Thin-Film Electrolytes for Reduced Temperature Solid Oxide Fuel Cells

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Abstract
Solid oxide fuel cells produce electricity at very high efficiency and have very low to negligible emissions, making them an attractive option for power generation for electric utilities. However, conventional SOFC's are operated at 1000 °C or more in order to attain reasonable power density. The high operating temperature of SOFC's leads to complex materials problems which have been difficult to solve in a cost-effective manner. Accordingly, there is much interest in reducing the operating temperature of SOFC's while still maintaining the power densities achieved at high temperatures. There are several approaches to reduced temperature operation including alternative solid electrolytes having higher ionic conductivity than yttria stablilized zirconia, thin solid electrolyte membranes, and improved electrode materials. Given the proven reliability of zirconia-based electrolytes (YSZ) in long-term SOFC tests, the use of stabilized zirconia electrolytes in reduced temperature fuel cells is a logical choice. In order to avoid compromising power density at intermediate temperatures, the thickness of the YSZ electrolyte must be reduced from that in conventional cells (100 to 200 μm) to approximately 4 to 10 μm. There are a number of approaches for depositing thin ceramic films onto porous supports including chemical vapor deposition/electrochemical vapor deposition, sol-gel deposition, sputter deposition, etc. In this paper we describe an inexpensive approach involving the use of colloidal dispersions of polycrystalline electrolyte for depositing 4 to 10 μm electrolyte films onto porous electrode supports in a single deposition step. This technique leads to highly dense, conductive, electrolyte films which exhibit near theoretical open circuit voltages in H2/air fuel cells. These electrolyte films exhibit bulk ionic conductivity, and may see application in reduced temperature SOFC's, gas separation membranes, and fast response sensors.