Operando X-Ray Diffraction Analysis and DFT Calculations of Ni Cathode Oxidation during Electrochemical CO₂ Reduction

In this study, Ni cathode oxidation during electrochemical CO 2 reduction (CO 2 electrolysis) in a solid oxide electrolysis cell was studied via ex situ and operando X-ray diffraction (XRD) analyses as well as first-principles density functional theory (DFT) calculations. The XRD NiO-Ni peak ratio was used to compare the degree of Ni oxidation. Ex situ XRD analysis was performed to study a Ni/yttria-stabilized zirconia (YSZ) cathode surface after CO 2 electrolysis at 1073 K, revealing that Ni oxidation was inhibited with increasing current density. Operando XRD analysis was performed to characterize the Ni/YSZ cathode surface during CO 2 electrolysis at 873 K, revealing that the NiO(200)-Ni(111) peak ratio was low (<0.05) during CO 2 electrolysis and rapidly increased to 0.7 without electrolysis after 10 min CO 2 exposure. These findings indicated that electrolysis enhanced the oxidation tolerance of the Ni cathode immediately after electrolysis initiation. DFT calculations showed that CO 2 reduction (CO 2 → CO + O (O was filled into a electrolyte vacancy)) at the triple-phase boundary was more favorable than Ni oxidation (Ni + CO 2 → NiO + CO) at the Ni (100) surface, suggesting that an increase in the CO 2 reduction rate improved oxidation tolerance.