Controlling Charge Transport in 2D Conductive MOFs─The Role of Nitrogen-Rich Ligands and Chemical Functionality

Conductive Metal-Organic Frameworks and Their Electrocatalysis Applications

Recently, electrically conductive metal-organic frameworks (EC-MOFs) have emerged as a wealthy library of porous frameworks with unique properties, allowing their use in diverse applications of energy conversion, including electrocatalysis. In this review, the electron conduction mechanisms in EC-MOFs are examined, while their electrical conductivities are considered. There have been various strategies to enhance the conductivities of MOFs including ligand modification, the incorporation of conducting materials, and the construction of multidimensional architectures. With sufficient conductivities being established for EC-MOFs, there have been extensive pursuits in their electrocatalysis applications, such as in the hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, N2 reduction reaction, and CO2 reduction reaction. In addition, computational modeling of EC-MOFs also exerts an important impact on revealing the synthesis-structure-performance relationships. Finally, the prospects and current challenges are discussed to provide guidelines for designing promising framework materials.

From 0D to 2D: microwave-assisted synthesis of electrically conductive metal-organic frameworks with controlled morphologies

Morphology control of electrically conductive metal-organic frameworks (EC-MOFs) can be a powerful means to tune their surface area and carrier transport pathways, particularly beneficial for energy conversion and storage. However, controlling EC-MOFs' morphology is underexplored due to the uncontrollable crystal nucleation and rapid growth kinetics. This work introduces a microwave-assisted strategy to readily synthesize Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with controlled morphologies. We controlled solvent compositions to facilitate particles' directional growth to 1D and 2D crystals. Meanwhile, we found that ultrasonication can manipulate crystal seeding, yielding 0D spherical Cu-HHTP crystals. Electronic conductivity measurements suggest that the isotropic nature of the 0D crystals allows a conductivity of 7.34 × 10-1 S cm-1, much higher than 1D and 2D counterparts. Additionally, the controlled 0D morphology enhanced the material's capacitance and effective surface area and significantly improved its photocurrent response. These findings underscore the pivotal impact of controlled morphology in optimizing EC-MOFs' physicochemical properties.

Precision-Engineered Construction of Proton-Conducting Metal-Organic Frameworks

Proton-conducting materials have attracted considerable interest because of their extensive application in energy storage and conversion devices. Among them, metal-organic frameworks (MOFs) present tremendous development potential and possibilities for constructing novel advanced proton conductors due to their special advantages in crystallinity, designability, and porosity. In particular, several special design strategies for the structure of MOFs have opened new doors for the advancement of MOF proton conductors, such as charged network construction, ligand functionalization, metal-center manipulation, defective engineering, guest molecule incorporation, and pore-space manipulation. With the implementation of these strategies, proton-conducting MOFs have developed significantly and profoundly within the last decade. Therefore, in this review, we critically discuss and analyze the fundamental principles, design strategies, and implementation methods targeted at improving the proton conductivity of MOFs through representative examples. Besides, the structural features, the proton conduction mechanism and the behavior of MOFs are discussed thoroughly and meticulously. Future endeavors are also proposed to address the challenges of proton-conducting MOFs in practical research. We sincerely expect that this review will bring guidance and inspiration for the design of proton-conducting MOFs and further motivate the research enthusiasm for novel proton-conducting materials.

Upgrading Structural Conjugation in Three-Dimensional Ni-Based Metal-Organic Frameworks for Promoting Electrical Conductivity and Specific Capacitance

Metal-organic frameworks (MOFs) have emerged as promising candidates for electrochemical energy storage and conversion due to their high specific surface areas, abundant active sites, and excellent chemical and structural tunability. However, the direct utilization of MOFs as electrochemical materials is a challenge because of the poor electroconductivity induced by the insulating nature of most organic linkers. Herein, a conjugated three-dimensional Ni-MOF {Ni(HBTC)(BPE)}n (Ni-BPE) with a 2-fold interpenetrating structure was developed via the coordination polymerization of Ni2+, a H3BTC ligand (1,3,5-benzenetricarboxylic acid), and a vinyl-functionalized bipyridine linker (1,2-di(4-pyridyl)ethylene, BPE). Ni-BPE displayed an enhanced conjugation system compared to analogous and insulated Ni-BPY that is constructed by the Ni-BTC layer and ordinary bipyridine linker (4,4'-bipyridine, BPY). Notably, upgrading structural conjugation promoted a dramatical ∼204 times increase in the electroconductivity of Ni-BPE compared to Ni-BPY. More importantly, Ni-BPE displayed a higher specific capacitance of 633.2 F g-1 (316.6 C g-1) at 1 A g-1, which exhibited a significant ∼1.5-fold enhancement than Ni-BPY. Furthermore, the asymmetric supercapacitor can reach a good energy density of 25.2 Wh kg-1 with a reasonable cycle stability of 71.0% over 5000 cycles. This work provides an effective method for optimizing the structure of insulating MOFs to enhance the electroconductivity and specific capacitance.

Thiatruxene-Based Conductive MOF: Harnessing Sulfur Chemistry for Enhanced Proton Transport

Functionalizing the organic building blocks of electrically conductive MOFs (EC-MOFs) can be a powerful method for adjusting the electronic structure and introducing a specific chemistry. However, designing EC-MOF linkers with reactive functional groups for postsynthetic modification is challenging due to the requirements of d-p conjugation. This work addresses such design limitations by synthesizing an EC-MOF, Cu-thiatruxene (Cu-thiaTRX). This conductive framework incorporated a truxene-based linker with heterocyclic sulfur, allowing for efficient conjugation and an electrical conductivity of 2.2 × 10-2 S cm-1. Harnessing sulfur chemistry in Cu-thiaTRX involves a two-step postsynthetic modification: oxidation and SNAr. The sulfinic groups introduced in the framework enabled tunable proton conductivity, leading to a 200-fold improvement. These results highlight the importance of a rational linker design for functionalization.

Fabricating Large-Area Thin Films of 2D Conductive Metal-Organic Frameworks

ConspectusRecent years have witnessed significant interest in two-dimensional metal-organic frameworks (MOFs) due to their unique properties and promising applications across various fields. These materials offer distinct advantages, including high porosity and excellent charge transport properties. Their tunability allows precise control over various factors, including the electronic structure adjustments and local reactivity modulation, facilitating a wide range of properties and applications, such as material sensing and spin dynamics control. Moreover, the precise crystal structure of 2D MOFs supports rational design and mechanism studies, providing insights into their potential applications and enhancing their utility in various scientific and technological endeavors.To fully unveil the latent capabilities of 2D MOFs and advance their applications across diverse fields, thin film synthesis is crucial. Thin films provide a platform for investigating the intrinsic electrical properties of 2D MOFs with anisotropic structures, enabling the exploration of their unique characteristics comprehensively. Additionally, thin films offer the advantage of minimizing interference at contacts and junctions, thereby enhancing the performance of 2D MOFs for various applications. Furthermore, the properties of thin films can vary with thickness, presenting an opportunity to tailor their characteristics based on specific requirements.In this Account, we present an overview of our research focusing on the synthesis of 2D conductive MOF thin films encompassing two primary methods: chemical vapor deposition and solution processing. The chemical vapor deposition method allows for one-step, all-vapor-phase processes resulting in MOFs with edge-on orientation, controllable film thicknesses ranging from 55 to 662.7 nm, and smooth, homogeneous surfaces. On the other hand, solution-processing introduces a novel MOF, Cu3(HHTATP)2, by reducing interlayer interactions through the addition of pendent Brønsted bases on a ligand, enabling spin coating for thin film synthesis. This method yields a concentrated 2D MOF solution, enabling spin coating for thin film synthesis, where reversible electrical conductivity changes occur through doping with an acid and dedoping with a base. Additionally, we discuss various other synthesis methods, such as interfacial synthesis, layer-by-layer assembly, and microfluidic-assisted synthesis, offering versatile approaches for fabricating large-area thin films with tailored properties. Finally, we address ongoing challenges and potential strategies for further advancement in 2D conductive MOF thin film synthesis. We hope that this Account provides insights for selecting synthesis methods tailored to specific purposes, contributes to the development of varied synthesis techniques, and facilitates the exploration of diverse applications, unlocking the full potential of 2D conductive MOFs for next-generation technologies.

Arresting dissolution of two-dimensional metal-organic frameworks enables long life in electrochemical devices

Two-dimensional conjugated metal-organic frameworks (2D cMOFs) are emerging as promising materials for electrochemical energy storage (EES). Despite considerable interest, an understanding of their electrochemical stability and the factors contributing to their degradation during cycling is largely lacking. Here we investigate three Cu-based MOFs and report that the dissolution of 2D cMOFs into electrolytes is a prevalent and significant degradation pathway. Several factors, such as the inherent solubility of ligands in electrolyte solvents and the duration of charge-discharge cycling exert a strong influence on the dissolution process. When these factors combine within a MOF, severely limited cycling stability is observed, with dissolution accounting for up to 80% of capacity degradation. Conversely, excellent cycling stability is observed when testing a Cu-MOF with a sparingly soluble ligand within an optimized potential window. Overall, these findings represent essential insights into the electrochemical stability of 2D cMOFs, offering crucial guidelines for their targeted development in EES applications.

Copper-Based Two-Dimensional Metal-Organic Frameworks for Fenton-like Photocatalytic Degradation of Methylene Blue under UV and Sunlight Irradiation

To efficiently degrade organic pollutants, photocatalysts must be effective under both ultraviolet (UV) radiation and sunlight. We synthesized a series of new metal-organic frameworks by using mild hydrothermal conditions. These frameworks incorporate three distinct bipyridyl ligands: pyrazine (pyr), 4,4'-bipyridine (bpy), and 1,2-bis(4-pyridyl)ethane (bpe). The resulting compounds are denoted as [Cu(pyz)(H2O)2MF6], [Cu(bpy)2(H2O)2]·MF6, and [Cu(bpe)2(H2O)2]·MF6·H2O [M = Zr (1, 3, and 5) and Hf (2, 4, and 6)]. All six compounds exhibited a two-dimensional crystal structure comprising infinitely nonintersecting linear chains. Compound 3 achieved 100% degradation of methylene blue (MB) after 8 min under UV irradiation and 100 min under natural sunlight in the presence of H2O2 as the electron acceptor. For compound 5, 100% MB degradation was achieved after 120 min under sunlight and 10 min under UV light. Moreover, reactive radical tests revealed that the dominant species involved in photocatalytic degradation are hydroxyl (•OH), superoxide radicals (•O2-), and photogenerated holes (h+). The photodegradation process followed pseudo-first-order kinetics, with photodegradation rate constants of 0.362 min-1 (0.039 min-1) for 3 and 0.316 min-1 (0.033 min-1) for 5 under UV (sunlight) irradiation. The developed photocatalysts with excellent activity and good recyclability are promising green catalysts for degrading organic pollutants during environmental decontamination.

De Novo Design and Facile Synthesis of Highly Crystalline 2D Conductive Metal-Organic Frameworks: A "Rotor-Stator" Strategy

Two-dimensional conductive metal-organic frameworks (2D c-MOFs), which feature high electrical conductivity and large charge carrier mobility, hold great promise in electronics and optoelectronics. Nevertheless, the limited solubility of commonly used planar ligands inevitably brings challenges in synthesis and purification and causes laborious coordination conditions for screening. Moreover, most reported 2D c-MOFs are polycrystalline powders with relatively low crystallinity and irregular morphology, hindering the unveiling of the detailed structure-function relationship. Herein, we developed a "rotor-stator" molecular design strategy to construct 2D c-MOFs using a delicately designed nonplanar biscarbazole ligand (8OH-DCB). Benefiting from the special "rotor-stator" structure of the ligand, crystals of Cu-DCB-MOF were successfully prepared, allowing for the precise determination of their crystal structure. Interestingly, the crystals of Cu-DCB-MOF can be obtained in various organic solvents, indicating excellent solvent compatibility. The versatility of the "rotor-stator" molecular design strategy was further demonstrated by another two new ligands with a "rotor-stator" structure, and afford corresponding 2D c-MOF crystals (Cu-DCBT-MOF and Cu-DCBBT-MOF). The current work presents a facile approach toward the rational design and direct construction of highly crystalline 2D c-MOFs using nonplanar ligands.