Understanding the processes governing performance and durability of solid oxide electrolysis cells.

Area of Science:

• Electrochemistry
• Materials Science
• Energy Conversion

Background:

• Solid Oxide Cells (SOCs) are crucial for reversible energy conversion.
• Understanding performance differences between fuel cell (FC-mode) and electrolysis cell (EC-mode) is vital.
• Ni-YSZ electrodes are commonly used in SOCs.

Purpose of the Study:

• To investigate the operational characteristics of Ni-YSZ electrode supported SOCs in both FC-mode and EC-mode.
• To analyze the kinetics and resistances under various gas mixtures and temperatures.
• To evaluate the impact of electrode porosity on SOC performance, particularly in EC-mode.

Main Methods:

• Polarization characterization of SOCs at 750 °C, 800 °C, and 850 °C.
• Testing with H2O/H2, CO2/CO, and mixed H2O/CO2/H2/CO gas atmospheres.
• Systematic variation of support structure porosity to assess diffusion and kinetic effects.

Main Results:

• Reduction kinetics (H2O, CO2) are slower than oxidation kinetics (H2, CO).
• CO2/CO mixtures exhibit slower kinetics than H2O/H2 mixtures.
• Diffusion resistance and low-frequency concentration resistance are significantly higher in EC-mode, leading to decreased activity.
• Support structure porosity directly impacts diffusion and kinetic resistances, especially under current application.

Conclusions:

• Kinetic and diffusion limitations are more pronounced in EC-mode operation.
• Electrode porosity plays a critical role in mitigating diffusion limitations in EC-mode.
• The findings suggest that general kinetic expressions for Ni-YSZ electrodes may be insufficient due to varying diffusion effects.
• Diffusion limitations may contribute to cell degradation by creating an increased reducing atmosphere at the interface.