Synthesis of Bimetallic Conductive 2D Metal-Organic Framework (Cox Niy -CAT) and Its Mass Production: Enhanced Electrochemical Oxygen Reduction Activity

MOF-on-MOF Growth: Inducing Naturally Nonpreferred MOFs and Atypical MOF Growth

ConspectusOverflowing metal-organic frameworks (MOFs) have been synthesized from a wide range of metal and organic components for specific purposes and intellectual curiosity. Each MOF has unique chemical and structural characteristics directed by the incorporated components, metal ions (or clusters), organic linkers, and their intrinsic coordination interactions. These incorporated components and structural characteristics are two pivotal factors influencing MOFs' fundamental properties and subsequent applications. Therefore, selecting the appropriate metal and organic components, considering their innate chemical and structural properties, is crucial to endow the final MOFs with the desired properties. Ultimately, producing MOFs with a desired structure using ideal components is the best approach to achieving the best MOFs tailored for specific purposes with desired properties. However, achieving MOFs with the intended structure from chosen components remains underdeveloped. In many cases, the resulting MOF structure is governed by the thermodynamically and/or kinetically preferred configuration (refers to a naturally preferred structure) of the chosen components and given reaction conditions. Additionally, producing hybrid MOFs with complex components, structures, and morphologies presents a great opportunity to obtain special MOFs with advanced properties and functions. In this Account, we outline our group's efforts over the past few years to develop naturally nonpreferred MOFs through the induced MOF-on-MOF growth process and atypical hybrid MOFs via nonstandard MOF-on-MOF growth. First, we highlight the prime strategy for producing naturally nonpreferred MOFs based on template-induced MOF-on-MOF growth. In this section, we discuss the two basic growth behaviors, isotropic and anisotropic growth of naturally nonpreferred MOFs, determined by the degree of matching between the cell lattices of the two MOFs. Second, we introduce the MOF farming concept for the productive cultivation and effective harvesting of naturally nonpreferred MOFs made by MOF-on-MOF growth. Here we discuss the importance of selecting the ideal MOF template for productive growth and developing an efficient method for harvesting cultivated MOFs. Next, we describe atypical anisotropic MOF-on-MOF growths between two MOFs with mismatched cell lattices. In this section, we introduce tip-to-middle MOF-on-MOF growth involving self-structural adjustment of the secondary MOF, logical inference of unidentified MOF structures based on MOF-on-MOF growth behavior and morphological features, and MOF-on-MOF growth accompanied by etching and transformation of the template. Finally, we discuss the perspectives and challenges of MOF-on-MOF growth and the synthesis of naturally nonpreferred MOFs. We hope that this Account offers valuable insights into the rational design and development of MOFs with desired structural and compositional characteristics, leading to the creation of ideal MOFs.

Hydrogen Bonds Induced Ultralong Stability of Conductive π-d Conjugated FeCo3(DDA)2 with High OER Activity

Conductive π-d conjugated metal-organic frameworks (MOFs) have attracted wide concerns in electrocatalysis due to their intrinsic high conductivity. However, the poor electrocatalytic stability is still a major problem that hinders the practical application of MOFs. Herein, a novel approach to enhancing the stability of MOF-based electrocatalyst, namely, the introduction of hydrogen bonds (H-bonds), is reported. Impressively, the π-d conjugated MOF FeCo3(DDA)2 (DDA = 1,5-diamino-4,8-dihydroxy-9,10-anthraceneedione) exhibits ultrahigh oxygen evolution reaction (OER) stability (up to 2000 h). The experimental studies demonstrate that the presence of H-bonds in FeCo3(DDA)2 is responsible for its ultrahigh OER stability. Besides that, FeCo3(DDA)2 also displays a prominent OER activity (an overpotential of 260 mV vs reversible hydrogen electrode (RHE) at a current density of 10 mA cm-2 and a Tafel slope of 46.86 mV dec-1). Density functional theory (DFT) calculations further indicate that the synergistic effect of the Fe and Co sites in FeCo3(DDA)2 contributes to its prominent OER performance. This work provides a new avenue of boosting the electrocatalytic stability of conductive π-d conjugated MOFs.

A Conductive 3D Dual-Metal π-d Conjugated Metal-Organic Framework Fe3(HITP)2/bpm@Co for Highly Efficient Oxygen Evolution Reaction

Although 2D π-d conjugated metal-organic frameworks (MOFs) exhibit high in-plane conductivity, the closely stacked layers result in low specific surface area and difficulty in mass transfer and diffusion. Hence, a conductive 3D MOF Fe3(HITP)2/bpm@Co (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) is reported through inserting bpm (4,4'-bipyrimidine) ligands and Co2+ into the interlayers of 2D MOF Fe3(HITP)2. Compared to 2D Fe3(HITP)2 (37.23 m2 g-1), 3D Fe3(HITP)2/bpm@Co displays a huge improvement in the specific surface area (373.82 m2 g-1). Furthermore, the combined experimental and density functional theory (DFT) theoretical calculations demonstrate the metallic behavior of Fe3(HITP)2/bpm@Co, which will benefit to the electrocatalytic activity of it. Impressively, Fe3(HITP)2/bpm@Co exhibits prominent and stable oxygen evolution reaction (OER) performance (an overpotential of 299 mV vs RHE at a current density of 10 mA cm-2 and a Tafel slope of 37.14 mV dec-1), which is superior to 2D Fe3(HITP)2 and comparable to commercial IrO2. DFT theoretical calculation reveals that the combined action of the Fe and Co sites in Fe3(HITP)2/bpm@Co is responsible for the enhanced electrocatalytic activity. This work provides an alternative approach to develop conductive 3D MOFs as efficient electrocatalysts.

Strategies to Improve Electrical Conductivity in Metal-Organic Frameworks: A Comparative Study

Metal-organic frameworks (MOFs), formed by the combination of both inorganic and organic components, have attracted special attention for their tunable porous structures, chemical and functional diversities, and enormous applications in gas storage, catalysis, sensing, etc. Recently, electronic applications of MOFs like electrocatalysis, supercapacitors, batteries, electrochemical sensing, etc., have become a major research topic in MOF chemistry. However, the low electrical conductivity of most MOFs represents a major handicap in the development of these emerging applications. To overcome these limitations, different strategies have been developed to enhance electrical conductivity of MOFs for their implementation in electronic devices. In this review, we outline all these strategies employed to increase the electronic conduction in both intrinsically (framework-modulated) and extrinsically (guests-modulated) conducting MOFs.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Synthesis of Electrical Conductive Metal-Organic Frameworks for Electrochemical Applications

Electrical conductivity is very important property of nanomaterials for using wide range of applications especially energy applications. Metal-organic frameworks (MOFs) are notorious for their low electrical conductivity and less considered for usage in pristine forms. However, the advantages of high surface area, porosity and confined catalytic active sites motivated researchers to improve the conductivity of MOFs. Therefore, 2D electrical conductive MOFs (ECMOF) have been widely synthesized by developing the effective synthetic strategies. In this article, we have summarized the recent trends in developing the 2D ECMOFs, following the summary of potential applications in the various fields with future perspectives.

Electrosynthesis of Hydrogen Peroxide Enabled by Exceptional Molecular Ni Sites in a Graphene-Supported Nickel Organic Framework

Electrosynthesis of hydrogen peroxide (H2O2) from 2e- transfer of the oxygen reduction reaction (2e--ORR) is a potential alternative to the traditional anthraquinone process. Two-dimensional (2D) metal-organic frameworks (MOFs) supported by carbon are frequently reported as promising 2e--ORR catalysts. Herein, a graphene-supported 2D MOF of Ni3(2,3,6,7,10,11-hexahydrotriphenylene)2 is synthesized through a common hydrothermal method, which exhibits high 2e--ORR performance. It is discovered that except for emerging MOFs, exceptional molecularly dispersed Ni sites coexist in the synthesis that have the same coordination sphere of the NiO4C4 moiety as the MOF. The molecular Ni sites are more catalytically active. The graphene support contains a suitable amount of residual oxygen groups, leading to the generation of those molecularly dispersed Ni sites. The oxygen groups exhibit a moderate electron-withdrawing effect at the outer sphere of Ni sites to slightly increase their oxidation state. This interaction decreases overpotentials and kinetically improves the selectivity of the 2e- reaction pathway.

Boosted ability of ZIF-8 for early-stage adsorption and degradation of chemical warfare agent simulants

Efficient adsorption of hazardous substances from the environment is crucial owing to the considerable risks they pose to both humans and ecosystems. Consequently, the development of porous materials with strong adsorption capabilities for hazardous substances, such as chemical warfare agents (CWAs), is pivotal for safeguarding human lives. Specifically, the early-stage adsorption proficiency of the adsorbents plays a vital role in determining their effectiveness as ideal adsorbents. Herein, we report the efficient adsorption of CWA simulants using thermally treated ZIF-8 (T-ZIF-8). The T-ZIF-8 samples were prepared by subjecting ZIF-8 to a simple thermal treatment, which resulted in a more positive surface charge with extra open metal sites. Although the pore volume of T-ZIF-8 decreased after thermal treatment, the positive surface charge of T-ZIF-8 proved advantageous for the adsorption of the CWA simulants. As a result, the adsorption capacity of T-ZIF-8 for the CWA simulants improved compared to that of pure ZIF-8. Notably, T-ZIF-8 exhibited a remarkably enhanced adsorption ability in the early stage of exposure to the CWA simulants, possibly due to the effective polar interactions between T-ZIF-8 and the simulants via the electron-rich components within the CWA simulants. Moreover, the enhanced adsorption capacity of T-ZIF-8 led to the fast degradation of simulant compared to pure ZIF-8. T-ZIF-8 also demonstrated excellent stability over three adsorption cycles. These findings highlight that T-ZIF-8 is an outstanding material for the early-stage adsorption and degradation of CWA simulants, offering high effectiveness and stability.

Two-Dimensional Electrically Conductive Metal-Organic Frameworks as Chemiresistive Sensors

Metal-organic frameworks (MOFs) have emerged as attractive chemical sensing materials due to their exceptionally high porosity and chemical diversity. Nevertheless, the utilization of MOFs in chemiresistive type sensors has been hindered by their inherent limitation in electrical conductivity. The recent emergence of two-dimensional conductive MOFs (2D c-MOFs) has addressed this limitation by offering enhanced electrical conductivity, while still retaining the advantageous properties of MOFs. In particular, c-MOFs have shown promising advantages for the fabrication of sensors capable of operating at room temperature. Thus, active research on gas sensors utilizing c-MOFs is currently underway, focusing on enhancing sensitivity and selectivity. To comprehend the potential of MOFs as chemiresistive sensors for future applications, it is crucial to understand not only the fundamental properties of conductive MOFs but also the state-of-the-art works that contribute to improving their performance. This comprehensive review delves into the distinctive characteristics of 2D c-MOFs as a new class of chemiresistors, providing in-depth insights into their unique sensing properties. Furthermore, we discuss the proposed sensing mechanisms associated with 2D c-MOFs and provide a concise summary of the strategies employed to enhance the sensing performance of 2D c-MOFs. These strategies encompass a range of approaches, including the design of metal nodes and linkers, morphology control, and the synergistic use of composite materials. In addition, the review thoroughly explores the prospects of 2D c-MOFs as chemiresistors and elucidates their remarkable potential for further advancements. The insights presented in this review shed light on future directions and offer valuable opportunities in the chemical sensing research field.

Ni-Rh-based bimetallic conductive MOF as a high-performance electrocatalyst for the oxygen evolution reaction

Metal-organic frameworks (MOFs) have recently been considered the promising catalysts due to their merits of abundant metal sites, versatile coordination groups, and tunable porous structure. However, low electronic conductivity of most MOFs obstructs their direct application in electrocatalysis. In this work, we fabricate an Ni-Rh bimetallic conductive MOF ([Ni2.85Rh0.15(HHTP)2]n/CC) grown in situ on carbon cloth. Abundant nanopores in the conductive MOFs expose additional catalytic active sites, and the advantageous 2D π-conjugated structure helps accelerate charge transfer. Owing to the introduction of Rh, [Ni2.85Rh0.15(HHTP)2]n/CC exhibited substantially improved oxygen evolution reaction (OER) activity and exhibited only an overpotential of 320 mV to achieve the current density of 20 mA cm-2. The remarkable OER performance confirmed by the electrochemical tests could be ascribed to the synergistic effect caused by the doped Rh together with Ni in [Ni2.85Rh0.15(HHTP)2]n/CC, thereby exhibiting outstanding electrocatalytic performance.

Induced Production of Atypical Naturally Nonpreferred Metal-Organic Frameworks and Their Detachment via Provoking Post-Mismatching

The structures of metal-organic frameworks (MOFs) are typically determined by the building blocks that compose them and the conditions under which they are formed. MOFs tend to adopt a thermodynamically and/or kinetically stable structure (naturally preferred form). Thus, constructing MOFs with naturally nonpreferred structures is a challenging task, as it requires avoiding the easier pathway toward a naturally preferred MOF. Herein, an approach to construct naturally nonpreferred dicarboxylate-linked MOFs employing reaction templates is reported. This strategy relies on the registry between the surface of the template and the cell lattice of a target MOF, which reduces the effort required to form naturally nonpreferred MOFs. Reactions of p-block trivalent metal ions (Ga3+ and In3+ ) with dicarboxylic acids typically produce preferred MIL-53 or MIL-68. However, the surface of UiO-67 (and UiO-66) template exhibits the well-defined hexagonal lattice, which induce the selective formation of a naturally nonpreferred MIL-88 structure. Inductively grown MIL-88s are purely isolated from the template via provoking a post-mismatch in their lattices and weakening the interfacial interaction between product and template. It is also discovered that an appropriate template for effective induced production of naturally nonpreferred MOFs shall be properly selected based on the cell lattice of a target MOF.

Defective MOF-74 with ancillary open metal sites for the enhanced adsorption of chemical warfare agent simulants

The development of effective porous adsorbents plays a vital role in eliminating hazardous substances from the environment. Toxic chemicals, including chemical warfare agents (CWAs), pose significant risks to both humans and ecosystems, highlighting the urgency to create efficient porous adsorbents. Therefore, substantial attention has been directed towards advancing adsorption techniques for the successful eradication of CWAs from the environment. Herein, we demonstrate a rational approach for enhancing the adsorption capability of a porous metal-organic framework (MOF) by employing ancillary open metal sites within the MOF structure. To generate defective MOF-74 (D-MOF-74) with ancillary open metal sites, some of the 2,5-dihydroxy-1,4-bezenedicarboxylic acid (DHBDC) linkers originally present in the MOF-74 structure were replaced with 1,4-benzenedicarboxylic acid (BDC) linkers. The absence of hydroxyl groups in the BDC linkers compared to the original DHBDC linkers creates ancillary open metal sites, which enhance the adsorption ability of D-MOF-74 for CWA simulants such as dimethyl methyl phosphonate, 2-chloroethyl ethyl sulfide, and methyl salicylate by providing effective interaction sites for the targeted molecules. However, excessive creation of open metal sites causes the collapse of the originally well-developed MOF-74 structure, resulting in a substantial reduction in its empty space and a subsequent decline in adsorption efficiency. Thus, to produce a defective MOF with the best performance, it is necessary to replace an appropriate amount of organic linker and create suitable open metal sites. Moreover, D-MOF-74 displays excellent recyclability during consecutive adsorption cycles without losing its original structure and morphology, suggesting that D-MOF-74 is an effective and stable material for the removal of CWA simulants.

Free-standing conductive nickel metal-organic framework nanowires as bifunctional electrodes for wearable pressure sensors and Ni-Zn batteries

Free-standing metal-organic frameworks (MOFs) with controllable structure and good stability are emerging as promising materials for applications in flexible pressure sensors and energy-storage devices. However, the inherent low electrical conductivity of MOF-based materials requires complex preparation processes that involve high-temperature carbonization. This work presents a simple method to grow conductive nickel MOF nanowire arrays on carbon cloth (Ni-CAT@CC) and use Ni-CAT@CC as the functional electrodes for flexible piezoresistive sensor. The resulting sensor is able to monitor human activity, including elbow bending, knee bending, and wrist bending. Besides, the soft-packaged aqueous Ni-Zn battery is assembled with Ni-CAT@CC, a piece of glass microfiber filters, and Zn foil acting as cathode, separator, and anode, respectively. The Ni-Zn battery can be used as a power source for finger pressure monitoring. This work demonstrates free-standing MOF-based nanowires as bifunctional fabric electrodes for wearable electronics.

Enhanced adsorption capacity of ZIF-8 for chemical warfare agent simulants caused by its morphology and surface charge

The effective separation of toxic chemicals, including chemical warfare agents (CWAs), from the environment via adsorption is of great importance because such chemicals pose a significant threat to humans and ecosystems. To this end, the development of effective porous adsorbents for CWA removal has received significant attention. Understanding the specific interactions between adsorbents and CWAs must precede for the development of effective adsorbents. Herein, we report the relationship between the adsorption capacity of porous ZIF-8 and its morphological and surface characteristics. Four types of ZIF-8, which have different morphologies (such as cubic, rhombic dodecahedron, and leaf- and plate-shaped samples), were selectively prepared. The four types of ZIF-8 were found to have different surface charges owing to dissimilarly exposed components on the surfaces and additionally incorporated components. The specific surface charges of ZIF-8 were found to be closely related to their adsorption capacities for CWA simulants such as 2-chloroethyl ethyl sulfide (CEES) and dimethyl methyl phosphonate (DMMP). Cubic ZIF-8, with the most positive surface charge among four ZIF-8 samples, exhibited the highest adsorption capacity for CEES and DMMP via the effective polar interaction. Moreover, ZIF-8 exhibited excellent recyclability without losing its adsorption capacity and without critical morphological or structural changes.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

2D Metal-Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.

Construction of defected MOF-74 with preserved crystallinity for efficient catalytic cyanosilylation of benzaldehyde

Numerous open metal sites and well-developed micropores are the two most significant characteristics that should be imparted to design metal-organic frameworks (MOFs) as effective catalysts. However, the construction of the best MOF catalyst with both these characteristics is challenging because the creation of numerous open metal sites generally triggers some structural collapse of the MOF. Herein, we report the construction of well-structured but defected MOFs through the growth of defected MOFs, where some of the original organic linkers were replaced with analog organic linkers, on the surface of a crystalline MOF template (MOF-on-MOF growth). Additional open metal sites within the MOF-74 structure were generated by replacing some of the 2,5-dihydroxy-1,4-bezenedicarboxylic acid presenting in MOF-74 with 1,4-benzenedicarboxylic acid due to the missing hydroxyl groups. And the resulting additional open metal sites within the MOF-74 structure resulted in enhanced catalytic activity for the cyanosilylation of aldehydes. However, the collapse of some of the well-developed MOF-74 structure was also followed by structural defects. Whereas, the growth of defected MOF-74 (D-MOF-74) on the well-crystallized MOF-74 template led to the production of relatively well-crystallized D-MOF-74. Core-shell type MOF-74@D-MOF-74 having abundant open metal sites with a preserved crystallinity exhibited the efficient catalytic cyanosilylation of several aldehydes. Additionally, MOF-74@D-MOF-74 displayed excellent recyclability during the consecutive catalytic cycles.

Metal-Organic Framework Materials for Production and Distribution of Ammonia

The efficient production of ammonia (NH₃) from dinitrogen (N₂) and water (H₂O) using renewable energy is an important step on the roadmap to the ammonia economy. The productivity of this conversion hinges on the design and development of new active catalysts. In the wide scope of materials that have been examined as catalysts for the photo- and electro-driven reduction of N₂ to NH₃, functional metal-organic framework (MOF) catalysts exhibit unique properties and appealing features. By elucidating their structural and spectroscopic properties and linking this to the observed activity of MOF-based catalysts, valuable information can be gathered to inspire new generations of advanced catalysts to produce green NH₃. NH₃ is also a surrogate for the hydrogen (H₂) economy, and the potential application of MOFs for the practical and effective capture, safe storage, and transport of NH₃ is also discussed. This Perspective analyzes the contribution that MOFs can make toward the ammonia economy.

Three-dimensional hierarchical flower-like bimetallic–organic materials in situ grown on carbon cloth and doped with sulfur as an air cathode in a microbial fuel cell

Herein, the output power density produced by Zn/Co-S-3DHFLM as the cathode catalyst of an MFC was higher than that of Co-3DHFLM.

Enhanced catalytic activity of MOF-74 via providing additional open metal sites for cyanosilylation of aldehydes

The preparation of metal-organic frameworks (MOFs) having many open metal sites is an excellent approach for the development of highly active MOF-based catalysts. Herein, well-defined rice-shaped MOF-74 microparticles having structural defects are prepared by incorporating two analogous organic linkers [2,5-dihydroxy-1,4-bezenedicarboxylic acid (DHBDC) and 2-hydroxy-1,4-benzenedicarboxylic acid (HBDC)] within the MOF-74 structure. The replacement of some of DHBDC in MOF-74 by HBDC causes the structural defects (excluding some of the bridged hydroxyl groups), and these structural defects provide the additional open metal sites within MOF-74. Finally, the additional open metal sites within MOF-74 result in the enhanced catalytic activity for the cyanosilylation of several aldehydes. A series of MOF-74s is prepared with various incorporated amounts of HBDC, and the optimum ratio between DHBDC and HBDC in MOF-74 to achieving the best catalytic performance is determined. In addition, the defected MOF-74 displays an excellent recyclability for the catalytic reaction.

Emerging Synthesis Strategies of 2D MOFs for Electrical Devices and Integrated Circuits

The development of advanced electronic devices is boosting many aspects of modern technology and industry. The ever-increasing demand for advanced electrical devices and integrated circuits calls for the design of novel materials, with superior properties for the improvement of working performance. In this review, a detailed overview of the synthesis strategies of 2D metal organic frameworks (MOFs) acquiring growing attention is presented, as a basis for expansion of novel key materials in electrical devices and integrated circuits. A framework of controllable synthesis routes to be implanted in the synthesis strategies of 2D materials and MOFs is described. In short, the synthesis methods of 2D MOFs are summarized and discussed in depth followed by the illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low-cost, and more environmentally friendly way which can open up their applicable opportunities as key materials in advanced electrical devices and integrated circuits, enabling their use in broad aspects of the society.

Two-Dimensional Metal-Organic Framework Nanosheets: Synthesis and Applications in Electrocatalysis and Photocatalysis

Two-dimensional metal-organic nanosheets (2D MONs) are an emerging class of ultrathin, porous, and crystalline materials. The organic/inorganic hybrid nature offers MONs distinct advantages over other inorganic nanosheets in terms of diversity of organic ligands and metal notes. Compared to bulk three-dimensional metal-organic frameworks, 2D MONs possess merits of high density and readily accessible catalytic sites, reduced diffusion pathways for reactants/products, and fast electron transport. These features endow MONs with enhanced physical/chemical properties and are ideal for heterogeneous catalysis. In this Review, state-of-the-art synthetic methods for the fabrication of 2D MONs were summarized. The advances of 2D MONs-based materials for electrocatalysis and photocatalysis, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂ RR), and electro-/photocatalytic organic transformations were systematically discussed. Finally, the challenges and perspectives regarding future design and synthesis of 2D MONs for high-performance electrocatalysis and photocatalysis were provided.

Gas sensing behavior of metal-catecholates based MOFs

We report on the usability aspect of triphenylene ligand-based metal-organic frameworks (MOF) as the potential gas sensing element in chemiresistive devices. Among various possibilities, we explored mono-metallic (Nickel-based) and bi-metallic (Nickel and copper-based) in room temperature gas sensing. Our investigations suggest that the chemiresistive device based on nickel catecholate MOFs were highly sensitive to ethyl alcohol gas in the concentration range of 5-100 ppm with decent sensing parameters such as response time, recovery time, repeatability, stability, etc. We also investigated bimetallic (Nickel and copper) catecholate based MOFs in gas sensing with different metallic content ratios (Cu: Ni:: 60:40 and 40:60). We found that the 1D Cu_0.6Ni_0.4-CAT nanostructures-based gas sensor to be selective towards H₂gas (0.2-7 ppm) at room temperature. We further explored the gas sensing abilities of Cu_0.4Ni_0.6-CAT based devices, and we found them to be selective towards NO₂gas. However, it was not possible to obtain the current versus concentration curve due to the gas molecules' aggressive chemisorption. However, the device could perform well (with a hysteresis error of ∼10%) for detecting NO gas (which has the 2nd best absolute response after NO₂). These results indicate that the ratio of metal ions in the MOF directly influences the sensing capabilities. Hence, rational synthetic variations in the metal content in MOF can lead to the design and develop highly selective and sensitive chemiresistive sensors.

Two-Dimensional Metal-Organic Frameworks as Ultrahigh-Performance Electrocatalysts for the Fuel Cell Cathode: A First-Principles Study

Except for metal-organic frameworks (MOFs) with traditional metal-nitrogen sites, MOFs with metal-oxygen sites may also possess good oxygen reduction reaction (ORR) catalytic activity due to their unique electronic structures. Herein, using density functional theory methods, the ORR performances of a series of M₃(HHTT)₂ (where M is a 3d, 4d, or 5d transition metal and HHTT is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene)) catalysts are explored. The binding energy (ΔE_species) results suggest that the binding energy of *OH (ΔE_*OH) shows a good linear relationship with the binding energies of *O and *OOH (ΔE_*O and ΔE_*OOH, respectively), indicating that ΔE_*OH can serve as a descriptor to reflect the catalytic activity of M₃(HHTT)₂. In addition, the volcano plot suggests that M₃(HHTT)₂ catalysts with a moderate binding strength of the intermediate *OH (0.6 eV < ΔE_*OH < 0.9 eV) show relatively high ORR activity. Therefore, four highly active ORR catalysts are screened out, namely, Fe₃(HHTT)₂, Co₃(HHTT)₂, Rh₃(HHTT)₂, and Ir₃(HHTT)₂, which possess very small overpotentials of 0.35, 0.24, 0.31, and 0.29 V, respectively. Their potential-determining step is the reduction of O₂ to the intermediate *OOH. It is encouraging that the theoretically lowest overpotential of this kind of catalyst is 0.21 V, which is superior to that on Pt(111). Moreover, Co₃(HHTT)₂ has excellent poisoning-tolerance ability for impurity gases (CO, NO, and SO₂) as well as fuel molecules (CH₃OH and HCOOH).

Two-Dimensional Conjugated Metal-Organic Frameworks for Electrocatalysis: Opportunities and Challenges

A highly effective electrocatalyst is the central component of advanced electrochemical energy conversion. Recently, two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as a class of promising electrocatalysts because of their advantages including 2D layered structure with high in-plane conjugation, intrinsic electrical conductivity, permanent pores, large surface area, chemical stability, and structural diversity. In this Review, we summarize the recent advances of 2D c-MOF electrocatalysts for electrochemical energy conversion. First, we introduce the chemical design principles and synthetic strategies of the reported 2D c-MOFs, as well as the functional design for the electrocatalysis. Subsequently, we present the representative 2D c-MOF electrocatalysts in various electrochemical reactions, such as hydrogen/oxygen evolution, and reduction reactions of oxygen, carbon dioxide, and nitrogen. We highlight the strategies for the structural design and property tuning of 2D c-MOF electrocatalysts to boost the catalytic performance, and we offer our perspectives in regard to the challenges to be overcome.

A novel trigonal bipyramidal cage-based Zn(ii)-MOF featuring two types of trinuclear clusters with high gas sorption properties

A unique trigonal bipyramidal cage-based Zn(ii)-MOF built from a linear trinuclear pin-wheel cluster and a triangular trinuclear cluster was prepared and shows a moderate gas adsorption amounts and high selectivities towards C2Hn/CH4 and C2H2/CO2.

Improving the Activity of Electrocatalysts toward the Hydrogen Evolution Reaction, the Oxygen Evolution Reaction, and the Oxygen Reduction Reaction via Modification of Metal and Ligand of Conductive Two-Dimensional Metal-Organic Frameworks

Exploring efficient and stable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution and reduction reactions (OER and ORR) is vital to the development of renewable energy technologies. Herein, on the basis of density functional theory (DFT) calculations, we systematically investigated 30 TMN_xO_4-x-HTP (TM = Fe, Co, Ni, Ru, Rh and Pd; x = 0-4; HTP refers to hexatriphenylene) analogs of conductive two-dimensional (2D) metal-organic frameworks (MOFs) as potential catalysts for HER, OER, and ORR. The results show the good stabilities and metallic features of TMN_xO_4-x-HTP. The interaction strength between intermediates and catalysts governs the catalytic activities, which can be modulated by tuning the TM atom and the local coordination number of N/O in catalysts. RhN₃O₁-HTP is an efficient bifunctional catalyst for HER and OER, and RhN₁O₃-HTP is a promising bifunctional catalyst for OER and ORR. Our findings highlight a potentially efficient class of electrocatalysts based on 2D MOF materials.

One-dimensional metal-organic frameworks for electrochemical applications

Metal-organic frameworks (MOFs) are as a category of crystalline porous materials. Extensive interest has been devoted to energy storage and energy conversion applications owing to their unique advantages of periodic architecture, high specific surface area, high adsorption, high conductivity, high specific capacitance, and high porosity. One-dimensional (1D) nanostructures have unique surface effects, easily regulated size, good agglutination of the substrate, and other distinct properties amenable to the field of energy storage and conversion. Therefore, 1D nanostructures could further improve the characteristic properties of MOFs, and it is of great importance for practical applications to control the size and morphological characteristics of MOFs. The electrochemical application of 1D MOFs is mainly discussed in this review, including energy storage applications in supercapacitors and batteries and energy conversion applications in catalysis. In addition, various synthesis strategies for 1D MOFs and their architectures are presented.

Alkaline Metal Oxide Assisting the Ionothermal Method for Efficient Fe-NX/C Catalyst Preparation

Owing to low cost and high efficiency, nonprecious metal catalysts have been widely used in various types of fuel cells. To obtain a high-activity electrocatalyst, a simple method for the synthesis of iron-modified covalent triazine frameworks by the direct heating of a mixture of FeCl3, ZnCl2, ZnO, and m-phthalodinitrile is reported. The role and a possible evolution pathway of the oxygen of metallic oxides are well discussed. To further verify our assumption, the Fe3O4 microspherical nanomaterials were synthesized and the relative Fe-based catalyst (Fe-NX/C) was successfully obtained by the ionothermal polymerization method. Fe-NX/C exhibits an extraordinary oxygen reduction reaction (ORR) performance in acidic solution, with a half-wave potential of only 25 mV negative shifts compared with Pt/C, while the power density is approximately 56% of that of Pt/C catalysts under the proton exchange membrane fuel cell testing condition. This work represents a new strategy to synthesize high-performance Fe-based catalysts toward ORR.

Conductive Metal-Organic Frameworks: Electronic Structure and Electrochemical Applications

Smarter and minimization of devices are consistently substantial to shape the energy landscape. Significant amounts of endeavours have come forward as promising steps to surmount this formidable challenge. It is undeniable that material scientists were contemplating smarter material beyond purely inorganic or organic materials. To our delight, metal-organic frameworks (MOFs), an inorganic-organic hybrid scaffold with unprecedented tunability and smart functionalities, have recently started their journey as an alternative. In this review, we focus on such propitious potential of MOFs that was untapped over a long time. We cover the synthetic strategies and (or) post-synthetic modifications towards the formation of conductive MOFs and their underlying concepts of charge transfer with structural aspects. We addressed theoretical calculations with the experimental outcomes and spectroelectrochemistry, which will trigger vigorous impetus about intrinsic electronic behaviour of the conductive frameworks. Finally, we discussed electrocatalysts and energy storage devices stemming from conductive MOFs to meet energy demand in the near future.

A template-directed synthesis of metal-organic framework (MOF-74) ultrathin nanosheets for oxygen reduction electrocatalysis

Metal–organic frameworks (MOFs) have received wide attention for their promising applications in numerous fields due to their tailorable structure, metal centers and porosity. However, the low mass permeability, poor conductivity and blockage of active metal centers severely restrict the utilization of MOF systems in electrocatalysis. Two-dimensionalization can endow MOF materials extra unsaturated metal centers and enhanced electron-transfer ability, and could be an effective strategy to achieve high-performance MOF-based electrocatalysts. Herein, Ni-MOF-74 nanosheets are synthesized using layered double hydroxide (LDH) as a template to directly grow ultrathin structures. Benefiting from the two-dimensional structure, Ni-MOF-74 nanosheets with carbon substrate exhibit an enhanced ORR electrocatalytic property with positive half-wave potential (+0.83 V vs. RHE), a large current density (3.9 mA cm⁻²), four-electron selectivity and a promising long-term durability.

Layered double hydroxide (LDH) is employed as a layered template to synthesize ultrathin Ni-MOF-74 nanosheets. The two-dimensional structure brings numerous exposed unsaturated active sites and enhanced conductivity for Ni-MOF-74.

2D metal-organic framework-based materials for electrocatalytic, photocatalytic and thermocatalytic applications

Ultrathin two-dimensional metal-organic frameworks (2D MOFs) have recently attracted extensive interest in various catalytic fields (e.g., electrocatalysis, photocatalysis, thermocatalysis) due to their ultrathin thickness, large surface area, abundant accessible unsaturated active sites and tunable surface properties. Besides tuning the intrinsic properties of pristine 2D MOFs by changing the metal nodes and organic ligands, one of the hot research trends is to develop 2D MOF hybrids and 2D MOF-derived materials with higher stability and conductivity in order to further increase their activity and durability. Here, the synthesis of 2D MOF nanosheets is briefly summarized and discussed. More attention is focused on summaries and discussions about the applications of these 2D MOFs, their hybrids and their derived materials as electrocatalysts, photocatalysts and thermocatalysts. The superior properties and catalytic performance of these 2D MOF-based catalysts compared to their 3D MOF counterparts in electrocatalysis, photocatalysis and thermocatalysis are highlighted. The enhanced activities of 2D MOFs, their hybrids and derivatives come from abundant accessible active sites, a high density of unsaturated metal nodes, ultrathin thickness, and tunable microenvironments around the MOFs. Views regarding current and future challenges in the field, and new advances in science and technology to meet these challenges, are also presented. Finally, conclusions and outlooks in this field are provided.

ZIF-67 Derived MnO2 Doped Electrocatalyst for Oxygen Reduction Reaction

In this study, zeolitic imidazolate framework (ZIF-67) derived nano-porous carbon structures that were further hybridized with MnO2 were tested for oxygen reduction reaction (ORR) as cathode material for fuel cells. The prepared electrocatalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Energy Dispersive X-ray Analysis (EDX). Cyclic voltammetry was performed on these materials at different scan rates under dissolved oxygen in basic media (0.1 M KOH), inert and oxygen rich conditions to obtain their I–V curves. Electrochemical impedance spectroscopy (EIS) and Chronoamperometry was also performed to observe the materials’ impedance and stability. We report improved performance of hybridized catalyst for ORR based on cyclic voltammetry and EIS results, which show that it can be a potential candidate for fuel cell applications.

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs): chemistry and function for MOFtronics

Two-dimensional conjugated MOFs are emerging for multifunctional electronic devices that brings us “MOFtronics”, such as (opto)electronics, spintronics, energy devices.

Heterometallic coordination polymers as heterogeneous electrocatalysts

Heterometallic coordination polymers have been rapidly developed as heterogeneous electrocatalysts. This review highlights the synthesis strategies of these polymers and the relationships between structures and electrocatalytic performances.

2D framework materials for energy applications

In recent years a massive increase in publications on conventional 2D materials (graphene, h-BN, MoS2) is documented, accompanied by the transfer of the 2D concept to porous (crystalline) materials, such as ordered 2D layered polymers, covalent-organic frameworks, and metal-organic frameworks. Over the years, the 3D frameworks have gained a lot of attention for use in applications, ranging from electronic devices to catalysis, and from information to separation technologies, mostly due to the modular construction concept and exceptionally high porosity. A key challenge lies in the implementation of these materials into devices arising from the deliberate manipulation of properties upon delamination of their layered counterparts, including an increase in surface area, higher diffusivity, better access to surface sites and a change in the band structure. Within this minireview, we would like to highlight recent achievements in the synthesis of 2D framework materials and their advantages for certain applications, and give some future perspectives.

One-step ultrasonic synthesis of Co/Ni-catecholates for improved performance in oxygen reduction reaction

The inherent periodically arranged M-N_X, M-S_X and M-O_X units (M are usually Fe, Co, Ni, etc.) in metal-organic frameworks (MOFs) can be promising active centers in electrocatalysis. In previous studies, MOFs were usually constructed by energy-consuming hydro- or solvo-thermal reactions. Ultrasonic synthesis is a rapid and environment-friendly technique when envisaging MOFs' industrial applications. In addition, different synthetic pathways for MOFs may lead to difference in their microstructure, resulting in different electrocatalytic performance. Nevertheless, only a handful of MOFs were successfully prepared by ultrasonic synthesis and few were applied in electrochemical catalysis. Herein, we constructed Ni/Co-catecholates (Ni/Co-CATs) synthesized by one-step ultrasonic method (250 W, 40 KHz, 25 W/L, Ultrasonic clearing machine) and compared their performance in oxygen reduction reaction (ORR) with that of Ni/Co-CATs synthesized by hydrothermal method. Ni-CAT and Co-CAT prepared by ultrasonic showed the half-wave potential of -0.196 V and -0.116 V (vs. Ag/AgCl), respectively. The potentials were more positive than those prepared by hydro-thermal method. And they showed excellent electrochemical stability in neutral solution. The latter was only 32 mV lower than that of commercial Pt/C. The improved performance in ORR was attributed to higher specific surface area and mesopore volume as well as more structural defects generated in the ultrasonic synthesis process, which could facilitate their exposure of electrocatalytic active sites and their mass transport. This work gives some perspective into cost-effective synthetic strategies of efficient MOFs-based electrocatalysts.

Fe/Ni Bimetallic Organic Framework Deposited on TiO2 Nanotube Array for Enhancing Higher and Stable Photoelectrochemical Activity of Oxygen Evaluation Reaction

Photoelectrochemical (PEC) water splitting is a promising strategy to improve the efficiency of oxygen evolution reactions (OERs). However, the efficient adsorption of visible light as well as long-term stability of light-harvesting electrocatalysis is the crucial issue in PEC cells. Metal–organic framework (MOF)-derived bimetallic electrocatalysis with its superior performance has wide application prospects in OER and PEC applications. Herein, we have fabricated a nickel and iron bimetallic organic framework (FeNi-MOF) deposited on top of anodized TiO₂ nanotube arrays (TNTA) for PEC and OER applications. The FeNi-MOF/TNTA was incorporated through the electrochemical deposition of Ni²⁺ and Fe³⁺ onto the surface of TNTA and then connected with organic ligands by the hydrothermal transformation. Therefore, FeNi-MOF/TNTA demonstrates abundant photoelectrocatalytic active sites that can enhance the photocurrent up to 1.91 mA/cm² under 100 mW/cm² and a negligible loss in activity after 180 min of photoreaction. The FeNi-MOF-doped photoanode shows predominant photoelectrochemical performance due to the boosted excellent light-harvesting ability, rapid photoresponse, and stimulated interfacial energy of charge separation under the UV-visible light irradiation conditions. The results of this study give deep insight into MOF-derived bimetallic nanomaterial synthesis for photoelectrochemical OER and provide guidance on future electrocatalysis design.

Iron-doped NiCo-MOF hollow nanospheres for enhanced electrocatalytic oxygen evolution

The development of metal-organic frameworks (MOFs) as high-efficiency electrocatalysts for water splitting has attracted special attention due to their unique structural features including high porosity, large surface areas, high concentrations of active sites, uniform pore sizes and shapes, etc. Most of the related reports focus on the in situ generation of high-efficiency electrocatalysts by annealed MOFs. However, the pyrolysis process usually destroys the porous structure of MOFs and reduces the number of active sites due to the absence of organic ligands and agglomeration of metal centers. In this work, we prepared unique NiCo-MOF hollow nanospheres (NiCo-MOF HNSs) by a solvothermal method and further fabricated Fe-doped NiCo-MOF HNSs (Fe@NiCo-MOF HNSs) by a simple impregnation-drying method. Significant enhancement of electrocatalytic activity of Fe@NiCo-MOF HNSs was witnessed because of the doped Fe. Compared with the parent NiCo-MOF HNSs, the optimized Fe@NiCo-MOF HNSs exhibited a lower overpotential of 244 mV at 10 mA·cm^-2 with a smaller Tafel slope of 48.61 mV·dec^-1, which was lowered by ca. 90 mV due to the influence of Fe doping on the electronic structure of the active centers of Ni and Co. The above materials also displayed excellent stability without obvious activity decay for at least 16 hours. These findings present a new entry in the design and fabrication of high-efficiency MOF-based electrocatalysts for energy conversion.

Multimetal Incorporation into 2D Conductive Metal-Organic Framework Nanowires Enabling Excellent Electrocatalytic Oxidation of Benzylamine to Benzonitrile

As an important organic intermediate, benzonitrile (BN) is widely involved in organic synthetic chemistry and pharmaceutical and dyestuff industries. However, the exploration of a more efficient and controllable synthesis technique and the corresponding greener catalysts for the synthesis of BN still poses a great challenge. Herein, with multimetallic two-dimensional conductive metal-organic frameworks (2D cMOF) as anodic electrocatalysts, we develop a green, convenient, and highly efficient electrochemical synthesis strategy for BN. Thanks to the intrinsic 2D electrically conductive structure and the optimized the multimetallic coupling catalytic effect, the resulting multimetallic 2D cMOFs exhibit excellent benzylamine (BA) electrooxidation performance. Especially, the trimetallic 2D cMOF (NiCoFe-CAT) requires an ultralow potential of 1.29 V vs reversible hydrogen electrode (RHE) to achieve a 10 mA·cm^-2 current density, which indicates the fastest reaction and the most favorable thermodynamic condition. A very high yield (0.058 mmol·mg^-1·h^-1) and faradic efficiency (∼87%) of benzonitrile are both achieved during the BA electrooxidation reaction at 1.45 V. The reaction mechanism investigations indicated that the various redox mediators of M^II/M^III (Ni, Co, Fe) may be regarded as multimetal active species to promote BA conversion. Also, the excellent cycling durability of multimetallic 2D cMOFs further promotes their potential practical applications. These electrocatalytic performances are considered excellent and nearly surpass all other reported Ni-based inorganics or MOF-based electrocatalysts for the electrocatalytic oxidation of benzylamine.