Search

The performance and durability of low-temperature fuel cells seriously depend on catalyst support materials. Catalysts supported on high surface area carbons are widely used in low temperature fuel cells. However, the corrosion of carbonaceous catalyst-support materials such as carbon black has been recognized as one of the causes of performance degradation of low-temperature fuel cells, in particular under repeated start-stop cycles or high-potential conditions. To improve the stability of the carbon support, materials with a higher graphitic character such as carbon nanotubes and carbon nanofibers have been tested in fuel cell conditions. These nanostructured carbons show a several-fold lower intrinsic corrosion rate, however, do not prevent carbon oxidation, but rather simply decrease the rate. Due their high stability in fuel cell environment, ceramic materials (oxides and carbides) have been investigated as carbon-substitute supports for fuel cell catalysts. Moreover, the higher specific electrocatalytic activity of some ceramic supported metals than unsupported and carbon supported ones, suggests the possibility of a synergistic effect by supporting metal catalyst on ceramic supports. This paper presents an overview of ceramic materials tested as a support for fuel cell catalysts, with particular attention addressed to the electrochemical activity and stability of the supported catalysts.

The formation, structure and transport properties of LiCoO 2 are described. LiCoO 2 exhibits two crystal structures, depending on both the preparation method and synthesis temperature. High temperature lithium cobalt oxide (HT-LiCoO 2 ) has a hexagonal layered structure, while the low temperature oxide (LT-LiCoO 2 ) has a cubic spinel-related structure. The dependence of the morphological characteristics (grain size, size distribution, crystallinity) of LiCoO 2 on synthesis method as well as their effect on the electrochemical properties are extensively reviewed. As the electrochemical properties and the electrical conductivity strongly depend on the structure of the oxide, primary attention is given to lithium cobalt oxide with defect structure and lithium and oxygen nonstoichiometry.

Gravimetric, dilatometric, diffractometric and microscopic measurements have been used to study the role of thermal treatment in the preparation of lithiated nickel oxide cathodes for molten carbonate fuel cells. Clear evidence has been obtained that both the nominal composition of the reactive system and the thermal treatment during solid solution formation appreciably affect the cathodes microstructure. Starting from a careful analysis of the X-ray data, a reaction model is proposed which quite satisfactorily explains the microstructural aspects of the reaction. It is shown that there are three different processes at the origin of the cathodes microstructure and that the effect of the thermal treatment is to affect their relative weights.

Scanning electron microscopy and X-ray powder diffractometry have been used to characterize solid solutions of the type LixNi1−xO. Structural and profile parameters were refined from X-ray data to study both the composition and the homegeneity of the mixed oxides. It is shown that non-homogeneity is responsible for the appearance of broad and asymmetrical diffraction peaks and that better agreement between calculated and observed patterns is obtained if non-homogeneous mixed oxides are treated as if they consisted of solid solutions of different composition. The morphology, the mean composition and the homogeneity of the solid solutions as well change (in a non-monotonous way) as a function of the time of the isothermal treatment (T = 800°C) after which the samples were obtained.

Li x Ni 1- x O solid solutions are both of scientific and technological interest as they have been proposed as cathodic material in MCFC. In this work TG, XRD and SEM measurements have been used to study the thermal stability of several solid solutions (of different initial composition) obtained starting from the reactive system Ni/Li 2 CO 3 . Composition changes, due to both lithium and oxygen loss, take place as a consequence of samples annealing. In some cases a sensible disagreement between the compositions deduced by TG and XRD data is showing, which is ascribed to the presence, in the final solid solutions, of lattice defects others than substitutional ones (namely cation vacancies). The TG and XRD measurements have been then worked out to obtain both the correct composition and vacancies concentration of the solid solutions. It is shown that solid solutions with similar lithium content but different vacancies concentration can be obtained depending on the preparation conditions.

Clear evidence was obtained by dilatation and density measurements that the amount of lithium carbonate in the starting mixture sensibly affects the sintering behaviour of mixed lithium-nickel oxide cathodes. Porosimetric, diffractometric and microscopic measurements were then performed to understand and put on a quantitative basis such an influence. It is shown that, when the sintering temperature is low enough to prevent lithium oxide loss from the mixed oxide, cathodes total porosity increases with increasing lithium content up to a limiting composition beyond which a porosity decrease takes place. Lithium oxide loss, on the other hand, leads to a total porosity decrease which is related both to the initial lithium content and to the amount of the lithium oxide lost. Such a behaviour can be explained on the basis of the different mechanism by which the mixed oxide is obtained depending on the relative amounts of nickel and lithium carbonate in the starting mixture. Moreover it is shown that the observed total porosity changes are mainly a consequence of microporosity changes.