Recycling carbon dioxide into carbon monoxide as a feedstock for industry and transportation has become of great interest as a means to reduce the impact of humanity on the environment. Due to the high thermodynamic efficiency inherent in electrochemical reduction of CO₂ at high temperatures (~800 ^oC), solid oxide electrolysis (SOE) is an attractive method for CO production. Furthermore, in steam and CO₂ co-electrolysis, the hydrogen and CO products constitute syngas, of significant importance to industry. Additionally, SOE of CO₂ is actively being explored by the National Aeronautics and Space Administration (NASA) to generate O₂ for future missions to Mars. Mitigating degradation of SOE cell performance over time remains a key challenge of the technology. For example, carbon accumulation from CO disproportionation reaction (coking) on the cathode during reduction of CO₂, particularly problematic in dry conditions, leads to mechanical and electrochemical degradation of the cell. Other degradation modes, well known in the solid oxide fuel cell community, such as large electrode volume changes during redox with subsequent mechanical failure are also of concern. As a result, electrodes have been developed to counter these degradation modes in electrolysis and fuel cell operation. The majority of studies have largely been guided by empirical evidence indicating coke tolerant electrodes. In this presentation, we describe an investigation into the relationship between gas/solid interface compositional and operational factors that lead to the onset of coking.

In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to evaluate conditions leading to coking using CO/CO₂ mixtures at 300 mTorr total pressure at 450 ^oC. Electrical bias was applied to ceria based films, deposited using pulsed laser deposition on “buried” Pt electrodes, supported on ionic conducting substrates of yttria stabilized zirconia (YSZ) to in situ drive the CO₂ reduction reaction. With electrochemical APXPS, we tracked the evolution of surface chemical change during coking reaction on ceria thin film samples with different dopant types (Zr, Gd) and doping concentrations. The surface spectroscopy analysis yielded information about the relationship between onset of coking, surface defect concentration (e.g., oxygen vacancies), surface composition (e.g., relative cation and impurity), and area specific resistance. During CO disproportionation, or coking, surface Ce³⁺ concentration was only loosely correlated with the deposited carbon intensity and indicated a threshold amount of Ce³⁺ for coking onset. The observed threshold phenomenon deepens previous understanding of Ce³⁺ sites’ catalytic role in CO disproportionation since no carbon was observed even considerable surface cerium had been reduced. By combining Monte Carlo simulation, we have shown that the Ce³⁺−Ce³⁺ pair formation exhibited similar behavior against surface Ce³⁺ concentration as coking intensity, indicating that Ce³−Ce³⁺ pair could be the dominant catalytic structure for coking reaction on ceria surface. These new insights on coking mechanism would be beneficial for rational coking-resistant electrodes design for CO₂ electrolysis devices.