General Synthesis of Hierarchically Macro/Mesoporous Fe,Ni-Doped CoSe/N-Doped Carbon Nanoshells for Enhanced Electrocatalytic Oxygen Evolution.

Constructing hierarchical porosity and designing rational hybrid composition are effective strategies for enhancing the electrocatalytic performance of hybrid catalysts for electrochemical energy conversion. Here, we develop a multistep "molecule/ion-exchange" strategy toward the synthesis of hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon nanoshells with tunable pore structures and compositions. Polystyrene (PS)@Co-based amorphous coordination polymer (Co-CP) core-shell particles with hierarchically macro/mesoporous nanoshells are first prepared by ligand-molecule-exchange etching of the outer layers in PS@Co-based metal-organic framework precursors. Afterward, a liquid-solid dual-ion-exchange reaction of PS@Co-CP particles with [Fe(CN)6]3- and [Ni(CN)4]2- ions leads to the formation of PS@Co-CP/Co-Fe Prussian blue analogue (PBA)/Co-Ni PBA particles, which are further transformed into hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon particles via a vapor-solid selenization reaction. Moreover, this approach could be extended to synthesize different hierarchically porous core-shell composites with various morphologies and tailored compositions. Because of their unique hierarchically porous nanoarchitecture, these Fe,Ni-doped CoSe/N-doped carbon particles with optimized composition show enhanced performance for electrocatalytic oxygen evolution.

In Search of Excellence: Convex versus Concave Noble Metal Nanostructures for Electrocatalytic Applications.

Controlling the shape of noble metal nanoparticles is a challenging but important task in electrocatalysis. Apart from hollow and nanocage structures, concave noble metal nanoparticles are considered a new class of unconventional electrocatalysts that exhibit superior electrocatalytic properties as compared with those of conventional nanoparticles (including convex and flat ones). Herein, several facile and highly reproducible routes for synthesizing nanostructured concave noble metal materials reported in the literature are discussed, together with their advantages over noble metal nanoparticles with convex shapes. In addition, possible ways of optimizing the synthesis procedure and enhancing the electrocatalytic characteristics of concave metal nanoparticles are suggested. Nanostructured noble metals with concave features are found to show better catalytic activity and stability hence improve their practical applicability in electrocatalysis.

Mesoporous Nanoarchitectures for Electrochemical Energy Conversion and Storage.

Mesoporous materials have attracted considerable attention because of their distinctive properties, including high surface areas, large pore sizes, tunable pore structures, controllable chemical compositions, and abundant forms of composite materials. During the last decade, there has been increasing research interest in constructing advanced mesoporous nanomaterials possessing short and open channels with efficient mass diffusion capability and rich accessible active sites for electrochemical energy conversion and storage. Here, the synthesis, structures, and energy-related applications of mesoporous nanomaterials are the main focus. After a brief summary of synthetic methods of mesoporous nanostructures, the delicate design and construction of mesoporous nanomaterials are described in detail through precise tailoring of the particle sizes, pore sizes, and nanostructures. Afterward, their applications as electrode materials for lithium-ion batteries, supercapacitors, water-splitting electrolyzers, and fuel cells are discussed. Finally, the possible development directions and challenges of mesoporous nanomaterials for electrochemical energy conversion and storage are proposed.

Universal Access to Two-Dimensional Mesoporous Heterostructures by Micelle-Directed Interfacial Assembly.

Two-dimensional (2D) mesoporous heterostructures combining ultrathin nanosheet morphology, periodic porous surface structures, and diverse hybrid compositions have become increasingly important for renewable energy storage and electronics. However, it remains a great challenge to develop a universal method to prepare 2D mesoporous heterostructures. Herein, we report a composite-micelle-directed interfacial assembly method to synthesize heterostructures of an ultrathin 2D material covered with mesoporous monolayers assembled on both sides. To demonstrate the concept, we first fabricated a new sandwichlike carbon@MXene@carbon mesoporous heterostructure through the self-assembly of exfoliated MXene nanosheets and block copolymer F127/melamine-formaldehyde resin composite micelles and subsequent thermal treatment. Finally, we demonstrate that the carbon@MXene@carbon mesoporous heterostructured nanosheets manifest remarkably enhanced electrochemical performance as a cathode material for lithium-sulfur batteries.

General Formation of Macro-/Mesoporous Nanoshells from Interfacial Assembly of Irregular Mesostructured Nanounits.

Mesoporous core-shell nanostructures with controllable ultra-large open channels in their nanoshells are of great interest. However, soft template-directed cooperative assembly to mesoporous nanoshells with highly accessible pores larger than 30 nm, or even above 50 nm into macroporous range, remains a significant challenge. Herein we report a general approach for precisely tailored coating of hierarchically macro-/mesoporous polymer and carbon shells, possessing highly accessible radial channels with extremely wide pore size distribution from ca. 10 nm to ca. 200 nm, on diverse functional materials. This strategy creates opportunities to tailor the interfacial assembly of irregular mesostructured nanounits on core materials and generate various core-shell nanomaterials with controllable pore architectures. The obtained Fe,N-doped macro-/mesoporous carbon nanoshells show enhanced electrochemical performance for the oxygen reduction reaction in alkaline condition.

Metal Atom-Doped Co3 O4 Hierarchical Nanoplates for Electrocatalytic Oxygen Evolution.

Electrocatalysts based on hierarchically structured and heteroatom-doped non-noble metal oxide materials are of great importance for efficient and low-cost electrochemical water splitting systems. Herein, the synthesis of a series of hierarchical hollow nanoplates (NPs) composed of ultrathin Co3 O4 nanosheets doped with 13 different metal atoms is reported. The synthesis involves a cooperative etching-coordination-reorganization approach starting from zeolitic imidazolate framework-67 (ZIF-67) NPs. First, metal atom decorated ZIF-67 NPs with unique cross-channels are formed through a Lewis acid etching and metal species coordination process. Afterward, the composite NPs are converted to hollow Co3 O4 hierarchical NPs composed of ultrathin nanosheets through a solvothermal reaction, during which the guest metal species is doped into the octahedral sites of Co3 O4 . Density functional theory calculations suggest that doping of small amount of Fe atoms near the surface of Co3 O4 can greatly enhance the electrocatalytic activity toward the oxygen evolution reaction (OER). Benefiting from the structural and compositional advantages, the obtained Fe-doped Co3 O4 hierarchical NPs manifest superior electrocatalytic performance for OER with an overpotential of 262 mV at 10 mA cm-2 , a Tafel slope of 43 mV dec-1 , and excellent stability even at a high current density of 100 mA cm-2 for 50 h.

Confining Sub-Nanometer Pt Clusters in Hollow Mesoporous Carbon Spheres for Boosting Hydrogen Evolution Activity.

Electrochemical water splitting is considered as a promising approach to produce clean and sustainable hydrogen fuel. As a new class of nanomaterials with high ratio of surface atoms and tunable composition and electronic structure, metal clusters are promising candidates as catalysts. Here, a new strategy is demonstrated to synthesize active and stable Pt-based electrocatalysts for hydrogen evolution by confining Pt clusters in hollow mesoporous carbon spheres (Pt5 /HMCS). Such a structure would effectively stabilize the Pt clusters during the ligand removal process, leading to remarkable electrocatalytic performance for hydrogen production in both acidic and alkaline solutions. Particularly, the optimal Pt5 /HMCS electrocatalyst exhibits 12 times the mass activity of Pt in commercial Pt/C catalyst with similar Pt loading. This study exemplifies a simple yet effective approach to improve the cost effectiveness of precious-metal-based catalysts with stabilized metal clusters.

Synthesis of CuS@CoS2 Double-Shelled Nanoboxes with Enhanced Sodium Storage Properties.

Metal sulfides have received considerable attention for efficient sodium storage owing to their high capacity and decent redox reversibility. However, the poor rate capability and fast capacity decay greatly hinder their practical application in sodium-ion batteries. Herein, an elegant multi-step templating strategy has been developed to rationally synthesize hierarchical double-shelled nanoboxes with the CoS2 nanosheet-constructed outer shell supported on the CuS inner shell. Their structure and composition enable these hierarchical CuS@CoS2 nanoboxes to show boosted electrochemical properties with high capacity, outstanding rate capability, and long cycle life.

Co-Fe Alloy/N-Doped Carbon Hollow Spheres Derived from Dual Metal-Organic Frameworks for Enhanced Electrocatalytic Oxygen Reduction.

Metal-organic framework (MOF) composites have recently been considered as promising precursors to derive advanced metal/carbon-based materials for various energy-related applications. Here, a dual-MOF-assisted pyrolysis approach is developed to synthesize Co-Fe alloy@N-doped carbon hollow spheres. Novel core-shell architectures consisting of polystyrene cores and Co-based MOF composite shells encapsulated with discrete Fe-based MOF nanocrystallites are first synthesized, followed by a thermal treatment to prepare hollow composite materials composed of Co-Fe alloy nanoparticles homogeneously distributed in porous N-doped carbon nanoshells. Benefitting from the unique structure and composition, the as-derived Co-Fe alloy@N-doped carbon hollow spheres exhibit enhanced electrocatalytic performance for oxygen reduction reaction. The present approach expands the toolbox for design and preparation of advanced MOF-derived functional materials for diverse applications.

Metal-Organic Framework-Assisted Synthesis of Compact Fe2O3 Nanotubes in Co3O4 Host with Enhanced Lithium Storage Properties.

Transition metal oxides are promising candidates for the high-capacity anode material in lithium-ion batteries. The electrochemical performance of transition metal oxides can be improved by constructing suitable composite architectures. Herein, we demonstrate a metal-organic framework (MOF)-assisted strategy for the synthesis of a hierarchical hybrid nanostructure composed of Fe2O3 nanotubes assembled in Co3O4 host. Starting from MOF composite precursors (Fe-based MOF encapsulated in a Co-based host matrix), a complex structure of Co3O4 host and engulfed Fe2O3 nanotubes was prepared by a simple annealing treatment in air. By virtue of their structural and compositional features, these hierarchical composite particles reveal enhanced lithium storage properties when employed as anodes for lithium-ion batteries.

Formation of Hierarchical Co9S8@ZnIn2S4 Heterostructured Cages as an Efficient Photocatalyst for Hydrogen Evolution.

Here we demonstrate the delicate design and construction of hierarchical Co9S8@ZnIn2S4 heterostructured cages as an efficient photocatalyst for hydrogen evolution with visible light. Two photoactive sulfide semiconductors are rationally integrated into a hierarchical hollow structure with strongly coupled heterogeneous shells and two-dimensional ultrathin subunits. The unique architecture can efficiently facilitate the separation and transfer of light-induced charges, offer large surface area, and expose rich active sites for photocatalytic redox reactions. Owing to the distinctive structural and compositional benefits, the hierarchical Co9S8@ZnIn2S4 hollow heterostructures without using any cocatalysts show remarkable activity with a hydrogen-producing rate of 6250 µmol h-1 g-1 and high stability for photocatalytic water splitting.

Metal-Organic Framework Hybrid-Assisted Formation of Co3 O4 /Co-Fe Oxide Double-Shelled Nanoboxes for Enhanced Oxygen Evolution.

Rational design of complex metal-organic framework (MOF) hybrid precursors offers a great opportunity to construct various functional nanostructures. Here, a novel MOF-hybrid-assisted strategy to synthesize Co3 O4 /Co-Fe oxide double-shelled nanoboxes is reported. In the first step, zeolitic imidazolate framework-67 (ZIF-67, a Co-based MOF)/Co-Fe Prussian blue analogue (PBA) yolk-shell nanocubes are formed via a facile anion-exchange reaction between ZIF-67 nanocube precursors and [Fe(CN)6 ]3- ions at room temperature. Subsequently, an annealing treatment is applied to prepare Co3 O4 /Co-Fe oxide double-shelled nanoboxes. Owing to the structural and compositional benefits, the as-derived Co3 O4 /Co-Fe oxide double-shelled nanoboxes exhibit enhanced electrocatalytic performance for oxygen evolution reaction in alkaline solution.

Construction of ZnIn2S4-In2O3 Hierarchical Tubular Heterostructures for Efficient CO2 Photoreduction.

We demonstrate the rational design and construction of sandwich-like ZnIn2S4-In2O3 hierarchical tubular heterostructures by growing ZnIn2S4 nanosheets on both inner and outer surfaces of In2O3 microtubes as photocatalysts for efficient CO2 photoreduction. The unique design integrates In2O3 and ZnIn2S4 into hierarchical one-dimensional (1D) open architectures with double-heterojunction shells and ultrathin two-dimensional (2D) nanosheet subunits. This design accelerates the separation and transfer of photogenerated charges, offers large surface area for CO2 adsorption, and exposes abundant active sites for surface catalysis. Benefiting from the structural and compositional merits, the optimized ZnIn2S4-In2O3 photocatalyst exhibits outstanding performance for reductive CO2 deoxygenation with considerable CO generation rate (3075 µmol h-1 g-1) and high stability.

Realization of Walnut-Shaped Particles with Macro-/Mesoporous Open Channels through Pore Architecture Manipulation and Their Use in Electrocatalytic Oxygen Reduction.

Herein we report a simple dual-soft-template approach to prepare walnut-shaped macro-/mesoporous polydopamine particles with diameter of ca. 270 nm, highly accessible bicontinuous channels and wide pore size distribution from ca. 20 nm to ca. 95 nm. This approach provides great opportunities to tailor the soft template-directed assembly processes and generate various polydopamine particles with controllable mesophase curvature. Walnut-shaped mesoporous carbon particles with large open mesochannels in the range of ca. 13 nm to ca. 50 nm can be fabricated by subsequent thermal treatment under nitrogen atmosphere. Lastly, we demonstrate that the as-derived walnut-shaped carbon particles manifest enhanced electrocatalytic performance for oxygen reduction reaction in alkaline electrolyte.

Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction.

Constructing atomically dispersed platinum (Pt) electrocatalysts is essential to build high-performance and cost-effective electrochemical water-splitting systems. We present a novel strategy to realize the traction and stabilization of isolated Pt atoms in the nitrogen-containing porous carbon matrix (Pt@PCM). In comparison with the commercial Pt/C catalyst (20 weight %), the as-prepared Pt@PCM catalyst exhibits significantly boosted mass activity (up to 25 times) for hydrogen evolution reaction. Results of extended x-ray absorption fine structure investigation and density functional theory calculation suggest that the active sites are associated with the lattice-confined Pt centers and the activated carbon (C)/nitrogen (N) atoms at the adjacency of the isolated Pt centers. This strategy may provide insights into constructing highly efficient single-atom catalysts for different energy-related applications.

Hierarchical Hollow Nanoprisms Based on Ultrathin Ni-Fe Layered Double Hydroxide Nanosheets with Enhanced Electrocatalytic Activity towards Oxygen Evolution.

The oxygen evolution reaction (OER) is involved in various renewable energy systems, such as water-splitting cells and metal-air batteries. Ni-Fe layered double hydroxides (LDHs) have been reported as promising OER electrocatalysts in alkaline electrolytes. The rational design of advanced nanostructures for Ni-Fe LDHs is highly desirable to optimize their electrocatalytic performance. Herein, we report a facile self-templated strategy for the synthesis of novel hierarchical hollow nanoprisms composed of ultrathin Ni-Fe LDH nanosheets. Tetragonal nanoprisms of nickel precursors were first synthesized as the self-sacrificing template. Afterwards, these Ni precursors were consumed during the hydrolysis of iron(II) sulfate for the simultaneous growth of a layer of Ni-Fe LDH nanosheets on the surface. The resultant Ni-Fe LDH hollow prisms with large surface areas manifest high electrocatalytic activity towards the OER with low overpotential, small Tafel slope, and remarkable stability.

Formation of Hierarchical In2S3-CdIn2S4 Heterostructured Nanotubes for Efficient and Stable Visible Light CO2 Reduction.

We demonstrate rational design and fabrication of hierarchical In2S3-CdIn2S4 heterostructured nanotubes as efficient and stable photocatalysts for visible light CO2 reduction. The novel self-templated strategy, including sequential anion- and cation-exchange reactions, integrates two distinct sulfide semiconductors into hierarchical tubular hybrids with homogeneous interfacial contacts and ultrathin two-dimensional (2D) nanosheet subunits. Accordingly, the hierarchical heterostructured nanotubes facilitate separation and migration of photoinduced charge carriers, enhance the adsorption and concentration of CO2 molecules, and offer rich active sites for surface redox reactions. Benefiting from these structural and compositional features, the optimized hierarchical In2S3-CdIn2S4 nanotubes without employing noble metal cocatalysts in the catalytic system manifest remarkable performance for deoxygenative reduction of CO2 with high CO generation rate (825 µmol h-1 g-1) and outstanding stability under visible light irradiation.

Formation of Single-Holed Cobalt/N-Doped Carbon Hollow Particles with Enhanced Electrocatalytic Activity toward Oxygen Reduction Reaction in Alkaline Media.

Design and construction of metal-organic framework (MOF) composite precursors have recently been considered as a promising strategy for the preparation of different structured metal/carbon-based functional materials. Here, an MOF composite-assisted strategy to synthesize single-holed cobalt/N-doped carbon hollow particles is reported. The yolk-shell polystyrene@zeolitic imidazolate framework-67 (PS@ZIF-67) composite precursors are first synthesized, followed by a controlled pyrolysis to obtain cobalt/N-doped carbon hollow particles with a large single hole on each shell. Moreover, the MOF-coating approach reported in this work can be extended to prepare various core-shell ZIF-67 composites with different structures and compositions. Benefiting from the structural and compositional advantages, the as-derived single-holed cobalt/N-doped carbon hollow particles manifest superior electrocatalytic oxygen reduction performance with high activity and excellent durability.

Mesoporous Carbon@Titanium Nitride Hollow Spheres as an Efficient SeS2 Host for Advanced Li-SeS2 Batteries.

The introduction of a certain proportion of selenium into sulfur-based cathodes is an effective strategy for enhancing the integrated battery performance. However, similar to sulfur, selenium sulfide cathodes suffer from poor cycling stability owing to the dissolution of reaction intermediate products. In this study, to exploit the advantages of SeS2 to the full and avoid its shortcomings, we designed and synthesized a hollow mesoporous carbon@titanium nitride (HMC@TiN) host for loading 70 wt % of SeS2 as a cathode material for Li-SeS2 batteries. Benefiting from both physical and chemical entrapment by hollow mesoporous carbon and TiN, the HMC@TiN/SeS2 cathode manifests high utilization of the active material and excellent cycling stability. Moreover, it exhibits promising areal capacity (up to 4 mAh cm-2 ) with stable cell performance in the high-mass-loading electrode.

Complex Nanostructures from Materials based on Metal-Organic Frameworks for Electrochemical Energy Storage and Conversion.

Metal-organic frameworks (MOFs) have drawn tremendous attention because of their abundant diversity in structure and composition. Recently, there has been growing research interest in deriving advanced nanomaterials with complex architectures and tailored chemical compositions from MOF-based precursors for electrochemical energy storage and conversion. Here, a comprehensive overview of the synthesis and energy-related applications of complex nanostructures derived from MOF-based precursors is provided. After a brief summary of synthetic methods of MOF-based templates and their conversion to desirable nanostructures, delicate designs and preparation of complex architectures from MOFs or their composites are described in detail, including porous structures, single-shelled hollow structures, and multishelled hollow structures, as well as other unusual complex structures. Afterward, their applications are discussed as electrode materials or catalysts for lithium-ion batteries, hybrid supercapacitors, water-splitting devices, and fuel cells. Lastly, the research challenges and possible development directions of complex nanostructures derived from MOF-based-templates for electrochemical energy storage and conversion applications are outlined.