Solid-oxide fuel cells electrical conductivity

In the 1960s, work on solid oxide fuel cells were conducted by the Westing-house Electric Corporation, where researchers Ruka and Weissbart built a fuel cell with electrolyte of 85% Zr02 and 15% CaO. Layers made from porous platinum were used as electrodes. Cell surface was 2.5 cm with a thickness of 15 mm. The construction of this cell is shown in Fig. 1.11. 

FIGURE 2.7 Change of electrical conductivity at 1000°C with respect to Ni volume content for Ni-YSZ cermets sintered for 2 h at 1200, 1250, 1300, and 1350°C, respectively. (From Pratihar, S.K. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 99(19) 513—521. Reproduced by permission of ECS-The Electrochemical Society.)... 

Pratihar SK, Baus RN, Mazumder S, and Maiti HS. Electrical conductivity and microstructure of Ni-YSZ anode prepared by liquid dispersion method. In Singhal SC, Dokiya M, editors. Proceedings of the Sixth International Symposium on Solid Oxide Fuel cells (SOFC-VI), Pennington, NJ The Electrochemical Society, 1999 99(19) 513-521. 

Tintinelli A, Rizzo C, and Giunta G. Ni-YSZ porous cermets microstructure and electrical conductivity. In Bossel U, editor. Proceedings of the first European Solid Oxide Fuel Cells Forum. Lucerne, Switzerland European Fuel Cell Forum, 1994 455 164. 

High temperature solid oxide fuel cells (SOFCs) have become of great interest as a potentially economical, clean and efficient means of producing electricity in a variety of commercial and industrial applications (Singhal, 1991). A SOFC is based upon the ability of oxide ions to be conducted through a solid at elevated temperatures. Oxide ion conductivity was observed in Zr02 9 mol% YjOj by Nernst as early as 1899. In 1937, Bauer... 

A comprehensive analysis of solid oxide fuel cells phenomena requires an effective multidisciplinary approach. Chemical reactions, electrical conduction, ionic conduction, gas phase mass transport, and heat transfer take place simultaneously and are tightly coupled. 

Yasuda, I. and Hishinuma, M., Electrical conductivity and chemical stability of calcium chromate hydroxyl apatite, Cas(Cr04)30H, and problems caused by the apatite formation at the electrode/separator interface in solid oxide fuel cells, Solid State Ionics 80, 1995, 141. 

Another type of electrical conductivity observed in ceramics is ionic conductivity, which often occurs appreciably at elevated temperature a widely used material exhibiting this behavior is zirconia doped with other oxides such as calcia (CaO) or yttria (Y2O3). For this material, atomic oxygen is the mobile ionic species. Doped zirconia finds widespread use as oxygen sensors, especially as part of automobile emission control systems, where the oxygen content of the exhaust gas is monitored to control the air-to-fuel ratio. Other applications of ionic conducting ceramics are as the electrolyte phases in solid-oxide fuel cells and in sodium-sulfur batteries. 

Since these first reports, Iwahara and other investigators have studied the conductivities (both ionic and electronic), conduction mechanism, deuterium isotope effect, and thermodynamic stability of these materials. The motivation for most of this work derives from the desire to utilize these materials for high temperature, hydrogen-fiieled solid oxide fuel cells. In a reverse operation mode, if metal or metal oxide electrodes are deposited onto a dense pellet of this material and heated to temperature T, the application of an electric potential to the electrodes will cause a hydrogen partial pressure difference across the pellet according to the Nemst equation ... 

Since the electronic conductivity of nanocrystalline ScSZ becomes significant in reducing atmosphere (Fig.3), its contribution to the electrical transport should be considered. This can be discussed based on the dependence of the ionic transference number, t,- = cr,- / (oxygen activity. Such information is important for the development of ScSZ solid electrolyte for Solid Oxide Fuel Cells. Figure 4 presents the relationship between the ionic transference number and oxygen activity, which has been determined based on the presented conductivity measurements and the defect model [13]. 

Point defects are particularly important in ceramics because of the role they can play in determining the properties of a material. The entire semiconductor industry is possible because of minute concentrations of point defects that are added to Si the dopants determine if the Si is n-type, p-type, or semi-insulating they determine the electrical properties. Solid-oxide fuel cells work because of the large concentrations of oxygen vacancies present the vacancies provide fast ion conduction pathways. CZ is cubic because of the presence of point defects that stabilize the cubic structure. 

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