No abstract

High-performance oxygen electrodes for solid oxide fuel cells (SOFC) require a highly efficient material selection for the oxygen reduction reaction that takes places near the cathode/electrolyte interface. La1-xSrxCo1-yFeyO3-δ (LSCF) delivers fast oxygen-exchange kinetics and is therefore utilized as cathode material for high performing state-of-the-art anode-supported fuel cells. The main disadvantage of LSCF is its instability against Zr-based electrolytes. A diffusion barrier interlayer consisting of Gd doped cerium oxide (GDC) is therefore inserted to prevent the formation of an insulating SrZrO3 phase. This study presents new results on the importance of the GDC manufacturing conditions. A complex interaction of GDC with LSCF and YSZ during fabrication is found with a tremendous influence on the performance of the cathode/electrolyte interface of SOFCs.

The oxygen reduction reaction of mixed ionic-electronic conducting (MIEC) cathodes Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is investigated by detailed electrochemical impedance spectroscopy and focused ion beam tomography. The oxygen transport parameters of these materials are usually determined for model systems, such as dense bulks or thin films. However in the present study, probable differences regarding the time and thermal history of the samples (e.g. ambient poisoning gases, grain coarsening, secondary phase or surface segregation, etc.) were avoided by the in situ sintering of electrodes with nominal stoichiometry under synthetic air. The microstructural parameters of the electrodes are obtained by 3D FIB-SEM reconstruction tomography and subsequently used in combination with the Adler-Lane-Steele analytical model to calculate a simulated cathode resistance. Large discrepancies are observed compared to the electrochemical impedance measurements. In particular, the electrochemical impedance measurements do not show a Gerischer behavior, as expected for MIEC materials controlled by a coupled surface-exchange and bulk diffusion. Analysis by distribution function of relaxation times (DRT) reveals four individual processes taking place, indicating a surface-exchange controlled behavior with a reaction zone similar to the particle size. Bulk diffusion (D) and surface-exchange (k) coefficients from literature are critically discussed and tentative surface-exchange coefficients (k) for both MIECs are given. High-temperature devices such as solid oxide fuel cells (SOFCs) and oxygen transport membranes (OTMs) are promising systems for working towards zero-emissions power generation. The selected materials require high chemical and microstructural stability, as well as high catalytic activity during the oxygen reduction reaction (ORR), under normal operational conditions (i.e. operational temperature and oxygen partial pressure). Particularly interesting are the mixed ionic-electronic conducting (MIEC) materials, based on the ABO 3 perovskite structure. Their material properties can be flexibly customized, giving solid solutions with high ionic-electronic conductivity and hence good transport properties. More specifically, La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) has become established as the stateof-the-art SOFC cathode, while Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) has presented outstanding oxygen permeation flux in its cubic phase. 1-3The surface-exchange coefficient (k) and the oxygen diffusion coefficient (D) provide key information about the oxygen transport mechanism that dominates the cathodic ORR. Many publications have assessed the D and k values of MIEC materials, most commonly by; electrical conductivity relaxation (ECR) measurements for dense bulks [4][5][6][7][8] and temperature for LSCF cathodes. 17 Here it should be pointed out that the well-known ALS model can only be applied if the measured impedance shows a Gerischer impedance, which indicates tha...