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Solid oxide electrolysis cells (SOECs) are promising systems for the selective electrochemical conversion of CO2, or mixed streams of CO2 and H2O, into high energy products, such as CO and H2. These products can be converted to fuels and chemicals using existing technologies. Although promising, state-of-the-art SOEC Ni-based cathode electrocatalysts are affected by poor redox stability leading to limited long-term performance. Due to their favorable redox properties, mixed ionic-electronic conducting (MIEC) oxides are promising alternatives to minimize these adverse effects. However, the electrochemical activities of MIECs are inferior to the Ni-based cathode electrocatalysts. Thus, improvement of the electrochemical performance of MIEC-based SOEC electrocatalysts is needed and requires an understanding of the factors that govern their activity. Herein, we investigate the effect of the B-site cations of LaBO3 perovskites on their electrochemical activity toward the reduction of CO2 in SOECs. We correlate changes in the B-site composition to the nature of adsorbed species and active sites on the oxide surfaces by integrating in situ CO2 Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) studies with density functional theory (DFT)-based calculations. These studies provide insights critical toward devising ways to improve the performance of MIEC-based SOEC cathodes for the electroreduction of CO2.

Electrochemical reduction of CO2 using solid oxide electrolysis cells (SOECs) has emerged as an attractive approach for converting CO2 to high energy molecules, such as CO, a key precursor for the synthesis of fuels and chemicals using the commercially established Fischer–Tropsch process. The in situ generation of syngas (CO and H2) has also been demonstrated in SOECs through the coelectrolysis of CO2 and H2O. However, conventional Ni-based SOEC cathodes exhibit high overpotential losses associated with CO2 activation, leading to the disproportional activation of CO2 and H2O during coelectrolysis, facilitating the equilibrium-limited thermochemical reverse water gas shift (RWGS) reaction. Thus, identification of factors that govern CO2 activation on transition metal electrocatalysts is important toward optimizing the performance of SOEC cathodes for modulated production of syngas. Herein, we experimentally assess the electrocatalytic performance of monometallic transition metal electrocatalysts (Fe, Ni, and Pd) toward electrochemical CO2 reduction in SOECs with the aim of understanding the electrocatalyst characteristics that govern this performance. We report that metal oxophilicity (a property correlated to the strength of metal–oxygen bonding) plays an important role in the energetics associated with electrochemical CO2 reduction and electrocatalyst deactivation via oxidation. We suggest that a compromise in the oxophilicity of the metal is required to achieve optimal electrochemical activity and stability because CO2 activation is facile on highly oxophilic transition metals to the left of Ni (i.e., Fe); however, strong oxygen binding on these metals leads to their deactivation via oxidation. Potential approaches that facilitate the electronic structure modulation of transitional metals to optimize their surface oxophilicity, such as alloying, are suggested.

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.

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