Clean and high-efficiency energy conversion devices are receiving great attention to address the global energy problem and achieve carbon neutrality. In recent years, solid oxide electrolysis cells (SOECs), an energy conversion device, have been widely studied for H2O reduction. However, the degradation under long operation is one of the biggest challenges for the commercialization of SOEC. Thus, it is important to clarify the mechanisms and relevant factors contributing to performance degradation of SOEC.One of the most studied degradation phenomena in SOECs is Ni microstructural change in the Ni-YSZ cathode. For example, the coarsening of Ni particles leads to the decrease of TPB and the Ni recedes from the electrolyte leading to an increase of the electrolyte’s thickness that increasing the R ohmic [1,2]. Both of the degradation caused by the Ni migration that led to microstructure change. Nevertheless, the microstructure of Ni-YSZ porous electrode is so complicated that it is difficult to understand the microstructure changes by direct observation. Thus, instead of the porous electrode, the model electrode can be utilized to simplify the microstructure and geometry, which can make the observation of Ni morphological change more intuitive.To avoid the complexity of Ni-YSZ porous electrode, in this study, a three-dimensional porous electrode is flattened into a two-dimensional comb-shaped model electrode, which allows direct observation of the morphological changes that take place in a complex electrode structure.In this study, laser microscope and scanning electron microscope (SEM) were used to observe the morphological change of Ni electrode. Under different conditions, no obvious Ni migration was found after the long-term operation. However, the Ni and YSZ interface was partially separated. This phenomenon may be caused by low wettability in SOEC mode, or it may be caused by the accumulation of oxygen vacancies leading to an increase of Ni contact angle to form a gap.Chronoamperometry (CA) and Impedance electrochemical measurements were used to study the electrochemical degradation of patterned cells during long term operation. The polarization resistance increased under different H2O and H2 concentration at 1073 K. At 10% H2O concentration, impedance results show that degradation mainly occurred in the first 25 hours. However, at 5% H2O concentration, the increase in polarization resistance was more uniform during 125h. Similarly, the CA results also show that the current decreased more significantly in the first 25 hours at 10% H2O concentration but maintains a certain upward trend at 5% H2O concentration until 125 hours. This result suggests that higher H2O concentration may lead to faster initial cell degradation. Because higher H2O concentration can cause the microstructural change of Ni/YSZ in a shorter period of time, such as Ni coarsening or Ni-YSZ interface detachment. Experiments with other oxides materials are currently underway. Different ceramics may possess different ionic and electronic conductivity and shows the effect of the interaction between Ni and the ceramics that may affect the Ni microstrutural change. The results will be compared with YSZ electrolyte in the future.AcknowledgementThis study was supported by JST, the establishment of university fellowships for the creation of science technology innovation, grant number JPMJFS2102 and NEDO, grant number JPNP16002.Reference[1] D. The, S. Grieshammer, M. Schroeder, M. Martin, M. Al Daroukh, F. Tietz, J. Schefold, A. Brisse, Microstructural comparison of solid oxide electrolyser cells operated for 6100 h and 9000 h, J . P ower Sources , 275 (2015) 901-911.[2] M.P. Hoerlein, M. Riegraf, R. Costa, G. Schiller, K.A. Friedrich, A parameter study of solid oxide electrolysis cell degradation: Microstructural changes of the fuel electrode, Electrochim . Acta , 276 (2018) 162-175.
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Jan 18, 2025
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