Electrochemical investigation of the influence of gas compositions on industrial-sized fuel electrode supported single cells under co- and CO2-electrolysis conditions

Abstract

Solid oxide electrolysis cells (SOECs) offer high efficiency for Power-to-X applications, but their performance under varying gas compositions—especially in co- and CO₂-electrolysis—requires further clarification. This study addresses this challenge by investigating the electrochemical behavior of fuel-electrode-supported, industrial-sized planar SOECs (81 cm${}^2$ active area) during steam, co-, and CO₂-electrolysis at 800 °C. Two SOECs were examined using polarization curves, electrochemical impedance spectroscopy (EIS) with total harmonic distortion, and Distribution of Relaxation Times (DRT), complemented by temperature and gas composition analysis. In addition, a cell was analyzed with Scanning Electron Microscopy (SEM) prior to experiments to provide supporting structural information on the cell. This comprehensive approach enabled detailed insight into reaction mechanisms, resistive losses, and electrode processes under different gas atmospheres. Co-electrolysis experiments were conducted with varying H₂O/CO₂ ratios, using either CO or H₂ in the gas mixture to isolate their individual effects. In CO₂-electrolysis, cells were tested with CO₂/CO ratios of 80/20 and 100/0. Transitions between steam-, co-, and CO₂-electrolysis modes were also studied. The results show that gas composition—particularly the partial pressures of H₂O, CO₂, H₂, and CO—significantly affects cell performance. Lower H₂O/CO₂ ratios increased area-specific resistance (ASR) and led to more endothermic operation. The presence of H₂ maintained ASR near steam electrolysis levels, while CO increased it towards CO₂-electrolysis values. DRT analysis revealed additional peaks related to CO₂/CO processes. CO₂-electrolysis exhibited the highest ASR, indicating greater resistive losses. These findings enhance understanding of SOEC operation under realistic conditions and support their optimization for industrial use.