles were mainly present near the surface rather than in the internal space, the difference in the relative proton conductivity and the relative methanol permeability was largest. It is supposed that the existence of the polypyrrole particles near the surface such as thin film was favorable to reducing methanol crossover with maintaining proton conductivity. As a result of that, N/P 003 had higher performance than Nafion under the specific condition.

A Computer Simulation Study of the Mesoscopic Structure of the Polyelectrolyte Membrane Nafion

We studied the mesoscopic structure of the perfluorinated sulfonic acid membrane Nafion containing water using a dissipative particle dynamics (DPD) simulation. A Nafion polymer is modeled by connecting coarse-grained particles, which correspond to the hydrophobic backbone of polytetrafluoroethylene and perfluorinated side chains terminated by hydrophilic end particles of sulfonic acid groups. Water is also modeled by the same size particle as adopted in the Nafion model, corresponding to a group of four H2O molecules. The Flory–Huggins χ-parameters between DPD particles are estimated from the mixing energy calculation using an atomistic simulation. In the DPD simulation, water particles and hydrophilic particles of Nafion side chains spontaneously form aggregates and are embedded in the hydrophobic phase of the Nafion backbone. This structure is a bicontinuous phase of Nafion and water regions and has a continuous path in the cavity of water in any direction. Although this sponge-like structure is essentially identical to the cluster-network model proposed from the experimental studies, the shape of the water clusters is not spherical but irregular, and the water regions are indistinguishable structures of water clusters and their channels. The cluster size and its dependence on the water content are in good agreement with experimental reports; therefore, the simulated mesoscopic structure is confirmed to be a highly possible one.