Accelerated calender life time testing for SOFC: Impact of overpotential

Kurzfassung

Substantial research activities in the last decades were dedicated to improve the durability of Solid Oxide Fuel Cells (SOFCs) to reach commercial viability targets. However, a lifetime of 40.000 – 80.000 h is needed to be economically competitive with current technologies and the estimation of lifetime as well as reliability of cells still remain critical issues [1]. Accelerated tests (AT) are a valuable approach to derive reliability and lifetime information and are routinely applied for various technologies, e.g. batteries and microelectronics [2]. Different methodologies are employed in this area, i.e. qualitative ATs to assess device reliability, and quantitative ATs which perform testing at a specific accelerating parameter and utilize models to allow extrapolation to the nominal operating level [3]. Yet, so far this method is not commonly used in the field of SOFC. Many experimental results show that in the state-of-the-art SOFC technology, microstructural changes of the fuel electrode (Ni coarsening, loss of Ni-Ni contact) and Cr-poisoning of the oxygen electrode seem to be the core degradation phenomena [4,5]. While the fuel electrode was shown to dominate the initial degradation, the contribution of the cathode progressively takes over. The initial degradation rate often seen in SOFC voltage degradation tests causes challenges in reporting degradation rates and forces long-term measurements due to the underlying coarsening phenomenon [6]. Therefore, it is certainly beneficial to focus on accelerating the initial degradation trend to significantly shorten needed testing times. Various previous studies - not only on Ni-YSZ electrodes but also on Ni-supported catalysts - suggest the use of steam partial pressure in the fuel and temperature as possible acceleration variables [7]. However, as SOFC can be operated in either galvano- or potentiostatic mode, current density or overpotential are additional relevant parameters in the context of ATs. This study is dedicated to evaluate the impact of fuel electrode overpotential on fuel electrode degradation via comparison of four 1000 h tests and proposes a methodology to assess this specific parameter for durability tests and lifetime prediction.