Infrared Spectroscopy Techniques in the Characterization of SOFC Functional Ceramics

structure is almost completely obtained after the combustion reaction, as observed by X-ray diffraction. The band at 1620 cm -1 corresponds to the stretching vibration frequency of coordinated H 2 O ( δ HOH ). The bands related to absorbed water (3400 cm -1 ) were also present in both samples. Before calcination at 900 °C for 4 h, the LSCF powder exhibited strong absorption bands at 1381 cm -1 and 1454 cm -1 , which indicate the characteristic vibration of NO 3-1 and the stretching of the CO 32- , respectively. Based on these results, it is possible to conclude that during LSCF synthesis, some nitrates can react with carbon dioxide to form carbonaceous phases which decompose after further calcination at 900 °C for 4 h. Therefore, FTIR results are consistent with XRD analyses. The present book is a definitive review in the field of Infrared (IR) and Near Infrared (NIR) Spectroscopies, which are powerful, non invasive imaging techniques. This book brings together multidisciplinary chapters written by leading authorities in the area. The book provides a thorough overview of progress in the field of applications of IR and NIR spectroscopy in Materials Science, Engineering and Technology. Through a presentation of diverse applications, this book aims at bridging various disciplines and provides a platform for collaborations among scientists.


Introduction
Infrared spectroscopy is certainly one of the most important analytical techniques available nowadays for scientists. One of the greatest advantages of infrared spectroscopy is that virtually any sample in any physical state can be analyzed. The technique is based on the vibrations of atoms of a molecule. An infrared spectrum is obtained by passing infrared radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy. The energy at which any peak in an absorption spectrum appears corresponds to the frequency of a vibration of a part of a sample molecule (Stuart, 2004).
Fourier transform infrared spectroscopy (FTIR) has been widely used for in situ analysis of adsorbed species and surface reactions. Infrared spectroscopy techniques have being used for the characterization of solid oxide fuel cells (SOFCs). FTIR is utilized to identify the structure of the SOFC electrode and electrolyte surface (Resini et al., 2009;Guo et al., 2010). Liu and co-workers (Liu et al., 2002) were pioneers in the in situ surface characterization by FTIR under SOFC operating conditions. The development of high-performance electrode and electrolyte materials for SOFC is an important step towards reducing the fuel cell operation temperature to the low and intermediate range (500 -700 ºC). As the operating temperature is reduced, many cell parts, such as the auxiliary components can be easily and cost-efficiently produced. To meet long operational lifetime, material compatibility and thermomechanical resistance would be less critical as the range of possibilities for lower temperature increases. To that end, recent research at UFRN, Natal, Brazil has successfully focused on novel synthesis processes based on microwave-assisted combustion and modified polymeric precursor methods in order to synthesize high performance cobaltite-based composite cathodes for low-intermediarytemperature SOFCs.
This book chapter reviews how powerful infrared spectroscopy techniques play a fundamental role in the novel synthesis approaches developed during the last 5 years (2007 -2011) by this research group. Synthesized high performance cathode powders had their features investigated using different materials characterization techniques, such as Fourier Transform Infrared Spectroscopy (FTIR), FAR and MID-Infrared spectroscopy.

Solid oxide fuel cells and its components
Fuel cells are highly efficient power generation devices which convert chemical energy of gaseous fuels (hydrogen, fossil fuels and ethanol among others) directly into electric power in a silent and environmentally friendly way. These electrochemical devices are promising alternatives to traditional mobile and stationary power sources, such as internal combustion engines and coal burning power plants. Among the various types of fuel cells, solid oxide fuel cells (SOFCs) have advantages linked to high energy conversion efficiency and excellent fuel flexibility because of their high operating temperature compared to other types of fuel cells. Fig. 1 depicts a typical SOFC single cell produced by our research group. It consists of three basic components: two porous electrodes (anode and cathode) and a solid electrolyte. Each one of these components must fulfil specific performance requirements such as microstructural stability during preparation and operation; chemical and physical compatibility, i.e., similar thermal expansion coefficients; adequate porosity and catalytic activity to achieve the highest performance (Minh & Takahashi, 1995;Singhal, 2000).
The SOFC electrolyte material, typically yttria stabilized zirconia (YSZ) or rare earth doped ceria, must be an electronic insulating but ion-conducting ceramic that allows only protons or oxygen ions to pass through. Furthermore, the electrolyte material must be dense to separate the air and fuel, chemically and structurally stable over a wide range of partial pressures of oxygen and temperatures. The cathode (air electrode) material, typically lanthanum manganites and cobaltites, has to be electrocatalyst for oxygen reduction into oxide ions. When an oxygen ionic conducting oxide is adopted as electrolyte, these ions diffuse through the material to the anode (fuel electrode), driven by the differences in oxygen chemical potential between fuel and air constituents of the cell, where they electrochemically oxidize fuels such as hydrogen, methane, and hydrocarbons. The released electrons flow through an external circuit to the cathode to complete the circuit. The complete mechanism of electric power production in a SOFC is also shown in Fig. 1.
SOFCs have attracted significant attention in the last 20-30 years due to their high efficiency, fuel flexibility and environmental advantages (Minh, 2004;Molenda et al., 2007;McIntosh & Gorte, 2004). However, typical SOFCs operate at 1000 ºC. Elevated operating temperatures introduce a series of difficulties such as sintering of the electrodes and high reactivity between cell components. For these reasons, there is a considerable research interest in reducing the operating temperature of these devices down to the range between 500 and 800 °C, which characterizes intermediate temperature solid oxide fuel cells (IT-SOFCs) or even lower, which would imply the use of inexpensive metallic materials, rapid start-up and shut-down, minimization of thermal degradation and reactions between cell components, and longer operational lifetime. Furthermore, as the operation temperature is reduced, system reliability increases, increasing the possibility of using SOFCs for a wide variety of applications, including residential and automotive devices. On the other hand, reduced operating temperatures reduces the overall electrochemical performance due to increased www.intechopen.com Fig. 1. A typical SOFC single cell with its basic components (anode, cathode and electrolyte) and the mechanism of energy production. ohmic losses and electrode polarization losses associated with thermally activated processes of both ionic transport and electrode reactions. In special, the electrochemical activity of the cathode dramatically deteriorates with decreasing temperature for typical strontium-doped lanthanum manganite (LSM) based electrodes (Singhal 2000;Steele, 2000;Steele & Heinzel, 2001).
Thus, to achieve acceptable performance of IT-SOFCs, reducing the electrolyte resistance and electrode polarization losses are two key points. Losses attributed to electrolytes can be minimized by decreasing the thickness of electrolyte layers or substituting traditional YSZ by other electrolytes, such as doped ceria and apatite-like materials. The overall electrochemical limitation would then be ruled by electrode polarization losses. Due to the higher activation energy and lower reaction kinetics for oxygen reduction at the cathode compared with those of the fuel oxidation at the anode, the polarization loss from the air electrode (cathode) that limits the overall cell performance (Ivers-Tiffee et al., 2001;Tsai & Barnett, 1997). Therefore, the development of new cathode materials with high electrocatalytic activity for oxygen reduction becomes a critical issue for the development of IT-SOFCs.

Development of cathode materials
The development of high performance cathodes is based on reducing its thickness from hundreds to few micrometers associated with the addition of an electrolyte material in the formation of composite cathodes. These are interesting approaches that can improve electrode performance for the reduction of oxygen. The latter allows the extension of the triple phase boundaries (TPB) from the electrolyte/cathode interface deep into the bulk of the electrode, permitting electrochemical reactions to take place within the electrode. The www.intechopen.com preparation of composite cathodes by the combination of strontium-doped lanthanum manganite (LSM) with yttria stabilized zirconia (YSZ) or even samarium oxide-doped ceria (SDC) has been intensively studied in the last years. Whereas rare earth doped ceria, both samarium (SDC) and gadolinium doped ceria (CGO), have higher ionic conductivity than YSZ. Thus, composite cathodes such as LSM-SDC or LSM-CGO have better electrochemical performance than LSM-YSZ, and are particularly indicated for IT-SOFCs (Zhang et al., 2007;Chen et al., 2008;Wang et al., 2007).

Lanthanum manganite and cobaltite based cathodes
Lanthanum manganites (LaMnO 3 ), particularly strontium-doped lanthanum manganites (La 1−x Sr x MnO 3 -LSM), are widely used as cathode materials in SOFCs operating at high temperatures (800 -1000 °C), which is the temperature range where yttria-stabilized z i r c o n i a ( Y S Z ) i s c o m m o n l y u s e d a s e l e c t r o l y t e . D o p i n g S r i n t o L a M n O 3 considerably increases electrical conductivity because of the increased number of holes. LSM cathodes present high stability, electro-catalytic activity for O 2 reduction at high temperatures and thermal expansion coefficient reasonably similar to YSZ electrolyte (Escobedo et al., 2008;Jiang, 2003;Minh, 1993). Even thought LSM cathodes have been successfully used as functional materials in high temperature SOFCs, they have been replaced for more efficient lanthanum strontium cobaltite ferrites (LSCF). LSMs are poor ionic conductors and the electrochemical reactions are limited to the region close to triple phase boundaries (TPB). On the other hand, LSCF is a mixed electronic/ionic conductor (MEIC) with appreciable ionic conductivity. The exchange of oxygen ions occurs at the electrode surface with the diffusion of oxygen through the mixed conductor Shao et al., 2009).
Due to low ionic conductivity and high activation energy to oxygen dissociation at low temperatures, cathodes made of LSM and LSCF are normally combined with the electrolyte material forming a composite cathode. The composite cathodes containing ceria are more attractive to low and intermediate temperatures (500 -700 °C) than the ones containing YSZ, which has lower ionic conductivity in such temperatures (Yang et al., 2007;Xu et al., 2006;Xu et al., 2005;Chen et al., 2007;Zhang et al., 2008;Lin et al., 2008).
Among the extensive number of chemical synthesis routes available for the preparation of LSM and LSCF powders, the polymeric precursor method (Pechini method), solid state reaction, spray pyrolysis and combustion synthesis have been successful used (Conceição et al., 2009;Cela et al., 2009;Grossin & Noudem, 2004;Guo et al., 2006;Macedo et al., 2009;Conceição et al., 2011). In spite of the proved efficiency of the polymeric precursor method to produce monophasic nanometric powders, alternative synthesis methods mainly using microwave technology have being implemented in an attempt to minimize energy consumption and total powder production time . An alternative method for preparing nanoparticles with particle sizes of order of nanometers using low annealing temperatures has been developed using commercial gelatin as a polymerizing agent. This method has been named by the authors as soft chemical route (Medeiros et al., 2004;Maia et al., 2006).
Regarding the development of LSM based composite cathodes, several works have reported the preparation of LSM-SDC cathodes from the powders obtained by different synthesis methods (Ye et al., 2007;Chen et al., 2007;Xu et al., 2009). However, no reports on the www.intechopen.com preparation of LSM-SDC films from powders synthesized using commercial gelatin have been encountered. This chapter presents recent results in obtaining cathodes based on LSM and SDC powders obtained by the modified Pechini method using commercial gelatin as polymerizing agent. LSM-SDC films were prepared using different amounts of ethyl cellulose as pore former. Among the many methods which can be used to produce SOFC cathodes, slurry spin coating was selected due to its fast processing time and high uniformity over the surface. In this technique, a high spin rotation quickly lays the slurry on the substrate which dries in short times, hindering particle agglomeration, which also contributes to the uniform distribution of particles along the green film. The characteristics of not only LSM, SDC and LSCF powders, but also LSM-SDC spin-coated and LSCF-SDC screen-printed films have been investigated by different techniques.

Infrared spectroscopy techniques applied in the SOFC research
The fundamental studies of the catalytic reactions mechanisms that occur near the threephase boundary in the anode of a solid oxide fuel cell still deserving attention and investigation. Many in situ spectroscopies such as Raman and infrared spectroscopy are routinely used in catalysis research to characterize surface intermediates and reaction mechanisms. It is very difficult to apply in situ spectroscopy techniques to an operating SOFC anode (Atkinson et al., 2004). Recently research groups (Liu et al., 2002;Guo et al., 2010) presented their methodologies on the possible ways to apply the infrared emission spectroscopy to characterize working SOFC anodes.
Guo and co-workers (Guo et al., 2010) worked recently developing a novel experimental technique to measure in situ surface deformation and temperature on the anode surface of a SOFC button cell, along with cell electrochemical performance under operating conditions. An adaptation of a SOFC button cell test apparatus was integrated with a Sagnac interferometric optical setup and IR thermometer. This optical technique was capable of in situ, noncontact, electrode surface deformation, and temperature measurement under SOFC operating conditions. The surface deformation measurement sensitivity is half-wavelength and is immune to temperature fluctuation and environmental vibration. The experimental data can be used for the validation and further development of SOFC structural and electrochemical modeling analyses (Guo et al., 2010).
Other application of infrared spectroscopy techniques, for example FTIR, is for functional ceramic, electrodes and electrolyte, characterization. Through analyze of the absorption bands in low wave number region the oxygen-metal linkage are associated; the enlarged bands are attributed to the stretching vibration of hydrogen-bonded OH groups present. Moreover, the residual carbon from the synthesis not eliminated in the thermal treatment processes, can also be observed in some bands, for example associated to carboxylate anion (COO-) stretching and the C-O groups or to stretching vibration of the C-O bonds.

Preparation and characterization of functional materials for SOFC
During the last years the research of the group on SOFC development has been focused on the synthesis of LaSrMnO 3 (LSM), SmCeO 2 (SDC) and LaSrCoFeO 3 (LSCF) powders by two different methods. LSM and SDC powders have been prepared by the modified Pechini www.intechopen.com method using gelatin as a polymerizing agent. LSCF powder has been synthesized by microwave-assisted combustion. The electrochemical performance of a single electrolytesupported SOFC containing LSCF-SDC composite cathode film has also been investigated.
The raw materials used to synthesize LSM and SDC powders by the modified Pechini method, in which gelatin replaces ethylene glycol, were lanthanum, strontium, manganese (VETEC, Brazil), samarium and cerium (Sigma-Aldrich, Germany) nitrates. For synthesizing LSM, a solution of manganese citrate was firstly prepared from manganese nitrate and citric acid with a molar ratio of 1:3 (metal/citric acid) under stirring for 2 h at 70 °C. Stoichiometric amounts of the other citrates were added one by one at 1 hour intervals. The temperature was slowly increased up to 80 ºC and then gelatin was added at a weight ratio of 40:60 (gelatin/citric acid). The solution was stirred on a hot plate, which allowed the temperature to be controlled until a polymeric resin was formed. This resin was pre-calcined at 300 °C for 2 h, forming a black solid mass which was grounded into a powder and calcined at 500, 700 or 900 °C for 4 h to obtain the LSM perovskite structure. SDC powder was obtained following the same procedure but calcined between 700 and 900 ºC.
To further understand the synthesis mechanism used in this work, which uses gelatin instead of ethylene glycol, it is necessary to review the traditional polymeric precursor method proposed by Pechini (Pechini, 1967). The Pechini method involves the formation of stable metal-chelate complexes with certain alpha-hydroxycarboxyl acids, such as citric acid, and polyesterification in the presence of a polyhydroxy alcohol, such as ethylene glycol, to form a polymeric resin. The metal cations are homogeneously distributed in the polymeric resin, which is then calcined to yield the desired oxides. The most common materials used as source of cations are nitrate salts since they can be fully removed at low temperatures (400 -500 ºC). The synthesis mechanism of the modified Pechini method used in this work can be explained in three basic steps, as shown in Fig. 2. It stands out by its simplicity and low cost, using only citric acid, gelatin and metal nitrates as reagents.
In the first step, a solution of metallic citrate is prepared from the mixture of deionized water, citric acid and the salts of each metal ion. The citric acid acts as a chelating agent to remove metal ions from the solution. Chelation is the ability that a chemical substance has to form a ring-like structure with a metal ion, resulting in a compound with different chemical properties compared to the original metal, which prevents it from following different chemical routes. The chelating agent acts as a crayfish which traps the metal in its claws. When metal ions are in solution, they are surrounded by water molecules which avoid the establishment of new bonds. Thus, the chelating agent replaces such water molecules (bonds) and forms a ring-like structure, resulting in the phenomenon known as chelation. The number of binding sites that can form coordination bonds in the metal determines the number of rings formed.
The second step of this synthesis mechanism is the gelatin hydrolysis. Gelatin is a natural polymer, composed of a mixture of high molecular weight polypeptides (proteins) obtained by controlled hydrolysis of collagen fiber. Therefore, it can be used for cheap aqueous polymeric precursor synthesis in order to attain nanocrystalline powders. As illustrated in Fig. 2, the gelatin hydrolysis corresponds to the breaking of peptide bonds in the presence of water. For easier understanding, the second step reaction, in this illustration, is taking place in a low molecular weight peptide. An extra hydrogen ion is necessary to react with the -NH 2 group on the left-hand end of the peptide, the one not involved in the peptide bond. By www.intechopen.com scaling this up to a polypeptide (a protein chain), each of the peptide bonds will be broken in exactly the same way. This means that a mixture of the amino acids that makes up the protein will form, although in the form of their positive ions because of the presence of hydrogen ions from the citric acid. The presence of positive ions (not shown in Fig.2) is due to the fact that an amino acid has both a basic amine group and an acidic carboxylic acid group. There is an internal transference of a hydrogen ion from the -COOH group to the -NH 2 group to grant both a negative and positive charge to the ion. This is called a zwitterion. This is how amino acids exist even in the solid state. After being dissolved in water, the gelatin forms a simple solution which also contains this ion. A zwitterion is a compound with no overall electrical charge, but which contains separate parts, positively and negatively charged. As the pH decreases by adding an acid to a solution of an amino acid, the -COO-side of the zwitterion picks up a hydrogen ion, forming the desirable compound.
In the third step, proteins act as amides and react with the free hydroxyl of the citric acid. The hydroxyl of the citric acid reacts like the hydroxyl of an alcohol in a standard esterification process. During the reaction of the amide with the citric acid, the substitution of the amide hydroxyl by an alkoxy radical (-OR) takes place. At the end of the process, the oxygen in the -OH group of the citric acid remains in the chain whereas the hydroxyl oxygen in the amino acids is eliminated in the form of water. This mechanism results in long chains containing metal cations tightly bond and evenly distributed.
To synthesize LSCF powders, an aqueous solution of La(NO 3 ) 3 , Sr(NO 3 ) 2 , Co(NO 3 ) 2 , and Fe(NO 3 ) 3 (VETEC, Brazil) and urea as fuel was heated up to 100 ºC under stirring for 10 minutes. Afterwards, the becker was placed in a microwave oven set to 810 W and 2.45 GHz. Self-ignition takes place in about 1 minute. The resulting powder was then calcined at 900 ºC for 4 h in order to remove carbon residues remaining in the ash and to convert the powder to the desired LSCF phase with a well-defined crystalline perovskite structure.
LSM, SDC and LSCF powders were characterized by XRD using a Shimadzu XDR-7000 diffractometer and scanning electron microscopy (SEM-SSX 550, Shimadzu). Infrared spectra were also recorded with FTIR (IR Prestige-21, Shimadzu) in the 400 -4600 cm −1 spectral range. Specific surface area measurements were performed only for the LSM powders. An infrared reflectance spectrum of a LSM pellet prepared from a powder calcined at 900 °C was recorded with a Fourier-transform spectrometer (Bomem DA 8-02) equipped with a fixed-angle specular reflectance accessory (external incidence angle of 11.5°).

Preparation and characterization of composite cathodes
Composite powders consisted of 50 wt.% cathode (LSM or LSCF) and 50 wt.% SDC were prepared by mixing in a ball mill for 24 h in order to deposit LSM-SDC and LSCF-SDC composite films by spin coating (LSM-SDC) and screen printing (LSCF-SDC). Prior to spin coating deposition, ceramic suspensions of LSM-SDC composite powders were prepared using ethanol and different amounts of ethyl cellulose as pore-forming material. Ethyl cellulose was added in the weight ratios of 4, 8 and 10 wt.% with respect to the total solid weight. With the purpose of studying the influence of the milling process on the particle size distribution, another mixture was prepared without milling and without the addition of ethyl cellulose. Before coating, the ceramic suspensions were ultrasonically treated and then deposited by spin coating onto YSZ substrates. Commercially available YSZ powder (Tosoh Corporation, Japan) was compressed into pellets (13 mm in diameter) under uniaxial pressure (74 MPa) and then sintered in air at 1450 °C for 4 h to increase its mechanical strength for application as ceramic substrate. In the deposition process, 25 layers were applied using initial and final rotation speeds of 500 rpm for 15 seconds and 5000 rpm for 30 seconds, respectively. The composite films were attained after sintering at 1150 °C for 4 h. LSCF-SDC cathodes were screen-printed onto YSZ electrolytes and sintered at 950 °C for 4 h. The morphological characterization of the surface and cross section of LSM-SDC composite cathodes was performed using a field emission gun scanning electron microscope (FEG-SEM, Zeiss-Supra 35) and a scanning electron microscope (SEM-SSX 550, Shimadzu). www.intechopen.com

Preparation and characterization of a SOFC single cell
A preliminary performance test was carried out to qualitatively evaluate the prepared LSCF-SDC composite cathode. A single SOFC supported on commercial, 200µm thick, YSZ electrolyte (Kerafol, Germany) with Ni-YSZ anode and LSCF-SDC composite cathode screen printed was used. To assemble the single cell, first the anode suspension was printed and the half cell was sintered at 1300 °C for 4 h. The anode was reduced during cell operation. Later on, the LSCF-SDC cathode was printed on the other electrolyte side and sintered at 950°C for 4 h to avoid undesirable reactions. An in-house test station was used in the performance evaluation. The single cell was tested using dry hydrogen as fuel and oxygen as oxidant, in the ratio of 40ml/min H 2 and 40ml/min O 2 . The current-voltage characteristic of the cell was measured using linear sweep voltammetry (LSV) over a temperature range of 800 to 950 °C. The microstructure of the cathode/electrolyte interface after the performance test was examined by SEM.

Characterization of functional materials for SOFC
XRD patterns of the calcined LSM powders can be observed in Fig. 3. The as-synthesized material depicts the main diffraction peaks characteristic of the rhombohedral structure with space group R 3 c (Konysheva et al., 2009) and the respective JCPDS (Joint Committee on Powder Diffraction Standards) chart number 53-0058. Secondary phases were found and identified as SrCO 3 and Mn 3 O 4 . The presence of SrCO 3 was identified at 500 and 700 ºC by comparison with JCPDS pattern number 84-1778. The formation of strontium carbonate is due to the reaction between SrO and CO 2 produced during organic compound decomposition. Mn 3 O 4 is present only in the powder calcined at 900 °C and was identified by comparison with pattern JCPDS 89-4837. It probably came from the change of oxidation state of the Mn ion. Vargas and co-workers (Vargas et al., 2008) have reported that www.intechopen.com temperatures higher than 1000 ºC are necessary to remove all the carbon present in the LSM powders. Lower calcination temperatures result in free carbon or as carbonates from the degradation of citrates. Table 1 lists the specific surface area (S BET ), average crystallite size (D XRD ), average particle size (d BET ), microstrain and the d BET /D XRD ratio for LSM powders calcined at 700 and 900 °C. There is a clear dependency between the average crystallite size and the calcination temperature. The thermal treatment temperature is directly proportional to the crystallite size, because of the crystallization process that occurs in high temperatures. Therefore, the specific surface area decreases with the onset of particle sintering. High particle sizes promote lower surface area to volume ratio, which results in small deformations in the crystalline structure parameters. The d BET /D XRD ratio, which indicates the degree of particle agglomeration, increases with the calcination temperature. In a previous study , d BET /D XRD equal to 12 was established for the LSM powder synthesized with ethylene glycol as the polymerizing agent and calcined at 900 °C. This result shows that the substitution of ethylene glycol for gelatin reduces powder agglomeration, improving the potential to prepare ceramic suspensions to be deposited as cathode films with good adherence to the substrate.
LSM powders were observed by scanning electron microscopy (Fig. 4). SEM micrographs revealed the presence of soft particles agglomerates. Fig. 4(d) displays a FEG-SEM high magnification image of the LSM powder calcined at 900 °C. As it can be seen, this powder reveals strong agglomeration of particles with an estimated size lower than 100 nm. Results from SEM analyses are in good agreement with specific surface area measurements.
In the FTIR spectra of the LSM powders calcined between 500 and 900 ºC, displayed in Fig. 5   The infrared reflectance spectrum in the FAR and MID ranges is presented in Fig. 6. The almost continuous decrease of the reflectivity as a function of the wave number shows that this spectrum is dominated by a conduction mechanism (Gosnet et al., 2008). Moreover, first www.intechopen.com order phonon features characteristic of the crystal lattice are clearly superposed on the decreasing reflectivity curve below 700 cm -1 . The reflectivity line shape of these phonons is rather characteristic of conducting materials and denotes possibly electron-phonon coupling in the LSM powder calcined at 900 °C, as also observed in La 0.7 Ca 0.3 MnO 3 ceramics (Kim et al., 1996). On the other hand, the two modes above 1000 cm -1 are likely to be defect modes from impurities due to unreacted materials, probably Mn 3 O 4 , as observed by XRD.
The XRD patterns of SDC powders, as synthesized by the modified Pechini method and after calcination from 700 to 900 ºC, are shown in Fig. 7. XRD results revealed no peaks corresponding to secondary phases, i.e., there are no obvious peaks from phases other than SDC (JCPDS 75-0158) until the detection limit of the X-ray diffraction. The Rietveld refinement of the diffraction data using Maud program indicates that SDC powders exhibit cubic structures with space group Fm-3m and crystallite sizes in the range of 7-38 nm. Typical particle morphology of the SDC powder calcined at 900 °C is shown in Fig. 7(b), from which it can be seen that the particles are nearly spherical and strongly agglomerated, as expected for nanoparticles synthesized by the polymeric precursor method. FEG-SEM images also showed that the spherical particles are lower than 50 nm.
The FTIR spectra of SDC powders synthesized by the modified Pechini method and calcined at different temperatures are shown in Fig. 8. The peaks at around 3400 and 1640 cm -1 correspond to the H-O stretching and H-O-H bending vibration, respectively. They indicate that water or hydroxyl groups still existed not only in the as-prepared, but also in all calcined samples. A low intensity band at 1380 cm -1 in the as-prepared powder might indicate the physical adsorption of CO 2 or be caused by residual NO 3- (Chen et al., 2006). However, this band is eliminated after heat treatment. Therefore, although a synthesis temperature as low as 350 °C was sufficient to produce the SDC phase, powders with higher purity were obtained by calcining above 700 °C.   9 shows the XRD patterns of LSCF powder both as-prepared and calcined at 900 °C for 4 h. Prior to calcination and after only 1 minute of microwave irradiation, the powder already exhibited mainly the perovskite-type structure. In addition, some peaks of deleterious phases (LaCoO 4 and SrCO 3 ) were detected before calcination. The formation of SrCO 3 can be attributed to the reaction of Sr(NO 3 ) 2 and CO 2 . Compared with the as-prepared powder, the LSCF powder after calcination displayed higher crystallization. Since carbonaceous phases www.intechopen.com may dramatically deteriorate the cathode performance for SOFCs due to its poisoning effect on the cathode surface, further calcination at 900 °C under air for 4 h was carried out in order to eliminate the SrCO 3 impurity, driving its decomposition to CO 2 . Accompanied by the formation of SrCO 3 , another K 2 NiF 4 -structured phase with many small peaks was identified in the as-prepared LSCF powder. This phase was ascribed as La 2 CoO 4 , based on JCPDS card 34-1296. It is important to mention that it has good electronic and oxygen ionic conductivity and activity for oxygen reduction, being therefore, expected to have negligible effect on the cathode performance of LSCF (Borovskikh et al., 2003;Vashook et al., 2000). In any case, it is observed that both secondary phases decomposed completely after calcination. Fig. 10 shows SEM images of the LSCF powder after calcination at 900 °C. As expected for ceramic materials synthesized by the combustion method, the powder presents a porous microstructure consisting of small uniform particles. The porous structure of this material can be attributed to significant gas evolution during the combustion reaction. Fig. 11 shows the FTIR spectra for the as-prepared and calcined LSCF powders. The strong absorption bands in the low wave number region (~ 595 cm -1 ) are associated with metaloxygen (M-O) bonds characteristic of the perovskite structure. The similar absorption intensity of this band confirms that the perovskite-type structure is almost completely obtained after the combustion reaction, as observed by X-ray diffraction. The band at 1620 cm -1 corresponds to the stretching vibration frequency of coordinated H 2 O (δ HOH ). The bands related to absorbed water (3400 cm -1 ) were also present in both samples. Before calcination at 900 °C for 4 h, the LSCF powder exhibited strong absorption bands at 1381 cm -1 and 1454 cm -1 , which indicate the characteristic vibration of NO 3 -1 and the stretching of the CO 3 2-, respectively. Based on these results, it is possible to conclude that during LSCF synthesis, some nitrates can react with carbon dioxide to form carbonaceous phases which decompose after further calcination at 900 °C for 4 h. Therefore, FTIR results are consistent with XRD analyses.

Characterization of composite cathodes
LSM, SDC and LSCF powders synthesized by the above mentioned methods were used to prepare LSM-SDC and LSCF-SDC composite films onto YSZ substrates. A previous study performed by Ye and co-workers (Ye et al., 2007) demonstrated that the amount of ethyl cellulose in LSM-SDC slurries is restricted by cracking of the surface of the films. These authors observed that cracking took place when the amount of pore former reached 15 wt.% and is caused by the large amount of organics which evaporates during sintering. Therefore, in the present study, LSM-SDC slurries with a maximum of 10 wt.% ethyl cellulose were used. Fig. 12 shows FEG-SEM images of the porous structure of LSM-SDC composite www.intechopen.com  (Figure 12b-d) it can be observed that both particle agglomeration and pore size decrease as the ethyl cellulose content increases. Moreover, the particle size is lower than 200 nm. Typically, if the slurry contains 10 wt.% ethyl cellulose (Fig. 12d), the film exhibits a highly porous surface morphology and uniform pore structure, which are essential conditions for obtaining high-performance cathodes. The electrochemical evaluation of these composite cathodes are currently under way. Fig. 13 shows cross-sectional SEM images of the LSM-SDC cathodes containing 0 and 10 wt.% ethyl cellulose after milling LSM and SDC powders. The film without addition of ethyl cellulose (Fig. 13a) is tightly adhered to the substrate and shows few pores. However, the film containing 10 wt.% ethyl cellulose (~ 10 μm thick) is also well adhered to the YSZ substrate and exhibits higher pore uniformity than that in Fig. 13a. Such a porous microstructure fulfils the need for a SOFC cathode, which possess high active surface area, while permitting rapid diffusion of oxygen through the porous cathode film. www.intechopen.com

Characterization of a SOFC single cell
A SEM micrograph of the cathode/electrolyte interface and preliminary results on the electrochemical activity of YSZ electrolyte-supported SOFCs containing Ni-YSZ anode and a LSCF-SDC composite cathode are shown in Fig. 14. As it can be seen in Fig. 14(a), the composite film not only has good adhesion to the electrolyte, but also possesses a porous microstructure which is required for the oxidant electrochemical reduction. It indicates that such a composite film can have a good performance as SOFC cathode. By the LSV technique, qualitative information about electrochemical activity of this SOFC was acquired. The power density curves (Fig. 14b) revealed that maximum power densities were 19, 26, 36 and 46 mW/cm 2 at 800, 850, 900 and 950 ºC. It is possible to compare these first results with literature data and safely state that the LSCF-SDC cathode composite is qualitatively better than other plain standard materials or cathode composites already reported. It should also be mentioned that the result obtained at 800 ºC is similar to that reported by Muccillo et al (Muccillo et al., 2006) for a SOFC single cell with LSM-YSZ cathode, Ni-YSZ anode and 70 µm thick YSZ electrolyte. Nonetheless, voltage and power density values cannot be compared to literature data because of the thick (200 µm) electrolyte used as cell support, which is responsible for high ohmic polarization. The improvement in cell performance is attributed to the good catalytic activity of the novel LSCF-SDC composite film prepared from the powders obtained by different synthesis methods.

Conclusions
Results showed that LSM, SDC and LSCF powders were successfully synthesized by modified Pechini and microwave-assisted combustion methods. The milling process of the LSM-SDC composite powders and the addition of ethyl cellulose to the slurries seem to reduce particle agglomeration and pore size, that could improve the triple phase boundary (TPB) and, consequently, the electrochemical efficiency of the cathode. According to the results gathered herein, LSM-SDC films containing 10 wt.% ethyl cellulose (maximum concentration in order to avoid surface cracks) exhibited uniform and continuous pore structure and excellent adhesion to the YSZ substrate. The composite films produced presented porous microstructures desirable for the electrochemical reduction of the oxidant. Homogeneity and agglomeration absence or phase segregation were also characteristics found in the composite films prepared.
FTIR technique was successfully applied for powder characterization. Results were in agreement with XRD data, demonstrating that the FTIR is also a powerful tool for evaluating the crystalline structure evolution after a chemical synthesis. Moreover, the data could also indicate that water or hydroxyl groups still existed for some calcinated samples as well as physical adsorption of CO 2 , giving a qualitative information of purity of the samples. The present book is a definitive review in the field of Infrared (IR) and Near Infrared (NIR) Spectroscopies, which are powerful, non invasive imaging techniques. This book brings together multidisciplinary chapters written by leading authorities in the area. The book provides a thorough overview of progress in the field of applications of IR and NIR spectroscopy in Materials Science, Engineering and Technology. Through a presentation of diverse applications, this book aims at bridging various disciplines and provides a platform for collaborations among scientists.

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