Solid oxide fuel cells (SOFCs) have the promise to improve energy efficiency and to provide society with a clean energy producing technology. The high temperature of operation (500−1000 °C) enables the solid oxide fuel cell to operate with existing fossil fuels and to be efficiently coupled with turbines to give very high efficiency conversion of fuels to electricity. Solid oxide fuel cells are complex electrochemical devices that contain three basic components, a porous anode, an electrolyte membrane, and a porous cathode. In this short review, a survey of the types and properties of materials that have been considered for each of these components is presented with an emphasis on the requirements for operation at intermediate temperature (500−800 °C). Some directions for future research are discussed.

Materials for Solid Oxide Fuel Cells

High temperature solid oxide fuel cells (SOFCs), typified by developers such as Siemens Westinghouse and Rolls-Royce, operate in the temperature region of 850–1000 °C. For such systems, very high efficiencies can be achieved from integration with gas turbines for large-scale stationary applications. However, high temperature operation means that the components of the stack need to be predominantly ceramic and high temperature metal alloys are needed for many balance-of-plant components. For smaller scale applications, where integration with a heat engine is not appropriate, there is a trend to move to lower temperatures of operation, into the so-called intermediate temperature (IT) range of 500–750 °C. This expands the choice of materials and stack geometries that can be used, offering reduced system cost and, in principle, reducing the corrosion rate of stack and system components.This review introduces the IT-SOFC and explains the advantages of operation in this temperature regime. The main advances made in materials chemistry that have made IT operation possible are described and some of the engineering issues and the new opportunities that reduced temperature operation affords are discussed.This tutorial review examines the advances being made in materials and engineering that are allowing solid oxide fuel cells to operate at lower temperature. The challenges and advantages of operating in the so-called ‘intermediate temperature’ range of 500–750 °C are discussed and the opportunities for applications not traditionally associated with solid oxide fuel cells are highlighted. This article serves as an introduction for scientists and engineers interested in intermediate temperature solid oxide fuel cells and the challenges and opportunities of reduced temperature operation.

Intermediate temperature solid oxide fuel cells

Progress in material selection for solid oxide fuel cell technology: A review

Transport properties of solid oxide electrolyte ceramics: a brief review

Key issues relating to intrinsic defects, dopant incorporation, and lithium ion migration in the LiFePO4 electrode material have been investigated using well-established atomistic modeling techniques. Our simulation model shows good reproduction of the observed olivine-type structure of LiFePO4. The most favorable intrinsic defect is the Li−Fe “anti-site” pair in which a Li ion (on the M1 site) and an Fe ion (on the M2 site) are interchanged. This type of anti-site defect or “intersite exchange” has been observed in olivine silicates. The lowest Li migration energy is found for the pathway along the [010] channel, with a nonlinear, curved trajectory between adjacent Li sites. Trends in dopant substitution energetics of a range of cations with charges varying from +2 to +5 are also examined. Low favorable energies are found only for divalent dopants on the Fe site (such as Mn), which is in accord with experimental work. Our results suggest that, on energetic grounds, LiFePO4 is not tolerant to aliovalent d...

Atomic-scale investigation of defects, dopants, and lithium transport in the LiFePO4 olivine-type battery material

Computer modelling techniques have been used to examine the mechanistic features of oxygen ion transport in the La8Sr2Si6O26 and La9.33Si6O26 apatite-oxides at the atomic level. The potential model reproduces the observed complex structures of both phases, which are comprised of [SiO4] tetrahedral units and La/O channels. Defect simulations have examined the lowest energy interstitial and vacancy sites. The results suggest that oxygen ion migration in La8Sr2Si6O26 is via a vacancy mechanism with a direct linear path between O5 sites. Interstitial oxygen migration is predicted for La9.33Si6O26via a non-linear (sinusoidal-like) pathway through the La3/O5 channel. The simulations demonstrate the importance of local relaxation of [SiO4] tetrahedra to assist in the facile conduction of oxygen interstitial ions. In general, the modelling study confirms that the high ionic conductivity in silicate-based apatites (with oxygen excess or cation vacancies) is mediated by oxygen interstitial migration.

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Defect chemistry and oxygen ion migration in the apatite-type materials La9.33Si6O26 and La8Sr2Si6O26

Research into materials displaying oxide ion conductivity is attracting considerable attention due to their potential technological applications in devices such as Solid Oxide Fuel Cells. In this paper, recent work on apatite-type oxide ion conductors is reviewed, showing that a wide range of cation substitutions are possible, due to the flexibility of the apatite structure in accommodating a range of ion sizes. The conductivity studies on these doped samples show that to achieve high oxide ion conduction, non-stoichiometry in terms of cation vacancies and/or oxygen excess is required, with the latter resulting in the highest conductivities. In contrast to most common oxide ion conductors, e.g. perovskite and fluorite in which oxide ion conduction proceeds via oxygen vacancies, the research on these apatite systems suggests that the conductivity involves interstitial oxide ions. With further optimization of these materials, particularly in terms of the Ge-containing systems, significant improvements in conductivity are likely, leading to the very real possibility of the application of apatite-type electrolytes in fuel cell and other applications. © 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 4: 373–384; 2004: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20028

Development of apatite-type oxide ion conductors

An apatite for fast oxide ion conduction

Apatite-type silicates have been attracting considerable interest as a new class of oxide ion conductor, whose conduction is mediated by interstitial oxide ions. We report here the first 29Si solid state NMR studies of these materials with a systematic investigation of thirteen compositions. Our results indicate a correlation between the silicon environment and the observed conductivity. Specifically, samples which show poor conductivity demonstrate a single NMR resonance, whereas fast ion conducting compositions show more complex NMR spectra. For the oxygen excess samples La9M(SiO4)6O2.5 (M = Ca, Sr, Ba) two peaks are observed at chemical shifts of ≈−77.5 and −80.5 ppm, with the second peak correlated with a silicate group adjacent to an interstitial oxygen site. On Ti doping to give La9M(SiO4)6−x(TiO4)xO2.5 (x = 1,2) the second peak disappears, which is consistent with the “trapping” of interstitial oxygens by Ti and the consequent lowering in oxide ion conductivity. The samples La9.33(SiO4)6O2 and La9.67(SiO4)6O2.5 show a further third weak peak at a chemical shift (≈−85.0 ppm) consistent with the presence of some [Si2O7]6− units in these samples, due to condensation of two [SiO4]4− units. The effect of such condensation of [SiO4]4− units will be the creation of additional interstitial oxide ion defects, i.e. 2 [SiO4]4−
				→ [Si2O7]6−
				+ Oint2−. Overall, the results further highlight the importance of the [SiO4]4− substructure in these materials, and additionally suggest that 29Si NMR could potentially be used to screen apatite silicate materials for oxide ion conductivity

Solid state 29Si NMR studies of apatite-type oxide ion conductors

Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO3
				(A = Ca, Ba) and LaMO3
				(M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant–defect association at the atomic level. The energetics of dopant incorporation in AZrO3 show strong correlation with ion size. We predict Y3+ to be one of the most favourable dopants for BaZrO3 on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy–dopant pairs in BaZrO3 are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO3 is via a curved pathway around the edge of the ScO6 octahedron. Dopant–vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO3, but very low energies in LaGaO3. This points to greater trapping of the oxygen vacancies in doped LaScO3, perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data.

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J. R. Tolchard

Papers

Defect chemistry and oxygen ion migration in the apatite-type materials La9.33Si6O26 and La8Sr2Si6O26

Development of apatite-type oxide ion conductors

An apatite for fast oxide ion conduction

Solid state 29Si NMR studies of apatite-type oxide ion conductors

Doping and defect association in AZrO3(A = Ca, Ba) and LaMO3(M = Sc, Ga) perovskite-type ionic conductors