Nafion Surface Research Articles

Force field measurements within the exclusion zone of water

Water molecules play critical roles in many biological functions, such as protein dynamics, enzymatic activities, and cellular responses. Previous nuclear magnetic resonance and neutron scattering studies have shown that water molecules bind to specific sites on surfaces and form localized clusters. However, most current experimental techniques cannot measure dynamic behaviors of ordered water molecules on cell-size (10μm) scale. Recently, the long-distance effect of structured water has been demonstrated by Pollack and his colleagues. Namely, there is a structured water layer near the hydrophilic surface that can exclude solutes (Zheng et al, Adv Colloid Interface Sci 127:19-27, 2006; Pollack 2006, Adv Colloid Interface Sci 103:173-196, 2003). The repelling forces of water clusters inside this exclusion region are investigated in this study. With a laser tweezers system, we found the existence of an unexpected force fields inside the solute-free exclusion zone near a Nafion surface. Our results suggest that the water clusters could transduce mechanical signals on the micrometer range within the exclusion zone. This unexpected inhomogeneous force field near the hydrophilic surface would provide a new insight into cellular activities, leading to a potential new physical chemistry mechanism for cell biology.

Unprecedented Structural Sensitivity toward Average Terrace Width: Nafion Adsorption at Pt{hkl} Electrodes

Ultrathin Nafion overlayers ranging from 0 to 3.3 nm in thickness have been deposited on platinum single crystal electrodes in order to study their voltammetric behavior as a function of both Nafion surface coverage and platinum surface step density. Unusual structural sensitivity of Nafion-induced voltammetric peaks ascribable to Nafion interactions with step sites is observed as a function of average terrace width. For stepped surfaces containing both {111} and {100} terraces, average terrace widths of 2–3 atoms give rise to the most intense Nafion-step electrosorption peaks. Rapid quenching of the Nafion-step voltammetric peaks is observed as the average terrace width is gradually increased until for >5 atom wide terraces all such electrochemical features are completely attenuated. This rapid diminution of the step-Nafion voltammetric peak, within just three atoms of terrace width, is unprecedented, and a parallel with the “splitting” of hydrogen underpotential deposition (HUPD) step peaks in perchloric acid aqueous electrolyte for closely spaced steps is noted. On the basis of these measurements, it is suggested that Nafion is a molecular probe of adsorbed OH species on Pt electrodes.

Bending control of Nafion-based electroactive polymer actuator coated with multi-walled carbon nanotubes

Electrical bending control of Nafion-based ionic polymer–metal composite (IPMC) is quite difficult. Unlike a conventional fully hydrated noble metal-coated Nafion type IPMC, however, highly dehydrated silver-coated Nafion type IPMC exhibited better electrical bending controllability. Embedding of the multi-walled carbon nanotube (MWCNT) into Nafion surface promoted adsorption of a larger quantity of silver on the Nafion surface, since the MWCNT surface served as adsorption sites for silver. A MWCNT-embedded Nafion coated with such a large quantity of silver (SCNT-Naf) exhibited large bending curvature under an applied voltage when in a highly dehydrated state, because of large scale induction of silver redox reaction. We could even achieve autonomous bending curvature control of the highly dehydrated SCNT-Naf quantitatively by automatically monitoring total charge imposed on it.

Effect of Thermal Annealing on Proton Conduction in Ion Exchange Membranes

ABSTRACTThe configurations of proton channel network on the surface of Nafion® membranes were studied using current sensing atomic force microscopy after the membranes were annealed at elevated temperatures, aimed at understanding the effect of aging process in the membranes. The results reveal that proton conductance of the membranes becomes more uniform and the proton channels become chain-like aligning in parallel to the membrane surface. Accompanied to the configuration changes, the proton conductivity of the membrane shows an increase. As the annealing continues, the chain-like configuration for the proton channels persists but the conductance of the membranes decreases. The time constant of the conductivity decay decreases with increase of the annealing temperature. The observed changes can be attributed to reorientation of proton channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes.

Kinetic study of Nafion degradation by Fenton reaction

To predict the durability of polymer electrolyte membranes in fuel cells, the degradation reactions of Nafion 117 films were studied as oxidation reactions with hydroxyl radicals as oxidation accelerators. The radical species were generated by the Fenton reaction between hydrogen peroxide (H2O2) and iron ions (Fe2+). The Nafion degradation kinetics were estimated by fluorine ion (F−) generation. The H2O2 and Nafion degradation reactions fit a pseudo-first-order rate constant. The values of the activation energy and frequency factor are 85kJmol−1 and 3.97×108s−1 for H2O2 decomposition in the presence of a Nafion film and 97kJmol−1 and 9.88×108s−1 for F− generation. The Nafion surface morphology became rough after reaction for 12h; small cracks, approximately 100μm in length, were observed at temperatures below 60°C. These cracks connected to make larger gaps of approximately 1mm at temperatures above 70°C. We also found a linear relationship between H2O2 consumption and F− generation. The rate constant is temperature dependent and expressed as ln(d[F−]/d[decomposed H2O2])=−19.5×103K−1+42.8. F− generated and H2O2 consumed along with the Nafion degradation conditions can be predicted using this relation.

Analysis of Oxygen Dissolution Rate from Gas Phase into Nafion Surface and Development of an Agglomerate Model

To clarify the rate determining process for the oxygen transport in a catalyst layer of PEFCs, an oxygen dissolution resistance from gas phase into Nafion surface was measured. The oxygen dissolution resistance, which was estimated form the relationship between Nafion thickness on Pt electrode and the diffusion-limited current density on the electrode, was found to account for a large part of the total oxygen transport loss. By introducing the oxygen dissolution resistance, an agglomerate model was able to predict I-V performance of a PEFC using a reasonable size of agglomerate.

Analysis of Oxygen Dissolution Rate from Gas Phase into Nafion Surface and Development of an Agglomerate Model

not Available.

Surface Structure of Nafion in Vapor and Liquid

The microstructure of Nafion varies in response to changes in hydration. Thus, it undergoes a transition from tightly packed bundles of inverted micelles with aqueous cores and fused hydrophobic shells ("macaroni bundles") at low hydrations to normal type ("spaghetti") micelles at high hydrations. It was postulated recently that a similar "macaroni-spaghetti" transition, i.e., breakup of surface-aligned macaroni to randomly oriented spaghetti, takes place at the polymer surface when the external medium is changed from vapor to liquid water, which can explain some puzzling features of Nafion and similar microphase-separated ionomers. The resulting (nonequilibrium) structures may remain confined to a few nanometers thick surface region. Here, this picture is corroborated using grazing-incidence small-angle X-ray scattering (GISAXS), contact angle, and atomic force microscopy (AFM). The enhanced alignment of bundles adjacent to the surface in vapor, similar to the effect of biaxial stretching, is elucidated by GISAXS of spin-cast Nafion films. It is inferred from the characteristic change in relative intensities and position of the ionomer peak in the X-Y (in-plane) and Z (out-of-plane) directions with varying X-ray penetration depths into the film. However, contact angle measurements show that the relatively smooth and very hydrophobic surface of Nafion in vapor transforms to a hydrophilic surface, when vapor as the external medium is replaced with liquid water. In addition, AFM indicates that the surface roughness significantly increases in liquid. The results demonstrate that the surface region of Nafion and similar microphase-separated materials may be indeed subject to drastic structural variations, even though the extremely slow relaxation of the solid matrix may preclude propagation of such changes into the bulk. These effects may have a profound effect on the macroscopic characteristics of Nafion membranes, such as hydration and conductivity, as well as their functioning as ion-selective barriers in electrochemical and other applications.

Humidity-Dependent Structure of Surface Water on Perfluorosulfonated Ionomer Thin Film Studied by Sum Frequency Generation Spectroscopy

Humidity-dependent water structure on the Nafion thin film surface was determined at room temperature by using sum frequency generation spectroscopy (SFG). When the Nafion thin film was exposed to a low relative humidity (RH) environment, a peak at 3720 cm−1 due to the “dangling OH” or “free OH” of water molecules and a peak centered around 3600 cm−1, which was assigned to be due to water molecules interacting with sulfonate groups of the Nafion surface, were observed. The intensities of these peaks increased as RH was increased. A broad peak centered around 3200−3300 cm−1 due to water adsorbed on hydrophobic, i.e., fluorocarbon, sites of Nafion was observed when the Nafion thin film was exposed to a high RH environment (RH > 60%). The behavior of surface water is compared with that of water in bulk Nafion.

Surface Characterization of Poly(acrylic acid) Grafted to Photo-oxidized Perfluorosulfonic Acid Membrane Used in Fuel Cells

AbstractPerfluorosulfonic acid membrane (Nafion®-117) was first surface modified with atmospheric pressure UV photo-oxidation or low-pressure vacuum UV photo-oxidation downstream from an Ar microwave plasma, and then graft polymerized with acrylic acid. X-ray photoelectron spectroscopy (XPS) was used to analyze the modified Nafion surface and poly(acrylic acid) grafted to the modified surface.

Low water swelling and high methanol resistant proton exchange membrane fabricated by cross-linking of multilayered polyelectrolyte complexes

Multilayered polyelectrolyte complexes (PECs) modified proton exchange membranes are successfully prepared from alternating deposition of oppositely charged polyelectrolyte chitosan (CS) and sulfonated poly (aryl ether ketone) bearing carboxyl groups (SPAEK-C) on the surface of Nafion ®117 via the layer-by-layer (LBL) method. The cross-linked PEC-coated Nafion ® membranes (c-PEC-coated Nafion ® membranes) are prepared from heat-induced cross-linking between CS and SPAEK-C via formation of amide bonds. Fourier transform infrared spectroscopy (FT-IR) verifies that cross-linking between ammonium groups of CS and carboxyl groups of SPAEK-C occurs by the formation of amide bonds. The morphology of the membranes is studied in detail by SEM. The c-PEC-coated Nafion ® membranes exhibit low water uptake (23.5%, 80 °C), water swelling (14.0%, 80 °C) and methanol permeability (2.7 × 10 −7 cm 2 s −1). Proton conductivity of the c-PEC-coated Nafion ® membranes increase at elevated temperature, reaching the value of 0.131 S cm −1 at 80 °C. These results show that cross-linking of layered CS/SPAEK-C films can effectively reduce water uptake, water swelling and methanol permeability while improve the dimensional stability and maintain high proton conductivity.

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.

High-resolution imaging of ion conductivity of Nafion ® membranes with electrochemical atomic force microscopy

Conductive electrochemical AFM images demonstrating the complex nature and structure of Nafion surface conductivity are presented. Nanoscale regions with high currents determining the overall total membrane current can be distinguished from majority domains with lower currents and non-conductive areas. The different conductive domains form ordered structures and show a specific dynamic behaviour. These observations were compared to the structural and electrical models in the literature. None of the models is able to explain all aspects of the current images. The existence of inverted micelles seems to be quite probable since the formation of agglomerates like chains and larger ordered clusters is clearly visible. This aspect is best described by the model of Schmidt-Rohr and Chen. In addition, the highly dynamic behaviour and distribution of conductive channels of Nafion leading to the formation of new current pathways also indicates the formation of different meso-phases with a high local fluctuation rate. The other discussed models also predict structural features which are in agreement with our observations like the formation of super-structures and agglomeration of fibers. The structural characterisation reflects the situation at or near the membrane surface and might differ from the bulk structure since the surface energy may have a large influence on the formation of structures during the membrane solidification process. The quite large dynamics of conductivity changes of Nafion reflected in the formation of new current pathways even at room temperature leads to the assumption that the internal structure of Nafion is subject to significant changes due to humidity and temperature variations. The local variation of individual structures may reflect the variation of concentration of hydrophilic and hydrophobic groups during membrane solidification. The minimization of surface free energy during a self-assembling process is essential for the formation of different phases and subsequent structures like chains, etc. as well as higher order clustering.

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

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

Study of Electrical Resistance on the Surface of Nafion 115® Membrane Used as Electrolyte in PEMFC Technology Part II: Surface Response Methodology

Study of Electrical Resistance on the Surface of Nafion 115® Membrane Used as Electrolyte in PEMFC Technology Part II: Surface Response Methodology

Study of Electrical Resistance on the Surface of Nafion 115® Membrane Used as Electrolyte in PEMFC Technology Part I: Statistical Inference

Study of Electrical Resistance on the Surface of Nafion 115® Membrane Used as Electrolyte in PEMFC Technology Part I: Statistical Inference

직접 메탄올 연료전지용 나피온 막의 폴리아닐린/Sulfonated Poly(ether sulfone) 다층 자기조립 박막에 의한 표면 개질

본 연구에서는 나피온 막을 통한 메탄올 투과도를 감소시키기 위하여 양전하를 띠는 polyaniline(PANi)와 음전하를 갖는 sulfonated Poly(ether sulfone)(SPES)으로부터 다층 자기조립 박막법으로 나피온 막의 표면을 개질하였다. PANi과 SPES는 열적 화학적으로 안정하며 매우 강직한 특성을 띄고 있어, 나피온 막 표면에 다층 자기조립 박막 형성시 메탄올 투과도 감소 및 치수 안정성의 향상을 기대할 수 있다. 자외선-가시광선 흡광분석에 의하여 PANi와 SPES의 다층박막의 형성이 균일하게 이루어짐을 확인 하였다. TEM 분석을 통하여 다층 자기조립 박막의 bilayer당 두께가 약 10 nm 정도임을 확인 하였다. 개질된 나피온 막은 순수한 나피온 막에 비하여 15%의 이온전도도 감소가 일어났지만 67%의 높은 메탄올 투과도 감소를 나타내어 2.5배 높은 선택도를 보였다. 5 M 메탄올을 연료로 사용한 연료전지 성능시험에서 개질된 나피온 막은 순수한 나피온 막에 비해 최대 전력 밀도가 <TEX>$30^{\circ}C$</TEX>에서는 2.4배 증가, <TEX>$60^{\circ}C$</TEX>에서는 1.4배 향상을 나타내었다. In this study, Nafion membrane was modified to prevent methanol crossover by layer-by-layer self assembly using polyaniline (PANi) as a polycation and sulfonated poly(ether sulfone) (SPES) as a polyanion onto the Nafion surface. Since PANi and SPES possess thermal and chemical stability and rigid backbone, their layer-by-layer self-assembled films on the Nafion are expected to reduce methanol permeability and to increase mechanical stability. UV-Vis absorption spectroscopy verified a linear build-up of the multilayers of PANi and SPES. We found that the thickness per bilayer was about 10 nm by TEM measurement. Although modified Nafion membrane exhibited 15% decrease of proton conductivity, it reduceded 67% of methanol permeability compared to that of the pristine Nafion membrane, resulting in 2.5 times larger selectivity. At the performance test of the fuel cell using 5M methanol as a fuel, the modified Nafion membrane showed 2.4 times higher maximum power density at <TEX>$30^{\circ}C$</TEX> and 1.4 times larger at <TEX>$60^{\circ}C$</TEX> than the pristine Nafion.

Self-assembled polyelectrolyte multilayered films on Nafion with lowered methanol cross-over for DMFC applications

The paper is concerned with the deposition of self-assembled polyelectrolyte multilayer on Nafion membrane by layer-by-layer (LbL) technique with lowered methanol cross-over for direct methanol fuel cell (DMFC) applications. The formation of self-assembled multilayered film on Nafion was characterized by UV–vis spectroscopy and it was found that the polyelectrolyte layers growth on the Nafion surface regularly. Furthermore, the proton conductivity and methanol cross-over measurements were carried out for characterization of the LbL self-assembled composite membranes. The results showed that the concentration and pH of the polyelectrolytes significantly affect the proton conductivity and methanol barrier properties of the composite membranes. 10 −1 monomol polyelectrolyte concentration and pH 1.8 was found to be optimum deposition conditions considering proton conductivity and methanol permeation properties of the LbL self-assembled composite membranes. The methanol permeability of the 10 bi-layers of PAH1.8/PSS1.8 deposited LbL self-assembly composite membrane was significantly suppressed and found to be 4.41 × 10 −7 cm 2/s while the proton conductivity value is in acceptable range for fuel cell applications.

Wetting and Absorption of Water Drops on Nafion Films

Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70 degrees between advancing and receding values were obtained by the Wilhelmy plate technique. Sessile drop measurements were consistent with the advancing contact angle; the sessile drop contact angle was 108 degrees . Water drop adhesion, as measured by the detachment angle on an inclined plane, showed much stronger water adhesion on Nafion than Teflon. Sessile water and methanol drops caused dry Nafion films to deflect. The flexure went through a maximum with time. Flexure increased with contact area of the drop, but was insensitive to the film thickness. Methanol drops spread more on Nafion and caused larger film flexure than water. The results suggest that the Nafion surface was initially hydrophobic but water and methanol drops caused hydrophilic sulfonic acid domains to be drawn to the Nafion surface. Local swelling of the film beneath the water drop caused the film to buckle. The maximum flexure is suggested to result from motion of a water swelling front through the Nafion film.

Direct observation of deformation of nafion surfaces induced by methanol treatment by using atomic force microscopy

We successfully characterized the effect of methanol treatment on the nanoscopic structures of a nafion film, which is widely used in direct methanol fuel cells (DMFCs). Atomic force microscopy (AFM) was used to repetitively image a particular region of a nafion sample before and after methanol solutions were dropped onto the nafion film and dried in air. When the surface was treated with 20% methanol for 5 min, many nanopores appeared on the surface. The number of nanopores increased when the sample was treated twice or thrice. By repetitive AFM imaging of a particular region of the same sample, we found that the shapes of the nanopores were deformed by the repeated methanol treatment, although the size of the nanopores had not significantly changed. The creation of the nanopores was affected by the concentration of methanol. Our results directly visualized the effects of methanol treatment on the surface structures of a nafion film at nanoscale levels for the first time.