Exsolution of Ni Nanoparticles from A-Site-Deficient Layered Double Perovskites for Dry Reforming of Methane and as an Anode Material for a Solid Oxide Fuel Cell

Exsolution is a promising technique to design metal nanoparticles for electrocatalysis and renewable energy. In this work, Ni-doped perovskites, (Pr_0.5Ba_0.5)_1-x/2Mn_1-x/2Ni_x/2O_3-δ with x = 0, 0.05, 0.1, and 0.2 (S-PBMNx), were prepared to design exsolution systems as solid oxide fuel cell anodes and for catalysis applications. X-ray diffraction and transmission electron microscopy (TEM) analyses demonstrated that correlating A-site deficiency with Ni content can effectively induce exsolution of all Ni under H₂ atmosphere at T ∼ 875 °C, yielding the reduced (exsolved) R-PBMNx materials. On heating the exsolution systems in air, metal incorporation in the oxide lattice did not occur; instead, the Ni nanoparticles oxidized to NiO on the layered perovskite surface. The lowest area-specific resistance (ASR) under wet 5% H₂/N₂ in symmetrical cells was observed for R-PBMN0.2 anode (ASR ∼ 0.64 Ω cm² at 850 °C) due to the highest Ni particle density in the R-PBMNx series. The best performance for dry reforming of methane (DRM) was also obtained for R-PBMN0.2, with CH₄ and CO₂ conversion rates at 11 and 32%, respectively, and the highest production of H₂ (37%). The DRM activity of R-PBMN0.2 starts at 800 °C and is sustained for up to at least 5 h operation with little carbon deposition (0.017 g·gcat^-1·h^-1). These results clearly demonstrate that varying Ni-doping in layered double perovskite oxides is an effective strategy to manipulate the electrochemical performance and catalytic activity for energy conversion purposes.

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.

Hydrogen Oxidation Pathway Over Ni-Ceria Electrode: Combined Study of DFT and Experiment

Ni–ceria cermets are potential anodes for intermediate-temperature solid oxide fuel cells, thanks to the catalytic activity and mixed conductivities of ceria-based materials associated with the variable valence states of cerium. However, the anodic reaction mechanism in the Ni–ceria systems needs to be further revealed. Via density functional theory with strong correlated correction method, this work gains insight into reaction pathways of hydrogen oxidation on a model system of Ni₁₀-CeO₂(111). The calculation shows that electrons tend to be transferred from Ni₁₀ cluster to cerium surface, creating surface oxygen vacancies. Six pathways are proposed considering different adsorption sites, and the interface pathway proceeding with hydrogen spillover is found to be the prevailing process, which includes a high adsorption energy of −1.859 eV and an energy barrier of 0.885 eV. The density functional theory (DFT) calculation results are verified through experimental measurements including electrical conductivity relaxation and temperature programmed desorption. The contribution of interface reaction to the total hydrogen oxidation reaction reaches up to 98%, and the formation of Ni–ceria interface by infiltrating Ni to porous ceria improves the electrochemical activity by 72% at 800°C.

Infiltrated Ni0.08Co0.02CeO2-x@Ni0.8Co0.2 Catalysts for a Finger-Like Anode in Direct Methane-Fueled Solid Oxide Fuel Cells

Direct utilization of methane in solid oxide fuel cells (SOFCs) is greatly impeded by the grievous carbon deposition and the much depressed catalytic activity. In this work, a promising anode, taking finger-like porous YSZ as the anode substrate and impregnated Ni_0.08Co_0.02Ce_0.9O_2-δ@Ni_0.8Co_0.2O as the novel catalyst, is fabricated via the phase conversion-combined tape-casting technique. This anode shows commendable mechanical strength and excellent catalytic activity and stability toward the methane conversion reactions, which is attributed to the exsolved alloy nanoparticles and the active oxygen species on the reduced Ni_0.08Co_0.02Ce_0.9O_2-δ catalyst as well as the facilitated methane transport rooting in the special open-pore microstructure of the anode substrate. Strikingly, this button cell delivers an excellent peak power density of 730 mW cm^-2 at 800 °C in 97% CH₄/3% H₂O fuel, only 9% lower than that in 97% H₂/3% H₂O. Our work shed new light on the SOFC anode developments.

Mixed Ionic-Electronic Conductivity, Redox Behavior and Thermochemical Expansion of Mn-Substituted 5YSZ as an Interlayer Material for Reversible Solid Oxide Cells

Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO₂)_0.95(Y₂O₃)_0.05]_1−x[MnO_y]_x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn^2+/3+→Mn²⁺ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x < 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion.

Route to High-Performance Micro-solid Oxide Fuel Cells on Metallic Substrates

Micro-solid oxide fuel cells based on thin films have strong potential for use in portable power devices. However, devices based on silicon substrates typically involve thin-film metallic electrodes which are unstable at high temperatures. Devices based on bulk metal substrates overcome these limitations, though performance is hindered by the challenge of growing state-of-the-art epitaxial materials on metals. Here, we demonstrate for the first time the growth of epitaxial cathode materials on metal substrates (stainless steel) commercially supplied with epitaxial electrolyte layers (1.5 μm (Y₂O₃)_0.15(ZrO₂)_0.85 (YSZ) + 50 nm CeO₂). We create epitaxial mesoporous cathodes of (La_0.60Sr_0.40)_0.95Co_0.20Fe_0.80O₃ (LSCF) on the substrate by growing LSCF/MgO vertically aligned nanocomposite films by pulsed laser deposition, followed by selectively etching out the MgO. To enable valid comparison with the literature, the cathodes are also grown on single-crystal substrates, confirming state-of-the-art performance with an area specific resistance of 100 Ω cm² at 500 °C and activation energy down to 0.97 eV. The work marks an important step toward the commercialization of high-performance micro-solid oxide fuel cells for portable power applications.

Metal Exsolution to Enhance the Catalytic Activity of Electrodes in Solid Oxide Fuel Cells

Exsolution is a novel technology for attaching metal catalyst particles onto ceramic anodes in the solid oxide fuel cells (SOFCs). The exsolved metal particles in the anode exhibit unique properties for reaction and have demonstrated remarkable stabilities under conditions that normally lead to coking. Despite extensive investigations, the underlying principles behind exsolution are still under investigation. In this review, the present status of exsolution materials for SOFC applications is reported, including a description of the fundamental concepts behind metal incorporation in oxide lattices, a listing of proposed mechanisms and thermodynamics of the exsolution process and a discussion on the catalytic properties of the resulting materials. Prospects and opportunities to use materials produced by exsolution for SOFC are discussed.

Multiple Effects of Iron and Nickel Additives on the Properties of Proton Conducting Yttrium-Doped Barium Cerate-Zirconate Electrolytes for High-Performance Solid Oxide Fuel Cells

Transition metal oxides have been used as sintering aids for proton-conducting barium cerate-zirconates, which are promising electrolyte materials for low-temperature solid oxide fuel cells (SOFCs) and high-performance electrochemical membrane reactors. However, the effects of the additives on properties other than the density of the electrolytes have been ignored. Here, we report our findings that transition metal additives also affect the electrical properties, stability, and even catalytic activity of proton-conducting ABO₃-type perovskites. BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) is selected as the basic material, and 2 mol % of Ni_1-xFe_x (x range: from 0 to 1.0) oxides and 4 mol % of FeO_1.5 are, respectively, added into BCZY to prepare electrolytes of anode-supported SOFCs. All of the electrolytes with additives can be densified after sintering at 1400 °C for 5 h, while BCZY without additive is porous. X-ray diffraction (XRD) spectra show that Ni and Fe are doped into the lattice of BCZY. For the first time, we find a positive function of Fe additive in BCZY that it not only acts as a good sintering aid but also improves the electrical performance and stability of the BCZY electrolyte in CO₂ and H₂O at reduced temperatures. The cell with the 2 mol % Ni_0.5Fe_0.5-doped BCZY electrolyte, with an unoptimized cathode, gives a power density of 973 mW cm^-2 at 700 °C, 120 mW cm^-2 at 450 °C, and 45 mW cm^-2 at 350 °C. It operates under a constant current of 800 mA cm^-2 at 650 °C for over 200 h, during which the voltage decreases from 0.73 to 0.71 V. A newly discovered densified layer, formed in the cathode during the SOFC operation, may cause the degradation.

Bio-inspired gas sensing: boosting performance with sensor optimization guided by "machine learning"

The performance of existing gas sensors often degrades in field conditions because of the loss of measurement accuracy in the presence of interferences. Thus, new sensing approaches are required with improved sensor selectivity. We are developing a new generation of gas sensors, known as multivariable sensors, that have several independent responses for multi-gas detection with a single sensor. In this study, we analyze the capabilities of natural and fabricated photonic three-dimensional (3-D) nanostructures as sensors for the detection of different gaseous species, such as vapors and non-condensable gases. We employed bare Morpho butterfly wing scales to control their gas selectivity with different illumination angles. Next, we chemically functionalized Morpho butterfly wing scales with a fluorinated silane to boost the response of these nanostructures to the vapors of interest and to suppress the response to ambient humidity. Further, we followed our previously developed design rules for sensing nanostructures and fabricated bioinspired inorganic 3-D nanostructures to achieve functionality beyond natural Morpho scales. These fabricated nanostructures have embedded catalytically active gold nanoparticles to operate at high temperatures of ≈300 °C for the detection of gases for solid oxide fuel cell (SOFC) applications. Our performance advances in the detection of multiple gaseous species with specific nanostructure designs were achieved by coupling the spectral responses of these nanostructures with machine learning (a.k.a. multivariate analysis, chemometrics) tools. Our newly acquired knowledge from studies of these natural and fabricated inorganic nanostructures coupled with machine learning data analytics allowed us to advance our design rules for sensing nanostructures toward the required gas selectivity for numerous gas monitoring scenarios at room and high temperatures for industrial, environmental, and other applications.

Co3O4-Impregnated NiO-YSZ: An Efficient Catalyst for Direct Methane Electrooxidation

Co₃O₄-impregnated NiO-YSZ (yttria-stabilized zirconia) is a possible electrocatalyst for direct methane electrooxidation with both high catalytic activity and the ability to mitigate coking. The physical and electrochemical properties of Co₃O₄-impregnated NiO-YSZ anodes are investigated and benchmarked against NiO-YSZ and CeO₂-impregnated NiO-YSZ anodes. The following methane electrooxidation activity trend: Co₃O₄-impregnated NiO-YSZ > CeO₂-impregnated NiO-YSZ > NiO-YSZ with i_o (exchange current density) values of 88, 83, and 2 mA cm^-2, respectively, is obtained in the high overpotential region. The high activity of Co₃O₄-impregnated NiO-YSZ is attributed to the changes in the electronic structure and microstructure with the incorporation of nickel into the lattice of Co₃O₄ as observed using X-ray photoelectron spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. Co₃O₄-impregnated NiO-YSZ also demonstrated the least coking during operation, confirming its utility as a methane electrooxidation catalyst.

Nanostructured BaCo0.4Fe0.4Zr0.1Y0.1O3-δ Cathodes with Different Microstructural Architectures

Lowering the operating temperature of solid oxide fuel cells (SOFCs) is crucial to make this technology commercially viable. In this context, the electrode efficiency at low temperatures could be greatly enhanced by microstructural design at the nanoscale. This work describes alternative microstructural approaches to improve the electrochemical efficiency of the BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) cathode. Different electrodes architectures are prepared in a single step by a cost-effective and scalable spray-pyrolysis deposition method. The microstructure and electrochemical efficiency are compared with those fabricated from ceramic powders and screen-printing technique. A complete structural, morphological and electrochemical characterization of the electrodes is carried out. Reduced values of area specific resistance are achieved for the nanostructured cathodes, i.e., 0.067 Ω·cm² at 600 °C, compared to 0.520 Ω·cm² for the same cathode obtained by screen-printing. An anode supported cell with nanostructured BCFZY cathode generates a peak power density of 1 W·cm⁻² at 600 °C.

Turning Detrimental Effect into Benefits: Enhanced Oxygen Reduction Reaction Activity of Cobalt-Free Perovskites at Intermediate Temperature via CO2-Induced Surface Activation

A minor amount of CO₂ in air usually causes a detrimental effect on oxygen activation over a solid oxide fuel cell (SOFC) cathode because insulating surface carbonate is easily formed, which inhibits charge transfer during the oxygen reduction reaction (ORR). In this study, we report that the detrimental effect due to the CO₂ interaction with perovskite oxide can be turned into a beneficial effect for facilitating ORR through tailoring the material composition of the perovskite. More specifically, for cobalt-free SrSc_0.025Nb_0.075Fe_0.9O_3-δ (SSNF), the exposure to the CO₂ atmosphere results in the formation of a minor amount of surface strontium carbonate mainly in the form of a nanofilm over the perovskite surface, which protects the electrode from further corrosion by CO₂, thus achieving a relatively stable performance even under a 10% CO₂-containing air atmosphere. When CO₂-free air is restored, the SrCO₃ is successfully decomposed at intermediate temperatures. As a result, the surface reaction kinetics is recovered to the initial degree while the charge transfer process is obviously improved. An area-specific resistance of only 0.07 Ω cm² is achieved at 650 °C after the CO₂-induced surface activation, much smaller than the original value of 0.13 Ω cm². In addition, the CO₂-treated electrode shows a fairly stable performance for ORR under a subsequent CO₂-free air atmosphere. To create such a beneficial effect, it is critical to tailor the degree of interaction of the perovskite surface with CO₂, while the benchmark Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) shows a too strong interaction with CO₂ with the formation of bulk-phase-like carbonate, which failed to decomposed even when restored with a CO₂-free atmosphere at intermediate temperatures, and as a result, worsened the ORR activity after the CO₂ treatment.

Hybrid Electrochemical Deposition Route for the Facile Nanofabrication of a Cr-Poisoning-Tolerant La(Ni,Fe)O3-δ Cathode for Solid Oxide Fuel Cells

Cr poisoning of cathode materials is one of the main degradation issues hampering the operation of solid oxide fuel cells (SOFCs). To overcome this shortcoming, LaNi_0.6Fe_0.4O_3-δ (LNF) has been developed as an alternative cathode material owing to its superior chemical stability in Cr environments. In this study, we develop a hybrid electrochemical deposition technique to fabricate a nanostructured LNF-gadolinium-doped ceria (GDC) (n-LNF-GDC) cathode with enhanced active reaction sites for the oxygen reduction reaction. For this purpose, Fe and Ni cations are co-deposited onto an electrically conductive carbon nanotube-modified GDC backbone by electroplating, whereas La cations are successively deposited through a chemically assisted electrodeposition method. The proposed method involves a low-temperature (900 °C) calcination step of electrodeposited cations, which avoids the need of fabricating a GDC diffusion barrier layer which is otherwise needed to avoid the formation of insulating phases (e.g., La₂Zr₂O₇) when fabricating by conventional high-temperature (≥1000 °C) sintering. Scanning electron microscopy images reveal a unique nanofibrous structure of n-LNF-GDC, which is believed to play an instrumental role in enhancing the electrochemical characteristics by increasing the active triple-phase boundaries. An anode-supported SOFC with the n-LNF-GDC cathode showed the superior performance of 0.984 W cm^-2 at an intermediate temperature of 750 °C as compared to the power densities of 0.495 and 0.874 W cm^-2 produced by LNF-GDC and state-of-the-art La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF)-GDC composite cathodes fabricated by conventional sintering. A short-term accelerated Cr-poisoning durability test indicated good electrochemical stability of n-LNF-GDC, whereas LSCF exhibited severe degradation. The electrochemically engineered nanostructured n-LNF-GDC can serve as an effective cathode for SOFCs to achieve high performance and long-term durability.

Gas-Permeable Inorganic Shell Improves the Coking Stability and Electrochemical Reactivity of Pt toward Methane Oxidation

Solid oxide fuel cells produce electricity directly by oxidizing methane, which is the most attractive natural gas fuel, and metal nanocatalysts are a promising means of overcoming the poor catalytic activity of conventional ceramic electrodes. However, the lack of thermal and chemical stability of nanocatalysts is a major bottleneck in the effort to ensure the lifetime of metal-decorated electrodes for methane oxidation. Here, for the first time, this issue is addressed by encapsulating metal nanoparticles with gas-permeable inorganic shells. Pt particles approximately 10 nm in size are dispersed on the surface of a porous La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) electrode via wet infiltration and are then coated with an ultrathin Al₂O₃ layer via atomic layer deposition. The Al₂O₃ overcoat, despite being an insulator, significantly enhances the immunity to carbon coking and provides high activity for the electrochemical oxidation of methane, thereby reducing the reaction impedance of the Pt-decorated electrode by more than 2 orders of magnitude and making the electrode activity of the Pt-decorated sample at 650 °C comparable with those reported at 800 °C for pristine LSCM electrodes. These observations provide a new perspective on strategies to lower the operation temperature, which has long been a challenge related to hydrocarbon-fueled solid oxide fuel cells.

Unraveling the Mechanism of Water-Mediated Sulfur Tolerance via Operando Surface-Enhanced Raman Spectroscopy

While several proton-conducting anode materials have shown excellent tolerance to sulfur poisoning, the mechanism is still unclear due largely to the inability to probe miniscule amounts of sulfur-containing species using conventional surface characterization techniques. Here we present our findings in unraveling the mechanism of water-mediated sulfur tolerance of a proton conductor under operating conditions empowered by surface-sensitive, operando surface-enhanced Raman spectroscopy (SERS) coupled with impedance spectroscopy. Contrary to the conventional view that surface-adsorbed sulfur is removed mainly by oxygen anions, it is found that -SO₄ groups on the surface of the proton conductor are converted to SO₂ by a water-mediated process, as confirmed by operando SERS analysis and density functional theory (DFT)-based calculations. The combination of operando SERS performed on a model electrode and theoretical computation offers an effective approach to investigate into complex mechanisms of electrode processes in various electrochemical systems, providing information vital to achieve the rational design of better electrode materials.

Enhanced Anode Performance and Coking Resistance by In Situ Exsolved Multiple-Twinned Co-Fe Nanoparticles for Solid Oxide Fuel Cells

The broad and large-scale application of solid oxide fuel cells (SOFCs) technology hinges significantly on the development of highly active and robust electrode materials. Here, Ni-free anode materials decorated with metal nanoparticles are synthesized by in situ reduction of Fe-doping Sr₂CoMo_1-xFe_xO_6-δ (x = 0, 0.05, 0.1) double perovskite oxides under a reducing condition at 850 °C. The exsolved nanoparticles from the Sr₂CoMo_0.95Fe_0.05O_6-δ (SCMF0.05) lattice are Co-Fe alloys with rich multiple-twinned defects, significantly enhancing the catalytic activity of the SCMF0.05 anode toward the oxidation of H₂ and CH₄. The electrolyte-supported single cell with the reuduced SCMF0.05 anode reaches peak power densities as high as 992.9 and 652.3 mW cm^-2 in H₂ and CH₄ at 850 °C, respectively, while maintaining superior stability (∼50 h at 700 °C). The reduced SCMF0.05 anode also demonstrates excellent coking resistance in CH₄, which can be attributed to the increased oxygen vacancies due to Fe doping and the effective catalysis of multiple-twinned Co-Fe alloy nanoparticles for reforming of CH₄ to H₂ and CO. The findings in this work may provide a new insight for the design of highly active and durable anode catalysts in SOFCs.

Formation of Nanocomposite Solid Oxide Fuel Cell Cathodes by Preferential Clustering of Cations from a Single Polymeric Precursor

Conventional composite cathodes used in solid oxide fuel cells (SOFCs) are fabricated by co-sintering of electrocatalyst and ionic conductor powders at 1100-1250 °C. The relatively high-temperature heat treatments required to ensure bonding among the powders and between the powders and electrolyte results in the formation of resistive phases and coarse microstructures corresponding to short triple-phase boundary (TPB) length and, consequently, low oxygen reduction activity. In the present work, to achieve long TPBs and avoid resistive phase formation, we propose to fabricate nanocomposite La_0.8Sr_0.2MnO₃-Ce_0.8Sm_0.2O₂ (LSM-SDC) and La_0.8Ca_0.2MnO₃-Ce_0.8Sm_0.2O₂ (LCM-SDC) thin film cathodes by a low-temperature method, which involves the use of a single polymeric precursor solution containing all the respective cations. Owing to the molecular level mixing and the liquid lack of any powder-based starting material, we envision that preferential clustering of cations forming nanoscale electrocatalyst and ionic conductor particles will take place upon heat treatment at relatively low temperatures of 600-800 °C. Here, we report for the first time in the literature, a correlation between the heat-treatment temperature-phase evolution-cluster formation-surface chemistry evolution and electrochemical activity of nanocomposite thin film cathodes fabricated from a single polymeric precursor. Our experiments reveal that highest electrochemical activity is achieved when the electrocatalyst phase is poorly crystallized, complete clustering of cations takes place, and A-site dopant segregation at the surface is minimal.

Ultrathin Atomic Layer-Deposited CeO2 Overlayer for High-Performance Fuel Cell Electrodes

Obtaining a catalyst with high activity and thermal stability is essential for high-performance energy conversion devices operating at an elevated temperature. Herein, the design and fabrication of a heterogeneous catalyst with an ultrathin CeO₂ overlayer via atomic layer deposition (ALD) on Pt electrodes for low-temperature solid oxide fuel cells (LT-SOFCs) is reported. The cell with a CeO₂-overcoated (five ALD cycles) Pt cathode shows lower activation resistance by 50% after a 10 h operation and higher thermal stability by a factor of 2 compared with the cell with a Pt-only cathode, which is known to be the best single catalyst at 450 °C. Eventually, a thin-film SOFC with a highly active and stable CeO₂-overcoated cathode based on an anodized aluminum oxide (AAO) substrate demonstrates a high peak power density of 800 mW cm^-2 at 500 °C, which is the highest performance ever reported for an AAO-based SOFC at this temperature.

Vanadium-Doped Strontium Molybdate with Exsolved Ni Nanoparticles as Anode Material for Solid Oxide Fuel Cells

Vanadium-doped strontium molybdate (SVM) has been investigated as a potential anode material for solid oxide fuel cells due to its high electronic conductivity of about 1000 S cm^-1 at 800 °C in reducing atmospheres. In this work, NiO is introduced to SVM with the B-site excess design to induce in situ growth of Ni nanoparticles in the anodic operational conditions. The Ni particles are exsolved from the parent oxide phase as clearly demonstrated with various techniques including X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The exsolved Ni nanoparticles significantly boost the electrocatalytic activity toward fuel oxidation reaction, improving the peak power density by 160% from 0.21 to 0.56 W cm^-2 at 800 °C when using H₂ as the fuel, meanwhile reducing the total interfacial polarization resistance by 56% from 0.81 to 0.36 Ω cm². The Ni-exsolved SVM anode also shows excellent catalytic activity toward H₂S-containing and hydrocarbon fuels, providing peak power densities of 0.43, 0.36, and 0.22 W cm^-2 at 800 °C for H₂-50 ppm H₂S, syngas, and ethanol, respectively. In addition, the cell with the Ni-exsolved SVM anode presents a stable power output, indicating that the Ni-SVM is a potential SOFC anode electrocatalyst for various fuels.

Approaching Durable Single-Layer Fuel Cells: Promotion of Electroactivity and Charge Separation via Nanoalloy Redox Exsolution

Single-layer fuel cells (SLFCs) based on mixed semiconductors and ionic conductors demonstrate  simplified material preparation and fabrication procedure and possess high performance potentially. However, the operational stability and principle of SLFCs have not yet been convinced of either commercialization or fundamental interests. We hereby report on the employment of a perovskite oxide-based phase-structured redox-stable semiconductor prior to determining a possible solution that improves the durability of the SLFC. Feasible working principles are established and an in-depth understanding of the short-circuit-free phenomenon in SLFCs with the mixed ionic and electronic conductors is provided. Additionally, a smart material design and cell structure processing are also proposed. An extended nonstop testing period of up to 2 days confirms the project feasibility and improved durability of the SLFCs, achieved by replacing the unstable lithiated oxide phase with redox-stable perovskite oxide, though the electrochemical performance is sacrificed. The precipitated metal/alloy nanoparticle on perovskite oxide not only improves the electrode reaction kinetics but also facilitates the charge separation and ionic conduction in SLFCs, consequently enhancing the fuel cell performance and electrical efficiency. The results confirmed the potential of stable operation for future practical deployment of SLFCs via appropriate selection of material and cell structure design. It is greatly believed that the physical junction plays a crucial role in overcoming the internal short-circuit issue of SLFCs.

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.

Advanced Fuel Cell Based on New Nanocrystalline Structure Gd0.1Ce0.9O2 Electrolyte

Lowering the operating temperature is a universal R&D challenge for the development of low-temperature (<600 °C) solid oxide fuel cells (SOFCs) that meet the demands of commercialization. Regarding the traditional electrolyte materials of SOFCs, bulk diffusion is the main ionic conduction mechanism, which is primarily affected by the bulk density and operating temperatures. In this study, we report a new mechanism for the Ce_0.9Gd_0.1O_2-δ (GDC) electrolyte based on a nanocrystalline structure with surface or grain boundary conduction, exhibiting an extremely high ionic conductivity of 0.37 S·cm^-1 at 550 °C. The fuel cell with the nanocrystalline structure GDC electrolyte (0.5 mm in thickness) can deliver a remarkable peak power density of 591.8 mW·cm^-2 at 550 °C, which is approximately 3.5 times higher than that for the cell with the GDC electrolyte densified at 1550 °C. An amorphous layer enriched by oxygen vacancies was found at the surface of the nano-GDC particles in the fuel cell test atmosphere, which was attributed to the ion conduction channel of the grain boundary diffusion. The ionic conduction at the interfaces between the particles was discovered to be the dominant conduction mechanism of the nanocrystalline structure GDC electrolyte. Oxygen ions and protons were determined to be the charge carriers in this interfacial conduction phenomenon, and the conduction of oxygen ions was dominant.

A-Site Ordered Double Perovskite with in Situ Exsolved Core-Shell Nanoparticles as Anode for Solid Oxide Fuel Cells

A highly active anode material for solid oxide fuel cells resistant to carbon deposition is developed. Co-Fe co-doped La_0.5Ba_0.5MnO_3-δ with a cubic-hexagonal heterogeneous stucture is synthesized through the Pechini method. An A-site ordered double perovskite with Co_0.94Fe_0.06 alloy-oxide core-shell nanoparticles on its surface is formed after reduction. The phase transition and the exsolution of the nanoparticles are investigated with X-ray diffraction, thermogravimetric analysis, and high-resolution transmission electron microscope. The exsolved nanoparticles with the layered double-perovskite supporter show a high catalytic activity. A single cell with that anode and a 300 μm thick La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ electrolyte layer exhibits maximum power densities of 1479 and 503 mW cm^-2 at 850 °C with wet hydrogen and wet methane fuels, respectively. Moreover, the single cell fed with wet methane exhibits a stable power output at 850 °C for 200 h, demonstrating a high resistance to carbon deposition of the anode due to the strong anchor of the exsolved nanoparticles on the perovskite parent. The oxide shell also preserves the metal particles from coking.

Understanding Structure-Activity Relationships in Sr1- xY xCoO3-δ through in Situ Neutron Diffraction and Electrochemical Measurements

In this work, we report a systematic study on temperature-dependent local structural evolution, oxygen stoichiometry, and electrochemical properties of an oxygen-deficient perovskite Sr_0.7Y_0.3CoO_3-δ (SYC30) for oxygen electrocatalysis. The obtained results are then closely compared with its analogue Sr_0.9Y_0.1CoO_3-δ (SYC10) of different crystal structures to establish structure-activity relationships. The comparison shows that both SYC30 and SYC10 consist of alternate layers of oxygen-deficient Co1-polyhedra and oxygen-saturated Co2-octahedra with Co1-polyhedra being responsible for V_o^•• migration. It is also found that the distribution and concentration of oxygen vacancies within the Co1-layer are, respectively, less symmetrical and lower in SYC30 than those in SYC10, making the former unfavorable for oxygen transport. A molecular orbital energy analysis reveals that the energy gap between Fermi level and O 2p level in the active Co1-polyhedra is larger in SYC30 than that in SYC10, further suggesting that SYC10 is a better oxide-ion conductor and thus a better electrocatalyst for oxygen reduction reaction, which is unambiguously confirmed by the subsequent electrochemical measurements.