Catalyst-induced enhancement of direct methane synthesis in solid oxide electrolyser

CO₂ valorisation to fuels and chemicals in solid oxide electrolytic cells (SOECs) has been a topic of great interest for the past few years. Methane is one such highly valuable fuel, but its in-situ generation in SOECs is an emerging technology. For the first time, we explain how the introduction of a methanation catalyst metal into the cathode affects the electrokinetics of in-situ methane generation in an electrolyte-supported symmetric tube cell while electrolysing a mixture of H₂/CO₂. Fe–Ag and Fe–GDC–Ag electrodes were studied in the applied voltage range of 1.3–1.8 V and temperature range of 500–700°C. While Fe guaranteed good catalytic activity towards methanation reactions, Ag ensured good electrical conductivity of the electrode. Addition of GDC to the cathode remarkably improved CO₂ electrolysis and modified Fe work function under loaded conditions, thus increasing methane generation to a maximum of 2.5% at 600°C and 1.6 V. For the Fe–Ag electrode, gaseous mass transport at the cathode was the rate-limiting step up to ~1.4 V, whereas at higher potentials, CO₂ electrolysis at the triple-phase boundary became the rate-determining step. Short-term cell testing at 1.7 V and 500°C revealed the presence of a Fe₃C phase on the cathode, which affected the electrochemical performance of the cell.