Understanding redox cycling behavior of Ni-YSZ anodes at 500 degrees C in solid oxide fuel cells by electrochemical impedance analysis

Abstract

Solid oxide fuel cells (SOFCs) are promising energy conversion devices because of their high electrical efficiency, even for small power systems. However, when the anode is exposed to reduction and oxidation (redox) cycles, the Ni phase causes a large microstructural change as a result of its chemical expansion and contraction. This negatively affects the electrochemical performance. However, most studies have focused on the redox cycling behaviors of SOFCs at high operation temperatures (>= 800 degrees C). Therefore, in this study, we investigate the degradation behavior of the SOFC anode during redox cycles at 500 degrees C. To identify the individual steps of the electrochemical processes of the anode, in-situ monitored impedance spectra were analyzed using the distribution of relaxation time method at various oxygen and hydrogen partial pressures. Consequently, the electrode polarization process was deconvoluted into five sub-processes. During the redox cycles, three major peaks were altered: gas phase diffusion in the anode substrate (10(-1)-10(1) Hz), gas diffusion coupled with charge transfer reaction and ionic transport (10(2)-10(3) Hz) and charged species across the Ni-yttria stabilized zirconia interface at the anode (10(3)-10(4) Hz). The major degradation of the electrode performance at 500 degrees C was attributed to the increase in gas phase diffusion resistance due to Ni phase aggregation and the decrease in porosity in the anode during the redox cycles. This was confirmed by microstructural analysis. By contrast, the other two processes (10(2)-10(3) and 10(3)-10(4) Hz) compensated each other, thus having negligible effect on performance degradation. Graphic abstract