טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShaul Raz
SubjectMixed Ionic Electronic Conduction in Electrodes for Solid
Oxide Fuel Cells
DepartmentDepartment of Physics
Supervisor Professor Emeritus Riess Ilan


Abstract

The electrode overpotential in solid oxide fuel cells (SOFC) depends both on the electrode material and the solid electrolyte (SE). In this work we examine the effect of changes in the electronic (electron/hole) conductivity of the SE surface region at the electrode interface on the electrode overpotential. Two ways for modifying the electronic conductivity are discussed. One is doping and the other by change of stoichiometry. Doping is achieved by ion implantation. Change of stoichiometry is generated by applying a high voltage to partially blocking electrodes. The latter are made of gold. Doping is shown to modify the total conductivity introducing electronic conduction by a hopping mechanism. The effect of change in stoichiometry is examined both with respect to the SE conductivity and to the electrode overpotential. It is shown that an increase in the electronic conductivity of the SE reduces the electrode overpotential, but has also an observe effect on the ionic conduction in the SE.

  Only part of the SE surface is involved in the electrode reaction, it is the part near the electrode pores edge denoted as the triple phase boundary (TPB). Changes in the SE surface conductivity affect the TPB width. Both doping and changes in the stoichiometry change the electron Fermi level. Changing the Fermi level in the SE surface may affect charge transfer process if they occur on the SE rather than on the electrode material.  We have developed a photo lithographic process for preparing electrodes with well defined patterns down to 2mm having a different TPB length. These electrodes are meant to allow examination of the dependence of the overpotential on TPB length. The method was applied to gold electrodes. Preliminary tests with electrodes prepared by this method are described