Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells

Rutile-Type Co0.5Ti0.5NbO4-Based Cathode with In Situ Exsolved Metal Particles for Direct CO2 Electrolysis

Rutile-type Co0.5Ti0.5NbO4 (CTO)-based materials doped with Fe3+ or Ni2+ were investigated as cathode electrodes to modify their electrical conductivity and electrocatalysis toward CO2 splitting. Higher electric conductivity was found in Co0.4Fe0.2Ti0.4NbO4 (CTO-Fe, 0.78 S cm-1) and Co0.25Ni0.25Ti0.5NbO4 (CTO-Ni, 2.10 S cm-1) compared to CTO (0.49 S cm-1) after the reduction at 800 °C in Ar-5% H2. Co and Co-Ni particles exsolved in situ from the surface of CTO, CTO-Fe, and CTO-Ni after reduction. CTO-Ni and CTO-Fe cathodes did better in the CO2 electrolysis at 800 °C than the CTO one, but the CTO-Ni cell was unstable after 10 h of operation due to the carbon deposition that blocked the electrode. The cell with CTO-Fe demonstrated a good stability for CO2 splitting in 100 h. This work demonstrates that rutile-type CTO-based cathodes are promising to provide an efficient and candidate oxide cathode for the electrolysis of CO2.

Copper-Enhanced CO2 Electroreduction in SOECs

The development of a Co-free and Ni-free electrocatalyst for carbon dioxide electrolysis would be a turning point for the large-scale commercialization of solid-oxide electrolysis cells (CO2-SOECs). Indeed, the demand for cobalt and nickel is expected to become critical by 2050 due to automotive electrification. Currently, the reference materials for CO2-SOEC electrodes are perovskite oxides containing Mn or Co (anodes) and Ni-YSZ cermets (cathodes). However, issues need to be addressed, such as structural degradation and/or carbon deposition at the cathode side, especially at high overpotentials. This work designs the 20 mol % replacement of iron by copper in La0.6Sr0.4FeO3-δ as a multipurpose electrode for CO2-SOECs. La0.6Sr0.4Fe0.8Cu0.2O3-δ (LSFCu) is synthesized by the solution combustion method, and iron partial substitution with copper is evaluated by X-ray powder diffraction with Rietveld refinement, X-ray photoelectron spectroscopy, thermogravimetric analyses, and electrical conductivity assessment. LSFCu is tested as the SOEC anode by measuring the area-specific resistance versus T and pO2. LSFCu structural, electrical, and electrocatalytic properties are also assessed in pure CO2 for the cathodic application. Finally, the proof of concept of a symmetric LSFCu-based CO2-SOEC is tested at 850 °C, revealing a current density value at 1.5 V of 1.22 A/cm2, which is remarkable when compared to similar Ni- or Co-containing systems.

Unlocking the Potential of A-Site Ca-Doped LaCo0.2Fe0.8O3-δ: A Redox-Stable Cathode Material Enabling High Current Density in Direct CO2 Electrolysis

Massive carbon dioxide (CO2) emission from recent human industrialization has affected the global ecosystem and raised great concern for environmental sustainability. The solid oxide electrolysis cell (SOEC) is a promising energy conversion device capable of efficiently converting CO2 into valuable chemicals using renewable energy sources. However, Sr-containing cathode materials face the challenge of Sr carbonation during CO2 electrolysis, which greatly affects the energy conversion efficiency and long-term stability. Thus, A-site Ca-doped La1-xCaxCo0.2Fe0.8O3-δ (0.2 ≤ x ≤ 0.6) oxides are developed for direct CO2 conversion to carbon monoxide (CO) in an intermediate-temperature SOEC (IT-SOEC). With a polarization resistance as low as 0.18 Ω cm2 in pure CO2 atmosphere, a remarkable current density of 2.24 A cm-2 was achieved at 1.5 V with La0.6Ca0.4Co0.2Fe0.8O3-δ (LCCF64) as the cathode in La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM) electrolyte (300 μm) supported electrolysis cells using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as the air electrode at 800 °C. Furthermore, symmetrical cells with LCCF64 as the electrodes also show promising electrolysis performance of 1.78 A cm-2 at 1.5 V at 800 °C. In addition, stable cell performance has been achieved on direct CO2 electrolysis at an applied constant current of 0.5 A cm-2 at 800 °C. The easily removable carbonate intermediate produced during direct CO2 electrolysis makes LCCF64 a promising regenerable cathode. The outstanding electrocatalytic performance of the LCCF64 cathode is ascribed to the highly active and stable metal/perovskite interfaces that resulted from the in situ exsolved Co/CoFe nanoparticles and the additional oxygen vacancies originated from the Ca2Fe2O5 phase synergistically providing active sites for CO2 adsorption and electrolysis. This study offers a novel approach to design catalysts with high performance for direct CO2 electrolysis.

Structural, Interfacial, and Electrochemical Stability of La0.3Ca0.7Fe0.7Cr0.3O3-δ Electrode Material for Application as the Oxygen Electrode in Reversible Solid Oxide Cells

A detailed study aimed at understanding the structural, interfacial, and electrochemical performance stability of La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) electrode material for application as the oxygen electrode in reversible solid oxide cells (RSOCs) is presented. Specifically, emphasis is placed on the stability of the LCFCr oxygen electrode during oxygen evolution (electrolysis mode), whereby many known electrode materials are known to fail due to delamination. The porous microstructure of the electrode was characterized by nanoscale X-ray microscopy (XRM) to reveal the percentage porosity, pore connectivity, average pore size, and electrochemical surface area, etc. Under polarization in a two-electrode symmetrical-cell configuration, while the working electrode was under anodic polarization, a very stable performance was observed at a cell potential of 0.2 V, although increasing the cell potential to 0.65 V caused significant performance degradation. This degradation was reversible when the cell was run at open circuit for 10 h. High-resolution transmission electron microscopy and wavelength dispersive spectroscopy revealed that the working electrode (LCFCr)/electrolyte (GDC) interface was structurally and chemically stable after hundreds of hours under polarization with no interdiffusion of the various species observed across the interface, hence rendering LCFCr a viable alternative for the oxygen electrode in RSOCs.