Cobalt–free La0.5Sr0.5Fe0.9Mo0.1O3– electrode for symmetrical SOFC running on H2 and CO fuels

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Enhancing the Electrochemical Performance in Symmetrical Solid Oxide Cells through Nanoengineered Redox-Stable Electrodes with Exsolved Nanoparticles

Symmetrical solid oxide cells (SSOCs) have recently gained significant attention for their potential in energy conversion due to their simplified cell configuration, cost-effectiveness, and excellent reversibility. However, previous research efforts have mainly focused on improving the electrode performance of perovskite-type electrodes through different doping strategies, neglecting microstructural optimization. This work presents novel approaches for the nanostructural tailoring of (La0.8Sr0.2)0.95Fe1-xTixO3-δ (LSFTx, x = 0.2 and 0.4) electrodes using a single-step spray-pyrolysis deposition process. By incorporating these electrodes into a Ce0.9Gd0.1O1.95 (CGO) porous backbone or employing a nanocomposite architecture with nanoscale particle size, we achieved significant improvements in the polarization resistance (Rp) compared with traditional screen-printed electrodes. To further boost the fuel oxidation performance, a Ni-doping strategy, coupled with meticulous microstructural optimization, was implemented. The exsolution of Ni nanoparticles under reducing conditions resulted in remarkable Rp values as low as 0.34 and 0.11 Ω cm2 in air and wet H2 at 700 °C, respectively. Moreover, an electrolyte-supported cell with symmetrical electrodes demonstrated a stable maximum power density of 617 mW cm-2 at 800 °C. These findings highlight the importance of combining electrode composition optimization with advanced morphology control in the design of highly efficient and durable SSOCs.

Tailoring the Stability of Ti-Doped Sr2Fe1.4TixMo0.6-xO6-δ Electrode Materials for Solid Oxide Fuel Cells

In this work, the stability of Sr₂(FeMo)O_6-δ-type perovskites was tailored by the substitution of Mo with Ti. Redox stable Sr₂Fe_1.4Ti_xMo_0.6-xO_6-δ (x = 0.1, 0.2 and 0.3) perovskites were successfully obtained and evaluated as potential electrode materials for SOFCs. The crystal structure as a function of temperature, microstructure, redox stability, and thermal expansion properties in reducing and oxidizing atmospheres, oxygen content change, and transport properties in air and reducing conditions, as well as chemical stability and compatibility towards typical electrolytes have been systematically studied. All Sr₂Fe_1.4Ti_xMo_0.6-xO_6-δ compounds exhibit a regular crystal structure with Pm-3m space group, showing excellent stability in oxidizing and reducing conditions. The increase of Ti-doping content in materials increases the thermal expansion coefficient (TEC), oxygen content change, and electrical conductivity in air, while it decreases the conductivity in reducing condition. All three materials are stable and compatible with studied electrolytes. Interestingly, redox stable Sr₂Fe_1.4Ti_0.1Mo_0.5O_6-δ, possessing 1 μm grain size, low TEC (15.3 × 10^-6 K^-1), large oxygen content change of 0.72 mol·mol^-1 between 30 and 900 °C, satisfactory conductivity of 4.1-7.3 S·cm^-1 in 5% H₂ at 600-800 °C, and good transport coefficients D and k, could be considered as a potential anode material for SOFCs, and are thus of great interest for further studies.

Magnesium-Doped Sr2(Fe,Mo)O6-δ Double Perovskites with Excellent Redox Stability as Stable Electrode Materials for Symmetrical Solid Oxide Fuel Cells

In this work, magnesium-doped Sr₂Fe_1.2Mg_0.2Mo_0.6O_6-δ and Sr₂Fe_0.9Mg_0.4Mo_0.7O_6-δ double perovskites with excellent redox stability have been successfully obtained. The physicochemical properties including: crystal structure properties, redox stability, thermal expansion properties in oxidizing and reducing conditions, oxygen content as a function of temperature and transport properties, as well as the chemical compatibility with typical electrolytes have been systematically investigated. The in situ oxidation of reduced samples using high-temperature XRD studies shows the crystal structure of materials stable at up to a high-temperature range. The in situ reduction and oxidation of sinters with dilatometer measurements prove the excellent redox stability of materials, with the thermal expansion coefficients measured comparable with electrolytes. The oxygen nonstoichiometry δ of compounds was determined and recorded in air and argon up to 900 °C. Sr₂Fe_1.2Mg_0.2Mo_0.6O_6-δ oxide presents satisfactory values of electrical conductivity in air (56.2 S·cm^-1 at 600 °C) and reducing conditions (10.3 S·cm^-1 at 800 °C), relatively high coefficients D and k, and good ionic conductivity (cal. 0.005 S·cm^-1 at 800 °C). The stability studies show that both compounds are compatible with Ce_0.8Gd_0.2O_1.9 but react with the La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-d electrolyte. Therefore, the magnesium-doped double perovskites with excellent redox stability can be potentially qualified as electrode materials for symmetrical SOFCs and are of great interest for further investigations.

Exploring the defect equilibrium and charge transport in electrode material La0.5Sr0.5Fe0.9Mo0.1O3-δ

Perovskite-type La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ synthesized via glycine nitrate combustion and sintered at 1350 °C was found to have an orthorhombic lattice, which transforms upon heating into a rhombohedral and then a cubic one. The oxygen content and electrical conductivity in this oxide were measured in the range of oxygen partial pressures from 10^-20 to 0.5 atm at 750-950 °C by coulometric titration and four-probe dc techniques, respectively. The oxygen content data were used to model the defect equilibrium in the oxide. Oxidation, charge disproportionation and electron exchange reactions between iron and molybdenum were assumed by the model to be involved in the formation of defects. The experimental data were well approximated with the model and the concentrations of charge carriers in La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ were determined to be used for the electrical conductivity analysis. The average mobility of oxygen ions and n- and p-type charge carriers was determined to be about 10^-5, 0.007, and 0.07 cm² V^-1 s^-1 with an activation energy of 0.80 ± 0.02, 0.34 ± 0.01, and 0.23 ± 0.01 eV, respectively. Comparison with La_0.5Sr_0.5FeO_3-δ shows that 10% Mo substitution provides a substantial increase in both the concentration and mobility of n-type carriers, which results in an almost threefold increase in electron conductivity under reducing conditions, while maintaining a high level of ionic conductivity.

LSCF–WO3 semiconductor composite electrolytes for low-temperature solid oxide fuel cells†

The La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–WO₃ semiconductor composite was applied as an electrolyte for low-temperature solid oxide fuel cells (LTSOFCs). The study results revealed that the fuel cell could output a maximum power density (P_max) of 812 mW cm⁻² when the weight ratio of LSCF to WO₃ was 8 : 2 (8LSCF–2WO₃), and its open-circuit voltage (OCV) was higher than 1.0 V. This indicated that there was no short circuit problem in this fuel cell device and 80 wt% LSCF existed in the electrolyte layer. This was mainly due to the suppressed electronic conductivity and increased ionic conductivity of the composite as compared with LSCF due to the introduction of the WO₃ wide band semiconductor. The oxygen ionic conductivity of the 8LSCF–2WO₃ electrolyte was 0.337 S cm⁻¹, which is much higher than that of the pure LSCF material. According to the XPS analysis results, a higher oxygen vacancy content at the heterointerface between LSCF and WO₃ contributed to the increased ionic conductivity.

SOFC with 8LSCF–2WO₃ electrolyte exhibited an excellent power density (P_max) of 812 mW cm⁻² at 550 °C due to the increased ionic conductivity and decreased activation energy.

Unveiling the key factor for the phase reconstruction and exsolved metallic particle distribution in perovskites

To significantly increase the amount of exsolved particles, the complete phase reconstruction from simple perovskite to Ruddlesden-Popper (R-P) perovskite is greatly desirable. However, a comprehensive understanding of key parameters affecting the phase reconstruction to R-P perovskite is still unexplored. Herein, we propose the Gibbs free energy for oxygen vacancy formation in Pr_0.5(Ba/Sr)_0.5TO_3-δ (T = Mn, Fe, Co, and Ni) as the important factor in determining the type of phase reconstruction. Furthermore, using in-situ temperature & environment-controlled X-ray diffraction measurements, we report the phase diagram and optimum 'x' range required for the complete phase reconstruction to R-P perovskite in Pr_0.5Ba_0.5-xSr_xFeO_3-δ system. Among the Pr_0.5Ba_0.5-xSr_xFeO_3-δ, (Pr_0.5Ba_0.2Sr_0.3)₂FeO_4+δ - Fe metal demonstrates the smallest size of exsolved Fe metal particles when the phase reconstruction occurs under reducing condition. The exsolved nano-Fe metal particles exhibit high particle density and are well-distributed on the perovskite surface, showing great catalytic activity in fuel cell and syngas production.

High-Efficiency of Bi-Functional-Based Perovskite Nanocomposite for Oxygen Evolution and Oxygen Reduction Reaction: An Overview

High efficient, low-cost and environmentally friendly-natured bi-functional-based perovskite electrode catalysts (BFPEC) are receiving increasing attention for oxygen reduction/oxygen evolution reaction (ORR/OER), playing an important role in the electrochemical energy conversion process using fuel cells and rechargeable batteries. Herein, we highlighted the different kinds of synthesis routes, morphological studies and electrode catalysts with A-site and B-site substitution co-substitution, generating oxygen vacancies studies for boosting ORR and OER activities. However, perovskite is a novel type of oxide family, which shows the state-of-art electrocatalytic performances in energy storage device applications. In this review article, we go through different types of BFPECs that have received massive appreciation and various strategies to promote their electrocatalytic activities (ORR/OER). Based on these various properties and their applications of BFPEC for ORR/OER, the general mechanism, catalytic performance and future outlook of these electrode catalysts have also been discussed.

Electrochemical properties of La0.5Sr0.5Fe0.95Mo0.05O3−δ as cathode materials for IT-SOEC

Solid oxide electrolysis cells (SOECs) are a new type of high-efficiency energy conversion device that can electrolyze CO₂ efficiently and convert electricity into chemical energy. However, the lack of efficient and stable cathodes hinders the practical application of CO₂ electrolysis in SOECs. Herein, a novel perovskite oxide La_0.5Sr_0.5Fe_0.95Mo_0.05O_3−δ (LSFMo) is synthesized and used as a cathode for SOECs. The introduction of Mo significantly improves the CO₂ tolerance of the material in a reducing atmosphere and solves the problem of SrCO₃ generation in the La_0.5Sr_0.5FeO_3−δ material. Mo ion doping promotes the conductivity in a reducing atmosphere and increases the oxygen deficiencies of the material, which lowers the ohmic resistance (R_s) of the material and significantly improves the CO₂ adsorption and dissociation in the middle-frequency of polarization resistance (R_p). For example, R_p decreases from 0.49 to 0.24 Ω cm² at 800 °C under 1.2 V. Further, the reduction of R_s and R_p increases the performance improvement, and the current density is increased from 1.56 to 2.13 A cm⁻² at 800 °C under 2 V. Furthermore, LSFMo shows reasonable short-term stability during the 60 h stability test.

Mo doping solves the SrCO₃ generation of LSF as SOEC cathode and increases the oxygen deficiencies of the material, which make LSFMo possess higher electrolytic performance.