**1.5 Techniques to obtain SOFC materials**

144 Electrochemical Cells – New Advances in Fundamental Researches and Applications

transport, and usually increase the catalytic activity. Some compositions can be highlighted, such as: Zr1-x-yTixYyO2 (Tao & Irvine, 2002), La1-xSrxA1-yMyO3 (A: Cr ou Fe; M: Ru, Cr ou Mn) (Sauvet & Fouletier, 2001), Sr1-xYxTiO3 (Hui & Petric, 2001), e La1-xSrxTiO3 (Canales-Vázquez

The cathode of a fuel cell is the interface between air (or oxygen) and the electrolyte. Its main functions are to catalyze the reaction of oxygen reduction and to transport the

In the same way of all the materials used in solid oxide fuel cells, the cathode must present

Minor differences between the thermal expansion coefficients of various components in

High catalytic activity for oxygen reduction and stability in highly oxidizing

The materials, perovskite-type ABO3, commonly used as cathodes in fuel cells are solid oxide ceramics based on lanthanum manganite (LaMnO3) substituting A for Sr ions. This material fills most of the requirements for its use as cathode for ceramic fuel cells operating at temperatures around 1000 °C. The ionic conductivity of materials based on LaMnO3 is significantly smaller than the ionic conductivity of YSZ electrolytes, but the ionic conductivity increases significantly substituting Mn for Co. The diffusion coefficients of oxygen ions in lanthanum cobaltite can reach 4-6 orders of magnitude higher when compared to those of lanthanum manganites with similar doping (Carter, 1992). Other materials have been used as cathode in SOFC, such as: Lanthanum strontium ferrite (LSF), (LaSr)(Fe)O3, Lanthanum strontium cobaltite (LSC), (LaSr)CoO3, Lanthanum strontium cobaltite ferrite (LSCF), (LaSr)(CoFe)O3, Lanthanum strontium manganite ferrite (LSMF), (LaSr)(MnFe)O3, Samarium strontium cobaltite (SSC), (SmSr)CoO3, Lanthanum calcium cobaltite ferrite (LCCF), (LaCa)(CoFe)O3, Praseodymium strontium manganite (PSM), (PrSr)MnO3, Praseodymium strontium manganite ferrite (PSMF), (PrSr)(MnFe)O3

The design of fuel cells with solid oxide electrolyte must be based on the concept of oxygen ion conduction through the electrolyte, with ions O2- migrating from the cathode to the

The materials that have been studied the most are: yttria stabilized zirconia, doped ceria with gadolinium and lanthanum gallate doped with strontium and magnesium. The solid oxide fuel cells can, in principle, operate in a wide temperature range between 500 °C and

anode, where they react with the fuel (H2, CO, etc..) generating an electrical current.

generated electrons to the interconnect (external circuit).

Phase and microstructure stability during the operation.

 High electrical conductivity, both electronic and ionic Porous microstructure during the entire operation of the cell

et al., 2003).

**1.4.2 Cathode** 

the cell.

Chemical stability.

atmospheres.

(Stambouli & Traversa, 2002b).

**1.4.3 Electrolyte** 

certain general characteristics:

Low cost and ease of fabrication.

The methods employed in the deposition of thin films of oxides can be divided into two major groups based on the nature of the deposition processes. Physical methods of deposition: physical vapor deposition (PVD) (Kueir-Weei et al., 1997), ion beam (Xiaodong et al., 2008) and sputtering (Haiqian et al., 2010). The chemical methods of deposition, which can be subdivided as to the nature of the precursor: gas phase and solution. The gas phase methods: chemical vapor deposition (CVD) (Bryant, 1977) and atomic layer epitaxy (ALE) (Suntola, 1992). The solution methods: spray pyrolysis (Chamberlin & Skarman, 1966), solgel (Brinker et al., 1990) and electrodeposition.

The table 1 shows de main advantages and disadvantages of some technique that can used to obtain SOFC materials.

The technique of spray pyrolysis can be used to obtain both, dense or porous oxide films, and to produce ceramic coatings and powders. Compared to other deposition techniques, spray pyrolysis is a simple method for operational control. It is also cost-effective, especially regarding the cost of system implementation. Furthermore, deposition in multi-layers can be easily obtained by this versatile technique.

Fuel Cell: A Review and a New Approach

film (Yu & Liao, 1998).

Fig. 2. Aerosol transport model.

occur, in order to form the desired final material.

substrate surface, forming a porous crust (Perednis, 2003).

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 147

investigated the mechanism of SnO2 film growth (Sears & Gee, 1988). In case of excessively rapid evaporation of the solvent along the way, the droplet size decreases and precipitates precursor salt on the edges of the drop, causing the deposition of precipitates on the substrate surface. This phenomenon is extremely deleterious to obtain dense and homogeneous films, since the particles formed in the atomizer-substrate path add to the

On the other hand, if the drops are sprayed against the substrate with sufficiently high strength, spread lightly, maintain an evaporation rate equivalent to the solute precipitation rate, the solute nucleates and precipitates homogeneously, creating a dense and continuous

A model of the possible transport situations of aerosol from the atomizer, toward the heated substrate can be seen in Figure 2. In zone I, the droplet is too large, has a very slow solvent evaporation rate and results in the formation of a brittle precipitate. In the second case, the drops have size and strength suitable for spraying, forming homogeneously aggregates of precipitates (zone II). And finally, in zone III, the drops are too small and not strong enough

The substrate temperature can be considered the most important factor to determine the solvent evaporation rate. It is directly linked to the time the solvent takes to spread over the surface of the substrate and the speed at which it evaporates after being spread. In addition, the substrate temperature should be high enough so that the salt decomposition reactions

It is known that low substrate temperatures provide an excellent scattering of the solution, however, the film layer that is formed is very rich in solvent, taking too much time to evaporate. After stopping the solution spraying, the film is still wet and the local rise in temperature causes the solvent to evaporate, diminishing the volume and contributing to the formation of tension in the film. The strong adhesion between film and substrate prevents free contraction of the film, promoting the formation of cracks (Neagu et al., 1981).

to reach the substrate, causing particles to appear before reaching the substrate.


Table 1. Advantages and disadvantages of some techniques used to obtain SOFC materials.
