Chromium Poisoning of LaMnO₃‐Based Cathode within Generalized Approach

Abstract

Recent progress of the NEDO project on durability/reliability of SOFC stacks is reported with an emphasis on the achievement of Mitsubishi Heavy Industries' segment‐in‐series cells in which the lanthanum manganite cathode has been improved. Durability tests were made by CRIEPI on their cells with/without doped ceria interlayer to check plausible effects of microstructure change and of chromium poisoning. Improved cells exhibit essentially no degradation for 10,000 h and also strong tolerance against the Cr contamination from stainless steel tubes. These features are discussed within the generalized degradation model developed inside the NEDO project. In particular, the extremely small overpotential can be considered to be effective in lowering the Cr poisoning by reducing the driving forces for the electrochemical Cr deposition at active sites. Insertion of doped ceria is also useful in preventing the Cr deposition or enhancing the volatilization of deposited Cr with cathodically emitted water vapors from ceria. Thermodynamic considerations reveal that the initial composition of LSM cathode is important to determine the microstructure change due to the chromium dissolution into the B‐sites in the perovskite lattice. Discussions are made on a role of doped ceria to prevent deterioration of Mn‐dissolved electrolyte by lowering the Mn dissolution into YSZ.

Current Status of Rapid Evaluation of Durability of Six SOFC Stacks within Nedo Project

“…This is not intrinsic to particular stacks. For example, two MHPS cells are plotted in Fig.1; one is with Cr, the other without Cr (5). This suggests that the cathode degradation behavior affects strongly this correlation.…”

“…For establishing the durability, a test in CRIEPI focuses a longer operation. Although this stack utilizes the manganite-based cathode, recent tests and analyses reveal that essentially no chromium poisoning effects appear even in the presence of chromium sources at the same operation temperature (5,8). Kyocera systems are commercially installed in an order of 10,000 units.…”

“…The presence of interlayer provides a small overpotential which should correspond to the driving force for chromium poisoning. As described elsewhere (3)(4)(5), the ceria-based interlayer can emit water vapors from the electrochemical sites (12) and this prevents the side electrochemical reactions causing the poisoning, namely deposition of Cr 2 O 3 .…”

“…This requires applying more precise techniques to identify such a tiny change. It should be also recognized that there appears a new feature of degradations found in the previous projects (4,5); after removing major sources of degradations, there remain complicated phenomena in which many different factors govern one degradation; some factor acts as poisoning, others to avoid poisoning. To properly simulate such degradation up to 90,000 h, it is necessary to obtain physicochemical understanding of such complicated processes.…”

Recent Achievements of NEDO Durability Project with an Emphasis on Correlation Between Cathode Overpotential and Ohmic Loss

“…Recent NEDO projects on durability/reliability of SOFC stacks , have revealed many interesting features associated with degradation phenomena appearing in a stack level . The first NEDO project in 2005–2007FY revealed that impurities accumulated in cell components show a variety of amount level depending on raw materials of cell components, processing, and operation conditions.…”