Enhancing CO2 Catalytic Adsorption on an Fe Nanoparticle-Decorated LaSrFeO4 + δ Cathode for CO2 Electrolysis

Enhancing the CO2 Adsorption of the Cobalt-Free Layered Perovskite Cathode for Solid-Oxide Electrolysis Cells Gains Excellent Stability under High Voltages via Oxygen-Defect Adjustment

Solid-oxide electrolysis cells are a clean energy conversion device with the ability to directly electrolyze the conversion of CO2 to CO efficiently. However, their practical applications are limited due to insufficient CO2 adsorption performance of the cathode materials. To overcome this issue, the A-site cation deficiency strategy has been applied in a layered perovskite PrBaFe1.6Ni0.4O6-δ (PBFN) cathode for direct CO2 electrolysis. The introduction of 5% deficiency at the Pr/Ba site leads to a significant increase in the concentration of oxygen vacancies (nonstoichiometric number δ of oxygen vacancies increased from 0.093 to 0.132), which greatly accelerates the CO2 adsorption performance as well as the O2- transport capacity toward the CO2 reduction reaction (CO2RR). CO2 temperature-programmed desorption indicates that A-site cation-deficient (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) shows a larger desorption peak area and a higher desorption temperature. PB95FN also exhibits a greater presence of carbonate in Fourier transform infrared (FT-IR) spectroscopy. The electrical conductivity relaxation test shows that the introduction of the 5% A-site deficiency effectively improves the surface oxygen exchange and diffusion kinetics of PB95FN. The current density of the electrolysis cell with the (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) cathode reaches 0.876 A·cm-2 under 1.5 V at 800 °C, which is 41% higher than that of PB100FN. Moreover, the PB95FN cathode demonstrates excellent long-term stability over 100 h and better short-term stability than PB100FN under high voltages, which can be ascribed to the enhanced CO2 adsorption performance. The PB95FN cathode maintains a porous structure and tightly binds to the electrolyte after stability testing. This study highlights the potential of regulating oxygen defects in layered perovskite PrBaFe1.6Ni0.4O6-δ cathode materials via incorporation of cation deficiency toward high-temperature CO2 electrolysis.

Revealing the detrimental CO2 reduction effect of La0.6Sr0.4FeO3-δ-derived heterostructure in solid oxide electrolysis cells

Solid oxide electrolysis cells hold unique Faraday efficiency and favored thermodynamic/kinetics for CO2 reduction to CO. Perovskite oxide-based composite materials are promising alternatives to Ni-based cermet electrodes in SOECs. However, contrary results of the electrocatalytic activity over single-phase perovskite oxide exist and the rationale of the negative effect is not well revealed. In this work, two-phase perovskite materials with various complementary properties and unique interfaces are self-assembled, which was realized by "subtractive" defect-driven phase separation. The obtained heterostructure electrodes showed reduced performance over that of single-phase materials although the cyclic stability was improved. The main reasons for the performance degradation are the decrease of electrical conductivity, oxygen vacancy concentration while increasing the average valence state of B-site Fe cations, and electrode surface Sr aggregation. This work highlights the self-assembly method and insight into the rational design and synthesis of active electrodes/catalysts for CO2 conversion in solid oxide cells.

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.

A High-Entropy Layered Perovskite Coated with In Situ Exsolved Core-Shell CuFe@FeOx Nanoparticles for Efficient CO2 Electrolysis

Solid oxide electrolysis cells (SOECs) are promising energy conversion devices capable of efficiently transforming CO2 into CO, reducing CO2 emissions, and alleviating the greenhouse effect. However, the development of a suitable cathode material remains a critical challenge. Here a new SOEC cathode is reported for CO2 electrolysis consisting of high-entropy Pr0.8 Sr1.2 (CuFe)0.4 Mo0.2 Mn0.2 Nb0.2 O4-δ (HE-PSCFMMN) layered perovskite uniformly coated with in situ exsolved core-shell structured CuFe alloy@FeOx (CFA@FeO) nanoparticles. Single cells with the HE-PSCFMMN-CFA@FeO cathode exhibit a consistently high current density of 1.95 A cm-2 for CO2 reduction at 1.5 V while maintaining excellent stability for up to 200 h under 0.75 A cm-2 at 800 °C in pure CO2 . In situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations confirm that the exsolution of CFA@FeO nanoparticles introduces additional oxygen vacancies within HE-PSCFMMN substrate, acting as active reaction sites. More importantly, the abundant oxygen vacancies in FeOx shell, in contrast to conventional in situ exsolved nanoparticles, enable the extension of the triple-phase boundary (TPB), thereby enhancing the kinetics of CO2 adsorption, dissociation, and reduction.

Enhancing Electrochemical CO2 Reduction on Perovskite Oxide for Solid Oxide Electrolysis Cells through In Situ A-Site Deficiencies and Surface Carbonate Deposition Induced by Lithium Cation Doping and Exsolution

Solid oxide electrolysis cells (SOECs) hold enormous potential for efficient conversion of CO2 to CO at low cost and high reaction kinetics. The identification of active cathodes is highly desirable to promote the SOEC's performance. This study explores a lithium-doped perovskite La0.6- x Lix Sr0.4 Co0.7 Mn0.3 O3-δ (x = 0, 0.025 0.05, and 0.10) material with in situ generated A-site deficiency and surface carbonate as SOEC cathodes  for CO2 reduction. The experimental results indicate that the SOEC with the La0.55 Li0.05 Sr0.4 Co0.7 Mn0.3 O3-δ cathode exhibits a current density of 0.991 A cm-2 at 1.5 V/800 °C, which is an improvement of ≈30% over the pristine sample. Furthermore, SOECs based on the proposed cathode demonstrate excellent stability over 300 h for pure CO2 electrolysis. The addition of lithium with high basicity, low valance, and small radius, coupled with A-site deficiency, promotes the formation of oxygen vacancy and modifies the electronic structure of active sites, thus enhancing CO2 adsorption, dissociation process, and CO desorption steps as corroborated by the experimental analysis and the density functional theory calculation. It is further confirmed that Li-ion migration to the cathode surface forms carbonate and consequently provides the perovskite cathode with an impressive anti-carbon deposition capability, as well as electrolysis activity.

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.

Ti/Ni co-doped perovskite cathode with excellent catalytic activity and CO2 chemisorption ability via nanocatalysts exsolution for solid oxide electrolysis cell

Carbon dioxide (CO₂) gas is the main cause of global warming and has a significant effect on both climate change and human health. In this study, Ni/Ti co-doped Sr_1.95Fe_1.2Ni_0.1Ti_0.2Mo_0.5O_6-δ (SFNTM) double perovskite oxides were prepared and used as solid oxide electrolysis cell (SOEC) cathode materials for effective CO₂ reduction. Ti-doping enhances the structural stability of the cathode material and increases the oxygen vacancy concentration. After treatment in 10% H₂/Ar at 800°C, Ni nanoparticles were exsolved in situ on the SFNTM surface (Ni@SFNTM), thereby improving its chemisorption and activation capacity for CO₂. Modified by the Ti-doping and the in situ exsolved Ni nanoparticles, the single cell with Ni@SFNMT cathode exhibits improved catalytic activity for CO₂ reduction, exhibiting a current density of 2.54 A cm⁻² at 1.8 V and 800°C. Furthermore, the single cell shows excellent stability after 100 h at 1.4 V, indicating that Ni/Ti co-doping is an effective strategy for designing novel cathode material with high electrochemical performance for SOEC.

Minimized thermal expansion mismatch of cobalt-based perovskite air electrodes for solid oxide cells

The mismatch of thermal expansion coefficients (TECs) between cobalt-containing perovskite air electrodes and electrolytes is a great challenge for the development of thermo-mechanically durable solid oxide cells (SOCs). In this work, we propose a facile design principle to directly grow highly dispersed Co reactive sites onto ion-conducting scaffolds and confine the dimension of active centres within nanoscale. As a representative, the Co-socketed BaCe_0.7Zr_0.2Y_0.1O_3-δ perovskite (denoted as R-BCZY-Co) was constructed via a consecutive sol-gel and in situ exsolution approach. Combined XRD, H₂-TPR, SEM and TEM results confirm the emergence of Co nanoparticles on a BCZY matrix without the segregation of a secondary Co-rich phase. The symmetric half-cell measurement suggests that R-BCZY-Co air electrode with the optimal Co content of 10 mol% exhibits a 7-fold promoted oxygen activation performance with a polarization resistance of ∼0.17 Ω cm² at 750 °C. The TEC mismatch between fabricated R-BCZY-Co electrodes and BCZY electrolytes is minimized down to only ∼11.4%, which is significantly lower than that of other representative counterparts. Moreover, the detailed XPS result proves that the architecture of exsolved Co on BCZY possesses a higher concentration of surface oxygen vacancy, which further benefits the kinetics of ion diffusion and oxygen absorption.