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Diffusion Layer (diffusion + layer)
Kinds of Diffusion Layer Selected AbstractsAnodic Stripping Voltammetry Using a Vibrating ElectrodeELECTROANALYSIS, Issue 13 2007Conrad Abstract This work proposes a vibrating microwire electrode as working electrode in stripping voltammetry. The vibration was found to maintain a constant and thin (1,2,,m) diffusion layer during the deposition step. The electrode vibration eliminated the need for external stirring of the solution, thus facilitating in situ detection in the environment. The vibration was effected by fixing a low-voltage (3,V), asymmetric, electrical rotor to the working electrode (a gold microwire of either 5 or 25,,m). The sensitivity of the vibrated electrode was ca. 22×greater than stationary. Measurements of copper (4,nM) by anodic stripping voltammetry using the vibrating electrode had a low standard deviation (1% for n=6) indicating that the diffusion layer had only minor variability. The agitation mechanism was unaffected by water moving at >2,m s,1 and by water pressure equivalent to a depth of >40,m, indicating its suitability for in situ measurements. The vibrating probe was used for in situ detection of copper by anodic stripping voltammetry to a depth of 6,m. Using a 5,min deposition time, the limit of detection for labile copper was 38,pM. [source] Investigation of a Novel Catalyst Coated Membrane Method to Prepare Low-Platinum-Loading Membrane Electrode Assemblies for PEMFCsFUEL CELLS, Issue 2 2009X. Leimin Abstract In this work, a novel catalyst coated membrane (CCM) approach,a catalyst-sprayed membrane under irradiation (CSMUI),was developed to prepare MEAs for proton exchange membrane fuel cell (PEMFC) application. Catalyst ink was sprayed directly onto the membrane and an infrared light was used simultaneously to evaporate the solvents. The resultant MEAs prepared by this method yielded very high performance. Based on this approach, the preparation of low-platinum-content MEAs was investigated. It was found that for the anode, even if the platinum loading was decreased from 0.2 to 0.03,mg,cm,2, only a very small performance decrease was observed; for the cathode, when the platinum loading was decreased from 0.3 to 0.15,mg,cm,2, just a 5% decrease was detected at 0.7,V, but a 35% decrease was observed when the loading was decreased from 0.15 to 0.06,mg,cm,2. These results indicate that this approach is much better than the catalyst coated gas diffusion layer (GDL) method, especially for the preparation of low-platinum-content MEAs. SEM and EIS measurements indicated ample interfacial contact between the catalyst layer and the membrane. [source] Effect of anode current collector on the performance of passive direct methanol fuel cellsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 8 2009Qin-Zhi Lai Abstract The effect of anode current collector on the performance of passive direct methanol fuel cell (DMFC) was investigated in this paper. The results revealed that the anode of passive DMFC with perforated current collector was poor at removing the produced CO2 bubbles that blocked the access of fuel to the active sites and thus degraded the cell performance. Moreover, the performances of the passive DMFCs with different parallel current collectors and different methanol concentrations at different temperatures were also tested and compared. The results indicated that the anode parallel current collector with a larger open ratio exhibited the best performance at higher temperatures and lower methanol solution concentrations due to enhanced mass transfer of methanol from the methanol solution reservoir to the gas diffusion layer. However, the passive DMFC with a smaller open ratio of the parallel current collector exhibited the best performance at lower temperatures and higher methanol solution concentrations due to the lower methanol crossover rate. Copyright © 2009 John Wiley & Sons, Ltd. [source] In situ micro Raman investigation of electrochemically formed halide and pseudohalide films on mercury electrodes,JOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2002Alexandre G. Brolo Oxidation of pure mercury electrodes was followed by in situ surface-unenhanced Raman spectroscopy. A special spectroelectrochemical cell allowed the observation of spectral features of the oxidation products using a Raman microprobe. The electrochemical behaviour of mercury in basic solution, halide (F,, Cl,, Br, and I,) and pseudohalide (SCN,) media was investigated. Insoluble films were formed during the anodic processes in all these systems. The identities of the electrochemically generated films were clearly determined from the Raman spectra. The main product of the electrochemical oxidation of mercury in basic solutions was solid HgO. The anodic behaviour of mercury in the presence of halide yielded insoluble Hg2X2 compounds (where X is the halide). The time dependence of the Raman signals indicated a diffusion-controlled growth of the halide films. The oxidation of mercury in SCN, solutions also produced a solid precipitate [Hg2(SCN)2]. Raman bands due to soluble mercury(II) thiocyanate complexes [Hg(SCN)x2,x], present in the electrochemical diffusion layer, were also observed. Copyright © 2002 John Wiley & Sons, Ltd. [source] In situ observation of water distribution and behaviour in a polymer electrolyte fuel cell by synchrotron X-ray imagingJOURNAL OF SYNCHROTRON RADIATION, Issue 4 2008Taihei Mukaide In situ visualization of the distribution and behaviour of water in a polymer electrolyte fuel cell during power generation has been demonstrated using a synchrotron X-ray imaging technique. Images were recorded using a CCD detector combined with a scintillator (Gd2O2S:Tb) and relay lens system, which were placed at 2.0,m or 2.5,m from the fuel cell. The images were measured continuously before and during power generation, and data on cell performance was recorded. The change of water distribution during power generation was obtained from X-ray images normalized with the initial state of the fuel cell. Compared with other techniques for visualizing the water in fuel cells, this technique enables the water distribution and behaviour in the fuel cell to be visualized during power generation with high spatial resolution. In particular, the effects of the specifications of the gas diffusion layer on the cathode side of the fuel cell on the distribution of water were efficiently identified. This is a very powerful technique for investigating the mechanism of water flow within the fuel cell and the relationship between water behaviour and cell performance. [source] A Dual-Electrode Approach for Highly Selective Detection of Glucose Based on Diffusion Layer Theory: Experiments and SimulationCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005Kang Wang Dr. Abstract A dual-electrode configuration for the highly selective detection of glucose in the diffusion layer of the substrate electrode is presented. In this approach, a glassy carbon electrode (GCE, substrate) modified with a conductive layer of glucose oxidase/Nafion/graphite (GNG) was used to create an interference-free region in its diffusion layer by electrochemical depletion of interfering electroactive species. A Pt microelectrode (tip, 5 ,m in radius) was located in the diffusion layer of the GNG-modified GCE (GNG-G) with the help of scanning electrochemical microscopy. Consequently, the tip of the electrode could sense glucose selectively by detecting the amount of hydrogen peroxide (H2O2) formed from the oxidization of glucose on the glucose oxidase layer. The influences of parameters, including tip,substrate distance, substrate potential, and electrolyzing time, on the interference-removing efficiency of this dual-electrode approach have been investigated systematically. When the electrolyzing time was 30 s, the tip,substrate distance was 1.8,a (9.0 ,m) (where a is the radius of the tip electrode), the potentials of the tip and substrate electrodes were 0.7 V and 0.4 V, respectively, and a mixture of ascorbic acid (0.3,mM), uric acid (0.3,mM), and 4-acetaminophen (0.3,mM) had no influence on the glucose detection. In addition, the current,time responses of the tip electrode at different tip,substrate distances in a solution containing interfering species were numerically simulated. The results from the simulation are in good agreement with the experimental data. This research provides a concept of detection in the diffusion layer of a substrate electrode, as an interference-free region, for developing novel microelectrochemical devices. [source] Diffusion couple studies of the Ti-Bi-Zn systemCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2004G. P. Vassilev Abstract The system Ti-Bi-Zn has been investigated using diffusion couples consisting of solid Ti and liquid (Bi+Zn) phase. The diffusion paths at 400, 500, 700 and 800 °C have been traced by means of electron microprobe analyses. The growth constants of the diffusion layers are roughly assessed. The phase diagram data obtained in this investigation are compared with previous studies of equilibrated alloys. The existence of the ternary compound TiBiZn has been confirmed. The formation of another phase with approximate formulae Ti4Bi3Zn to Ti9Bi7Zn4 has been observed at high temperatures. The latter compound as well as the ternary extension of the TiXBiY (X , 5, Y , 6) phase react easily with air. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Effect of the Anode Loading and Method of MEA Fabrication on DMFC PerformanceFUEL CELLS, Issue 3 2007T. V. Reshetenko Abstract The influence of the Pt-Ru anode loading and MEA preparation techniques on direct methanol fuel cell (DMFC) performance is studied. Two different anode catalyst layer preparation techniques are employed. One is the direct coating of anode catalyst ink on a membrane to form a catalyst coated membrane, CCManode, and the other is the coating of the ink on the diffusion layers, which generates a catalyst coated substrate, CCSanode. The power density of a combined CCManode/CCScathode MEA is higher than for a CCSanode/CCScathode MEA. The main difference in the performance is observed in the high current density region, where two-phase flow is present and mass transfer processes govern the performance. The CCManode and CCSanode have different macroscopic structures, while showing the same microscopic morphology. Based on their morphological differences, it is expected that the combination of the CCManode and carbon paper provides the more homogeneous removal of CO2 at high currents. The authors suggest that the application of the CCManode with an optimal anode loading improves anode mass transfer, reduces methanol crossover, and enhances the electrochemical reactions. [source] Modelling Approach for Planar Self-Breathing PEMFC and Comparison with Experimental Results,FUEL CELLS, Issue 4 2004A. Schmitz Abstract This paper presents a model-based analysis of a proton exchange membrane fuel cell,(PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self-breathing, operation. This planar fuel cell is fabricated using printed circuit board,(PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components,(O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB,(COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water. [source] A Review of Mathematical Models for Hydrogen and Direct Methanol Polymer Electrolyte Membrane Fuel CellsFUEL CELLS, Issue 1-2 2004K.Z. Yao Abstract This paper presents a review of the mathematical modeling of two types of polymer electrolyte membrane fuel cells: hydrogen fuel cells and direct methanol fuel cells. Models of single cells are described as well as models of entire fuel cell stacks. Methods for obtaining model parameters are briefly summarized, as well as the numerical techniques used to solve the model equations. Effective models have been developed to describe the fundamental electrochemical and transport phenomena occurring in the diffusion layers, catalyst layers, and membrane. More research is required to develop models that are validated using experimental data, and models that can account for complex two-phase flows of liquids and gases. [source] Thermodynamic optimization of internal structure in a fuel cellINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 4 2004Jose V. C. Vargas Abstract This paper shows that the internal structure (relative sizes, spacings) of a fuel cell can be optimized so that performance is maximized at the global level. The optimization of flow geometry begins at the smallest (elemental) level, where the fuel cell is modelled as a unidirectional flow system. The polarization curve, power and efficiency are obtained as functions of temperature, pressure, geometry and operating parameters. Although the model is illustrated for an alkaline fuel cell, it may be applied to other fuel cell types by changing the reaction equations and accounting for the appropriate energy interactions. The optimization of the internal structure is subjected to fixed total volume. There are four degrees of freedom in the optimization, which account for the relative thicknesses of the two (anode and cathode) diffusion layers, two reaction layers and the space occupied by the electrolyte solution. The available volume is distributed optimally through the system so that the total power is maximized. Numerical results show that the optima are sharp, and must be identified accurately. Temperature and pressure gradients play important roles, especially as the fuel and oxidant flow paths increase. The optimized internal structure is reported in dimensionless form. Directions for future improvements in flow architecture (constructal design) are discussed. Copyright © 2004 John Wiley & Sons, Ltd. [source] Platinum Nanoelectrodes Embedded in an Insulating Alumina Matrix: An Innovative Approach,CHEMICAL VAPOR DEPOSITION, Issue 4 2005A. Battiston A novel strategy, based on MOCVD, has been employed for preparation of ensembles of platinum nanoelectrodes. Pt/Al2O3 films with different Pt loading were deposited in just one step on glass capillaries. Nanocomposite films with varying Pt/Al atomic ratios were investigated by both cyclic voltammetry and TEM. Ensembles of nanoelectrodes with either overlap or no overlap of the diffusion layers of each nanoelectrode are obtained, depending on the Pt loading. From TEM measurements average Pt particle size of 3 , 7 nm was determined (Figure). [source] | ||||||||||