Showing papers on "Nafion published in 2003"

Solubilization of carbon nanotubes by Nafion toward the preparation of amperometric biosensors.

Abstract: The ability to solubilize single-wall and multiwall carbon nanotubes (CNT) in the presence of the perfluorinated polymer Nafion is described. Such use of Nafion as a solubilizing agent for CNT overcomes a major obstacle for creating CNT-based biosensing devices. Their association with Nafion does not impair the electrocatalytic properties of CNT. The resulting CNT/Nafion modified glassy-carbon electrodes exhibit a strong and stable electrocatalytic response toward hydrogen peroxide. The marked acceleration of the hydrogen peroxide redox process is very attractive for the operation of oxidase-based amperometric biosensors, as illustrated for the highly selective low-potential (−0.05 V vs Ag/AgCl) biosensing of glucose. These findings open the door for using CNT in a wide range of chemical sensors and nanoscale electronic devices.

Proton Conduction Mechanisms at Low Degrees of Hydration in Sulfonic Acid–Based Polymer Electrolyte Membranes

Abstract: ▪ Abstract The need to operate polymer electrolyte membrane (PEM) fuel cells at temperatures above 100°C, where the amount of water in the membrane is restricted, has provided much of the motivation for understanding the mechanisms of proton conduction at low degrees of hydration. Although experiments have not provided any direct information, numerous theoretical investigations have begun to provide the basis for understanding the mechanisms of proton conduction in these nano-phase-separated materials. Both the hydrated morphology and the nature of the confined water in the hydrophilic domains influence proton dissociation from the acidic sites (i.e., −SO3H), transfer to the water environment, and transport through the membrane. The following molecular processes are discussed in connection to their role in the conduction of protons in sulfonic acid–based polymer electrolyte membranes (PEMs): (a) local chemistry of the hydrophilic side chains; its effect on the dissociation of the proton and eventual stabi...

Comparative experimental study of ionic polymer–metal composites with different backbone ionomers and in various cation forms

Abstract: An ionic polymer–metal composite (IPMC) consisting of a thin perfluorinated ionomer (usually, Nafion or Flemion) strip, platinum, and/or gold plated on both faces and neutralized by a certain amount of appropriate cations undergoes large bending motion when, in a hydrated state, a small electric field is applied across its thickness. When the same membrane is suddenly bent, a small voltage of the order of millivolts is produced across its surfaces. Hence IPMCs can serve as soft bending actuators and sensors. This coupled electrical–chemical–mechanical response of IPMCs depends on the structure of the backbone ionic polymer, the morphology and conductivity of the metal electrodes, the nature of the cations, and the level of hydration (or other solvent uptake). We have carried out extensive experimental studies on both Nafion- and Flemion-based IPMCs in various cation forms, seeking to understand the fundamental properties of these composites, to explore the mechanism of their actuation, and finally, to optimize their performance for various potential applications. The results of some of these tests on both Nafion- and Flemion-based IPMCs with alkali-metal or alkyl-ammonium cations are reported here. Compared with Nafion-based IPMCs, Flemion-based IPMCs with fine dendritic goldelectrodes have higher ion-exchange capacity, better surface conductivity, higher hydration capacity, and higher longitudinal stiffness. They also display greater bending actuation under the same applied voltage. In addition, they do not display a reverse relaxation under a sustained dc voltage, which is typical of Nafion-based IPMCs in alkali-metal form. Flemion IPMCs thus are promising composites for application as bending actuators.

Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading

Abstract: The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization. determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.

Effect of acidification treatment and morphological stability of sulfonated poly(arylene ether sulfone) copolymer proton‐exchange membranes for fuel‐cell use above 100 °C

Abstract: Directly copolymerized wholly aromatic sulfonated poly(arylene ether sulfone) copolymers derived from 4,4'-biphenol, 4,4'-dichlorodiphenyl sulfone, 3,3'-disulfonated, and 4,4'-dichlorodiphenyl sulfone (BPSH) were evaluated as proton-exchange membranes for elevated temperature operation (100-140 °C). Acidification of the copolymer from the sulfonated form after the nucleophilic step (condensation) copolymerization involved either immersing the solvent-cast membrane in sulfuric acid at 30 °C for 24 h and washing with water at 30 °C for 24 h (method 1) or immersion in sulfuric acid at 100 °C for 2 h followed by similar water treatment at 100 °C for 2 h (method 2). The fully hydrated BPSH membranes treated by method 2 exhibited higher proton conductivity, greater water absorption, and less temperature dependence on proton conductivity as compared with the membranes acidified at 30 °C. In contrast, the conductivity and water absorption of a control perfluorosulfonic acid copolymer (Nafion 1135) were invariant with treatment temperature; however, the conductivity of the Nafion membranes at elevated temperature was strongly dependent on heating rate or temperature. Tapping-mode atomic force microscope results demonstrated that all of the membranes exposed to high-temperature conditions underwent an irreversible change of the ionic domain microstructure, the extent of which depended on the concentration of sulfonic acid sites in the BPSH system. The effect of aging membranes based on BPSH and Nafion at elevated temperature on proton conductivity is also discussed.

Electrogenerated chemiluminescence from tris(2,2'-bipyridyl)ruthenium(II) immobilized in titania-perfluorosulfonated ionomer composite films

Abstract: Electrochemical behavior and electrogenerated chemiluminescence (ECL) of tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)32+) immobilized in sol−gel-derived titania (TiO2)−Nafion composite films coated on a glassy carbon electrode have been investigated. The electroactivity of Ru(bpy)32+ ion exchanged into the composite films and its ECL behavior were strongly dependent upon the amount of Nafion incorporated into the TiO2−Nafion composite films. The ECL sensor of Ru(bpy)32+ immobilized in a TiO2−Nafion composite with 50% Nafion content showed the maximum ECL intensities for both tripropylamine (TPA) and sodium oxalate in 0.05 M phosphate buffer solution at pH 7. Detection limits were 0.1 μM for TPA and 1.0 μM for oxalate (S/N = 3) with a linear range of 3 orders of magnitude in concentration. The present ECL sensor showed improved ECL sensitivity and long-term stability, as compared to the ECL sensors based on pure Nafion films. The present Ru(bpy)32+ ECL sensor based on TiO2−Nafion (50%) composite films was ap...

A Fuel Cell Development for Using Borohydrides as the Fuel

Abstract: A fuel cell was developed using borohydride solutions as the fuel. The cell consisted of an anode made of a Zr-Ni alloy, a cathode made of Pt/C, and a form Nafion membrane as the electrolyte. The borohydride-fueled cell showed an open-circuit voltage of 1.3 V, compared with 1.0 V for a hydrogen gas-fueled one. The anode exhibited a small polarization property compared with the cathode. The cathode polarization was the main reason for the cell voltage drop with increasing currents. When a Nafion membrane was used as the electrolyte, it was confirmed that cations were the charge carrier in it. Compared with Nafion 112 membrane, Nafion 117 membrane demonstrated a considerable resistance to borohydride crossover and resulted in acceptable cell performance. However, there are several problems such as evolution during operation, crossover, NaOH accumulation at the cathode, and accumulation at the anode in recent systems. Further effort is needed to develop the fuel cell using borohydrides as the fuel. © 2003 The Electrochemical Society. All rights reserved.

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

Abstract: 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.

Fixation of nanosized proton transport channels in membranes

Abstract: The structure of proton transport channels is controlled and fixed in sulfonated poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer membranes. Microphase-separated SBS membranes were cross-linked by UV irradiation in the presence of a photoinitiator and then sulfonated by acetyl sulfate. Sulfonated cross-linked SBS (scSBS) membranes with fixed nanosized proton transport channels were then prepared. The scSBS membranes show good proton conductivity, comparable to that of Nafion, and a low methanol permeability, more than 1 order of magnitude smaller than that of Nafion. Sulfonated non-cross-linkable poly(styrene-b-(ethylene-r-butylene)-b-styrene) membranes and sulfonated cross-linked poly(styrene-r-butadiene) membranes, which have less developed proton transport channels, were also prepared for comparison. The effects of the presence and size of the proton transport channels on the proton conductivity and methanol permeability were investigated. It is concluded that both the structure and the fix...

Degradation mechanism of polystyrene sulfonic acid membrane and application of its composite membranes in fuel cells

Abstract: The lifetime behavior of a H2/O2 proton exchange membrane (PEM) fuel cell with polystyrene sulfonic acid (PSSA) membrane have been investigated in order to give an insight into the degradation mechanism of the PSSA membrane. The distribution of sulfur concentration in the cross section of the PSSA membrane was measured by energy dispersive analysis of X-ray, and the chemical composition of the PSSA membrane was characterized by infrared spectroscopy before and after the lifetime experiment. The degradation mechanism of the PSSA membrane is postulated as: the oxygen reduction at the cathode proceeds through some peroxide intermediates during the fuel cell operation, and these intermediates have strong oxidative ability and may chemically attack the tertiary hydrogen at the α-carbon of the PSSA; the degradation of the PSSA membrane mainly takes place at the cathode side of the cell, and the loss of the aromatic rings and the SO3− groups simultaneously occurs from the PSSA membrane. A new kind of the PSSA-Nafion composite membrane, where the Nafion membrane is bonded with the PSSA membrane and located at the cathode of the cell, was designed to prevent oxidation degradation of the PSSA membrane in fuel cells. The performances of fuel cells with PSSA-Nafion101 and PSSA-recast Nafion composite membranes are demonstrated to be stable after 835 h and 240 h, respectively.

Fast Ion Conduction in Layer-By-Layer Polymer Films

Abstract: The layer-by-layer (LBL) deposition technique has been applied to the design of polymer electrolyte films appropriate for electrochemical applications such as sensors and electrochromic cells. In this work, we describe the properties of three LBL polymer electrolyte systems assembled from cationic layers of linear poly(ethylene imine) (LPEI), with anionic layers of Nafion, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), and poly(acrylic acid) (PAA). The assembly behavior of these systems was carefully examined, and ionic conductivity was determined using impedance spectroscopy. The influences of assembly conditions and water plasticization on ion conduction were elaborated. Room-temperature ionic conductivity greater than 10-5 S/cm can be achieved within LPEI/PAMPS and LPEI/PAA films, which is 2 orders of magnitude greater than the highest values previously described in LBL films. By manipulating a unique assembly mechanism, high ionic conductivity can be achieved in LPEI/PAMPS films at low pl...

Sulfonated Poly(ether ether ketone) Membranes for Direct Methanol Fuel Cells

Abstract: Sulfonated poly(ether ether ketone) (SPEEK) with different degrees of sulfonation has been prepared and evaluated as proton exchange membrane electrolytes in direct methanol fuel cells (DMFCs). The membranes have been characterized by ion-exchange capacity, proton conductivity, and liquid uptake measurements. The proton conductivity of the SPEEK membranes increases with increasing sulfonation level, and are lower than that of Nafion. The percent liquid uptake increases with increasing temperature, methanol concentration, and degree of sulfonation. Within a narrow range of sulfonation of ∼50%, the SPEEK membranes exhibit electrochemical performances comparable to or exceeding that of Nafion at 65°C, making it an attractive low-cost alternative to Nafion. The better performance of the SPEEK membranes is due to the suppression of methanol permeability as indicated by a lower methanol crossover current density at the cathode.

Self-Humidifying Electrolyte Membranes for Fuel Cells Preparation of Highly Dispersed TiO 2 Particles in Nafion 112

Abstract: We propose self-humidifying polymer electrolyte membranes (PEM) with highly dispersed nanometer-sized Pt and/or metal oxides for polymer electrolyte fuel cells (PEFCs) operated with dry H 2 and O 2 The Pt particles were expected to suppress the crossover by the catalytic recombination of H 2 and O 2 , while the oxide particles (TiO 2 ) that have hygroscopic property were expected to adsorb the water produced at Pt particles together with that produced at the cathode reaction and to release the water once the PEM needs water The preparation protocol of TiO 2 nanoparticles in a commercial Nafion 112 membrane via in situ sol-gel reactions was developed, resulting in a transparent membrane with uniform distribution of TiO 2 in the PEM Water adsorbability increased more than two times by dispersing only 2 wt % TiO 2 in the PEM That newly prepared TiO 2 -PEM cooperated with highly dispersed Pt nanoparticles (Pt-TiO 2 -PEM) was confirmed to perform a self-humidifying operation in a PEFC with dry H 2 and O 2