State-of-the-Art La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode for SOFC: Microstructural and Electrochemical Properties
Authors: Celikbilek Ozden , Dessemond Laurent , Djurado Elisabeth 
Published: February 2020

Solid Oxide Fuel Cells (SOFCs) have drawn much attention because they convert chemical energy into electrical energy efficiently in eco-friendly way. The performance of a SOFC depends on several combined factors in each cell compartments (i.e. anode, cathode and electrolyte). Especially the polarization resistance of the cathode becomes significant for intermediate temperature operations (500 – 700 °C). For this reason, the cathode reaction kinetics and influence of microstructure of state-of-the-art cathode materials have been extensively researched through comparative experimental studies and numerical simulations in the literature. Porous cathodes comprising mixed ionic-electronic conductors (MIEC) are commonly preferred in these energy conversion devices operating at intermediate temperature. They grabbed particular attention since the oxygen reduction reaction (ORR) is extended at the electrode/electrolyte interface, beyond the triple phase boundary (TPB) points. Earlier in our group, reaction kinetics of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) deposited by electrostatic spray deposition (ESD) technique was studied in two different microstructures, i.e; the coral and columnar-types [1-3]. Thanks to nanostructured microstructure features, one of the lowest polarization resistance (Rpol) values compared to literature was achieved.
The present study concerns the influence of the thickness of columnar LSCF films on the area specific polarization resistance (ASR) of the cathode. Symmetrical cells of LSCF were deposited on dense Ce0.9Gd0.1O2-δ (CGO) electrolyte. The cathodes were consisted of double layer films of LSCF deposited sequentially by ESD technique as cathode functional layer (CFL) and by screen printing (SP) technique as current collecting layer (CCL) as shown in Figure 1. The thickness of CFL was varied between 0.5 and 18 μm by controlling the time of ESD deposition from 20᾿, 40᾿, 1h, 1h45᾿, 3h and 4h30’. Thickness of CCL was varied by SP from 20 to 50 μm. The resulting electrode thicknesses and microstructures were investigated by scanning electron microscope and microstructural parameters (porosity, specific surface area) were calculated by 3D reconstruction of the images obtained by FIB/SEM tomography.
The electrode performance in terms of ASR as well as current constriction effects were monitored by electrochemical impedance spectroscopy (EIS).
The results are discussed in terms of microstructure, adherence at the electrode/electrolyte interface, conductivity and optimized cathode performance. The hierarchical porous character of CFL is highlighted with macro porosity separating the columns and nano porosity within the columns [4]. The CCL provides a flatter surface than the CFL, thus improving the current collection by increasing the contact points between the surface and current collecting grid. We have shown that the electrochemically active layer in LSCF nanostructured columnar cathodes deposited by ESD concentrates on only a couple hundred nanometers away from the electrode/electrolyte interface [4]. One can conclude that the optimized films require at least 4-5 µm CFL thickness and 36 µm total thickness (CFL-CCL). In this geometry, the optimized ASR value of 0.065 Ω cm2 at 600 °C was achieved for double layer cathode characterized by a porosity amount of approximately 22 % and a specific surface area of 19 μm-1 with a columnar-type microstructure [4]. This ASR value is still amongst the highest specific surface area values achieved for SOFC cathodes.

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Figure 1