Showing papers on "Nafion published in 2010"

Tunable High Performance Cross-Linked Alkaline Anion Exchange Membranes for Fuel Cell Applications

Abstract: Fuel cells are energy conversion devices that show great potential in numerous applications ranging from automobiles to portable electronics. However, further development of fuel cell components is necessary for them to become commercially viable. One component critical to their performance is the polymer electrolyte membrane, which is an ion conductive medium separating the two electrodes. While proton conducting membranes are well established (e.g., Nafion), hydroxide conducting membranes (alkaline anion exchange membranes, AAEMs) have been relatively unexplored by comparison. Operating under alkaline conditions offers significant efficiency benefits, especially for the oxygen reduction reaction; therefore, effective AAEMs could significantly advance fuel cell technologies. Here we demonstrate the use of ring-opening metathesis polymerization to generate new cross-linked membrane materials exhibiting high hydroxide ion conductivity and good mechanical properties. Cross-linking allows for increased ion incorporation, which, in turn supports high conductivities. This facile synthetic approach enables the preparation of cross-linked materials with the potential to meet the demands of hydrogen-powered fuel cells as well as direct methanol fuel cells.

Nonhumidified Intermediate Temperature Fuel Cells Using Protic Ionic Liquids

Abstract: In this paper, the characterization of a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), as a proton conductor for a fuel cell and the fabrication of a membrane-type fuel cell system using [dema][TfO] under nonhumidified conditions at intermediate temperatures are described in detail. In terms of physicochemical and electrochemical properties, [dema][TfO] exhibits high activity for fuel cell electrode reactions (i.e., the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR)) at a Pt electrode, and the open circuit voltage (OCV) of a liquid fuel cell is 1.03 V at 150 degrees C, as has reported in ref 27. However, diethylmethylammonium bis(trifluoromethane sulfonyl)amide ([dema][NTf(2)]) has relatively low HOR and ORR activity, and thus, the OCV is ca. 0.7 V, although [dema][NTf(2)] and [dema][TfO] have an identical cation ([dema]) and similar thermal and bulk-transport properties. Proton conduction occurs mainly via the vehicle mechanism in [dema][TfO] and the proton transference number (t(+)) is 0.5-0.6. This relatively low t(+) appears to be more disadvantageous for a proton conductor than for other electrolytes such as hydrated sulfonated polymer electrolyte membranes (t(+) = 1.0). However, fast proton-exchange reactions occur between ammonium cations and amines in a model compound. This indicates that the proton-exchange mechanism contributes to the fuel cell system under operation, where deprotonated amines are continuously generated by the cathodic reaction, and that polarization of the cell is avoided. Six-membered sulfonated polyimides in the diethylmethylammonium form exhibit excellent compatibility with [dema][TfO]. The composite membranes can be obtained up to a [dema][TfO] content of 80 wt % and exhibit good thermal stability, high ionic conductivity, and mechanical strength and gas permeation comparable to those of hydrated Nafion. H(2)/O(2) fuel cells prepared using the composite membranes can successfully operate at temperatures from 30 to 140 degrees C under nonhumidified conditions, and a current density of 250 mA cm(-2) is achieved at 120 degrees C. The protic ionic liquid and its composite membrane are a possible candidate for an electrolyte of a H(2)/O(2) fuel cell that operates under nonhumidified conditions.

Electrocatalytic Measurement Methodology of Oxide Catalysts Using a Thin-Film Rotating Disk Electrode

Abstract: Transition-metal oxides can exhibit high electrocatalytic activity for reactions such as the oxygen reduction reaction (ORR) in alkaline media. It is often difficult to measure and compare the activities of oxide catalysts on either per mass or per surface area basis, because of the poorly defined oxygen transport to and within porous oxide electrodes of several tens of micrometers thickness. In this study, a methodology was developed to compare the ORR activities of submicrometer-sized transition-metal oxides. Thin films of LaNiO 3 , LaCu 0.5 Mn 0.5 O 3 , and La 0.75 Ca 0.25 FeO 3 oxide particles were bonded to glassy carbon via an ion-exchanged Nafion binder, and their mass and specific ORR activities were extracted from rotating disk electrode measurements. We found that the specific activity of LaNi0 3 was much higher than that of La 0.75 Ca 0.25 FeO 3 and LaCu 0.5 Mn 0.5 O 3 . The projection of LaNiO 3 in the actual fuel cell cathode was presented, which was shown to be competitive with current platinum-based cathodes.

Solution chemistry of self-assembled graphene nanohybrids for high-performance flexible biosensors.

Abstract: We report the preparation of free-standing flexible conductive reduced graphene oxide/Nafion (RGON) hybrid films by a solution chemistry that utilizes self-assembly and directional convective-assembly. The hydrophobic backbone of Nafion provided well-defined integrated structures, on micro- and macroscales, for the construction of hybrid materials through self-assembly, while the hydrophilic sulfonate groups enabled highly stable dispersibility (∼0.5 mg/mL) and long-term stability (2 months) for graphene. The geometrically interlocked morphology of RGON produced a high degree of mechanical integrity in the hybrid films, while the interpenetrating network constructed favorable conduction pathways for charge transport. Importantly, the synergistic electrochemical characteristics of RGON were attributed to high conductivity (1176 S/m), facilitated electron transfer (ET), and low interfacial resistance. Consequently, RGON films obtained the excellent figure of merit as electrochemical biosensing platforms for...

On the micro-, meso-, and macroporous structures of polymer electrolyte membrane fuel cell catalyst layers.

Abstract: In this work, N2 adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer−Emmett−Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles ( 50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume,...

Super Proton Conductive High-Purity Nafion Nanofibers

Abstract: In this paper, we report the high proton conductivity of a single high-purity Nafion nanofiber (1.5 S/cm), which is an order of magnitude higher than the bulk Nafion film (∼0.1 S/cm). We also observe a nanosize effect, where proton conductivity increases sharply with decreasing fiber diameter. X-ray scattering provides a rationale for these findings, where an oriented ionic morphology was observed in the nanofiber in contrast to the isotropic morphology in the bulk film. This work also demonstrates the successful fabrication of high-purity Nafion nanofibers (∼99.9 wt %) via electrospinning and higher humidity sensitivity for nanofibers compared to the bulk. These results should have a significant impact on fuel cells and sensors.

Highly Sensitive and Selective Dopamine Biosensor Fabricated with Silanized Graphene

Abstract: A new type of chemically modified graphene, EDTA-modified reduced graphene (EDTA-RG), was synthesized by silanization of graphene with N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA-silane). It was found that the presence of EDTA on the graphene surface enables the formation of a very stable suspension of EDTA-RG in Nafion/ethanol solution. When deposited onto a glass carbon electrode surface, a very stable, uniform thin film of EDTA-RG−Nafion composite was successfully obtained. The electrochemical behavior of this EDTA-RG−Nafion modified electrode, electrochemical catalysis, ionic selectivity, and biocompatibility, have been investigated using a variety of electrochemical techniques. The ion selectivity was investigated by using a negatively charged probe [Fe(CN)6]3−/4−, a positively charged probe, Ru(bpy)32+, and two biomolecules, dopamine and ascorbic acid. The thin film of EDTA-RG−Nafion composite exhibits high ion selectivity. Performances of EDTA−graphene−Nafion modified electrodes ...

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Block. Synthesis and Properties

Abstract: Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks were synthesized. The degree of polymerization of hydrophobic blocks (X) was controlled to be 15, 30, and 60 and that of hydrophilic blocks (Y) to be 4, 8, 12, and 16. Morphological observation by scanning transmission microscopy (STEM) and small-angle X-ray scattering (SAXS) showed that high local concentration of sulfonic acid groups within the hydrophilic blocks enhanced phase separation between the hydrophobic and hydrophilic blocks. Rodlike hydrophilic aggregates were found to be interconnected very well, which resulted in high proton conductivity even at low relative humidity (RH). The ionomer membrane with X30Y8 and 1.86 mequiv/g of ion exchange capacity (IEC) showed 0.03 S/cm at 80 °C and 40% RH, which was a comparable or higher proton conductivity than that of the state-of-the-art perfluorinated ionomer (Nafion) membrane. The longer blocks induced higher proton conductivity; howev...

Three Phase Interfaces at Electrified Metal−Solid Electrolyte Systems 1. Study of the Pt(hkl)−Nafion Interface

Abstract: A voltammetric fingerprinting approach has been used to probe the nature of Pt−Nafion three phase interfaces for Pt(hkl) and polycrystalline platinum surfaces. Nature of adsorbing species is identified as the sulfonate anions via CO charge displacement technique. The affinity for the sulfonate anions to adsorb on the electrode surface is investigated. Adsorption strength of the sulfonate anions with the electrode surface is compared with other strongly adsorbing anions such as (bi) sulfates and chlorides. Various factors that influence the adsorption properties of the sulfonate anions are studied. Nature and strength of the anion interaction with various surface geometries is also discussed. A physical model is presented to describe the observed phenomena.

A Dusty Fluid Model for Predicting Hydroxyl Anion Conductivity in Alkaline Anion Exchange Membranes

Abstract: Advances in metal-cation-free, quaternary ammonium, polymer alkaline anion exchange membranes (AAEMs) have provided a recent resurgence of interest in the alkaline fuel cell (AFC). The alkaline environment supported by the AAEM offers several potential advantages, including opportunities for the use of non-noble metal catalysts with high energy density and logistically favorable fuels and oxidants, such as methanol and air. However, recent experimental literature has shown that the AAEM derived AFCs have considerable resistive losses that can be attributed to the AAEM. This work describes a dusty fluid model used to predict AAEM conductivities as a function of relative humidity and membrane properties in an initial attempt at forming a framework for understanding the processes at work. A percolation model is used to account for the membrane structure. The model is validated using Nafion 115 conductivity data.

Chemical Degradation of Nafion Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR Spectroscopy

Abstract: A new ex situ method has been developed to mimic the degradation of the polymer membranes in polymer electrolyte membrane fuel cells (PEMFCs), caused by the cross-leakage of H2 and O2. In this ex situ setup, it is possible to expose membranes to flows of different gases with a controlled temperature and humidity. H+-form Nafion films with and without an electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion-exchange capacity measurements were performed. The molecular mobility changes of the ionomer membrane upon degradation were examined for the first time by variable-temperature 19F NMR line-shape, T1 and T1ρ relaxation experiments. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage) and w...

Structure-relaxation interplay of a new nanostructured membrane based on tetraethylammonium trifluoromethanesulfonate ionic liquid and neutralized nafion 117 for high-temperature fuel cells.

Abstract: In this report, the electrical performance at T > 100 °C and low relative humidity of proton-conducting Nafion-based membranes was improved by preparing new materials based on Nafion 117 (N117) neutralized with triethylammonium (TEA+) and doped with the ionic liquid (IL) trifluoromethanesulfonate of triethylammonium (TEA-TF). In particular, a new two-step protocol for the preparation of [N117x−(TEA+)x/(TEA-TF)y] is proposed. [N117x−(TEA+)x/(TEA-TF)y] membrane is composed of ca. 30 wt % of TEA-TF. The structure of the different nanophases composing the materials and their interactions were investigated by FT-IR ATR and micro-Raman spectroscopy. The thermal stability, water uptake, and mechanical properties of the membranes were studied by thermogravimetric analysis and dynamic mechanical analysis measurements. With respect to pristine N117, the thermal and mechanical properties of the proposed materials were improved. The electric response of [N117x−(TEA+)x/(TEA-TF)y] was studied by broad band dielectric s...

Surface Structure of Nafion in Vapor and Liquid

Abstract: 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 ...

Oxygen reduction reaction at three-phase interfaces.

Abstract: The kinetics of the oxygen reduction reaction (ORR) is studied at metal-supporting electrolyte-Nafion three-phase interfaces. We first demonstrate that the sulfonate anions of Nafion are specifically adsorbed on a wide range of surfaces ranging from Pt(hkl) single-crystal surfaces, Pt-poly, Pt-skin [produced on a Pt(3)Ni(111) surface by annealing in ultrahigh vacuum, UHV] to high-surface-area nanostructured thin-film (NSTF) catalysts. The surface coverage by sulfonate and the strength of the Pt-sulfonate interaction are strongly dependent on the geometry and the nature of the Pt surface atoms. Also, they are found to behave analogous to (bi)sulfate anion-specific adsorption on these surfaces, where for the Pt(hkl) surfaces, the trend is Pt(111)>Pt(110)>Pt(100) and for the Pt-skin surface on Pt(3)Ni(111), the interaction strength is found to be Pt-skin

An inorganic–organic proton exchange membrane for fuel cells with a controlled nanoscale pore structure

Abstract: Proton exchange membrane fuel cells have the potential for applications in energy conversion and energy storage, but their development has been impeded by problems with the membrane electrode assembly. Here, we demonstrate that a silicon-based inorganic–organic membrane offers a number of advantages over Nafion—the membrane widely used as a proton exchange membrane in hydrogen fuel cells—including higher proton conductivity, a lack of volumetric size change, and membrane electrode assembly construction capabilities. Key to achieving these advantages is fabricating a silicon membrane with pores with diameters of ∼5–7 nm, adding a self-assembled molecular monolayer on the pore surface, and then capping the pores with a layer of porous silica. The silica layer reduces the diameter of the pores and ensures their hydration, resulting in a proton conductivity that is two to three orders of magnitude higher than that of Nafion at low humidity. A membrane electrode assembly constructed with this proton exchange membrane delivered an order of magnitude higher power density than that achieved previously with a dry hydrogen feed and an air-breathing cathode.