S.P.S. Badwal

Zirconia-based solid electrolytes: microstructure, stability and ionic conductivity

The oxygen-ion conducting properties of cubic, partially stabilized and tetragonal zirconia materials have been described. The ionic conductivity depends on the dopant type (size, valence), dopant concentration and phase assemblage. A brief account of the phase assemblage expected in commonly used binary and some ternary zirconia (with CaO, MgO, Y2O3, Sc2O3 and rare-earth oxides) systems is given. A large number of zirconia-based solid electrolytes show time-dependent conductivity behavior above about 700°C either as a result of the materials being in the two-phase field or due to ordering process. The effect of powder and ceramic fabrication and post-sinter annealing on the bulk phase assemblage and grain boundary segregation and their impact on the lattice and grain boundary resistivities has been considered.

Interaction between chromia forming alloy interconnects and air electrode of solid oxide fuel cells

The performance of solid oxide fuel cells, especially on the air side, degrades very rapidly when chromia forming alloys are used as interconnects in contact with electrodes. The effect of chromia forming alloys on the performance of doped LaMnO3 air electrode has been studied at the current fuel cell operating temperatures in the vicinity of 900–1000 °C. Interactions between a chromium-rich alloy and doped LaMnO3 were studied during testing of fuel cell stacks as well as in specially designed electrochemical experiments. On completion of these experiments interconnect/electrode and electrode/ electrolyte interfaces as well as interconnect, electrode and electrolyte materials were examined with scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and optical microscopy. A mechanism for cell performance degradation based on these observations has been proposed.

Solid oxide electrolyte fuel cell review

Solid oxide fuel cells are a promising technology for electric power generation in the 21st century. The principles of SOFC operation, stack designs, materials, status of development and future challenges are discussed.

Stability of solid oxide fuel cell components

Abstract The solid oxide fuel cell technology offers substantial potential for clean and efficient power generation. The high operating temperature (700–1000°C) is beneficial for co-generation of both electricity and high-grade heat at user sites, thus, increasing total system efficiency to above 85%. However, the high operating temperature imposes severe restrictions on materials, which can otherwise be effectively used for construction of the complete device. Degradation of fuel cell performance occurs over a period of time and is related to the deterioration of material properties and interfacial reactions between various fuel cell components. In this paper, a short overview on the stability of various fuel cell components in real operating environments is given.

An electrode kinetics study of H2 oxidation on Ni/Y2O3–ZrO2 cermet electrode of the solid oxide fuel cell

Hydrogen oxidation on Ni/3 mol% Y2O3–ZrO2 (Ni/Y-TZP) cermet electrodes has been studied in moist H2 environments. The impedance behaviour is characterised by two clearly separated arcs in the frequency domain, indicating that the overall electrode reaction is controlled by at least two rate limiting processes in series. The electrode process associated with the low frequency arc is mainly dependent on the H2 concentration. Moreover, the electrode conductivity of the low frequency arc (σL) is independent of temperature and the polarisation potential. Based on these observations, the electrode process associated with the low frequency arc has been attributed to hydrogen dissociative adsorption/diffusion on the surface of Ni particles. The electrode process of the high frequency arc is also affected by the H2 concentration and by oxygen partial pressure (PO2), however in contrast to the low frequency arc, the electrode conductivity associated with the high frequency electrode process (σH) showed a strong temperature dependence with an activation energy of ∼162 kJ mol−1 and it increases with increasing polarisation potential. This arc has been attributed to hydrogen transfer from the Ni electrode surface to the Y-TZP electrolyte surface, followed by a charge transfer process on the electrolyte surface near the interface region. The results of this study clearly demonstrate that the Y-TZP in the Ni cermet electrodes has little effect on the reaction mechanism, but plays an important role in modifying the electrode microstructure and kinetics of the fuel oxidation reaction.

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.

Hydrogen Oxidation at the Nickel and Platinum Electrodes on Yttria‐Tetragonal Zirconia Electrolyte

H{sub 2} oxidation has been studied for Pt and Ni electrodes for different H{sub 2}/H{sub 2}O ratios at 1,000 C in solid oxide fuel cells using yttria-tetragonal zirconia electrolyte by galvanostatic current interruption and electrochemical impedance spectroscopy. The results clearly indicate that the mechanism and kinetics of the H{sub 2} oxidation reaction are strongly dependent on the catalytic activities of electrode materials, electronic conductivity of the electrolyte surface, and the water content in H{sub 2} gas. The effect of water vapor in H{sub 2} gas on the reaction kinetics is very much dependent on the electrode materials and is related to the partial pressure of oxygen. A reaction mechanism with two rate-limiting steps has been proposed and discussed.

Grain boundary resistivity in zirconia-based materials: effect of sintering temperatures and impurities

Abstract The role of grain boundary phases and their influence on grain boundary resistivity in ZrO 2 -based electrolyte systems have been investigated by impedance spectroscopy and microstructural analysis. The effect of sintering temperature on the grain boundary and intragrain resistivity of seven ZrO 2 -Y 2 O 3 systems varying in impurity and Y 2 O 3 contents has been described. For each system six to eight different sintering temperatures between 1200 and 1700 °C were used. Changes in the intragrain resistivity with increasing sintering temperature were small and related mainly to densification of the ceramic. The grain boundary resistivity exhibited a complex behaviour which was different for different ceramics and varied with the impurity content. The grain boundary resistivity versus sintering temperature plots showed inflection or peaks. This type of behaviour has been attributed to the dynamic nature of the grain boundary phase (composition, location and wetting properties all of which change with the sintering temperature and atmosphere, cooling rates from the sintering temperature and subsequent heat treatments) and the amount and type of impurities in the starting powders. The activation energy for oxygen-ion conduction across grain boundaries was independent of the impurity content or nature of the glassy phase. It has been shown that the oxygen-ion conduction across grain boundaries takes place through direct grain to grain contact rather than through the intermediate grain boundary glassy phase which has an extremely low oxygen-ion conductivity.

Yttria-zirconia: Effect of microstructure on conductivity

Complex impedance measurements and detailed analysis of the grain-boundary microstructure have been made on fully stabilized yttria-zirconia sintered bodies as a function of grain size. The prereacted yttria-zirconia powder used in this study was obtained from a commercial source. The powder has very high reactivity and starts sintering around 1200° C. The densification process is complete around 1350° C but the grain growth continues almost linearly with sintering temperature. The grain size variation obtained was between 1 and 30 μm. The grain-boundary resistivity when plotted against grain size showed an inflection in the vicinity of 1500° C sintering temperature. These results have been explained in terms of the grain-boundary microstructure changing with the sintering temperature. The thickness of the grain-boundary layer determined from impedance data and transmission electron micrographs are in reasonably good agreement. The activation energy for the grain-boundary resistivity was only slightly higher than that for the lattice resistivity.

Electrical conductivity of single crystal and polycrystalline yttria-stabilized zirconia

The conductivity of several single crystal and polycrystalline Y2O3-ZrO2 samples has been studied by complex impedance and four-probe direct current techniques. For single crystals only one arc, due to lattice conductivity, was observed in the complex impedance representation. Polycrystalline materials showed a second arc, due to grain boundary resistance, the extent of which decreased as the impurity concentration was reduced and as the electrolyte microstructure improved. The activation energies for the volume and total conductivity of the purest polycrystalline samples were similar and agreed with those for the single crystals. These values, however, decreased by 20 to 25 kJ mol−1 on going from low (<550° C) to high (>850° C) temperatures. The change in the activation energy with temperature is thought to be due to a gradual transition between an association region, where vacancies are bound to dopant cations, and a dissociation region where vacancies are free and mobile.