Symmetric Catalyst Design Employing Ir Nanoparticles on Black WO3- x Nanofiber Support for Boosting Water Electrolysis.

The efficient evolution of gaseous hydrogen and oxygen from water is required to realize sustainable energy conversion systems. To address the sluggish kinetics of the multielectron transfer reaction, bifunctional catalyst materials for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) should be developed. Herein, a tailored combination of atomically minimized iridium catalysts and highly conductive black WO3- x nanofiber supports are developed for the bifunctional electrolyzer system. Atomic Ir catalysts, particularly those that activate the OER, minimize the utilization of precious metals. The oxygen-deficient black WO3- x NF support, which boosts the HER, offers increased electronic conductivity and favorable nucleation sites for Ir loading. The Ir-black WO3- x NFs exhibit increased double-layer capacitance, a significantly reduced onset potential, lower Tafel slope, and stable cyclability for both the OER and HER, compared to large-sized Ir catalysts loaded on white WO3 nanofibers. This study offers a strategy for developing an optimal catalyst material with suitable supports for high-performance and economical water electrolysis systems for achieving carbon-negative targets.

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.

Cathodes Coating Layer with Li-Ion Diffusion Selectivity Employing Interactive Network of Metal-Organic Polyhedras for Li-Ion Batteries.

Surface modification of cathodes using Ni-rich coating layers prevents bulk and surface degradation for the stable operation of Li-ion batteries at high voltages. However, insulating and dense inorganic coating layers often impede charge transfer and ion diffusion kinetics. In this study, the fabrication of dual functional coating materials using metal-organic polyhedra (MOP) with 3D networks within microporous units of Li-ion batteries for surface stabilization and facile ion diffusion is proposed. Zr-based MOP is modified by introducing acyl groups as a chemical linkage (MOPAC), and MOPAC layers are homogenously coated by simple spray coating on the cathode. The coating allow the smooth transport of electrons and ions. MOPAC effectively suppress side reactions between the cathode and electrolyte and protect active materials against aggressive fluoride ions by forming a Li-ion selective passivation film. The MOPAC-coated Ni-rich layered cathode exhibited better cycle retention and enhanced kinetic properties than pristine and MOP-coated cathodes. Reduction of undesirable gas evolution on the cathode by MOPAC is also verified. Microporous MOPAC coating can simultaneously stabilize both the bulk and surface of the Ni-rich layered cathode and maintain good electrochemical reaction kinetics for high-performance Li-ion batteries.

Coarse-Graining of Imaginary Time Feynman Path Integrals: Inclusion of Intramolecular Interactions and Bottom-up Force-Matching.

Feynman's imaginary time path integral formalism of quantum statistical mechanics and the corresponding quantum-classical isomorphism provide a tangible way of incorporating nuclear quantum effect (NQE) in the simulation of condensed matter systems using well-developed classical simulation techniques. Our previous work has presented the many-body coarse-graining of path integral (CG-PI) theory that builds an isomorphism between the quantum partition function of N distinguishable particles and the classical partition function of 2N pseudoparticles. In this present work, we develop a generalized version of the many-body CG-PI theory that incorporates many-body interactions in the force field. Based on the new derivation, we provide a numerical CG-PI (n-CG-PI) modeling strategy parametrized from the underlying path integral molecular dynamics (PIMD) trajectories using force matching and Boltzmann inversion. The n-CG-PI models for two liquid systems are shown to capture well both the intramolecular and intermolecular structural correlations of the reference PIMD simulations. The generalized derivation of the many-body CG-PI theory and the n-CG-PI model presented in this work extend the scope of the CG-PI formalism by generalizing the previously limited theory to incorporate force fields of realistic molecular systems.

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.

Controllable Insertion Mechanism of Expanded Graphite Anodes Employing Conversion Reaction Pillars for Sodium-Ion Batteries.

Controlling the structural and reaction characteristics of carbonaceous anode materials is essential to realizing alternative alkali-ion batteries. In this study, we report on expanded graphite material employing MoSx conversion reaction pillars (EG-MoSx) inserted into the interlayers and assess them as potential anode candidates for Na-ion batteries. We succeed in a tailored control of the insertion characteristics between one-phase reaction and two-phase reaction by modifying the crystal structure of EG-MoSx under different thermal treatment conditions. EG-MoSx-900 anode with an enlarged interlayer of ∼5.38 Šdelivers an exceptionally high capacity of 501 mAh g-1. We successfully solve the irreversible capacity issues of the expanded graphite materials by forming chemical preformation of the solid electrolyte interface (SEI) layer on the electrode surface, thereby significantly increasing coulombic efficiencies of thermally tuned EG-MoSx (52.20 → 97.25%). We elucidate the electrochemical mechanism and structural properties of the EG-MoSx anode materials by ex situ characterizations. Inserting active sulfide pillars enables us to overcome the performance limitations of existing Na-ion battery technologies, and we expect that this strategy will be applied to realize another family of alkali-ion batteries.

Lithium-Air Batteries: Air-Breathing Challenges and Perspective.

Lithium-oxygen (Li-O2) batteries have been intensively investigated in recent decades for their utilization in electric vehicles. The intrinsic challenges arising from O2 (electro)chemistry have been mitigated by developing various types of catalysts, porous electrode materials, and stable electrolyte solutions. At the next stage, we face the need to reform batteries by substituting pure O2 gas with air from Earth's atmosphere. Thus, the key emerging challenges of Li-air batteries, which are related to the selective filtration of O2 gas from air and the suppression of undesired reactions with other constituents in air, such as N2, water vapor (H2O), and carbon dioxide (CO2), should be properly addressed. In this review, we discuss all key aspects for developing Li-air batteries that are optimized for operating in ambient air and highlight the crucial considerations and perspectives for future air-breathing batteries.

Super-Expansion of Assembled Reduced Graphene Oxide Interlayers by Segregation of Al Nanoparticle Pillars for High-Capacity Na-Ion Battery Anodes.

The applicability of Na-ion batteries is contingent on breakthroughs in alternative electrode materials that have high capacities and which are economically viable. Unfortunately, conventional graphite anodes for Li-ion battery systems do not allow Na-ion accommodation into their interlayer space owing to the large ionic radius and low stabilizing energy of Na in graphite. Here, we suggest a promising strategy for significantly increasing Na capacity by expanding the axial slab space of graphite. We successfully synthesized reconstructed graphite materials via self-assembly of negative graphite oxide (GO) flakes and Al cation (positive) pillars and by subsequent chemical reaction of the obtained Al-GO materials. Al pillars, atomically distributed in graphite interlayers, can extend the slab space by up to ∼7 Å, which is a 2-fold interlayer distance of pristine graphite. An exceptionally high capacity of 780 mAh/g is demonstrated for reconstructed graphite anodes with Al pillars, compared with rGO materials (210 mAh/g). We investigated the electrochemical reaction mechanism and structural changes associated with discharge and charge to emphasize the benefit of using reconstructed graphite as anodes in Na-ion batteries. Our strategy of modifying the interlayer distance by introducing metallic pillars between the layers can help address the low capacity of carbonaceous anodes.

Coarse-graining of many-body path integrals: Theory and numerical approximations.

Feynman's imaginary time path integral approach to quantum statistical mechanics provides a theoretical formalism for including nuclear quantum effects (NQEs) in simulation of condensed matter systems. Sinitskiy and Voth [J. Chem. Phys. 143, 094104 (2015)] have presented the coarse-grained path integral (CG-PI) theory, which provides a reductionist coarse-grained representation of the imaginary time path integral based on the quantum-classical isomorphism. In this paper, the many-body generalization of the CG-PI theory is presented. It is shown that the N interacting particles obeying quantum Boltzmann statistics can be represented as a system of N pairs of classical-like pseudoparticles coupled to each other analogous to the pseudoparticle pair of the one-body theory. Moreover, we present a numerical CG-PI (n-CG-PI) method applying a simple approximation to the coupling scheme between the pseudoparticles due to numerical challenges of directly implementing the full many-body CG-PI theory. Structural correlations of two liquid systems are investigated to demonstrate the performance of the n-CG-PI method. Both the many-body CG-PI theory and the n-CG-PI method not only present reductionist views of the many-body quantum Boltzmann statistics but also provide theoretical and numerical insight into how to explicitly incorporate NQEs in the representation of condensed matter systems with minimal additional degrees of freedom.

Heme biomolecule as redox mediator and oxygen shuttle for efficient charging of lithium-oxygen batteries.

One of the greatest challenges with lithium-oxygen batteries involves identifying catalysts that facilitate the growth and evolution of cathode species on an oxygen electrode. Heterogeneous solid catalysts cannot adequately address the problematic overpotentials when the surfaces become passivated. However, there exists a class of biomolecules which have been designed by nature to guide complex solution-based oxygen chemistries. Here, we show that the heme molecule, a common porphyrin cofactor in blood, can function as a soluble redox catalyst and oxygen shuttle for efficient oxygen evolution in non-aqueous Li-O2 batteries. The heme's oxygen binding capability facilitates battery recharge by accepting and releasing dissociated oxygen species while benefiting charge transfer with the cathode. We reveal the chemical change of heme redox molecules where synergy exists with the electrolyte species. This study brings focus to the rational design of solution-based catalysts and suggests a sustainable cross-link between biomolecules and advanced energy storage.

Solution-processed titanium carbide MXene films examined as highly transparent conductors.

MXenes, a new family of two-dimensional structures, have recently gained significant attention due to their unique physical properties suitable for a wide range of potential applications. Here we introduce Ti3C2Tx delaminated monolayers as ultrathin transparent conductors with properties exceeding comparable reduced graphene oxide films. Solution processed Ti3C2Tx films exhibit sheet resistances as low as 437 Ω sq-1 with 77% transmittance at 550 nm. Field effect transistor measurements confirm that these films have a metallic nature, which makes them suitable as electrodes. We show using Kelvin Probe Atomic Force Microscopy that the work function of delaminated Ti3C2Tx flakes (with OH terminal groups) is 5.28 ± 0.03 eV. These results demonstrate that solution-processed Ti3C2Tx conducting films could open up a new direction for the next generation of transparent conductive electrodes.

Dimensional Effects of MoS2 Nanoplates Embedded in Carbon Nanofibers for Bifunctional Li and Na Insertion and Conversion Reactions.

Controlling structural and morphological features of molybdenum disulfide (MoS2) nanoplates determines anode reaction performance for Li-ion and Na-ion batteries. In this work, we investigate dimensional effects of MoS2 nanoplates randomly embedded in twisted mesoporous carbon nanofibers (MoS2@MCNFs) on Li and Na storage properties. Considering dimensions of the MoS2 nanoplates (e.g., interlayer, lateral distance, and slabs of stacking in number), we controlled thermolysis temperature to synthesize the MoS2 nanoplates with different geometry and optimize them in the hybrid anode for delivering high performance. The MoS2@MCNFs electrode exhibits reversible Li and Na capacities greater than 1000 cycles even at high current density of 1.0 A g-1 (1221.94 mAh g-1 with capacity retention of 95.6% for Li-ion batteries and 447.29 mAh g-1 with capacity retention of 87.11% for Na-ion batteries). We elucidated the insertion, conversion, and interfacial reaction characteristics of the thermosensitive MoS2 nanoplates in the MCNFs, especially associated with a reversible capacity. Our study will hint at rational design of the nanostructured MoS2 electrodes and focus on significance of their dimensional effects on anode performance.

Tailored Combination of Low Dimensional Catalysts for Efficient Oxygen Reduction and Evolution in Li-O2 Batteries.

The development of efficient bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key issue pertaining high performance Li-O2 batteries. Here, we propose a heterogeneous electrocatalyst consisting of LaMnO3 nanofibers (NFs) functionalized with RuO2 nanoparticles (NPs) and non-oxidized graphene nanoflakes (GNFs). The Li-O2 cell employing the tailored catalysts delivers an excellent electrochemical performance, affording significantly reduced discharge/charge voltage gaps (1.0 V at 400 mA g(-1) ), and superior cyclability for over 320 cycles. The outstanding performance arises from (1) the networked LaMnO3 NFs providing ORR/OER sites without severe aggregation, (2) the synergistic coupling of RuO2 NPs for further improving the OER activity and the electrical conductivity on the surface of the LaMnO3 NFs, and (3) the use of GNFs providing a fast electronic pathway as well as improved ORR kinetics.

A New Design Strategy for Observing Lithium Oxide Growth-Evolution Interactions Using Geometric Catalyst Positioning.

Understanding the catalyzed formation and evolution of lithium-oxide products in Li-O2 batteries is central to the development of next-generation energy storage technology. Catalytic sites, while effective in lowering reaction barriers, often become deactivated when placed on the surface of an oxygen electrode due to passivation by solid products. Here we investigate a mechanism for alleviating catalyst deactivation by dispersing Pd catalytic sites away from the oxygen electrode surface in a well-structured anodic aluminum oxide (AAO) porous membrane interlayer. We observe the cross-sectional product growth and evolution in Li-O2 cells by characterizing products that grow from the electrode surface. Morphological and structural details of the products in both catalyzed and uncatalyzed cells are investigated independently from the influence of the oxygen electrode. We find that the geometric decoration of catalysts far from the conductive electrode surface significantly improves the reaction reversibility by chemically facilitating the oxidation reaction through local coordination with PdO surfaces. The influence of the catalyst position on product composition is further verified by ex situ X-ray photoelectron spectroscopy and Raman spectroscopy in addition to morphological studies.

Pt and Pd catalyzed oxidation of Li2O2 and DMSO during Li-O2 battery charging.

Rechargeable Li-O2 and Li-air batteries require electrode and electrolyte materials that synergistically promote long-term cell operation. In this study, we investigate the role of noble metals Pt and Pd as catalysts in the Li-O2 oxidation process and their compatibility with dimethyl sulfoxide (DMSO) based electrolytes. We identify a basis for low potential Li2O2 evolution followed by oxidative decomposition of the electrolyte to form carbonate side products.

Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold.

In this study, we present a facile and scalable approach to fabricate omniphobic nanofiber membranes by constructing multilevel re-entrant structures with low surface energy. We first prepared positively charged nanofiber mats by electrospinning a blend polymer-surfactant solution of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and cationic surfactant (benzyltriethylammonium). Negatively charged silica nanoparticles (SiNPs) were grafted on the positively charged electrospun nanofibers via dip-coating to achieve multilevel re-entrant structures. Grafted SiNPs were then coated with fluoroalkylsilane to lower the surface energy of the membrane. The fabricated membrane showed excellent omniphobicity, as demonstrated by its wetting resistance to various low surface tension liquids, including ethanol with a surface tension of 22.1 mN/m. As a promising application, the prepared omniphobic membrane was tested in direct contact membrane distillation to extract water from highly saline feed solutions containing low surface tension substances, mimicking emerging industrial wastewaters (e.g., from shale gas production). While a control hydrophobic PVDF-HFP nanofiber membrane failed in the desalination/separation process due to low wetting resistance, our fabricated omniphobic membrane exhibited a stable desalination performance for 8 h of operation, successfully demonstrating clean water production from the low surface tension feedwater.

Heterogeneous WSx/WO3 Thorn-Bush Nanofiber Electrodes for Sodium-Ion Batteries.

Heterogeneous electrode materials with hierarchical architectures promise to enable considerable improvement in future energy storage devices. In this study, we report on a tailored synthetic strategy used to create heterogeneous tungsten sulfide/oxide core-shell nanofiber materials with vertically and randomly aligned thorn-bush features, and we evaluate them as potential anode materials for high-performance Na-ion batteries. The WSx (2 ≤ x ≤ 3, amorphous WS3 and crystalline WS2) nanofiber is successfully prepared by electrospinning and subsequent calcination in a reducing atmosphere. To prevent capacity degradation of the WSx anodes originating from sulfur dissolution, a facile post-thermal treatment in air is applied to form an oxide passivation surface. Interestingly, WO3 thorn bundles are randomly grown on the nanofiber stem, resulting from the surface conversion. We elucidate the evolving morphological and structural features of the nanofibers during post-thermal treatment. The heterogeneous thorn-bush nanofiber electrodes deliver a high second discharge capacity of 791 mAh g(-1) and improved cycle performance for 100 cycles compared to the pristine WSx nanofiber. We show that this hierarchical design is effective in reducing sulfur dissolution, as shown by cycling analysis with counter Na electrodes.

Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces.

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

Ultrathin Nanotube/Nanowire Electrodes by Spin-Spray Layer-by-Layer Assembly: A Concept for Transparent Energy Storage.

Fully integrated transparent devices require versatile architectures for energy storage, yet typical battery electrodes are thick (20-100 µm) and composed of optically absorbent materials. Reducing the length scale of active materials, assembling them with a controllable method and minimizing electrode thickness should bring transparent batteries closer to reality. In this work, the rapid and controllable spin-spray layer-by-layer (SSLbL) method is used to generate high quality networks of 1D nanomaterials: single-walled carbon nanotubes (SWNT) and vanadium pentoxide (V2O5) nanowires for anode and cathode electrodes, respectively. These ultrathin films, deposited with ∼2 nm/bilayer precision are transparent when deposited on a transparent substrate (>87% transmittance) and electrochemically active in Li-ion cells. SSLbL-assembled ultrathin SWNT anodes and V2O5 cathodes exhibit reversible lithiation capacities of 23 and 7 µAh/cm(2), respectively at a current density of 5 µA/cm(2). When these electrodes are combined in a full cell, they retain ∼5 µAh/cm(2) capacity over 100 cycles, equivalent to the prelithiation capacity of the limiting V2O5 cathode. The SSLbL technique employed here to generate functional thin films is uniquely suited to the generation of transparent electrodes and offers a compelling path to realize the potential of fully integrated transparent devices.

Simple synthesis of highly catalytic carbon-free MnCo2O4@Ni as an oxygen electrode for rechargeable Li-O2 batteries with long-term stability.

An effective integrated design with a free standing and carbon-free architecture of spinel MnCo2O4 oxide prepared using facile and cost effective hydrothermal method as the oxygen electrode for the Li-O2 battery, is introduced to avoid the parasitic reactions of carbon and binder with discharge products and reaction intermediates, respectively. The highly porous structure of the electrode allows the electrolyte and oxygen to diffuse effectively into the catalytically active sites and hence improve the cell performance. The amorphous Li2O2 will then precipitate and decompose on the surface of free-standing catalyst nanorods. Electrochemical examination demonstrates that the free-standing electrode without carbon support gives the highest specific capacity and the minimum capacity fading among the rechargeable Li-O2 batteries tested. The Li-O2 cell has demonstrated a cyclability of 119 cycles while maintaining a moderate specific capacity of 1000 mAh g(-1). Furthermore, the synergistic effect of the fast kinetics of electron transport provided by the free-standing structure and the high electro-catalytic activity of the spinel oxide enables excellent performance of the oxygen electrode for Li-O2 cells.