Effect of Steam to Carbon Dioxide Ratio on the Performance of a Solid Oxide Cell for H₂O/CO₂ Co-Electrolysis

The mixture of H₂ and CO, the so-called syngas, is the value-added product of H₂O and CO₂ co-electrolysis and the feedstock for the production of value-added chemicals (mainly through Fischer-Tropsch). The H₂/CO ratio determines the process in which syngas will be utilized and the type of chemicals it will produce. In the present work, we investigate the effect of H₂O/CO₂ (steam/carbon dioxide, S/C) ratio of 0.5, 1 and 2 in the feed, on the electrochemical performance of an 8YSZ electrolyte-supported solid oxide cell and the H₂/CO ratio in the outlet, under co-electrolysis at 900 °C. The B-site iron doped lanthanum strontium chromite La₀.₇₅Sr₀.₂₅Cr₀.₉Fe₀.₁O_3-δ (LSCF) is used as fuel electrode material while as oxygen electrode the state-of-the art LSM perovskite is employed. LSCF is a mixed ionic-electronic conductor (MIEC) operating both under a reducing and oxidizing atmosphere. The cell is electrochemically characterized under co-electrolysis conditions both in the presence and absence of hydrogen in the feed of the steam and carbon dioxide mixtures. The results indicate that under the same concentration of hydrogen and different S/C ratios, the same electrochemical performance with a maximum current density of approximately 400 mA cm^-2 is observed. However, increasing p(H₂) in the feed results in higher OCV, smaller iV slope and R_p values. Furthermore, the maximum current density obtained from the cell does not seem to be affected by whether H₂ is present or absent from the fuel electrode feed but has a significant effect on the H₂/CO ratio in the analyzed outlet stream. Moreover, the H₂/CO ratio seems to be identical under polarization at different current density values. Remarkably, the performance of the LSCF perovskite fuel electrode is not compromised by the exposure to oxidizing conditions, showcasing that this class of electrocatalysts retains their reactivity in oxidizing, reducing, and humid environments.