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.

Electrochemically Activatable Liquid Organic Hydrogen Carriers and Their Applications.

Hydrogen has been chosen as an environmentally benign energy source to replace fossil-fuel-based energy systems. Since hydrogen is difficult to store and transport in its gaseous phase, thermochemical liquid organic hydrogen carriers (LOHCs) have been developed as one of the alternative technologies. However, the high temperature and pressure requirements of thermochemical LOHC systems result in huge energy waste and impracticality. This Perspective proposes electrochemical (EC)-LOHCs capable of more efficient, safer, and lower temperature and pressure hydrogen storage/utilization. To enable this technology, several EC-LOHC candidates such as isopropanol, phenolic compounds, and organic acids are described, and the latest research trends and design concepts of related homo/hetero-based electrocatalysts are discussed. In addition, we propose efficient fuel-cell-based systems that implement electrochemical (de)hydrogenation of EC-LOHCs and present prospects for relevant technologies.

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.

Dynamically Stable Active Sites from Surface Evolution of Perovskite Materials during the Oxygen Evolution Reaction.

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La1-xSrxCoO3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoOxHy) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity-stability factor for CoOxHy/LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.

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.

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.

Schwannoma With Cystic Changes in the Pterygomandibular Space.

Schwannomas are tumors that arise from Schwann cells. Although schwannomas can occur almost anywhere in the body where nerve cells are present, they rarely occur in the head and neck region, including the oral and maxillofacial region. Cystic changes in schwannomas are extremely rare. This report is on a case of schwannoma with cystic changes that occurred in the pterygomandibular space.A 46-year-old woman presented with a complaint of limited mouth opening and pain on the left side of the mandible for 3 months. On panoramic radiography, radiolucency was seen on the left mandibular ramus. On enhanced computed tomography, a 4 × 3 cm cystic mass was found along the inner side of the left mandibular ramus area. Magnetic resonance imaging showed a multiseptated, well-demarcated cystic lesion on the inner side of the mandibular ramus on the left side. Under general anesthesia, the tumor was excised. The final diagnosis was schwannoma with cystic changes. Lower lip hypoesthesia occurred postoperatively. At the 1-year postoperative follow-up, maximum mouth opening was increased to 44 mm, and lip hypoesthesia was improved.

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.

Tuning the oxygen reduction activity of the Pt-Ni nanoparticles upon specific anion adsorption by varying heat treatment atmospheres.

Heat treatment of Pt based nanoparticles under various conditions is one of the conventional ways to modify the electrocatalytic properties for enhancement of the oxygen reduction reaction (ORR). However, the effect of the heat treatment atmosphere on the ORR activity especially upon specific anion adsorption still remains unclear. This paper investigates the Pt-Ni bimetallic nanoparticles (Pt2Ni1), under various heat treatment atmospheres, as enhanced cathodic electrocatalysts for the high temperature-proton exchange membrane fuel cell (HT-PEMFC) using a phosphoric acid doped polybenzimidazole (p-PBI) membrane. The X-ray spectroscopic measurement showed the variations of the electronic structures of Pt-Ni nanoparticles under the heat treatment condition. In the half-cell measurement, the argon treated electrocatalyst demonstrated the highest catalytic activity owing to the appropriate electronic interaction between Pt and Ni. The single cell test with a p-PBI membrane, at 160 °C, also confirmed the excellent oxygen reduction reactivity and durability of the argon-treated Pt-Ni nanoparticles. This result suggested that the alteration of the electronic structure by a proper heat treatment atmosphere upon specific anion adsorption decisively influenced the ORR activity both at half-cell and single-cell scales.

Investigating Electrode-Ionomer Interface Phenomena for Electrochemical Energy Applications.

The endeavor to develop high-performance electrochemical energy applications has underscored the growing importance of comprehending the intricate dynamics within an electrode's structure and their influence on overall performance. This review investigates the complexities of electrode-ionomer interactions, which play a critical role in optimizing electrochemical reactions. Our examination encompasses both microscopic and meso/macro scale functions of ionomers at the electrode-ionomer interface, providing a thorough analysis of how these interactions can either enhance or impede surface reactions. Furthermore, this review explores the broader-scale implications of ionomer distribution within porous electrodes, taking into account factors like ionomer types, electrode ink formulation, and carbon support interactions. We also present and evaluate state-of-the-art techniques for investigating ionomer distribution, including electrochemical methods, imaging, modeling, and analytical techniques. Finally, the performance implications of these phenomena are discussed in the context of energy conversion devices. Through this comprehensive exploration of intricate interactions, this review contributes to the ongoing advancements in the field of energy research, ultimately facilitating the design and development of more efficient and sustainable energy devices.

Correlation between platinum nanoparticle surface rearrangement induced by heat treatment and activity for an oxygen reduction reaction.

Heat treatment of nanoparticles could induce the surface rearrangement for more stable facet exposure induced by thermodynamics. By changing the heat treatment environment, we confirmed the correlation between the oxygen reduction activity and the effect of surface oxide and the degree of surface rearrangement of Pt nanoparticles. Native surface oxide was not a critical factor for oxygen reduction activity. However, the degree of surface rearrangement could affect the activity, which was confirmed by the surface sensitive techniques such as CO(ad) oxidation and potential of zero total charge. Analysis indicated that the driving force for nanoparticle surface rearrangement was affected by the heat treatment environment such as gas, in our case.

Chemical tuning of electrochemical properties of Pt-skin surfaces for highly active oxygen reduction reactions.

Pt-skin surfaces were successfully fabricated by the chemical deposition of additional Pt on corrugated Pt-Ni nanoparticles with Pt-skeleton surfaces. Compared to the Pt-skin formed by heat annealing, the chemically-tuned Pt-skin had a higher Pt coordination number and surface crystallinity, which resulted in superior ORR activity and durability.

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.

Noble Metal-Based Multimetallic Nanoparticles for Electrocatalytic Applications.

Noble metal-based multimetallic nanoparticles (NMMNs) have attracted great attention for their multifunctional and synergistic effects, which offer numerous catalytic applications. Combined experimental and theoretical studies have enabled formulation of various design principles for tuning the electrocatalytic performance through controlling size, composition, morphology, and crystal structure of the nanoparticles. Despite significant advancements in the field, the chemical synthesis of NMMNs with ideal characteristics for catalysis, including high activity, stability, product-selectivity, and scalability is still challenging. This review provides an overview on structure-based classification and the general synthesis of NMMN electrocatalysts. Furthermore, postsynthetic treatments, such as the removal of surfactants to optimize the activity, and utilization of NMMNs onto suitable support for practical electrocatalytic applications are highlighted. In the end, future direction and challenges associated with the electrocatalysis of NMMNs are covered.

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.

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.

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction.

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.