Takao Esaka

Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production

Abstract Some sintered oxides based on SrCeO3 were found to exhibit proton conduction on exposing them to a hydrogen-containing atmosphere at high temperature. The verification of proton conduction was made by studying the emf of various gas cells using the specimen diaphragm as an electrolyte. These materials could be applied to the electrolyte for steam electrolysis to produce hydrogen gas.

Diffusion coefficient measurement of lithium ion in sintered Li1.33Ti1.67O4 by means of neutron radiography

Neutron radiography (NR) using cold neutron beam was applied to the tracer diffusion coefficient measurement of lithium ion in Li1.33Ti1.67O4. The diffusion couples with different lithium isotope concentrations were annealed at 860 to 900°C and the isotope profiles of lithium ion in the sample were measured by NR. The diffusion profiles obtained for the samples annealed at higher temperatures show good agreement with Fick’s law, whereas the diffusion was not carried out well at the interface for the lower-temperature-annealed samples. The values of diffusion coefficients obtained from the profile fitting were a little smaller than those expected from the electric conductivity. The NR method was found to be the most useful method to measure the tracer diffusion coefficient of lithium ion in solids.

Formation of high oxide ion conductive phases in the sintered oxides of the system Bi2O3Ln2O3 (Ln = LaYb)

The electrical conduction in various phases of the system Bi/sub 2/O/sub 3/-Ln/sub 2/O/sub 3/ (Ln = La, Nd, Sm, Dy, Er, or Yb) was investigated by measuring ac conductivity and the emf of the oxygen gas concentration cell. High-oxide-ion conduction was observed in the rhombohedral and face-centered cubic (fcc) phase in these systems. The fcc phase could be stabilized over a wide range of temperature by adding a certain amount of Ln/sub 2/O/sub 3/. In these cases, the larger the atomic number of Ln, the lower the content of Ln/sub 2/O/sub 3/ required to form the fcc solid solution, except in the case of Yb/sub 2/O/sub 3/. The oxide ion conductivity of this phase decreased with increasing content of Ln/sub 2/O/sub 3/. Maximum conductivity was obtained at the lower limit of the fcc solid solution formation range in each system, which was more than one order of magnitude higher than those of conventional stabilized zirconias. Lattice parameters of the fcc phase were calculated from the x-ray diffraction patterns. The relationship between the oxide ion conductivity and the lattice parameter was also discussed.

Formation of protons in SrCeO3-based proton conducting oxides. Part II. Evaluation of proton concentration and mobility in Yb-doped SrCeO3

Abstract In order to evaluate the proton concentration in SrCeO3-based oxides, protons in the oxides were expelled in the form of water vapor by raising temperature in a flowing oxygen gas and the total amount of water vapor evolved was measured. While the solubility of water vapor in pure SrCeO3 was negligibly small, Yb-doped proton-conducting oxides dissolved an appreciable amount of water vapor. The determined proton concentration for SrCe0.95Yb0.05O3−α was about 2 mol% at 600°C and about 1 mol% at 1000°C. The equilibrium constant for proton and hole formation and their mobilities were estimated, and these parameters fitted to the conduction behavior measured previously by us.

Mixed conduction and oxygen permeation in the substituted oxides for CaTiO3

Mixed conduction in substituted perovskite-type oxides CaTi1−xMxO3−α (M=Fe, Co and Ni) was studied by means of electrochemical methods. Of these materials, Fe-substituted oxides showed the highest oxide ion conductivity as well as high electronic conductivity. The oxide ion transport numbers of the oxides were lower than 0.6 over the whole composition range measured in air. Electrochemical oxygen permeation in such a material could be confirmed by gas chromatography. Extraction of oxygen from air was carried out using the mixed conductor. The oxygen extraction rate in this method was higher than that using a silicone film.

Structural Analysis of Pure and Electrochemically Lithiated SiO Using Neutron Elastic Scattering

Neutron elastic scattering was used to determine the structure of as-received and lithiated amorphous SiO which has been proposed as an anode material for lithium ion secondary batteries. Based on a comparison between the total radial distribution functions [RDF(r)] of SiO and SiO 2 , it was suggested that amorphous SiO is composed of a three-dimensional SiO 4 tetrahedral network similar to silica (SiO 2 ) glass and metallic silicon clusters, and that the latter were finely dispersed in the SiO 4 matrix. On the other hand, electrochemically lithiated SiO showed a typical lithium negative correlation together with the disappearance of the Si-Si correlation in the RDF(r) which indicated that lithium predominantly reacted with the metallic silicon to form Li-Si alloys.

High temperature type protonic conductor based on SrCeO3 and its application to the extraction of hydrogen gas

Protonic and electronic conductivities of SrCe0.95Yb0.05O3−α ceramic in hydrogen gas were investigated at high temperatures. The protonic conductivity was 2 orders of magnitude higher than electronic conductivity. When the ceramic was exposed to hydrogen gas on one sinde and to oxygen gas on another side (fuel cell condition), the electronic conductivity increases with increasing partial pressure of oxygen suggesting that the charge carriers were positive holes. Using this ceramic as a solid electrolyte diaphragm, electrochemical hydrogen extractor was constructed by way of experiment and we could extract hydrogen from the pyrolyzed gas of CO + H2O mixture, ethane or steam. A bench-scale steam electrolyzer was fabricated using the protonic conductor, and pure hydrogen gas could be extracted in a rate of a few l/hr.

Oxide ion conduction in the solid solution based on the scheelite-type oxide PbWO4

Abstract Electrical conduction in substituted scheelite-type oxides was studied by way of electrochemical method. Many substituted samples based on CaWO4, CaMoO4, BaWO4, etc. showed the mixed phase, where enhanced electrical conduction was not observed. High oxide ion conduction was observed in Pb1−xLaxWO4+x/2 or Pb1−xLa2x/3WO4 having the scheelite-type tetragonal structure. The maximum conductivity is 4.2×10−2S cm−1 at 800°C for Pb0.8La0.2WO4.1. The oxide ion conduction was considered to be due to the interstitial oxide ions formed by substitution of Pb2+ by La3+. These results are discussed in relation to the crystal phases.

Ionic conduction in substituted scheelite-type oxides

In a search for new high temperature ionic conductors, various substituted scheelite-type samples were prepared by the solid state reaction and their electrical conduction was investigated. Typical scheelite-type oxides designated as ABO 4 (A = Ca, Sr, Ba; B = W, Mo) are generally classified into the oxysalts. However, some oxides having cations with almost the same electronegativities, such as Pb and W in PbWO 4 , can be regarded as double oxides with a deformed fluorite-type structure. Therefore, in order to create appropriate conductors in each oxide type, A 2+ B 6+ O 4 , alternative substitutions were tried: A 1-x M + 2x BO 4 for the former and A 1-x M 3+ x BO 4-x/2 for the latter. As a result, new types of ionic conductors were found: lithium ion conductors and oxide ion conductors in scheelite-type solid solutions based on ABO 4 (A = Ca, Sr, Ba; B = W, Mo) and PbWO 4 , respectively. Lithium ion conduction was confirmed visually by neutron radiography.

Powder neutron diffraction study of Ln-substituted PbWO4 oxide ion conductors

Abstract Structure refinements have been carried out on pure and lanthanide-substituted PbWO4 using powder neutron diffraction, and the defect structures of La- and Pr-substituted PbWO4 are discussed. The Rietveld refinement structure of pure PbWO4 agreed well with previous results. In the Pb1−xLaxWO4+x/2 system, the oxide ion interstitials seemed to be rather localized around z=0.3 of the 8e position (0 0 z) or the adjacent general position of the scheelite structure-type with I41/a symmetry. The deduced interstitial position is considered in terms of the oxide ion conduction path. A similar defect structure has been determined for the praseodymium-substituted compound, Pb1−xPrxWO4+x/2. In contrast, Pb1−xLa2x/3WO4-type substitution resulted in the formation of a cation-deficient structure.