Electrochemical open circuit voltage (OCV) characterization of SOFC materials

In this paper, we are reporting an extensive characterization, by means of open circuit voltage measurements, of Ce_0.8Gd_0.2O₂, La_0.9Sr_0.1Ga_0.8Mg_0.2O₃, and La₂Mo_0.6W_1.4O₉ oxide-ions and BaCe_0.8Y_0.2O₃ and BaCe_0.55Zr_0.3Y_0.15O₃ proton-conducting electrolyte materials for solid oxide fuel cell (SOFC) applications. This simple and common technique, well known for a long time in the electrochemical study of solid oxide fuel cells, has been here proposed for the electrical characterization of these ceramic materials, in order to define their ionic transport numbers, the maximum voltage performances, the thermal and chemical stability, and also to suggest the ideal temperature range for different applications, as in the electrochemical devices, sensors, and SOFC field. In the paper, controlled and reproducible working conditions have been applied in a wide range of temperature, by means of ultrapure gas (H₂ and O₂), under operational conditions found in real SOFC devices and, mainly, without the usual problems related to the chemical compatibility, the depolarization efficiency, and the high current density required to the electrode materials in the design of a more efficient SOFC device.     

Facile preparation of SGC composite as anode for lithium-ion batteries

The silicon/graphite/carbon (SGC) composite was successfully prepared by ball-milling combined with pyrolysis technology using nanosilicon, graphite, and phenolic resin as raw materials. The structure and morphology of the as-prepared materials are characterized by X–ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM). Meanwhile, the electrochemical performance is tested by constant current charge–discharge technique, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) measurements. The electrodes exhibit not only high initial specific capacity at a current density of 100 mA g^?1, but also good capacity retention in the following 50 cycles. The EIS results indicate that the electrodes show low charge transfer impedance R_sf?+?R_ct. The results promote the as-prepared SGC material as a promising anode for commercial use.     

A one-chamber solid electrolyte fuel cell for chemical cogeneration

A one chamber solid oxide fuel cell (SOFC) which works in a flow of a mixture of CH₄ and air has been studied in the present author's laboratory. This new type cell consists of Pt | Solid Electrolyte | Au, in which both electrodes are exposed to the CH₄₊₂ai r mixture with a CH₄/O₂ ratio of 2 at elevated temperatures, and it generates an electric power producing hydrogen and carbon monoxide (synthetic gas) with a H₂/CO ratio of about 2. A large difference in catalytic activity for the partial oxidation of CH₄ between the Pt and Au electrode materials leads to an oxygen concentration cell which can generate an electric power between them. In this paper the author describes such a one-chamber fuel cell with respect to their structural feature, performance as a fuel cell and their component materials citing the experimental results made by his laboratory's group. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

A novel concept for in situ gas-phase laser Raman spectroscopy for solid oxide fuel cell research

A planar solid oxide fuel cell (SOFC) operated with hydrogen at T = 1,123 K was equipped with an optically transparent anode flow field to apply species concentration measurements by 1D laser Raman scattering. The flow channels had a cross section of 3 mm × 4 mm and a length of 40 mm. The beam from a pulsed high-power frequency-doubled Nd:YAG laser (λ = 532 nm) was directed through one channel and the Raman-scattered light from different molecular species was imaged onto an intensified CCD camera. The main goal of the study was an assessment of the potential of this experimental configuration for a quantitative determination of local gas concentrations. The paper describes the configuration of the optically accessible SOFC, the laser system and optical setup for 1D Raman spectroscopy as well as the challenges associated with the measurements. Important aspects like laser pulse shaping, signal background and signal quality are addressed. Examples of measured species concentration profiles are presented.     

Processing and characterization of ultra-thin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC

Sub-micron yttria-stabilized zirconia (YSZ) electrolyte layer was prepared by a liquid state deposition method and with an average thickness of 0.5 μm to improve the performance of the anode-supported solid oxide fuel cell (SOFC). The YSZ precursors, containing yttrium and zirconium species and an additive, poly-vinyl-pyrrolidone (PVP), were spin-coated on a Ni/YSZ anode substrate. Several properties, including crystalline phases, microstructures, and current–voltage (I–V) characteristics, were investigated. The thin film of 4 mol% Y₂O₃-doped ZrO₂ (4YSZ) consisted of cubic, tetragonal, and a trace of monoclinic phases, and showed a crack-free layer after sintering at 1300 °C. The anode supported SOFC, which consists of the Ni–YSZ anode, 4YSZ electrolyte, and Pt/Pd cathode, showed power densities of 477 mW/cm² at 600 °C, and 684 mW/cm² at 800 °C. Otherwise, the surface cracks of the other YSZ-coated samples (e.g. 8YSZ) can be repaired by a multi-coating method.     

固体氧化物燃料电池堆多物理场耦合模拟的高效电化学-传质耦合算法

A multiphysics model for a production scale planar solid oxide fuel cell (SOFC) stack is important for the SOFC technology, but usually requires an unpractical amount of computing resource. The major cause for the huge computing resource requirement is identified as the need to solve the cathode O₂ transport and the associated electrochemistry. To overcome the technical obstacle, an analytical model for solving the O₂ transport and its coupling with the electrochemistry is derived. The analytical model is used to greatly reduce the numerical mesh complexity of a multiphysics model. Numerical test shows that the analytical approximation is highly accurate and stable. A multiphysics numerical modeling tool taking advantage of the analytical solution is then developed through Fluent®. The numerical efficiency and stability of this modeling tool are further demonstrated by simulating a 30-cell stack with a production scale cell size. Detailed information about the stack performance is revealed and brie y discussed. The multiphysics modeling tool can be used to guide the stack design and select the operating parameters.     

Influence of colloidal templates on the impedance spectroscopic behaviour of Pr0.7Sr0.3Fe0.8Ni0.2O3 for solid oxide fuel cell applications

The creation of porous materials with three-dimensional periodicity has been identified as being of potential interest for increasing the overall performance of solid oxide fuel cells (SOFC). In this work, we have investigated the formation of pore systems in the nanometer scale by replicating colloidal templates. Templating methods have been used to prepare iron-nickel-based perovskite Pr_0.7Sr_0.3Fe_0.8Ni_0.2O₃ material with nanoporous microstructure. Polymethyl methacrylate (PMMA), polystyrene (PS) and polycarboxylate (PC) microspheres with different diameters were used as pore formers. These samples were synthesized and characterized by thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction, transmission electron microscopy and field emission scanning electron microscopy. The polarization resistance of the materials was studied by Electrochemical Impedance Spectroscopy. The study demonstrated that templated porosity is maintained and highly influences on the impedance spectroscopic behaviour, being the material synthesized with policarboxylate microspheres the most interesting of the three used templates for SOFC applications.     

“Cold” CO<Subscript>2</Subscript> laser ablation of green-state LTCC—experimental verification of improved model and comparison of various LTCC materials

New experimental results in support of the universal mechanism of “cold” laser ablation for machining of various commercial green ceramic materials (LTCC) are presented in this paper. The “cold” ablation model was mathematically formulated and employed to derive an ablation curve equation. The model was tested by CO₂ laser ablation of a custom-made green-state alumina ceramic featuring varying binder content. An excellent fit of ablation curve to experimental data was obtained, yielding insight into process energetics and an ablative measurement method of absorption coefficient. The analysis was applied to a sample of commercial LTCC materials. The ablation results were practically identical for all materials in agreement with the prediction of the model, with the high rates of >100 micron/shot at repetition >1 kHz and accuracy comparable with the ceramic grain size. This work provides evidence that the CO₂ laser processing has a great potential to become a key low-cost precision processing method for the existing LTCC-based electronic devices (micro-via drilling, general cutting and scribing) and for the new generation of LTCC-based devices comprising micro-fluidics, micro-mechanics, opto-electronics and meta-material structures.     

Fabrication and characterization of transparent Tm,Ho:YAG ceramic

Transparent Tm,Ho:YAG ceramic with excellent optical properties was fabricated by solid-state reaction method using high-purity commercial powders as raw materials. Optical absorption and 2.0 μm emission spectra of the ceramic specimen have been studied. The strongest emission corresponds to ⁵ I ₇ → ⁵ I ₈ transition of Ho³⁺ centered at 2.06 μm. As a result, Tm,Ho:YAG ceramic shows potential application in 2.0 μm laser devices.     

Info-gap robust design with load and model uncertainties

This paper develops a new structural design concept which incorporates uncertainties in both the load and the structural model parameters. Info-gap models of uncertainty are used to represent uncertainty in the power spectral density of the load and in parameters of the vibration model of the structure. It is demonstrated that any design which optimizes functional performance will also minimize the robustness to uncertainty. Since uncertainties are prevalent in many applications, this paper argues that it is necessary to satisfy critical performance requirements (rather than to optimize performance), and to maximize the robustness to uncertainty. The design implications of this robust-satisficing approach are demonstrated with several heuristic structural design examples. It is shown that design preferences depend upon performance requirements: preferences between designs can be reversed when performance requirements change. Also, we show that the info-gap robustness function provides an attractive tool for adjudicating between conflicting objectives in multi-criteria design.     

Modeling and simulation of planar SOFC to study the electrochemical properties

In this paper, modeling and simulations are carried out using COMSOL Multiphysics. A three-dimensional model is developed for a planar intermediate temperature (IT) solid oxide fuel cell (SOFC). A parametric study has been carried out to analyze the performance of SOFC.Simulations reveal some promising features and enhanced performance of SOFC. It is shown that the maximum value of power (4–3.3) kW/m² still remains higher with significant rise of temperature (600 °C–1000 °C), nearly 0.15 kW/m² is the very small loss of power per 100 °C rise of temperature. Results have shown that the electrolytic current density is (6700–5500) A/m² for peak value of power (4–3.3) kW/m² with increase of temperature (600 °C–1000 °C). For model validation we have plotted a comparison of average current density.     

Potentiality of SBN textured ceramics for pyroelectric applications

Textured Sr_xBa_1−xNb₂O₆ (SBN) ferroelectric ceramics with x = 0.53 and 0.63 were fabricated by hot forging process. The objective was to obtain, in the ceramic form, the strong anisotropy of the electric properties that these materials possess in the single crystal form. Properties such as electric permittivity, pyroelectric coefficient and dielectric loss showed an anisotropy between the perpendicular and parallel direction with respect to the pressure axis (applied pressure during the forging of the ceramics). A high pyroelectric coefficient, comparable with these published for SBN single crystal with the same composition, was obtained for the SBN53/47 ceramic, when measured in the perpendicular direction to the pressing axis. From the calculus of the pyroelectric figures of merit, it was possible to conclude that the textured SBN53/47 ceramic has a high potential to be used as pyroelectric elements. This ceramic, cut in the perpendicular direction to the pressing axis, possess high potential as fast pulse detector but the same ceramic, cut in the parallel direction to the pressing axis, has better properties to be used as large area and point detectors.     

Non-conventional solid state fuel cells: Materials & technology

Investigation of “Non-conventional material” for fuel cells, such as oxide-salt-ceramic composites and ceria based or perovskite oxides with different dopants, leads to a much lower fuel cell operating temperature compared to conventional high temperature, ∼1000 °C, solid oxide fuel cells (SOFCs), which provides the new possibilities for facilitating SOFC commercialisation. This work is essentially an effort to develop new types of solid oxide ion and proton fuel cells (SOFC and SPFC) at fairly low temperatures, <800 °C, or intermediate temperature, 400 to 800 °C. The conventional high temperature SOFCs using yttria-stabilised zirconia (YSZ) materials, and low temperature SPFCs (<200 °C) using polymer membrane electrolytes have complex material and system problems from either special high temperature requests or expensive technology and reforming systems. This research is intended to provide materials and technology along new routes for so-called non-conventional fuel cell systems, to facilitate solid state fuel cell cmmercialisation. The fuel cell research on these non-conventional systems is promising. This paper, based on recent achievements in research on materials and technology, summaries the developnt of material systems and new fuel cell devices regarding their potential marketability in the near future. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Left-handedness in K-type multilevel system in the presence of spontaneously generated coherence

The left-handedness in atomic vapor medium consisting of five energy levels in K-type configuration is studied in this work. The theoretical modeling has been done using density matrix approach in which the spontaneously generated coherence (SGC) is also included. The system shows negative electrical permittivity (?) as well as negative magnetic permeability (μ) and thus displays the negative refractive index (n) for certain values of system parameters in the presence of SGC. The parameters (?, μ, n) quantifying left-handedness in the system become more negative if the SGC is strong. Under the off-resonant conditions these parameters become less negative and the left-handedness of system degrades.     

Theory of laterally nonuniform MOS transistor under weak inversion: A technique for determination of interface parameters

The well-known model of current-voltage (I-V) characteristics of a MOS transistor (MOST) in weak inversion [1] was modified with regard to lateral nonuniformity of the semiconductor surface potential. A simple technique for determining the fluctuation parameter and the spectral density of interface states from drain-current (output) and drain-gate (transfer) single-threshold I-V characteristics is developed. Combined with measurements of the MOST threshold voltage, it makes possible the calculation of the effective oxide charge. The technique is fairly accurate and is useful for IC process control.     

Influence of microporosity on the strength properties of SiC ceramic materials

A series of silicon carbide ceramic samples with variable characteristics of the microporosity and strength, such as the ballistic strength σ _B and the static strength σ _S , are investigated. The dependences of the strength on the integral porosity for ceramic materials are determined. It is established that the strength (both σ _B and σ _S ) is directly proportional to the average length of the bridges between micropores. The mechanism of the influence of microporosity on the strength of the ceramic materials is elucidated. According to this mechanism, interpore bridges are concentrators of stresses and, hence, are broken when a load is applied to the ceramic material. Numerous breakings of bridges bring about the failure of the ceramic body. The average stress concentration coefficient is estimated as a function of the integral porosity of the ceramic material. It is demonstrated that the static strength of the ceramic material is determined by the presence of large micropores (50–100 μm).     

Cathode overpotential investigation by means of “Built-in” potential electrode

The objective of the present work is the development of a “built-in” potential electrode method for direct measurements of the cathode and anode overpotentials and the corresponding interface resistances of solid oxide fuel cells (SOFC). The studies were performed on a yttria-stabilised zirconia (YSZ) electrolyte-supported SOFC using La_0.8Sr_0.2MnO₃ as cathode, GDC as protecting layer and Ni-ScSZ cermet as anode. The mesh potential electrode was placed inside the YSZ membrane near the cathode side. Using the combination of the I-U and the impedance measurements with the built-in potential electrode technique, the temperature dependencies of the electrodes and electrolyte contributions to the total cell resistance were determined.     

Zirconia stabilized by Y and Mn: A microstructural characterization

Cubic stabilized ZrO₂ with 8 mol% Y₂O₃ (YSZ) is commonly used as an electrolyte in solid oxide fuel cells (SOFC). One of the most promising cathode materials is La,Sr-manganite (LSM). During manufacture and operation of the SOFC, Mn diffuses from the LSM into YSZ. The structural changes caused by the presence of Mn in the electrolyte under various oxygen partial pressures have been examined by X-ray diffraction. Microstructures of YSZ containing Mn have been examined by transmission electron microscopy and surface changes of the electrolyte due to Mn diffusion were studied by scanning electron microscopy. The results are discussed with respect to properties and stability of the electrolyte and the SOFC. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Technical and economic evaluation of a methane solid oxide fuel cell

Solid oxide fuel cells offer the possibility of high temperature synthesis of chemical products with cogeneration of electricity, a process known as chemical cogeneration. This research primarily addresses the likelihood of upscaling present bench-scale experimental results of methane fuelled SOFCs. Methane coupling, i.e. production of C₂ hydrocarbons, is one of the possibilities of chemical cogeneration. In evaluating the co-generating SOFC, the methane-coupling design was compared to two other possible competing designs, namely, a regular SOFC plant (complete oxidation of methane to CO₂ and H₂O) and an SOFC plant coproducing synthesis gas. It was found that the rate of return for the regular fuel cell exceeded 25% whereas that for the ethylene plant was about 21%. The synthesis gas plant was well behind at about 17%. The reasons that have so far prohibited large-scale application of such systems are discussed. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995.