Showing papers on "Nafion published in 2006"

Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (nafion and PTFE) in single chamber microbial fuel cells.

Abstract: Cathode catalysts and binders were examined for their effect on power densities in single chamber, air-cathode, microbial fuel cells (MFCs). Chronopotentiometry tests indicated thatthe cathode potential was only slightly reduced (20-40 mV) when Pt loadings were decreased from 2 to 0.1 mg cm(-2), and that Nafion performed better as a Pt binder than poly(tetrafluoroethylene) (PTFE). Replacing the precious-metal Pt catalyst (0.5 mg cm(-2); Nafion binder) with a cobalt material (cobalt tetramethylphenylporphyrin, CoTMPP) produced slightly improved cathode performance above 0.6 mA cm(-2), but reduced performance (<40 mV) at lower current densities. MFC fed batch tests conducted for 35 cycles (31 days) using glucose showed that replacement of the Nafion binder used for the cathode catalyst (0.5 mg of Pt cm(-2)) with PTFE reduced the maximum power densities (from 400 +/- 10 to 480 +/- 20 mW m(-2) to 331 +/- 3 to 360 +/- 10 mW m(-2)). When the Pt loading on cathode was reduced to 0.1 mg cm(-2), the maximum power density of MFC was reduced on average by 19% (379 +/- 5 to 301 +/- 15 mW m(-2); Nafion binder). Power densities with CoTMPP were only 12% (369 +/- 8 mW m(-2)) lower over 25 cycles than those obtained with Pt (0.5 mg cm(-2); Nafion binder). Power densities obtained using with catalysts on the cathodes were approximately 4 times more than those obtained using a plain carbon electrode. These results demonstrate that cathodes used in MFCs can contain very little Pt, and that the Pt can even be replaced with a non-precious metal catalyst such as a CoTMPP with only slightly reduced performance.

Effects of membrane cation transport on pH and microbial fuel cell performance.

Abstract: Due to the excellent proton conductivity of Nafion membranes in polymer electrolyte membrane fuel cells (PEMFCs), Nafion has been applied also in microbial fuel cells (MFCs). In literature, however, application of Nafion in MFCs has been associated with operational problems. Nafion transports cation species other than protons as well, and in MFCs concentrations of other cation species (Na+, K+, NH4+, Ca2+, and Mg2+) are typically 10(5) times higher than the proton concentration. The objective of this study, therefore, was to quantify membrane cation transport in an operating MFC and to evaluate the consequences of this transport for MFC application on wastewaters. We observed that during operation of an MFC mainly cation species other than protons were responsible for the transport of positive charge through the membrane, which resulted in accumulation of these cations and in increased conductivity in the cathode chamber. Furthermore, protons are consumed in the cathode reaction and, consequently, transport of cation species other than protons resulted in an increased pH in the cathode chamber and a decreased MFC performance. Membrane cation transport, therefore, needs to be considered in the development of future MFC systems.

Aliphatic/Aromatic Polyimide Ionomers as a Proton Conductive Membrane for Fuel Cell Applications

Abstract: To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78−2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 °C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm-1 was obtained for 1 at 140 °C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1−2 orders of magnitude lower than that of Na...

Proton conduction in rare-earth ortho-niobates and ortho-tantalates

Abstract: Some oxides contain sufficient equilibrium concentrations of protons in wet atmospheres to show useful proton conduction at elevated temperatures1. As an example, Y-doped BaCeO3 has shown promising performance as a thin-film electrolyte in fuel cells at intermediate temperatures (400–600 ∘C)2. In contrast to proton-conducting polymers (for example, Nafion(R)) and acid salts (for example, CsHSO4), such oxidic ceramics are stable at sufficiently elevated temperatures that electrode kinetics are fast and insensitive to poisoning, but they tend to be basic (Ba-based or Sr-based) compounds with poor chemical and mechanical stability3. In search of more stable proton-conducting materials, we have investigated several acceptor-doped rare-earth ortho-niobates and ortho-tantalates, RE1−xAxMO4 (M=Nb,Ta). We show that this class of materials shows mixed protonic, native ionic and electronic conduction depending on conditions. Both the low-temperature monoclinic and high-temperature tetragonal polymorphs show proton conduction. The proton conductivity is dominant in wet atmospheres below roughly 800∘C and the highest proton conductivity of approximately 10−3Scm−1 was found for Ca-doped LaNbO4. These transport characteristics can be used in sensors and fuel cells provided that the electrolyte film thickness is in the micrometre range.

Nafion/Zeolite Nanocomposite Membrane by in Situ Crystallization for a Direct Methanol Fuel Cell

Abstract: A method for preparing Nafion/acid functionalized zeolite Beta (NAFB) nanocomposite membranes by in situ hydrothermal crystallization is described. The nanocomposite membranes have a slightly lower proton conductivity but a markedly lower methanol permeability (ca. 40% reduction). When tested with 1 M methanol feed, the direct methanol fuel cells (DMFCs) with the NAFB composite membranes have a slightly higher open circuit voltage (OCV; 3%) and much higher maximum power density (21%) than those with the pure Nafion 115 membranes. With a higher methanol concentration (5 M), the DMFCs with the NAFB composite membranes demonstrate a 14% higher OCV and a 93% higher maximum power density than those with the pure Nafion 115 membranes. Compared with the commercial Nafion 115 membranes, the NAFB composite membranes have slightly lower tensile strength but higher elastic modulus.

Steady-state dc and impedance investigations of H2/O2 alkaline membrane fuel cells with commercial Pt/C, Ag/C, and Au/C cathodes.

Abstract: The performances of H2/O2 metal−cation−free alkaline anion-exchange membrane (AAEM) fuel cells operated with commercially available Au/C and Ag/C cathodes are reported for the first time. Of major significance, the power density obtained with 4 mg cm-2 Ag/C (60% mass) cathodes was comparable to that obtained with 0.5 mg cm-2 Pt/C (20% mass) electrodes, whereas the performance when using the same Ag/C cathode in a Nafion-based acidic membrane electrode assembly (MEA) was poor. These initial studies demonstrate that the oxygen reduction electrokinetics are improved when operating Pt/C cathodes at high pH in AAEM-based fuel cells as compared with operation at low pH (in Nafion-based proton-exchange membrane fuel cells). The results of in situ alternating current impedance spectroscopy were core to the assignment of the source of the limited performances of the AAEM-based fuel cells as being the limited supply of water molecules to the cathode reaction sites. Minimizing the thickness of the AAEM improved the ...

Metallic Nanoparticle−Carbon Nanotube Composites for Electrochemical Determination of Explosive Nitroaromatic Compounds

Abstract: Metal nanoparticles (Pt, Au, or Cu) together with multiwalled and single-walled carbon nanotubes (MWCNT and SWCNT) solubilized in Nafion have been used to form nanocomposites for electrochemical detection of trinitrotoluene (TNT) and several other nitroaromatics. Electrochemical and surface characterization by cyclic voltammetry, AFM, TEM, SEM, and Raman spectroscopy confirmed the presence of metal nanoparticles on CNTs. Among various combinations tested, the most synergistic signal effect was observed for the nanocomposite modified glassy carbon electrode (GC) containing Cu nanoparticles and SWCNT solubilized in Nafion. This combination provided the best sensitivity for detecting TNT and other nitroaromatic compounds. Adsorptive stripping voltammetry for TNT resulted in a detection limit of 1 ppb, with linearity up to 3 orders of magnitude. Selectivity toward the number and position of the nitro groups in different nitroaromatics was very reproducible and distinct. Reproducibility of the TNT signal was w...

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Abstract: Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elas- tic and plastic deformation of Nafion-based materials decreases with both the tempera- ture and water content. Nafion/titania composites have slightly higher elastic moduli. The composite membranes exhibit less strain hardening than Nafion. Composite mem- branes also show a reduction in the long-time creep of � 40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3-20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of � 0.55 MPa in 125-lm membranes. The resistivity of Nafion increases when the mem- brane is placed under a load. At 23 8C and 100% relative humidity, the resistivity of Nafion increases by � 15% under an applied stress of 7.5 MPa. There is a substantial hy- steresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical con- straints, and these constraints can impact fuel cell performance. V C 2006 Wiley Periodicals,

Function and Characterization of Metal Oxide−Nafion Composite Membranes for Elevated-Temperature H2/O2 PEM Fuel Cells

Abstract: Metal-oxide-recast Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFC) from 80 to 130 °C and at relative humidities ranging from 75 to 100%. Membranes of nominal 125 μm thickness were prepared by suspending a variety of metal oxide particles (SiO2, TiO2, Al2O3, and ZrO2) in solubilized Nafion. The composite membranes were characterized using electrochemical, X-ray scattering, spectroscopic, mechanical, and thermal analysis techniques. Membrane characteristics were compared to fuel cell performance. These studies indicated a specific chemical interaction between polymer sulfonate groups and the metal oxide surface for systems that provide a good elevated-temperature (i.e., fuel-cell operation above 120 °C) performance. Composite systems that incorporate either a TiO2 or a SiO2 phase produced superior elevated-temperature, low-humidity behavior compared to that of a simple Nafion-based fuel cell. Improved temperature tolerance permits the intr...

Electrocatalysis of Borohydride Oxidation on Colloidal Pt and Pt-Alloys (Pt-Ir, Pt-Ni, and Pt-Au) and Application for Direct Borohydride Fuel Cell Anodes

Abstract: Colloidal Pt and Pt-alloys (Pt-Au, Pt-Ni, and Pt-Ir, 1:1 atomic ratio) supported on Vulcan XC-72 (20% wt metal load) were prepared according to the Bonneman method and investigated for electrocatalytic activity with respect to borohydride oxidation for fuel cell applications. Voltammetry on static and rotating electrodes, chronopotentiometry, and chronoamperometry were performed on the colloidal catalysts immobilized on glassy carbon with the help of Nafion 117. The BH - 4 electro-oxidation mechanism is complex and it could involve, depending on the catalyst, a number of species such as BH - 4 directly, BH 3 OH - , and H 2 (the latter two species formed in catalytic hydrolysis). Direct borohydride fuel cell experiments using a 2 M NaOH-2 M NaBH 4 solution on the anode side, 5 mg cm - 2 colloidal anode catalyst load, Nafion 117 membrane, and an O 2 gas diffusion cathode with 4 mg cm - 2 Pt, showed that Pt-Ir and Pt-Ni were the most active anode catalysts, giving a cell voltage of 0.53 V at 100 mA cm - 2 and 333 K.

SANS Study of the Effects of Water Vapor Sorption on the Nanoscale Structure of Perfluorinated Sulfonic Acid (NAFION) Membranes

Abstract: Several poly(perfluorosulfonic acid) membranes (NAFION, EW = 1100) with the same sulfonic acid content were systematically investigated with SANS under in-situ water vapor sorption and/or with bulk water to quantify the effects of relative humidity (RH), membrane processing (melt-extruded and solution-casting), prehistory (pretreated at 80 °C and as-received), and thickness on the nanoscale structure at room temperature. The sorption isotherm (water uptake vs RH) of the membranes showed a strong correlation between the interionic domain distance (Lion) and RH. The melt-extruded membranes showed evidence of partial alignment of better organized ionic domains than those solution-cast. Pretreating the membranes resulted in a larger Lion and a broader scattering over the entire range of RH. The ionic peak of the melt-extruded membranes (as-received and pretreated) became more symmetric and narrower with sorption time. Diffusion coefficients of water vapor, based on structural evolution and Fick's second law, ...

Mesoscale simulation of morphology in hydrated perfluorosulfonic acid membranes.

Abstract: Current fuel cell proton exchange membranes rely on a random network of conducting hydrophilic domains to transport protons across the membrane. Despite extensive investigation, details of the structure of the hydrophilic domains in these membranes remain unresolved. In this study a dynamic self-consistent mean field theory has been applied to obtain the morphologies of hydrated perfluorosulfonic acid membranes (equivalent weight of 1100) as a model system for Nafion at several water contents. A coarse-grained mesoscale model was developed by dividing the system into three components: backbone, side chain, and water. The interaction parameters for this model were generated using classical molecular dynamics. The simulated morphology shows phase separated micelles filled with water, surrounded by side chains containing sulfonic groups, and embedded in the fluorocarbon matrix. The size distribution and connectivity of the hydrophilic domains were analyzed and the small angle neutron scattering (SANS) pattern was calculated. At low water content (lambda 8), the domains deform into elliptical and barbell shapes as they merge. The simulated morphology, hydrophilic domain size and shape are generally consistent with some experimental observations.

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

Abstract: This report describes a study of the effect of SiO2 nanopowders on the mechanism of ionic motion and interactions taking place in hybrid inorganic−organic membranes based on Nafion. Five nanocompos...

Transport Properties of Sulfonated Poly(styrene-b-isobutylene-b-styrene) Triblock Copolymers at High Ion-Exchange Capacities

Abstract: Transport properties of sulfonated poly(styrene-b-isobutylene-b-styrene) (S-SIBS) triblock copolymers were examined as a function of ion-exchange capacity (IEC), specifically at high IECs (up to ∼2 mequiv/g). The proton conductivity of S-SIBS was ∼1 order of magnitude higher than sulfonated polystyrene at similar IECs and 3-fold higher than Nafion 117 at an IEC of 2 mequiv/g. However, all polymers in this study possessed similar selectivities (i.e., proton conductivity/methanol permeability) regardless of chemistry or morphology. Small-angle X-ray scattering reveals a periodic-to-nonperiodic transition in S-SIBS with an anisotropic lamellar morphology oriented in the plane of the membrane at IECs ranging from 0.5 to 1 mequiv/g and an isotropic cocontinuous morphology at IECs ranging from 1.1 to 2 mequiv/g. This morphological transition coincides with a discontinuity in the IEC-dependent transport properties. In addition, S-SIBS transport properties were measured after solution casting from 15 different so...

On the Development of Naphthalene-Based Sulfonated Polyimide Membranes for Fuel Cell Applications

Abstract: This article reviews the recent progress made over the past years based on naphthalene-based sulfonated polyimides (SPIs) in terms of proton conductivity, membrane swelling behavior, membrane stability toward water, and fuel cell performance in polymer electrolyte fuel cells (PEFCs) or direct methanol fuel cells (DMFCs). The structure-property relationship of SPI membranes is discussed in details with respect to the chemical structure of various sulfonated diamines and morphology of SPI membranes from the viewpoints of viscosity, mechanical strength and proton conductivity. Ion exchange capacity (IEC), basicity of sulfonated diamine, configuration (para-, meta-, or ortho-orientation) and chemical structure of polymer chain (linear or net-work) show great influence on the water stability and mechanical strength of SPI membrane. The SPIs with a branched/crosslinked structure and derived from highly basic sulfonated diamines display reasonably high water stability of more than 200–300 h in water at 130 °C, suggesting high potential as PEMs operating at temperatures up to 100 °C. The SPI membranes have fairly high proton conductivity at higher relative humidities and low methanol permeability. The water and methanol crossover through membrane under the fuel cell operation conditions is not controlled by electro-osmosis due to proton transport but by diffusion due to activity difference. This is quite different from the case of perfluorosulfonated membranes such as Nafion and results in the advantageous effects on fuel cell performance. SPI membranes displayed high PEFC performances comparable to those of Nafion 112. In addition, SPI membranes displayed higher performances in DMFC systems with higher methanol concentration (20–50 wt %), which is superior to Nafion and have high potential for DMFC applications at mediate temperatures (40–80 °C).

Characterization of the solvation and transport of the hydrated proton in the perfluorosulfonic acid membrane nafion.

Abstract: The solvation and transport properties of the sulfonate−hydronium ion pair have been studied in hydrated Nafion through molecular dynamics simulation. Explicit proton and charge delocalization of the excess proton transport, via the Grotthuss hopping mechanism, were treated using the self-consistent multistate empirical valence bond (SCI-MS-EVB) method. The nature of the sulfonate−hydronium ion pair was characterized through analysis of free-energy profiles. It was found that, in general, the excess proton is solvated between two water molecules of a Zundel moiety while in the contact ion pair position, but then it transitions to an Eigen-like configuration in the solvent-separated pair position. Furthermore, the positive charge associated with the excess proton passes between the contact and solvent-separated ion pair positions through the Grotthuss mechanism rather than simple vehicular diffusion. The total proton diffusion was decomposed into vehicular and Grotthuss components and were found to be of t...

Effect of Water on the Low Temperature Conductivity of Polymer Electrolytes

Abstract: The proton conductivity of radiation-grafted ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid (ETFE-g-PSSA) and Nafion 117 membranes between 25 and -37 degrees C is reported. The freezing of water in the membranes, which strongly depends on the internal acid concentration, results in a 4-fold decrease in proton conductivity. The activation energies before and after the freezing of the membranes are approximately 0.15 and 0.4 eV, consistent with proton transport through liquid water and strongly bound water, respectively. Differential scanning calorimetry data show that up to 14 H(2)O molecules per H(+)/SO(3)(-) group remain unfrozen at subzero temperatures and are believed to be responsible for the low temperature conductivity that is observed. These results indicate that proton conductivity in membranes may be achieved via strongly bound and highly polarized water.

Investigation of Low-Temperature Proton Transport in Nafion Using Direct Current Conductivity and Differential Scanning Calorimetry

Abstract: The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membrane's water content per acid group, λ, and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above λ ∼ 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with λ. Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity.