Análisis de instrumentos aplicativos en el software geogebra

In this chapter, we have successfully synthesized La0.6Sr0.4Ga0.3Fe0.7O3 with a simple, controllable and easily scalable wet chemistry route: preparation with amounts from 0.5 to 15g have been carried out and the obtained results demonstrated the high reproducibility of the method. We have investigated several parameters of the synthesis, setting the minimum amount of nitric acid at 4.5ml per gram of product for a complete combustion, and the minimum calcination temperature at 900°C for a completely pure phase. The calcination temperature limits the superficial area, that is useful in particular for catalytic purposes, to 9m2_{/g. Calcination temperature can} be lowered if minor impurities are tolerated or whether the material will undergo cycles of reduction/oxidation, in this case the perovskite auto-arranges towards the complete purity and the BET specific surface area is about twice. Investigations on the powders prior to calcination indicated that after the combustion the perovskitic phase have already been formed, but persists a massive organic fraction that requires a temperature treatment for its removal. The resistance of the material to the reduction was tested between 800 and 1000°C, and the obtained results outline an exceptional resistance to reduction compared to the other perovskites: operation of the material in hydrogen atmospheres at 800°C leads only to minor structural

modification. This, joint to the peculiar ability of this perovskite to recover the original structure after a simple oxidation treatment, makes it particularly suitable for applications even in very reducing atmospheres. No differences of stability have been observed as a function of the different preparation conditions.

3 Tuning functionality through

nano-composition:

MO

x

(M=Mn and Fe)+ LSGF as

electrodes in Solid Oxide Fuel

Cells

NANOCOMPOSITES: A FLEXIBLE MEAN FOR TUNING THE PROPERTIES OF A MATERIAL

THE DISCOVERY OF UNEXPECTED AND VERY COMPLEX INTERACTIONS ON THE SUBSTRATE

AN ACCURATE INVESTIGATION OF THE NEW PROPERTIES

EIS INVESTIGATIONS OF THE PREPARED MATERIALS AS SOFC CATHODES

3.1 Introduction

The accurate selection of the element constituting a perovskite can confer new properties to the material[176], modify existing ones [177] or stabilize desired phases[178]. Most of the currently employed perovskite-based materials result from an elaborate optimization process. In this work, a different modification technique has been studied: functionalization through nanocomposition. The properties and behaviour of the nanocomposites and of the pristine perovskite have been compared with the aim of developing and optimizing a performant cathode to be used in SOFCs. After a detailed investigation, the effect on material properties of the thermal treatments, carried out to obtain the final button cell, has been correlated with the electrochemical performance of the materials. Finally, the catalytic activity in a model reaction as methane oxidation was studied with the aim of developing symmetric SOFCs directly fed with fuels different than hydrogen, such as methane or biogas.

The perovskite La0.6Sr0.4Ga0.3Fe0.7O3 (LSGF) derives from simpler LaFeO3: the addition of Sr pushes Fe atoms to oxidation state 4+ and induces the formation of oxygen vacancies increasing electronic and ionic conductivity, whereas gallium

increases the stability towards reducing and chemically aggressive environments[150], [151]. LSGF is already known for its catalytic activity towards methane oxidation[179] (this is a common feature among ferrites[170], [180]) and it is studied as a material for dense membranes to be applied in oxygen purification, or in membrane reactors (for methane partial oxidation reactors, as an example)[139], [181]. Catalytic activity was related to the Fe-redox couple and to oxygen mobility. Application of LSGF as a SOFC cathode has already been proposed, especially coupled with LSGM as electrolyte[182], [183] to avoid the formation of insulating phases at the interfaces (with electrolyte and interconnect). Some problem related to thermal expansion during a phase transition at intermediate temperature could however arise[184]. Because of the interesting catalytic and conductivity properties, the potential uses of LSGF are wide, covering different fields from catalysis to energy production and storage.

Impregnation[185] is a process commonly used to enhance the performances of a material, and consists in depositing a new phase on an existing substrate from a drying solution: the solution evaporates, and the solid precursor deposits on the substrate. Often, the impregnated phase contains a noble metal, which most of the times is indeed the real active phase of the material, while the substrate gives mainly mechanical strength; a notable example of this is Pt impregnated on carbon in Polymer Electrolyte Fuel Cells[71] and in general automotive catalysts, on inert [186]– [188] but also on active [189] substrates. It is possible to distinguish between a wet and a dry impregnation[190], whether the used precursors solution is in excess (wet) or has a limited volume corresponding to the porosity of the substrate (dry).In this case, wet impregnation has been chosen because it allows to easily control the amount of deposited phase on powder substrates.

In this chapter, simple iron and manganese oxides are deposited, by wet impregnation, on LSGF, a support which is not inert but catalytic and electrocatalytically active, with the aim of improving the SOFC’s electrode performance. The choice of non-noble, cheap impregnated phases is driven by the need of avoiding or limiting the use of expensive and/or critical elements (CRM)[191].

Tuning functionality through nano-composition: MOx (M=Mn and Fe)+ LSGF as electrodes in Solid Oxide Fuel Cells

71 Beside economic reasons, Fe and Mn oxides have been chosen because of their

catalytic activity[192]–[198].

The impregnated LSGF powders (named LSGF + FeOx and LSGF + MnOx) has been studied by means of X-Ray Diffraction (XRD), Temperature Programmed Reduction (TPR), X-Ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X- Ray Analysis (EDX). The surface modification subsequent to the oxide deposition have been studied also by means of N2 adsorption isotherms and SEM. The nanocomposites have been tested in catalytic activity towards methane oxidation. In previous works LSGF has been already tested in oxygen excess condition[179]; during these tests, an excess of methane has been used, in order to assess materials selectivity also in conditions where partial oxidation would be favoured. The effect of impregnation on electrocatalytic properties has been studied using the nanocomposite materials as SOFC cathodes in symmetrical electrolyte (CGO – cerium 90% gadolinium 10% mixed oxide) supported cells. CGO has been chosen to easily compare results with other materials’ performances, considered the only limited literature available on LSGM. Since deposition of electrodes involves treatment at high temperatures potentially able to modify the LSGF composite, the powders (both LSGF and impregnated LSGF ones) have been treated under the same conditions and thoroughly examined to monitor closely any modification of the material consequent to each preparation step. In particular the XRD vs temperature investigation allowed to monitor the structural behaviour of the nanocomposites during the thermal treatment necessary for the SOFC preparation.