Cobalt-free perovskite cathode materials SrFe1−xTixO3−δ and performance optimization for intermediate-temperature solid oxide fuel cells

Novel Anion-Doped Cathode Material SrFe1-x Si x O3-δF y for Intermediate-Temperature Solid Oxide Fuel Cells

SrFe1-x Si x O3-δF y cathode materials (x = 0.05, 0.1, 0.15; y = 0, 0.1, 0.5) were prepared via a solid-state method. X-ray diffraction results show that the synthesized F doping samples were perovskite structure. X-ray photoelectron spectroscopy findings show that F- anions were doped into SrFe1-x Si x O3-δ. Transmission electron microscopy and energy-dispersive spectroscopy were performed to analyze the microstructure and element distribution in the materials, respectively. Double-layer composite cathode symmetric cells were prepared through a screen printing method. Scanning electron microscopy images revealed that the double-layer composite cathode adhered well to the electrolyte. The doping with F- can increase the coefficient of thermal expansion of SrFe1-x Si x O3-δ. The electrochemical impedance spectroscopy results indicate that the oxygen transport capacity of the SrFe0.95Si0.05O3-δ material can be improved by doping with F-, but such a method can decrease the oxygen transport capacity of SrFe0.9Si0.1O3-δ. At 800 °C, the peak power density of the single cell supported by an anode and SrFe0.9Si0.1O3-δF0.1 as the cathode reached 388.91 mW/cm2. Thus, the incorporation of F- into SrFe1-x Si x O3-δ cathode materials can improve their electrochemical performance and enable their application as cathode materials for solid-oxide fuel cells.

Bonding State and Thermal Expansion Coefficient of Mn-Doped Ba0.5Sr0.5FeO3-δ Perovskite Oxides for IT-SOFCs

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba0.5Sr0.5Fe1-xMnxO3-δ (BSFMnx, x = 0-0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400-950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the Oads/Olat ratio decreased, and the binding energy of the Olat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal-oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm-1 (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm2 at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10-6 K-1, which is similar to that of Ce0.8Sm0.2O2 (SDC) (12.2 × 10-6 K-1). Mn doping increased the metal-oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC.

Flexible All-Solid-State Asymmetric Supercapacitors Based on PPy-Decorated SrFeO3-δ Perovskites on Carbon Cloth

The defective structure and high oxygen vacancy concentration of SrFeO3-δ perovskite enable fast ion-electron transport, but its low conductivity still hinders the high electrochemical performance. Herein, to enhance the conductivity of SrFeO3-δ-based electrodes, polypyrrole-modified SrFeO3-δ perovskite on carbon cloth (PPy@SFO@CC) has been successfully fabricated by electrodeposition of polypyrrole (PPy) on the surface of SFO@CC. The optimal PPy700@SFO@CC electrode exhibits a specific capacitance of 421 F g-1 at 1 A g-1. It was found that the outside PPy layer not only accelerates the electron transport and ion diffusion but also creates more oxygen vacancies in SrFeO3-δ, enhancing the charge storage performance significantly. Moreover, the NiCo2O4@CC//PPy700@SFO@CC device maintains a specific capacitance of 63.6% after 3000 cycles, which is ascribed to the weak adhesion forces between the active materials and carbon cloth. Finally, the all-solid-state flexible supercapacitor NiCo2O4@CC//PPy700@SFO@CC is constructed with PVA-KOH as the solid electrolyte, delivering an energy density of 16.9 W h kg-1 at a power density of 984 W kg-1. The flexible supercapacitor retains 69% of its specific capacitance after 1000 bending and folding times, demonstrating a certain degree of foldability. The present study opens new avenues for perovskite oxide-based flexible all-solid-state supercapacitors.

Self-regeneration of supported transition metals by a high entropy-driven principle

The sintering of Supported Transition Metal Catalysts (STMCs) is a core issue during high temperature catalysis. Perovskite oxides as host matrix for STMCs are proven to be sintering-resistance, leading to a family of self-regenerative materials. However, none other design principles for self-regenerative catalysts were put forward since 2002, which cannot satisfy diverse catalytic processes. Herein, inspired by the principle of high entropy-stabilized structure, a concept whether entropy driving force could promote the self-regeneration process is proposed. To verify it, a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox is constructed as a host model, and interestingly in situ reversible exsolution-dissolution of supported metallic species are observed in multi redox cycles. Notably, in situ exsolved transition metals from high entropy Zr0.5(NiFeCuMnCo)0.5Ox support, whose entropic contribution (TΔSconfig = T⋆12.7 J mol-1 K-1) is predominant in ∆G, affording ultrahigh thermal stability in long-term CO2 hydrogenation (400 °C, >500 h). Current theory may inspire more STWCs with excellent sintering-resistance performance.

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.

Insights in to the Electrochemical Activity of Fe-Based Perovskite Cathodes toward Oxygen Reduction Reaction for Solid Oxide Fuel Cells

The development of novel oxygen reduction electrodes with superior electrocatalytic activity and CO2 durability is a major challenge for solid oxide fuel cells (SOFCs). Here, novel cobalt-free perovskite oxides, BaFe1−xYxO3−δ (x = 0.05, 0.10, and 0.15) denoted as BFY05, BFY10, and BFY15, are intensively evaluated as oxygen reduction electrode candidate for solid oxide fuel cells. These materials have been synthesized and the electrocatalytic activity for oxygen reduction reaction (ORR) has been investigated systematically. The BFY10 cathode exhibits the best electrocatalytic performance with a lowest polarization resistance of 0.057 Ω cm2 at 700 °C. Meanwhile, the single cells with the BFY05, BFY10 and BFY15 cathodes deliver the peak power densities of 0.73, 1.1, and 0.89 W cm−2 at 700 °C, respectively. Furthermore, electrochemical impedance spectra (EIS) are analyzed by means of distribution of relaxation time (DRT). The results indicate that the oxygen adsorption-dissociation process is determined to be the rate-limiting step at the electrode interface. In addition, the single cell with the BFY10 cathode exhibits a good long-term stability at 700 °C under an output voltage of 0.5 V for 120 h.

A Cobalt-Free Multi-Phase Nanocomposite as Near-Ideal Cathode of Intermediate-Temperature Solid Oxide Fuel Cells Developed by Smart Self-Assembly

An ideal solid oxide fuel cell (SOFC) cathode should meet multiple requirements, i.e., high activity for oxygen reduction reaction (ORR), good conductivity, favorable stability, and sound thermo-mechanical/chemical compatibility with electrolyte, while it is very challenging to achieve all these requirements based on a single-phase material. Herein, a cost-effective multi-phase nanocomposite, facilely synthesized through smart self-assembly at high temperature, is developed as a near-ideal cathode of intermediate-temperature SOFCs, showing high ORR activity (an area-specific resistance of ≈0.028 Ω cm² and a power output of 1208 mW cm^-2 at 650 °C), affordable conductivity (21.5 S cm^-1 at 650 °C), favorable stability (560 h operation in single cell), excellent chemical compatibility with Sm_0.2 Ce_0.8 O_1.9 electrolyte, and reduced thermal expansion coefficient (≈16.8 × 10^-6 K^-1 ). Such a nanocomposite (Sr_0.9 Ce_0.1 Fe_0.8 Ni_0.2 O_{3- δ} ) is composed of a single perovskite main phase (77.2 wt%), a Ruddlesden-Popper (RP) second phase (13.3 wt%), and surface-decorated NiO (5.8 wt%) and CeO₂ (3.7 wt%) minor phases. The RP phase promotes the oxygen bulk diffusion while NiO and CeO₂ nanoparticles facilitate the oxygen surface process and O^2- migration from the surface to the main phase, respectively. The strong interaction between four phases in nanodomain creates a synergistic effect, leading to the superior ORR activity.

Insights into the CO2 Stability-Performance Trade-Off of Antimony-Doped SrFeO3-δ Perovskite Cathode for Solid Oxide Fuel Cells

One major challenge for the further development of solid oxide fuel cells is obtaining high-performance cathode materials with sufficient stability against reactions with CO₂ present in the ambient atmosphere. However, the enhanced stability is often achieved by using material systems exhibiting decreased performance metrics. The phenomena underlying the performance and stability trade-off has not been well understood. This paper uses antimony-doped SrFeO_3-δ as a model material to shed light on the relationship between the structure, stability, and performance of perovskite-structured oxides which are commonly used as cathode materials. X-ray absorption revealed that partial substitution of Fe by Sb leads to a series of changes in the local environment of the iron atom, such as a decrease in the iron oxidation state and increase in the oxygen coordination number. Theoretical calculations show that the structural changes are associated with an increase in both the oxygen vacancy formation energy and metal-oxygen bond energy. The area-specific resistance (ASR) of the perovskite oxide increases with Sb doping, indicating a deterioration of the oxygen reduction activity. Exposure of the materials to CO₂ leads to depressed oxygen desorption and an increased ASR, which becomes less pronounced at higher Sb doping levels. Origin of the stability-performance trade-off is discussed based on the structural parameters.

Recent Progresses in Electrocatalysts for Water Electrolysis

Abstract

The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.

Graphical Abstract

Niobium Doped Lanthanum Strontium Ferrite as A Redox-Stable and Sulfur-Tolerant Anode for Solid Oxide Fuel Cells

The Nb-doped lanthanum strontium ferrite perovskite oxide La_0.8 Sr_0.2 Fe_0.9 Nb_0.1 O_3-δ (LSFNb) is evaluated as an anode material in a solid oxide fuel cell (SOFC). The effects of Nb partial substitution in the crystal structure, the electrical conductivity, and the valence of Fe ions are studied. LSFNb exhibits good structural stability in a severe reducing atmosphere at 800 °C, suggesting that high-valent Nb can effectively promote the stability of the lattice structure. The concentration of Fe²⁺ increases after Nb doping, as confirmed by X-ray photoelectron spectroscopy. The maximum power density of a thick Sc-stabilized zirconia (ScSZ) electrolyte-supported single cell reached 241.6 mW cm^-2 at 800 °C with H₂ as fuel. The cell exhibited excellent stability for 100 h continuous operation without detectable degeneration. Scanning electron microscopy clearly revealed exsolution on the LSFNb surface after operation. Meanwhile, LSFNb particles agglomerated significantly during long-term stability testing. Impedance spectra suggested that both the LSFNb anode and the (La_0.75 Sr_0.25 )_0.95 MnO_3-δ /ScSZ cathode underwent an activation process during long-term testing, through which the charge transfer ability increased significantly. Meanwhile, low-frequency resistance (R_L ) mainly attributed to the anode (80 %) significantly increased, probably due to the agglomeration of LSFNb particles. The LSFNb anode exhibits excellent anti-sulfuring poisoning ability and redox stability. These results demonstrate that LSFNb is a promising anode material for SOFCs.