A ceramic fuel cell in an all solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. Current ceramic fuel cells use an oxygen-ion conductor or a proton conductor as the electrolyte and operate at high temperatures (>600°C). Ceramic fuel cells, commonly referred to as solid-oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. This paper reviews the science and technology of ceramic fuel cells and discusses the critical issues posed by the development of this type of fuel cell. The emphasis is given to the discussion of component materials (especially, ZrO2 electrolyte, nickel/ZrO2 cermet anode, LaMnO3 cathode, and LaCrO3 interconnect), gas reactions at the electrodes, stack designs, and processing techniques used in the fabrication of required ceramic structures.

Ceramic Fuel Cells

Transparent conductors as solar energy materials: A panoramic review

Despite being first demonstrated over 160 years ago, and offering significant environmental benefits and high electrical efficiency, it is only in the last two decades that fuel cells have offered a realistic prospect of being commercially viable. The solid oxide fuel cell (SOFC) offers great promise and is presently the subject of intense research activity. Unlike other fuel cells the SOFC is a solid-state device which operates at elevated temperatures. This review discusses the particular issues facing the development of a high temperature solid-state fuel cell and the inorganic materials currently used and under investigation for such cells, together with the problems associated with operating SOFCs on practical hydrocarbon fuels.

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Solid oxide fuel cells

The state of the art development of interconnect materials for solid oxide fuel cells (SOFCs) is reviewed. The specific interconnect requirements from a materials design point of view are described in detail. Doped-LaCrO 3 ceramics considered as interconnects for electrolyte-supported planar SOFC suffer from their extreme sensitivity to oxygen partial pressure and high manufacturing cost. Advent of anode-supported planar SOFC allows lower operating temperatures and great efforts have been directed towards the development of metallic interconnects. Chromia formed on the surface of chromium-based alloys are intrinsically too volatile to be viable as interconnects for SOFCs operating above 800 °C. Some intermediate phases occurring between iron-based interconnect and cathode significantly increase the contact resistance. Metallic materials that are capable of developing stable oxide scales with acceptable growth rate and reasonably high electrical conductivity over the expected SOFC lifetime need to be developed.

Development of interconnect materials for solid oxide fuel cells

We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

High-temperature measurements have been performed on the electronically conducting oxide system ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cr}{\mathrm{O}}_{3}$ ($0l~xl~0.4$). The electrical conductivity displayed thermally activated temperature dependence between 300 and 1900 K with activation energies that varied between 0.11 and 0.19 eV. Seebeck coefficients measured over a similar temperature range showed little temperature dependence, an indication that the carrier mobility, rather than the carrier concentration, is thermally activated. The magnetic susceptibility was Curie-Weiss-like between 600 and 1100 K with an effective moment that decreased with an increase in strontium concentration. These results are shown to be consistent with transport by the adiabatic hopping of small polarons.

Localized level hopping transport in La(Sr)Cr O 3

Low-temperature magnetic susceptibilities yield an effective magnetic moment per W atom of (3.24\ifmmode\pm\else\textpm\fi{}0.10)${\ensuremath{\mu}}_{B}$ for both ${\mathrm{W}}_{0.05}$${\mathrm{V}}_{0.95}$${\mathrm{O}}_{2}$ and ${\mathrm{W}}_{0.08}$${\mathrm{V}}_{0.92}$${\mathrm{O}}_{2}$. This value suggests a transfer of two d-shell electrons from the W to neighboring V ions to form ${\mathrm{V}}^{3+}$-${\mathrm{W}}^{6+}$ and ${\mathrm{V}}^{3+}$-${\mathrm{V}}^{4+}$ pairs along the a axis of the monoclinic low-temperature phase. Studies of the extended x-ray-absorption fine-structure pattern for various compositions of different W-cation fraction x, and of the area of the W ${L}_{\mathrm{III}}$-edge resonance as well, confirm that the valence of W is 6+. Also, examination of the position of the V K edge versus composition for these alloys and other compounds indicates that the concentration of ${\mathrm{V}}^{3+}$ ions increases with W-cation fraction x. Thus, upon alloying ${\mathrm{VO}}_{2}$ with W, the reduction in the temperature of the metal-semiconductor transition by W appears to be due to charge transfer. From x-ray small-angle scattering it was found that there are no W-atom clusters present. The high-field magnetization at low temperature suggests that alloying causes this oxide to become a spin glass.

Local atomic and electronic arrangements in WxV1-xO2.

Diffuse neutron scattering measurements have been made on ferromagnetic iron-chromium alloys containing 15-, 30-, 50-, and 73-at.% Cr with the long-wavelength neutron spectrometer at Harwell. The results have been analyzed, within the framework of the Marshall model, to determine both the average moments at the iron and chromium sites and the disturbances in the individual moments arising from fluctuations in the near-neighbor environent. The average moment per iron atom rises to a small maximum around 15-at.% Cr and then decreases as the chromium concentration increases. The average chromium moment is negative and becomes smaller in magnitude as the chromium concentration increases. The sum of the disturbances at the iron and chromium sites changes sign as a function of concentration. The range of the disturbance appears to be smaller in chromium-rich than in iron-rich alloys.

Ferromagnetism in iron-chromium alloys. II. Neutron scattering studies

Bulk magnetization measurements were made between 5 and 300 K in fields up to 14 kOe on a series of iron-chromium alloys containing from 2- to 70-at.% Cr. The temperature dependence of the magnetization was analyzed to yield spin-wave stiffness coefficients that are systematically different from values obtained directly by inelastic-neutron-scattering measurements. The concentration dependence of the mean moment is approximately linear and is close to the Slater-Pauling curve. However, subtle deviations from linearity reflect the concentration dependence of the individual iron and chromium moments, which is presented in part II. The concentration dependence of the moments can be approximately reproduced by existing coherent-potential-approximation calculations.

A. T. Aldred

Papers

Localized level hopping transport in La(Sr)Cr O 3

Local atomic and electronic arrangements in WxV1-xO2.

Ferromagnetism in iron-chromium alloys. II. Neutron scattering studies

Ferromagnetism in iron-chromium alloys. I. Bulk magnetization measurements

Temperature dependence of the magnetization of nickel