**1.3.2 Planar layout**

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

Inside the planar design of SOFC cells, the development of new materials and techniques of production allowed an evolution of the configurations of planar cells.

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

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

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

#### Fuel Cell: A Review and a New Approach About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 143

development of third generation SOFC (3G-SOFC) interconnect support type, allowing a reduction in the ceramic components thickness, accompanied by a reduction in operating temperature of the cell (800 °C) and the use of metallic interconnects. In this design, the electrolyte thickness has less than 20 µm and both, anode and cathode thickness, has about 50 µm (Wang, 2004). Figure 1 shows a schematic configuration of planar SOFCs.

Fig. 1. Representation of the configuration for the three-generations of planar SOFCs.
