Authors: Teruhisa Horita
Source: ECS Meeting Abstracts (article, 2025-11-24)
Field: Materials Science — Materials Chemistry

Progress on the Evaluation Methods for Durability of Advanced SOFC Stacks Introduction Among many technical issues to commercializing the SOFC system &amp; stacks, durability is one of the key factors to achieve reliability of SOFC systems. From FY2020, the NEDO Japanese national project has started to establish the evaluation and analytical methods of durability in the advanced SOFC stack with long-term operation and high efficiency. The goal of this project is the following three points: (1) to develop the advanced evaluation and analytical methods for the cell stacks which show a lifetime over 15 years (130 kh) with high efficiency of over 65% LHV, (2) to develop the evaluation and analytical methods for dynamic operation mode such as rapid start-stop and load cycling, and (3) to develop the advanced evaluation methods for cell-stack durability. This paper reports the recent progress of this project. Project scheme The project is conducted by the collaboration consortium style among industries, universities, and laboratories. The research topic covers the fundamental mechanism of degradation of materials, cells, and stacks to the application level of specific technologies in each company’s cell stack. Therefore, we have strict data information control by the open-close strategy in the consortium. Figure 1 shows a schematic diagram of the project structure. Three themes are constructed to facilitate the research; 1) Developments of evaluation methods of high-efficient &amp; long-life stacks, 2) Dev. of evaluation methods for multi-purpose applications, and 3) Advanced analysis methods for stacks. Results and Achievements Durability tests of practical stacks T The stack developers supplied three practical stacks and tested them for nearly 20,000 hours with high fuel utilization (Uf~85%). Relatively stable performances were reported for these stacks (less than 0.4% /1000 hrs of the voltage drop) and durability factors were considered with degradation mechanisms at cells and stacks. Even in high fuel utilization, the main factors in the stacks are the cathodic overpotentials and ohmic resistances. These resistance increases were distinct in the long-term operation. At the long operation over 15,000 hours, sometimes stacks showed an increase of anodic polarization due to the Ni migration in the Ni-YSZ anode. This long-term operation of the stack showed the first demonstration of high fuel utilization and it showed the possibility of high-efficiency operation in the practical stacks. Advanced simulation methods for stack durability Based on the stack durability tests and degradation mechanism analyses, we first constructed the advanced simulation method. This method includes both multi-scale simulation and several degradation factors. From the nano-meter level of microstructures, 3D reconstruction techniques are applied with FIB-SEM images before and after long-term operation. Triple phase boundary (TPB) active lengths are considered for these microstructures and evaluate the performance of the cells. These concepts are applied to the cell level where several ten mm level sizes. Then, simulation methods are applied to the stack level with the cell to demonstrate the temperature distribution in the stacks. When considering the degradation factors, several factors were also considered, and these factors are related to each other during operation. The advanced simulation first includes the effects of multi-degradation factors in the stacks Accelerated stress testing To evaluate the lifetime of the cell stacks, it is effective to construct the accelerated stress testing. In this project, current density increases were considered at certain time intervals, assuming the local temperature increase at the electrode/electrolyte interfaces. The current density increase surely facilitates the degradation and it is effective to use this method as an accelerated stress testing. Protocols for rapid start and stop A special effort is made to establish an appropriate evaluation method for rapid start-stop in SOFCs. About a 30-minutes start is assumed (R. T. to 600 o C, ramping at 20 K/min) for this evaluation. This rapid evaluation gives a large temperature gradient in the cells, and the evaluation methods included the possibility of cell reliability in terms of mechanical strength. Advanced degradation analyses methods This study applied several advanced degradation analysis methods for precise clarification of the degradation mechanism. Several new degradation mechanisms have been clarified, such as cathode degradation by Sr migration in operation and phase transformation in ZrO 2 -based materials, Ni migration in Ni-YSZ cermet, etc. Especially, new analytical methods are based on AI machine learning for the microstructure image acquisition and evaluation of long-term operation. The presentation will show more information about the measurement methods and the related market. Conclusion Through this project, new evaluation &amp; analysis methods have been developed for the degradation of the advanced stacks (durability of 15 years and LHV 65%). These methods contribute to measuring the degradation rates in normal long-term operation and dynamic operation with load cycling. After completing the new analytical methods, the advanced SOFC stacks were commercialized with the compensation of reliability. The new trend of machine learning was also applied to the image analyses and degradation analyses. Acknowledgement: This paper was supported by the NEDO project (JPNP20003） References Horita, ECS Transactions, Vol.103, (1)1023-1031 (2021). Horita, ECS Transactions, Vol.111 (6),1347-1356 (2023). Horita, Proceedings of the 31 st Fuel Cell Symposium in Japan, 1B6, pp.63-66 (2024). Figure 1 Figure 2 Durability ; Schematic ; Reliability (semiconductor) ; Failure mode and effects analysis ; Evaluation methods ; Stack (abstract data type) Advancements in Solid Oxide Fuel Cells ; Thermal Expansion and Ionic Conductivity ; Fuel Cells and Related Materials