In situ synthesis of methane using Ag–GDC composite electrodes in a tubular solid oxide electrolytic cell: new insight into the role of oxide ion removal

Abstract

The conversion of waste CO₂ into energy carrier fuels (“electrofuels”) using renewable energy (RE) in solid oxide electrolytic cells (SOECs) is a fast-emerging technology. Methane is one such potential electrofuel under consideration for the transport and local storage of RE. Most of the synthetic methane generation routes under investigation are two-step processes utilizing SOECs as a source of either H₂ or syngas (H₂/CO mixture) that undergoes methanation in a subsequent thermochemical reactor. However, the technology for direct one-step in situ synthesis of methane in SOECs is still at an early stage. This work demonstrates, for the very first time, purely electrolytic one-step methane generation in a symmetric, tubular SOEC in the temperature range of 500–700 °C in the absence of any methanation catalyst simply by electrolysing a mixture of H₂ and CO₂. The non-attainment of methane at OCV and in contrast methane generation under applied potential indicate that the phenomenon is completely driven by electrochemical processes. Interestingly, at all temperatures, the first trace of methane was detected at a certain minimum value of current density, and that value of current density increased non-linearly with temperature. The extent of methane generation appears to be effectively shifted by increasing the rate of oxide ion removal from the cell. Thus, we hypothesize that the electrochemical oxygen pumping phenomenon is a facilitator of such a direct methane synthesis reaction envisaged during in situ methanation in SOECs.