(Invited) Elucidation of Degradation Mechanisms of Ni Cermet Electrode during Co-Electrolysis Using Cross-Section Model Electrodes
Authors: Sato Takashi , Bisaka Nanako , Budiman Riyan Achmad , Yamaguchi Mina , Yashiro Keiji , Kawada Tatsuya 
Published: December 2024

One of the main problems in the commercialization of solid oxide H2O/CO2 co-electrolysis cells is performance degradation during long-term operation which mainly comes from the nickel migration and carbon deposition in the fuel electrode. In order to suppress the degradation, it is essential to understand its mechanism. However, practical porous cermet electrodes have a complex three-dimensional structure, making it difficult to separate the multiple causes of degradation. Therefore, a model electrode is used to understand the degradation phenomena. While previous studies used glid pattern electrodes [1], we use comb-shaped model electrodes. We developed a comb-shaped Ni electrode on the YSZ thin film that simulate a porous Ni/YSZ into a flat surface [2]. The comb model electrode has a structure in which anode and cathode thin-film electrodes are formed on a YSZ thin-film in the form of a comb. It allows the mechanism of Ni migration and carbon deposition to be revealed in the cross-sectional direction. To further understand the degradation mechanism, the morphological change of the electrode was captured in real-time by operando evaluation using a comb-shaped model cell to evaluate the distribution of migrated nickel and deposited carbon.
The model electrode was fabricated by sputtering YSZ thin film on single-crystal MgO, on which thin-film patterned electrodes of Ni and Pt were fabricated by photolithography. Ni acts as WE (working electrode) and Pt as CE (counter electrode), WE and CE are 40 mm apart. To eliminate the contribution of the counter electrode, the Pt reference electrode was deposited in the middle of the YSZ electrolyte. The measurements were done under two different conditions: 1. Water electrolysis and 2. Co-electrolysis. The measurement was carried out at 1073 K with 10% H2O + 1% H2 + 99% Ar for water electrolysis and 10% H2O + 10% H2 + 20% CO2 + 70% Ar for co-electrolysis. The long-term measurements were performed for 100 h at -0.462 V from RE (reference electrode) to WE in water electrolysis, -0.516 V from RE to WE in co-electrolysis where the impedance measurements were taken for every 5 h by electrochemical workstation (Biologic SP-300). During the measurement, a laser microscope (Keyence VK-X3000, Japan) recorded the microstructure change for 100 h.
In both steam electrolysis and co-electrolysis, the tip of the Ni electrode was raised by electrolysis, suggesting that Ni was exfoliated from the YSZ by the electrolysis of water vapor and carbon dioxide. In the co-electrolysis, carbon deposition was observed at the tip of the Ni electrode by EDX and EPMA results. This carbon deposition is concentrated at the tip of the comb, suggesting that the electrode reaction is concentrated at the interface between the fuel electrode and the electrolyte. The calculated reaction area length obtained from the transmission line model (TLM) was close to the range where the electrode was delaminated and carbon deposition was observed, suggesting that more quantitative analysis would allow the comb model electrode to reveal the conditions under which degradation phenomena occur.

Acknowledgement

This study was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, grant number JPNP16002.

References

[1] Z. Ouyang et al., Journal of Power Sources, 529 (2022) 231228
[2] R. A. Budiman et al., ECS Trans., 111(6) (2023) 1509