Kenichi Ushiki

New concept for simplifying SOFC system

Abstract A novel design for the solid oxide fuel cell (SOFC) has been proposed. By attaching palladium and gold electrodes on the same face of BaCe0.8Gd0.2O3−α electrolyte and feeding a mixture of CH4 and O2 (CH4:O2 mole ratio = 2:1) to the cell at 950 °C, electric power could be generated from the cell. The main advantages are that the ohmic resistance of the cell decreased with reducing distance between the two electrodes and that some unit cells were connected in series and parallel with one another on the same electrolyte. In addition, the surface ionic conduction of oxide ion was studied from the standpoint of the surface morphology of the electrolyte.

A novel cell design for simplifying SOFC system

A novel design for the solid oxide fuel cell (SOFC) has been proposed. By attaching palladium and gold electrodes on the same face of BaCe0.8Gd0.2O3−α electrolyte and feeding a mixture of CH4 and O2 (CH4:O2 mole ratio = 2:1) to the cell at 950 °C, an electric power could be generated from the cell. The main advantages are that the ohmic resistance of the cell decreased with reducing distance between the two electrodes and that some unit cells were connected in series and parallel with one another on the same electrolyte.

Electrochemical oxygen pump using CeO2-based solid electrolyte for NOx detection independent of O2 concentration

Abstract The electrochemical oxygen pump has been studied using Ceo 0.8 Sm 0.2 O 1.9 as a solid electrolyte and gold metal as an electrode. The O 2 concentration in the flow of a mixture of 1000 ppm NO, 1–10% O 2 , 5% H 2 O and 5% CO 2 at a flow-rate of 20 ml min −1 was electrochemically controlled to a constant value of 5% by applying anodic or cathodic currents to the cell at 900 °C. During oxygen pumping, NO was not electrolyzed at all and the pumping amount of oxygen obeyed Faradays law. A potentiometic- or amperometric-type sensor for NOx , when introduced into the resulting gas stream, exhibited an output signal without influence of the O 2 concentration.

Electrochemical removal of NO in the presence of excess O2, H2O and CO2 using Sm2O3-doped CeO2 as a solid electrolyte

Abstract Electrochemical removal of NO from a gas stream containing 2% O 2 , 5% H 2 O and 5% CO 2 has been carried out between 400 and 800 °C using a single-compartment reactor, constructed from CeO 2 -based solid electrolyte with two palladium electrodes. At all temperatures studied, NO was decomposed to N 2 at the palladium cathode by applying a direct current to the reactor. The dependence of the conversion of NO to N 2 on the concentration of NO, O 2 , H 2 O or CO 2 was investigated at 450 °C in detail.

Medium-temperature electrolysis of NO and CH4 under lean-burn conditions using ytrria-stabilized zirconia as a solid electrolyte

Electrolysis of NO and CH4 in the presence of excess O2 has been studied using a single-compartment reactor, constructed from yttria-stabilized zirconia (YSZ), with two palladium electrodes. The palladium electrodes were attached by an electroless plating method. Experiments were conducted between 550 and 900 °C with a mixture of 1000 ppm NO, 1000 ppm CH4, 2% O2, 5% H2O and 5% CO2 in argon at a flow rate of 50 ml min–1. The reduction of NO to N2 and the oxidation of CH4 to CO2 were promoted by applying a direct current between the two palladium electrodes. The current efficiency for the reduction of NO was about 1.5% at all measured temperatures, while that for the oxidation of CH4 decreased from 2.7 to 0.3% with increasing temperature from 550 to 900 °C. The dc resistance of the reactor was mainly due to the ohmic resistance of the electrolyte. The transport properties of O2– through the electrolyte followed Faradays law under operating conditions. These results were strongly dependent on the morphology and thickness of the palladium electrode.

Ultra-fine grinding of La0.8Sr0.2MnO3 oxide by vibration mill

Abstract The grinding operation of La 0.8 Sn 0.2 MnO 3 (LSM) powder by a vibration mill has been carried out with the intention of giving a higher surface area and a good thermal stability. The specific surface area of the LSM powder increased with operation time and reached up to 34 m 2 g −1 for 350 h. XRD measurement and chemical analysis showed that during the grinding operation, the crystal structure of the LSM powder was kept and that the extent of the contamination was negligibly small. In CH 4 oxidation at 400°C and N 2 O reduction at 450°C over the ground LSM powder, both conversions were raised proportional to the increasing specific surface area. Furthermore, the thermal stability of the ground LSM powder was improved by grinding it together with CeO 2 powder. The fine dispersion of CeO 2 particles around the LSM particles effectively depressed the sintering of the LSM powder at elevated temperatures.

Membrane reactor for oxidative coupling of CH4 with an oxide ion–electron hole mixed conductor

A membrane reactor for the oxidative coupling of CH4 has been constructed with an oxide ion–electron hole mixed conductor, BaCe0.8Gd0.2O3–α. 10% CH4 diluted with Ar and an O2–Ar mixture at a given ratio were fed into opposite sides of the membrane at 1173 K. The formation rate of C2–hydrocarbons (ethane and especially ethene) in the CH4 compartment increased with Po2 in the O2–Ar compartment. This enhancement was due to electrochemical oxygen permeation, causing the conductor to short-circuit itself, resulting form the mixed conduction. For comparison, 10% CH4 and a small amount of O2 were co-fed into one side of the membrane. The membrane operation gave a C2 selectivity two times that of the co-feed operation for all CH4 conversions. From a catalytic study using BaCe1–xGdxO3 –α powders as catalyst, it was found that increasing both the oxide-ion and electron-hole conductivities enhanced the formation of C2 hydrocarbons, but reduced that of CO2. On the basis of these results, a mechanism for the oxidative coupling of CH4 of the mixed-conductor membrane was proposed.

Electrochemical removal of NO and CH4 in the presence of excess O2, H2O and CO2 using Sm2O3-doped CeO2 as a solid electrolyte

Electrochemical removal of NO and CH4 from a gas stream containing excess O2, H2O and CO2 has been carried out between 400 and 800 °C using a single-compartment reactor, which is constructed from CeO2-based solid electrolyte with two palladium electrodes. At all temperatures studied, both NO and CH4 were decomposed by applying a direct current to the reactor. NO was decomposed to N2 in two different ways depending on the applied current. At low currents, NO was electrolysed together with O2 at the palladium cathode; and, at high currents, NO was catalytically reduced over the palladium surface free from adsorbed oxygen. By investigating the influences of the concentration of NO, H2O, CO2, CH4 or C3H8 contained in the reactant gas to these two decompositions, their mechanisms are discussed in detail.

Oxidative coupling of CH4 using alkali-metal ion conductors as a solid electrolyte

The oxidative coupling of CH4 to C2H4 and C2H6(C2-hydrocarbons) has been carried out using an Li+ ion conductor as a solid electrolyte. A single-compartment cell is constructed from (Li2O)0.17(BaO)0.07(TiO2)0.76(LBT) ceramic with two gold electrodes. Alternative current voltages are applied between the two electrodes with a mixture of 8.3% CH4 and 1.6% O2 in argon. The experiments are carried out under various conditions. The formation of C2-hydrocarbons is the most effectively enhanced at a temperature of 850 °C or a frequency of 10 Hz. For example, the conversion of CH4 and the selectivity of C2-hydrocarbons at 3 V are 2 and 1.5 times more than those at the open circuit, respectively. The applications of a dc voltage to a two-compartment cell suggested that the active sites are generated via the cathodic reaction. The mechanism for the formation of C2-hydrocarbons is discussed in detail.

Preparation of nanometre-sized titania dispersed silica

Since nanometre-sized semiconductor particles exhibit quantum-size effect, they have been attracting much interest in recent years. Chalcogenide semiconductors such as CdS have been studied intensely [1±3]. Investigations on oxide semiconductors were limited for some oxides such as TiO2, ZnO, WO3 and Fe2O3 [4±7]. To the authors knowledge, there has been no study on the dispersion of oxide semiconductor nano-particles into a solid matrix. In this paper, we chose titania as an oxide semiconductor and tried to synthesize nanometre-sized titania (anatase) colloidal particles. Dispersion of these anatase particles into a silica matrix was also tried. Titanium tetraisopropoxide (TTIP, Kanto Chemicals) was used as received. TTIP was diluted with 20 ml of reagent grade ethanol and dried with molecular sieve 3A before use. The amount of TTIP was varied between 2 ml and 10 ml. The solution was dropped into 50 ml of HCl or HNO3 (concentration; 0.1±2 N) with a micro tube pump at the rate of 50 ml hÿ1. The dropping procedure was conducted at room temperature in a closed vessel purged with dry nitrogen gas. Silica sol was prepared by the acidic hydrolysis of tetraethoxysilane (TEOS, Kanto Chemicals). 18.9 ml of TEOS was diluted with 10 ml of ethanol and hydrolysed with 10 ml of 1 N HNO3. 100 ìl of HF (46%) was added after 30 min of the hydrolysis in order to accelerate the polymerization of silica. Titania sol was mixed with the silica sol so that the titania content was 1, 5 and 10 wt %. Transparency of the sol was not lost by the mixing. Since both the silica and titania particles were positively charged under the low pH conditions, they did not ̄oculate to form precipitates. The mixed sol was cast into plastic containers and kept at 40 8C to change into a gel within a few hours. The gel was kept at the same temperature to dry by the gradual evaporation of the solvents. After drying, transparent thin gel plates were obtained. The size of the titania particles was observed in a transmission electron microscope (TEM, JEOL JEM2000CX). X-ray powder diffraction (XRD) patterns were obtained on a MAC Science X-ray diffractometer MXP3 (CuKá, operated at 40 kV and 20 mA). The powder sample was obtained by evaporating the solvent of the colloid under reduced pressure with a rotary evaporator. Optical absorption spectra of the colloids were measured on an ultraviolet-visible (UVVIS) spectrophotometer (JASCO Ubest 50) between the wavelengths of 200 and 500 nm. The titania particle dispersed silica gel was heated at 500 8C, 750 8C and 1000 8C for 1 h. After heating, these gels were examined by X-ray powder diffraction, UV-VIS spectra and TEM observation with the same instruments used for the colloidal particles. The formation of transparent titania colloids depended on the acid concentration. When the acid concentration was 0.1 N, unpeptized precipitate remained. In the case of high acid concentrations (1 N and 2 N), the titania precipitate was peptized immediately to form a transparent colloid. Colloidal particles were observed for all the sol samples by TEM observation. Fig. 1 shows a typical TEM micrograph of the titania colloids. This sample was prepared with 4 ml of TTIP and 1 N, HCl. Formation of crystalline particles was observed in this micrograph. A crystal lattice was observed in these particles and one particle consisted of a few crystallites with a size of 2±3 nm. The diameters of the agglomerated particles were about 5 nm. These transparent colloids were stable for about for one day at room temperature. Fig. 2 shows the XRD pattern of the same colloid shown in Fig. 1. The crystalline phase of the colloidal particles was identi®ed to be anatase from this XRD pattern. Fig. 3 shows the UV-VIS spectra of these titania colloids. These samples were peptized by 1 N HCl. The absorption spectra were normalized by Equation 1 and the absorption coef®cient was shown in Fig. 3.