Controllable CO ₂ conversion in high performance proton conducting solid oxide electrolysis cells and the possible mechanisms

To solve the increasing greenhouse problem and to achieve sustainable carbon cycling, effective conversion of CO ₂ through chemical or electrochemical ways is key. In this study, efficient and controllable conversion of CO ₂ mainly to CO and CH ₄ has been demonstrated in a proton conducting solid oxide electrolysis cell (P-SOEC) using BaZr _0.8 Y _0.2 O _3-δ (BZY) as the electrolyte and SrEu ₂ Fe _1.8 Co _0.2 O _7-δ as the anode, in which an excellent current density of 1.23 A cm ^-2 at 1.5 V was achieved at 550 °C and 100 hours of smooth operation is demonstrated. Compared with the pure steam electrolysis, impedance spectral investigations indicate that the presence of CO ₂ in the cathode actually accelerates the electrode reactions, in contrast with that in a regular O-SOEC. This may be attributed to the higher adsorption of CO ₂ and more effective conversion of protons over the BZY electrolyte. With the increase of electrolysis current, formation of both CO and CH ₄ are enhanced, contradictory to the deduction based on thermodynamic calculations in which the concentration of CH ₄ increases while that of CO reduces. In situ Raman and in situ diffuse reflectance FTIR spectroscopy (in situ DRIFTS) was conducted, and reaction routes for CO ₂ were then proposed. Continuously replenished protons, which steadily and efficiently react with CO ₃ ^2- to form-OCO- and finally CO, are suggested to play a critical role in the conversion of CO ₂ and the formation of CO in the P-SOEC. Our results shed new light on future effective conversion of CO ₂ .