M.Sc Thesis


M.Sc StudentTzvia Beitner
SubjectDetermining the Active Cathode Area in Solid Oxide Fuel
Cells
DepartmentDepartment of Energy
Supervisors Professor Tsur Yoed
Professor Emeritus Riess Ilan
Full Thesis textFull thesis text - English Version


Abstract

In order to improve the performance of solid oxide fuel cells, there is a need to deepen our understanding of the cathode reaction, which is rate-limiting. The purpose of this work is to develop and demonstrate a direct electrochemical technique of measuring the cathode triple-phase boundary between electrode, electrolyte and gas phase.

Interdigitated gold micro-electrodes were fabricated on yttria-stabilized zirconia as gold is ion-blocking, allowing isolation of the active cathode area to the triple-phase boundary.

A geometric model was developed to provide a mathematical relation between the resistance of oxygen ion motion between two interdigitated electrodes and their spacing, from which it is possible to find a number value for the width of the triple-phase boundary at given conditions of temperature and dc bias.

Three-electrode measurements were performed, allowing electrical measurement between two interdigitated gold electrodes while at the same time driving dc current between the gold electrodes and a silver counter electrode. Electrochemical impedance spectroscopy and cyclic voltammetry measurements were performed. Electrochemical impedance spectroscopy results demonstrate two main features - a large low-frequency arc and a smaller high-frequency arc.

The effect of varying the measurement temperature, dc bias and inter-electrode spacing on the impedance spectroscopy results was examined. The results suggest that the low frequency arc reflects the electrochemical process at the gold electrodes, as it is strongly affected by dc bias and does not appear to be affected by varying the inter-electrode spacing. The high frequency arc reflects the impedance of oxygen ion motion in the electrolyte, as it shows only a weak dependence on the dc bias but is strongly affected by varying the inter-electrode spacing.

The impedance spectra are analyzed using the Impedance Spectroscopy Genetic Program developed in Professor Yoed Tsur’s group. Of particular interest is the resistance of the high frequency feature, as this is determined to be the relevant parameter for obtaining information about the triple-phase boundary width. In order to obtain the triple-phase boundary width, impedance spectroscopy must be measured under identical conditions for several interdigitated electrode pairs with different spacings between the stripes. It is noted that due to the logarithmic dependence of the resistance on the inter-electrode spacing and the small number of electrode spacings examined, the error in the measurement of the triple-phase boundary width is large, even for high-quality data. It appears that the triple-phase boundary width decreases with increasing temperature and increasing negative dc bias, whereas it increases with increasing positive dc bias. However, at this point it is impossible to determine this with certainty, due to the large error in the obtained values for the triple-phase boundary width. The triple-phase boundary width was determined to be on the order of 10-1 - 10-2 µm at 300ºC.


Cyclic voltammetry measurements confirm ionic motion in the electrolyte at the temperatures used in this work, and demonstrate a build-up of potential between the two interdigitated Au electrodes under applied dc bias, consistent with the simulated and measured results for the potential distribution of the system in the three-electrode setup.