Chromium Poisoning of Cathodes in Solid Oxide Fuel Cells and its Mitigation Employing CuMn1.8O4 Spinel Coatings on Interconnects
Authors: Wang Ruofan , Sun Zhihao , Lu Yanchen , Pal Uday Bhanu , Basu Soumendra Nath , Gopalan Srikanth 
Published: February 2020

Intermediate temperature solid oxide fuel cells (IT-SOFCs) enable the use of chromia-forming alloys in interconnects and balance-of-plant (BoP) materials. However, depending on the SOFC operating conditions, the chromium (Cr) containing species can transport and deposit in the SOFC cathodes and deteriorate their performances. In order to investigate the Cr-poisoning phenomena, anode-supported planar SOFCs with Sr-doped LaMnO3 (LSM) + yttria-stabilized zirconia (YSZ) cathode active layer and LSM cathode current collector layer were fabricated and electrochemically tested in contact with Crofer interconnect at 800 °C. The cathode atmosphere (dry air or 10% humidified air) and current condition (no current or constant cathodic current) were varied in tests on identical cells, and the performance degradations under different test conditions were compared. It was found that both humidity and cathodic current can promote Cr poisoning. Microstructure characterizations also confirmed major amounts of Cr-containing deposits at the cathode/electrolyte interfaces of the cell tested with cathodic current and/or humidity. Free energy minimization calculations and thermogravimetric experiments were performed to determine the chromium vapor species that form over the chromia-forming alloy interconnect and result in the chromium deposition. Based on these results, the roles of humidity and cathodic current in chromium poisoning are evaluated, and a performance degradation mechanism associated to chromium vapor species dissociation at the cathode/electrolyte interface is proposed.

In order to mitigate the effect of Cr-poisoning, a protective Cu-Mn spinel coating was applied on the interconnect surface. The Cu-Mn spinel coating has good thermal compatibility, high stability and electronic conductivity at the SOFC operating temperature. The candidate interconnect material (Crofer22H meshes) with no protective coating, those with commercial CuMn2O4 spinel coating and the ones with lab-developed CuMn1.8O4spinel coating were investigated. The lab-developed CuMn1.8O4spinel coating was deposited on the Crofer22H mesh by electrophoretic deposition and densified by a reduction and re-oxidation process. Anode-supported SOFCs with LSM based cathode with different Crofer22H meshes (bare, commercially coated CuMn2O4, and lab-coated CuMn1.8O4) were electrochemically tested at 800 °C with a constant cathodic current (0.5 A/cm2) for total durations of up to 216 hours, and the cell performances were characterized by current-voltage measurements every 24 hours. Cross sections of the Crofer22H meshes and the SOFC cathodes were examined before and after the cell tests by scanning electron spectroscopy and energy dispersive X-ray spectroscopy. The results of the mitigating effects of the two types of Cu-Mn spinel interconnect coatings on Cr-poisoning of the SOFC cathodes were compared. It was observed that the performance of the denser lab-developed Cu-Mn spinel coating was distinctly better showing significantly less chromium containing deposits at the cathode/electrolyte interface after the test.