Showing papers in "Fuel Cells in 2006"

The Mechanism of Forming H2O from H2 and O2 over a Pt Catalyst via Direct Oxygen Reduction

Abstract: Density functional theory (DFT) was used with the B3LYP gradient-corrected exchange–correlation functional to study the mechanism for the reaction of H2 + O2 → H2O over a Pt catalyst via direct oxygen reduction. Within these studies we first examined the binding characteristics and energetics for each likely intermediate chemisorbed on the Pt(111) surface, modeled by a 35 atom cluster: O, H, O2, H2, OH, OOH, H2O. Then, the pathways for the dissociation processes of the various intermediates on the Pt35 cluster were calculated. For convenience in comparing different reaction steps, these energetics were used to calculate heats of formation (ΔHf), which were combined with the dissociation barriers. Two main reaction pathways were found for the formation of H2O from H2 and O2: •(OO-Dissociation: Here O2 adsorbs on the surface, dissociates, and finally reacts with H sequentially to firstly form OH and then water. The rate-determining step (RDS) for this mechanism is the Oad + Had → OHad with a barrier of 31.66 kcal mol–1 and not the dissociation of O2ad (barrier of 15.02 kcal mol–1). •OOH-Formation: Here O2 reacts firstly with Had to form OOHad, which then dissociates to form OHad and Oad (the RDS, with a barrier of 17.13 kcal mol–1), which finally reacts with another Had to form water. Thus, under gas-phase conditions, the OOH-Formation mechanism is found to be the most favorable.

Break down of losses in thin electrolyte SOFCs

Abstract: The contributions of the individual components of the cell (anode, cathode, and electrolyte) to the cell resistance were determined experimentally, directly from impedance spectra obtained from a full cell It was an anode supported thin electrolyte cell, consisting of a YSZ electrolyte, a Ni/YSZ cermet anode, and a LSM composite cathode Additional, qualitative information was obtained using symmetric cells with LSM composite electrodes The investigations were carried out in the temperature interval from 700 to 850 °C The electrolyte and anode activation energies obtained were 09 and 11 eV, respectively, which is in relatively good agreement with literature values The anode resistance was 024 Ω cm2 and the cathode resistance was 058 Ω cm2 at 700 °C, corresponding to 23% and 56% of the total resistance, respectively

Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane

Abstract: Intrinsically proton conducting polymers with imidazole as proton solvent tethered to a polysiloxane backbone via a flexible spacer have been synthesized. Apart from the standard characterization also their thermal properties and transport behavior have been investigated. The materials exhibit proton conductivity as a consequence of self-dissociation of the imidazole moieties and “structure diffusion” of the resulting defects. In particular, no liquid phase such as water or monomeric imidazole is needed for the observed proton conductivities. To study the influence of the tether structure on the transport properties, cyclic oligomers and open chain polymers with different spacer lengths have been synthesized. The materials are thermally stable up to 200 °C and become soft around room temperature. The conductivity exhibits VTF and WLF behavior with maximum conductivities around σ = 1.5.10–3 S cm–1 at T = 160 °C. The activation volume of the conductivity as derived from pressure dependent measurements is found to be unusually high. The lowest activation volumes and the highest conductivities are observed for the materials with the highest segmental mobilities, i.e. the longest spacers. Proton self-diffusion coefficients as obtained from PFG NMR diffusion measurements are significantly higher than expected from the proton conductivities obtained by dielectric spectroscopy. This corresponds to unusually high Haven ratios which have been interpreted by correlated proton transfers allowing for fast proton diffusion while minimizing the separation of ionic charge carriers.

Thin Film Microelectrodes in SOFC Electrode Research

Abstract: Experimental studies using conventional (porous) solid oxide fuel cell (SOFC) electrodes are often rather difficult to interpret in terms of a mechanistic understanding of the electrochemical polarization phenomena. Owing to the complex morphology and structure of porous electrodes, a quantitative determination of the properties of the electrochemical materials rather than of the effective electrode properties is far from being straightforward. Micro-patterned epitaxially grown thin film electrodes offer new possibilities in this field of research. This is particularly true for microelectrodes of several 10 μm diameter, fabricated lithographically from thin films. They allow well-defined geometry-dependent experiments, minimize the importance of ohmic drops, and avoid the necessity of a reference electrode. Further, they turn out to be particularly well-suited for statistical studies, as well as for the investigation of irreversible processes. Hence, thin-film microelectrodes are an excellent tool for the investigation of SOFC electrode materials. This is exemplified by p(O2)-, voltage-, and geometry-dependent measurements on (La0.8Sr0.2)0.92MnO3 and La0.6Sr0.4Co0.8Fe0.2O3–δ microelectrodes on single-crystal yttria-stabilized zirconia electrolytes.

Growth Mechanisms and Electrical Conductivity of Oxide Scales on Ferritic Steels Proposed as Interconnect Materials for SOFC's

Abstract: The oxidation behaviour of two ferritic steel variants was studied at 800 °C in air and the results were compared with data obtained for two Cr-based materials. The mechanisms of scale formation were investigated for oxidation times ranging from a few minutes up to 6,000 hours. A number of conventional analysis techniques such as optical metallography, scanning electron microscopy, and X-ray diffraction were used for scale characterization, in combination with two-stage oxidation studies using an 18O-tracer. It was found that the growth rates of the scales on the two steels were not only governed by the main scale forming alloying elements, Cr and Mn, but to a substantial extent by minor additions of Si and Al. At the test temperature of 800 °C these latter elements affect scale formation, although they are not directly incorporated in the surface scales. Independent of a detailed alloy composition, the conductivities of the scales on the ferritic steels were found to be higher than those of the surface scales formed on the Cr-based materials studied.

Carbonyl Tailored Electrocatalysts

Abstract: In recent years, a number of carbonyl clusters have been used as precursors to synthesize electrocatalysts of the nanolength scale for fuel cell reactions. This short review aims to delineate the concepts underlying the synthesis of these type of materials, highlighting the rationale of this type of research. Some selected examples regarding fuel cell reactions at the anode and the cathode are discussed.

Material Aspects of the Design and Operation of Direct Borohydride Fuel Cells

Abstract: The direct borohydride fuel cell (DBFC) has attracted increasing interest as a potential high power source for mobile and portable applications. Engineering design plays an important role in the development of the DBFC. This paper reports data for the selection of anode, cathode, and membrane materials for the DBFC. The best DBFC performance is achieved with a Au anode, a Pt cathode, and a 3541P ion exchange membrane. The use of non-precious catalysts, e.g., Ag, leads to promising results.

Internal Reforming of Methane at Ni/YSZ and Ni/CGO SOFC Cermet Anodes

Abstract: The internal reforming of methane on Ni/CGO and Ni/YSZ anodes was investigated with single cells operated at steam to carbon ratios from 0 to 3 and at temperatures of 800 °C and 950 °C. The incorporation of gas extraction ports allowed the measurement of the local gas composition in the anode gas compartment by gas chromatography. The methane conversion is presented as a function of feed gas composition, temperature, gas flow velocity, and electrical load. The impact of the anode material on the reforming reaction and on cell performance is shown. Methane conversion along the Ni/CGO anode was calculated with a one-dimensional model; the required kinetic parameters were obtained by data fitting.

Solid oxide fuel cell performance under severe operating conditions

Abstract: The performance and degradation of Solid Oxide Fuel Cells (SOFC) were studied under severe operating conditions. The cells studied were manufactured in a small series by ECN, in the framework of the EU funded CORE-SOFC project. The cells were of the anode-supported type with a double layer LSM cathode. They were operated at 750 °C or 850 °C in hydrogen with 5% or 50% water at current densities ranging from 0.25 A cm–2 to 1 A cm–2 for periods of 300 hours or more. The area specific cell resistance, corrected for fuel utilisation, ranged between 0.20 Ω cm2 and 0.34 Ω cm2 at 850 °C and 520 mV, and between 0.51 Ω cm2 and 0.92 Ω cm2 at 750 °C and 520 mV. The degradation of cell performance was found to be low (ranging from 0 to 8%/1,000 hours) at regular operating conditions. Voltage degradation rates of 20 to 40%/1,000 hours were observed under severe operating conditions, depending on the test conditions. Data analysis revealed a critical cell voltage of ca 750 mV, above which the degradation rates were trivial, but below which they were significant. Some cells were also tested using a different procedure to that usually applied at Riso. This gave a different aging behaviour, indicating that the detailed test circumstances may be decisive to the outcome.

Alternative Catalysts and Carbon Support Material for PEMFC

Abstract: In order to investigate the possibility of increasing the reactivity for oxygen reduction reaction (ORR) of the cathode in a PEMFC a series of Pt/C catalysts was prepared using water-in-oil micro ...

Effect of Gas Dilution on PEM Fuel Cell Performance and Impedance Response

Abstract: In this work, an experimental study of gas dilution on the performance of a polymer electrolyte membrane (PEM) fuel cell was carried out. Impedance spectra were analysed depending on the dilution of either hydrogen or oxygen by nitrogen. By comparison of the impedance spectra obtained, the three loops were attributed to hydrogen oxidation, oxygen reduction and a low frequency diffusion process. Hydrogen oxidation was shown to be more complicated when the anode side was fed by a simulated reforming gas than with the same dilution rate with nitrogen.

Curvature of Planar Solid Oxide Fuel Cells during Sealing and Cooling of Stacks

Abstract: Thin solid oxide fuel cells (SOFCs), with a planar multi-layer structure, typically exhibit curvature behavior and develop residual stresses originating from the thermal mismatch of the materials involved. The curvature effects oppose successful stack operation, which also relies on the permanent physical contact of geometrically stable planar cells. Curvature studies have been carried out as part of the project “Component Reliability in Solid Oxide Fuel Cell Systems for Commercial Operation” (CORE-SOFC). Various aspects of the curvature of unconstrained cells and cells fixed in stacks were considered. The experiments and theoretical analyses included the curvature changes due to anode reduction and the suppression of cell curvature, either by a compensation layer or by an additional surface load. The critical stresses in the layers are addressed and results are reported for different cell thicknesses and lengths. Cell flatness can be achieved with a compensation layer but the average residual tensile stress in the anode increases. On the other hand finite element analysis (FEA) indicates that relatively high loads are necessary to completely suppress cell curvature. Furthermore, analytical and FEA results on the curvature changes of sealed cells during cooling are presented and the impact of variations in sealing geometry is illustrated. The results are compared to in-situ observations of model stacks, where curvature and strain are analyzed using image correlation.

Preparation of Porosity-Graded SOFC Anode Substrates

Abstract: Porosity graded anode substrates for solid oxide fuel cells are considered to optimise the gas transport through the substrate by maintaining a high electrochemical activity for fuel oxidation at the anode/solid electrolyte interface. In this work, the fabrication of porosity graded anode substrates, made from nickel oxide and yttria-modified zirconia and produced by dry uni-axial pressing, are described. Using carbon as pore formers and adjusting the particle size distribution in the ceramic NiO-YSZ masses, samples with gradually changing porosity are built up. The sintering behaviour of the individual layers is analysed and partly adjusted so the multi-layer support can be co-fired together with the YSZ electrolyte layer. In the oxidized state, four-layer, porosity graded anode supported half-cells with a dense YSZ electrolyte are demonstrated.

Comparison between Nafion® and a Nafion® Zirconium Phosphate Nano‐Composite in Fuel Cell Applications

Abstract: A comparative investigation of the electrical, mechanical, and chemical behaviour of zirconium phosphate-Nafion® composite membranes and Nafion® by means of ex-situ measurements, as well as with fuel cell operation, reveals a slight reduction of ionic conductivity, a significant improvement of mechanical stability, and increased water retention for the composite materials. The overall efficiency at 130 °C is increased during direct methanol fuel cell (DMFC) operation because the reduction in the ionic conductivity is overcompensated for by the decrease in methanol crossover. With H2 as the fuel, the slight reduction in overall efficiency corresponds to the decrease in ionic conductivity. The dimensional stability of the membrane and the membrane electrode assembly (MEA) is significantly improved for operating temperatures above 100 °C. A model for the microstructure-property relation for PFSA-Zr(HPO4)2 · n H2O composite membranes is presented, based on the experimental results from membranes with varying filler contents and distributions, obtained through different synthesis routes. It is aimed at the improvement of water distribution in the membrane upon fuel cell operation.

Electrode Activation and Passivation of Solid Oxide Fuel Cell Electrodes

Abstract: The performance of anode-supported cells with a composite LSM-YSZ cathode and an LSM current collector was investigated. Over the first 48 hours, after the application of a constant current, the cell voltage was observed to increase by up to 20%. When the current was switched off, the cell resistance increased significantly over the next four days at open circuit conditions. Apparently, at OCV conditions cell passivation occurs. The cell gradually reactivates, once the current is switched on again. Part of this activation / passivation process is fast enough to influence the resistance of the cell during i–V measurements (over less than 1 hour) and a considerable hysteresis is observed in the cell voltage during these measurements. Impedance spectroscopy was used to investigate the activation / passivation process. It was found that the series resistance and the part of the polarisation impedance above approximately 100 Hz were not influenced by the activation / passivation process. The part of the polarisation impedance between 1 and 100 Hz was highly influenced by the activation / passivation process and during cell polarisation this part of the polarisation impedance was up to 40% lower than at open circuit conditions. This frequency range of the spectrum was also sensitive to the oxygen partial pressure at the cathode side, indicating that it is the cathode that activates and passivates.

Covalent‐Ionically Cross‐linked Poly(Etheretherketone)‐Basic Polysulfone Blend Ionomer Membranes

Abstract: Sulfinated-sulfonated PEEK was synthesized via partial reduction of sulfochlorinated PEEK with aqueous Na2SO3. From these polymers, covalent-ionically cross-linked ionomer blend membranes were prepared by mixing the sulfinated-sulfonated PEEK with different base-modified polysulfones (PSU-base), followed by cross-linking with 1,4-diiodobutane. These membranes have been compared with covalently cross-linked membranes which were obtained by the cross-linking of sulfinated-sulfonated PEEK and with ionically cross-linked membranes obtained by mixing sulfonated PEEK with different PSU bases. The membranes have been characterized in terms of thermal stability by means of thermogravimetry (TGA) and coupled TGA-FTIR, in terms of cross-linking extent by extraction experiments, in terms of proton conductivity by impedance spectroscopy, and finally, in terms of water uptake by gravimetric analysis.

Testing of Ni‐Plated Ferritic Steel Interconnect in SOFC Stacks

Abstract: Stack tests were run at 850 °C for periods from 80 hours to 1,150 hours to develop contacting procedures and at the same time evaluate the performance of a 5 μm electroplated nickel coating on a ferritic Fe22Cr interconnect. The metallic nickel coating reacted relatively quickly during the initial heating to 1,030 °C. During this time, 20–70 μm thick surface layers of austenitic steel were formed, which were covered by a 1–4 μm chromia layer on the anode side and by a layer of mixed Cr-Fe-Ni-spinels over a 1–4 μm chromia layer on the cathode side. The microstructure and composition of the protective scale on the cathode side was susceptible to pitting-type corrosion patterns, which may limit the life expectancy to less than 2,000 hours for the 200 μm thick interconnect tested. The initial area-specific resistances (ASR) at the interconnect/cathode current collector interface and the interconnect/anode current collector interface were very low (2 and 10 mΩ cm2, respectively). The cathode side interface resistance increased over the 1,150 hours by ∼7 mΩ cm2/103�h, but the anode side interface resistance decreased during the first 600 hours of the experiment before it started to show a slight increase, < 1 mΩ cm2/103�h, maintaining values below 1 mΩ cm2.

Thermo‐Mechanical Model of Solid Oxide Fuel Cell Fed with Methane

Abstract: Internal steam reforming induces an inhomogeneous temperature distribution in SOFC cell. A thermo-mechanical model has been developed to determine both the temperature gradient and the stress field within an anode supported cell structure fed with methane. Residual stresses due to manufacturing process and thermal stresses induced by the mismatch in thermal expansion coefficients have been taken into account. Mechanical external loading of the cell has been neglected. The results of this study have shown a cooling area at the inlet of the cell due to the endothermic steam reforming reaction. However, in our conditions of simulation, the temperature gradient in the cell has been found relatively low (0.2 °C mm–1). Consequently, thermal stresses in the three layers of the cell are mainly due to the heating from room temperature to the operating one. Mechanical calculations have been performed considering a low compressive residual stress in the thin electrolyte at room temperature. In these conditions, a high tensile stress has been calculated in the thin electrolyte at operating temperature. Its value is nearly homogenous all over the layer and exceeds the limit strength of the Yttria Stabilised Zirconia. The brittle fracture of electrolyte layer in one thermal cycle has been analysed by a Weibull distribution. The failure probability reaches 66% for a 60 micrometer electrolyte thickness. However, this result depends strongly on the layer thickness. Moreover, a singular stress field analysis has allowed determining an important tensile stress at the anode edge (on the circumferential free surface). It has been shown that fracture can be initiated at defects in this tensile surface, if the fracture toughness of the anodic cermet is inferior to 0.41 MPa √m.

New Composite DMFC Anode with PEDOT as a Mixed Conductor and Catalyst Support

Abstract: The conducting polymer PEDOT is tested as a carbon substitute material for the DMFC anode. Electrochemical experiments are carried out in a three-electrode half-cell at room temperature. The first part of this paper focuses on the characterization of PEDOT and PEDOT-PSS as a diffusion layer material. PEDOT/C cyclic voltammograms are recorded in 1 M H2SO4 in the range between the hydrogen evolution and polymer over-oxidation potentials. The incorporation of the PSS counter-ion into the polymer matrix leads to higher impedance values compared to PEDOT without PSS. The second part deals with the study of PEDOT as a Pt catalyst support. It is found that the partial over-oxidation of PEDOT enhances the activity of Pt for methanol oxidation. This can be explained by a change in the morphology of PEDOT that probably improves methanol diffusion within the reaction layer. The degradation of the conducting polymer is confirmed by SEM, EDX, and FTIR measurements.

Corrosion Behaviour of Stainless Steel Plates in PEMFC Working Conditions

Abstract: Stainless steels commonly used as bipolar plates in PEMFC may corrode and produce metallic ions in anodic and cathodic media. In order to avoid or to limit as low as possible these pollutions, which reduce the fuel cell efficiency, studies are performed to understand occurring phenomena. The characterizations of the different stainless steels behaviour are obtained by means of electrochemical techniques in synthetic solutions representative of anodic and cathodic media.

SOFC Modelling and Numerical Simulation of Performances

Abstract: The main purpose of this work is to establish the macroscopic impact of solid oxide thin layer electrolytes, which are proposed to reduce SOFC (solid oxide fuel cell) operating temperature. This work deals with the planar technology configuration. Fuel and oxidant flow through numerous small tubes. The electrical performance analysis of such an elementary fuel cell consist in establishing the relation between tension and current and the influence of parameters such as tubes dimensions, thickness and electrical conductivity of thin layers and fuel used. This study uses fundamental laws of species and energy conservation, as well as electrochemical and electrical laws. Stack performance calculation is mainly a thermal and electrical calculation. This is done here by using Fluent® code calculations and C programming. Particular attention is given to the thermal properties, because of the importance of the dilatation problem in SOFC technology.

On the Suitability of Hydrous Ruthenium Oxide Supports to Enhance Intrinsic Proton Conductivity in DMFC Anodes

Abstract: Hydrous ruthenium oxides have been investigated as novel support materials for fuel cell electrocatalysts for use in DMFC applications. These oxides were chosen in particular due to their potential intrinsic proton conductivity. Pt nanoparticles have been deposited onto the new support, and the resulting catalysts characterized both structurally and electrochemically. The Pt nanoparticles are sized between 3–4 nm and are highly dispersed on the support. Transmission electron micrographs show that the individual Pt nanoparticles are covered by an amorphous coating layer – probably hydrous ruthenium oxide, in good agreement with the XPS data. Electrochemical measurements on model electrodes indicate that proton conductivity of the supporting material is strongly affected by interdiffusion of methanol. Nevertheless, initial tests on membrane electrode assemblies (MEAs) showed improved performance, particularly with respect to internal resistance, when compared to Pt-Ru black. The catalyst showed very high activity in CO stripping experiments performed on a full MEA, suggesting high catalyst utilization, despite the comparatively low Nafion® content used in the electrode layer.

On the Role of Reactant Transport and (Surface) Alloy Formation for the CO Tolerance of Carbon Supported PtRu Polymer Electrolyte Fuel Cell Catalysts

Abstract: The role of atomic scale intermixing for the electrocatalytic activity of bimetallic PtRu anode catalysts in reformate operated polymer electrolyte fuel cells (PEFC) was investigated, exploiting the specific properties of colloid based catalyst synthesis for the selective preparation of alloyed and non-alloyed bimetallic catalysts. Three different carbon supported PtRu catalysts with different degrees of Pt and Ru intermixing, consisting of (i) carbon supported PtRu alloy particles (PtRu/C), (ii) Pt and Ru particles co-deposited on the same carbon support (Pt+Ru/C), and (iii) a mixture of carbon supported Pt and carbon supported Ru (Pt/C+Ru/C) as well as the respective monometallic Pt/C and Ru/C catalysts were prepared and characterized by electron microscopy (TEM), X-ray absorption spectroscopy, and CO stripping. Their performance as PEFC anode catalysts was evaluated by oxidation of a H2/2%CO gas mixture (simulated reformate) under fuel cell relevant conditions (elevated temperature, continuous reaction and controlled reactant transport) in a rotating disk electrode (RDE) set-up. The CO tolerance and H2 oxidation activity of the three catalysts is comparable and distinctly different from that of the monometallic catalysts. The results indicate significant transport of the reactants, COad and/or OHad, between Pt and Ru surface areas and particles for all three catalysts, with only subtle differences from the alloy catalyst to the physical mixture. The high activity and CO tolerance of the bimetallic catalysts, through the formation of bimetallic surfaces, is explained, e.g., by contact formation in nanoparticle agglomerates or by material transport and subsequent surface decoration/surface alloy formation during catalyst preparation, conditioning, and operation. The instability and mobility of the catalysts under these conditions closely resembles concepts in gas phase catalysis.

Binary Mixtures of Carbon Supported Pt and Ru Catalysts for PEM Fuel Cells

Abstract: Commercially available carbon-supported pure Pt and pure Ru catalysts were heat treated in order to modify their individual particle sizes, mixed in a mortar and tested for their suitability as anode catalysts in polymer electrolyte fuel cells (PEMFCs). The procedure chosen for mixing monometallic catalysts should bring more flexibility to the preparation of catalysts, which could thus be easily adjusted to the working conditions. The catalyst mixtures were structurally characterized using a combination of X-ray diffraction (XRD) and transmission electron microscopy (TEM), and indicated an increase in particle size with heat treatment temperature, while the size distribution remained sufficiently narrow. The particle size effect on the resulting activity of the catalyst mixtures was investigated by current-voltage measurements (U/i curves) in single cell fuel cells. A mixture of the as-received platinum and ruthenium gave the best results, which was almost comparable to a commercial Pt-Ru alloy catalyst purchased from E-TEK inc. Only a minor decrease in cell performance was seen during long term operation. Following operation, the X-ray diffraction patterns show reflections of the Pt fcc phase, but not the hcp Ru phase. This may be explained by leaching of the pure ruthenium catalysts and re-decoration on Pt nanoparticles, or by formation of an amorphous Ru oxide or, less likely, Pt-Ru alloy during operation. However, these structural changes do not seem to significantly affect the cell performance.

Ageing Mechanisms of Proton Exchange Membrane Used in Fuel Cell Applications

Abstract: Degradation processes of both sulfonated polyimide (sPI) and sulfonated polyetheretherketone (sPEEK) membranes were studied in-situ in fuel cells. Ex-situ ageing protocols were developed for both polymers and model compounds. On the one hand sulfonated polyimides are shown to be mainly sensitive to hydrolysis though an additional thermo activated oxidation process occurs in fuel cell conditions and at least accelerates the degradation. On the other hand sulfonated polyetheretherketone are insensitive to hydrolysis but greatly degraded in an oxidative medium.

Development of Hybrid Polymer Electrolyte Membranes Based on the Semi-Interpenetrating Network Concept

Abstract: Hybrid inorganic/organic polymer electrolyte membranes for potential fuel cell applications are prepared by centrifugal casting from solutions of sulfonated polyetheretherketone (SPEEK) (DS 64%) and polyethoxysiloxane (PEOS) in dimethylacetamide, following the concept of a semi-interpenetrating network. The in situ transformation of PEOS into SiO2 occurs in a “water free” process. The morphology of the films obtained is controlled by the phase segregation process, determined by the rate of evaporation of the solvent and by the transformation of PEOS into SiO2-particles. The latter process is influenced by the presence of a catalyst. Moreover, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole is added to the mixture to enhance the interaction between SPEEK and PEOS and to influence the membrane morphology. The size and size-distribution of the SiO2 particles formed in the organic matrix are examined by means of transmission electron microscopy. The TEM investigations show a strongly reduced particle size when N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole is added to the mixture. Proton conductivity measurements are performed on the membranes by impedance spectroscopy in an open set-up that allows measurements along the longitudinal direction of the sample. All the samples show a plateau in impedance at medium frequencies that represents the proton conducting process. Nafion® 115 is measured in the same set-up for comparison.

Preparation and Characterization of Stable Ionomers and Ionomer Membranes for Fuel Cells

Abstract: Membranes based on sulfonated arylene polymers and their cross-linked blends were prepared. embedding the ionomers into textile or porous ptfe matrix and/or doping with zrp nanoparticles reduced the swelling, and improved the thermal stability. ptfe reinforcement led to an excellent single cell performance, which is promising for pemfc applications. some of the membranes performed better than nafion(r). the net methanol crossover correlates with the membranae thickness and composition, but surprisingly, suppression of the methanol crossover alone, does not enhance u/i performance. esr experiments of oxidative degradation of the monomer by oh center dot at ph > 10 leads to phenoxyl and semi-quinone radicals. this study focuses on the case where o-2(center dot-) or o-1(2) are the damaging species, i.e. above ph 11.7, and the dominant reaction is that involving the aromatic ring. under conditions of fuel starvation, the acid protons get depleted, which may lead to high local ph values, even in an overall acidic environment. it is therefore important to be aware of the entirely different form of degradation chemistry at high ph. in the presence of both ho center dot radicals and oxygen, complete degradation of the aromatic rings can be observed, within hours. in view of this, perfluorinated membranes are more inert. cross-linked arylene polymer blend membranes and ptfe-reinforced arylene ionomer membranes can be regarded as promising for further development, due to their high stability and excellent pemfc performance.

Selective Cathode Catalysts for Mixed‐reactant Alkaline Alcohol Fuel Cells

Abstract: A highly selective Ag-W2C/C electrocatalyst for oxygen reduction is developed for potential application in mixed-reactant alcohol fuel cells. The catalysts are prepared by an intermittent microwave heating (IMH) method. Both the W2C/C and Ag-W2C/C show catalytic activity for oxygen reduction in alkaline media. The introduction of W2C into the Ag/C results in an enhanced activity that is evidenced by a positive movement in the onset potential and an increased current density at the same conditions. The novelty of the Ag-W2C/C catalyst is the high selectivity for oxygen reduction in the presence of alcohol.