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Controllable CO₂ conversion in high performance proton conducting solid oxide electrolysis cells and the possible mechanisms

Shi, Nai, Xie, Yun, Huan, Daoming, Yang, Yi, Xue, Shuangshuang, Qi, Zeming, Pan, Yang, Peng, Ranran, Xia, Changrong, Lu, Yalin
Journal of materials chemistry A 2019 v.7 no.9 pp. 4855-4864
Fourier transform infrared spectroscopy, Raman spectroscopy, adsorption, anodes, carbon cycle, carbon dioxide, carbon monoxide, carbonates, cathodes, electrochemistry, electrolysis, electrolytes, greenhouses, methane, protons, reflectance, steam, thermodynamics
To solve the increasing greenhouse problem and to achieve sustainable carbon cycling, effective conversion of CO₂ through chemical or electrochemical ways is key. In this study, efficient and controllable conversion of CO₂ mainly to CO and CH₄ has been demonstrated in a proton conducting solid oxide electrolysis cell (P-SOEC) using BaZr₀.₈Y₀.₂O₃₋δ (BZY) as the electrolyte and SrEu₂Fe₁.₈Co₀.₂O₇₋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 CO₂ 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 CO₂ and more effective conversion of protons over the BZY electrolyte. With the increase of electrolysis current, formation of both CO and CH₄ are enhanced, contradictory to the deduction based on thermodynamic calculations in which the concentration of CH₄ 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 CO₂ were then proposed. Continuously replenished protons, which steadily and efficiently react with CO₃²⁻ to form–OCO– and finally CO, are suggested to play a critical role in the conversion of CO₂ and the formation of CO in the P-SOEC. Our results shed new light on future effective conversion of CO₂.