Application of GDC-YDB bilayer and LSM-YDB cathode for intermediate temperature solid oxide fuel cells

Yttria-doped bismuth (YDB) and gadolinia-doped ceria (GDC) are investigated as a bilayer electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). LSM-YDB is used as a cathode material in order to improve the poor ionic conduction of LSM and the compatibility with the YDB electrolyte. The performance of the bilayer cell was measured under humidified H₂ (3 % H₂O) atmosphere and an operating temperature between 500 °C and 650 °C. The polarization resistance and ohmic resistance of the GDC-YDB bilayer cell were 0.189 Ωcm² and 0.227 Ωcm² at 650 °C, respectively. The bilayer cell showed 0.527 Wcm^?2 in the maximum power density at 650 °C, which is about two times higher than the single-layer cell of 0.21 Wcm^?2. The OCV of the bilayer cell was 0.89 V at 650 °C, suggesting that the electronic conduction caused by the reduction of ceria was successfully suppressed by the YDB layer. The introduction of an YDB-GDC bilayer cell with LSM-YDB cathode thus appears to be a promising method for improving the performance of GDC-based SOFCs and reducing operating temperature.     

Acceptor-doped ceria deposited on a porous Ni film as a possible micro-SOFC electrolyte

Micro-SOFCs, miniaturized solid oxide fuel cells (SOFCs) for low temperature operation, are being developed as a power source for portable electronics. Reducing the thickness of the electrolyte and the adoption of acceptor-doped ceria as an electrolyte material are important to minimize the Ohmic resistance at low temperature. Acceptor-doped ceria thin-films are often deposited on nano-porous metal substrates to reduce cracking of the thin electrolyte. However, due to the difficulty of depositing a pore-free electrolyte on a porous medium, the cells often show the low open circuit voltages (OCVs). In this study, we have deposited ～1 μm-thick Gd-doped ceria on a nano-porous nickel film to assess whether a thin-film, metal-supported GDC can be deposited as a pore-free layer and would thus be suitable as an electrolyte of micro-SOFCs. The Ni-supported GDC cell showed an OCV of ～0.92 V at 450 °C under a hydrogen/air gradient. The high OCV verifies that the thin-electrolyte layer, deposited on porous Ni using the pulsed laser deposition (PLD) method, is dense enough to prevent gas leakage as also observed in its microstructure.     

Preparation and characterization of copper based cermet anodes for use in solid oxide fuel cells at intermediate temperatures

Two Cu-based anode cermets suitable for direct hydrocarbon oxidation in Solid Oxide Fuel Cells (SOFC) based on yttria stabilized zirconia (YSZ) electrolyte were tested in the temperature range (500–800°C). The ceramic components were CeO₂ and the perovskite La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−d (LSCM). The cermets were made in both the form of pellets and films applied onto the YSZ electrolytes. Pellets exhibited good mechanical strength and resistance to fracture in both oxidized and reduced state. Cu–LSCM cermets exhibited good redox cycling behavior between 700–800°C. Reduction temperature plays a significant role on final morphology with Cu segregation occurring at 800°C. Cu–LSCM films were found to exhibit lower polarization resistances than Cu–CeO₂ under 5% H₂. Examination of the data revealed a poorer contact of the Cu–CeO₂ electrode with the YSZ surface than the Cu–LSCM electrode. Reduction temperature should be less than 750°C to ensure suitable microstructure and adhesion of both film electrodes with the electrolyte.     

Sintering behavioral and electrochemical performances of LSM-BSB composite cathode for IT-SOFCs

In this paper, La_0.8Sr_0.2MnO₃-Ba_0.1Bi_0.9O_1.5-?? (LSM-BSB) composite cathode was prepared and characterized for intermediate temperature solid oxide fuel cells (IT-SOFCs). XRD results show that no reaction occurred between LSM and BSB at 900°C. SEM results show that the LSM-BSB composite cathode formed good contact with YSZ electrolyte after sintered at 900°C for 2 h, which significantly reduced the sintering temperature of cathode. Compared with the LSM-YSZ electrode sintered at 1200°C for 2 h, LSM-BSB electrode exhibits better electrochemical performance. At 800°C, the area specific resistance (ASR) of the LSM-BSB30 electrode is about 0.168 ??cm², which is nearly 1.5 times lower than that of LSM-YSZ composite cathode.     

Structural and electrical properties of Sr(Ti, Fe)O3-δ materials for SOFC cathodes

Doped strontium titanates are very versatile materials. Iron doped SrTiO₃ can be used, for example, as a material for resistive gas sensors and fuel cell electrodes. In this paper, two compositions based on Fe doped SrTiO₃ were studied as possible candidates for cathode application in SOFCs. Namely, SrTi_0.65Fe_0.35O₃ and SrTi_0.50Fe_0.50O₃ were examined. A chemical reactivity between electrode and YSZ electrolyte material was investigated, since Sr containing cathode materials in contact with YSZ electrolyte are prone to form insulating phases. Electrical conductivity of bulk samples showed relatively low total conductivities of 0.4 S cm⁻¹ and ~2 S cm⁻¹ for STF35 and STF50 respectively. Suitability for cathode application was studied by Electrochemical Impedance Spectroscopy in a symmetrical electrode configuration. Area specific resistance (ASR) was determined in the temperature range from 600°C to 800°C. At 790°C samples show polarization ASR of approximately 0.1 Ω cm². It can be expected that further reduction of electrode ASR can be obtained by introduction of ceria barrier layer and tailoring of the electrode microstructure.     

Low temperature metal-organic chemical vapor deposition of Ru thin films as electrode for high dielectric capacitors

Abstract

Low temperature metal-organic chemical vapor deposition (MOCVD) process of Ru films for use as electrode material was studied using a noble dome type reactor, liquid delivery technique and a new precursor. The films were grown at temperatures ranging from 275°C to 480°C in which film growth was controlled by a surface chemical reaction with a small activation energy of 0.21 eV. The root-mean-squared surface roughness was as low as 23 Å for a film grown at 290°C on a SiO₂ surface.     

Chemical Vapor Deposition of Ru Bottom Electrode for Ferroelectric Bi4 − x La x Ti3O12 Capacitors

Ruthenium films formed by metalorganic chemical vapor deposition were investigated, taking account of the application to the bottom electrode of ferroelectric capacitors. Ruthenium films were deposited using a liquid-type source of Ru[EtCp]₂ in a cold-wall type reactor with infrared lamps. A smooth and flat Ru film was successfully formed on a SiO₂-covered Si substrate without a seed layer. As the deposition temperature increased to 400°C, the crystallinity of the Ru film improved and the film exhibited high c-axis orientation. After forming a Bi_{4 ? x} La _x Ti₃O₁₂ (BLT) film by a sol-gel technique, a Pt/BLT/Ru capacitor was fabricated on the Ru film. Good hysteresis loops with 2P _r = 20 μC/cm² and 2V _c = 3.4 V were successfully obtained and the ferroelectric property did not depend on the deposition temperature of Ru in the temperature range from 325°C to 400°C. On the contrary, the leakage current density was significantly suppressed down to 1/100 as the deposition temperature of Ru increased from 325°C to 400°C.     

CO gas sensing properties in Pd-added ZnO sensors

Unadded and 0.5 mol% Pd-added ZnO bulk and thin films were prepared by sintering and sputtering, respectively, and their CO gas sensing properties were investigated. The effects of Pd addition, sensing temperature (100–500 °C), and humidity on the CO gas response were discussed. In the bulk sensors, Pd-addition lowered the temperature for the maximum CO gas response (sensitivity) from 400 to 300 °C, whereas the thin film sensors (unadded and Pd-added) exhibited maximum gas response at 200 °C. The Pd-addition enhanced the CO gas response in thin film sensors, and it was also effective for reducing the interference from humidity in both bulk and thin film sensors.     

Highly enhanced electrochemical performance of silicon-free platinum–yttria stabilized zirconia interfaces

In the drive to achieve economically viable solid oxide fuel cells, efforts have been directed towards substantially decreasing their operating temperature. Unfortunately, these efforts have been hindered by extremely sluggish electrode kinetics at reduced temperatures. In this report, we show that silicon impurities on the surface of the electrolyte play a critical role in influencing electrode kinetics. More specifically, improvements by as much as three orders of magnitude are reported for the performance of platinum electrodes on yttria-stabilized zirconia electrolytes prepared as high purity thin films with a largely Si-free surface. These improvements in performance are estimated to enable operation of a solid oxide fuel cell down to approximately 400 °C.     

Performance of a novel dot matrix fuel cell using an inorganic micro‐proton conductor

The power generation properties of a novel dot matrix fuel cell using an inorganic micro‐proton conductor were evaluated in dry gas mixtures of hydrogen and oxygen during room‐temperature operation. The single dot matrix fuel cell was composed of aggregates of micro‐electrolyte dots filling pores arranged in a matrix form on a Teflon or polyimide substrate with Pt/C and Pt catalytic electrodes. Micro‐electrolyte dots were prepared by the sol–gel method using titanium phosphorus oxides as the proton conductive hybrid materials. The open‐circuit voltage of the single cell became higher when using a small dot diameter and achieved a maximum of 500 mV with an electrolyte dot density of 17 dots/cm² in the dry gas mixtures during room‐temperature operation. This value corresponds to about one‐half of the theoretical electromotive force. Moreover, the current density of the single cell increased with the dot diameter such that it grew to 8 mA/cm² at a dot diameter of 500 µm. As a result, dot matrix fuel cells connected in series and parallel were found to achieve the cell performance of high‐energy density such as used by high‐energy microchips. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Microporous activated carbons from used coffee grounds for application to electric double‐layer capacitors

Electrochemical double‐layer capacitors (EDLCs) are devices that store enormous amounts of charge electrostatically when a potential is applied between electrodes of very high surface area (typically made of porous carbon) and an electrolyte. Wider commercialization of this technology has been held back by the lack of ultralow‐cost electrode materials. We demonstrate that used coffee grounds can be processed to form low‐cost electrodes. The surface and electrochemical characteristics of microporous activated carbons from used coffee grounds (CGCs) were measured. First, optimal times and temperatures for carbonization and activation were identified on the basis of Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size distribution. Second, CGCs were used as polarized electrodes in EDLCs, whose capacitances were evaluated using cyclic voltammetry. The results show that carbonization for 1 h at 600 °C with a heating rate of 300 °C/h, followed by CO₂ activation for 2 h at 1000 °C, affords the highest BET surface area (1867 m²/g) compared to other works. The produced CGCs have many micropores of less than 2 nm across, which contribute to the formation of an electric double layer. Capacitors made using these CGCs show the highest capacitance (103 F/g) in 0.8 M (C₂H₅)₄NBF₄/PC as an organic electrolyte, which is much higher than the ∼80 F/g typically used in organic‐electrolyte‐based commercial EDLCs, suggesting that coffee grounds are a useful electrode material. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Low temperature process and thin SBT films for ferroelectric memory devices

Abstract

At crystallization temperatures of about 800°C bismuth layered oxide SrBi₂Ta₂O₉ (SBT) deposited by MOD develops good ferroelectric properties for use in FeRAM devices. But scaling down the film thickness of SBT below 150 nm only shorts are measured at this crystallization temperture after top electrode deposition. Working Pt/SBT/Pt-capacitors are achieved by reducing the crystallization temperature. Also temperatures of 800°C are too high for integration of the SBT module in a stacked capacitor architecture for high density memory devices. Therefore, a process is needed to reduced the crystllization temperature of SBT, called ”Low Temperature Process“.

In this work the electric properties of spin-on processed SBT crystallized in a temperature window from 650°C up to 800°C are investigated. As shown by XRD, transtion of the nonferroelectric Fluorite phase to the Aurivillius phase takes place at approximately 625°C. Increasing the cystallization temperature gives better crystaallized SBT films with bigger SBT graains. However, film prosity is also increasing with temperature. Electrical results of stoichiometric variations of SBT are presented. SEM pictures show that cluster formation is correlated with less film porosity at lower temperatures.     

Preparation and electrochemical properties of La<Subscript>1.0</Subscript>Sr<Subscript>1.0</Subscript>FeO<Subscript>4+δ</Subscript> as cathode material for intermediate temperature solid oxide fuel cells

Cathodic material La_1.0Sr_1.0FeO_4+δ for an intermediate temperature solid oxide fuel cell (IT-SOFC) was prepared via the glycine-nitrate process and characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM). XRD results showed that no reaction occurred between the La_1.0Sr_1.0FeO_4+δ electrode and Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1000 °C. SEM results showed that the electrode formed good contact with the SDC electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of La_1.0Sr_1.0FeO_4+δ were measured using electrochemical impedance spectroscopy (EIS) and steady state polarization measurement. At 700 °C, the polarization resistance was about 3.90 Ωcm², and the lowest polarization overpotential was 57 mV at a current density of 55 mA cm⁻².     

EFFECT OF ANNEALING ON THE ELECTRICAL PROPERTIES OF SrTiO3 THIN FILMS PRODUCED BY ION BEAM SPUTTERING

ABSTRACT

Thin film capacitors with SrTiO₃ (STO) as dielectric and Pt as electrode material have been prepared by ion beam sputtering. The as-deposited film is amorphous and exhibits a crystallization temperature around 321°C as proved by X-ray diffraction. The effect of post annealing on the crystalline quality of the films was systematically studied by x-ray diffraction and Atomic Force microscopy (AFM). The temperature and frequency dependent dielectric properties were measured from 30°C to 200°C and 0.01 Hz to 10⁵ Hz, respectively. The influence of the microstructure of SrTiO₃ thin films on their electrical properties was investigated through an extensive characterization. The electrical properties of SrTiO₃ films appear to be strongly depending on the annealing temperatures. The capacitance voltage (C-V) characteristics reveal an improvement of capacitance density with increasing the annealing temperature.     

Sol-gel PZT thin films on nickel alloy electrodes

Abstract

The Ni alloy electrode was used for a bottom electrode of PZT thin films prepared by sol-gel process. Although PZT films were crystallized on soda–lime glass substrates with the alloy electrodes at a relatively low temperature of 500°C, second phases of Pb₃O₄ and ZrTiO₄ were produced on the electrode in addition to the perovskite PZT phase. In order to prevent the second phases forming, the heat treatment time of the electrode was increased to obtain the thicker Al₂O₃ layer on the alloy electrode. The second phases decreased with increasing the heat treatment time; however, the phases did not disappear. When BaTiO₃ films were inserted between the electrodes and PZT films, the PZT single phase was obtained. The tan δ of the films decreased with decreasing the amount of the second phases, finally it became 3.9%, the film of which possessed a remanent polarization of 20 μC/cm².     

Performance of several types of fuel cells and factor analysis of performance

There are four types of fuel cells: Polymer Electrolyte Fuel Cells (PEFC), Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), and Solid Oxide Fuel Cells (SOFC). The performance of these fuel cells has not been compared. Equations that are able to express the performance of these fuel cells accurately were derived or modified from the latest published performance data. The cell voltages of the four fuel cells were estimated by these equations, resulting in high‐temperature fuel cells such as MCFC and SOFC having higher energy conversion efficiencies than low‐temperature fuel cells such as PEFC and PAFC. This difference originates from low cathode polarization, overcoming both a decrease of open circuit voltage with increasing temperature and higher Nernst losses for high‐temperature fuel cells of approximately 85 mV than those for low‐temperature fuel cells due to steam generation in the anode chamber in the high‐temperature fuel cells. A generalized relationship between cell voltage and operating temperature was derived, stating that the cell voltage is almost constant between 500 °C and 1000 °C. A fuel cell which has protons as a migration species in the electrolyte and works between 250 °C and 500 °C would give a performance comparable with high‐temperature fuel cells due to lower Nernst losses than those for high‐temperature fuel cells. © 2001 Scripta Technica, Electr Eng Jpn, 138(1): 24–33, 2002     

Preparation of BST thin films on Pt electrode on Si wafer with down-flow LSMCVD reactor

Abstract

A novel type of down-flow LSMCVD (Liquid Source Mist CVD) reactor was developed to prepare a high dielectric BST thin film on Pt electrode on Si wafer. Barium acetate [Ba (OOCCH₃)₂], strontium acetate [Sr (OOCCH₃)₂], and titanium isoproxide [Ti (OC₃H₇ ⁱ )₄] were used as metal sources. Metal sources were dissolved in acetic acid, 1-butanol, or 2-methoxyethanol. BST [Ba/(Ba + Sr) = 0.7] film annealed on Pt/Ti/SiO₂/Si above 650°C was polycrystalline. BST film has a (110) preferred orientation with increasing temperature. Surface roughness of BST film and grain size increased with increasing temperature. The metal-oxygen bond was formed at 650°C as shown in the spectra of FTIR. The depth profiles of elements of BST thin films indicated a uniform composition throughout the film. BST films annealed at 750°C showed a dielectric constant and a tanδ of 390 (thickness: 150 nm) and 0.06 at a frequency of 100 kHz, respectively. The behavior of capacitance of the BST film with bias voltage showed paraelectric property. BST film annealed at 750°C had the leakage current density of 3.2 (μA/cm²) at a bias voltage of 2V.     

Influence of Annealing Temperature on Structural,Morphological and NH3 Sensing Properties of Nanostructured WO3 Porous Films

The tungsten trioxide (WO₃) precursor was prepared by sol-gel method with tungsten powder as the raw material, and the WO₃ gas sensing films were obtained by a dip coating method and annealing precursor in air. X-ray diffraction (XRD) spectra indicate that with increasing annealing temperature the triclinic structure of as-prepared sample was transformed into monoclinic or orthorhombic phase. The images of scanning electron microscopy (SEM) exhibit that the WO₃ grain sizes increase from less than 100 nm to several micrometers with increasing annealing temperature. The influences of applied frequency, annealing and operating temperature on NH₃ gas sensing properties of the nanostructured WO₃ porous films were investigated. The results indicate that the gas sensing film annealed at 500°C express high sensitivity, fast response and recovery speed to NH₃ at operating temperature 250°C.     

Bottom electrode structures of Pt/Ru and Ru deposited on polycrystalline silicon by MOCVD for DRAM capacitor

Abstract

The electrode structures of Pt/Ru and Ru on polycrystalline silicon (poly-Si) were prepared by metalorganic chemical vapor deposition (MOCVD) for high dielectric constant (Ba, Sr)TiO₃(BST) capacitor integration. The electrode structures of Pt/Ru/poly-Si annealed above 700°C for 1h in oxygen atmosphere showed a smooth surface ·microstructure without any second phases on the platinum. The specific contact resistance of Pt/Ru and poly-Si in Pt/Ru/poly-Si structures annealed at 800°C was about 1.5 × 10^?5 Ω-cm². The step coverage of Ru film deposited at 150°C was 76% and those of Pt film deposited at 300°C on Ru (deposited at 150°C) was about 61.3%.     

Electro-Optic Properties of Heteroepitaxial Pb(Zr,Ti)O3/La0.5Sr0.5CoO3 Film Structures

Vertical ferroelectric Pb(Zr,Ti)O₃ (PZT) 1 μm thick film capacitor was fabricated by pulsed laser deposition technique (PLD) onto conducting La_0.5Sr_0.5CoO₃(LSCO) 100 nm thick bottom electrode on both side polished YAlO₃ + 1% Nd₂O₃ (Nd:YAlO₃) single crystal substrate to operate as a Pockels cell optical modulator. On top of the PZT film, semitransparent 30 nm thick Au electrode was deposited by thermal evaporation. Intensity of the chopped 670 nm polarized laser radiation transmitted through the Au/PZT/LSCO/Nd:YAlO₃ cell was measured at various temperatures and bias voltage applied. Applying 20 V (200 kV/cm) across the capacitive cell, modulation of the transmitted light as high as 3% was achieved while the voltage tunability measured at 1 kHz from C-V characteristics was about 70%. Thermo-optical measurements performed for PZT/Nd:YAlO₃ sample in the range up to 400°C showed the phenomenon of critical opalescence in the vicinity of Curie temperature at 208°C. Optical transmission through the PZT film biased with electric field was studied in the range 400 to 1000 nm. Film thickness, refraction index and absorption coefficient have been determined from the interference pattern observed in the PZT transmission spectrum. A simple model yields the dispersion relation for the electro-optic coefficient.