Facile Approach to Enhance Activity and CO2 Resistance of a Novel Cobalt-Free Perovskite Cathode for Solid Oxide Fuel Cells

Developing high-performance and cost-effective cathodes is ever-increasingly vital for the advancement of intermediate-temperature solid oxide fuel cells (IT-SOFCs). To facilitate the popularization of nonprecious metallic and cobalt-free oxygen reduction electrodes, herein, we propose a novel perovskite-based BaFeO_3-δ (BF) matrix, Ba_0.75Sr_0.25Fe_0.875Y_0.125O_3-δ (BSFY), as a highly active cathode for IT-SOFCs. To our satisfaction, the BSFY electrode showcases a low area-specific resistance of 0.063 Ω cm², as well as a high peak power density of 1288 mW cm^-2 at 600 °C, yielding a more than threefold improvement compared to that of its BF counterpart (371 mW cm^-2). The long-term durability test highlights its practicability under the IT operating condition. When tested in 10 vol % CO₂-containing air, the BSFY electrode exhibits impressive resistance against contaminants within 50 h (<0.4 Ω cm² with a deterioration rate of ∼0.00011 Ω cm² min^-1). Coupled with its reversible response between pure air and the contaminant, the BSFY cathode is expected to be a promising cobalt-free alternative with high CO₂ resistance for IT-SOFCs.

One Pot-Synthesized Ag/Ag-Doped CeO2 Nanocomposite with Rich and Stable 3D Interfaces and Ce3+ for Efficient Carbon Dioxide Electroreduction

Electrochemical CO₂ reduction (ECR) technology is promising to produce value-added chemicals and alleviate the climate deterioration. Interface engineering is demonstrated to improve the ECR performance for metal and oxide composite catalysts. However, the approach to form a substantial interface is still limited. In this work, we report a facile one-pot coprecipitation method to synthetize novel silver and silver-doped ceria (Ag/CeO₂) nanocomposites. This catalyst provides a rich 3D interface and high Ce³⁺ concentration (33.6%), both of which are beneficial for ECR to CO. As a result, Ag/CeO₂ exhibits a 99% faradaic efficiency and 10.5 A g^-1 mass activity to convert CO₂ into CO at an overpotential of 0.83 V. The strong interfacial interaction between Ag and CeO₂ may enable the presence of surface Ce³⁺ and guarantee the improved durability during the electrolysis. We also develop numerical simulation to understand the local pH effect on the ECR performance and propose that the superior ECR performance of Ag/CeO₂ is mainly due to the accelerated CO formation rate rather than the suppressed hydrogen evolution reaction.

La0.6Sr0.4Co0.2Fe0.8O3-δ/CeO2 Heterostructured Composite Nanofibers as a Highly Active and Robust Cathode Catalyst for Solid Oxide Fuel Cells

The lack of highly active and robust catalysts for the oxygen reduction reaction (ORR) at the intermediate temperatures significantly hinders the commercialization of solid oxide fuel cells (SOFCs). Here, we report a novel heterostructured composite nanofiber cathode composed of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and CeO₂ nanoparticles, synthesized by using a coaxial electrospinning technique, which exhibits remarkably enhanced ORR activity and durability as compared to single LSCF powder and nanofibers. This cathode achieves a polarization resistance of 0.031 Ω cm² at 700 °C, approximately 1/5 of that for the LSCF powder cathode (0.158 Ω cm²). Such enhancement can be attributed to the continuous paths provided by nanofibers for efficient mass/charge transport and the interdiffusion of La and Ce at the heterointerface which leads to more oxygen vacancy formation. Furthermore, the anode-supported cell with the LSCF/CeO₂ composite cathode shows excellent stability (0.4 V for ∼200 h at 600 °C) because of suppression of Sr segregation in LSCF by introducing CeO₂ and the structure of heterogeneous nanofibers. These results indicate that the microstructure design of this heterostructured composite nanofiber for LSCF/CeO₂ is extremely effective for enhancing ORR activity and stability. This finding may provide a new strategy for the microstructure design of highly active and robust ORR catalysts in SOFCs.

Interface interactions and enhanced room temperature ferromagnetism of Ag@CeO2 nanostructures

Enhancement of room temperature ferromagnetism (RTFM) has been achieved with core-shell metal-oxide nanoparticles (Ag@CeO₂). To enhance the magnetic properties, interfacial charge transfer is achieved via the formation of a core-shell interface. Furthermore, by varying the shell thicknesses, additional control of the RTFM can be obtained. The Ag@CeO₂ core-shell nanoparticles are synthesized successfully via a two-step method. Ag nanoparticles (NPs) are first synthesized on a TiO₂ substrate by a thermally assisted photoreduction method, and then CeO₂ NPs are deposited on the surface of Ag NPs by chemical reduction. No surfactants or organic compounds are used during the synthesis. At the interface between the core and the shell, electron transfers from the Ag-p orbital to the Ag-d and Ce-f orbitals are evidenced by X-ray absorption spectroscopy and electron energy loss spectroscopy. Such interfacial charge transfer results in enhanced room temperature ferromagnetism in the Ag@CeO₂ core-shell NPs compared to the magnetism arising for bare Ag or CeO₂ NPs. This study suggests that tailoring the interface, the surface and their coupling in nanostructured metal-oxide core shell nanoparticles is an effective way to enhance their magnetic properties.

Evaluation of the CO2 Poisoning Effect on a Highly Active Cathode SrSc(0.175)Nb(0.025)Co(0.8)O(3-δ) in the Oxygen Reduction Reaction

A solid oxide fuel cell (SOFC) is a highly efficient device for converting chemical energy to electrical energy. In addition to the efforts to reduce the operating temperature of SOFCs to below 600 °C, research studies of the basic mechanism of CO2 poisoning on cathode materials are envisioned to improve the operation of dual-chamber SOFCs using ambient air. In this work, we comparatively studied the CO2 poisoning effect on two highly active perovskites SrSc(0.175)Nb(0.025)Co(0.8)O(3-δ) (SSNC) and Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) (BSCF), using complementary characterization techniques, e.g., powder X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), CO2-temperature-programmed desorption (CO2-TPD), and electrochemical impedance spectroscopy (EIS). The SSNC cathode shows better tolerance to CO2 as compared with BSCF, which is attributed to the absence of Ba, higher average metal-oxygen bond energy (ABE) of SSNC, and the higher acidity of Nb(5+) cations, whereas the oxygen vacancy concentration plays a less important role.

Promotion of Oxygen Reduction by Exsolved Silver Nanoparticles on a Perovskite Scaffold for Low-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have potential to be the cleanest and most efficient electrochemical energy conversion devices with excellent fuel flexibility. To make SOFC systems more durable and economically competitive, however, the operation temperature must be significantly reduced, which depends sensitively on the development of highly active electrocatalysts for oxygen reduction reaction (ORR) at low temperatures. Here we report a novel silver nanoparticle-decorated perovskite oxide, prepared via a facile exsolution process from a Sr0.95Ag0.05Nb0.1Co0.9O3-δ (SANC) perovskite precursor, as a highly active and robust ORR electrocatalyst for low-temperature SOFCs. The exsolved Sr0.95Ag0.05Nb0.1Co0.9O3-δ (denoted as e-SANC) electrode is very active for ORR, achieving a very low area specific resistance (∼0.214 Ω cm(2) at 500 °C). An anode-supported cell with the new heterostructured cathode demonstrates very high peak power density (1116 mW cm(-2) at 500 °C) and stable operation for 140 h at a current density of 625 mA cm(-2). The superior ORR activity and stability are attributed to the fast oxygen surface exchange kinetics and the firm adhesion of the Ag nanoparticles to the Sr0.95Nb0.1Co0.9O3-δ (SNC0.95) support. Moreover, the e-SANC cathode displays improved tolerance to CO2. These unique features make the new heterostructured material a highly promising cathode for low-temperature SOFCs.

Compositional engineering of perovskite oxides for highly efficient oxygen reduction reactions

Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO(3-δ) is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO(3-δ) has been applied as an effective way to improve physicochemical properties and electrochemical performance. Most findings however are obtained only from experimental observations, and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic conductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport, and electrochemical reactivity are experimentally characterized. In-depth analysis of doping level effect is also undertaken by first-principles calculations. Among the compositions, SrCo0.95Sc0.05O(3-δ) shows the best oxygen kinetics and corresponds to the minimum fraction of Sc for stabilization of the oxygen-vacancy-disordered structure. The results strongly support that B-site doping of SrCoO(3-δ) with a small amount of redox-inactive cation is an effective strategy toward the development of highly active mixed conducting perovskites for efficient solid oxide fuel cells and oxygen transport membranes.