1,720,981 research outputs found
Inductance correction in impedance study of solid oxide fuel cells
No full textA procedure for evaluation and elimination of errors, caused by parasitic inductance and resistance in EIS studies of two solid oxide fuel cells (SOFC) materials: yttria stabilized zirconia (YSZ) electrolyte and lanthanum strontium manganite (LSM)/YSZ composite cathode is presented in this paper. It is shown that for these low impedance systems the parasitic inductance can affect not only the high frequencies but also the middle and low ones. The parasitic errors correction procedure increases significantly the reliability of the electrochemical impedance spectroscopy (EIS) results
Fuel cell e.g. mixed conduction membrane fuel cell, for producing electric energy for stationary applications, has channel whose cross section has minimum size larger than specific value so as to discharge from diaphragm to outside of cell
No full textNOVELTY - The cell (1) has a porous central diaphragm (30) whose surfaces (32, 35) are in contact with an electrolyte (20) and a cathode (50). The electrolyte has material conducting M ions and the diaphragm has material conducting both M and N ions. Rectilinear channels (52) pass through the cathode and connected to the diaphragm and a free surface of the cathode. Minimum size of a cross-section of one of the channels is larger than 20 micrometers so as to enable a product i.e. water, from reaction of the ions to be discharged from the diaphragm to outside the cell through the channels.
USE - Fuel cell e.g. mixed conduction membrane fuel cell, for producing electric energy for stationary applications. Can also be used for long term onboard applications e.g. car.
ADVANTAGE - The minimum size of the cross-section of one of the channels is larger than 20 micrometers so as to enable the product resulting from reaction of the ions to be discharged from the diaphragm to outside the cell through the channels in an efficient manner, thus increasing power density of the fuel cell. The electrolyte and the central membrane are manufactured by using single operation e.g. sintering, so as to simplify manufacturing of the fuel cell and improve mechanical resistance and durability of the assembly
Overview of current trends in SOFC materials
No full textBecause of their potential to reduce the environmental impact and geopolitical consequences of the use of fossil fuels, fuel cells have emerged as sustainable power generation systems. Like a combustion engine, a fuel cell uses a chemical fuel as its energy source; but like a battery, the chemical energy is directly converted into electrical energy. In addition to high efficiency and low emissions, fuel cells are attractive for their modular and distributed nature, and zero noise pollution. SOFC single cells and stack systems are multilayer structures consisting of ceramic and metallic materials with different well defined electrical transport properties. All components have to show well-adjusted thermal expansion behaviour, chemical compatibility of material interfaces and chemical stability in the prevailing temperature and gas atmosphere. Performance and long-term stability improvements at affordable costs can be obtained by lowering the operating temperature. This goal requires drastic reduction of electrodes polarisation resistance as well as highly-conducting electrolyte materials. The use of new and nanostructured material and the application of appropriate technologies for the production of optimized microstructure becomes essential for highly efficient SOFCs operating in the medium and low temperature range. A review of state-of-art materials is given together with a perspective presentation of innovative materials and technologies for new types of SOFC
Study of the rate limiting step of the cathodic process in anode supported solid oxide fuel cell
No full textThe oxygen reduction (OR) mechanism at the Sr-doped LaMnO3 (LSM) and yttria stabilized zirconia (YSZ) composite cathode for high temperature solid oxide fuel cells is still uncertain, despite of the great deal of work carried out over the last years about this system. In previous works, we tested a half-cell (with a YSZ electrolyte pellet) in a typical three-electrode configuration: It was observed that the portion of the composite cathode volume involved in the reaction depends on the operating temperature. Moreover we analyzed part of the impedance data by the differential impedance analysis, which does not need a preliminary working hypothesis. The results suggested that significant limitations in the oxygen ion transport occur in the LSM pure material, which are not observed in the composite YSZ/LSM cathode. In this study, we investigate the behavior of the LSM/YSZ system in a Ni/YSZ cermet anode-supported hW-cell with yttria stabilized zirconia a (8YSZ) electrolyte and a screen printed LSM/YSZ composite cathode. The aim is to individuate and characterize the cathodic contribution from the overall impedance response, varying the partial pressure of the reactant gases, to obtain additional information about the OR mechanism from the p(O2) dependence. By a possible interpretation of the oxygen reaction mechanism, a comparative study of the cathode behavior with previous results is performed. Copyright © 2008 by ASME
Differential impedance analysis for the study of the rate limiting step of electrodic process in sofc cathodes
No full textThe increasing demands for more efficient low temperature Solid Oxide Fuel Cells (SOFC) focused the investigations towards the development of systems with higher conductivities at lower temperatures. The cathode-electrolyte interface is of great importance for the operation of the device at lower temperatures. It is essential to develop high performance electrodes because at such temperatures the electrode reaction rate is slower. There are two main approaches for the description of the cathode reaction of mixed conducting porous electrodes. The classical approach follows the "three-phase boundary" concept, which allows the involvement of gas-phase species at the electrochemical interface, but in the same time needs an operation with one-dimensional interface among three phases. The limitations of the tpb concept are bypassed by break down of the electrode reaction into individual steps including charge-transfer across a two-dimensional interface as well as adsorption, solid state and gas diffusion. This approach is successfully applied in the impedance studies of cathode reaction of porous mixed conducting electrode, using equivalent circuit model description, where the non-charge transfer steps are treated as series of parallel combinations of charge-transfer elements. Although ensuring a good fit of measured with calculated data, in many cases this approach could be regarded as a formal description and not as a tool for elucidating the dominating mechanism of the complex process. Recently a non-charge transfer approach, known as ALS model was proposed for characterization of oxygen reduction on single phased porous mixed conducting oxide electrodes. It was found both theoretically and experimentally that the electrode polarization losses are associated mainly with the generation and transport of oxygen ions within the cathode material, while the actual interfacial charge-transfer is very fast provided the interface is not contaminated. Gas phase diffusion becomes dominant below 1 % oxygen in N2. In this work the electrochemistry of oxygen reduction on porous composite electrodes consisting of La(1-x)SrxMnO3-? (LSM) and Y-stabilised Zirconia (YSZ) has been analysed. Half cells consisting of YSZ electrolyte pellets and slurry coated cathodes were tested with a three electrodes configuration. The composite cathodes considered in this study have a fixed volume ratio LSM/YSZ equal to 1. Impedance measurements were analyzed by the technique of the Differential Impedance Analysis (DIA), which does not need a preliminary working hypothesis. The application of DIA gives information about the dominant phenomena, based on comparative study of the cathode behaviour of LSM and of composite materials. The analysis of the electrochemical data suggests that adsorption of oxygen or ionic transport could be the key phenomena in the cathodic process
Electrochemical testing of an innovative dual membrane fuel cell design in reversible mode
No full textSolid oxide fuel Cells (SOFC) are intrinsically reversible which makes them attractive for the development of reversible devices (rSOC). The main hurdles that have to be overcome are the higher degradation in electrolyzer (EL) mode and the slow and difficult switching form mode to mode. This work aims at the development and experimental validation of a concept for rSOC based on a new dual membrane fuel cell (dmFC) design which can overcome the existing problems of the classical SOFC. The kernel of the system is additional chamber - central membrane (CM) for water formation/evacuation in FC mode and injection in El mode. Its optimization in respect of microstructure and geometry in laboratory conditions is carried out on button cells. The electrochemical performance is evaluated based on volt-ampere characteristics (VACs) combined with impedance measurements in different working points. The influence of a catalyst in the water chamber is also examined. The VACs which give integral picture of the cell performance are in excellent agreement with the impedance studies which ensure deeper and quantitative information about the processes, including information about the rate limiting step. The results from the optimization of the water chamber show that the combination of design and material brings to important principle advantages in respect to the classical rSOC – better performance in electrolyzer mode combined with instantaneous switching
Parametrical Coordinates and Microsamples to Investigate Real SOFCs in Operating Stacks
No full textOne of the issues defining the performance of an SOFC stack is to estimate and check during operation the behavior of all components (i.e. cell, sealant, interconnects). Each component has at least one function which is not strictly related to the power or heat generation but all are strategic due to their contribution to the degradation of the whole stack. The model based approach leads to identify zones of the stack by their operating parameters (e.g. T, j, P) corresponding to an univocal material response. In this paper, an experiment combining the parametrical coordinates with real samples in order to contribute to the study of real stacks is presented. The sample and the experiment itself are cost effective and offer the opportunity to soundly contribute to the assessment of stack components behavior in the "real world". The research was carried out in the frame of the FCH-JU project ENDURANCE
Impedance studies of cathode/electrolyte behaviour in SOFC
No full textThis paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity Ea increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with Ea independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher Ea at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in Ea for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched. © 2007 Elsevier Ltd. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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