Showing papers on "Nafion published in 2008"

Parallel cylindrical water nanochannels in Nafion fuel-cell membranes.

Abstract: The structure of the Nafion ionomer used in proton-exchange membranes of H(2)/O(2) fuel cells has long been contentious. Using a recently introduced algorithm, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic 'ionomer peak' arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. At 20 vol% water, the water channels have diameters of between 1.8 and 3.5 nm, with an average of 2.4 nm. Nafion crystallites (approximately 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of approximately (5 nm)(2). Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures.

Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts

Abstract: In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Nowadays, acidic polymer electrolytes, typically Nafion, are widely used. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost and hampering the widespread application of fuel cells. Here, we report a type of polymer electrolyte fuel cells (PEFC) employing a hydroxide ion-conductive polymer, quaternary ammonium polysulphone, as alkaline electrolyte and nonprecious metals, chromium-decorated nickel and silver, as the catalyst for the negative and positive electrodes, respectively. In addition to the development of a high-performance alkaline polymer electrolyte particularly suitable for fuel cells, key progress has been achieved in catalyst tailoring: The surface electronic structure of nickel has been tuned to suppress selectively the surface oxidative passivation with retained activity toward hydrogen oxidation. This report of a H2–O2 PEFC completely free from noble metal catalysts in both the positive and negative electrodes represents an important advancement in the research and development of fuel cells.

Mass Transport through a Proton Exchange Membrane (Nafion) in Microbial Fuel Cells

Abstract: Proton exchange membranes (PEMs) are one of the most important components in microbial fuel cells (MFCs), since PEMs physically separate the anode and cathode compartments while allowing protons to transport to the cathode in order to sustain an electrical current. The Nafion 117 membrane used in this study is generally regarded as having excellent proton conductivity, though many problems for its application in MFCs remain. We investigated problems associated with Nafion including: oxygen leakage from cathode to anode, substrate loss, cation transport and accumulation rather than protons, and biofouling. It was found that Nafion was quite permeable to oxygen. The oxygen mass transfer coefficient (KO) and the oxygen diffusion coefficient (DO) for Nafion was estimated as KO = 2.80 × 10−4 cm/s and DO = 5.35 × 10−6 cm2/s, respectively when a 50 mM phosphate buffer was used as the catholyte. The MFC with distilled water instead of phosphate buffer showed similar values (KO = 2.77 × 10−4 cm/s, DO = 5.27 × 10−6...

Design of an Electrochemical Cell Making Syngas ( CO + H2 ) from CO2 and H2O Reduction at Room Temperature

Abstract: An electrolysis-cell design for simultaneous electrochemical reduction of CO 2 and H 2 O to make syngas (CO + H 2 ) at room temperature (25°C) was developed, based on a technology very close to that of proton-exchange-membrane fuel cells (PEMFC), i.e., based on the use of gas-diffusion electrodes so as to achieve high current densities. While a configuration involving a proton-exchange membrane (Nafion) as electrolyte was shown to be unfavorable for CO 2 reduction, a modified configuration based on the insertion of a pH-buffer layer (aqueous KHCO 3 ) between the silver-based cathode catalyst layer and the Nafion membrane allows for a great enhancement of the cathode selectivity for CO 2 reduction to CO [ca. 30 mA/cm 2 at a potential of -1.7 to -1.75 V vs SCE (saturated-calomel reference electrode)]. A CO/H 2 ratio of 1/2, suitable for methanol synthesis, is obtained at a potential of ca. -2 V vs SCE and a total current density of ca. 80 mA/cm 2 . An issue that has been identified is the change in product selectivity upon long-term electrolysis. Results obtained with two other cell designs are also presented and compared.

Advanced Mesostructured Hybrid Silica−Nafion Membranes for High-Performance PEM Fuel Cell

Abstract: Organic−inorganic hybrid membranes of Nafion and mesoporous silica containing sulfonic acid groups were synthesized using the sol−gel process with the goal of increasing the proton conductivity and water retention at higher temperatures and lowering relative humidities as well as improving the dimensional stability. These hybrid membranes were prepared via in situ co-condensation of tetraethoxysilane and chlorosulfonylphenethylsilane via self-assembly route using organic surfactants as templates for the tuning of the architecture of the silica or hybrid organosilica components. In this paper, we describe the elaboration and characterization of new hybrid membranes all the way from the precursor solution to the evaluation of the fuel cell performances. These hybrid materials were extensively characterized with the determination of their physicochemical and electrochemical properties. The membrane containing functionalized silica showed a higher ionic exchange capacity and greater water management than stan...

Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vanadium ion crossover for vanadium redox flow batteries

Abstract: The crossover of vanadium ions through proton-exchange membranes such as those of Nafion is the chief reason that results in the low energy efficiency and high self-discharge rate of vanadium redox flow batteries (VRB). With respect to applicability, the ideal proton-exchange membrane used in VRB should possess simultaneously high proton conductivity and low vanadium ion permeability. Here, we report a novel approach using a polyelectrolyte layer-by-layer self-assembly technique to fabricate a barrier layer onto the surface of Nafion membrane by alternate adsorption of polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS), which can suppress the crossover of vanadium ions. The Nafion–[PDDA-PSS]n membrane (n = the number of multilayers) obtained shows much lower vanadium ion permeability compared with plain Nafion membrane. Accordingly, the VRB with Nafion–[PDDA-PSS]n membrane exhibits a higher coulombic efficiency (CE) and energy efficiency (EE) together with a slower self-discharge rate than that of Nafion system. The highest CE of 97.6% and EE of 83.9% can be achieved at charge–discharge current density of 80 mA cm−2 and 20 mA cm−2, respectively.

Polymer Electrolyte Fuel Cells Using Nafion‐Based Composite Membranes with Functionalized Carbon Nanotubes

Abstract: Fuel efficiency: Enhanced proton conductivity is obtained by the incorporation of single-walled carbon nanotubes prefunctionalized with sulfonic acid groups (S-SWCNTs) into a Nafion matrix (see scheme). The acid content of the CNT connects the hydrophobic regions of the membrane, thus providing a network for proton mobility. The polymer electrolyte membranes also have improved mechanical stability in H 2 /O 2 fuel cells.

Gated proton transport in aligned mesoporous silica films

Abstract: Modulated proton transport plays significant roles in biological processes such as ATP synthesis as well as in technologically important applications including, for example, hydrogen fuel cells. The state-of-the-art proton-exchange membrane is the sulphonated tetrafluoroethylene copolymer Nafion developed by DuPont in the late 1960s, with a high proton conductivity. However, actively switchable proton conduction, a functional mimic of the ion transport within a cell membrane, has yet to be realized. Herein, we report the electrostatic gating of proton transport within aligned mesoporous silica thin film. It is observed that surface-charge-mediated transport is dominant at low proton concentrations. We have further demonstrated that the proton conduction can be actively modulated by two-fourfold with a gate voltage as low as 1 V. Such artificial gatable ion transport media could have potential applications in nanofluidic chemical processors, biomolecular separation and electrochemical energy conversion.

A Novel Sulfonated Carbon Composite Solid Acid Catalyst for Biodiesel Synthesis

Abstract: A novel sulfonated carbon composite solid acid was successfully prepared by the pyrolysis of a polymer matrix impregnated with glucose followed by sulfonation. The title catalyst has higher acid site density, better esterification activity of both small and large free fatty acids (acetic acid and palmitic acid), and better reusability than the previously reported carbon-based catalyst prepared by sulfonating pyrolyzed sugar. This catalyst also exhibited higher esterification activity than tungstated zirconia (WZ) and Silica-Supported Nafion (Nafion®SAC-13). The higher activity of the sulfonated carbon composite solid acid catalyst was clearly due to the presence of a much higher acid site density than any of the other catalysts.

Durability of Perfluorosulfonic Acid and Hydrocarbon Membranes: Effect of Humidity and Temperature

Abstract: The effect of humidity on the chemical stability of two types of membranes [i.e., perfluorosulfonic acid type (PFSA, Nafion 112) and biphenyl sulfone hydrocarbon type, (BPSH-35)] was studied by subjecting the membrane electrode assemblies (MEAs) to open-circuit voltage (OCV) decay and potential cycling tests at elevated temperatures and low inlet-gas relative humidities. The BPSH-35 membranes showed poor chemical stability in ex situ Fenton tests compared to that of Nafion membranes. However, under fuel cell conditions, BPSH-35 MEAs outperformed Nafion 112 MEAs in both the OCV decay and potential cycling tests. For both membranes, (i) at a given temperature, membrane degradation was more pronounced at lower humidities and (ii) at a given relative humidity operation, increasing the cell temperature accelerated membrane degradation. Mechanical stability of these two types of membranes was also studied using relative humidity (RH) cycling. Due to decreased swelling and contraction during wet-up and dry-out cycles, Nafion 112 lasted longer than BPSH-35 membranes in the RH cycling test.

Fuel cells: Proton channels

Abstract: The structure of Nafion, the polymer electrolyte membrane used in some fuel cells, has been extensively debated over recent decades. Now, a model reveals the nanoscale arrangement that could explain the excellent transport properties of the material.

Degradation Mitigation in Polymer Electrolyte Membranes Using Cerium Oxide as a Regenerative Free-Radical Scavenger

Abstract: The efficacy of CeO 2 nanoparticles in mitigating free-radical-induced polymer electrolyte membrane (PEM) degradation is investigated. Commercially obtained CeO 2 and nanoparticles synthesized in-house were incorporated within a recast Nafion membrane. Membrane electrode assemblies were prepared using Nafion and Nafion-CeO 2 composite membranes (0.5, 1, and 3 wt % CeO 2 ). The composite membranes exhibited very similar proton conductivities (∼35 mS/cm) and hydrogen crossover (∼ 1 mA/cm 2 ) as Nafion. However, the fluoride emission rate (from accelerated tests) was lowered by more than 1 order of magnitude upon addition of CeO 2 into the Nafion membrane, suggesting that CeO 2 nanoparticles have tremendous potential to greatly enhance membrane durability.

Polysulfones grafted with poly(vinylphosphonic acid) for highly proton conducting fuel cell membranes in the hydrated and nominally dry state

Abstract: Mechanically strong and flexible membranes with very high local concentrations of immobilized proton-conducting phosphonic acid have been achieved by grafting poly(vinylphosphonic acid) side chains onto polysulfones. The graft copolymers were prepared by anionic polymerization of diethyl vinylphosphonate initiated from lithiated polysulfones, followed by quantitative cleavage of the ester functions. The resulting phosphonic acid units acted like monoprotic acids to indicate a high level of intramolecular interactions, and the phase-separated nature of the copolymers was shown by dual glass transitions. Fully polymeric membranes were conveniently cast from solution and showed high proton conductivities, e.g., 5 mS/cm under nominally dry conditions at 120 °C and up to 93 mS/cm under 100% relative humidity at the same temperature. The corresponding conductivities measured for Nafion 115 under the same conditions were 0.04 and 105 mS/cm, respectively. The former membranes furthermore showed high thermal stabi...

Non-Fickian Water Vapor Sorption Dynamics by Nafion Membranes

Abstract: Dynamics of water absorption from a saturated vapor and water desorption into dry air for Nafion 1100 EW ionomers have been measured for film thicknesses between 51 and 606 μm and at temperatures ranging from 30 to 90 °C. Water absorption and desorption exhibit two distinct non-Fickian characteristics: (1) desorption is 10 times faster than absorption and (2) the normalized mass change does not collapse to a single master curve when plotted against time normalized by membrane thickness squared, t/𝓁 2, for either absorption or desorption. Water desorption data were fit well by a model in which diffusion is rapid and interfacial mass transport resistance is the rate-limiting process for water loss. Water absorption is described by a two-stage process. At early times, interfacial mass transport controls water absorption, and at longer times, water absorption is controlled by the dynamics of polymer swelling and relaxation.

Flexible Carbon Nanotube-Polymer Composite Films with High Conductivity and Superhydrophobicity Made by Solution Process

Abstract: CNT/Nafion nanocomposite film made by solution process exhibits high conductivity and superhydrophobicity. The highest water contact angle reaches 165.3 +/- 1.9 degrees. The wettability of the film can be controlled by simply varying the filtering rate and the content ratio of Nafion to CNT. We also develop a novel optical method to directly observe the air-solid-liquid interface for the first time. The extraordinary mechanical strength provided by the polymer helps the film retain its conductivity and superhydrobicity after 1000 bending cycles.

Electrospinning and Solution Properties of Nafion and Poly(acrylic acid)

Abstract: In this study, the electrospinning performance and solution properties of Nafion and its blend with another polyelectrolyte, poly(acrylic acid) (PAA), were investigated. Attempts to electrospin pure Nafion at various polymer concentrations (5−35 wt %), solvents, neutralization, and electrospinning conditions resulted in electrospraying rather than electrospinning. However, a polymer solution blend (5 wt %) of Nafion and PAA resulted in beaded fibers at 8 wt % PAA and smooth electrospinning above 12% PAA. Fiber sizes of the blend increased from 90 to 600 nm with increasing PAA content. Dynamic light scattering on pure Nafion solutions in various solvents reveals large aggregates (i.e., dispersion) of various sizes due to polymer backbone and ionic interactions. The lack of sufficient polymer chain entanglement evidenced by low viscosity and aggregate formation in Nafion solutions prohibits fiber formation during electrospinning. The addition of PAA to Nafion modifies the ionic strength of the solvent resul...

Wetting and absorption of water drops on Nafion films.

Abstract: Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70° 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°. 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 ma...

Doping and de-doping of carbon nanotube transparent conducting films by dispersant and chemical treatment

Abstract: Single-walled carbon nanotubes (SWCNTs) dispersed with Nafion in a solvent mixture containing de-ionized water and 1-propanol (bisolvent) were sprayed on a poly(ethylene terephthalate) substrate to fabricate flexible transparent conducting films (TCFs). Different SWCNT-to-Nafion ratios were used to optimize the film performance of transparence and sheet resistance. The TCFs were then immersed in nitric acid. These steps resulted in p-type doping due to the presence of Nafion in the SWCNT network and de-doping (removal of doping effect) by the acid treatment. X-Ray photoelectron and Raman spectroscopy confirmed that the de-doping effect occurred with the partial removal of Nafion from the nanotube surface by the nitric acid treatment, which improved the film conductivity by a factor of ∼4 with negligible change in transmittance.

A Biomimetic Actuator Based on an Ionic Networking Membrane of Poly(styrene-alt-maleimide)-Incorporated Poly(vinylidene fluoride)†

Abstract: A novel electro-active polymer actuator employing the ionic networking membrane of poly(styrene-alt-maleimide) (PSMI)-incorporated poly(vinylidene fluoride) (PVDF) was developed to improve the electrical and mechanical performance of the artificial muscles. The main drawback of the previous ionic polymer-metal composite actuator was the straightening-back and relaxation under the constant voltage excitation. The present ionic networking membrane actuator overcomes the relaxation of the ionic polymer-metal composite actuator under the constant voltage and also shows much larger tip displacement than that of the Nafion-based actuator. Under the simple harmonic stimulus, the measured mechanical displacement was comparable to that of the Nafion-based actuator. The excellent electromechanical response of the current polymer actuator is attributed to two factors: the inherent large ionic-exchange capacity and the unique hydrophilic nano-channels of the ionic networking membrane. The electro-active polymer actuator of PSMI-incorporated PVDF can be a promising smart material and may possibly diversify niche applications in biomimetic motion.