Study of electrochemical performance of strontium doped lanthanum cobalt oxides using electrochemical impedance spectroscopy and microelectrode array cell design.

摘要

Mixed ionic-electronic conducting materials (MIEC) with perovskite structure are promising cathode candidates for intermediate temperature Solid Oxide Fuel Cells (SOFC) operating between 600°C∼800°C. Due to its mixed conducting nature, the electrode performance is controlled by oxygen surface exchange reaction, bulk diffusion and/or surface diffusion processes, and electrode porous structure. Understanding the correlation between performance control mechanisms and electrode's physical and chemical properties is crucial for practical applications. This study tries to provide more quantitative and qualitative understanding of performance limiting processes, kinetic and transport properties of La1-xSr xCoO3-delta (LSC) materials under practical operating and material processing conditions. This information will benefit the engineering design and development of MIEC porous electrodes for high temperature electrochemical applications.;The electrochemical performance of La0.8Sr0.2CoO 3-delta and La0.8Sr0.2CoO3-delta were studied under various temperature and gas conditions by electrochemical impedance spectroscopy (EIS) using a novel microelectrode array test cell design. This design effectively reduces impedance measurement error induced by reference electrode placement in thin planar test cells. Effects of firing temperature on electrode's porous structure and surface chemical conditions were characterized with SEM, BET, and FIB-SEM. Long term degradation behavior was also studied by EIS. The chemical impedance was quantitatively characterized using the apex height and characteristic frequency from measured impedance spectra. The surface exchange rate, effective oxygen vacancy diffusion coefficient, and active electrode thickness were calculated using a two dimensional bulk diffusion model. The results show that the active electrode thickness is comparable to the electrode's pore structure. The kinetic and transport properties depend on electrode's chemical composition, processing condition (firing temperature), and testing conditions. The oxygen transport process of La0.6Sr 0.4CoO3-delta is mostly under solid phase oxygen vacancy diffusion control. For La0.8Sr0.2CoO3-delta' the bulk diffusion transport assumption breaks down. The results suggest that the surface diffusion pathway may have significant contribution to the oxygen diffusion transport process and overall electrode performance.