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Title: STABLE HIGH CONDUCTIVITY BILAYERED ELECTROLYTES FOR LOW TEMPERATURE SOLID OXIDE FUEL CELLS

Solid oxide fuel cells (SOFCs) are the future of energy production in America. They offer great promise as a clean and efficient process for directly converting chemical energy to electricity while providing significant environmental benefits (they produce negligible CO, HC, or NOx and, as a result of their high efficiency, produce about one-third less CO{sub 2} per kilowatt hour than internal combustion engines). Unfortunately, the current SOFC technology, based on a stabilized zirconia electrolyte, must operate in the region of 1000 C to avoid unacceptably high ohmic losses. These high temperatures demand (a) specialized (expensive) materials for the fuel cell interconnects and insulation, (b) time to heat up to the operating temperature and (c) energy input to arrive at the operating temperature. Therefore, if fuel cells could be designed to give a reasonable power output at lower temperatures tremendous benefits may be accrued, not the least of which is reduced cost. The problem is, at lower temperatures the conductivity of the conventional stabilized zirconia electrolyte decreases to the point where it cannot supply electrical current efficiently to an external load. The primary objectives of the proposed research is to develop a stable high conductivity (>0.05 S cm{sup -1} at 550more » C) electrolyte for lower temperature SOFCs. This objective is specifically directed toward meeting the lowest (and most difficult) temperature criteria for the 21st Century Fuel Cell Program. Meeting this objective provides a potential for future transportation applications of SOFCs, where their ability to directly use hydrocarbon fuels could permit refueling within the existing transportation infrastructure. In order to meet this objective we are developing a functionally gradient bilayer electrolyte comprised of bismuth oxide on the air side and ceria on the fuel side. Bismuth oxide and doped ceria are among the highest ionic conducting electrolytes and in fact bismuth oxide based electrolytes are the only known solid oxide electrolytes to have an ionic conductivity that meets the program conductivity goal. We have previously demonstrated that this concept works, that a bismuth oxide/ceria bilayer electrolyte provides near theoretical open circuit potential (OCP) and is stable for 1400 h of fuel cell operation under both open circuit and maximum power conditions. More recently, we developed a computer model to determine the defect transport in this bilayer and have found that a bilayer comprised primarily of the more conductive component (bismuth oxide) is stable for 500 C operation. In this first year of the project we are obtaining necessary thermochemical data to complete the computer model as well as initial SOFC results based on thick 1-2 mm single and bilayer ceria/bismuth oxide electrolytes. We will use the computer model to obtain the optimum relative layer thickness as a function of temperature and air/fuel conditions. SOFCs will be fabricated with 1-2 mm single and bilayer electrolytes based on the modeling results, tested for OCP, conductivity, and stability and compared against the predictions. The computer modeling is a continuation of previous work under support from GRI and the student was available at the inception of the contract. However, the experimental effort was delayed until the beginning of the Spring Semester because the contract was started in October, 2 months after the start of our Fall Semester, and after all of the graduate students were committed to other projects. The results from both of these efforts are described in the following two sections: (1) Experimental; and (2) Computer Modeling.« less
Authors:
Publication Date:
OSTI Identifier:
809195
DOE Contract Number:
AC26-99FT40712
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 1 Oct 2000
Research Org:
University of Florida (US)
Sponsoring Org:
(US)
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; AVAILABILITY; BISMUTH OXIDES; COMBUSTION; ELECTROLYTES; FUEL CELLS; IONIC CONDUCTIVITY; OXIDES; SIMULATION; SOLID OXIDE FUEL CELLS