Efficient symmetrical electrodes of PrBaFe2-Co O5+δ (x=0, 0.2,0.4) for solid oxide fuel cells and solid oxide electrolysis cells

Suppressing Structure Delamination for Enhanced Electrochemical Performance of Solid Oxide Cells

Reversible solid oxide cells (rSOCs) have significant potential as efficient energy conversion and storage systems. Nevertheless, the practical application of their conventional air electrodes, such as La0.8Sr0.2MnO3-δ (LSM), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), and PrBa0.8Ca0.2Co2O5+δ (PBCC), remains unsatisfactory due to interface delamination during prolonged electrochemical operation. Using micro-focusing X-ray absorption spectroscopy (µ-XAS), a decrease (increase) in the co-valence state from the electrode surface to the electrode/electrolyte interface is observed, leading to the above delamination. Utilizing the one-pot method to incorporate an oxygen-vacancy-enriched CeO2 electrode into these air electrodes, the uniform distribution of the Co valence state is observed, alleviating the structural delamination. PBCC-CeO2 electrodes exhibited a degradation rate of 0.095 mV h-1 at 650 °C during a nearly 500-h test as compared with 0.907 mV h-1 observed during the 135-h test for PBCC. Additionally, a remarkable increase in electrolysis current density from 636 to 934 mA cm-2 under 1.3 V and a maximum power density from 912 to 989 mW cm-2 upon incorporating CeO2 into PBCC is also observed. BSCF-CeO2 and LSM-CeO2 also show enhanced electrochemical performance and prolonged stability as compared to BSCF and LSM. This work offers a strategy to mitigate the structural delamination of conventional electrodes to boost the performance of rSOCs.

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.

Syngas Production from CO2 and H2O via Solid-Oxide Electrolyzer Cells: Fundamentals, Materials, Degradation, Operating Conditions, and Applications

Highly efficient coelectrolysis of CO2/H2O into syngas (a mixture of CO/H2), and subsequent syngas conversion to fuels and value-added chemicals, is one of the most promising alternatives to reach the corner of zero carbon strategy and renewable electricity storage. This research reviews the current state-of-the-art advancements in the coelectrolysis of CO2/H2O in solid oxide electrolyzer cells (SOECs) to produce the important syngas intermediate. The overviews of the latest research on the operating principles and thermodynamic and kinetic models are included for both oxygen-ion- and proton-conducting SOECs. The advanced materials that have recently been developed for both types of SOECs are summarized. It later elucidates the necessity and possibility of regulating the syngas ratios (H2:CO) via changing the operating conditions, including temperature, inlet gas composition, flow rate, applied voltage or current, and pressure. In addition, the sustainability and widespread application of SOEC technology for the conversion of syngas is highlighted. Finally, the challenges and the future research directions in this field are addressed. This review will appeal to scientists working on renewable-energy-conversion technologies, CO2 utilization, and SOEC applications. The implementation of the technologies introduced in this review offers solutions to climate change and renewable-power-storage problems.

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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MESH Headings Collapse

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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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Exsolution of Co-Fe Alloy Nanoparticles on the PrBaFeCoO5+δ Layered Perovskite Monitored by Neutron Powder Diffraction and Catalytic Effect on Dry Reforming of Methane

Reversible exsolution and dissolution of metal nanoparticles (NPs) in complex oxides have been investigated as an efficient strategy to improve the performance and durability of the catalysts for thermal and electrochemical energy conversion. Here, in situ exsolution of Co-Fe alloy NPs from the layered perovskite PrBaFeCoO_5+δ (PBFC) and their dissolution back into the oxide host have been monitored for the first time by in situ neutron powder diffraction and confirmed by X-ray diffraction and electron microscopy. Catalytic tests for dry reforming of methane showed stable operation over ∼100 h at 800 °C with negligible carbon deposition (<0.3 mg/g_cat h). The CO₂ and CH₄ conversions are among the highest achieved by layered double perovskites. The cyclability of the PBFC catalyst and the potential to improve the catalytic activity by adjusting the composition, size, and the NP distribution would pave the way for highly efficient energy conversion applications.

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.

Water Electrolysis toward Elevated Temperature: Advances, Challenges and Frontiers

Since severe global warming and related climate issues have been caused by the extensive utilization of fossil fuels, the vigorous development of renewable resources is needed, and transformation into stable chemical energy is required to overcome the detriment of their fluctuations as energy sources. As an environmentally friendly and efficient energy carrier, hydrogen can be employed in various industries and produced directly by renewable energy (called green hydrogen). Nevertheless, large-scale green hydrogen production by water electrolysis is prohibited by its uncompetitive cost caused by a high specific energy demand and electricity expenses, which can be overcome by enhancing the corresponding thermodynamics and kinetics at elevated working temperatures. In the present review, the effects of temperature variation are primarily introduced from the perspective of electrolysis cells. Following an increasing order of working temperature, multidimensional evaluations considering materials and structures, performance, degradation mechanisms and mitigation strategies as well as electrolysis in stacks and systems are presented based on elevated temperature alkaline electrolysis cells and polymer electrolyte membrane electrolysis cells (ET-AECs and ET-PEMECs), elevated temperature ionic conductors (ET-ICs), protonic ceramic electrolysis cells (PCECs) and solid oxide electrolysis cells (SOECs).

Structural Features and Defect Equilibrium in Cubic PrBa1−xSrxFe2O6−δ

The structure, oxygen non-stoichiometry, and defect equilibrium in perovskite-type PrBa_1−xSr_xFe₂O_6−δ (x = 0, 0.25, 0.50) synthesized at 1350 °C were studied. For all compositions, X-ray diffraction testifies to the formation of a cubic structure (S.G. Pm3¯m), but an electron diffraction study reveals additional diffuse satellites around each Bragg spot, indicating the primary incommensurate modulation with wave vectors about ±0.43a*. The results were interpreted as a sign of the short-order in both A-cation and anion sublattices in the areas of a few nanometers in size, and of an intermediate state before the formation of an ordered superstructure. An increase in oxygen deficiency was found to promote the ordering, whereas partial substitution of barium by strontium caused the opposite effect. The oxygen content in oxides as a function of oxygen partial pressure and temperature was measured by coulometric titration, and the data were used for the modeling of defect equilibrium in oxides. The simulation results implied oxygen vacancy ordering in PrBa_1−xSr_xFe₂O_6−δ that is in agreement with the electron diffraction study. Besides oxidation and charge disproportionation reactions, the reactions of oxygen vacancy distribution between non-equivalent anion positions, and their trapping in clusters with Pr³⁺ ions were taken into account by the model. It was demonstrated that an increase in the strontium content in Pr_0.5Ba_0.5−xSr_xFeO_3−δ suppressed ordering of oxygen vacancies, increased the binding energy of oxygen ions in the oxides, and resulted in an increase in the concentration of p-type carriers.

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.