Oxidation Process of Ni Cathode in CO2 Electrolysis in SOEC: X-Ray and DFT Study

Abstract

Solid oxide electrolysis cells (SOECs) can convert H2O and CO2 to syngas(H2/CO) at high temperature, promising a way in the power-to-gas process. A Ni electrode such as nickel/yttria-stabilized zirconia (Ni/YSZ) and nickel/scandia-stabilized zirconia (Ni/ScSZ) have widely used as a cathode for SOECs. Meanwhile, the cathode faced severe oxidizing condition due to a high steam and CO2 content. H2, a reduction agent, can inhibit the cathode oxidation; however, an additional H2 decreases a total energy efficiency. Oxidation tolerance is an important parameter for the material candidate as the cathode. A combination study of X-ray and DFT study is a promising way to understand oxidation mechanisms of Ni cathode, which provides an useful hint in the cathode design with oxidation tolerance.In this study, oxidation process of Ni cathode in CO2 electrolysis was studied using X-ray and density functional theory (DFT). Ni cathodes (Ni/YSZ, Ni/ScSZ) were used in H2/CO2 environment under the electrolysis mode. After CO2 electrolysis, Ni cathode surface was analyzed using SEM/EDS and X-ray diffraction (XRD). Influence of H2/CO2 ratio and operating condition on cathode oxidization was studied in the electrolysis mode. All calculations here are performed employing the plane wave density functional theory (DFT) method implemented in the Vienna Ab-Initio Simulation Package (VASP).XRD results showed that the cathode oxidation was progressed at H2/CO2 ratio of 0, whereas the cathode oxidation was inhibited when the H2/CO2 ratio was higher than 0.75 at 800 oC. In addition, it was suggested that the electrolysis mode, in which oxygen was moved from the cathode to the anode, had a role in inhibiting the cathode oxidation. In the DFT calculation, an activation energy of CO2 decomposition at the triple phase boundary (CO2→CO+O,vacancy) was much lower than that at the Ni surface (CO2→CO+O,Ni), indicating that CO2 electrolysis was more preferable than Ni oxidation. It was shown that the electrolysis mode can enhance the oxidation tolerance of Ni cathode. Moreover, Ni/YSZ and Ni/ScSZ cathode were also studied in terms of CO2 electrolysis and oxidation. The activation energy of CO2 decomposition of Ni/ScSZ was lower than that of Ni/YSZ mainly because O can easily enter the vacancy in the ScSZ. It was shown that the oxidation can be controlled from the electrolyte structure in the electrolysis mode.