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.

Solid oxide fuel cell (SOFC) is a highly efficient electrochemical device that can be used to directly convert chemical energy of hydrogen and hydrocarbon fuels into electricity in an environmentally friendly manner. While the typical operating temperatures of SOFCs have been reduced to 600–850 ℃, ferritic stainless steels (FSS), such as ZMG 232, AISI 441, SUS430, etc., can be utilized as interconnect materials, due to their high electrical conductivity, low material/manufacturing cost, excellent mechanic strength and similar coefficient of thermal expansion (CTE) to other cell components. However, it still exists several issues during their operation at 600–850 ℃, including continuous growth of Cr₂O₃ scale, Cr migration to cathode and dimensional tolerance at interconnect-cathode interface. These issues will cause serious performance degradation for stacks. To solve these problems, a dense protective coating and a porous contact layer are typically prepared between the metallic interconnect and the cathode. The dense protective coating is utilized to inhibit the growth of Cr₂O₃ scale and prevent Cr migration to the cathode, while the porous contact layer can provide better electrical pathway to decrease the power losses and compensate for the dimensional tolerance between the interconnect and the cathode.

In this study, MnCoNiFeCu alloy powder was selected as the precursor materials for dense protective coating and porous contact layer. A coating/contact dual-layer structure was fabricated through reactive co-sintering in the simulated SUS 430 interconnect/coating/contact/cathode/cathode support test cells. The precursors were calcined in air at 900 ℃ for 2 h to study phase evolution and microstructure of the protective coating and the contact layer. The phases in the sintered layers were characterized by using X-ray diffraction (XRD), whereas scanning electron microscopy (SEM) featured with energy-dispersive spectroscopy (EDS) was utilized to analyze the microstructure and obtain the compositional information. The test cells were fabricated to examine the electrical performance of the dual-layer structure via Area-Specific Resistance (ASR). After the initial sintering, the furnace temperature was then dropped to 800 ℃ for isothermal oxidation for 1000 h and ten thermal cyclic test. The tested samples were then epoxy-mounted, sectioned, and polished for examining cross-sectional microstructure after oxidation. EDS line scans near at the interconnect-coating and contact-LSM cathode interfaces were obtained to identify the possible interdiffusion between the cell components. Furthermore, the effectiveness of the dual layer in inhibiting the growth of the Cr₂O₃ layer and blocking Cr migration from the interconnect to cathode was also assessed.

XRD results of the protective coating and contact layer after thermal conversion revealed that the sintered samples predominantly consisted of spinel phase, with minor oxides, while no metallic phases were detected. Specifically, the protective coating exhibited a majority of the spinel phase along with CuO and CoO, as confirmed by EDS mappings, indicating the aggregation of Cu and Co on the surface. In contrast, the contact layer primarily contained the spinel phase and CuO, with EDS mappings indicating uniform distribution of Fe, Co, Ni, Mn and Cu, while no delamination was observed. In ASR measurements, the tested sample exhibited stable behavior with an ASR of only 22.04–22.71 mΩ·cm² during the 1000 h isothermal oxidation, while the thermal cycling led a dramatic increase in ASR. Once the ASR measurement was completed, the sample cross-sectional surface was characterized. For isothermal oxidation, a dense protective coating and a relatively porous contact layer were observed between the interconnect and cathode. The dual-layer structure was well-boned with the interconnect and cathode after the isothermal oxidation, indicating that the double-layer structure exhibited exceptional thermal compatibility with the adjacent cell components. Importantly, no Cr was detected within both the contact layer and cathode, further confirming the effectiveness of the double-layer structure in inhibiting the migration of Cr from the interconnect to cathode. Conversely, the thermal cycling test sample exhibited serious cracking at the interface between the porous contact layer and the LSM cathode, which was responsible for the rapid increase in ASR during thermal cycling testing.

In this study, MnCoNiFeCu alloy powder was utilized as the precursor material to develop a dense protective coating and a porous contact layer simultaneously through reactive co-sintering, forming a (Mn,Co,Ni,Fe,Co)₃O₄-based dual-layer structure. The sample exhibited stable behavior with an ASR of only 22.04–22.71 mΩ·cm² after 1000 h of isothermal oxidation, while the thermal cycling led a dramatic increase ASR in. Notably, the growth of the Cr₂O₃ scale was dramatically suppressed, while no Cr was detected in the LSM cathode, confirming the effectiveness of the thermally converted dual-layer structure in blocking the migration of Cr. The HEA used as the dense protective coating and porous contact layer offers several advantages, including uniform microstructure, improved electrical performance, exceptional Cr-blocking capability and simple fabrication process. In the future study, more efforts should focus on optimizing the elements in the precursor alloy to further enhance the uniformity and CTE matching of the dual-layer structure after sintering.