MOF‐Derived Metal Oxide Composites for Advanced Electrochemical Energy Storage

Over the past two decades, metal–organic frameworks (MOFs), a type of porous material, have aroused great interest as precursors or templates for the derivation of metal oxides and composites for the next generation of electrochemical energy storage applications owing to their high specific surface areas, controllable structures, and adjustable pore sizes. The electrode materials, which affect the performance in practical applications, are pivotal components of batteries and supercapacitors. Metal oxide composites derived from metal–organic frameworks possessing high reversible capacity and superior rate and cycle performance are excellent electrode materials. In this Review, potential applications for MOF‐derived metal oxide composites for lithium‐ion batteries, sodium‐ion batteries, lithium–oxygen batteries, and supercapacitors are studied and summarized. Finally, the challenges and opportunities for future research on MOF‐derived metal oxide composites are proposed on the basis of academic knowledge from the reported literature as well as from experimental experience.     

Metal–Organic Framework‐Derived Materials for Sodium Energy Storage

Recently, sodium‐ion batteries (SIBs) are extensively explored and are regarded as one of the most promising alternatives to lithium‐ion batteries for electrochemical energy conversion and storage, owing to the abundant raw material resources, low cost, and similar electrochemical behavior of elemental sodium compared to lithium. Metal–organic frameworks (MOFs) have attracted enormous attention due to their high surface areas, tunable structures, and diverse applications in drug delivery, gas storage, and catalysis. Recently, there has been an escalating interest in exploiting MOF‐derived materials as anodes for sodium energy storage due to their fast mass transport resulting from their highly porous structures and relatively simple preparation methods originating from in situ thermal treatment processes. In this Review, the recent progress of the sodium‐ion storage performances of MOF‐derived materials, including MOF‐derived porous carbons, metal oxides, metal oxide/carbon nanocomposites, and other materials (e.g., metal phosphides, metal sulfides, and metal selenides), as SIB anodes is systematically and completely presented and discussed. Moreover, the current challenges and perspectives of MOF‐derived materials in electrochemical energy storage are discussed.     

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.     

Thermally Stable Metal‐Organic Framework‐Templated Synthesis of Hierarchically Porous Metal Sulfides: Enhanced Photocatalytic Hydrogen Production

Porous nanostructured materials are demonstrated to be very promising in catalysis due to their well accessible active sites. Thermally stable metal‐organic frameworks (MOFs) as hard templates are successfully utilized to afford porous metal oxides and subsequently metal sulfides by a nanocasting method. The resultant metal oxides/sulfides show considerable Brunauer–Emmett–Teller (BET) surface areas, by partially inheriting the pore character of MOF templates. Preliminary investigation on the obtained hierarchically porous CdS for water splitting, as a proof of concept, demonstrates its much higher activity than both corresponding bulk and nanosized counterparts, under visible light irradiation. Given the structural diversity and tailorability of MOFs, such synthetic approach may open an avenue to the synthesis of advanced porous materials for functional applications.     

Self‐Template‐Directed Metal–Organic Frameworks Network and the Derived Honeycomb‐Like Carbon Flakes via Confinement Pyrolysis

Metal–organic frameworks (MOFs) have become a research hotspot since they have been explored as convenient precursors for preparing various multifunctional nanomaterials. However, the preparation of MOF networks with controllable flake morphology in large scale is not realized yet. Herein, a self‐template strategy is developed to prepare MOF networks. In this work, layered double‐metal hydroxide (LDH) and other layered metal hydroxides are used not only as a scaffold but also as a self‐sacrificed metal source. After capturing the abundant metal cations identically from the LDH by the organic linkers, MOF networks are in situ formed. It is interesting that the MOF network‐derived carbon materials retain the flake morphology and exhibit a unique honeycomb‐like macroporous structure due to the confined shrinkage of the polyhedral facets. The overall properties of the carbon networks are adjustable according to the tailored metal compositions in LDH and the derived MOFs, which are desirable for target‐oriented applications as exemplified by the electrochemical application in supercapacitors.     

State‐of‐the‐Art Advances and Challenges of Iron‐Based Metal Organic Frameworks from Attractive Features,Synthesis to Multifunctional Applications

Metal organic frameworks (MOFs), as an original kind of organic–inorganic porous material, are constructed with metal centers and organic linkers via a coordination complexation reaction. Among uncountable MOF materials, iron‐containing metal organic frameworks (Fe‐MOFs) have excellent potential in practical applications owing to their many fascinating properties, such as diverse structure types, low toxicity, preferable stability, and tailored functionality. Here, recent research progresses of Fe‐MOFs in attractive features, synthesis, and multifunctional applications are described. Fe‐MOFs with porosity and tailored functionality are discussed according to the design of building blocks. Four types of synthetic methods including solvothermal, hydrothermal, microwave, and dry gel conversion synthesis are illustrated. Finally, the applications of Fe‐MOFs in Li‐ion batteries, sensors, gas storage, separation in gas and liquid phases, and catalysis are elucidated, focusing on the mechanism. The aim is to provide prospects for extending Fe‐MOFs in more practical applications.     

Porous Pt‐Ni Nanowires within In Situ Generated Metal‐Organic Frameworks for Highly Chemoselective Cinnamaldehyde Hydrogenation

Although chemoselective hydrogenation of unsaturated aldehydes is the major route to highly valuable industrially demanded unsaturated alcohols, it is still challenging, as the production of saturated aldehydes is more favorable over unsaturated alcohols from the view of thermodynamics. By combining the structural features of porous nanowires (NWs) and metal‐organic frameworks (MOFs), a unique class of porous Pt‐Ni NWs in situ encapsuled by MOFs (Pt‐Ni NWs@Ni/Fex‐MOFs) is designed to enhance the unsaturated alcohols selectivity in the cinnamaldehyde (CAL) hydrogenation. A detailed catalytic study shows that the porous Pt‐Ni NWs@Ni/Fe_x‐MOFs exhibit volcano‐type activity and selectivity in CAL hydrogenation as a function of Fe content. The optimized porous PtNi_2.20 NWs@Ni/Fe₄‐MOF is highly active and selective with 99.5% CAL conversion and 83.3% cinnamyl alcohol selectivity due to the confinement effect, appropriate thickness of MOF and its optimized electronic structure, and excellent durability with negligible activity and selectivity loss after five runs.     

Bimetallic Metal‐Organic Frameworks: Probing the Lewis Acid Site for CO2 Conversion

A highly porous metal‐organic framework (MOF) incorporating two kinds of second building units (SBUs), i.e., dimeric paddlewheel (Zn₂(COO)₄) and tetrameric (Zn₄(O)(CO₂)₆), is successfully assembled by the reaction of a tricarboxylate ligand with Zn^II ion. Subsequently, single‐crystal‐to‐single‐crystal metal cation exchange using the constructed MOF is investigated, and the results show that Cu^II and Co^II ions can selectively be introduced into the MOF without compromising the crystallinity of the pristine framework. This metal cation‐exchangeable MOF provides a useful platform for studying the metal effect on both gas adsorption and catalytic activity of the resulted MOFs. While the gas adsorption experiments reveal that Cu^II and Co^II exchanged samples exhibit comparable CO₂ adsorption capability to the pristine Zn^II‐based MOF under the same conditions, catalytic investigations for the cycloaddition reaction of CO₂ with epoxides into related carbonates demonstrate that Zn^II‐based MOF affords the highest catalytic activity as compared with Cu^II and Co^II exchanged ones. Molecular dynamic simulations are carried out to further confirm the catalytic performance of these constructed MOFs on chemical fixation of CO₂ to carbonates. This research sheds light on how metal exchange can influence intrinsic properties of MOFs.     

Thermal Instability Induced Oriented 2D Pores for Enhanced Sodium Storage

Hierarchical porous structures are highly desired for various applications. However, it is still challenging to obtain such materials with tunable architectures. Here, this paper reports hierarchical nanomaterials with oriented 2D pores by taking advantages of thermally instable bonds in vanadium‐based metal–organic frameworks (MOFs). High‐temperature calcination of these MOFs accompanied by the loss of coordinated water molecules and other components enables the formation of orderly slit‐like 2D pores in vanadium oxide/porous carbon nanorods (VO_x/PCs). This unique combination leads to an increase of the reactive surface area. In addition, optimized VO_x/PCs demonstrate high‐rate capability and ultralong cycling life for sodium storage. The assembled full cells also show high capacity and cycling stability. This report provides an effective strategy for producing MOFs‐derived composites with hierarchical porous architectures for energy storage.     

Metal–Organic‐Framework‐Based Catalysts for Photoreduction of CO2

Photoreduction of CO₂ into reusable carbon forms is considered as a promising approach to address the crisis of energy from fossil fuels and reduce excessive CO₂ emission. Recently, metal–organic frameworks (MOFs) have attracted much attention as CO₂ photoreduction‐related catalysts, owing to their unique electronic band structures, excellent CO₂ adsorption capacities, and tailorable light‐absorption abilities. Recent advances on the design, synthesis, and CO₂ reduction applications of MOF‐based photocatalysts are discussed here, beginning with the introduction of the characteristics of high‐efficiency photocatalysts and structural advantages of MOFs. The roles of MOFs in CO₂ photoreduction systems as photocatalysts, photocatalytic hosts, and cocatalysts are analyzed. Detailed discussions focus on two constituents of pure MOFs (metal clusters such as Ti–O, Zr–O, and Fe–O clusters and functional organic linkers such as amino‐modified, photosensitizer‐functionalized, and electron‐rich conjugated linkers) and three types of MOF‐based composites (metal–MOF, semiconductor–MOF, and photosensitizer–MOF composites). The constituents, CO₂ adsorption capacities, absorption edges, and photocatalytic activities of these photocatalysts are highlighted to provide fundamental guidance to rational design of efficient MOF‐based photocatalyst materials for CO₂ reduction. A perspective of future research directions, critical challenges to be met, and potential solutions in this research field concludes the discussion.     

Ternary Alloys Encapsulated within Different MOFs via a Self‐Sacrificing Template Process: A Potential Platform for the Investigation of Size‐Selective Catalytic Performances

Functional nanoparticles encapsulated within metal–organic frameworks (MOFs) as an emerging class of composite materials attract increasing attention owing to their enhanced or even novel properties caused by the synergistic effect between the two functional materials. However, there is still no ideal composite structure as platform to systematically analyze and evaluate the relation between the enhanced catalytic performance of composites and the structure of MOF shells. In this work, taking RhCoNi ternary alloy nanoflowers, for example, first the RhCoNi@MOF composite catalysts sheathed with different structured MOFs via a facile self‐sacrificing template process are successfully fabricated. The structure type of MOF shells is easily adjustable by using different organic molecules as etchant and coordination reagent (e.g., 2,5‐dihydroxyterephthalic acid or 2‐methylimidazole), which can dissolve out the Co or Ni element in the alloy template in a targeted manner, thereby producing ZIF‐67(Co) or MOF‐74(Ni) shells accordingly. With the difference between the two MOF shells in the aperture sizes, the as‐prepared two RhCoNi@MOF composites preform distinct size selectivity during the alkene hydrogenation. This work would help us to get more comprehensive understanding of the intrinsic role of MOFs behind the enhanced catalytic performance of nanoparticle@MOF composites.     

Highly Adsorptive,MOF‐Functionalized Nonwoven Fiber Mats for Hazardous Gas Capture Enabled by Atomic Layer Deposition

While metal‐organic frameworks (MOFs) show great potential for gas adsorption and storage, their powder form limits deployment opportunities. Integration of MOFs on polymeric fibrous scaffolds will enable new applications in gas adsorption, membrane separation, catalysis, and toxic gas sensing. Here, we demonstrate a new synthesis route for growing MOFs on fibrous materials that achieves high MOF loadings, large surface areas and high adsorptive capacities. We find that a nanoscale coating of Al₂O₃ formed by atomic layer deposition (ALD) on the surface of nonwoven fiber mats facilitates nucleation of MOFs on the fibers throughout the mat. Functionality of MOFs is fully maintained after integration, and MOF crystals are well attached to the fibers. Breakthrough tests for HKUST‐1 MOFs [Cu₃(BTC)₂] on ALD‐coated polypropylene fibers reveal NH₃ dynamic loadings up to 5.93 ± 0.20 mol/kg_(MOF+fiber). Most importantly, this synthetic approach is generally applicable to a wide range of polymer fibers (e.g., PP, PET, cotton) and MOFs (e.g., HKUST‐1, MOF‐74, and UiO‐66).     

Bottom‐Up Fabrication of Semiconductive Metal–Organic Framework Ultrathin Films

Though generally considered insulating, recent progress on the discovery of conductive porous metal–organic frameworks (MOFs) offers new opportunities for their integration as electroactive components in electronic devices. Compared to classical semiconductors, these metal–organic hybrids combine the crystallinity of inorganic materials with easier chemical functionalization and processability. Still, future development depends on the ability to produce high‐quality films with fine control over their orientation, crystallinity, homogeneity, and thickness. Here self‐assembled monolayer substrate modification and bottom‐up techniques are used to produce preferentially oriented, ultrathin, conductive films of Cu‐CAT‐1. The approach permits to fabricate and study the electrical response of MOF‐based devices incorporating the thinnest MOF film reported thus far (10 nm thick).     

Encapsulation,Visualization and Expression of Genes with Biomimetically Mineralized Zeolitic Imidazolate Framework‐8 (ZIF‐8)

Recent work in biomolecule‐metal–organic framework (MOF) composites has proven to be an effective strategy for the protection of proteins. However, for other biomacromolecules such as nucleic acids, the encapsulation into nano MOFs and the related characterizations are in their infancy. Herein, encapsulation of a complete gene‐set in zeolitic imidazolate framework‐8 (ZIF‐8) MOFs and cellular expression of the gene delivered by the nano MOF composites are reported. Using a green fluorescent protein (GFP) plasmid (plGFP) as a proof‐of‐concept genetic macromolecule, successful transfection of mammalian cancer cells with plGFP for up to 4 days is shown. Cell transfection assays and soft X‐ray cryo‐tomography (cryo‐SXT) demonstrate the feasibility of DNA@MOF biocomposites as intracellular gene delivery vehicles. Expression occurs over relatively prolonged time points where the cargo nucleic acid is released gradually in order to maintain sustained expression.     

In Situ Transformation of MOFs into Layered Double Hydroxide Embedded Metal Sulfides for Improved Electrocatalytic and Supercapacitive Performance

Direct adoption of metal‐organic frameworks (MOFs) as electrode materials shows impoverished electrochemical performance owing to low electrical conductivity and poor chemical stability. In this study, we demonstrate self‐templated pseudomorphic transformation of MOF into surface chemistry rich hollow framework that delivers highly reactive, durable, and universal electrochemically active energy conversion and storage functionalities. In situ pseudomorphic transformation of MOF‐derived hollow rhombic dodecahedron template and sulfurization of nickel cobalt layered double hydroxides (NiCo‐LDHs) lead to the construction of interlayered metal sulfides (NiCo‐LDH/Co₉S₈) system. The embedment of metal sulfide species (Co₉S₈) at the LDH intergalleries offers optimal interfacing of the hybrid constituent elements and materials stability. The hybrid NiCo‐LDH/Co₉S₈ system collectively presents an ideal porous structure, rich redox chemistry, and high electrical conductivity matrix. This leads to a significant enhancement in its complementary electrocatalytic hydrogen evolution and supercapacitive energy storage properties. This work establishes the potential of MOF derived scaffold for designing of novel class hybrid inorganic–organic functional materials for electrochemical applications and beyond.     

Defective Porous Carbon Polyhedra Decorated with Copper Nanoparticles for Enhanced NIR‐Driven Photothermal Cancer Therapy

Currently, there is tremendous interest in the discovery of new and improved photothermal agents for near‐infrared (NIR)‐driven cancer therapy. Herein, a series of novel photothermal agents, comprising copper nanoparticles supported on defective porous carbon polyhedra are successfully prepared by heating a Cu‐BTC metal–organic framework (MOF) precursor at different temperatures (t) in the range 400–900 °C under an argon atmosphere. The copper nanoparticle size and carbon defect concentration in the obtained products (denoted herein as Cu@CPP‐t) increase with synthesis temperature, thus imparting the Cu@CPP‐t samples with distinct NIR absorption properties and photothermal heating responses. The Cu@CPP‐800 sample shows a remarkable photothermal conversion efficiency of 48.5% under 808 nm laser irradiation, representing one of the highest photothermal efficiencies yet reported for a carbon‐based photothermal agent. In vivo experiments conducted with tumor bearing nude Balb/c mice confirm the efficacy of Cu@CPP‐800 as a very promising NIR‐driven phototherapy agent for cancer treatment. Results encourage the wider use of MOFs as low cost precursors for the synthesis of carbon‐supported metal nanoparticle composites for photothermal therapy.     

Engineering Homochiral Metal‐Organic Frameworks for Heterogeneous Asymmetric Catalysis and Enantioselective Separation

Owing to the potential applications in technological areas such as gas storage, catalysis, separation, sensing and nonlinear optics, tremendous efforts have been devoted to the development of porous metal‐organic frameworks (MOFs) over the past ten years. Homochiral porous MOFs are particularly attractive candidates as heterogeneous asymmetric catalysts and enantioselective adsorbents and separators for production of optically active organic compounds due to the lack of homochiral inorganic porous materials such as zeolites. In this review, we summarize the recent research progress in homochiral MOF materials, including their synthetic strategy, distinctive structural features and latest advances in asymmetric heterogeneous catalysis and enantioselective separation.     

Confined Heteropoly Blues in Defected Zr‐MOF (Bottle Around Ship) for High‐Efficiency Oxidative Desulfurization

The Keggin‐type polyoxometalates (POMs) are effective catalysts for oxidative desulfurization (ODS) and confining these POMs in metal–organic frameworks (MOFs) is a promising strategy to improve their performances. Herein, postsynthetic modification of POMs confined in MOFs by adding thiourea creates more unsaturated metal sites as defects, promoting ODS catalytic activity. Additional modification by confining 1‐butyl‐3‐methyl imidazolium POMs in MOFs is performed to obtain higher ODS activity, owing to the affinity between electron‐rich thiophene‐based compounds and electrophilic imidazolium compounds. The ODS catalytic activities of four Zr‐MOF‐based composites (bottle around ship) including phosphomolybdate acid (PMA)/UiO‐66, [Bmim]₃PMo₁₂O₄₀/UiO‐66, PMA/Thiourea/UiO‐66, and [Bmim]₃PMo₁₂O₄₀/Thiourea/UiO‐66 are therefore investigated in detail. In order to explore the catalytic mechanism of these MOF composites, their microstructures and electronic structures are probed by various techniques such as X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared, Raman, scanning electron microscope, transmission electron microscope, BET, X‐ray photoelectron spectroscopy, EPR, UV–vis, NMR spectra, and H₂‐temperature‐programmed reduction. The results reveal that phosphomolybdate blues and imidazolium phosphomolybdate blues with different Mo⁵⁺/Mo⁶⁺ ratios with the Keggin structure are confined in defected UiO‐66 for all four composites. This approach can be applied to design and synthesize other POMs/MOFs composites as efficient catalysts.     

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–N_x 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.     

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.