Failure Mode and Effect Analysis of Composite Cathode on Anode-Supported SOFCs under the Accelerated Durability Testing

Compared to other-type of fuel cells, solid oxide fuel cells (SOFCs) have many advantages, such as higher efficiency and performance, environmental friendless, fuel flexibility and no use of precious metal catalysts [1]. For making the commercialization of SOFCs, however there is still forced to improve durability of components with high performance. In particular, researches for durability are only in the early stages, compared to the effort of developing materials to reduce operating temperature with enhanced performance.

Recently, studies of SOFC durability have extended to understand main degradation mechanisms of components. The degradation phenomena of Ni cermets anode is often observed in low concentrations of hydrogen at high current density. Chen et al.[2] reported that the poor tolerance of Ni-ceria cermets may result in delamination of each layer of SOFC components by variation of anode volume. Other researchers examined that chemical reactions of LSM with YSZ during hot-temperature SOFC operations form insulating phases like SrZrO₃ or La₂ZrO₇ [3, 4]. This phenomena lead to decrease SOFC performance with high polarization loss. The humidified air also occurs degradation of cathode materials because of variable hydration properties or formation of secondary phases [5, 6].

In this study, degradation mechanisms of the neodymium doped ceria (NDC) based anode-supported SOFCs with cathodes related to perovskite structure are investigated by accelerated degradation testing methods. As the preparation of experiments, NiO-NDC cermets are used for anode substrates and NDC is synthesized by combustion process [7]. The NDC slurries are deposited on the substrate as an electrolyte. These green pellets are sintered at 1550 ^°C for 4 h with a 2 ^°C min^-1 ramp rate in air. To compare influences of durability on the cathode materials, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and Nd(Ba_0.5Sr_0.5)Co_1.5Fe_0.5O_5+δ (NBSCF) are prepared with NDC. The mixtures are screen-printed on anode-supported cells and sintered at 1150 ^°C for 2 h. A sintered cell is loaded on a reactor and sealed by glass rings.

Durability of anode-supported SOFCs is investigated under various failure modes such as hydrogen and air depletion and highly humidified air. The experiments are carried out periodical turn- on/off of hydrogen (and air) in anode (and cathode) side under constant current density of 180 mA cm^-2 at 650 ^°C. To find main degradation mechanism after failure tests, the impedance spectra and overpotential analysis are performed and the microstructure analysis is also carried out by SEM, TEM and XRD [8, 9]. Those experiments contribute to find design parameters for highly durable SOFC components.

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* Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park).