The Electrochemical Influence and Oxycen Exchange Properties of Mixed Conducting Electrode Materials on Solid Oxide Electrolytes

Solid electrolytes based on oxygen ion conductors find extensive applications as devices for oxygen sensor, oxygen pumps, solid oxide fuel cells and electrocatalytic reactors. When using ceramic ionic conductors for applications, frequent attempts are made to improve the performance of the electrochemical system by apply'rig mixed conductors as thin top layers instead of the conventional ir~ert noble metal electrodes [ 1-4]. A development of a new oxygen ion conductor will therefore be accompanied by research for suitable electrode materials. It has been argued that materials most suited for electrode applications are to be found among the mixed conducting oxides [4,5]. One of the mixed conducting oxides studied is terbia-stabilized &bismuth(III) oxide as it follows from earlier research that the Bi203 based electrolytes most probably have good oxygen exchange properties. This material was also chosen for its compatibility with the electrolyte used i.e. Bi203 stabilized with 25% Er203 (BE25). Probably in this way interfacial polarization effects between the mixed conducting oxide and ~he solid electrolyte can be minimized [ 6 ]. In order to study the performance of this system, current against overvoltage measurements were carried out. An attempt has been made to check the influence of the electrode electrolyte interfacz on the performance of the electrode. The measurements were made using a disk shaped BE 25 electrolyte samples with a diameter of 12 mm and with a working electrode of 400 nm thickness sputtered on one side of the electrolyte. Bi203 stabilized with 50% Tb203 (BTS0) was used as electrode. The electrode electrolyte combinations were annealed at 1023 K. The samples were placed in a three electrode celt with gold annular working and counter electrodes ef I0 mm outer diarac~er and 6 mm inner diameter making contact on either side of the electrolyte sample. A point shaped reference electrode was placed in the center of the working electrode. This configuration was chosen for it minimizes the potential drop between the reference and the working electrode [ 7 ]. To ensure good contact between the contacts of the cell and the sample, porous gold layers of 300-400 nm of the same shape and size as the contacts of the cell were sputtered onto the electrolyte/electrode combination. To study the influence of the electrode eiectroiyte interI%ce, a full circular layer of BT50 or an annular shaped BY50 layer of 10 turn outer and 5 mm inner diameter were suputtered onto BE25 samples, Using these two electrode structures for current overvoltage (I-V) measurements a difference in the virtual position of the reference electrode is generated, in case of the ring shaped BT50 electrode, the polarization due to the electrolyte elec-