SOC Degradation: Long-Term and Small-Scale Effects

Abstract

In the last ten years, various stack tests were operated at Forschungszentrum Jülich with duration times of 15,000h to 100,000h in fuel cell and electrolysis mode. Intensive post-test analysis and partial impedance analysis during stack operation revealed degradation effects with respect to the electrodes, the electrolyte, and the cell contact. In this paper, we present results from four different stacks and their various levels of degradation, which are mostly diffusion-related. In particular, three different elements were identified as causing visible degradation effects: Ni in electrolysis mode, and Mn and Sr in fuel cell mode. In electrolysis mode, Ni migrates away from the electrolyte/electrode interface, thus increasing electrolyte thickness and enhancing ohmic cell resistance. In fuel cell mode, Mn originating from various sources diffuses towards the electrode/electrolyte interface and leads to foreign phase formation. This in turn results in electrolyte cracking or detachment. Finally, Sr separates from Sr-containing air electrode materials and diffuses towards the electrolyte where it also forms secondary phases.