Chromium poisoning of the air electrode remains a significant obstacle to the long-term performance of solid oxide fuel cells (SOFCs). Many strategies to mitigate this effect have been investigated. However, they require the introduction and development of new materials and components. Furthermore, these methods do not ensure reliable SOFC performance for a sufficient amount of time for commercial viability. An avenue that has not been previously well explored is the in-situ removal of Cr-rich deposits. Here, electrochemical cleaning, a new poisoning mitigation method, is investigated. During cleaning, a mild anodic bias reverses the electrochemical deposition reactions that form chromium-rich deposits. Chromium vapor species are reformed, freeing up the active sites and recovering cell performance. Cells with LSM/YSZ composite air electrodes were exposed to Cr vapors at 800°C and then subjected to electrochemical cleaning. Changes to cell performance were assessed using current-voltage (IV) measurements. Post-test chromium quantification was conducted using energy dispersive x-ray spectroscopy (EDS). Scanning electron microscopy (SEM) revealed that electrochemical cleaning removes Cr2O3, one of two types of Cr-rich deposits that form in LSM-based cells. Using thermogravimetric analysis (TGA) and x-ray diffraction crystallography (XRD), the chemical decomposition of the other type of Cr-rich deposit, Mn, Cr spinel, was investigated as a deposit removal strategy. MnCr2O4 is not thermodynamically stable below 540°C under pure oxygen, forming Cr2O3 and Mn2O3. It was found that the rate of decomposition is quite low and likely not practically feasible on larger spinel particles. The easier method to fully recover cell performance is to electrochemically clean the cell at a frequency high enough that prevents the formation of a significant amount of Mn, Cr spinel (MnCr2O4). Thus, the potential for a diagnostic tool that determines the onset of spinel formation was investigated. A distribution of relaxation times (DRT) analysis was used to better understand physical changes that occur at the LSM and YSZ phases during cell activation and poisoning. Two DRT peaks were attributed to two specific oxygen reduction reaction (ORR) pathways, supported by known property changes that occur during cell operation. It was found that the deposition of Cr-rich deposits on the YSZ surface causes a positive frequency shift in the DRT peak associated with the ORR pathway starting at the YSZ/gas interface. Finally, the analysis and conclusions found for Cr poisoning in LSM-based cells is applied in a discussion on chromium poisoning mitigation and reversal strategies more Cr-tolerant mixed electronic/ionic conducting (MIEC) electrode materials.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/44784 |
| Date | 24 May 2022 |
| Creators | Sugimoto, Michelle |
| Contributors | Pal, Uday |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
Page generated in 0.0931 seconds