**2. Materials and methods**

148 Electrochemical Cells – New Advances in Fundamental Researches and Applications

However, if the substrate temperature is high, it will reduce the time of solvent evaporation to a point when the solvent partially evaporates before touching the substrate, inhibiting the spread. In cases where the temperature is too high, the droplets of solution not even touch the substrate, all the solvent is evaporated on the way between the atomizer and the surface

The choice of an intermediate temperature is necessary to obtain a film with the desired characteristics. This should consider the solvent used, so it evaporates after a light scattering

In recent years, research has been aimed at further reducing the operating temperature of the cell, leading to the development of a new class of planar SOFC, the intermediate temperature (IT-SOFC). This configuration provides an even greater reduction in operation temperature, allowing a large-scale use of metal interconnects. However, the ionic conductivity of the electrolyte is greatly affected by reducing the temperature. In this context, there are two ways to develop electrolytes for IT-SOFC: by changing the electrolyte to a material that has a high conductivity; or by reducing of the electrolyte thickness. In IT-SOFC cells, the electrolyte thickness should be as thin as possible (less than 10 µm has been suggested). In this case, the cell can operate at temperatures between 600 °C and 800 °C

In recent years, research has been aimed at further reducing the operating temperature of the cell, leading to the development of a new class of planar SOFC, the intermediate temperature (IT-SOFC). This configuration provides an even greater reduction in operation temperature, allowing a large-scale use of metal interconnects. However, the ionic conductivity of the electrolyte is greatly affected by reducing the temperature. In this context, there are two ways to develop electrolytes for IT-SOFC: by changing the electrolyte to a material that has a high conductivity; or by reducing the electrolyte thickness. In IT-SOFC cells, the electrolyte thickness should be as thin as possible (less than 10 µm has been suggested). In this case, the cell can operate at temperatures between 600 °C and 800 °C.

For the case of electrolyte change, materials with better ionic conductivity than YSZ have been studied to replace it. LSGM has been successful when applied with a metallic Ni anode, presenting ionic conductivity in temperatures around 400 °C (Sasaki et al., 2008; Tuker, 2010). The problem with such material is its low chemical stability. Another exhaustly studied material for SOFC electrolyte is gadolinium-doped ceria (CGO). This material has better ionic conductivity than YSZ and can be used in lower temperatures. However, at 600 °C the Ce reduction occurs. This reduction confers electric conduction to

The greatest challenge nowadays is to reach fuel cells operating around 700 °C without efficiency losses. The operation temperature can be lowered when the electrolyte thickness is reduced and the densification optimized. Several researches aim the obtaining of thin and

In the present study, some results about the elaboration of the YSZ thin film by spray

the material and the cell can suffer a short circuit (Tuker, 2010; Yuan et al., 2010).

and only particles are deposited on the substrate (Perednis & Gauckler, 2004).

of the solution on the substrate.

**1.6 Research directions** 

(Cooper et al., 2008).

dense electrolytes (Gaudon et al., 2006).

pyrolysis process will be presented.

The spray pyrolysis setup consisted mainly of the following parts: a spraying unit, a liquid feeding unit, and a temperature control unit (Figure 3). The spray unit consisted of an airbrush (Campbell Hausfeld) using an air blast atomizer. The liquid feeding unit is the precursor solution, constituted by yttrium chloride (YCl3.6H2O) (Aldrich Chemicals) and zirconium acetylacetonate (Zr(C6H7O2)4) (Aldrich Chemicals) dissolved in three different solvents: (1) mixture of ethanol (C2H5OH) (FMaia) and propylene glycol (C3H8O2) (Proton) (1:1 vol.%); (2) mixture of ethanol and 2-methoxy, 1-propanol (C4H10O2) (Aldrich Chemicals) (1:1 vol.%); (3) mixture of ethanol and diethylene glycol monobutyl ether (C8H18O3) (Aldrich Chemicals) (1:1 vol.%). The Table 2 shows the boiling temperatures of any individual solvents used in this work.

Fig. 3. Spray pyrolysis experimental apparatus.


Table 2. Boiling temperatures of the solvents used.

The solutions were prepared according to the stoichiometry required to the films (ZrO2)0.92(Y2O3)0.08 (Perednis & Gauckler, 2004) and adopting a final concentration of salts in solution of 0.1 mol.L-1. The precursor solution was maintained under stirring and heating at 50 ºC in a hotplate stirrer (Fisaton), in order to obtain the complete dissolution of the salts and decrease the heat loss of the substrate. Finally, the temperature control unit consisted in a hotplate, used for heating the substrate. A thermostat controlled the hotplate temperature and the substrate temperature was monitored by an infrared pyrometer. The precursor solution was sprayed on the heated LSM porous substrate, in order to obtain the YSZ films.

The substrate temperature is determining in the morphology of films obtained, since it is directly related to the solvent evaporation rate, and a quick evaporation of the solvent promotes the formation of particles instead of forming a continuous film. On the other hand, the slow evaporation of the solvent, promotes crack formation in the film. Previous studies were made to determine the optimum temperature to obtain continuous films. For this reason, in this chapter, a single temperature was studied, 350 °C.

Fuel Cell: A Review and a New Approach

stretch of COO-group (Farhikhte, 2010).

Zone I

treatment at 700 °C for 2 hours.

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 151

However, after the heat treatment at 700 °C for 2 hours, the crystallization of zirconia was observed for all the solutions tested. There was no influence of the solvent used in the stabilization of the zirconia phase. The overlapping of the tetragonal and cubic zirconia peaks impedes the determination of the predominant stabilized phase (Wattnasiriwech et al., 2006). Given the importance of determining the stable phase, other techniques can be used to complement the analysis of YSZ X-ray diffraction. In this work Fourier Transform

The FT-IR spectrum (Figure 5) shows a very pronounced peak around 471 cm-1 (zone IV). Comparing it to FT-IR spectra for cubic zirconia, presented in the literature (Khollam et al., 2001), it is possible to see the presence of vibrational frequencies resulting from metaloxygen bonds characteristic of this phase around 471 cm-1, revealing that the heat treatment at 700 °C allowed the stabilization of the zirconia cubic phase for the films prepared. The band located at 3455 cm-1 (zone I) can be attributed to bonds O - H, possibly due to the presence of solvent excess adsorbed in the film. The bands presented around 2330 cm-1 (zone II) correspond to adsorbed atmospheric CO2, according to (Andrade et al., 2006). In the region from 1630 to 1619 cm-1 (zone III), the bands correspond to asymmetric and symmetric

Infrared Spectroscopy (FT-IR) was used to determinate the zirconia phase stabilized.

Fig. 5. FT-IR spectrum Of YSZ obtained from different precursor solutions after heat

Zone II

Zone III

Zone IV

**3.1.2 Fourier Transform Infrared Spectroscopy (FT-IR)** 

Three other important parameters influence in the solvent evaporation rate and in the attainment of dense and continuous films: solution flow rate, nozzle distance and air pressure,. In this study, these parameters were kept constant, and their values were, respectively, 35 mL.h-1, 250 mm and 3 kgf.cm-2.

The YSZ films are amorphous, after deposition, and a heat treatment was required in order to stabilize the zirconia cubic phase. This heat treatment was performed at 700 °C for two hours in a furnace with a constant heating rate of 2 °C.min-1 and slow cooling.

Two protocols were used for the film deposition: one-step deposition and multi-layer deposition. The first protocol consists in a deposition of 50 mL of precursor solution followed by heat treatment and the other protocol consists in a deposition of 150 mL of precursor solution in three sequential steps with intermediate heat treatment after each deposition.

The microstructure and the morphology of the films were evaluated by Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM).
