ZIF‐8/ZIF‐67‐Derived Co‐Nx‐Embedded 1D Porous Carbon Nanofibers with Graphitic Carbon‐Encased Co Nanoparticles as an Efficient Bifunctional Electrocatalyst

Herein, an approach is reported for fabrication of Co‐N_x‐embedded 1D porous carbon nanofibers (CNFs) with graphitic carbon‐encased Co nanoparticles originated from metal–organic frameworks (MOFs), which is further explored as a bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Electrochemical results reveal that the electrocatalyst prepared by pyrolysis at 1000 °C (CoNC‐CNF‐1000) exhibits excellent catalytic activity toward ORR that favors the four‐electron ORR process and outstanding long‐term stability with 86% current retention after 40 000 s. Meanwhile, it also shows superior electrocatalytic activity toward OER, reaching a lower potential of 1.68 V at 10 mA cm^?2 and a potential gap of 0.88 V between the OER potential (at 10 mA cm^?2) and the ORR half‐wave potential. The ORR and OER performance of CoNC‐CNF‐1000 have outperformed commercial Pt/C and most nonprecious‐metal catalysts reported to date. The remarkable ORR and OER catalytic performance can be mainly attributable to the unique 1D structure, such as higher graphitization degree beneficial for electronic mobility, hierarchical porosity facilitating the mass transport, and highly dispersed CoN_xC active sites functionalized carbon framework. This strategy will shed light on the development of other MOF‐based carbon nanofibers for energy storage and electrochemical devices.     

The Influence of Carbon Nitride Nanosheets Doping on the Crystalline Formation of MIL‐88B(Fe) and the Photocatalytic Activities

In this research, bulk graphitic carbon nitride (g‐C₃N₄) is exfoliated and transferred to the carbon nitride nanosheets (CNNSs), which are then coupled with MIL‐88B(Fe) to form the hybrid. From the results of the powder X‐ray diffraction, scanning electronic microscopy and thermogravimetric analysis, it is found that the doping of CNNSs on the surface of MIL‐88(Fe) could maintain the basic structure of MIL‐88B(Fe), and the smaller dimension of CNNSs might influence the crystallization process of metal‐organic frameworks (MOFs) compared to bulk g‐C₃N₄. Besides, the effects of the CNNSs incorporation on photocatalysis are also investigated. Through the photoluminescence spectra, electrochemical measurements, and photocatalytic experiments, the hybrid containing 6% CNNSs is certified to possess the highest catalytic activity to degrade methylene blue and reduce Cr(VI) under visible light. The improvement of the photocatalytic performance can be attributed to the matched energy level which favors the formation of the heterojunctions. Besides, it promotes the charge migration such that the contact between MOFs and CNNSs is more intimate, which can be inferred from the electronic microscopy images. Finally, a possible photocatalytic mechanism is put forward by the relative calculation and the employment of the scavengers to trap the active species.     

Ultrathin 2D Zirconium Metal–Organic Framework Nanosheets: Preparation and Application in Photocatalysis

Synthesizing ultrathin 2D metal–organic framework nanosheets in high yields has received increasing research interest but remains a great challenge. In this work, ultrathin zirconium‐porphyrinic metal–organic framework (MOF) nanosheets with thickness down to ≈1.5 nm are synthesized through a pseudoassembly–disassembly strategy. Owing to the their unique properties originating from their ultrathin thickness and highly exposed active sites, the as‐prepared ultrathin nanosheets exhibit far superior photocatalysis performance compared to the corresponding bulk MOF. This work highlights new opportunities in designing ultrathin MOF nanosheets and paves the way to expand the potential applications of MOFs.     

Electric Power Generation through the Direct Interaction of Pristine Graphene‐Oxide with Water Molecules

Converting ubiquitous environmental energy into electric power holds tremendous social and financial interests. Traditional energy harvesters and converters are limited by the specific materials and complex configuration of devices. Herein, it is presented that electric power can be directly produced from pristine graphene oxide (GO) without any pretreatment or additives once encountering the water vapor, which will generate an open‐circuit‐voltage of up to 0.4–0.7 V and a short‐circuit‐current‐density of 2–25 µA cm^?2 on a single piece of GO film. This phenomenon results from the directional movement of charged hydrogen ions through the GO film. The present work demonstrates and provides an extremely simple method for electric energy generation, which offers more applications of graphene‐based materials in green energy converting field.     

High strength and ductility of 34CrNiMo6 steel produced by laser solid forming

Because of the excellent mechanical properties of 34CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility. In this study, 34CrNiMo6 Steel having high strength and ductility is produced by laser solid forming (LSF) with a quenching-tempering (QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%, respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample.     

Facile growth of homogeneous Ni(OH)<Subscript>2</Subscript> coating on carbon nanosheets for high-performance asymmetric supercapacitor applications

The growth of a Ni(OH)₂ coating on conductive carbon substrates is an efficient way to address issues related to their poor conductivity in electrochemical capacitor applications. However, the direct growth of nickel hydroxide coatings on a carbon substrate is challenging, because the surfaces of these systems are not compatible and a preoxidation treatment of the conductive carbon substrate is usually required. Herein, we present a facile preoxidation-free approach to fabricate a uniform Ni(OH)₂ coating on carbon nanosheets (CNs) by an ion-exchange reaction to achieve the in situ transformation of a MgO/C composite to a Ni(OH)₂/C one. The obtained Ni(OH)₂/CNs hybrids possess nanosheet morphology, a large surface area (278 m²/g), and homogeneous elemental distributions. When employed as supercapacitors in a three-electrode configuration, the Ni(OH)₂/CNs hybrid achieves a large capacitance of 2,218 F/g at a current density of 1.0 A/g. Moreover, asymmetric supercapacitors fabricated with the Ni(OH)₂/CNs hybrid exhibit superior supercapacitive performances, with a large capacity of 198 F/g, and high energy density of 56.7 Wh/kg at a power density of 4.0 kW/kg. They show excellent cycling stability with 93% capacity retention after 10,000 cycles, making the Ni(OH)₂/CNs hybrid a promising candidate for practical applications in supercapacitor devices.

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Numerical and experimental investigations in electromagnetic riveting with different rivet dies

In this work, the numerical simulations and electromagnetic riveting (EMR) experiments were conducted to investigate microstructure evolution and the forming mechanism of adiabatic shear bands (ASBs). And the effects of rivet dies on microstructure distributions in formed heads and mechanical properties of riveted structures were systematically explored. The impact velocity and deformation distribution results demonstrated that the proposed numerical method was accurate and reliable. The simulation results showed the slope angle of rivet dies notably affected the plastic flow of materials, and then determined the microstructure distribution in formed heads. The combined effects of inhomogeneous plastic flow and thermal softening were accounted for the forming of ASBs. The formed heads had two obvious ASBs (upper and lower ASB) for the 40° rivet die and flat rivet die. The formed heads only had the lower ASB and no clear upper for the 60° rivet die and 80° rivet die. The pull-out test results showed that the specific rivet die could improve the mechanical properties of the EMR joints, which contribute to the engineering applications of EMR riveted structures.     

Molten-salt chemical exfoliation process for preparing two-dimensional mesoporous Si nanosheets as high-rate Li-storage anode

Two-dimensional (2D) materials have attracted enormous attention due to their functional applications in energy storage. In this work, a low-temperature molten-salt chemical exfoliation methodology is developed for producing free-standing 2D mesoporous Si through deintercalation of CaSi₂ in excess molten AlCl₃ at 195 °C. The average dimension of these sheets is 1.5 μm, and the thickness of a single sheet is approximately 10 nm. The as-prepared 2D Si has a Brunauer–Emmett–Teller surface area of 154 m²·g^?1 and an average pore size of 5.87 nm. With this unique structure, the 2D Si exhibits superior Li-storage performance, including a reversible capacity of 2,974 mA·h·g^?1 at 0.2 C, reversible capacities of 2,162, 1,947, and 1,527 mA·h·g^?1 at 0.8, 2, and 5 C after 200 cycles, and a capacity retention of 357 mA·h·g^?1 even at 30 C (90 A·g^?1).

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