Experimental and theoretical study of LSM/YSZ cathodes behaviour, based on morphological and electrochemical investigation

An experimental and theoretical study carried out on composite Lanthanum Strontium Manganite (LSM) and Yttria-stabilised Zirconia (YSZ) electrodes is presented. The elctrocatalytic activity of cathodes of different thicknesses is investigated by potentiodynamic polarisation and impedance spectroscopy measurements, extracted in different cell configurations as well as in different operating conditions of temperature, oxygen partial pressure and overpotentials. Both polarisation resistance (in impedance plots) and current (in Tafel plots) show a dependence on electrodes thickness, with the most favourable values at around 40 μm. The electrochemical results obtained under cell polarisation make visible two different kinetic mechanisms in the oxygen reduction reaction: a Butler-Volmer behaviour at lower temperatures (<700°C), and a possible mass transfer controlling step when the temperature increases. Due to the influence of the performances on thickness, the distribution of the morphological properties along the cathode thickness is investigated, observing the scanning electrode microscope images of LSM/YSZ cathodes of different thicknesses. As a consequence of this image analysis, a variation of morphological properties is highlighted along the axial coordinate, suggesting a variation of the solid volume fraction along the cathode thickness. This porosity variation will affect other morphological properties, namely density of active contact sites and tortuosity of electrons and ions paths. A mathematical model is developed, which takes in account the morphological properties and reproduces the dependence of the polarisation resistance on thickness, as determined by impedance spectroscopy. The model characterises charge and mass transfer and electrochemical kinetics using effective parameters (conductivity, diffusivity, exchange current) related to morphology and material properties by percolation theory.