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A Quantitative Tunneling/Desorption Model for the Exchange Current at the Porous Electrode/Beta"- Alumina/Alkali Metal Gas Three Phase Zone at 700–1300K

Published online by Cambridge University Press:  15 February 2011

R. M. Williams ,
M. A. Ryan ,
C. Saipetch  and
H. G. LeDuc
R. M. Williams
Affiliation:
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109roger. m.williams@jpl.nasa.gov
M. A. Ryan
Affiliation:
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109roger. m.williams@jpl.nasa.gov
C. Saipetch
Affiliation:
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109roger. m.williams@jpl.nasa.gov
H. G. LeDuc
Affiliation:
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109roger. m.williams@jpl.nasa.gov
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Abstract

The exchange current observed at porous metal electrodes on sodium or potassium beta"-alumina solid electrolytes in alkali metal vapor is quantitatively modeled with a multi-step process with good agreement with experimental results. No empirically adjusted parameters were used, although some physical parameters have poor precision. Steps include: (1) diffusion of Na+ ions to the reaction site; (2) stretching of the Na+ ionic bond with the β3"-alumina surface to reach a configuration suitable for accepting an electron to form a surface bound Na0 atom; (3) electron tunneling from the Mo electrode to the ions; and (4) desorption of Na0 atoms weakly bound at the reaction site on the β"-alumina surface; (5) electron tunneling between Na0 or Na+ on the defect block and (6) Na0 and/or Na+ mobility on the spinel block surface may extend the reaction area substantially.

The rate is increasingly dominated by the region close to the three-phase boundary as temperature increases and the rate near the three-phase boundary increases fastest, because desorption has a higher energy than reorganization. At high temperatures, surface diffusion of Na+ ions from the defect block edges to the spinel block edges is responsible for an increase in the total effective reaction zone area near the three-phase boundary.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 453: Symposium R – Solid State Chemistry of Inorganic Materials , 1996 , 597
Copyright
Copyright © Materials Research Society 1997

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References

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