A Gas-Steamed MOF Route to P-Doped Open Carbon Cages with Enhanced Zn-Ion Energy Storage Capability and Ultrastability.

Carbon micro/nanocages have received great attention, especially in electrochemical energy-storage systems. Herein, as a proof-of-concept, a solid-state gas-steamed metal-organic-framework approach is designed to fabricate carbon cages with controlled openings on walls, and N, P dopants. Taking advantage of the fabricated carbon cages with large openings on their walls for enhanced kinetics of mass transport and N, P dopants within the carbon matrix for favoring chemical adsorption of Zn ions, when used as carbon cathodes for advanced aqueous Zn-ion hybrid supercapacitors (ZHSCs), such open carbon cages (OCCs) display a wide operation voltage of 2.0 V and an enhanced capacity of 225 mAh g-1 at 0.1 A g-1 . Also, they exhibit an ultralong cycling lifespan of up to 300 000 cycles with 96.5% capacity retention. Particularly, such OCCs as electrode materials lead to a soft-pack ZHSC device, delivering a high energy density of 97 Wh kg-1 and a superb power density of 6.5 kW kg-1 . Further, the device can operate in a wide temperature range from -25 to + 40 °C, covering the temperatures for practical applications in daily life.

Single-Atom Catalysts Derived from Metal-Organic Frameworks for Electrochemical Applications.

Single-atom catalysts (SACs) have received tremendous attention due to their extraordinary catalytic performances. The synthesis of this kind of catalysts is highly desired and challenging. In the last few years, metal-organic frameworks (MOFs) have been demonstrated as a promising precursor for fabricating SACs. In this review, the progress and recent advances in the synthesis of MOF-derived SACs and their electrochemical applications are summarized. First, the synthetic approaches based on MOFs and accessible characterization techniques for SACs as well as their advantages/disadvantages are discussed. Then, the electrochemical applications of these MOF-derived SACs including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), CO2 reduction reaction (CO2 RR), nitrogen reduction reaction (NRR), and other energy-related reactions are reviewed. Finally, insights into the current challenges and future prospects of this field are briefly presented.

Uniformly bimetal-decorated holey carbon nanorods derived from metal-organic framework for efficient hydrogen evolution.

The hydrogen evolution reaction (HER) as a fundamental process in electrocatalysis plays a significant role in clean energy technologies. For an energy-efficient HER, it demands an effective, durable, and low-cost catalyst to trigger proton reduction with minimal overpotential and fast kinetics. Here, we successfully fabricate a highly efficient HER catalyst of N-C/Co/Mo2C holey nanorods with Co/ß-Mo2C nanoparticles uniformly embedded in nitrogen-doped carbon (N-C/Co/Mo2C) by pyrolyzing the molybdate-coordinated zeolitic imidazolate framework (ZIF-67/MoO42-) holey nanorods, which result from the reaction between CoMoO4 and MeIM in a methanol/water/triethylamine mixed solution. The uniform distribution of MoO42- in the ZIF-67/MoO42- enables Co/ß-Mo2C nanoparticles to be well-distributed within nitrogen-doped carbon holey nanorods. This synthetic strategy endows the N-C/Co/Mo2C catalyst with uniformly decorated bimetal, thus attaining excellent HER electrocatalytic activities with a small overpotential of 142.0 mV at 10 mA cm-2 and superior stability in 1.0 mol L-1 KOH aqueous solution.

A Zinc-Dual-Halogen Battery with a Molten Hydrate Electrolyte.

Halogen redox couples offer several advantages for energy storage such as low cost, high solubility in water, and high redox potential. However, the operational complexity of storing halogens at the oxidation state via liquid-phase media hampers their widespread application in energy-storage devices. Herein, an aqueous zinc-dual-halogen battery system taking the advantages of redox flow batteries (inherent scalability) and intercalation chemistry (high capacity) is designed and fabricated. To enhance specific energy, the designed cell exploits both bromine and chlorine as the cathode redox couples that are present as halozinc complexes in a newly developed molten hydrate electrolyte, which is distinctive to the conventional zinc-bromine batteries. Benefiting from the reversible uptake of halogens at the graphite cathode, exclusive reliance on earth-abundant elements, and membrane-free and possible flow-through configuration, the proposed battery can potentially realize high-performance massive electric energy storage at a reasonable cost.

Nanoribbon Superstructures of Graphene Nanocages for Efficient Electrocatalytic Hydrogen Evolution.

Two-dimensional carbon architectures are attracting tremendous interests for various promising applications due to their outstanding electronic and mechanical properties, although it is a great challenge to rationally devise facile and operative methodologies to engineer their structural traits owing to complex synthetic processes. Herein, for the first time, we fabricate two-dimensional carbon nanoribbons via direct thermal exfoliation of one-dimensional Ni-based metal-organic framework (MOF) nanorods, in which interconnected graphitic carbon nanocages are self-assembled into a belt-like superstructure with carbon-encapsulated Ni nanoparticles immobilized on the surface. Due to the unparalleled structural superiority, the MOF-derived carbon nanobelts exhibit excellent catalytic performances in electrocatalytic hydrogen evolution. Importantly, the practical synthetic strategy may trigger the rapid development of carbon-based superstructures in many frontier fields.

MOF-Mediated Fabrication of a Porous 3D Superstructure of Carbon Nanosheets Decorated with Ultrafine Cobalt Phosphide Nanoparticles for Efficient Electrocatalysis and Zinc-Air Batteries.

Superstructures have attracted great interest owing to their potential applications. Herein, we report the first scalable preparation of a porous nickel-foam-templated superstructure of carbon nanosheets decorated with ultrafine cobalt phosphide nanoparticles. Uniform two-dimensional (2D) Co-metal organic framework (MOF) nanosheets (Co-MNS) grow on nickel foam, followed by a MOF-mediated tandem (carbonization/phosphidation) pyrolysis. The resulting superstructure has a porous 3D interconnected network with well-arranged 2D carbon nanosheets on it, in which ultrafine cobalt phosphide nanoparticles are tightly immobilized. A single piece of this superstructure can be directly used as a self-supported electrode for electrocatalysis without any binders. This "one-piece" porous superstructure with excellent mass transport and electron transport properties, and catalytically active cobalt phosphide nanoparticles with ultrasmall size (3-4 nm), shows excellent trifunctional electrocatalytic activities for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR), achieving great performances in water splitting and Zn-air batteries.

Fabricating Dual-Atom Iron Catalysts for Efficient Oxygen Evolution Reaction: A Heteroatom Modulator Approach.

Understanding the thermal aggregation behavior of metal atoms is important for the synthesis of supported metal clusters. Here, derived from a metal-organic framework encapsulating a trinuclear FeIII 2 FeII complex (denoted as Fe3 ) within the channels, a well-defined nitrogen-doped carbon layer is fabricated as an ideal support for stabilizing the generated iron nanoclusters. Atomic replacement of FeII by other metal(II) ions (e.g., ZnII /CoII ) via synthesizing isostructural trinuclear-complex precursors (Fe2 Zn/Fe2 Co), namely the "heteroatom modulator approach", is inhibiting the aggregation of Fe atoms toward nanoclusters with formation of a stable iron dimer in an optimal metal-nitrogen moiety, clearly identified by direct transmission electron microscopy and X-ray absorption fine structure analysis. The supported iron dimer, serving as cooperative metal-metal site, acts as efficient oxygen evolution catalyst. Our findings offer an atomic insight to guide the future design of ultrasmall metal clusters bearing outstanding catalytic capabilities.

A Honeycomb-Like Bulk Superstructure of Carbon Nanosheets for Electrocatalysis and Energy Storage.

Superstructures have attracted extensive attention because of their potential applications in materials science and biology. Herein, we fabricate the first centimeter-sized porous superstructure of carbon nanosheets (SCNS) by using metal-organic framework nanoparticles as a template and polyvinylpyrrolidone as an additional carbon source. The SCNS shows a honeycomb-like morphology with wall-sharing carbon cages, in each cavity of which a porous carbon sphere is encapsulated. A single piece of SCNS is directly used as the electrode for a two-electrode symmetrical supercapacitor cell without any binders and supports, benefiting from its advantage in ultra-large geometric size, and the Fe-immobilized SCNS exhibits excellent catalytic performances for oxygen reduction reaction and in a Zn-air battery. This synthetic strategy presents a facile approach for preparing functional SCNS at centimetric scale with controllable morphologies and compositions favoring the fabrication of energy devices.

Single-Atom Iron Catalysts on Overhang-Eave Carbon Cages for High-Performance Oxygen Reduction Reaction.

Single-atom catalysts have drawn great attention, especially in electrocatalysis. However, most of previous works focus on the enhanced catalytic properties via improving metal loading. Engineering morphologies of catalysts to facilitate mass transport through catalyst layers, thus increasing the utilization of each active site, is regarded as an appealing way for enhanced performance. Herein, we design an overhang-eave structure decorated with isolated single-atom iron sites via a silica-mediated MOF-templated approach for oxygen reduction reaction (ORR) catalysis. This catalyst demonstrates superior ORR performance in both alkaline and acidic electrolytes, comparable to the state-of-the-art Pt/C catalyst and superior to most precious-metal-free catalysts reported to date. This activity originates from its edge-rich structure, having more three-phase boundaries with enhanced mass transport of reactants to accessible single-atom iron sites (increasing the utilization of active sites), which verifies the practicability of such a synthetic approach.

Synthesis of a Hierarchically Porous C/Co3 O4 Nanostructure with Boron Doping for Oxygen Evolution Reaction.

Fabricating a low-cost and highly efficient electrocatalyst is of importance for the development of renewable energy devices. In this work, we have synthesized an ultrafine cobalt oxide nanocatalyst (5-10 nm) doped with boron (BC/Co3 O4 ) by using a metal-organic framework as a precursor, which exhibits an excellent catalytic activity for oxygen evolution reaction (OER). Owing to the improvement of accessible active sites by boron doping, the synthesized catalyst can reach a current density of 10 mA cm-2 at 1.54 V with a low overpotential of 310 mV, superior than those of commercial RuO2 and N-doped C/Co3 O4 . This work provides a facile way to develop highly efficient catalysts for electrochemical reactions.

Synthesis of micro/nanoscaled metal-organic frameworks and their direct electrochemical applications.

As a new class of crystalline porous materials, metal-organic frameworks (MOFs) have received great attention owing to their unique advantages of ultrahigh surface area, large pore volume and versatile applications. Developing different strategies to control the morphology and size of MOFs is very important for their practical applications. Recently, micro/nanosized MOFs have been regarded as promising candidates for electrode materials with excellent performances, which not only bridge the gap between fundamental MOF science and forward-looking applications, but also provide an opportunity to make clear the relationship between morphologies and properties. This review focuses on the design and fabrication of one-, two- and three-dimensional MOFs at micro/nanoscale, and their direct applications in batteries, supercapacitors and electrocatalysis. A discussion on challenges and future prospects of the synthesis and electrochemical applications of micro/nanoscaled MOF materials is presented.

A Hydrangea-Like Superstructure of Open Carbon Cages with Hierarchical Porosity and Highly Active Metal Sites.

Carbon micro-/nanocages have attracted great attention owing to their wide potential applications. Herein, a self-templated strategy is presented for the synthesis of a hydrangea-like superstructure of open carbon cages through morphology-controlled thermal transformation of core@shell metal-organic frameworks (MOFs). Direct pyrolysis of core@shell zinc (Zn)@cobalt (Co)-MOFs produces well-defined open-wall nitrogen-doped carbon cages. By introducing guest iron (Fe) ions into the core@shell MOF precursor, the open carbon cages are self-assembled into a hydrangea-like 3D superstructure interconnected by carbon nanotubes, which are grown in situ on the Fe-Co alloy nanoparticles formed during the pyrolysis of Fe-introduced Zn@Co-MOFs. Taking advantage of such hierarchically porous superstructures with excellent accessibility, synergetic effects between the Fe and the Co, and the presence of catalytically active sites of both metal nanoparticles and metal-Nx species, this superstructure of open carbon cages exhibits efficient bifunctional catalysis for both oxygen evolution reaction and oxygen reduction reaction, achieving a great performance in Zn-air batteries.

Fabrication of a Spherical Superstructure of Carbon Nanorods.

Hierarchical superstructures in nano/microsize have attracted great attention owing to their wide potential applications. Herein, a self-templated strategy is presented for the synthesis of a spherical superstructure of carbon nanorods (SS-CNR) in micrometers through the morphology-preserved thermal transformation of a spherical superstructure of metal-organic framework nanorods (SS-MOFNR). The self-ordered SS-MOFNR with a chestnut-shell-like superstructure composed of 1D MOF nanorods on the shell is synthesized by a hydrothermal transformation process from crystalline MOF nanoparticles. After carbonization in argon, the hierarchical SS-MOFNR transforms into SS-CNR, which preserves the original chestnut-shell-like superstructure with 1D porous carbon nanorods on the shell. Taking the advantage of this functional superstructure, SS-CNR immobilized with ultrafine palladium (Pd) nanoparticles (Pd@SS-CNR) exhibits excellent catalytic activity for formic acid dehydrogenation. This synthetic strategy provides a facile method to synthesize uniform spherical superstructures constructed from 1D MOF nanorods or carbon nanorods for applications in catalysis and energy storage.

Superlong Single-Crystal Metal-Organic Framework Nanotubes.

Nanotubes have attracted great attention. Here, we report the fabrication of the first single-crystal metal-organic framework (MOF) nanotubes. Superlong single-crystal cobalt-organic framework (Co-MOF) nanotubes, which have a diameter of ∼70 nm and length of 20-35 µm with parallel multichannels (window size: 1.1 nm), have been successfully synthesized via an amorphous MOF-mediated recrystallization approach. The synthesized MOF nanotubes can be used as a nanocolumn for separation of large molecules. Carbonization of the Co-MOF nanotubes in an argon atmosphere preserves the 1D morphology, affording long carbon nanofibers. A hierarchical architecture composed of carbon nanofibers wrapped by carbon nanotubes (20-30 nm in diameter and 200-300 nm in length) with cobalt nanoparticles on the top is formed by the carbonization of the Co-MOF nanotubes along with dicyandiamide as a nitrogen and a secondary carbon source. The resulting hierarchical dendrites with carbon nanofiber trunks and carbon nanotube branches exhibit excellent electrocatalytic activity for oxygen reduction reaction and exceptional applications in rechargeable Zn-air batteries. This work demonstrates a new strategy to fabricate MOF nanotubes and relative 1D nanostructures.

Mesoporous iron oxide nanofibers and their loading capacity of curcumin.

Mesoporous iron oxide nanofibers were obtained by calcination of electrospun precursors at various temperatures. Their microstructure is influenced by the calcination temperature. As the calcination temperature is at 350 degrees C, the resultant iron oxide nanofibers largely consist of magnetic Fe3O4 and gamma-Fe2O3, with a specific surface area of about 120 m2/g and magnetization of about 66.5 Am2/kg. When the precursor calcined at 450 degrees C, the pure mesoporous alpha-Fe2O3 nanofibers with a specific surface area of about 92 m2/g are obtained and they show a high loading for curcumin. All the adjusted R-squares for the pseudo-second-order model overtop 0.99 in the initial curcumin ethanol solution concentrations of 30, 40 and 60 microg/mL, which suggests the pseudo-second-order kinetics model fit the adsorption kinetics of curcumin onto the mesoporous alpha-Fe2O3 nanofibers, and the adsorption can reach equilibrium in 60 min. While, Langmuir model (R2 = 0.9980) fits well the curcumin adsorption isotherm onto alpha-Fe2O3 mesoporous nanofibers, and the adsorption capacity is up to 12.48 mg/g at the curcumin concentration of 60 microg/mL.

Preparation of La1-xKxFeO3 microtubes and their adsorption kinetics of methyl blue.

The nanocrystalline La1-xKxFeO3 (x < or = 0.2) microtubes with a high specific surface area were prepared by the citrate-gel and thermal transformation process. These microtubes were characterized by X-ray diffraction (XRD), Brunauere-Emmette-Teller method (BET), and field emission scanning electron microscopy (FE-SEM). With the increase in K content (x) from 0 to 0.2, the average grain size decreases from 32.4 to 24.4 nm, and the specific surface area increases from 8.9 to 36.4 m2/g. The adsorption of methyl blue was analyzed by UV visible spectrophotometer. The adsorption capacity increases with the increase of the substituted-K content and the surface area of the La1-xKxFeO3 microtubes. The adsorption results shows that all the La1-xKxFeO3 microtubes exhibit a high adsorption activity for methyl blue, with the value of x ranging from 0 to 0.2, the adsorbance increases from 139.7 to 173.7 mg/g at the initial methyl blue concentration of 0.5 mg/mL in aqueous solution, and the kinetics data related to the adsorption of methyl blue onto the La1-xKxFeO3 microtubes are in good agreement with the pseudo-second-order kinetic model in the initial methyl blue concentration of 0.1-0.5 mg/mL.

Formation and soot combustion of honeycomb-like LaFeO3 microfibers.

The nanocrystalline, honeycomb-like, perovskite LaFeO3 microfibers with a fibre diameter about 1-2 microm and channel sizes about 180-220 nm on the cross-section were prepared by the citrate-gel process. These microfibers were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and Brunauere-Emmette-Teller (BET) method. After calcined at a low temperature of 550 degrees C for 6 hours, the single phase of perovskite LaFeO3 microfibers is formed and the grain size increases from 27 to 38 nm with the calcination temperature increasing from 500 to 650 degrees C. The catalytic activity for soot combustion was analyzed by thermo-gravimetric method (TG), and the LaFeO3 microfibers calcined at 600 degrees C exhibits the highest catalytic activity for soot combustion, with a lowest T50 (393 degrees C) and T90 (434 degrees C). The formation mechanism of the honeycomb-like structure is analyzed and these honeycomb-like microfibers can be used as advanced catalysts, absorbents, filters and microreactors.