Galenda, Alessandro (2008) Advanced Perovskite Materilas For Intermediate Temperature Solid Oxide Fuel Cells. [Tesi di dottorato]
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The research program developed during the Ph.D. School is focussed on the study of advanced materials for applications in the intermediate temperature solid oxide fuel cells (IT-SOFCs).
Fuel cells (FCs) are often considered as the best solution to produce clean energy starting from various primary resources. FCs are employed for the direct production of electric power by electrochemical conversion of the potential energy of a fuel. Fuel cells work as a common galvanic cell: the fuel is oxidised at the anode and the combustive (usually air) is reduced at the cathode.
Among the various kind of fuel cells, solid oxide fuel cells are very interesting thanks to their singular properties, such as the high output powers (reaching megawatts) and the excellent efficiency (until about 70% with the co-generation). Another interesting characteristic is the ability to work with different type of fuels. Beyond hydrogen (whose usage involves the well know difficulties concerning production, transportation and storage), SOFCs can operate with alcohols (such as methanol and ethanol) or hydrocarbons. This can offer significant opportunity in renewable energy field taking into consideration fuels derived from bio-masses and urban or industrial rubbish.
A common SOFC usually works at very high temperature (800÷1100°C). Anyway, many studies have been carried out to develop new materials able to guarantee the best performances at lower temperatures (500÷700°C) and build the new generation of SOFCs: the so-called Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Nevertheless it is, necessary to develop new electrolyte materials characterized by high anionic conduction at lower temperatures and new electrodes with electronic, or better mixed ionic-electronic conductivity (MIEC) and a suitable activity toward fuel oxidation and combustive reduction.
In the present study, several perovskite based oxide materials have been considered. These particular compounds show a wide range of interesting chemical and physical properties. Moreover, these characteristics can be tuned employing different constituting elements and different kinds and amounts of dopant elements.
Taking into account the literature research outcomes, two kinds of perovskites have been studied: gallates and cuprates. The first ones are lanthanum gallate doped with strontium and iron and with strontium and copper (La0.8Sr0.2Ga0.8Fe0.2O3-?, named LSGF, La0.8Sr0.2Ga0.8Cu0.2O3-?, LSGC), while the second types derive from lanthanum cuprate (LaCu0.8Co0.2O3-?, LCC1, and La2Cu0.8Co0.2O4-?, LCC2).
The samples have been prepared by means of two wet-chemistry procedure (Pechini process and the Polyacrylamide Gel method) to avoid using the high temperature ceramic route and to study the influence of the preparation procedure.
The obtained catalysts were characterized by means of X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Diffuse Reflectance Infrared spectroscopy Fourier Transform (DRIFT) Spectroscopy.
In general, XRD revealed the presence, beside the desired one, of minor phases whose amount and typology is influenced by the composition and doping. In the case of LCC1, in contrast, a mixture of La2Cu0.8Co0.2O4-? and CuO, was obtained instead of LaCu0.8Co0.2O3-?. XPS investigation testifies the surface segregation of strontium as carbonate and of lanthanum as oxide and hydroxide. Copper is present as copper oxide both in LCC1 and LCC2. As a general consideration, the presence of carbonate species and hydroxyl groups is mainly a surface phenomenon.
Interesting information have been obtained from the catalytic tests. The reactivity of the materials has been investigated toward methanol and ethanol under several conditions: tests with pure alcohol vapours, under oxidising atmosphere (enriching the carrier gas with O2) and in steam reforming conditions, have been carried out at several temperatures between RT and 400°C. The experiments were performed employing a home made continuous flow reactor monitoring the exit stream by IR and QMS.
Significant differences have been observed between the samples obtained by means of the two different preparation procedures: the results, as a whole, indicate that the samples obtained by Gel procedure show a higher activity.
Both in alcohol oxidation (carried out with oxygen) and in alcohol steam reforming the higher activity of cuprate based materials is evident. LSGF and LSGC, in contrast, exhibit lower reactivity. It has also to be considered that a certain poisoning of the catalysts surfaces was observed as a consequence of the interaction with the reaction products (carbon dioxide, as an example). This is particularly true when the reaction is carried out with the only alcohols or under steam reforming conditions.
The cathodic activity was investigated by measuring the oxygen permeability throughout the materials pressed as a pellet. The permeation mechanism is specific for O2 and provides useful information concerning both redox and transport properties (for oxide anions) for the investigated material. Permeability measurements were carried out employing a home made reactor. This is expressly conceived, realized and optimized during the PhD term. A detailed study concerning the materials (ceramic macor) and fittings has been done (paying particular attention to the pasting of the samples on the ceramic support). The tests have been monitored by means of QMS and show particularly interesting results for the cuprates.
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