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

Evolution and Reconstruction of Air-Electrode Surface Composition in Reversible Protonic Ceramic Cells: Mechanisms, Impacts on Catalytic Performance, and Optimization Strategies - A Review

Reversible protonic ceramic cells (R-PCCs) are at the forefront of electrochemical conversion devices, capable of reversibly and efficiently converting chemical energy into electricity at intermediate temperatures (350-700 °C) with zero carbon emissions. However, slow surface catalytic reactions at the air-electrode often hinder their performance and durability. The electrode surface is not merely an extension of the bulk structure, equilibrium reconstruction can lead to significantly different crystal-plane terminations and morphologies, which are influenced by material's intrinsic properties and external reaction conditions. Understanding electrode surface evolution at elevated temperatures in water-containing, oxidative atmospheres presents significant importance. In this review, a comprehensive summary of recent processes in applying advanced characterization techniques for high-temperature electrode surfaces is provided, exploring the correlations between surface evolution and performance fluctuations by examining the structural evolution and reconstruction of various air-electrode surfaces associated with degradation and activation phenomena, offering insights into their impact on electrode performance. Furthermore, reported strategies and recent advances in enhancing the electrochemical performance of R-PCCs through engineering air-electrode surfaces is discussed. This review offers valuable insights into surface evolution in R-PCCs and is expected to guide future developments in high-temperature catalysis, solid-state ionics, and energy materials.

Lewis Basicity Enhanced SrFeO3-δ-Based Perovskite Fuel Electrode for Surface and Bulk Processes Co-Accelerated CO2 Reduction in Solid Oxide Electrolysis Cells

Electroreduction of CO2 to CO by solid oxide electrolysis cells (SOEC) is an effective means to realize carbon neutralization. However, the sluggish kinetics at SOEC fuel electrode impedes its further practical application. Herein, the doping strategy of cesium ion (Cs1+) is employed to develop a series of perovskite-type fuel electrode materials, i.e., CsxSr1-xFe0.9Nb0.1O3-δ (x = 0.05, 0.1, 0.15). Combining the results of experiments and theoretical calculations, it is found that the introduction of Cs1+ into A-site of SrFeO3-δ-based perovskite accelerates the reaction kinetics of CO2 adsorption and dissociation due to increased lattice oxygen basicity caused by the low electronegativity of Cs1+. In addition, in comparison to Sr2+, the larger ionic radius and lower valence of Cs1+ is beneficial to decrease formation energy of oxygen vacancy and migration barrier of oxygen ion in perovskite bulk. Because of those merits brought by Cs1+ doping, the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte-supported electrolysis cells with Cs0.1Sr0.9Fe0.9Nb0.1O3-δ fuel electrode presents satisfied current density of 2205 mA cm-2 at 1.6 V and 850 °C. The stable long-term operation of electrolysis cells is also demonstrated at applied current density of 600 mA cm-2 and 800 °C for 100 h.

Recent Advances in Barium Cobaltite-Based Perovskite Oxides as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are considered as advanced energy conversion technologies due to the high efficiency, fuel flexibility, and all-solid structure. Nevertheless, their widespread applications are strongly hindered by the high operational temperatures, limited material selection choices, inferior long-term stability, and relatively high costs. Therefore, reducing operational temperatures of SOFCs to intermediate-temperature (IT, 500-800 °C) range can remarkably promote the practical applications by enabling the use of low-cost materials and enhancing the cell stability. Nevertheless, the conventional cathodes for high-temperature SOFCs display inferior electrocatalytic activity for oxygen reduction reaction (ORR) at reduced temperatures. Barium cobaltite (BaCoO3-δ)-based perovskite oxides are regarded as promising cathodes for IT-SOFCs because of the high free lattice volume and large oxygen vacancy content. However, BaCoO3-δ-based perovskite oxides suffer from poor structural stability, inferior thermal compatibility, and insufficient ionic conductivity. Herein, an in-time review about the recent advances in BaCoO3-δ-based cathodes for IT-SOFCs is presented by emphasizing the material design strategies including functional/selectively doping, deficiency control, and (nano)composite construction to enhance the ORR activity/durability and thermal compatibility. Finally, the currently existed challenges and future research trends are presented. This review will provide valuable insights for the development of BaCoO3-δ-based electrocatalysts for various energy conversion/storage technologies.

Self-Assembled Controllable Cu-Based Perovskite/Calcium Oxide Hybrids with Strong Interfacial Interactions for Enhanced CH4 Electrosynthesis

Cu-based perovskite oxide catalysts show promise for CO2 electromethanation, but suffer from unsatisfactory CH4 selectivity and poor stability. Here, we report self-assembled, controllable Cu-based perovskite/calcium oxide hybrids with strongly interacting interfaces for high-performance CH4 electrosynthesis. As proof-of-concept catalysts, the La2CuO4/(CaO)x (x from 0.2 to 1.2) series has tunable CaO phase concentrations and thus controllable interface sizes. The La2CuO4 and CaO components are intimately connected at the interface, leading to strong interfacial interactions mainly manifested by marked electron transfer from Ca2+ to Cu2+. In CH4 electrosynthesis, their activity and selectivity show a volcano-type dependence on the CaO phase concentrations and are positively correlated with the interface sizes. Among them, the La2CuO4/(CaO)0.8 delivers the optimal activity and selectivity for CH4, together with good stability, much better than those of a physical-mixture counterpart and most reported Cu-based perovskite oxides. Moreover, La2CuO4/(CaO)0.8 stands out as one of the most effective Cu-based catalysts for CH4 electrosynthesis, achieving a high CH4 selectivity of 77.6% at 300 mA cm-2. Our experiments and theoretical calculations highlight the significant role of self-assembly-induced strong interfacial interactions in promoting *CO adsorption/hydrogenation, intensifying resistance to structural degradation, and consequently underpinning the achievement of such optimized performance.

Perovskite nanocomposites: synthesis, properties, and applications from renewable energy to optoelectronics

The oxide and halide perovskite materials with a ABX3 structure exhibit a number of excellent properties, including a high dielectric constant, electrochemical properties, a wide band gap, and a large absorption coefficient. These properties have led to a range of applications, including renewable energy and optoelectronics, where high-performance catalysts are needed. However, it is difficult for a single structure of perovskite alone to simultaneously fulfill the diverse needs of multiple applications, such as high performance and good stability at the same time. Consequently, perovskite nanocomposites have been developed to address the current limitations and enhance their functionality by combining perovskite with two or more materials to create complementary materials. This review paper categorizes perovskite nanocomposites according to their structural composition and outlines their synthesis methodologies, as well as their applications in various fields. These include fuel cells, electrochemical water splitting, CO2 mitigation, supercapacitors, and optoelectronic devices. Additionally, the review presents a summary of their research status, practical challenges, and future prospects in the fields of renewable energy and electronics.

Ultrafast Sintered Composite Cathode Incorporating a Negative Thermal Expansion Material for High-Performance Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs) offer a promising, clean, and efficient energy conversion solution. However, thermal mismatch between cathodes and electrolytes remains a critical obstacle, leading to interfacial damage such as cracking and delamination. Incorporating negative thermal expansion (NTE) materials into the cathode can mitigate this issue. The challenge lies in integrating NTE materials without compromising electrochemical performance or causing unwanted reactions during sintering. This study introduces a novel BaFe0.9Zr0.1O3-δ (BFZ)-NdMnO3-δ composite cathode fabricated using an ultrafast high-temperature sintering (UHS) process. This approach mitigates thermal expansion while boosting the cathode's catalytic activity compared to a single-phase BFZ cathode. The resulting fuel cell achieves a high peak power density of ∼550 mW cm-2 at 600 °C and demonstrates excellent stability during a 100 h test at 550 °C. These findings highlight the potential of UHS for developing high-performance, thermally compatible cathode materials that advance the field of PCFCs.

In situ construction of A-site high-entropy perovskites with interfacial CeO2 for a high-performance IT-SOFC air-electrode

Developing an intermediate-temperature solid oxide fuel cell (IT-SOFC) is one of the most promising ways of achieving carbon neutrality, but its air-electrode is restricted by the conflict between the sluggish catalytic activity and durability. Herein, an A-site high-entropy perovskite composite La0.2Ba0.2Sr0.2Ca0.2Ce0.2-xCoO3-δ-xCeO2 (LBSCCC-CeO2) air-electrode material is fabricated via a one-step self-constructing strategy, which shows excellent oxygen reduction activity and stability due to the high-entropy structure and the synergy effect between LBSCCC and interfacial CeO2. This work provides a new way of fabricating high-performance air-electrodes in IT-SOFCs.

Revealing the detrimental CO2 reduction effect of La0.6Sr0.4FeO3-δ-derived heterostructure in solid oxide electrolysis cells

Solid oxide electrolysis cells hold unique Faraday efficiency and favored thermodynamic/kinetics for CO2 reduction to CO. Perovskite oxide-based composite materials are promising alternatives to Ni-based cermet electrodes in SOECs. However, contrary results of the electrocatalytic activity over single-phase perovskite oxide exist and the rationale of the negative effect is not well revealed. In this work, two-phase perovskite materials with various complementary properties and unique interfaces are self-assembled, which was realized by "subtractive" defect-driven phase separation. The obtained heterostructure electrodes showed reduced performance over that of single-phase materials although the cyclic stability was improved. The main reasons for the performance degradation are the decrease of electrical conductivity, oxygen vacancy concentration while increasing the average valence state of B-site Fe cations, and electrode surface Sr aggregation. This work highlights the self-assembly method and insight into the rational design and synthesis of active electrodes/catalysts for CO2 conversion in solid oxide cells.

Synergistic Effects of In-Situ Exsolved Ni-Ru Bimetallic Catalyst on High-Performance and Durable Direct-Methane Solid Oxide Fuel Cells

Direct-methane solid oxide fuel cells (CH4-SOFCs) have gained significant attention as methane, the primary component of natural gas (NG), is cheap and widely available and the natural gas infrastructures are relatively mature. However, at intermediate temperatures (e.g., 600-650 °C), current CH4-SOFCs suffer from low performance and poor durability under a low steam-to-carbon ratio (S/C ratio), which is ascribed to the Ni-based anode that is of low catalytic activity and prone to coking. Herein, with the guidance of density functional theory (DFT) studies, a highly active and coking tolerant steam methane reforming (SMR) catalyst, Sm-doped CeO2-supported Ni-Ru (SCNR), was developed. The synergy between Ni and Ru lowers the activation energy of the first C-H bond activation and promotes CHx decomposition. Additionally, Sm doping increases the oxygen vacancy concentration in CeO2, facilitating H2O adsorption and dissociation. The SCNR can therefore simultaneously activate both CH4 and H2O molecules while oxidizing the CH* and improving coking tolerance. We then applied SCNR as the CH4-SOFC anode catalytic reforming layer. A peak power density of 733 mW cm-2 was achieved at 650 °C, representing a 55% improvement compared to that of pristine CH4-SOFCs (473 mW cm-2). Moreover, long-term durability testing, with >2000 h continuous operation, was performed under almost dry methane (5% H2O). These results highlight that CH4-SOFCs with a SCNR catalytic layer can convert NG to electricity with high efficiency and resilience.

Self-Assembly Dual Active Site Nanocomposite Anode Ce0.6Mn0.3Fe0.1O2-δ/NiFe/MnOx for Electrooxidative Dehydrogenation of Ethane to Ethylene

As the demand for ethylene grows continuously in industry, conversion of ethane to ethylene has become more and more important; however, it still faces fundamental challenges of low ethane conversion, low ethylene selectivity, overoxidation, and instability of catalysts. Electrooxidative dehydrogenation of ethane (EODHE) in a solid oxide electrolysis cell (SOEC) is an alternative process. Here, a multiphase oxide Ce0.6Mn0.3Fe0.1O2-δ-NiFe-MnOx has been fabricated by a self-assembly process and utilized as the SOEC anode material for EODHE. The highest ethane conversions reached 52.23% with 94.11% ethylene selectivity at the anode side and CO with 10.9 mL min-1 cm-2 at the cathode side, at 1.8 V at 700 °C. The remarkable electrooxidative performance of CMF-NiFe-MnOx is ascribed to the NiFe alloy and MnOx nanoparticles and improvement of the concentration of oxygen vacancies within the fluorite substrate, generating dual active sites for C2H6 adsorption, dehydrogenation, and selective transformation of hydrogen without overoxidizing the ethylene generated. Such a tailored strategy achieves no significant degradation observed after 120 h of operation and constitutes a promising basis for EODHE.

Nanotechnologies in ceramic electrochemical cells

Although they are emerging technologies for achieving high-efficiency and green and eco-friendly energy conversion, ceramic electrochemical cells (CECs), i.e. solid oxide electrolysis cells (SOECs) and fuel cells (SOFCs), are still fundamentally limited by their inferior catalytic activities at low temperature, poor thermo-mechanical stability, high material cost, etc. The materials used in electrolytes and electrodes, which are the most important components in CECs, are highly associated with the cell performances. Therefore, rational design of electrolytes and electrodes with excellent catalytic activities and high stabilities at relatively low cost is a meaningful and valuable approach for the development of CECs. Nanotechnology is a powerful tool for improving the material performances in CECs owing to the favourable effects induced by the nanocrystallization of electrolytes and electrodes. Herein, a relatively comprehensive review on the nanotechnologies implemented in CECs is conducted. The working principles of CECs and the corresponding challenges were first presented, followed by the comprehensive insights into the working mechanisms of nanocrystalline materials in CECs. Then, systematic summarization and analyses of the commonly used nano-engineering strategies in the fabrication of CEC materials, including physical and chemical methods, were provided. In addition, the frontiers in the research of advanced electrolyte and electrode materials were discussed with a special emphasis on the modified electrochemical properties derived from nanotechnologies. Finally, the bottlenecks and the promising breakthroughs in nanotechnologies were highlighted in the direction of providing useful references for rational design of nanomaterials for CECs.

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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Ni/NiO Exsolved Perovskite La0.2Sr0.7Ti0.9Ni0.1O3-δ for Semiconductor-Ionic Fuel Cells: Roles of Electrocatalytic Activity and Physical Junctions

A semiconductor-ionic fuel cell (SIFC) is recognized as a promising technology and an alternative approach to reduce the operating temperature of solid oxide fuel cells. The development of alternative semiconductors substituting easily reduced transition metal oxide is a great challenge as high activity and durability should be satisfied simultaneously. In this study, the B-site Ni-doped La_0.2Sr_0.7Ti_0.9Ni_0.1O_3-δ (LSTN) perovskite is synthesized and used as a potential semiconductor for SIFC. The in situ exsolution and A-site deficiency strategy enable the homogeneous decoration of Ni/NiO nanoparticles as reactive sites to improve the electrode reaction kinetics. It also supports the formation of basic ingredient of the Schottky junction to improve the charge separation efficiency. Furthermore, additional symmetric Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) electrocatalytic electrode layers significantly enhance the electrode reaction activity and cells' charge separation efficiency, as confirmed by the superior open circuit voltage of 1.13 V (close to Nernst's theoretical value) and peak power density of 650 mW cm^-2 at 550 °C, where the latter is one order of magnitude higher than NCAL electrode-free SIFC. Additionally, a bulk heterojunction effect is proposed to illustrate the electron-blocking and ion-promoting processes of the semiconductor-ionic composite electrolyte in SIFCs, based on the energy band values of the applied materials. Overall, we found that the energy conversion efficiency of novel SIFC can be remarkably improved through in situ exsolution and intentional introduction of the catalytic functionality.

Interfacial electron transfer in heterojunction nanofibers for highly efficient oxygen evolution reaction

Efficient catalysts for the oxygen evolution reaction (OER) are critical to the progress of electrochemical devices for clean energy conversion and storage. Although heterogeneous electrocatalysts have superior activity, it is a great challenge to elucidate electron transfer at surface catalytic sites and intrinsic mechanisms. Herein, we demonstrate a new type of heterostructure electrocatalyst in which Sr_0.9Ce_0.05Fe_0.95Ru_0.05O₃ fibers are hybridized with in situ grown RuO₂ nanoparticles (SCFR-RuO₂). We investigate its unique structure, electron transfer mechanisms related to the highly OER activity by combining experimental and theoretical calculations. Remarkably, SCFR-RuO₂ shows an optimized OER overpotential of 295 mV at 10 mA cm^-2. The promoted electron transfer and OER kinetics are ascribed to the coupling of electronic effects at the SCFR-RuO₂ heterostructure. A strong triangular relationship among overpotential-Tafel slope-work function is proposed to be a potential descriptor of OER activity in SCFR-RuO₂. These insights provide guidelines for tuning the OER performance via modified work functions in perovskite electrocatalysts.

Toward Superb Perovskite Oxide Electrocatalysts: Engineering of Coupled Nanocomposites

A significant issue that bedeviled the commercialization of renewable energy technologies, ranging from low-temperature water electrolyzers to high-temperature solid oxide cells, is the lack of high-performance catalysts. Among various candidates that could tackle such a challenge, perovskite oxides are rising-star materials because of their unique structural and compositional flexibility. However, single-phase perovskite oxides are challenging to satisfy all the requirements of electrocatalysts concurrently for practical applications, such as high catalytic activity, excellent stability, good ionic and electronic conductivities, and superior chemical/thermo-mechanical robustness. Impressively, perovskite oxides with coupled nanocomposites are emerging as a novel form offering multifunctionality due to their intrinsic features, including infinite interfaces with solid interaction, tunable compositions, flexible configurations, and maximum synergistic effects between assorted components. Considering this new configuration has attracted great attention owing to its promising performances in various energy-related applications, this review timely summarizes the leading-edge development of perovskite oxide-based coupled nanocomposites. Their state-of-art synthetic strategies are surveyed and highlighted, their unique structural advantages are highlighted and illustrated through the typical oxygen reduction reaction and oxygen evolution reactions in both high and low-temperature applications. Opinions on the current critical scientific issues and opportunities in this burgeoning research field are all provided.

Electrokinetic Insights into the Triple Ionic and Electronic Conductivity of a Novel Nanocomposite Functional Material for Protonic Ceramic Fuel Cells

Triple ionic and electronic conductivity (TIEC) in cathode materials for protonic ceramic fuel cells (PCFCs) is a desirable feature that enhances the spatial expansion of active reaction sites for electrochemical oxygen reduction reaction. The realization of optimal TIEC in single-phase materials, however, is challenging. A facile route that facilitates the optimization of TIEC in PCFC cathodes is the strategic development of multiphase cathode materials. In this study, a cubic-rhombohedral TIEC nanocomposite material with the composition Ba(CeCo)_0.4 (FeZr)_0.1 O_{3- δ} (BCCFZ) is designed via self-assembly engineering. The material consists of a mixed ionic and electronic conducting phase, BaCo_{1-( x + y + z )} Ce_x Fe_y Zr_z O_{3- δ} (M-BCCFZ), and a dominant proton-conducting phase, BaCe_{1-( x + y + z )} Co_x Zr_y Fe_z O_{3- δ} (H-BCCZF). The dominant cerium-rich H-BCCFZ phase enhances the material's oxygen vacancy concentration and the proton defects formation and transport with a low enthalpy of protonation of -30 ± 9 kJ mol^-1 . The area-specific resistance of the BCCFZ symmetrical cell is 0.089 Ω cm² at 650 °C in 2.5% H₂ O-air. The peak power density of the anode-supported single cell based on BCCFZ cathode reaches 1054 mW cm^-2 at 650 °C with good operation stability spanning over 500 h at 550 °C. These promote BCCFZ as a befitting cathode material geared toward PCFC commercialization.

Improved Durability of High-Performance Intermediate-Temperature Solid Oxide Fuel Cells with a Ba-Doped La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode

As a device for direct conversion of chemical energy into electrical energy, the solid oxide fuel cell (SOFC) contributes positively to the sustainable development strategy. However, the commercialization of fuel cells is still impeded by severe cathode degradation caused by its limited stability at operating temperatures and being prone to Cr-poisoning from Cr-containing alloy interconnectors commonly used in these cells. This paper reports the development of a high-durability Ba-doped LSCF(La0.6Sr0.4Co0.2Fe0.8O3-δ) cathode material under realistic fuel cell operating conditions in the presence of the Cr alloy. In particular, when tested in a symmetrical cell constructed of Ba-doped LSCF, the polarization resistance of the cell remains very low at 0.06 Ω cm2 after being tested at 800 °C for 120 h exposed to Cr in 3% humidified air. In contrast, for the undoped LSCF under the same testing conditions, the polarization resistance of the cell increases ∼10 times from 0.22 Ω cm2 of the pristine cell to 2.18 Ω cm2 after Cr-exposure testing. Furthermore, when tested in an anode-supported complete cell as a cathode under typical SOFC operation conditions at 750 °C, the cell with the Ba-doped LSCF cathode displays significantly low degradation rates of 0.00056% h-1 (without Cr) and 0.00310% h-1 (with Cr); both are much lower than that of the cell using the undoped LSCF cathode (0.00124% h-1 without Cr and 0.01082% h-1 with Cr). This enhanced durability and Cr-tolerance exhibited by the Ba-doped LSCF cathode stem from its higher crystal structure stability and improved chemical resistance compared to undoped LSCF.

Structural Engineering of Cobalt-Free Perovskite Enables Efficient and Durable Oxygen Reduction in Solid Oxide Fuel Cells

Developing low-cost, efficient, and durable cobalt-free perovskite oxides for oxygen reduction reaction at intermediate-to-low temperatures is crucial to enhance the viability of solid oxide fuel cells (SOFCs), a promising ingredient for establishing a more sustainable future. Herein, a highly active and robust cobalt-free perovskite Ba_0.75 Sr_0.25 Fe_0.95 P_0.05 O_3-δ (BSFP) oxygen electrode via a facile co-doping strategy for intermediate-to-low temperature SOFCs (ILT-SOFCs) is reported by a combined experimental and theoretical approach. Attributed to stable and oxygen defect-rich structure, and remarkable intrinsic oxygen transport kinetics, the BSFP cathode shows exceptional catalytic performance, including record-level power output among iron-based perovskite cathodes (1464 mW cm^-2 at 600 °C), low area-specific resistance (≈0.1 Ω cm² at 600 °C), robust stability both in symmetrical and single cell configurations, and outstanding CO₂ tolerance/reversibility. The first-principle calculations validate the role of co-doping of strontium and phosphorus for the high activity and durability. Central to this work is the combined experiment-calculation approach to point to an effective strategy in the development of highly active and stable perovskite-type cathodes for ILT-SOFCs and related applications.

Kinetic Atomic-Scale Reproducibility of the Oxygen Reduction Reaction Process and a Newly Suggested Strong Correlation Descriptor: A Case Study of BaCo0.75Fe0.25O3-δ

The sluggish oxygen reduction reaction (ORR) is a central issue for energy conversion technologies, particularly in the cathodes of solid oxide fuel cells. The recognition of atomic-level kinetics of the ORR is the key solution. Herein, we take BaCo_0.75Fe_0.25O₃ (BCF) perovskite cathode as a model to investigate the ambiguous ORR mechanism by density function theory and ab initio molecular dynamics. The oxygen dissociation process was found as the rate-determining step, and the performance of BCF series perovskite could be well-characterized by the dissociation barrier energy. Further electronic structure analysis demonstrated that the Pr (Nd)-O_dis bond accepted electrons during the oxygen dissociation process, resulting in reduction of the barrier energy. Finally, strong correlations between rare earth 4f electrons and B-site transition metal 3d electrons were found to be another underlying descriptor to determine the electrochemical activity. We expected that the method could be universally applied to design or screen other high-performance perovskite cathodes.

Realizing Simultaneous Detrimental Reactions Suppression and Multiple Benefits Generation from Nickel Doping toward Improved Protonic Ceramic Fuel Cell Performance

Anode-supported protonic ceramic fuel cells (PCFCs) are highly promising and efficient energy conversion systems. However, several challenges need to be overcome before these systems are used more widely, including the poor sintering of recently developed proton-conducting oxides and the decreased proton conductivity due to detrimental reactions between the nickel from anode and the electrolyte occurring during high-temperature co-sintering. Herein, a Ni doping strategy to increase the electrolyte sintering, suppress the detrimental phase reactions, and generate stable Ni nanoparticles for enhanced performance is proposed. A nickel-doped perovskite oxide is developed with the nominal composition of Ba(Zr_0.1 Ce_0.7 Y_0.1 Yb_0.1 )_0.95 Ni_0.05 O_{3- δ} . Acting as a sintering aid, such a small amount of nickel effectively improves the sintering of the electrolyte. Concomitantly, reactions between nickel and the Ni-doped ceramic phase are suppressed, turning detrimental phase reactions into benefits. The nickel doping further promotes the formation of Ni nanoparticles, which enhance the electrocatalytic activity of the anode toward the hydrogen oxidation reaction and improve the charge transfer across the anode-electrolyte interface. As a result, highly efficient PCFCs are developed. The innovative anode developed in this work also shows favorable activity toward ammonia decomposition, making it highly promising for use in direct ammonia fuel cells.

Minimized thermal expansion mismatch of cobalt-based perovskite air electrodes for solid oxide cells

The mismatch of thermal expansion coefficients (TECs) between cobalt-containing perovskite air electrodes and electrolytes is a great challenge for the development of thermo-mechanically durable solid oxide cells (SOCs). In this work, we propose a facile design principle to directly grow highly dispersed Co reactive sites onto ion-conducting scaffolds and confine the dimension of active centres within nanoscale. As a representative, the Co-socketed BaCe_0.7Zr_0.2Y_0.1O_3-δ perovskite (denoted as R-BCZY-Co) was constructed via a consecutive sol-gel and in situ exsolution approach. Combined XRD, H₂-TPR, SEM and TEM results confirm the emergence of Co nanoparticles on a BCZY matrix without the segregation of a secondary Co-rich phase. The symmetric half-cell measurement suggests that R-BCZY-Co air electrode with the optimal Co content of 10 mol% exhibits a 7-fold promoted oxygen activation performance with a polarization resistance of ∼0.17 Ω cm² at 750 °C. The TEC mismatch between fabricated R-BCZY-Co electrodes and BCZY electrolytes is minimized down to only ∼11.4%, which is significantly lower than that of other representative counterparts. Moreover, the detailed XPS result proves that the architecture of exsolved Co on BCZY possesses a higher concentration of surface oxygen vacancy, which further benefits the kinetics of ion diffusion and oxygen absorption.

High-Performance Perovskite Composite Electrocatalysts Enabled by Controllable Interface Engineering

Single-phase perovskite oxides that contain nonprecious metals have long been pursued as candidates for catalyzing the oxygen evolution reaction, but their catalytic activity cannot meet the requirements for practical electrochemical energy conversion technologies. Here a cation deficiency-promoted phase separation strategy to design perovskite-based composites with significantly enhanced water oxidation kinetics compared to single-phase counterparts is reported. These composites, self-assembled from perovskite precursors, comprise strongly interacting perovskite and related phases, whose structure, composition, and concentration can be accurately controlled by tailoring the stoichiometry of the precursors. The composite catalyst with optimized phase composition and concentration outperforms known perovskite oxide systems and state-of-the-art catalysts by 1-3 orders of magnitude. It is further demonstrated that the strong interfacial interaction of the composite catalysts plays a key role in promoting oxygen ionic transport to boost the lattice-oxygen participated water oxidation. These results suggest a simple and viable approach to developing high-performance, perovskite-based composite catalysts for electrochemical energy conversion.

Effectively Promoting Activity and Stability of a MnCo2O4-Based Cathode by In Situ Constructed Heterointerfaces for Solid Oxide Fuel Cells

The development of multiphase composite electrocatalysts plays a key role in achieving the efficient and durable operation of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Herein, a self-assembled nanocomposite is developed as the oxygen reduction reaction (ORR) catalyst for IT-SOFCs through a coprecipitation method. The nanocomposite is composed of a doped (Mn_0.6Mg_0.4)_0.8Sc_0.2Co₂O₄ (MMSCO) spinel oxide (84 wt %), an orthorhombic perovskite phase (11.3 wt %, the spontaneous combination of PrO₂ additives and spinel), and a minor Sc₂O₃ phase (4.7 wt %). The surface of the (Mn_0.6Mg_0.4)_0.8Sc_0.2Co₂O₄ phase is activated by the self-assembled nanocoating with many heterogeneous interfaces. Thence, the ORR kinetics is obviously accelerated and an area-specific resistance (ASR) of ∼0.11 Ω cm² is obtained at 750 °C. Moreover, a single cell with the cathode shows a peak power density (PPD) of 1144.1 mW cm^-2 at 750 °C, much higher than that of the cell with the MnCo₂O₄ cathode (456.2 mW cm^-2). An enhanced stability of ∼120 h (0.8 A cm^-2, 750 °C) is also achieved, related to the reduced thermal expansion coefficient (13.9 × 10^-6 K^-1). The improvement in ORR kinetics and stability can be attributed to the refinement of grains, the formation of heterointerfaces, and the enhancement of mechanical compatibility.

A Direct n-Butane Solid Oxide Fuel Cell Using Ba(Zr0.1Ce0.7Y0.1Yb0.1)0.9Ni0.05Ru0.05O3-δ Perovskite as the Reforming Layer

Hydrocarbon-fueled solid oxide fuel cells (SOFCs) that can operate in the intermediate temperature range of 500-700 °C represent an attractive SOFC device for combined heat and power applications in the industrial market. One of the ways to realize such a device relies upon exploiting an in situ steam reforming process in the anode catalyzed by an anti-carbon coking catalyst. Here, we report a new Ni and Ru bimetal-doped perovskite catalyst, Ba(Zr_0.1Ce_0.7Y_0.1Yb_0.1)_0.9Ni_0.05Ru_0.05O_3-δ (BZCYYbNRu), with enhanced catalytic hydrogen production activity on n-butane (C₄H₁₀), which can resist carbon coking over extended operation durations. Ru in the perovskite lattice inhibits Ni precipitation from perovskite, and the high water adsorption capacity of proton conducting perovskite improves the coking resistance of BZCYYbNRu. When BZCYYbNRu is used as a steam reforming catalyst layer on a Ni-YSZ-supported anode, the single fuel cell not only achieves a higher power density of 1113 mW cm^-2 at 700 °C under a 10 mL min^-1 C₄H₁₀ continuous feed stream at a steam to carbon (H₂O/C) ratio of 0.5 but also shows a much better operational stability for 100 h at 600 °C compared with those reported in the literature.

Pr-Doping Motivating the Phase Transformation of the BaFeO3-δ Perovskite as a High-Performance Solid Oxide Fuel Cell Cathode

Intermediate temperature solid oxide fuel cells (IT-SOFCs) have been extensively studied due to high efficiency, cleanliness, and fuel flexibility. To develop highly active and stable IT-SOFCs for the practical application, preparing an efficient cathode is necessary to address the challenges such as poor catalytic activity and CO2 poisoning. Herein, an efficient optimized strategy for designing a high-performance cathode is demonstrated. By motivating the phase transformation of BaFeO3-δ perovskites, achieved by doping Pr at the B site, remarkably enhanced electrochemical activity and CO2 resistance are thus achieved. The appropriate content of Pr substitution at Fe sites increases the oxygen vacancy concentration of the material, promotes the reaction on the oxygen electrode, and shows excellent electrochemical performance and efficient catalytic activity. The improved reaction kinetics of the BaFe0.95Pr0.05O3-δ (BFP05) cathode is also reflected by a lower electrochemical impedance value (0.061 Ω·cm2 at 750 °C) and activation energy, which is attributed to high surface oxygen exchange and chemical bulk diffusion. The single cells with the BFP05 cathode achieve a peak power density of 798.7 mW·cm-2 at 750 °C and a stability over 50 h with no observed performance degradation in CO2-containing gas. In conclusion, these results represent a promising optimized strategy in developing electrode materials of IT-SOFCs.

Thermal-expansion offset for high-performance fuel cell cathodes

One challenge for the commercial development of solid oxide fuel cells as efficient energy-conversion devices is thermo-mechanical instability. Large internal-strain gradients caused by the mismatch in thermal expansion behaviour between different fuel cell components are the main cause of this instability, which can lead to cell degradation, delamination or fracture^1-4. Here we demonstrate an approach to realizing full thermo-mechanical compatibility between the cathode and other cell components by introducing a thermal-expansion offset. We use reactive sintering to combine a cobalt-based perovskite with high electrochemical activity and large thermal-expansion coefficient with a negative-thermal-expansion material, thus forming a composite electrode with a thermal-expansion behaviour that is well matched to that of the electrolyte. A new interphase is formed because of the limited reaction between the two materials in the composite during the calcination process, which also creates A-site deficiencies in the perovskite. As a result, the composite shows both high activity and excellent stability. The introduction of reactive negative-thermal-expansion components may provide a general strategy for the development of fully compatible and highly active electrodes for solid oxide fuel cells.