One-Step Pyrolysis Construction of Bimetallic Atom-Cluster Sites for Boosting Bifunctional Catalytic Activity in Zn-Air Batteries.

Due to their unique advantages, single atoms and clusters of transition metals are expected to achieve a breakthrough in catalytic activity, but large-scale production of active materials remains a challenge. In this work, a simple solvent-free one-step annealing method is developed and applied to construct diatomic and cluster active sites in activated carbon by utilizing the strong anchoring ability of phenanthroline to metal ions, which can be scaled for mass productions. Benefiting from the synergy between the different metals, the obtained sub-nano-bimetallic atom-cluster catalysts (FeNiAC -NC) exhibit high oxygen reduction reactions (ORR) activity (E1/2 = 0.936 V vs. RHE) and a small ORR/oxygen evolution reaction (OER) potential gap of only 0.594 V. An in-house pouch Zn-air battery is assembled using an FeNiAC -NC catalyst, which demonstrates a stability of 1000 h, outperforming previous reports. The existence of clusters and their effects on catalytic activity is analyzed by density functional theory calculations to reveal the chemistry of nano-bimetallic atom-cluster catalysts.

Molecular Dipoles as a Surface Flattening and Interface Stabilizing Agent for Lithium-Metal Batteries.

Reaching the border of the capable energy limit in existing battery technology has turned research attention away from the rebirth of unstable Li-metal anode chemistry in order to achieve exceptional performance. Strict regulation of the dendritic Li surface reaction, which results in a short circuit and safety issues, should be achieved to realize Li-metal batteries. Herein, this study reports a surface-flattening and interface product stabilizing agent employing methyl pyrrolidone (MP) molecular dipoles in the electrolyte for cyclable Li-metal batteries. The excellent stability of the Li-metal electrode over 600 cycles at a high current density of 5 mA cm-2 has been demonstrated using an optimal concentration of the MP additive. This study has identified the flattening surface reconstruction and crystal rearrangement behavior along the stable (110) plane assisted by the MP molecular dipoles. The stabilization of the Li-metal anodes using molecular dipole agents has helped develop next-generation energy storage devices using Li-metal anodes, such as Li-air, Li-S, and semi-solid-state batteries.

Correlating Surface Structures and Electrochemical Activity Using Shape-Controlled Single-Pt Nanoparticles.

We report a method for synthesizing and studying shape-controlled, single Pt nanoparticles (NPs) supported on carbon nanoelectrodes. The key advance is that the synthetic method makes it possible to produce single, electrochemically active NPs with a vast range of crystal structures and sizes. Equally important, the NPs can be fully characterized, and, therefore, the electrochemical properties of the NPs can be directly correlated to the size and structure of a single shape. This makes it possible to directly correlate experimental results to first-principles theory. Because just one well-characterized NP is analyzed at a time, the difficulty of applying a theoretical analysis to an ensemble of NPs having different sizes and structures is avoided. In this article, we report on two specific Pt NP shapes having sizes on the order of 200 nm: concave hexoctahedral (HOH) and concave trapezohedral (TPH). The former has {15 6 1} facets and the latter {10 1 1} facets. The electrochemical properties of these single NPs for the formic acid oxidation (FAO) reaction are compared to those of a single, spherical polycrystalline Pt NP of the same size. Finally, density functional theory, performed prior to the electrochemical studies, were used to interpret the experimental results of the FAO experiments.

Black Tungsten Oxide Nanofiber as a Robust Support for Metal Catalysts: High Catalyst Loading for Electrochemical Oxygen Reduction.

Black valve metal oxides with low oxygen vacancies are identified to be promising for various industrial applications, such as in gas sensing, photocatalysis, and rechargeable batteries, owing to their high reducibility and stability, as well as considerable fractions of low-valent metal species and oxygen vacancies in their lattices. Herein, the nanofiber (NF) of black oxygen-deficient tungsten trioxide (WO3- x ) is presented as a versatile and robust support for the direct growth of a platinum catalyst for oxygen reduction reaction (ORR). The nonstoichiometric, poorly crystallized black WO3- x NFs are prepared by electrospinning the W precursor into NFs followed by their low-temperature (650 °C) reductive calcination. The black WO3- x NFs have adequate electrical conductivity owing to their decreased bandgap and amorphous structure. Remarkably, the oxygen-deficient surface (surface O/W = 2.44) facilitates the growth of small Pt nanoparticles, which resist aggregation, as confirmed by structural characterization and computational analysis. The Pt-loaded black WO3- x NFs outperform the Pt-loaded crystalline white WO3- x NFs in both the electrochemical ORR activity and the accelerated durability test. This study can inspire the use of oxygen-deficient metal oxides as supports for other electrocatalysts, and can further increase the versatility of oxygen-deficient metal oxides.

Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency.

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNy nanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm-2) in an alkaline medium. Although pristine NCNF or CoOx@CoNy alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNy nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNy nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.

Intermetallic Pd3Pb nanocubes with high selectivity for the 4-electron oxygen reduction reaction pathway.

Pd-Based nanoparticles are excellent alternatives to the typically used Pt-based materials that catalyze fuel cell reactions. Specifically, Pd-based intermetallic nanomaterials have shown great promise as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media; however, their synthesis remains a challenge and shape-controlled nanoparticles are limited. Here, a low-temperature approach to intermetallic Pd3Pb nanocubes is demonstrated and their electrocatalytic properties evaluated for the ORR. The intermetallic Pd3Pb nanocubes outperformed all reference catalysts, with a mass activity of 154 mA mgPd-1 which is a 130% increase in activity compared to the commercial Pd/C reference and a 230% increase compared to Pd nanocubes. Tafel analysis reveals that the Pd3Pb nanocubes are highly selective for the 4-electron reduction pathway, with minimal HO2- formation. Density functional theory (DFT) calculations show that the increased activity for the intermetallic nanocubes compared to Pd is likely due to the weakening of OH* adsorption, decreasing the required overpotential. These results show that intermetallic Pd3Pb nanocubes are highly efficient for the 4-electron pathway of the ORR and could inspire the study of other shape-controlled intermetallics as catalysts for fuel cell applications.

Effects of ensembles, ligand, and strain on adsorbate binding to alloy surfaces.

Alloying elements with strong and weak adsorption properties can produce a catalyst with optimally tuned adsorbate binding. A full understanding of this alloying effect, however, is not well-established. Here, we use density functional theory to study the ensemble, ligand, and strain effects of close-packed surfaces alloyed by transition metals with a combination of strong and weak adsorption of H and O. Specifically, we consider PdAu, RhAu, and PtAu bimetallics as ordered and randomly alloyed (111) surfaces, as well as randomly alloyed 140-atom clusters. In these alloys, Au is the weak-binding component and Pd, Rh, and Pt are characteristic strong-binding metals. In order to separate the different effects of alloying on binding, we calculate the tunability of H- and O-binding energies as a function of lattice constant (strain effect), number of alloy-substituted sublayers (ligand effect), and randomly alloyed geometries (ensemble effect). We find that on these alloyed surfaces, the ensemble effect more significantly tunes the adsorbate binding as compared to the ligand and strain effects, with the binding energies predominantly determined by the local adsorption environment provided by the specific triatomic ensemble on the (111) surface. However, we also find that tuning of adsorbate binding from the ligand and strain effects cannot be neglected in a quantitative description. Extending our studies to other bimetallics (PdAg, RhAg, PtAg, PdCu, RhCu, and PtCu), we find similar conclusions that the tunability of adsorbate binding on random alloys is predominately described by the ensemble effect.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li-O2 Batteries.

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O2) batteries, the irreversible formation of Li2O2, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co4N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co4N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co4N-decorated CNF (Co4N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O2 cells. During cycling, metallic Co4N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co4N nanorods promote reversible formation/decomposition of film-type Li2O2, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g-1). Moreover, pouch-type Li-air cells using Co4N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Design of Reduction Process of SnO2 by CH4 for Efficient Sn Recovery.

We design a novel method for the CH4 reduction of SnO2 for the efficient recovery of Sn from SnO2 through a study combining theory and experiment. The atomic-level process of CH4-SnO2 interaction and temperature-dependent reduction behavior of SnO2 were studied with a combination of a multi-scale computational method of thermodynamic simulations and density functional theory (DFT) calculations. We found that CH4 was a highly efficient and a versatile reducing agent, as the total reducing power of CH4 originates from the carbon and hydrogen of CH4, which sequentially reduce SnO2. Moreover, as a result of the CH4 reduction of SnO2, a mixture of CO and H2 was produced as a gas-phase product (syngas). The relative molar ratio of the produced gas-phase product was controllable by the reduction temperature and the amount of supplied CH4. The laboratory-scale experimental study confirmed that CH4 actively reduces SnO2, producing 99.34% high-purity Sn and H2 and CO. Our results present a novel method for an efficient, green, and economical recycling strategy for Sn with economic value added that is held by the co-produced clean energy source (syngas).

Interface engineering for a rational design of poison-free bimetallic CO oxidation catalysts.

We use density functional theory calculations of Pt@Cu core@shell nanoparticles (NPs) to design bifunctional poison-free CO oxidation catalysts. By calculating the adsorption chemistry under CO oxidation conditions, we find that the Pt@Cu NPs will be active for CO oxidation with resistance to CO-poisoning. The CO oxidation pathway at the Pt-Cu interface is determined on the Pt NP covered with a full- and partial-shell of Cu. The exposed portion of the Pt core preferentially binds CO and the Cu shell binds O2, supplying oxygen for the reaction. The Pt-Cu interface provides CO-oxidation sites that are not poisoned by either CO or O2. Additional computational screening shows that this separation of reactant binding sites is possible for several other core@shell NPs. Our results indicate that the metal-metal interface within a single NP can be optimized for design of bifunctional catalytic systems with improved performance.

CO oxidation by MoS2-supported Au19 nanoparticles: effects of vacancy formation and tensile strain.

The mechanism of the catalytic oxidation of CO activated by MoS2-supported Au19 nanoparticles (NPs) was studied using density functional theory calculations. Of particular interest were the effects of the physical/chemical modification of a MoS2 support on the CO oxidation pathway and the activation of specific reactive centers, i.e., the Au atoms of Au19 or the Au-MoS2 perimeter sites. We systematically modified MoS2 by introducing an S vacancy or 5% tensile strain and studied the shift of each reaction step and the overall change in the reaction pathway and activity. Despite the lack of direct involvement of the Au-MoS2 perimeter in the reaction, the combination of an S vacancy and the tensile strain in the MoS2 support was found to improve the stability and catalytic activity of Au NPs for CO oxidation. The results show that support modification can provide information for new pathways for the rational design of Au-based catalysts.

Catalytic characteristics of AgCu bimetallic nanoparticles in the oxygen reduction reaction.

Intensive research on oxygen reduction reaction (ORR) catalysts has been undertaken to find a Pt substitute or reduce the amount of Pt. Ag nanoparticles are potential Pt substitutes; however, the weak oxygen adsorption energy of Ag prompted investigation of other catalysts. Herein, we prepared AgCu bimetallic nanoparticle (NP) systems to improve the catalytic performance and compared the catalytic performance of Ag, Cu, AgCu (core-shell), and AgCu (alloy) NP systems as new catalyst by investigating the adsorption energy of oxygen and the activation energy of oxygen dissociation, which is known to be the rate-determining step of ORR. By analyzing HOMO-level isosurfaces of metal NPs and oxygen, we found that the adsorption sites and the oxygen adsorption energies varied with different configurations of NPs. We then plotted the oxygen adsorption energies against the energy barrier of oxygen dissociation to determine the catalytic performance. AgCu (alloy) and Cu NPs exhibited strong adsorption energies and low activation-energy barriers. However, the overly strong oxygen adsorption energy of Cu NPs hindered the ORR.

Phase diagram and structural evolution of Ag-Au bimetallic nanoparticles: molecular dynamics simulations.

We studied the structural evolution of a 270-atom Ag-Au bimetallic nanoparticle (2 nm in size) with varying composition and temperature. The liquid to solid transition region and the solid-state structure were investigated using molecular dynamics simulations. To determine the exact transition temperature region, we applied the mean square displacement and structure deviation methods, as well as the generally used caloric curve of potential energy versus temperature. The results showed that a complete solid-solution phase diagram of the binary Ag-Au system was obtained. Irrespective of the composition, the freezing temperature of a Ag-Au bimetallic nanoparticle was lower than that of the bulk state by a margin of several hundred degrees, and three different solid-state structures are proposed in relation to the Au composition. Our phase diagram offers guidance for the application of Ag-Au nanoparticles.