Systematic evaluation of Co-free LnBaFe2O5+δ (Ln=Lanthanides or Y) oxides towards the application as cathodes for intermediate-temperature solid oxide fuel cells

A-Site Engineering of the High-Entropy Perovskite Pr0.4La0.4Ba0.4Sr0.4Ca0.4Fe2O5+δ Cathode for Intermediate-Temperature SOFCs

Mixed-oxygen ionic and electronic conduction is crucial for the cathode materials of solid oxide fuel cells, ensuring high efficiency and low-temperature operation. However, the electronic and oxygen ionic conductivity of traditional Fe-based layered perovskite cathode materials is low, resulting in insufficient oxygen reduction reactivity. Herein, a type of high-entropy perovskite oxide consisting of five equimolar metals, Pr0.4La0.4Ba0.4Sr0.4Ca0.4Fe2O5+δ (PLBSCF), a high-performance cobalt-free cathode derived from the PrBaFe2O5+δ (PBF), is proposed. Such A-site engineering could not only increase the oxygen vacancy concentration of PLBSCF but also give higher conductivity than PBF, thus significantly reducing the polarization impedance of the symmetric cell to only 0.052 Ω·cm2 at 750 °C. The good output performance of a single cell is also realized. The peak power density of the single cell with PLBSCF-Ce0.9Gd0.1O2-δ (GDC) as the cathode at 750 °C was 0.853 W·cm-2. Additionally, the single cell with the PLBSCF cathode exhibits a good durable performance of 100 h at 750 °C. Combining the distribution of relaxation time analysis, it can be seen that the enhancement of the oxygen reduction reaction is due to the reduction of intermediate-frequency and low-frequency resistance, indicating an improvement in the charge transfer process and adsorption/dissociation process of molecular oxygen.

Advancements in Perovskite-Based Cathode Materials for Solid Oxide Fuel Cells: A Comprehensive Review

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C Collapse

Key Words

• Ruddlesden-Popper phase
• Solid oxide fuel cell
• double perovskite
• electrochemical activity
• polarization resistance
• thermal expansion coefficient

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Affiliation(s)

Ayesha Samreen Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Muhammad Sudais Ali Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Muhammad Huzaifa Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Nasir Ali Research Center for Sensing Materials and Devices, Zhejiang Labs, Yuhang District, Nanhu, China
Bilal Hassan Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Fazl Ullah Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Shahid Ali Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan
Nor Anisa Arifin Materials Engineering and Testing Group, TNB Research Sdn Bhd, No.1, Kawasan Institusi Penyelidikan, Jln Ayer Hitam, 43000, Kajang, Selangor, Malaysia

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Fe-based double perovskite with Zn doping for enhanced electrochemical performance as intermediate-temperature solid oxide fuel cell cathode material

This study aims to investigate the implications of transition-metal Zn doping at the B-site on the crystal structure, average thermal expansion coefficient (TEC), electrocatalytic activity, and electrochemical performance of LaBaFe2O5+δ by preparing LaBaFe2-xZnxO5+δ (x = 0, 0.05, 0.1, 0.15, 0.2, LBFZx). The X-ray diffraction (XRD) results show that Zn2+ doping does not change the crystal structure, the unit cell volume increases, and the lattice expands. The X-ray photoelectron spectroscopy (XPS) and mineral titration results show that the oxygen vacancy concentration and Fe4+ content gradually increase with the increase in doping amount. TEC decreases with the increase in Zn2+ doping amount, and the TEC of LBFZ0.2 is 11.4 × 10-6 K-1 at 30-750 °C. The conductivity has the best value of 103 S cm-1 at the doping amount of x = 0.1. The scanning electron microscopy (SEM) images demonstrate that the electrolyte CGO(Gd0.1Ce0.9O1.95) becomes denser after high-temperature calcination, and the cathode material is well attached to the electrolyte. The electrochemical impedance analysis shows that Zn2+ doping at the B-site can reduce the (Rp) polarization resistance, and the Rp value of the symmetric cell with LaBaFe1.8Zn0.2O5+δ as cathode at 800 °C is 0.014 Ω cm2. The peak power density (PPD) value of the anode-supported single cell is 453 mW cm-2, which shows excellent electrochemical performance.

Cobalt-Free Double Perovskite Oxide as a Promising Cathode for Solid Oxide Fuel Cells

Double perovskite oxide PrBaFe₂O_5+δ is a potential cathode material for intermediate-temperature solid oxide fuel cells. To improve its electrochemical performance, the trivalent element Ga is investigated to partially replace Fe, forming PrBaFe_2-xGa_xO_5+δ (PBFGx, x = 0.05, 0.1, and 0.15). The doping effects on physicochemical properties and electrochemical properties are analyzed regarding the phase structures, element valence states, amount of oxygen vacancies, content of oxygen species, oxygen surface exchange coefficients (k_chem), electrochemical polarization resistance, and single-cell performance. Specifically, PBFG0.1 exhibits improved k_chem, such as a 19% improvement from 4.09 × 10^-4 to 4.86 × 10^-4 cm s^-1 at 750 °C, due to the increased concentration of reactive oxygen species and oxygen vacancies. Consequently, the interfacial polarization resistance is decreased by 28% from 0.057 to 0.041 Ω cm² at 800 °C. The subreaction steps of the oxygen reduction reaction in the PBFG0.1 cathode are further investigated, which suggests that the oxygen dissociation process is greatly enhanced by doping Ga. Meanwhile, doping Ga increases the peak power density of the anode-supported single cell by 36% from 629 to 856 mW cm^-2 at 800 °C. The single cell with the PBFG0.1 cathode also exhibits good stability in 100 h of long-term operation at 750 °C.

Promoted Performance of Layered Perovskite PrBaFe2O5+δ Cathode for Protonic Ceramic Fuel Cells by Zn Doping

Proton-conducting solid–oxide fuel cell (H-SOFC) is an alternative promising low-temperature electrochemical cell for renewable energy, but the performance is insufficient because of the low activity of cathode materials at low temperatures. A layered perovskite oxide PrBaFe1.9Zn0.1O5+δ (PBFZ) was synthesized and investigated as a promising cathode material for low-temperature H-SOFC. Here, the partial substitution of Fe by Zn further enhances the electrical conductivity and thermal compatibility of PrBaFe2O5+δ (PBF). The PBFZ exhibits improved conductivity in the air at intermediate temperatures and good chemical compatibility with electrolytes. The oxygen vacancy formed at the PBFZ lattice due to Zn doping enhances proton defects, resulting in an improved performance by extending the catalytic sites to the whole cathode area. A single cell with a Ni-BZCY anode, PBFZ cathode, and BaZr0.7Ce0.2Y0.1O3-δ (BZCY) electrolyte membrane was successfully fabricated and tested at 550–700 °C. The maximum power density and Rp were enhanced to 513 mW·cm−2 and 0.3 Ω·cm2 at 700 °C, respectively, due to Zn doping.

Layered Oxygen-Deficient Double Perovskites as Promising Cathode Materials for Solid Oxide Fuel Cells

Development of new functional materials with improved characteristics for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) is one of the most important tasks of modern materials science. High electrocatalytic activity in oxygen reduction reactions (ORR), chemical and thermomechanical compatibility with solid electrolytes, as well as stability at elevated temperatures are the most important requirements for cathode materials utilized in SOFCs. Layered oxygen-deficient double perovskites possess the complex of the above-mentioned properties, being one of the most promising cathode materials operating at intermediate temperatures. The present review summarizes the data available in the literature concerning crystal structure, thermal, electrotransport-related, and other functional properties (including electrochemical performance in ORR) of these materials. The main emphasis is placed on the state-of-art approaches to improving the functional characteristics of these complex oxides.

Enhanced Electrochemical Performance of the Fe-Based Layered Perovskite Oxygen Electrode for Reversible Solid Oxide Cells

Reversible solid oxide cells (RSOCs) present a conceivable potential for addressing energy storage and conversion issues through realizing efficient cycles between fuels and electricity based on the reversible operation of the fuel cell (FC) mode and electrolysis cell (EC) mode. Reliable electrode materials with high electrochemical catalytic activity and sufficient durability are imperatively desired to stretch the talents of RSOCs. Herein, oxygen vacancy engineering is successfully implemented on the Fe-based layered perovskite by introducing Zr⁴⁺, which is demonstrated to greatly improve the pristine intrinsic performance, and a novel efficient and durable oxygen electrode material is synthesized. The substitution of Zr at the Fe site of PrBaFe₂O_5+δ (PBF) enables enlarging the lattice free volume and generating more oxygen vacancies. Simultaneously, the target material delivers more rapid oxygen surface exchange coefficients and bulk diffusion coefficients. The performance of both the FC mode and EC mode is greatly enhanced, exhibiting an FC peak power density (PPD) of 1.26 W cm^-2 and an electrolysis current density of 2.21 A cm^-2 of single button cells at 700 °C, respectively. The reversible operation is carried out for 70 h under representative conditions, that is, in air and 50% H₂O + 50% H₂ fuel. Eventually, the optimized material (PBFZr), mixed with Gd_0.1Ce_0.9O₂, is applied as the composite oxygen electrode for the reversible tubular cell and presents excellent performance, achieving 4W and 5.8 A at 750 °C and the corresponding PPDs of 140 and 200 mW cm^-2 at 700 and 750 °C, respectively. The enhanced performance verifies that PBFZr is a promising oxygen electrode material for the tubular RSOCs.

Boosting the Electrochemical Performance of Fe-Based Layered Double Perovskite Cathodes by Zn2+ Doping for Solid Oxide Fuel Cells

Mixed oxygen ionic and electronic conduction is a vital function for cathode materials of solid oxide fuel cells (SOFCs), ensuring high efficiency and low-temperature operation. However, Fe-based layered double perovskites, as a classical family of mixed oxygen ionic and electronic conducting (MIEC) oxides, are generally inactive toward the oxygen reduction reaction due to their intrinsic low electronic and oxygen-ion conductivity. Herein, Zn doping is presented as a novel pathway to improve the electrochemical performance of Fe-based layered double perovskite oxides in SOFC applications. The results demonstrate that the incorporation of Zn ions at Fe sites of the PrBaFe₂O_5+δ (PBF) lattice simultaneously regulates the concentration of holes and oxygen vacancies. Consequently, the oxygen surface exchange coefficient and oxygen-ion bulk diffusion coefficient of Zn-doped PBF are significantly tuned. The enhanced mixed oxygen ionic and electronic conduction is further confirmed by a lower polarization resistance of 0.0615 and 0.231 Ω·cm² for PrBaFe_1.9Zn_0.1O_5+δ (PBFZ0.1) and PBF, respectively, which is measured using symmetric cells at 750 °C. Moreover, the PBFZ0.1-based single cell demonstrates the highest output performance among the reported Fe-based layered double perovskite cathodes, rendering a peak power density of 1.06 W·cm^-2 at 750 °C and outstanding stability over 240 h at 700 °C. The current work provides a highly effective strategy for designing cathode materials for next-generation SOFCs.

Compositional Engineering of a La1-xBaxCoO3-δ-(1-a) BaZr0.9Y0.1O2.95 (a = 0.6, 0.7, 0.8 and x = 0.5, 0.6, 0.7) Nanocomposite Cathodes for Protonic Ceramic Fuel Cells

Compositionally engineered a La1-xBaxCoO3-δ-(1-a) BaZr0.9Y0.1O2.95 (a = 0.6, 0.7, 0.8 and x = 0.5, 0.6, 0.7) (LBZ) nanocomposite cathodes were prepared by oxidation driven in situ exsolution of a single-phase material deposited on a BaZr0.9Y0.1O2.95 electrolyte. The processing procedure of the cathode was optimized by reducing the number of thermal treatments as the single-phase precursor was deposited directly on the electrolyte. The exsolution and firing of the cathodes occurred in one step. The electrochemical performance of symmetrical cells with the compositionally engineered cathodes was investigated by impedance spectroscopy in controlled atmospheres. The optimized materials processing gave web-like nanostructured cathodes with superior electrochemical performance for all compositions. The area specific resistances obtained were all below 12 Ω·cm2 at 400 °C and below 0.59 Ω·cm2 at 600 °C in 3% moist synthetic air. The resistances of the nominal 0.6 La0.5Ba0.5CoO3-δ-0.4 BaZr0.9Y0.1O2.95 and 0.8 La0.5Ba0.5CoO3-δ-0.2 BaZr0.9Y0.1O2.95 composite cathodes were among the lowest reported for protonic ceramic fuel cells cathodes in symmetrical cell configuration with ASR equal to 4.04 and 4.84 Ω·cm2 at 400 °C, and 0.21 and 0.27 Ω·cm2 at 600 °C, respectively.

High-Performance La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ Cathode Composites via an Exsolution Mechanism for Protonic Ceramic Fuel Cells

A novel exsolution process was used to fabricate complex all-oxide nanocomposite cathodes for Protonic Ceramic Fuel Cells (PCFCs). The nanocomposite cathodes with La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ nominal composition were prepared from a single-phase precursor via an oxidation-driven exsolution mechanism. The exsolution process results in a highly nanostructured and intimately interconnected percolating network of the two final phases, one proton conducting (BaZr1−zYzO3−δ) and one mixed oxygen ion and electron conducting (La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ), yielding excellent cathode performance. The cathode powder is synthesized as a single-phase cubic precursor by a modified Pechini route followed by annealing at 700 °C in N2. The precursor phase is exsolved into two cubic perovskite phases by further heat treatment in air. The phase composition and chemical composition of the two phases were confirmed by Rietveld refinement. The electrical conductivity of the composites was measured and the electrochemical performance was determined by impedance spectroscopy of symmetrical cells using BaZr0.9Y0.1O2.95 as electrolyte. Our results establish the potential of this exsolution method where a large number of different cations can be used to design composite cathodes. The La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr0.9Y0.1O2.95 composite cathode shows the best performance of 0.44 Ω∙cm2 at 600 °C in 3% moist synthetic air.

Designing a protonic ceramic fuel cell with novel electrochemically active oxygen electrodes based on doped Nd0.5Ba0.5FeO3−δ

Slight co-doping of Nd_0.5Ba_0.5FeO_3−δ results in enhancement of both ionic transport and electrochemical behavior, indicating that NBFCo is a promising electrode for proton-conducting electrolytes.

Evaluation of Ba-deficient PrBa1−xFe2O5+δ oxides as cathode materials for intermediate-temperature solid oxide fuel cells

Cobalt-free double perovskite oxides are promising cathode materials in intermediate-temperature solid oxide fuel cells, and often suffer from low activity in oxygen reduction reactions.

Layered perovskite Sm1−xLaxBaFe2O5+δ as cobalt-free cathodes for IT-SOFCs

The La-doped SmBaFe₂O_5+δ series of materials were investigated systematically to determine whether they may serve as effective cathodes.