Electrochemical Reduction of CO2 using Solid Oxide Electrolysis Cells: Insights into Catalysis by Nonstoichiometric Mixed Metal Oxides

Selective electrochemical reduction of CO2 using renewable energy sources to create platform molecules for synthesis of fuels and chemicals has become a contemporary research area of interest because of its potential for recycling and minimizing the adverse environmental impacts of CO2. Solid oxide electrolysis cells (SOECs) are solid-state electrochemical devices with significant potential in this area because of their ability to efficiently and selectively convert CO2 to CO or, when coupled with water electrolysis, to produce syngas (CO and H2). Both CO and syngas are precursors for the synthesis of fuels and chemicals using existing technologies. While promising, SOECs are limited by the instability of the state-of-the-art cathode electrocatalyst, Ni/yttria-stabilized zirconia (YSZ) cermet, due to its limited redox properties and deactivation by carbon deposits. Nonstoichiometric mixed ionic and electronic conducting oxides are promising alternatives because of their redox stability and resistance to deactivation by carbon. Herein, we summarize the literature in this area and derive trends that relate changes in composition and oxygen defects in these oxides to activity, selectivity, and stability for the electrochemical reduction of CO2 to CO in SOECs using both experimental and theoretical studies. We also evaluate the factors that present challenges in a direct comparison of the performance of SOEC cathode electrocatalysts for CO2 reduction reported in the literature and suggest possible solutions and standardized protocols for benchmarking the performance of SOECs. We conclude by summarizing and providing an overview of challenges in the field along with potential solutions and opportunities for electrochemical reduction of CO2 by nonstoichiometric mixed metal oxides in SOECs.