Hollow-Structured Metal Oxides as Oxygen-Related Catalysts

"Three-in-One" Multifunctional Hollow Nanocages with Colorimetric Photothermal Catalytic Activity for Enhancing Sensitivity in Biosensing

Immunochromatographic assays (ICAs) have been widely used in the field detection of mycotoxin contaminants. Nevertheless, the lack of multisignal readout capability and the ability of signaling tags to maintain their biological activity while efficiently loading antibodies remain a great challenge in satisfying diverse testing demands. Herein, we proposed a novel three-in-one multifunctional hollow vanadium nanomicrosphere (high brightness-catalytic-photothermal properties)-mediated triple-readout ICA (VHMS-ICA) for sensitive detection of T-2. As the key to this biosensing strategy, vanadium was used as the catalytic-photothermal characterization center, and natural polyphenols were utilized as the bridging ligands for coupling with the antibody while self-assembling with formaldehyde cross-linking into a hollow nanocage-like structure, which offers the possibility of realizing a three-signal readout strategy and improving the coupling efficiency to the antibody while preserving its biological activity. The constructed sensors showed a detection limit (LOD) of 2 pg/mL for T-2, which was about 345-fold higher than that of conventional gold nanoparticle-based ICA (0.596 ng/mL). As anticipated, the detection range of VHMS-ICA was extended about 8-fold compared with the colorimetric signal alone. Ultimately, the proposed immunosensor performed well in maize and oat samples, with satisfactory recoveries. Owing to the synergistic and complementary interactions between distinct signaling modes, the establishment of multimodal immunosensors with multifunctional tags is an efficient strategy to satisfy diversified detection demands.

Hollow-Structured and Polyhedron-Shaped High Entropy Oxide toward Highly Active and Robust Oxygen Evolution Reaction in a Full pH Range

High entropy metal oxides (HEO) are superior to many reactions involving multi-step elementary reactions. However, controlled synthesis of hollow-structured HEO catalysts, which offers large surface area and fast mass transfer kinetics, remains challenging and unexplored due to the complicated metal precursors. Herein, a metal organic framework-templated synthesis of hollow-structured and polyhedron-shaped HEO catalysts assembled with ultra-small nanoparticles, with up to ten metal elements, can be achieved, by taking advantage of the ion-exchange method. ZnFeNiCuCoRu-O HEO catalyst displays excellent activity and ultra-stability for oxygen evolution reaction in full pH range, with an overpotential of 170 mV at a current density of 10 mA cm-2 , a Tafel slope of 56 mV dec-1 , and a decay of activity by 7% in 30 h in alkaline medium, as well as a 12% and 8% decay in acidic and neutral medium, respectively. DFT calculation indicates that the energy barrier of the potential determining step on Ru-Fe bridge site is significantly lower than any other Ru-related bridge sites for the unique hollow structured HEO structures. This work highlights the importance of ion-exchange method in preparing highly stable and active hollow-structured HEOs catalysts toward highly efficient energy conversion and storage devices.

Cobalt Oxide-Based Electrocatalysts with Bifunctionality for High-Performing Rechargeable Zinc-Air Batteries

In recent years, the rapid growth in renewable energy applications has created a significant demand for efficient energy storage solutions on a large scale. Among the various options, rechargeable zinc-air batteries (ZABs) have emerged as an appealing choice in green energy storage technology due to their higher energy density, sustainability, and cost-effectiveness. Regarding this fact, a spotlight is shaded on air electrode for constructing high-performance ZABs. Cobalt oxide-based electrocatalysts on the air electrode have gained significant attention due to their extraordinary features. Particularly, exploration and integration of bifunctional behavior for energy storage has remarkably promoted both ORR and OER to facilitate the overall performance of the battery. The plot of this review is forwarded towards in-depth analysis of the latest advancements in electrocatalysts that are based on cobalt oxide and possess bifunctional properties along with an introduction of the fundamental aspects of ZABs, Additionally, the topic entails an examination of the morphological variations and mechanistic details mentioning about the synthesis processes. Finally, a direction is provided for future research endeavors through addressing the challenges and prospects in the advancement of next-generation bifunctional electrocatalysts to empower high-performing ZABs with bifunctional cobalt oxide.

Examining O[Formula: see text] adsorption on pristine and defective popgraphene sheets: A DFT study

CONTEXT

Popgraphene (PopG) is a two-dimensional carbon-based material with fused pentagonal and octagonal rings. Like graphene, it exhibits a metallic band gap and exceptional thermal, dynamic, and mechanical stability. Here, we theoretically study the electronic and structural properties of PopG monolayers, including their doped and vacancy-endowed versions, as O[Formula: see text] adsorbers. Our findings show that pristine and vacancy-endowed PopG sheets have a comparable ability to adsorb O[Formula: see text] molecules, with adsorption energies ranging from [Formula: see text]0.57 to [Formula: see text]0.59 eV (physisorption). In these cases, octagonal rings play a dominant role in the adsorption mechanism. Platinum and Silicon doping enhance the O[Formula: see text] adsorption in areas close to the octagonal rings, resulting in adsorption energies ranging from [Formula: see text]1.13 to [Formula: see text]2.56 eV (chemisorption). Furthermore, we computed the recovery time for the adsorbed O[Formula: see text] molecules. The results suggest that PopG/O[Formula: see text] interaction in pristine and vacancy-endowed cases can change the PopG electronic properties before O[Formula: see text] diffusion.

METHODS

Density Functional Theory (DFT) simulations, with Van der Waals corrections (DFT-D, within the Grimme scheme), were performed to study the structural and electronic properties of PopG/O[Formula: see text] systems using the DMol3 code within the Biovia Materials Studio software. The exchange and correlation functions are treated within the generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) functional. We used the double-zeta plus polarization (DZP) for the basis set in these cases. We also considered the BSSE correction through the counterpoise method and the nuclei-valence electron interactions by including semi-core DFT pseudopotentials.

Designed Concave Octahedron Heterostructures Decode Distinct Metabolic Patterns of Epithelial Ovarian Tumors

Epithelial ovarian cancer (EOC) is a polyfactorial process associated with alterations in metabolic pathways. A high-performance screening tool for EOC is in high demand to improve prognostic outcome but is still missing. Here, a concave octahedron Mn₂ O₃ /(Co,Mn)(Co,Mn)₂ O₄ (MO/CMO) composite with a heterojunction, rough surface, hollow interior, and sharp corners is developed to record metabolic patterns of ovarian tumors by laser desorption/ionization mass spectrometry (LDI-MS). The MO/CMO composites with multiple physical effects induce enhanced light absorption, preferred charge transfer, increased photothermal conversion, and selective trapping of small molecules. The MO/CMO shows ≈2-5-fold signal enhancement compared to mono- or dual-enhancement counterparts, and ≈10-48-fold compared to the commercialized products. Subsequently, serum metabolic fingerprints of ovarian tumors are revealed by MO/CMO-assisted LDI-MS, achieving high reproducibility of direct serum detection without treatment. Furthermore, machine learning of the metabolic fingerprints distinguishes malignant ovarian tumors from benign controls with the area under the curve value of 0.987. Finally, seven metabolites associated with the progression of ovarian tumors are screened as potential biomarkers. The approach guides the future depiction of the state-of-the-art matrix for intensive MS detection and accelerates the growth of nanomaterials-based platforms toward precision diagnosis scenarios.

Synthesis and Nanostructure Investigation of Hybrid β-Ga2 O3 /ZnGa2 O4 Nanocomposite Networks with Narrow-Band Green Luminescence and High Initial Electrochemical Capacity

The material design of functional "aero"-networks offers a facile approach to optical, catalytical, or and electrochemical applications based on multiscale morphologies, high large reactive area, and prominent material diversity. Here in this paper, the synthesis and structural characterization of a hybrid β-Ga₂ O₃ /ZnGa₂ O₄ nanocomposite aero-network are presented. The nanocomposite networks are studied on multiscale with respect to their micro- and nanostructure by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and are characterized for their photoluminescent response to UV light excitation and their electrochemical performance with Li-ion conversion reaction. The structural investigations reveal the simultaneous transformation of the precursor aero-GaN(ZnO) network into hollow architectures composed of β-Ga₂ O₃ and ZnGa₂ O₄ nanocrystals with a phase ratio of ≈1:2. The photoluminescence of hybrid aero-β-Ga₂ O₃ /ZnGa₂ O₄ nanocomposite networks demonstrates narrow band (λ_em  = 504 nm) green light emission of ZnGa₂ O₄ under UV light excitation (λ_ex  = 300 nm). The evaluation of the metal-oxide network performance for electrochemical application for Li-ion batteries shows high initial capacities of ≈714 mAh g^-1 at 100 mA g^-1 paired with exceptional rate performance even at high current densities of 4 A g^-1 with 347 mAh g^-1 . This study provides is an exciting showcase example of novel networked materials and demonstrates the opportunities of tailored micro-/nanostructures for diverse applications a diversity of possible applications.

Oxygen Vacancy Promoted O2 Activation over Mesoporous Ni-Co Mixed Oxides for Aromatic Hydrocarbon Oxidation

Whether the oxygen vacancies of heterogeneous catalysts improve their catalytic activity or not has recently been the topic of intense debate in the oxidation of hydrocarbons. We designed an effective strategy to construct mesoporous Ni-Co mixed oxides via a ligand-assisted self-assembly approach. The surface oxygen vacancy concentrations of the mesoporous Ni-Co mixed oxide catalysts were regulated by changing the doping amount of Ni or the reduction method, and the relationship between oxygen vacancies and catalytic activity was studied. Controlled experiments and DFT calculations revealed that oxygen molecules were more favorably adsorbed and activated on oxygen vacancies to form active oxygen species. Increasing the oxygen vacancy concentration within a certain range can effectively enrich the active oxygen species, therefore improving the oxidation rate of ethylbenzene. The optimized mCo₃O₄-0.1NiO catalyst exhibited a remarkable catalytic activity for the solvent-free oxidation of ethylbenzene to acetophenone, typically including 68.0% conversion and 95.4% selectivity (20 mg mCo₃O₄-0.1NiO, 10 mL ethylbenzene, and 0.6 MPa O₂).

Hollow Heterostructured Nanocatalysts for Boosting Electrocatalytic Water Splitting

The implementation of electrochemical water splitting demands the development and application of electrocatalysts to overcome sluggish reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Hollow nanostructures, particularly for hollow heterostructured nanomaterials can provide multiple solutions to accelerate the HER/OER kinetics owing to their advantageous merit. Herein, the recent advances of hollow heterostructured nanocatalysts and their excellent performance for water splitting are systematically summarized. Starting by illustrating the intrinsically advantageous features of hollow heterostructures, achievements in engineering hollow heterostructured electrocatalysts are also highlighted with the focus on structural design, interfacial engineering, composition regulation, and catalytic evaluation. Finally, some perspective insights and future challenges of hollow heterostructured nanocatalysts for electrocatalytic water splitting are also discussed.

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal-O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Fe Ions-Doped TiO2 Aerogels as Catalysts of Oxygen Reduction Reactions in Alkaline Solutions

Aerogels have interconnected networks and preeminent pore structures. When used as the catalysts for oxygen reduction reaction (ORR), they can facilitate the mass transfer and expose more active sites. Here, we synthesized the Fe-doped titanium oxide-based aerogels (TA/Fes) by the sol-gel method combined with thermal treatment. The specific surface areas of the TA/Fes ranged from 475 to 774 m²·g^-1, and the pore volumes varied from 0.96 to 1.72 cm³·g^-1. The doping effect of the Fe ions and the oxygen vacancies in anatase enhance the electrical conductivity, leading to the low Rct (313.3-828.2 Ω). All samples showed excellent stability (2.0-4.5 mV) and 4e^- pathway. The limiting current density of TA/Fe3 reached 5.34 mA·cm^-2, which was comparable to that of commercial Pt/C. The preparation method is inspiring and the as-prepared aerogel catalysts have potential in promoting the scale of fuel cells.

Hollow mesoporous atomically dispersed metal-nitrogen-carbon catalysts with enhanced diffusion for catalysis involving larger molecules

Single-atom catalysts (SACs) show great promise in various applications due to their maximal atom utilization efficiency. However, the controlled synthesis of SACs with appropriate porous structures remains a challenge that must be overcome to address the diffusion issues in catalysis. Resolving these diffusion issues has become increasingly important because the intrinsic activity of the catalysts is dramatically improved by spatially isolated single-atom sites. Herein, we develop a facile topo-conversion strategy for fabricating hollow mesoporous metal-nitrogen-carbon SACs with enhanced diffusion for catalysis. Several hollow mesoporous metal-nitrogen-carbon SACs, including Co, Ni, Mn and Cu, are successfully fabricated by this strategy. Taking hollow mesoporous cobalt-nitrogen-carbon SACs as a proof-of-concept, diffusion and kinetic experiments demonstrate the enhanced diffusion of hollow mesoporous structures compared to the solid ones, which alleviates the bottleneck of poor mass transport in catalysis, especially involving larger molecules. Impressively, the combination of superior intrinsic activity from Co-N₄ sites and the enhanced diffusion from the hollow mesoporous nanoarchitecture significantly improves the catalytic performance of the oxidative coupling of aniline and its derivatives.

Interfacial Assemble of Prussian Blue Analog to Access Hierarchical FeNi (oxy)-Hydroxide Nanosheets for Electrocatalytic Water Splitting

Developing facile methods for the synthesis of active and stable electrocatalysts is vitally important to realize overall water splitting. Here, we demonstrate a practical method to obtain FeNiOOH nanosheets on nickel foam (NF) as bifunctional electrocatalyst by growing a FeCo Prussian blue analog with further in situ oxidation under ambient conditions. The binder-free, self-standing FeNiOOH/NF electrode with hierarchical nanostructures requires low overpotentials of 260 mV and 240 mV at a current density of 50 mA cm-2 for oxygen evolution reaction and hydrogen evolution reaction, respectively, in 1.0 M KOH solution. Therefore, an alkaline water electrolyzer constructed by bifunctional FeNiOOH/NF electrode as both anode and cathode delivers 50 mA cm-2 under a cell voltage of 1.74 V with remarkable stability, which outperforms the IrO2-Pt/C-based electrolyzer. The excellent performance could be ascribed to the superior FeNiOOH intrinsic activity and the hierarchical structure. This work provides a cost-efficient surface engineering method to obtain binder-free, self-standing bifunctional electrocatalyst on commercial NF, which could be further extended to other energy and environment applications.

Hollow Mesoporous Manganese Oxides: Application in Cancer Diagnosis and Therapy

The precision, minimal invasiveness, and integration of diagnosis and treatment are critical factors for tumor treatment at the present. Although nanomedicine has shown the potential in tumor precision treatment, nanocarriers with high efficiency, excellent targeting, controlled release, and good biocompatibility still need to be further explored. Hollow mesoporous manganese oxides nanomaterials (HM-MONs), as an efficient drug delivery carrier, have attracted substantial attention in applications of tumor diagnosis and therapy due to their unique properties, such as tumor microenvironment stimuli-responsiveness, prominent catalytic activity, excellent biodegradation, and outstanding magnetic resonance imaging ability. The HM-MONs can not only enhance the therapeutic efficiency but also realize multimodal diagnosis of tumors. Consequently, it is necessary to introduce applications based on HM-MONs in cancer diagnosis and therapy. In this review, the representative progress of HM-MONs in synthesis is discussed. Then, several promising applications in drug delivery, bio-imaging, and bio-detection are highlighted. Finally, the challenges and perspectives of the anticancer applications are summarized, which is expected to provide meaningful guidance on further research.

Design and Synthesis of Hollow Nanostructures for Electrochemical Water Splitting

Electrocatalytic water splitting using renewable energy is widely considered as a clean and sustainable way to produce hydrogen as an ideal energy fuel for the future. Electrocatalysts are indispensable elements for large-scale water electrolysis, which can efficiently accelerate electrochemical reactions occurring at both ends. Benefitting from high specific surface area, well-defined void space, and tunable chemical compositions, hollow nanostructures can be applied as promising candidates of direct electrocatalysts or supports for loading internal or external electrocatalysts. Herein, some recent progress in the structural design of micro-/nanostructured hollow materials as advanced electrocatalysts for water splitting is summarized. First, the design principles and corresponding strategies toward highly effective hollow electrocatalysts for oxygen/hydrogen evolution reactions are highlighted. Afterward, an overview of current reports about hollow electrocatalysts with diverse architectural designs and functionalities is given, including direct hollow electrocatalysts with single-shelled, multi-shelled, or open features and heterostructured electrocatalysts based on hollow hosts. Finally, some future research directions of hollow electrocatalysts for water splitting are discussed based on personal perspectives.

Successive Anion/Cation Exchange Enables the Fabrication of Hollow CuCo2S4 Nanorods for Advanced Oxygen Evolution Reaction Electrocatalysis

Hollow CuCo₂S₄ nanorods (H-CCS-Ns) have been successfully developed via a facile successive anion/cation-exchange method. The outstanding electrocatalytic performance of H-CCS-Ns is mainly attributed to its distinctive hollow structure, which accelerates the electron transfer rate and provides abundant active sites. Moreover, a mechanism study indicates that H-CCS-Ns has highly active octahedral Co³⁺, and the existence of Co³⁺ cations optimizes the adsorption of oxygen-involved intermediates, making H-CCS-Ns a promising OER electrocatalyst. Optimized H-CCS-Ns only need an ultralow overpotential of 220 mV to drive a current density of 10 mA·cm^-2 and exhibit distinguished cycling stability with a negligible fluctuation for 30 h. More impressively, when H-CCS-Ns are assembled with Pt/C for overall water splitting, a voltage as low as 1.545 V is required at a current density of 10 mA·cm^-2, and the catalyst shows outstanding stability for as long as 38 h. This study offers a feasible strategy to design hollow spinel catalysts for efficient OER catalysis.

Reconstruction of spinel Co3O4 by inert Zn2+ towards enhanced oxygen catalytic activity

Zn_xCo_3-xO₄ (0 ≤ x≤ 1) coupled with nitrogen-doped hollow porous carbon spheres exhibits a superior oxygen catalytic activity. A Zn-air battery using Zn_0.6Co_2.4O₄/NHCS as a cathodic catalyst affords a high-power density (130 mW cm^-2) and excellent stability. The effect of reconstruction of catalytically active Co ions induced by Zn is well-investigated.

Advances in Understanding the Electrocatalytic Reconstruction Chemistry of Coordination Compounds

Coordination compounds including mainstream metal-organic frameworks and Prussian blue analogues receive extensive researches when they directly serve as electrocatalysts. Their reconstruction phenomena, that are closely associated with actual contributions and intrinsic catalytic mechanisms, are expected to be well summarized. Here, the recent advances in understanding reconstruction chemistry of coordination compounds are reviewed, including their main classifications and structural properties, reconstruction phenomena in electrocatalysis (e.g., oxygen/hydrogen evolution reaction, CO₂ reduction), influence factors of reconstruction parameters (e.g., reconstruction rate and reconstruction degree), and reconstruction-performance correlation. It is outlined that the reconstruction processes are influenced by electronic structure of coordination compounds, pH and temperature of testing solution, and applied potentials. The characterization techniques reflecting the evolution information before and after catalysis are also introduced for reconstruction-related mechanistic study. Finally, some challenges and outlooks on reconstruction investigations of coordination compounds are proposed, and the necessity of studying and understanding of these themes under actual working conditions of devices is highlighted.

Stepwise chemical oxidation to access ultrathin metal (oxy)-hydroxide nanosheets for the oxygen evolution reaction

Incorporation of ultrathin nanosheets with dopants/defects shows great potential to enable metal (oxy)-hydroxide electrocatalysts with enhanced oxygen evolution reaction (OER) performance via the regulation of atomic structure and bonding arrangements. However, it remains challenging in synthesis especially for such dual control and at large scale. In this study, we present a stepwise chemical oxidation route, involving phase transition and reconstruction processes, to access ultrathin CoOOH nanosheets with a thickness of ca. 4 nm and abundant oxygen vacancies. Other transition metals were also doped into CoOOH nanosheets through this strategy. Among them, the optimized FeCoOOH nanosheets demonstrated an efficient OER activity with overpotential as low as 252 mV (current density: 10 mA cm^-2) and excellent stability. A high and stable solar-to-hydrogen efficiency of 10.5% was acquired when FeCoOOH nanosheets were used as the anode in a constructed water splitting device driven by solar energy. This study offers a noble and facile strategy for potentially scalable preparation of atom-modulated ultrathin metal (oxy)-hydroxide nanosheets, and also demonstrates the OER applications.

Rational Design and Engineering of One-Dimensional Hollow Nanostructures for Efficient Electrochemical Energy Storage

The unique structural characteristics of one-dimensional (1D) hollow nanostructures result in intriguing physicochemical properties and wide applications, especially for electrochemical energy storage applications. In this Minireview, we give an overview of recent developments in the rational design and engineering of various kinds of 1D hollow nanostructures with well-designed architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties for different kinds of electrochemical energy storage applications (i.e. lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, lithium-selenium sulfur batteries, lithium metal anodes, metal-air batteries, supercapacitors). We conclude with prospects on some critical challenges and possible future research directions in this field. It is anticipated that further innovative studies on the structural and compositional design of functional 1D nanostructured electrodes for energy storage applications will be stimulated.

Electromagnetic absorbers with Schottky contacts derived from interfacial ligand exchanging metal-organic frameworks

Various types of polycrystals have been regarded as excellent electromagnetic (EM) microwave absorbents, while differentiated heterointerfaces among grains usually manipulate conductive loss and polarization relaxation, especially interfacial polarization. Herein, polar facets that dominated the optimization of EM attenuation were clarified by carefully designing polycrystalline Schottky junctions with metal-semiconductor contacts for the first time. An ingenious ligand exchange technique was utilized to construct Zn-MOF (ZIF-L) precursors for Fe-ZnO polycrystals, in which Fe-containing Fe(CN)₆^3- etching ligand acted as metallic source in Schottky junctions. By adjusting the Schottky contacts in polycrystals, the enhanced grain boundaries mainly induced stronger interfacial polarization and affected the microcurrent lightly. This is because Schottky barriers can cause local charge accumulation on heterointerfaces for polarization relaxation. Additionally, the coexistence of Zn and O vacancies brought a lot of lattice defects and distortions for dipole polarization. Thus, optimal EM wave absorbability was obtained by polycrystals with 8 h ligand exchange and an effective absorption band reaching 4.88 GHz. This work can provide guidance for designing advanced polycrystalline EM absorption materials and also highlight the mechanism and requirement of Schottky junctions dominating polarization.

Rapid synthesis of Cu2O hollow spheres at low temperature and their catalytic performance for the decomposition of ammonium perchlorate

A simple and efficient strategy for synthesizing high-performance crystalline catalyst Cu2O hollow spheres with a large specific surface area was demonstrated.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

Continuous Synthesis of Hollow High-Entropy Nanoparticles for Energy and Catalysis Applications

Mixing multimetallic elements in hollow-structured nanoparticles is a promising strategy for the synthesis of highly efficient and cost-effective catalysts. However, the synthesis of multimetallic hollow nanoparticles is limited to two or three elements due to the difficulties in morphology control under the harsh alloying conditions. Herein, the rapid and continuous synthesis of hollow high-entropy-alloy (HEA) nanoparticles using a continuous "droplet-to-particle" method is reported. The formation of these hollow HEA nanoparticles is enabled through the decomposition of a gas-blowing agent in which a large amount of gas is produced in situ to "puff" the droplet during heating, followed by decomposition of the metal salt precursors and nucleation/growth of multimetallic particles. The high active sites per mass ratio of such hollow HEA nanoparticles makes them promising candidates for energy and electrocatalysis applications. As a proof-of-concept, it is demonstrated that these materials can be applied as the cathode catalyst for Li-O₂ battery operations with a record-high current density per catalyst mass loading of 2000 mA g_cat. ^-1 , as well as good stability and durable catalytic activity. This work offers a viable strategy for the continuous manufacturing of hollow HEA nanomaterials that can find broad applications in energy and catalysis.

MOF-Assisted Synthesis of Highly Mesoporous Cr2O3/SiO2 Nanohybrids for Efficient Lewis-Acid-Catalyzed Reactions

The facile fabrication of porous solid acids is highly desired for replacing hazardous liquid acids for many acid-catalyzed reactions in the industry. Herein, we present a bottom-up strategy to construct ultrastable mesoporous Cr₂O₃/SiO₂ nanohybrids (denoted as Meso-Cr-Si-O) with highly dispersed Lewis acid sites by pyrolysis of a SiO₂@MIL-101 precursor prepared via nanocasting by a reverse double-solvent approach, which can guarantee the efficient encapsulation of SiO₂ nanoparticles (NPs) inside the MIL-101 pores. The pore environment of Meso-Cr-Si-O can be well tuned by simply controlling the amount of silica within the MIL-101 pores and the pyrolysis temperature. Pyridine adsorption experiments demonstrate that the density of Lewis acidic sites in the obtained Meso-Cr-Si-O is much higher than that of MIL-101-derived Cr₂O₃ NPs. Benefitting from its highly mesoporous nanostructure with abundant acid sites, the optimal Meso-Cr-Si-O exhibits a significantly improved catalytic activity for the Lewis-acid-catalyzed Meerwein-Ponndorf-Verley reduction of cyclohexanone with 4.5 times higher yield of cyclohexanol than that of the MIL-101-derived Cr₂O₃ NPs, representing the first efficient Cr₂O₃-based catalytic system for this reaction.

Anodic hydrazine electrooxidation boosted overall water electrolysis by bifunctional porous nickel phosphide nanotubes on nickel foam

Water electrolysis is an environmentally friendly and sustainable technique for ultra-pure hydrogen production, while expensive electrode materials and high driving voltage have seriously hindered its commercialization process. Here, Earth-abundant bifunctional porous Ni2P hollow nanotubes on nickel foam (Ni2P-HNTs/NF) electrocatalysts are synthesized through a facile self-template method and a phosphating process, which are perfectly combined with the hydrazine electrooxidation reaction (HzOR) boosted water electrolysis. Benefiting from the unique structural characteristic of open-framework and abundant step atoms, Ni2P-HNTs/NF achieves 10 mA cm-2 at 91 mV (vs. RHE) for the cathodic hydrogen evolution reaction and 18 mV (vs. RHE) for the anodic HzOR in a three electrode system, respectively. The corresponding two-electrode hydrazine electrolyzer produces 10 mA cm-2 with a total voltage of only 152 mV for ultra-pure hydrogen production, highlighting a cost-effective and energy-saving water electrolysis mode.

Designed Formation of Double-Shelled Ni-Fe Layered-Double-Hydroxide Nanocages for Efficient Oxygen Evolution Reaction

Delicate design of nanostructures for oxygen-evolution electrocatalysts is an important strategy for accelerating the reaction kinetics of water splitting. In this work, Ni-Fe layered-double-hydroxide (LDH) nanocages with tunable shells are synthesized via a facile one-pot self-templating method. The number of shells can be precisely controlled by regulating the template etching at the interface. Benefiting from the double-shelled structure with large electroactive surface area and optimized chemical composition, the hierarchical Ni-Fe LDH nanocages exhibit appealing electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte. Particularly, double-shelled Ni-Fe LDH nanocages can achieve a current density of 20 mA cm^-2 at a low overpotential of 246 mV with excellent stability.

KB-templated in situ synthesis of highly dispersed bimetallic NiFe phosphides as efficient oxygen evolution catalysts

KB-templated in situ synthesized highly dispersed bimetallic NiFe phosphides function as efficient oxygen evolution catalysts.

In situ construction of a poly-3,4-ethylenedioxythiophene skeleton on carbon nanotubes to improve long-term stability for oxygen reduction reaction

Although carbon nanomaterials show good potential as efficient electrocatalysts for oxygen reduction reaction (ORR), their inferior long-term stability due to their high surface energy resulting in compact restacking limits their practical performance.

Optimized Co2+(Td)–O–Fe3+(Oh) electronic states in a spinel electrocatalyst for highly efficient oxygen evolution reaction performance

The introduction of iron into Co₃O₄ can induce a change in the electronic states of Co³⁺, which is an effective means to regulate the oxygen evolution reaction activity.