Zirconia-based SOFCs

Despite the rather low ionic conductivity and, hence, the poor utilization of the bulk path, porous LSM cathodes show acceptable polarization resistances in zirconia-based SOFCs at 1000°C. However, the activation energy of the polarization resistance is rather high (ca. 1.8-2.1 eV [94, 95, 119, 120, 131, 132]) and the trend to lower the operation temperature of fuel cells requires... 

Typical electrolyte materials for SOFCs are oxides with low valence element substitutions, sometimes named acceptor dopants [13, 95] which create oxygen vacancies through charge compensation. For SOFC applications, there are various materials that have been explored as electrolyte, yttria-doped zirconia (YSZ) and gadolinium-doped ceria (GDC) are the most common materials used for the oxideconducting electrolyte. Above 800 °C, YSZ becomes a conductor of oxygen ions (02-) zirconia-based SOFC operates between 800 and 1100 °C. The ionic conductivity of YSZ is 0.02 S m at 800 °C and 0.1 S cm at 1000 °C. A thin electrolyte (25-50 (im) ensures that the contribution of electrolyte to the ohmic loss in the SOFC is kept to a minimum. 

The solid oxide electrolyte must be free of porosity that permits gas to permeate from one side of the electrolyte layer to the other, and it should be thin to minimize ohmic loss. In addition, the electrolyte must have a transport number for O as close to unity as possible, and a transport and a transport number for electronic conduction as close to zero as possible. Zirconia-based electrolytes are suitable for SOFCs because they exhibit pure anionic conductivity over a wide... 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

The most advanced SOFC s employ oxide ion conducting zirconia-based electrolytes. The conductivity of the electrolyte determines their operation temperature. The temperature dependence of the electrical conductivity for zirconia-hased oxides [12] is shown in Fig. 2. 

Room temperature monoclinic zirconia has little use as a SOFC electrolyte because it is predominantly an electronic conductor with low oxygen ion conductivity [15]. Cubic zirconia has high ionic conductivity but needs to be stabilized so that it retains its cubic structure at room temperature. Nemst discovered and reported in 1899 that mixtures of zirconia with other oxides such as magnesia showed high ionic conduction at high temperatures [16]. Two years later, he patented his further observation that the material composition (15% yttria and 85% zirconia) was suitable for electric-lamp glowers [17]. Westinghouse Electric Corporation has used a similar zirconia-based electrolyte in their SOFC development since 1962 [18]. 

As already discussed, the electrolyte is the most important component of a SOFC, the properties of which determine critical parameters such as cell performance and temperature of operation. Because of this, much time has been devoted to developing and understanding the materials and their properties and Raman spectroscopy has been a key tool. This section summarises the key results of studies that have used Raman spectroscopy to investigate electrolytes for SOFCs and is split into three parts. The first focuses on Zirconia based materials the conventional electrolyte of choice. The second will summarise the results of investigations on Ceria based electrolytes the frontrunner electrolyte for intermediate temperature SOFCs. Other novel electrolytes which have some potential for reduced temperature operation will be summarised in the final section. 

Much has been published on ceria-based fuel cells however, to date, only Ceres Power Ltd., Crawly, UK, is developing SOFCs with ceria-based electrolytes, whereas numerous companies are developing SOFCs with zirconia-based electrolytes. Ceres Power Ltd. is developing a ceria-based electrolyte on a metal support, where the target operating temperature is 500-600 Ceres Power Ltd. is focus-... 

Manganese-based perovskites are widely recognized as the materials best suited for the cathode of a high-temperature SOFC that uses a zirconia-based electrolyte and operates at temperatures higher than 800°C. In this section, (La, Sr)Mn03 (LSM) is chosen for further discussion. 

Although LSC is an attractive material for a SOFC cathode, its use is restricted because of the instability of LSC on zirconia-based electrolytes. It is well known that LSC reacts with YSZ and forms SrZrOs at the interface. When YSZ or ScSZ is used as the electrolyte, a protective interlayer is indispensable. 

Concerning the Ruddlesden-Popper series, Laberty et alP reported on the performance of SOFC cathodes with lanthanum-nickelate-based composites. They showed that lanthanum nickelate performs poorly when used as a single-phase cathode in yttria-stabilised, zirconia-based air-H2 button cells at 800 °C. However high power densities of up to 2.2 W cm were measured using a La2Ni04+ -CS0 composite bilayer cathode. 

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