Carbon dioxide capture by direct methanation in co-electrolysis using solid oxide cell

Reducing carbon emissions in the global economy's energy and industry sectors necessitates not just the implementation of innovative low-emission technologies but also the adoption of techniques for capturing and recycling CO2. The Power-to-X conversion allows the path to the reuse of CO2 originating from fossil fuels burning and other industrial processes, hence reducing the emission of the key greenhouse gas. The use of high-temperature electrochemical devices for such purposes is a promising solution, which has been already deployed as subsidized demo projects. In the present work, the direct electrolysis of the H2O-CO2-H2 mixtures with in situ conversion of CO2 and CO to CH4 was effectively conducted in the temperature range of 575–650 °C using commercial solid oxide half-cell with YSZ-type electrolyte on Ni-YSZ support with a custom lanthanum‑strontium cobaltite anode. The degree of carbon conversion in the atmosphere enriched with hydrogen was close to 90%, but under such conditions, the products are diluted to 5–6 vol% in H2. Reducing the temperature leads to a sufficient increase in the CH4 outcome, but it is constrained by the decline of the cell performance. Electrochemical processes were monitored by impedance spectroscopy and analyzed using the distribution of the relaxation times technique. Analysis of the distribution of relaxation times of impedance spectra showed that two parallel competitive processes can be attributed to CO2 conversion. Peak with characteristic time c.a. 0.1 s might be associated with diffusion-like mass transfer through cermet support, and it plays an important role in the electrochemical conversion at elevated temperature and high current density. Conversely, the peak observed in the vicinity of 5 ms becomes the limiting stage at lower temperature and probably is related to charge transfer or absorption and surface diffusion of carbon-containing species. Electrolysis current density might reach 0.4 A cm−2 even at 600 °C without explicit damage to the electrolyte or electrodes. A brief degradation test demonstrated a minor loss in the electrochemical performance of the cell. Thus, it was demonstrated that direct electrochemical conversion of CO2 to CH4 can be achieved with sufficient productivity using a semi-commercial cell with an active surface of 16 cm2 under conditions compatible with those existing in the state-of-the-art stacks of solid oxide cells.