Exsolution of Ru Nanoparticles on BaCe0.9 Y0.1 O3-δ Modifying Geometry and Electronic Structure of Ru for Ammonia Synthesis Reaction Under Mild Conditions.

Green ammonia is an efficient, carbon-free energy carrier and storage medium. The ammonia synthesis using green hydrogen requires an active catalyst that operates under mild conditions. The catalytic activity can be promoted by controlling the geometry and electronic structure of the active species. An exsolution process is implemented to improve catalytic activity by modulating the geometry and electronic structure of Ru. Ru nanoparticles exsolved on a BaCe0.9 Y0.1 O3-δ support exhibit uniform size distribution, 5.03 ± 0.91 nm, and exhibited one of the highest activities, 387.31 mmolNH3  gRu -1  h-1 (0.1 MPa and 450 °C). The role of the exsolution and BaCe0.9 Y0.1 O3-δ support is studied by comparing the catalyst with control samples and in-depth characterizations. The optimal nanoparticle size is maintained during the reaction, as the Ru nanoparticles prepared by exsolution are well-anchored to the support with in-plane epitaxy. The electronic structure of Ru is modified by unexpected in situ Ba promoter accumulation around the base of the Ru nanoparticles.

Vapor-Mediated Infiltration of Nanocatalysts for Low-Temperature Solid Oxide Fuel Cells Using Electrosprayed Dendrites.

Electrode architecturing for fast electrochemical reaction is essential for achieving high-performance of low-temperature solid oxide fuel cells (LT-SOFCs). However, the conventional droplet infiltration technique still has limitations in terms of the applicability and scalability of nanocatalyst implementation. Here, we develop a novel two-step precursor infiltration process and fabricate high-performance LT-SOFCs with homogeneous and robust nanocatalysts. This novel infiltration process is designed based on the principle of a reversible sol-gel transition where the gelated precursor dendrites are uniformly deposited onto the electrode via controlled nanoscale electrospraying process then resolubilized and infiltrated into the porous electrode structure through subsequent humidity control. Our infiltration technique reduces the cathodic polarization resistance by 18% compared to conventional processes, thereby achieving an enhanced peak power density of 0.976 W cm-2 at 650 °C. These results, which provide various degrees of freedom for forming nanocatalysts, exhibit an advancement in LT-SOFC technology.

Identification of an Actual Strain-Induced Effect on Fast Ion Conduction in a Thin-Film Electrolyte.

Strain-induced fast ion conduction has been a research area of interest for nanoscale energy conversion and storage systems. However, because of significant discrepancies in the interpretation of strain effects, there remains a lack of understanding of how fast ionic transport can be achieved by strain effects and how strain can be controlled in a nanoscale system. In this study, we investigated strain effects on the ionic conductivity of Gd0.2Ce0.8O1.9-δ (100) thin films under well controlled experimental conditions, in which errors due to the external environment could not intervene during the conductivity measurement. In order to avoid any interference from perpendicular-to-surface defects, such as grain boundaries, the ionic conductivity was measured in the out-of-plane direction by electrochemical impedance spectroscopy analysis. With varying film thickness, we found that a thicker film has a lower activation energy of ionic conduction. In addition, careful strain analysis using both reciprocal space mapping and strain mapping in transmission electron microscopy shows that a thicker film has a higher tensile strain than a thinner film. Furthermore, the tensile strain of thicker film was mostly developed near a grain boundary, which indicates that intrinsic strain is dominant rather than epitaxial or thermal strain during thin-film deposition and growth via the Volmer-Weber (island) growth mode.

Open-cell voltage and electrical conductivity of a protonic ceramic electrolyte under two chemical potential gradients.

BaZr0.8Y0.2O3-δ, which is a proton-conducting oxide used as an electrolyte for protonic ceramic fuel cells (PCFCs), possesses two mobile ionic charge carriers-oxygen ions and protons-in a crystalline lattice below 500 °C. The equilibrium concentrations of these charge carriers are dependent on water activity. This feature induces a complexity in the distribution of charge carriers within the electrolyte under the influence of the two chemical potential gradients of oxygen and water, which is a typical operating condition in PCFCs. This makes the theoretical derivations of the open-cell voltage and the electrical resistance of the electrolyte difficult. Here, we calculate the distributions of oxygen vacancies and protons across the electrolyte by solving diffusion equations based on the defect chemistry of BaZr0.8Y0.2O3-δ at 500 °C. We then extract the theoretical open-cell voltage and electrical conductivity of the electrolyte in a range of water and oxygen activities that is of interest for PCFCs.

A comprehensive investigation of direct ammonia-fueled thin-film solid-oxide fuel cells: Performance, limitation, and prospects.

Ammonia is a promising carbon-free hydrogen carrier. Owing to their nickel-rich anodes and high operating temperatures, solid oxide fuel cells (SOFCs) can directly utilize NH3 fuel-direct-ammonia SOFCs (DA-SOFCs). Lowering the operating temperature can diversify application areas of DA-SOFCs. We tested direct-ammonia operation using two types of thin-film SOFCs (TF-SOFCs) under 500 to 650°C and compared these with a conventional SOFC. The TF-SOFC with a nickel oxide gadolinium-doped ceria anode achieved a peak power density of 1330 mW cm-2 (NH3 fuel under 650°C), which is the best performance reported to date. However, the performance difference between the NH3 and H2 operations was significant. Electrochemical impedance analyses, ammonia conversion quantification, and two-dimensional multi-physics modeling suggested that reduced ammonia conversion at low temperatures is the main cause of the performance gap. A comparative study with previously reported DA-SOFCs clarified that incorporating a more active ammonia decomposition catalyst will further improve low-temperature DA-SOFCs.

Quantitative determination of lithium depletion during rapid cycling in sulfide-based all-solid-state batteries.

We propose a promising electrochemical analysis tool based on the distribution of relaxation times (DRT) to quantify interfacial resistances towards a comprehensive understanding of complex solid-state interfacial phenomena in sulfide-based all-solid-state batteries (ASSBs). Using DRT-assisted impedance analysis, we identify a new resistance component in the range of 102-103 Hz of 3.5 and 0.9 Ω in the absence and presence of a LiNbO3 layer, respectively, at 1C-rate. Experimental and computational studies confirm that this interfacial resistance results from lithium depletion in sulfide solid electrolytes. Furthermore, we expect our approach to provide new insights into complex interfacial phenomena in ASSBs.

Naturally diffused sintering aid for highly conductive bilayer electrolytes in solid oxide cells.

Solid oxide cells (SOCs) are promising sustainable and efficient electrochemical energy conversion devices. The application of a bilayer electrolyte comprising wide electrolytic oxide and highly conductive oxide is essential to lower the operating temperatures while maintaining high performance. However, a structurally and chemically ideal bilayer has been unattainable through cost-effective conventional ceramic processes. Here, we describe a strategy of naturally diffused sintering aid allowing the fabrication of defect-free doped-zirconia/doped-ceria bilayer electrolyte with full density and reduced interdiffusion layer at lower sintering temperature owing to the supply of small but appropriate amount of sintering aid from doped zirconia to doped ceria that makes the thermal shrinkages of both layers perfectly congruent. The resulting SOCs exhibit a minimal ohmic loss of 0.09 ohm cm2 and remarkable performances in both fuel cell (power density exceeding 1.3 W cm−2) and electrolysis (current density of −1.27 A cm−2 at 1.3 V) operations at 700°C.

Promotion of Pt/CeO2 catalyst by hydrogen treatment for low-temperature CO oxidation.

Low temperature CO oxidation reaction is known to be facilitated over platinum supported on a reducible cerium oxide. Pt species act as binding sites for reactant CO molecules, and oxygen vacancies on surface of cerium oxide atomically activate the reactant O2 molecules. However, the impacts of size of Pt species and concentration of oxygen vacancy at the surface of cerium oxide on the CO oxidation reaction have not been clearly distinguished, thereby various diverse approaches have been suggested to date. Here using the co-precipitation method we have prepared pure ceria support and infiltrated it with Pt solution to obtain 0.5 atomic% Pt supported on cerium oxide catalyst, and then systematically varied the size of Pt from single atom to ∼1.7 nm sized nanoparticles and oxygen vacancy concentration at surface of cerium oxide by controlling the heat-treatment conditions, which are temperature and oxygen partial pressure. It is found that Pt nanoparticles in range of 1-1.7 nm achieve 100% of CO oxidation reaction at ∼100 °C lower temperature compared to Pt single atom owing to the facile adsorption of CO but weaker binding strength between Pt and CO molecules, and the oxygen vacancy in the vicinity of Pt accelerates CO oxidation below 150 °C. Based on this understanding, we show that a simple hydrogen reduction at 550 °C for the single atom Pt supported on CeO2 catalyst induces the formation of highly dispersed Pt nanoparticles with size of 1.7 ± 0.2 nm and the higher concentration of surface oxygen vacancies simultaneously, enabling 100% conversion from CO to CO2 at 200 °C as well as 16% conversion even at 150 °C owing to the synergistic effects of Pt nanoparticles and oxygen vacancies.

Comprehensive Understanding of Cathodic and Anodic Polarization Effects on Stability of Nanoscale Oxygen Electrode for Reversible Solid Oxide Cells.

Degradation of oxygen electrode in reversible solid oxide cells operating in both electrolysis and fuel-cell modes is a critical issue that should be tackled. However, origins and mechanisms thereof have been diversely suggested mainly due to the difficulty in precise analysis of microstructural/compositional changes of porous electrode, which is a typical form in solid oxide cells. In this study, we investigate the degradation phenomena of oxygen electrode under electrolysis and fuel-cell long-term operations for 540 h, respectively, using a geometrically well-defined, nanoscale La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) dense film with a thickness of ∼70 nm. Based on assessments of electrochemical properties and analyses of microstructural and compositional changes after long-term operations, we suggest consolidated degradation mechanisms of oxygen electrode, including the phenomena of kinetic demixing/decomposition of LSCF, which is not readily observable in the typical porous-structured electrode.

Collateral hydrogenation over proton-conducting Ni/BaZr0.85Y0.15O3-δ catalysts for promoting CO2 methanation.

Despite the importance of CO2 methanation for eco-friendly carbon-neutral fuel recycling, the current technologies, relying on catalytic hydrogenation over metal-based catalysts, face technological and economical limitations. Herein, we employ the steam hydrogenation capability of proton conductors to achieve collateral CO2 methanation over the Ni/BaZr0.85Y0.15O3-δ catalyst, which is shown to outperform its conventional Ni/Al2O3 counterpart in terms of CH4 yield (8% higher) and long-term stability (3% higher for 150 h) at 400 °C while exhibiting a CH4 selectivity above 98%. Moreover, infrared and X-ray photoelectron spectroscopy analyses reveal the appearance of distinct mobile proton-related OH bands during the methanation reaction.

High-Temperature Current Collection Enabled by the in Situ Phase Transformation of Cobalt-Nickel Foam for Solid Oxide Fuel Cells.

For the commercial development of solid oxide fuel cells (SOFCs), cathode current collection has been one of the most challenging issues because it is extremely difficult to form continuous electric paths between two rigid components in a high-temperature oxidizing atmosphere. Herein, we present a Co-Ni foam as an innovative cathode current collector that fulfills all strict thermochemical and thermomechanical requirements for use in SOFCs. The Co-Ni foam is originally in the form of a metal alloy, offering excellent mechanical properties and manufacturing tolerance during stack assembly and startup processes. Then, it is converted to the conductive spinel oxide in situ during operation and provides nearly ideal structural and chemical characteristics as a current collector, gas distributor, and load-bearing component. The functionality and durability of the Co-Ni foam are verified by unit cell test and 1 kW-class stack operation, demonstrating performance that is equivalent to that of precious metals as well as an exceptional stability under dynamic conditions with severe temperature and current variations. This work highlights a cost-effective technique to achieve highly reliable electric contacts over the large area using the in situ metal-to-ceramic phase transformation that could be applied to various high-temperature electrochemical devices.

Catalytic behavior of metal catalysts in high-temperature RWGS reaction: In-situ FT-IR experiments and first-principles calculations.

High-temperature chemical reactions are ubiquitous in (electro) chemical applications designed to meet the growing demands of environmental and energy protection. However, the fundamental understanding and optimization of such reactions are great challenges because they are hampered by the spontaneous, dynamic, and high-temperature conditions. Here, we investigated the roles of metal catalysts (Pd, Ni, Cu, and Ag) in the high-temperature reverse water-gas shift (RWGS) reaction using in-situ surface analyses and density functional theory (DFT) calculations. Catalysts were prepared by the deposition-precipitation method with urea hydrolysis and freeze-drying. Most metals show a maximum catalytic activity during the RWGS reaction (reaching the thermodynamic conversion limit) with formate groups as an intermediate adsorbed species, while Ag metal has limited activity with the carbonate species on its surface. According to DFT calculations, such carbonate groups result from the suppressed dissociation and adsorption of hydrogen on the Ag surface, which is in good agreement with the experimental RWGS results.

Strain-Induced Tailoring of Oxygen-Ion Transport in Highly Doped CeO2 Electrolyte: Effects of Biaxial Extrinsic and Local Lattice Strain.

We explored oxygen-ion transport in highly doped CeO2 through density-functional theory calculations. By applying biaxial strain to 18.75 mol % CeO2:Gd, we predicted the average migration-barrier energy with six different pathways, with results in good agreement with those of experiments. Additionally, we found that the migration-barrier energy could be lowered by increasing the tetrahedron volume, including the space occupied by the oxygen vacancy. Our results indicate that the tetrahedron volume can be expanded by larger codopants, as well as biaxial tensile strain. Thus, the combination of thin-film structure and codoping could offer a new approach to accelerate oxygen-ion transport.

Constrained Sintering in Fabrication of Solid Oxide Fuel Cells.

Solid oxide fuel cells (SOFCs) are inevitably affected by the tensile stress field imposed by the rigid substrate during constrained sintering, which strongly affects microstructural evolution and flaw generation in the fabrication process and subsequent operation. In the case of sintering a composite cathode, one component acts as a continuous matrix phase while the other acts as a dispersed phase depending upon the initial composition and packing structure. The clustering of dispersed particles in the matrix has significant effects on the final microstructure, and strong rigidity of the clusters covering the entire cathode volume is desirable to obtain stable pore structure. The local constraints developed around the dispersed particles and their clusters effectively suppress generation of major process flaws, and microstructural features such as triple phase boundary and porosity could be readily controlled by adjusting the content and size of the dispersed particles. However, in the fabrication of the dense electrolyte layer via the chemical solution deposition route using slow-sintering nanoparticles dispersed in a sol matrix, the rigidity of the cluster should be minimized for the fine matrix to continuously densify, and special care should be taken in selecting the size of the dispersed particles to optimize the thermodynamic stability criteria of the grain size and film thickness. The principles of constrained sintering presented in this paper could be used as basic guidelines for realizing the ideal microstructure of SOFCs.

Extremely thin bilayer electrolyte for solid oxide fuel cells (SOFCs) fabricated by chemical solution deposition (CSD).

An extremely thin bilayer electrolyte consisting of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) is successfully fabricated on a sintered NiO-YSZ substrate. Major processing flaws are effectively eliminated by applying local constraints to YSZ nanoparticles, and excellent open circuit voltage and cell performance are demonstrated in a solid oxide fuel cell (SOFC) at intermediate operating temperatures.