Correlating the Onset of CO Disproportionation to Surface Chemistry on Ceria
Authors: Wang Jiayue , Bishop Sean R. , Lu Qiyang , Sun Lixin , Tsvetkov Nikolai , Vardar Gulin , Bliem Roland , Jansen Maximilian , Gallet Jean-Jacques , Bournel Fabrice , Waluyo Iradwikanari , Crumlin Ethan J , Yildiz Bilge 
Published: February 2020

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.