F.T. Ciacchi

Scandia–zirconia electrolytes for intermediate temperature solid oxide fuel cell operation

Abstract Conductivity of several compositions in the scandia–zirconia system (Sc 2 O 3 content between 7.0 and 11.0 mol%) has been investigated as a function of temperature and time at the nominal solid oxide fuel cell operating temperature of 850°C. For comparison, the stability of electrolyte compositions was also studied at 1000°C by monitoring conductivity as a function of time. All compositions showed conductivity degradation as a function of time both at 850 and 1000°C, however, it was much lower in compositions containing Sc 2 O 3 between 9.0 and 11.0 mol% especially at 1000°C. The maximum conductivity was observed for materials with compositions close to 9 mol% Sc 2 O 3 –ZrO 2 . The 10.0 and 11.0 mol% Sc 2 O 3 –ZrO 2 compositions showed the presence of a rhombohedral-phase (Sc 2 Zr 7 O 17 , the beta-phase) at room temperature. Hysteresis loops were observed in the conductivity–temperature plots around 500–600°C due to the transformation of the low conducting beta-phase to high conductivity cubic-phase on heating and the reverse on cooling. The contribution of grain boundary impedance in as-sintered and annealed specimens was relatively small for all specimens studied.

Evaluation of commercial zirconia powders forsolid oxide fuel cells

Abstract Yttria-zirconia electrolytes prepared from powders obtained from several suppliers have been evaluated for use in solid oxide fuel cells. Two compositions with Y2O3 content of 3 and 8 mol% have been studied. Ionic conductivities were measured with a four-probe dc technique over the temperature range of 400–1000°C and impedance spectroscopy over the range 300–450°C. In addition, the effect of annealing on the conductivity has been studied at the current fuel cell operating temperature of 1000°C. The microstructure (grain size, distribution, shape, pore size and its distribution) has been investigated with scanning electron microscopy. Most specimens could be densified to near theoretical density except for powders supplied by Magnesium Elektron which had somewhat lower density. At 1000°C the conductivity of specimens with 8 mol% Y2O3 content was higher by a factor of about three compared with 3 mol% Y2O3ZrO2 specimens. However, below 400°C the conductivities were comparable. The grain boundary resistivity was a function of the SiO2 content in the starting powders. As a consequence of annealing of specimens at 1000°C, an increase in both the grain boundary and the intragrain resistivity (measured at low temperatures) was observed but the effect was much higher on the grain boundary impedance especially for specimens with Y2O3 content in the vicinity of 8 mol%. At the fuel cell operating temperature of 1000°C, the difference in the conductivity of specimens prepared from powders supplied by different manufacturers was insignificant apart from the role of actual dopant content.

Oxygen-ion conducting electrolyte materials for solid oxide fuel cells

Oxygen-ion conducting solid electrolyte systems have been reviewed with specific emphasis on their use in solid oxide fuel cells. The relationships between phase assemblage, electrolyte stability and ionic conductivity have been discussed. The role of parameters such as sintering temperature and atmosphere which influence the segregation of impurities, present in the starting ceramic powders, at grain boundaries and at the external surface of the electrolyte compacts has been emphasised. The stability of various electrolyte materials in contact with other fuel cell components and in fuel environments has been discussed in detail. The ageing behaviour at fuel cell operating temperatures has been described. Data on ionic conductivity, mechanical and thermal properties have been presented for a number of electrolyte materials.

Investigation of the stability of ceria-gadolinia electrolytes in solid oxide fuel cell environments

Doped ceria-based materials are potential electrolytes for use in lower operating temperature (500-700 degrees C) solid oxide fuel cells because of their high ionic conductivity. In this study, impedance behaviour and microstructure of the (Ce0.8Gd0.2)O-1.9 exposed to mild fuel environments (H-2-N-2 mixtures) have been investigated. The exposure of specimens to H-2-N-2 mixtures at 1000 degrees C resulted in a substantial expansion of the lattice as a consequence of the reduction of Ce4+ to Ce3+, which in turn led to the development of microcracks and loss of continuity at the grain boundary region and increase in both the grain boundary (major effect) and the lattice (minor effect) resistivity. The behaviour for the grain boundary resistivity after the 800 degrees C exposure was somewhat similar although expansion of the lattice at 800 degrees C (or lower temperatures) was considerably less conspicuous. After exposure to H-2-N-2 atmosphere at lower temperatures (650 and 500 degrees C), although no significant increase in the grain boundary resistivity for exposures up to 1000 h was observed, the shape of the grain boundary are was clearly affected. The large increase in the grain boundary resistivity in reduced specimens has been attributed to the observed microcracking, loss of continuity between grains and possibly the formation of new phase regions with extremely poor oxygen-ion conductivity along grain boundaries during the reduction. The disruption to the microstructure is not recovered on subsequent oxidation in air

Characterisation, conductivity and mechanical properties of theoxygen-ion conductor La0.9Sr0.1Ga0.8Mg0.2O3-x

The new oxygen-ion conductor La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-x has been prepared by conventional solid-state reaction at high temperatures and characterised by X-ray diffraction, scanning and transmission electron microscopy, and conductivity (four-probe dc and impedance) measurements. The room-temperature structure is orthorhombic, space groupPnma (no. 62), with a=5.5391(7) A, b=7.8236(12) A, c=5.5224(7) A. The material undergoes a phase transition at 445 K to a rhombohedral structure. Mechanical property measurements at room temperature and at 1173 K give average strength measurements of 162±14 MPa and 55±11 MPa respectively. Conductivity and ionic transport number measurements confirm predominantly ionic conduction. The contribution from the grain boundary conductivity is extremely small at temperatures below 673 K. At 1073 K, an ionic conductivity value of 0.12 S cm -1 was recorded in air.

The system Y2O3-Sc2O3-ZrO2: Phase stability and ionic conductivity studies

Abstract The ionic conductivity of the system Y2O3-Sc2O3-ZrO2 has been investigated, as a function of temperature (400–1000°C) and time (at 1000°C), for a range of Y/Sc ratios (eight compositions) at a constant dopant level of 8 mol%. The conductivity of scandia-rich compositions was almost twice that of the fully stabilized Y2O3-ZrO2 but in all the specimens the conductivity deteriorated with time on annealing at 1000°C. In Sc2O3-rich compositions, a dopant-rich t′-phase is formed by diffusionless transformation from the cubic phase on cooling the ceramic from the sintering temperature. On annealing at 1000°C, this phase decomposes to low-dopant t-ZrO2 precipitates and a cubic zirconia solid-solution matrix. In Y2O3-rich compositions only the cubic phase is formed but this is not an equilibrium phase with respect to dopant distribution. On annealing a slight redistribution of the dopant level and precipitation of t-ZrO2 takes place. These changes in the microstructure are responsible for the observed conductivity ageing process. The decrease in the conductivity and increase in the activation energy with increasing Y2O3 above 850°C has been discussed in terms of the steric blocking effect of the larger Y3+ cation.

The system Y2O3-Sc2O3-ZrO2: Phase characterisation by XRD, TEM and optical microscopy

Abstract The phase assemblage in the ternary system Y2O3-Sc2O3-ZrO2 has been investigated as a function of the Sc 2 O 3 Y 2 O 3 ratio for several compositions with a constant total stabilizer (Y2O3 + Sc2O3) content of 8 mol%, using X-ray diffraction, transmission electron and optical microscopy. Considerable twinning was observed in as-sintered speciments with higher scandia content ( Sc 2 O 3 /Y 2 O 3 ≥ 5 3 . These compositions were indexed as having tetragonal symmetry. The twinning is associated with the formation of a dopant rich tetragonal phase (designated as the t′-ZrO2 phase) as a result of diffusionless transformation on cooling the material from the sintering temperature at which it had a cubic symmetry. On annealing Sc2O3-rich compositions at 1000°C for 2000 h, twinning disappeared and the specimens consisted of cubic matrix with fine precipitates of a low-dopant tetragonal phase (t-ZrO2) dispersed uniformly. The cubic matrix had a slightly higher dopant content than the corresponding t′-phase. The Y2O3-rich compositions in the as-sintered form had cubic symmetry and showed no twinning. On annealing, they decomposed to t-ZrO2 precipitates plus a cubic solid-solution matrix richer in the dopant. All the eight Y2O3/Sc2O3 compositions studied were in the two-phase field at the annealing temperature.

Performance of zirconia membrane oxygen sensors at low temperatures with nonstoichiometric oxide electrodes

Sensor tests on zirconia membrane probes incorporating nonstoichiometric oxides of the general formula (U, M)O2±x and having the fluorite structure have been described over the temperature range of 300 to 600° C and oxygen concentrations between 100 and 0.1%. The results have been compared with those for porous platinum electrodes. The sensors with (U, M)O2±x electrodes performed well, down to 350°C, whereas those with porous platinum electrodes showed large deviations from Nernstian behaviour even at 450°C. The variables studied included the effect of dopant concentration and type, surface area of the electrode and heat treatment. The electrolyte used in the sensor tests had at least an order of magnitude lower conductivity compared with the traditionally used electrolytes such as Y2O3−ZrO2 or Sc2O3−ZrO2. Despite this, the better performance of sensors provided with (U, M)O2±x electrodes suggests that the electrode/electrolyte interface plays a much more dominant role than the electrolyte.

A fully automated four-probe d.c. conductivity technique for investigating solid electrolytes

A fully automated four-probe d.c. conductivity technique has been described for investigating the conductivity of solid electrolytes as a function of temperature and time. The technique is equally applicable to semiconducting and superconducting materials. With full automation, the time for data acquisition is very short and the reproducibility is high. Several uncertainties associated with manual recording of data are reduced. Some applications of the technique have been described. Various sources of error associated with collection of data have been considered and limitations and advantages of the techniques have been discussed.

Tubular zirconia–yttria electrolyte membrane technology for oxygen separation

Abstract Oxygen separation technology has applications in health care, defence, on-site generation of gases with known oxygen concentration, food packaging (oxygen removal), aquaculture and a number of others. The market potential for this technology is of the order of several billion dollars globally. Ceramic membranes based on O 2− or O 2− /electronic conducting materials have the potential to service this market. Pure O 2− conducting ceramic membrane technology can be used not only for production of oxygen but also for oxygen removal in gas streams and enclosures as well as for oxygen level control to produce calibration gases. In this paper, the operation of several oxygen separation devices of tubular design constructed from zirconia–yttria electrolyte has been discussed for oxygen generation, oxygen removal and control. Life time tests performed for periods up to 8000 h have been described.