**1.4.1 Anode**

142 Electrochemical Cells – New Advances in Fundamental Researches and Applications

in the cell as already mentioned, is also responsible for its major disadvantages, such as the problems of material selection, high heat detritions and the impossibility of using metallic materials, which cost much less than the ceramic materials currently used (Aruna & Rajam,

The studies on solid oxide fuel cells have promoted considerable improvements on their characteristics and properties. Several layouts were developed to improve the characteristics

The solid oxide fuel cell is highly influenced by the temperature variation during operation, constantly suffering from thermal stress, which requires excellent compatibility between the thermal expansion coefficients of the cell components. The cylindrical shape of tubular SOFC contributes significantly to minimize the difference in the coefficients of thermal expansion, thus avoiding the formation of cracks and delamination. This model also makes unnecessary the use of sealant gases. On the other hand, the efficiency is impaired, since the

This design consists of flat electrodes and electrolyte, separated by thin interconnects. The components can be manufactured separately, giving the production simplicity. It has higher energy density than the tubular. A disadvantage is the long time needed for heating and

Inside the planar design of SOFC cells, the development of new materials and techniques of

The first generation of SOFC fuel cells (1G-SOFC) operated at temperatures of around 1000 °C. The system consisted of electrolyte support and the mechanical stability of the cell was given by the thickness of the electrolyte. In this design the anode and cathode were quite thin (around 50 µm), while the electrolyte had thickness of 100 µm to 200 µm. However, the operating temperature of the cell was a limiting factor in popularizing this type of energy

At high temperature, the thickness of the electrolyte around 200 µm did not provide a problem in the ionic conductivity. However, the reduction of cell operating temperature to 700 °C - 800 °C would result in a drastic reduction of the electrolyte conductivity, so, the second generation of planar SOFC (2G-SOFC) was developed aiming to reduce the electrolyte thickness. The mechanical stability of the cell can no longer be granted by the electrolyte, which now is less than 20 µm. For that, the second-generation cells are anode support type, where the anode is responsible for mechanical stability, with thickness between 300 µm and 1500 µm. In this

However, the costs of the cell are directly related to the costs of obtaining and producing ceramic materials present in their components, and an anode or cathode with very high thickness results in a substantial increase in production costs. This motivation led to the

of SOFC, considering the current needs and costs of production and efficiency.

path made by the electric current is increased, causing ohmic losses (Minh, 1993).

cooling of the cell, used to prevent the formation of cracks (Minh, 1993).

production allowed an evolution of the configurations of planar cells.

source, which led to the development of new designs for the cell (Wang, 2004).

generation, the cathode thickness was of 50 µm (Wang, 2004).

2008; Farooque & Maru, 2001; Srivastava et al., 1997 ).

**1.3 Evolution of SOFC** 

**1.3.1 Tubular layout** 

**1.3.2 Planar layout** 

The anode of a fuel cell is the interface between the fuel and electrolyte. The main functions of the anode are:


The anode material must possess, under the operating conditions of the fuel cell: good physical and chemical stability, chemical and structural compatibility with the electrolyte and interconnect, high ionic and electronic conductivity and catalytic activity for fuel oxidation (Ralph et al., 2001). The thermal stability is an important aspect to maintain the structural integrity throughout the temperature variations at which this component is subjected.

In general, the performance of the anode is defined by its electrical and electrochemical properties and therefore has a strong dependence on its microstructure. Thus, the control parameters, such as composition, size and distribution of particles and pores, are very important for optimizing the performance of the anode material of a solid oxide fuel cell.

Ceramic-metal composites, typically Ni-based, have been commonly used. Among them, the NiO-YSZ composite is the material of conventional fuel cells. Ni is also used because it has good electrical, mechanical and catalytic properties (Ralph et al., 2001). More recently, mixed conductors based on ceria (NiO-GDC) and transition metal perovskites (such as Fe, Mn, Cr and Ti) are being studied as potential candidates for anode materials for solid oxide fuel cells. However, to date, these materials do not present, in reducing atmospheres, values of electronic conductivity high enough for high performance fuel cell (Gong et al., 2010). The oxides of transition metals can have different oxidation states that can induce electron

Fuel Cell: A Review and a New Approach

**1.5 Techniques to obtain SOFC materials** 

gel (Brinker et al., 1990) and electrodeposition.

easily obtained by this versatile technique.

to obtain SOFC materials.

temperatures.

materials.

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 145

1000 °C. Thus, they can be divided into two types: operating at high temperatures (> 750 ºC) and intermediate temperatures (500 ºC to 750 ºC). One of the determinant factors for the operating temperature is the characteristic of the solid electrolyte. The ohmic losses associated with the electrolyte are important for the cell performance. In order to reduce the operating temperature of SOFC, aiming the use of more conventional steel alloys as interconnects at temperatures around 700 °C (Horita et al., 2008; Perednis & Gauckler, 2004), it is necessary to employ electrolytes with high oxygen ionic conductivity at low

YSZ is so far the most widely used solid electrolyte for application in high temperature SOFC. For many years, the zirconium oxide is already known as a conductor of oxygen ions. The yttria addition to the zirconia-yttria solid solution has two functions: to stabilize the cubic structure type fluorite and to form oxygen vacancies in concentrations proportional to the yttria content. These vacancies are responsible for high ionic conductivity. Yttria stabilized zirconia is a suitable ionic conductor at temperatures above 800 °C, since thin dense membranes (less than 20 µm) can be manufactured. These membranes should be free of impurities. The stabilized zirconia is chemically inert to most reactive gases and electrode

In view of the limitations encountered in using other types of ceramic conductors than yttria stabilized zirconia, its efficiency at low temperatures had to be improved. To reduce the operating temperature of the cell without affecting the efficiency of oxygen ion conduction, the electrolyte has to be as thin as possible in order to compensate the increase in ohmic losses (Huijsmans, 2001). Other advantages of fuel cells with thin electrolytes are reduction in material costs and improvement in the characteristics of the cells (Perednis & Gauckler, 2004). Therefore, the yttria stabilized zirconia is still a material with great prospects in the application as electrolyte in solid oxide fuel cells. Research about this type of material is aimed to improve its characteristics in order to adapt the needs of current applications.

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), sol-

The table 1 shows de main advantages and disadvantages of some technique that can used

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

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 et al., 2003).
