Electrical Characterization of Ultrathin Film Electrolytes for Micro-SOFCs

Abstract

The reliability of solid oxide fuel cells (SOFCs) particularly depends on the high quality of solid oxide electrolytes. The application of thinner electrolytes and multi electrolyte layers requires a more reliable characterization method. Most of the investigations on thin film solid electrolytes have been made for the parallel transport along the interface, which is not however directly related to the fuel cell performance of those electrolytes. In this work an array of ion-blocking metallic Ti/Au microelectrodes with about a 160 µm diameter was applied on top of an ultrathin (1 µm) yttria-stabilized-zirconia/gadolinium-doped-ceria (YSZ/GDC) heterolayer solid electrolyte in a micro-SOFC prepared by PLD as well as an 8-µm thick YSZ layer by screen printing, to study the transport characteristics in the perpendicular direction relevant for fuel cell operation. While the capacitance variation in the electrode area supported the working principle of the measurement technique, other local variations could be related to the quality of the electrolyte layers and deposited electrode points. While the small electrode size and low temperature measurements increaseed the electrolyte resistances enough for the reliable estimation, the impedance spectra appeared to consist of only a large electrode polarization. Modulus representation distinguished two high frequency responses with resistance magnitude differing by orders of magnitude, which can be ascribed to the gadolinium-doped ceria buffer electrolyte layer with a 200 nm thickness and yttria-stabilized zirconia layer of about 1 µm. The major impedance response was attributed to the resistance due to electron hole conduction in GDC due to the ion-blocking top electrodes with activation energy of 0.7 eV. The respective conductivity values were obtained by model analysis using empirical Havriliak-Negami elements and by temperature adjustments with respect to the conductivity of the YSZ layers.

Identification of an Actual Strain-Induced Effect on Fast Ion Conduction in a Thin-Film Electrolyte

“…Since the conductivity of the Nb-STO substrate is much higher than GDC thin films (σ­(Nb – STO) ≅ 150 Ω –1 cm –1 at room temperature), the contribution of pure ohmic resistance is negligible, and thus, the impedance spectra generally contains two major components consisting of a parallel RQ element (GDC film) at high frequencies and a constant phase element (electrode reaction) at low frequencies. For out-of-plane conductivity measurements, grain boundary contributions are not commonly observed, owing to a parallel conduction path to the grain boundary (no conduction across the grain boundary); thus, the overall impedance spectra can be divided into two contributions: resistance to ionic migration through the grain of the thin film at high frequencies and the interface resistance at the interface between the thin film and electrode at low frequencies. − Therefore, the resistance and capacitance of GDC thin film can be estimated by fitting the impedance spectra using the equivalent circuit in the inset of Figure a. The calculated capacitance of GDC thin film is in the range 3–70 nF, which corresponds to the value calculated using the relative permittivity of GDC and the geometric factor for the out-of-plane direction (A/l ≅ 7 m).…”