Design of Ultrathin Pt‐Based Multimetallic Nanostructures for Efficient Oxygen Reduction Electrocatalysis

Nanocatalysts with high platinum (Pt) utilization efficiency are attracting extensive attention for oxygen reduction reactions (ORR) conducted at the cathode of fuel cells. Ultrathin Pt‐based multimetallic nanostructures show obvious advantages in accelerating the sluggish cathodic ORR due to their ultrahigh Pt utilization efficiency. A focus on recent important developments is provided in using wet chemistry techniques for making/tuning the multimetallic nanostructures with high Pt utilization efficiency for boosting ORR activity and durability. First, new synthetic methods for multimetallic core/shell nanoparticles with ultrathin shell sizes for achieving highly efficient ORR catalysts are reviewed. To obtain better ORR activity and stability, multimetallic nanowires or nanosheets with well‐defined structure and surface are further highlighted. Furthermore, ultrathin Pt‐based multimetallic nanoframes that feature 3D molecularly accessible surfaces for achieving more efficient ORR catalysis are discussed. Finally, the remaining challenges and outlooks for the future will be provided for this promising research field.     

Scalable Design of Two‐Dimensional Oxide Nanosheets for Construction of Ultrathin Multilayer Nanocapacitor

Large size of capacitors is the main hurdle in miniaturization of current electronic devices. Herein, a scalable solution‐based layer‐by‐layer engineering of metallic and high‐κ dielectric nanosheets into multilayer nanosheet capacitors (MNCs) with overall thickness of ≈20 nm is presented. The MNCs are built through neat tiling of 2D metallic Ru_0.95O₂^0.2? and high‐κ dielectric Ca₂NaNb₄O₁₃^? nanosheets via the Langmuir–Blodgett (LB) approach at room temperature which is verified by cross‐sectional high‐resolution transmission electron microscopy (HRTEM). The resultant MNCs demonstrate a high capacitance of 40–52 µF cm^?2 and low leakage currents down to 10^?5–10^?6 A cm^?2. Such MNCs also possess complimentary in situ robust dielectric properties under high‐temperature measurements up to 250 °C. Based on capacitance normalized by the thickness, the developed MNC outperforms state‐of‐the‐art multilayer ceramic capacitors (MLCC, ≈22 µF cm^?2/5 × 10⁴ nm) present in the market. The strategy is effective due to the advantages of facile, economical, and ambient temperature solution assembly.     

Generalized Synthesis of Ultrathin Cobalt‐Based Nanosheets from Metallophthalocyanine‐Modulated Self‐Assemblies for Complementary Water Electrolysis

The development of effective approaches for preparing large‐area, self‐standing, ultrathin metal‐based nanosheets, which have proved to be favorable for catalytic applications such as water electrolysis, is highly desirable but remains a great challenge. Reported herein is a simple and versatile strategy to synthesize ultrathin Co₃O₄ and CoP NSs consisting of close‐packed nanoparticles by pyrolyzing cobalt(II) phthalocyanine/graphene oxide (CoPc/GO) assemblies in air and subsequent topotactic phosphidation while preserving the graphene‐like morphology. The strong π–π stacking interactions between CoPc and GO, and the inhibiting effect of the tetrapyrrole‐derived macrocycle for grain growth during the catalytic carbon gasification contribute to the NSs forming. The resulting homologous Co₃O₄ and CoP NSs display outstanding catalytic activity in alkaline media toward the oxygen evolution reaction and the hydrogen evolution reaction, respectively, ascribed to the richly exposed active sites, and the expedited electrolyte/ion transmission path. The integrated asymmetrical two‐electrode configuration also presents a superior cell voltage of 1.63 V at 10 mA cm^?2 for overall water splitting, accompanied with the excellent durability during long‐term cycling. Further evidences validate that this strategy is appropriate to fabricate graphene‐like ultrathin NSs of many other metal oxides, such as Fe₂O₃, NiO, MoO₃, and mixed‐metal oxides, for various applications.     

Ultrathin Calcium Silicate Hydrate Nanosheets with Large Specific Surface Areas: Synthesis,Crystallization, Layered Self‐Assembly and Applications as Excellent Adsorbents for Drug,Protein, and Metal Ions

A simple and low‐cost solution synthesis is reported for low‐crystalline 1.4 nm tobermorite‐like calcium silicate hydrate (CSH) ultrathin nanosheets with a thickness of ～2.8 nm and with a large specific surface area (SSA), via a reaction‐rate‐controlled precipitation process. The BET SSA of the CSH ultrathin nanosheets can reach as high as 505 m² g^?1. The CSH ultrathin nanosheets have little cytotoxicity and can be converted to anhydrous calcium silicate (ACS) ultrathin nanosheets with a well preserved morphology via a heat treatment process. The crystallinity of CSH ultrathin nanosheets can be improved by solvothermal treatment in water/ethanol binary solvents or a single solvent of water, producing well‐crystalline 1.1 nm tobermorite‐like CSH nanobelts or nanosheets. CSH ultrathin nanosheets acting as building blocks can self‐assemble into layered nanostructures via three different routes. The CSH ultrathin nanosheets are investigated as promising adsorbents for protein (hemoglobin, Hb), drug (ibuprofen, IBU), and metal ions (Cr³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺). The highest adsorbed percentages of Hb and IBU are found to be 83% and 94%, respectively. The highest adsorption capacities of Hb and IBU are found to be as high as 878 milligram Hb per gram CSH and 2.2 gram IBU per gram CSH, respectively. The ppm level metal ions can be totally adsorbed from aqueous solution in just a few minutes. Thus, the CSH ultrathin nanosheets are a promising candidate as excellent adsorbents in the biomedical field and for waste water treatment. Several empirical laws are summarized based on the adsorption profiles of Hb and IBU using CSH ultrathin nanosheets as the adsorbent. Furthermore, the ACS ultrathin nanosheets as adsorbents for Hb protein and IBU drug are investigated.     

Strongly Coupled 3D Hybrids of N‐doped Porous Carbon Nanosheet/CoNi Alloy‐Encapsulated Carbon Nanotubes for Enhanced Electrocatalysis

A novel 3D nanoarchitecture comprising in situ‐formed N‐doped CoNi alloy‐encapsulated carbon nanotubes (CoNi‐NCNTs) grown on N‐doped porous carbon nanosheets (NPCNs) is designed and constructed for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). When evaluated as an electrocatalyst for ORR, the hybrid shows efficient catalytic activity, high selectivity, superior durability, and strong tolerance against methanol crossover compared with the commercial Pt/C catalyst. Such good oxygen reduction reaction performance is comparable to most of the previously reported results and the synergistic effect is found to boost the catalytic performance. Moreover, the constructed hybrid exhibits an excellent ORR activity with a current density of 10 mA cm⁻² at 1.59 V and an onset potential of 1.57 V, even beyond the state‐of‐the‐art Ir/C catalyst in alkaline media. The enhancement in electrochemical performance can be attributed to the unique morphology and defect structures, high porosity, good conductive networks, and strongly interacting CoNi‐NCNT and NPCN in the hybrid. These results suggest the possibility for the development of effective nanocarbon electrocatalysts to replace commercial noble metal catalysts for direct use in fuel cells and water splitting devices.     

ZIF‐67‐Derived 3D Hollow Mesoporous Crystalline Co3O4 Wrapped by 2D g‐C3N4 Nanosheets for Photocatalytic Removal of Nitric Oxide

ZIF‐67‐derived 3D hollow mesoporous crystalline Co₃O₄ wrapped by 2D graphitic carbon nitride (g‐C₃N₄) nanosheets are prepared by low temperature annealing, and are used for the photocatalytic oxidation of nitric oxide (NO) at a concentration of 600 ppb. The p–n heterojunction between Co₃O₄ and g‐C₃N₄ forms a spatial conductive network frame and results in a broad visible light response range. The hollow mesoporous structure of Co₃O₄ contributes to the circulation and adsorption of NO, and the large specific surface area exposes abundant active sites for the reaction of active species. A maximum NO degradation efficiency of 57% is achieved by adjusting the mass of the Co₃O₄ precursor. Cycling tests and X‐ray diffraction indicate the high stability and recyclability of the composite, making it promising in environmental purification applications.     

Synergistic Effect of Mesoporous Co3O4 Nanowires Confined by N‐Doped Graphene Aerogel for Enhanced Lithium Storage

A one‐step multipurpose strategy is developed to realize a sophisticated design that simultaneously integrates three desirable components of nitrogen dopant, 3D graphene, and 1D mesoporous metal oxide nanowires into one hybrid material. This facile synthetic strategy includes a one‐step hydrothermal reaction followed by topotactic calcination. The utilization of urea as the starting reagent enables the precipitation of precursor nanowires and concurrent doping of nitrogen heteroatoms on graphene during hydrothermal reaction, while at the same time the graphene nanosheets are self‐assembled to afford a 3D scaffold. Detailed characterizations on the final calcined product are conducted to confirm the phase purity, porosity, nitrogen composition, and morphology. The integration of two building blocks, i.e., flexible graphene nanosheets and Co₃O₄ nanowires, enables various intertwining behaviors such as seaming, bridging, hooping, bundling, and sandwiching, of which synergistic effect substantially enhances electrical and electrochemical properties of the resultant hybrid. For lithium ion battery application of the hybrid, a remarkably high capacity more than 1200 mA h g^?1 (at 100 mA g^?1) is stabilized over 100 cycles with coulombic efficiency higher than 97%. Even during rapid discharge/charge processes (1000 mA g^?1), a reversible charge capacity of 812 mA h g^?1 is still retained after 230 cycles.     

Synergism of Geometric Construction and Electronic Regulation: 3D Se‐(NiCo)Sx/(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt‐based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru‐based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large‐scale applications. This paper describes the construction of the ultrathin defect‐enriched 3D Se‐(NiCo)S_x/(OH)_x nanosheets for overall water splitting through a facile Se‐induced hydrothermal treatment. Via Se‐induced fabrication, highly efficient Se‐(NiCo)S_x/(OH)_x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as‐prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm^?2 for OER and HER, respectively. Moreover, an overall water‐splitting device delivers a current density of 10 mA cm^?2 for ≈66 h without obvious degradation.     

Mesoporous Metal–Metalloid Amorphous Alloys: The First Synthesis of Open 3D Mesoporous Ni‐B Amorphous Alloy Spheres via a Dual Chemical Reduction Method

Selective hydrogenation of nitriles is an industrially relevant synthetic route for the preparation of primary amines. Amorphous metal–boron alloys have a tunable, glass‐like structure that generates a high concentration of unsaturated metal surface atoms that serve as active sites in hydrogenation reactions. Here, a method to create nanoparticles composed of mesoporous 3D networks of amorphous nickel–boron (Ni‐B) alloy is reported. The hydrogenation of benzyl cyanide to β‐phenylethylamine is used as a model reaction to assess catalytic performance. The mesoporous Ni‐B alloy spheres have a turnover frequency value of 11.6 h^?1, which outperforms non‐porous Ni‐B spheres with the same composition. The bottom‐up synthesis of mesoporous transition metal–metalloid alloys expands the possible reactions that these metal architectures can perform while simultaneously incorporating more Earth‐abundant catalysts.     

Incorporating Nitrogen‐Doped Graphene Quantum Dots and Ni3S2 Nanosheets: A Synergistic Electrocatalyst with Highly Enhanced Activity for Overall Water Splitting

A noble‐metal‐free electrocatalyst is fabricated via in situ formation of nanocomposite of nitrogen‐doped graphene quantum dots (NGQDs) and Ni₃S₂ nanosheets on the Ni foam (Ni₃S₂‐NGQDs/NF). The resultant Ni₃S₂‐NGQDs/NF can serve as an active, binder‐free, and self‐supported catalytic electrode for direct water splitting, which delivers a current density of 10 mA cm^?2 at an overpotential of 216 mV for oxygen evolution reaction and 218 mV for hydrogen evolution reaction in alkaline media. This bifunctional electrocatalyst enables the construction of an alkali electrolyzer with a low cell voltage of 1.58 V versus reversible hydrogen electrode (RHE) at 10 mA cm^?2. The experimental results and theoretical calculations demonstrate that the synergistic effects of the constructed active interfaces are the key factor for excellent performance. The nanocomposite of NGQDs and Ni₃S₂ nanosheets can be promising water splitting electrocatalyst for large‐scale hydrogen generation or other energy storage and conversion applications.