Robust battery interphases from dilute fluorinated cations.

Controlling solid electrolyte interphase (SEI) in batteries is crucial for their efficient cycling. Herein, we demonstrate an approach to enable robust battery performance that does not rely on high fractions of fluorinated species in electrolytes, thus substantially decreasing the environmental footprint and cost of high-energy batteries. In this approach, we use very low fractions of readily reducible fluorinated cations in electrolyte (∼0.1 wt%) and employ electrostatic attraction to generate a substantial population of these cations at the anode surface. As a result, we can form a robust fluorine-rich SEI that allows for dendrite-free deposition of dense Li and stable cycling of Li-metal full cells with high-voltage cathodes. Our approach represents a general strategy for delivering desired chemical species to battery anodes through electrostatic attraction while using minute amounts of additive.

Dynamic Electrochemical Interfaces for Energy Conversion and Storage.

Electrochemical energy conversion and storage are central to developing future renewable energy systems. For efficient energy utilization, both the performance and stability of electrochemical systems should be optimized in terms of the electrochemical interface. To achieve this goal, it is imperative to understand how a tailored electrode structure and electrolyte speciation can modify the electrochemical interface structure to improve its properties. However, most approaches describe the electrochemical interface in a static or frozen state. Although a simple static model has long been adopted to describe the electrochemical interface, atomic and molecular level pictures of the interface structure should be represented more dynamically to understand the key interactions. From this perspective, we highlight the importance of understanding the dynamics within an electrochemical interface in the process of designing highly functional and robust energy conversion and storage systems. For this purpose, we explore three unique classes of dynamic electrochemical interfaces: self-healing, active-site-hosted, and redox-mediated interfaces. These three cases of dynamic electrochemical interfaces focusing on active site regeneration collectively suggest that our understanding of electrochemical systems should not be limited to static models but instead expanded toward dynamic ones with close interactions between the electrode surface, dissolved active sites, soluble species, and reactants in the electrolyte. Only when we begin to comprehend the fundamentals of these dynamics through operando analyses can electrochemical conversion and storage systems be advanced to their full potential.

Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis.

Sustainable energy-conversion and chemical-production require catalysts with high activity, durability, and product-selectivity. Metal/oxide hybrid structure has been intensively investigated to achieve promising catalytic performance, especially in neutral or alkaline electrocatalysis where water dissociation is promoted near the oxide surface for (de)protonation of intermediates. Although catalytic promise of the hybrid structure is demonstrated, it is still challenging to precisely modulate metal/oxide interfacial interactions on the nanoscale. Herein, we report an effective strategy to construct rich metal/oxide nano-interfaces on conductive carbon supports in a surfactant-free and self-terminated way. When compared to the physically mixed Pd/CeO2 system, a much higher degree of interface formation was identified with largely improved hydrogen oxidation reaction (HOR) kinetics. The benefits of the rich metal-CeO2 interface were further generalized to Pd alloys for optimized adsorption energy, where the Pd3Ni/CeO2/C catalyst shows superior performance with HOR selectivity against CO poisoning and shows long-term stability. We believe this work highlights the importance of controlling the interfacial junctions of the electrocatalyst in simultaneously achieving enhanced activity, selectivity, and stability.

Carbon Shell on Active Nanocatalyst for Stable Electrocatalysis.

Electrocatalysis is a key process for renewable energy conversion and fuel production in future energy systems. Various nanostructures have been investigated to optimize the electrocatalytic activity and realize efficient energy use. However, the long-term stability of electrocatalysts is also crucial for the sustainable and reliable operation of energy devices. Nanocatalysts are degraded by various processes during electrocatalysis, which causes critical performance loss. Recent operando analyses have revealed the mechanisms of electrocatalyst failure, and specific structures have been identified as robust against degradation. Nevertheless, achieving both high activity and robust stability with the same nanostructure is challenging because the structure-property relationships that affect activity and stability are different. The optimization of electrocatalysis is often limited by a large trade-off between activity and stability in catalyst structures. Therefore, it is essential to introduce functional structural units into catalyst design to achieve electrochemical stability while preserving high activity.In this Account, we highlight the strategic use of carbon shells on catalyst surfaces to improve the stability during electrocatalysis. For this purpose, we cover three issues in the use of carbon-shell-encapsulated nanoparticles (CSENPs) as robust and active electrocatalysts: the origin of the improved stability, the identification of active sites, and synthetic routes. Carbon shells can shield catalyst surfaces from both (electro)chemical oxidation and physical agglomeration. By limiting the exposure of the catalyst surface to an oxidizing (electro)chemical environment, carbon shells can preserve the initial active site structure during electrocatalysis. In addition, by providing a physical barrier between nanoparticles, carbon shells can maintain the high surface area of CSENPs by reducing particle agglomeration during electrocatalysis. This barrier effect is also useful for constructing more active or durable structures by annealing without surface area loss. Compared to the clear stabilizing effect, however, the effect of the shell on active sites on the CSENP surface can be puzzling. Even when they are covered by a carbon shell that can block molecular adsorption on active sites, CSENP catalysts remain active and even exhibit unique catalytic behavior. Thus, we briefly cover recent efforts to identify major active sites on CSENPs using molecular probes. Furthermore, considering the membranelike role of the carbon shell, we suggest several remaining issues that should be resolved to obtain a fundamental understanding of CSENP design. Finally, we describe two synthetic approaches for the successful carbon shell encapsulation of nanoparticles: two-step and one-step syntheses. Both the postmortem coating of nanocatalysts (two-step) and the in situ formation via precursor ligands (one step) are shown to produce a durable carbon layer on nanocatalysts in a controlled manner. The strengths and limitations of each approach are also presented to promote the further investigation of advanced synthesis methods.The hybrid structure of CSENPs, that is, the active catalyst surface and the durable carbon shell, provides an interesting opportunity in electrocatalysis. However, our understanding of CSENPs is still highly limited, and further investigation is needed to answer fundamental questions regarding both active site identification and the mechanisms of stability improvement. Only when we start to comprehend the fundamental mechanisms underlying electrocatalysis on CSENPs will electrocatalysts be further improved for sustainable long-term device operation.

Facet-Defined Strain-Free Spinel Oxide for Oxygen Reduction.

Exposing facet and surface strain are critical factors affecting catalytic performance but unraveling the composition-dependent activity on specific facets under strain-controlled environment is still challenging due to the synthetic difficulties. Herein, we achieved a (001) facet-defined Co-Mn spinel oxide surface with different surface compositions using epitaxial growth on Co3O4 nanocube template. We adopted composition gradient synthesis to relieve the strain layer by layer, minimizing the surface strain effect on catalytic activity. In this system, experimental and calculational analyses of model oxygen reduction reaction (ORR) activity reveals a volcano-like trend with Mn/Co ratios because of an adequate charge transfer from octahedral-Mn to neighboring Co. Co0.5Mn0.5 as an optimized Mn/Co ratio exhibits both outstanding ORR activity (0.894 V vs RHE in 1 M KOH) and stability (2% activity loss against chronoamperometry). By controlling facet and strain, this study provides a well-defined platform for investigating composition-structure-activity relationships in electrocatalytic processes.

Structural Insights into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis.

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, a fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of their multi-element nature. Additional complexities arise when MMO catalysts have crystalline structures with two different metal site occupancies, such as the spinel structure, which makes it more challenging to investigate the origin of the (electro)catalytic activity of MMOs. Here, uniform-sized multi-metal spinel oxide nanoparticles composed of Mn, Co, and Fe as model MMO electrocatalysts are synthesized and the contributions of each element to the structural flexibility of the spinel oxides are systematically studied, which boosts the electrocatalytic oxygen reduction reaction (ORR) activity. Detailed crystal and electronic structure characterizations combined with electrochemical and computational studies reveal that the incorporation of Co not only increases the preferential octahedral site occupancy, but also modifies the electronic state of the ORR-active Mn site to enhance the intrinsic ORR activity. As a result, nanoparticles of the optimized catalyst, Co0.25 Mn0.75 Fe2.0 -MMO, exhibit a half-wave potential of 0.904 V (versus RHE) and mass activity of 46.9 A goxide -1 (at 0.9 V versus RHE) with promising stability.

Direct Synthesis of Intermetallic Platinum-Alloy Nanoparticles Highly Loaded on Carbon Supports for Efficient Electrocatalysis.

Compared to nanostructured platinum (Pt) catalysts, ordered Pt-based intermetallic nanoparticles supported on a carbon substrate exhibit much enhanced catalytic performance, especially in fuel cell electrocatalysis. However, direct synthesis of homogeneous intermetallic alloy nanocatalysts on carbonaceous supports with high loading is still challenging. Herein, we report a novel synthetic strategy to directly produce highly dispersed MPt alloy nanoparticles (M = Fe, Co, or Ni) on various carbon supports with high catalyst loading. Importantly, a unique bimetallic compound, composed of [M(bpy)3]2+ cation (bpy = 2,2'-bipyridine) and [PtCl6]2- anion, evenly decomposes on carbon surface and forms uniformly sized intermetallic nanoparticles with a nitrogen-doped carbon protection layer. The excellent oxygen reduction reaction (ORR) activity and stability of the representative reduced graphene oxide (rGO)-supported L10-FePt catalyst (37 wt %-FePt/rGO), exhibiting 18.8 times higher specific activity than commercial Pt/C catalyst without degradation over 20 000 cycles, well demonstrate the effectiveness of our synthetic approach toward uniformly alloyed nanoparticles with high homogeneity.

Epitaxially Strained CeO2 /Mn3 O4 Nanocrystals as an Enhanced Antioxidant for Radioprotection.

Nanomaterials with antioxidant properties are promising for treating reactive oxygen species (ROS)-related diseases. However, maintaining efficacy at low doses to minimize toxicity is a critical for clinical applications. Tuning the surface strain of metallic nanoparticles can enhance catalytic reactivity, which has rarely been demonstrated in metal oxide nanomaterials. Here, it is shown that inducing surface strains of CeO2 /Mn3 O4 nanocrystals produces highly catalytic antioxidants that can protect tissue-resident stem cells from irradiation-induced ROS damage. Manganese ions deposited on the surface of cerium oxide (CeO2 ) nanocrystals form strained layers of manganese oxide (Mn3 O4 ) islands, increasing the number of oxygen vacancies. CeO2 /Mn3 O4 nanocrystals show better catalytic activity than CeO2 or Mn3 O4 alone and can protect the regenerative capabilities of intestinal stem cells in an organoid model after a lethal dose of irradiation. A small amount of the nanocrystals prevents acute radiation syndrome and increases the survival rate of mice treated with a lethal dose of total body irradiation.

Design and synthesis of multigrain nanocrystals via geometric misfit strain.

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co3O4 nanocube core that carries a Mn3O4 shell on each facet. The individual shells are symmetry-related interconnected grains6, and the large geometric misfit between adjacent tetragonal Mn3O4 grains results in tilt boundaries at the sharp edges of the Co3O4 nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase7. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.

Design Principle of Fe-N-C Electrocatalysts: How to Optimize Multimodal Porous Structures?

The effect of porous structures on the electrocatalytic activity of N-doped carbon is studied by using electrochemical analysis techniques and the result is applied to synthesize highly active and stable Fe-N-C catalyst for oxygen reduction reaction (ORR). We developed synthetic procedures to prepare three types of N-doped carbon model catalysts that are designed for systematic comparison of the porous structures. The difference in their catalytic activity is investigated in relation to the surface area and the electrochemical parameters. We found that macro- and mesoporous structures contribute to different stages of the reaction kinetics. The catalytic activity is further enhanced by loading the optimized amount of Fe to prepare Fe-N-C catalyst. In both N-doped carbon and Fe-N-C catalysts, the hierarchical porous structure improved electrocatalytic performance in acidic and alkaline media. The optimized catalyst exhibits one of the best ORR performance in alkaline medium with excellent long-term stability in anion exchange membrane fuel cell and accelerated durability test. Our study establishes a basis for rationale design of the porous carbon structure for electrocatalytic applications.

Edge-Terminated MoS2 Nanoassembled Electrocatalyst via In Situ Hybridization with 3D Carbon Network.

Transition metal dichalcogenides, especially MoS2 , are considered as promising electrocatalysts for hydrogen evolution reaction (HER). Since the physicochemical properties of MoS2 and electrode morphology are highly sensitive factor for HER performance, designed synthesis is highly pursued. Here, an in situ method to prepare a 3D carbon/MoS2 hybrid catalyst, motivated by the graphene ribbon synthesis process, is reported. By rational design strategies, the hybrid electrocatalysts with cross-connected porous structure are obtained, and they show a high HER activity even comparable to the state-of-the-art MoS2 catalyst without appreciable activity loss in long-term operations. Based on various physicochemical techniques, it is demonstrated that the synthetic procedure can effectively guide the formation of active site and 3D structure with a distinctive feature; increased exposure of active sites by decreased domain size and intrinsically high activity through controlling the number of stacking layers. Moreover, the importance of structural properties of the MoS2 -based catalysts is verified by controlled experiments, validating the effectiveness of the designed synthesis approach.

Highly Durable and Active Pt-Based Nanoscale Design for Fuel-Cell Oxygen-Reduction Electrocatalysts.

Fuel cells are one of the promising energy-conversion devices due to their high efficiency and zero emission. Although recent advances in electrocatalysts have been achieved using various material designs such as alloys, core@shell structures, and shape control, many issues still remain to be resolved. Especially, material design issues for high durability and high activity are recently accentuated owing to severe instability of nanoparticles under fuel-cell operating conditions. To address these issues, fundamental understanding of functional links between activity and durability is timely urgent. Here, the activity and durability of nanoscale materials are summarized, focusing on the nanoparticle size effect. In addition to phenomenological observation, two major degradation origins, including atomic dissolution and particle size increase, are discussed related to the activity decrease. Based on the fundamental understanding of nanoparticle degradation, recent promising strategies for durable Pt-based nanoscale electrocatalysts are introduced and the role of each design for durability enhancement is discussed. Finally, short comments related to the future direction of nanoparticle issues are provided in terms of nanoparticle synthesis and analysis.

Electrochemically Synthesized Nanoporous Molybdenum Carbide as a Durable Electrocatalyst for Hydrogen Evolution Reaction.

Demands for sustainable production of hydrogen are rapidly increasing because of environmental considerations for fossil fuel consumption and development of fuel cell technologies. Thus, the development of high-performance and economical catalysts has been extensively investigated. In this study, a nanoporous Mo carbide electrode is prepared using a top-down electrochemical process and it is applied as an electrocatalyst for the hydrogen evolution reaction (HER). Anodic oxidation of Mo foil followed by heat treatment in a carbon monoxide (CO) atmosphere forms a nanostructured Mo carbide with excellent interconnections, and these structural characteristics lead to high activity and durability when applied to the HER. Additionally, characteristic behavior of Mo is observed; metallic Mo nanosheets form during electrochemical anodization by exfoliation along the (110) planes. These nanosheets are viable for chemical modification, indicating their feasibility in various applications. Moreover, the role of carbon shells is investigated on the surface of the electrocatalysts, whereby it is suggested that carbon shells serve as a mechanical barrier against the oxidative degradation of catalysts that accompanies unavoidable volume expansion.

Coffee Waste-Derived Hierarchical Porous Carbon as a Highly Active and Durable Electrocatalyst for Electrochemical Energy Applications.

Nitrogen-doped porous carbon materials have been highlighted as promising alternatives to high-cost platinum in various electrochemical energy applications. However, protocols to generate effective pore structure are still challenging, which hampers mass production and utilization of carbon materials. Here, we suggest a facile and effective method for hierarchical porous carbon by a single-step carbonization of coffee waste (CW) with ZnCl2. The CW, which is one of the most earth-abundant organic waste, can be successfully converted to nitrogen-doped porous carbon. It shows outstanding oxygen reduction activity and durability comparable to the state-of-the-art platinum, and the half-wave potential is also comparable to the best metal-free electrocatalysts in alkaline media. Finally, we apply it to counter electrode of dye-sensitized solar cell, whose photovoltaic efficiency surpasses the one made with conventional platinum electrode. We demonstrate the feasibility of our strategies for highly efficient, cheap, and environment-friendly electrocatalyst to replace platinum in various electrochemical energy applications.

Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst.

A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm-2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production.

High-Density Single-Layer Coating of Gold Nanoparticles onto Multiple Substrates by Using an Intrinsically Disordered Protein of α-Synuclein for Nanoapplications.

Functional graffiti of nanoparticles onto target surface is an important issue in the development of nanodevices. A general strategy has been introduced here to decorate chemically diverse substrates with gold nanoparticles (AuNPs) in the form of a close-packed single layer by using an omni-adhesive protein of α-synuclein (αS) as conjugated with the particles. Since the adsorption was highly sensitive to pH, the amino acid sequence of αS exposed from the conjugates and its conformationally disordered state capable of exhibiting structural plasticity are considered to be responsible for the single-layer coating over diverse surfaces. Merited by the simple solution-based adsorption procedure, the particles have been imprinted to various geometric shapes in 2-D and physically inaccessible surfaces of 3-D objects. The αS-encapsulated AuNPs to form a high-density single-layer coat has been employed in the development of nonvolatile memory, fule-cell, solar-cell, and cell-culture platform, where the outlying αS has played versatile roles such as a dielectric layer for charge retention, a sacrificial layer to expose AuNPs for chemical catalysis, a reaction center for silicification, and biointerface for cell attachment, respectively. Multiple utilizations of the αS-based hybrid NPs, therefore, could offer great versatility to fabricate a variety of NP-integrated advanced materials which would serve as an indispensable component for widespread applications of high-performance nanodevices.

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction.

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a "dual purpose" N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From in situ XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells.

Facile and gram-scale synthesis of metal-free catalysts: toward realistic applications for fuel cells.

Although numerous reports on nonprecious metal catalysts for replacing expensive Pt-based catalysts have been published, few of these studies have demonstrated their practical application in fuel cells. In this work, we report graphitic carbon nitride and carbon nanofiber hybrid materials synthesized by a facile and gram-scale method via liquid-based reactions, without the use of toxic materials or a high pressure-high temperature reactor, for use as fuel cell cathodes. The resulting materials exhibited remarkable methanol tolerance, selectivity, and stability even without a metal dopant. Furthermore, these completely metal-free catalysts exhibited outstanding performance as cathode materials in an actual fuel cell device: a membrane electrode assembly with both acidic and alkaline polymer electrolytes. The fabrication method and remarkable performance of the single cell produced in this study represent progressive steps toward the realistic application of metal-free cathode electrocatalysts in fuel cells.