Chromium poisoning in cathodes of solid oxide fuel cells: the role of current density, humidity, and cathode composition, and strategies for mitigation
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Power generation systems based on solid oxide fuel cells (SOFCs) offer a pathway to a highly efficient and pollution free energy economy. Operation of SOFCs at intermediate temperatures allow the use of metallic interconnects. However, the chromium oxide scale that forms on the metallic interconnect can volatilize and transport and deposit on the cathode, leading to cell performance degradation. The objectives of this dissertation are to understand the role of current density, cathode overpotential, humidity, and cathode composition on Cr-poisoning and suggest mitigation strategies based on this understanding. Conventional (La,Sr)MnO3 (LSM) cathode half-cells have been electrochemically studied to understand the mechanism of Cr-poisoning. The cells have been tested as a function of current density (and thus cathode overpotential), and humidity levels in the oxidant gas. The half-cell measurements have revealed that Cr-poisoning accelerated with cathode overpotential (i.e. current density) and humidity. Microstructural characterization of tested cells found evidence of Cr-rich species at the cathode/ electrolyte interface at high cathode overpotential and humidity. Based on the experimental results, a mechanism of Cr-poisoning has been proposed. With the objective of mitigating Cr-poisoning observed in LSM cathodes, lanthanum nickelate, La2NiO4+δ (LNO) has been studied as an alternative cathode material. Both half-cells and full single SOFCs featuring LNO as the working electrode/cathode, and ferritic stainless steel current collectors have been fabricated. The cells have been tested under the same conditions as the LSM cells. The chromium deposition at the cathode/ electrolyte interface was much reduced for LNO compared to LSM, and the cell performance of cell featuring LNO cathode continually improved with time in contrast to the LSM cell which started to degrade during cathodic current application. Based on the deconvolution of the polarization losses, it was concluded that the higher tolerance of the LNO cathode to Cr-poisoning compared to LSM, can be attributed to maintaining a low cathode activation polarization. The differences in the mechanisms of Cr-poisoning between LSM and LNO has been clarified. A two-pronged strategy combining chromium-tolerant cathodes and interconnect protective coatings is suggested to mitigate long-term performance degradation arising from chromium poisoning.