Impedance Contributions in SOEC Electrolysis Cells Based on EIS and DRT

Solid oxide electrolysis cells (SOECs) are among the most promising technologies for hydrogen production, particularly when coupled with renewable energy sources, such as solar and wind power. Operating at high temperatures, SOECs achieve high electrochemical efficiency, effectively converting electrical energy into hydrogen with lower operational costs. However, their long-term stability and performance degradation remain significant challenges, impeding large-scale commercialization. A fundamental factor influencing SOEC performance is the impedance contributions of its components. Therefore, a comprehensive understanding of the electrochemical impedance characteristics in SOECs is crucial for performance optimization and durability improvement.

In this study, electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) were employed to systematically analyze the impedance contributions of various SOEC components. Experimental measurements were conducted under different operating conditions, including temperature, fuel utilization, steam content and oxygen partial pressure at the oxygen electrode. The impedance response of the fuel and oxygen electrodes was analyzed to identify the dominant electrochemical processes and their respective frequency-dependent characteristics.

DRT analysis revealed multiple characteristic peaks, each corresponding to a distinct kinetic process: P1 (1×10³–1×10⁴ Hz) was associated with oxygen ion transport in the Ni-YSZ fuel electrode, P2 (1×10²–1×10³ Hz) corresponded to charge transfer reactions within the fuel electrode, P3 (50–100 Hz) reflected the charge transfer and ionic transport in the LSC oxygen electrode, P4 (1–10 Hz) represented gas diffusion within the fuel electrode, and P5 (0.1–1.0 Hz) was attributed to gas-phase diffusion in the oxygen electrode and gas conversion reactions in the fuel electrode. Based on these findings, an equivalent circuit model (ECM) was developed to accurately describe the electrochemical behavior of the SOEC. The proposed ECM incorporated a combination of resistive and capacitive elements to represent various charge transfer, mass transport and gas diffusion processes. The model successfully quantified the contributions of different polarization losses, offering valuable insights into the dominant limitations affecting the performance of SOEC. Moreover, it is demonstrated that increasing the operating temperature led to a significant reduction in polarization impedance, due primarily to enhanced oxygen ion conductivity and faster charge transfer kinetics. Similarly, increasing the steam content reduced gas-phase diffusion resistance, thereby improving overall SOEC efficiency. Conversely, higher fuel utilization ratios resulted in increased concentration polarization, emphasizing the importance of optimizing gas composition and flow rates for stable long-term operation.

This work provides a detailed electrochemical analysis of SOEC impedance contributions using advanced characterization techniques. The combination of EIS, DRT and ECM modeling offers a comprehensive framework for understanding the key kinetic processes governing the performance of SOEC. The findings contributed to the ongoing efforts to enhance SOEC stability, reduce polarization losses and improve overall efficiency, paving a way for more robust and commercially viable electrolysis systems.