Morphology and electrochemical activity of SOFC composite cathodes: I. experimental analysis

Abstract

This paper describes the first part of an experimental and theoretical study performed on composite Lanthanum Strontium Manganite (LSM) and Yttria-stabilized Zirconia (YSZ) electrodes. Cathode electrocatalytic activity was investigated using different cell configurations and carrying out potentiodynamic polarisation and electrochemical impedance spectroscopy measurements (EIS). Measurements were carried out at different oxygen partial pressures, overpotentials, temperatures and electrode geometries. In order to identify the main steps involved in cathodic oxygen reduction, the NLLS-Fit procedure was used. The results for different cell geometries agree with each other, suggesting a transition in the overall reaction mechanism, from charge transfer to mass transfer control, at a critical temperature of about 750 A degrees C. The experimental results also show a remarkable effect of electrode thickness on the overall reaction rate, throughout the temperature range tested. A grey level gradient along the thickness of the thicker electrodes were detected by analyzing microscopic images of the cells. These results, together with electrochemical measurements on cathodes with different thickness, confirm that morphology plays a key role in determining the performance of Solid Oxide Fuel Cells (SOFC) composite cathodes

Morphology and electrochemical activity of SOFC composite cathodes: II. Mathematical modelling

“…In contrast with these theoretical results on the effect of thickness on the performance of porous composite cathodes, experimental results presented in a companion paper [14] show a remarkably different behaviour. Polarisation resistance measured at different cathode thicknesses does not shows an asymptotic minimum, rather it is reported to be almost constant for thin cathodes (up to 10 lm thickness), sharply decrease for intermediate thicknesses, reach a minimum for 20 to 40 lm thickness ranges, and slightly increase in thicker cathodes.…”

“…much slower than oxygen diffusion and electron conduction), and (ii) uniform morphological properties are considered over the whole cathode thickness. LSM/YSZ porous composite cathodes experimentally characterised in a companion paper [14] do not show such an asymptotic minimum of polarisation resistance at varying thicknesses. Polarisation resistance measured over a wide range of operating temperatures (800-900°C) is almost constant in thin cathodes (with less than 10 lm thickness), it decreases in the intermediate range of thickness up to a minimum value observed at around 50 lm, then increases for thicker cathodes.…”

“…Variations of Polarisation resistance experimentally observed for different cathode thicknesses cannot be predicted when assuming uniform morphological properties of cathode materials [14]. The mathematical model presented in this work to give insight into these experimental results is based on the following assumptions:…”

“…To give insight to the results reported by Barbucci et al [14], this paper reports on theoretical investigations based on (i) experimental observations on the distribution of morphological properties along the thickness of porous composite cathodes, and (ii) a continuum approach to model SOFC porous composite cathodes accounting for variations of morphological properties along cathode thickness.…”

Synthesis of LSM–YSZ–GDC dual composite SOFC cathodes for high-performance power-generation systems

“…Electrochemical impedance measurements were performed on the symmetrical cells under an air atmosphere by ac impedance spectroscopy (Solartron SI 1260/1287) and cathode durability tests were performed according to the schedule shown in Fig. 2 [18][19][20][21][22][23]. Ni-YSZ anode-supported cells with LSM/GDC-YSZ dual composite cathode were also fabricated.…”