Ashok V. Joshi

Phase stability and oxygen transport characteristics of yttria- and niobia-stabilized bismuth oxide

As a part of an overall study to explore the potential application of stabilized Bi2O3 as oxygen separator in various electrochemical systems, an investigation of the stability and transport characteristics of yttria- and niobia-stabilized bismuth oxide was undertaken. Polycrystalline Bi2O3 samples containing 25mol % Y2O3 were fabricated by pressureless sintering powder compacts at 1000‡ C in air. Samples containing 15mol % Nb2O5 were also fabricated by pressureless sintering at 900‡ C in air. The resulting samples were dense and of an equiaxed microstructure with grain size in the range from 28Μm for the yttria-stabilized and 42Μm for the niobia-stabilized materials, respectively. X-ray diffraction of the as-sintered specimens showed them to be single phase with CaF2-type structure. Ionic conductivity was measured by an a.c. technique over a wide range of temperatures. It was observed that the ionic conductivity of the yttria-stabilized bismuth oxide was greater than that of the niobia-stabilized one.The specimens subsequently were annealed over a range of temperatures between 600‡ C and 700‡ C for up to several days. X-ray diffraction traces taken on the Y2O3-stabilized samples indicated that the original cubic solid solution had decomposed. The decomposition of the yttria-stabilized samples was also accompanied by the occurrence of exaggerated grain growth. The observed decomposition is not in agreement with the phase diagram available in the literature, according to which the cubic phase should be stable over the range of temperatures the samples were annealed in the present study. By contrast, Nb2O5-stabilized Bi2O3 remained cubic, although it appeared to have dissociated into two cubic solid solutions of slightly differing lattice parameters. There was no perceptible change in the grain size of the niobia-stabilized samples.Several electrolyte tubes made of the yttria- and niobia-stabilized bismuth oxide were electrolytically tested under a d.c. mode with silver electrodes. In tubes made of the yttriastabilized material, the current density decreased with time (under a constant applied voltage) at 650‡ C and at ⩽ 700‡C but did not at ⩾ 700‡C consistent with the observation that the material did not decompose at ⩾ 700‡ C but did at 650‡ C. At 600‡ C, the rate of decrease was slower than at 650‡ C indicating that the kinetics of phase decomposition is probably slower at 600‡ C. In the niobia-stabilized tubes the decrease in the current density was lower. This decrease is probably related to the apparent formation of two cubic solid solutions of slightly differing compositions.The present work shows that the published phase diagram of the Y2O3-Bi2O3 system is incorrect. The present results also suggest that for application to temperatures as low as 650‡ C (and possibly lower), electrolytes made with Nb2O5 as the stabilizer are preferable.

Solid Electrolyte Materials, Devices, and Applications

This paper outlines the development status, issues, and applications of several solid electrolyte electrochemical devices currently being developed by Ceramatec and its partners. Ceramatec and its commercial partner Air Products and Chemicals, Inc., (APCI) have successfully developed and demonstrated an electrochemical device that utilizes a ceria-based, solid electrolyte to separate oxygen from air [1, 2]. Other oxygen separator projects utilize ion transport membrane(s) (ITM) composed of mixed ionic and electronic conductors to transport oxygen ions across the membrane by means of a pressure differential driving force to generate high purity oxygen or a chemical reaction driving force to produce synthesis gas from methane (ITM Syngas).Ceramatec, in partnership with SOFCo, demonstrated kilowatt class solid oxide fuel cell (SOFC) stacks operating on a variety of fuels such as pipeline natural gas and reformed diesel. Ceramatec is presently working with Cummins and SOFCo to develop low cost modular fuel cells under the Department of Energy’s Solid-state Energy Conversion Alliance (SECA) initiative. Some of Ceramatec’s other programs are focused on development of gallate electrolyte based fuel cells [3] and metallic bipolar plates [4] for lower temperature operation.

Structure, microstructure and transport properties of mixed ionic-electronic conductors based on bismuth oxide Part I. Bi-Y-Cu-O system

Abstract An ionic-electronic mixed conductor consisting of a stabilized fcc Bi1.5Y0.5O3 ionic conductive matrix phase, and a tetragonal Bi2Cu2+1−nCu1+nO4−0.5n electronic conductive second phase has been developed. The ionic transference numbe r and conductivity of the composite material depend critically on the amount of the bismuth copper oxide second phase and its morphology.

Theoretical assessment of an oxygen heat engine: The effect of mass transport limitation

The efficiency of a closed, reversible cycle consisting of an isothermal expansion of a gas at a higher temperature followed by its compression at a lower temperature is Carnot-limited. If the expansion and compression can be achieved electrochemically through solid electrolytes, heat can be directly converted into electrical energy. A device with oxygen ion conducting electrolytes and oxygen gas as the working fluid may be called an oxygen heat engine in analogy with the sodium heat engine. Theoretical analysis of such a device is presented. The power is shown to exhibit a maximum at some critical pressure in the low pressure chamber. The maximum in power occurs due to a compromise between thermodynamic and kinetic factors. At pressures lower than the critical pressure, mass transport in the low pressure chamber limits power. At pressures greater than the critical, low Nernst potential limits power. The mass transport limitation can, in principle, be eliminated by proper design of the device and/or introduction of an oxygen containing compound with low equilibrium oxygen partial pressure in the low pressure chamber. For example, introduction of a mixture of H2O and H2 in the low pressure chamber makes high power densities possible. Theoretical assessment of this type of heat engine shows that power densities in excess of 200 mW/cm2 are possible.