Cr Poisoning in SOC Stacks
Authors: Yu Shangzhe , Schäfer Dominik , Kunz Felix , Eichel Rüdiger-A. 
Published: November 2022

Solid oxide cell stacks (SOCs) usually use a Cr-containing alloy as the interconnect, which requires the air electrode not only to exhibit satisfying electrochemical performance, but also a high resistance to Cr poisoning. This is because during operation volatile Cr species will diffuse from the oxide scales on the interconnect to the air electrode, leading to a fast degradation. Currently, Forschungszentrum Jülich (FZJ) applies an MCF (MnCo1.9 Fe0.1O3–δ) coating by means of atmospheric plasma-spraying (APS) on the interconnect. This coating reduces the degradation rate from 0.9 % / kh to ~ 0.3 % / kh. However, according to distribution of relaxation time (DRT) figures, Cr poisoning is still a major contribution to overall degradation in the beginning of the operation.

In order to further alleviate issues of Cr poisoning, an LSC (La0.6Sr0.4CoO3 –δ) air electrode was investigated. Based on previous studies, the LSC electrode was expected to be more resistant to Cr poisoning compared to LSCF (La0.58Sr0.4Co0.2Fe0.8O3 –δ). Long-term tests demonstrated a lower degradation rate for LSC air electrodes compared to LSCF air electrodes and LSC adsorbed less Cr than LSCF even if the operation time of the SOC stack with LSC air electrode was twice longer [1]. For improving the understanding of the degradation mechanisms of Cr poisoning on LSC and LSCF, an accelerated test was carried out. The test was done with a rainbow stack that used LSC (RU A) and LSCF (RU B) air electrodes in different repetitive units (RU) of the stack. To quickly degrade the air electrodes, no protective coatings were added on the interconnects and 1.4509 stainless steel was used instead of Crofer 22 APU. Besides, the operation condition (775 ºC, 0.75 A/cm2) was harsher than that of previous tests (730 ºC, 0.5 A/cm2). Due to fast degradation, the stack was shut down after only 1,700 h operation. Latest analysis of this stack showed the peak attributed to air electrode polarization in the DRT deconvolution for EIS spectra of RU A cells increased evidently while that of RU B cells barely changed. This was a contradictory finding to previous understanding as it indicated it was LSCF instead of LSC that was more resistant to Cr poisoning. A possible reason might be a lower chemical stability of LSC to volatile Cr species at high temperature and current density. To clearly reveal the reason, Cr distribution and deposited Cr amount in the air electrode needs to be determined. Post-test characterization, such as SEM, EDX and ICP-OES, is undergoing.

In addition, a steady-state multiphysical three-dimensional model of an SOC was developed with the open-source software OpenFOAM. Momentum transfer, mass transfer and electrochemical reactions were successfully included in the model. A coupled solver was applied to solve the ionic and electronic potential across different regions. The model was verified by the comparison of the simulation results of the OpenFOAM model with the simulation results of an equal model developed with the commercial software COMSOL, whereas the same mesh, geometry, and parameters were used. Also, the OpenFOAM model was validated by a comparison of the simulation results with experimental results under different operation modes (e.g., fuel cell and electrolysis modes) and temperatures. Assisted by the model, the spatial distribution of the local mole fraction of oxygen and local overpotential in the air electrode could be revealed, which was helpful in analyzing Cr poisoning. As expected, it was found that the overpotential mainly became non-zero at the interface between the air electrode and the electrolyte. Since Cr is mainly observed near the interconnect interface of the LSCF air electrode in post-test examinations, the Cr deposition is expected to primarily arise from chemical reactions. Plus, owing to higher current density on the rib, the mole fraction of oxygen was also lower under rib, where more Cr deposition can be triggered owing to lower oxygen partial pressure. Micro-kinetic modelling of the reduction of adsorption sites owing to Cr poisoning is under development.

After the governing equations related to degradation will be included, the model will become a transient-state model which is capable of predicting the voltage with evolution of time.

References:

Fang, Q., Menzler, N. H., & Blum, L. (2021). Degradation Analysis of Long-Term Solid Oxide Fuel Cell Stacks with Respect to Chromium Poisoning in La0.58Sr0.4Co0.2Fe0.8O3−δ and La0.6Sr0.4CoO3−δ Cathodes. Journal of The Electrochemical Society, 168(10), 104505.