Electrolysis of O2-Containing Low Concentration CO2 in Solid Oxide Electrolysis Cells with La-Based Hexaaluminate Cathode for Syngas Production

Abstract

Excessive emission of CO2, the primary greenhouse gas, has been widely believed to be the main contributor to global warming. Recently, direct-air-capture (DAC) of CO2 is emerging as a promising technique to mitigate the greenhouse problem. In this work, a new technology is developed where the CO2/air mixture captured by DAC1 is directly converted into high value-added syngas using solid oxide electrolysis cells (SOECs)2. In this new system, H2O and O2-containing CO2 (CO2/air mixture) are co-electrolyzed for syngas production in the cathode of SOECs, and in the anode, methane is partially oxidized into syngas to decrease the overpotential and electricity consumption.The cathode materials of SOECs need to keep stable under the condition of strong reduction and high humidity. meanwhile CO2 directly captured from ambient air is mixed with oxygen in the cathode, which poses a challenge to prevent the oxidation of cathode materials. Hexaaluminate3 (AAl12O19-δ or AAl12O17-δ), as a kind of composite metal oxides, is considered as one of the most promising high-temperature catalytic materials due to its unique layered crystal structure. Besides, Al3+ in its structure can be partially or completely substituted by transition metal ions, and the valence state and proportion of its elements and content of oxygen vacancy can be adjusted in a large range. Therefore, hexaaluminate catalysts with high catalytic activity can be designed by means of chemical tailoring according to the characteristics of chemical reactions and requirements for catalysts.In this work, La-hexaaluminate is employed as the cathode material for co-electrolyzing H2O and CO2/air mixture to produce syngas. A series of characterization methods are used to evaluate the basic physical properties of hexaaluminate, including phase structure, oxygen vacancy concentration, thermal stability, thermal expansion coefficient and chemical compatibility with electrolyte materials. Moreover, the stability of electrolytic cell in various operating conditions was investigated, the morphology of cathode and anode materials before and after the operation of the electrolytic cell were compared. The reason of performance attenuation of the electrolytic cell was preliminarily analyzed. The new scheme developed in the present work, provides a way for the effective utilization of O2-containing low concentration CO2 captured by DAC, which is not only useful for alleviating the greenhouse effect, but also gains extra fuels with the help of renewable energy.