Redox-active Metal-organic Frameworks Research Articles

All-purpose redox-active metal-organic frameworks as both cathodic and anodic host materials for advanced lithium-sulfur batteries

All-purpose redox-active metal-organic frameworks as both cathodic and anodic host materials for advanced lithium-sulfur batteries

Radical-Driven Crystal-Amorphous-Crystal Transition of a Metal-Organic Framework.

Self-assembly-based structural transition has been explored for various applications, including molecular machines, sensors, and drug delivery. In this study, we developed new redox-active metal-organic frameworks (MOFs) called DGIST-10 series that comprise π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligands and Ni2+ ions, aiming to boost ligand-self-assembly-driven structural transition and study the involved mechanism. Notably, during the synthesis of the MOFs, a single-crystal-amorphous-single-crystal structural transition occurred within the MOFs upon radical formation, which was ascribed to the fact that radicals prefer spin-pairing or through-space electron delocalization by π-orbital overlap. The radical-formation-induced structural transitions were further confirmed by the postsynthetic solvothermal treatment of isolated nonradical MOF crystals. Notably, the transient amorphous phase without morphological disintegration was clearly observed, contributing to the seminal structural change of the MOF. We believe that this unprecedented structural transition triggered by the ligand self-assembly magnifies the structural flexibility and diversity of MOFs, which is one of the pivotal aspects of MOFs.

A dual-functional Cu-BTC/COOH-MWCNTs ratiometric electrochemical sensing device for the detection of sunset yellow and tartrazine

A ratiometric electrochemical sensor for sunset yellow (SY) and tartrazine (TZ) was constructed by modifying a glassy carbon electrode with a composite of carboxylated multiwalled carbon nanotubes (COOH-MWCNTs) and a redox-active metal–organic frameworks (Cu-BTC MOFs). Cu-BTC served as an internal reference probe with a constant oxidation peak current. The sensor exhibited excellent linearity in the concentration range of 2–200 μM under optimized conditions. The detection limits for SY and TZ were found to be 0.73 μM and 0.78 μM, respectively. When the sensor was used to determine SY and TZ in two carbonated beverages, the resules were good and did not significantly differ from those obtained by UV–Vis spectrophotometry.

In situ growth of a redox-active metal-organic framework on electrospun carbon nanofibers as a free-standing electrode for flexible energy storage devices.

The rational construction of free-standing and flexible electrodes for application in electrochemical energy storage devices and next-generation supercapacitors is an emerging research focus. Herein, we prepared a redox-active ferrocene dicarboxylic acid (Fc)-based nickel metal-organic framework (MOF) on electrospun carbon nanofibers (NiFc-MOF@CNFs) via an in situ approach. This in situ approach avoided the aggregation of the MOF. The NiFc-MOF@CNF flexible electrode showed a high redox-active behavior owing to the presence of ferrocene and flexible carbon nanofibers, which led to unique properties, including high flexibility and lightweight. Furthermore, the prepared electrode was utilized in a supercapacitors (SC) without the use of any binder, which achieved a specific capacity of 460 C g-1 at 1 A g-1 with an excellent cyclic retention of 82.2% after 25 000 cycles and a good rate capability. A flexible asymmetric supercapacitor device was assembled, which delivered a high energy density of 56.25 W h kg-1 and a long-lasting cycling performance. Also, the prepared electrode could be used as a freestanding electrode in flexible devices at different bending angles. The obtained cyclic voltammetry curves showed negligible changes, indicating the high stability and good flexibility of the electrode. Thus, the use of the in situ strategy can lead to the uniform growth of redox-active MOFs or other porous materials on CNFs.

Construction of a redox-active metal-organic framework with an octanuclear lithium one-dimensional building block.

A stable lithium metal-organic framework, constructed using a redox-active N,N,N',N'-tetrakis(4-carboxyphenyl)-1,4-phenylenediamine linker and Li8 cluster-based one-dimensional rod secondary building unit, exhibits good stability and reversible redox activity. The Li8-MOF, which can be oxidized by AgNO3, has the potential to function as an electrochromic device, thereby advancing the development of smart MOF materials.

Charge-Transfer-Regulated Selective Solar Fuel Production in Aqueous Medium by a Tetrathiafulvalene-Based Redox-Active Metal–Organic Framework

Charge-Transfer-Regulated Selective Solar Fuel Production in Aqueous Medium by a Tetrathiafulvalene-Based Redox-Active Metal–Organic Framework

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks.

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Effectively facilitating the degradation of chloramphenicol by the synergism of Shewanella oneidensis MR-1 and the metal-organic framework

Electroactive bacteria (EAB) and metal oxides are capable of synergistically removing chloramphenicol (CAP). However, the effects of redox-active metal-organic frameworks (MOFs) on CAP degradation with EAB are not yet known. This study investigated the synergism of iron-based MOFs (Fe-MIL-101) and Shewanella oneidensis MR-1 on CAP degradation. 0.5 g/L Fe-MIL-101 with more possible active sites led to a three-fold higher CAP removal rate in the synergistic system with MR-1 (initial bacterial concentration of 0.2 at OD600), and showed a superior catalytic effect than exogenously added Fe(III)/Fe(II) or magnetite. Mass spectrometry revealed that CAP was transformed into smaller molecular weight and less toxic metabolites in cultures. Transcriptomic analysis showed that Fe-MIL-101 enhanced the expression of genes related to nitro and chlorinated contaminants degradation. Additionally, genes encoding hydrogenases and c-type cytochromes associated with extracellular electron transfer were significantly upregulated, which may contribute to the simultaneous bioreduction of CAP both intracellularly and extracellularly. These results indicated that Fe-MIL-101 can be used as a catalyst to synergize with EAB to effectively facilitate CAP degradation, which might shed new light on the application in the in situ bioremediation of antibiotic-contaminated environments.

2D MOFs Containing Bis(azabenzimidazole) and Dicarboxylate Moieties for the Efficient Oxygen Evolution Reaction and CO2Sorption

With the rapid advancement of electronic technology, the necessity for efficient electrode materials has been tremendously increasing. With this endeavor, the design of cost-saving, environment-friendly, and stable electrocatalysts from earth-abundant transition elements for water splitting is immensely desired. To this contribution, herein, we have aimed to design two stable Co(II)- and Ni(II)-containing redox-active metal–organic frameworks (Co-MOF and Ni-MOF) with a nitrogen-rich linear bis(azabenzimidazole) ligand and electron-rich dicarboxylic acids and explored the redox-active metal–organic frameworks (MOFs) as efficient heterogeneous catalysts for oxygen evolution reaction (OER) and CO2 sorption studies. The structural analysis of Co-MOF shows that it exhibits a double-walled square grid-type two-dimensional structure, whereas Ni-MOF exhibits a rectangular grid-type two-dimensional layered structure. Further, three-dimensional packing of both MOFs is formed by two-dimensional (2D) metal–ligand coordination layers via N–H···O hydrogen bonding interactions with the imidazole moiety and solvent molecules, resulting in one-dimensional cavities. These cavities have void spaces of 27.6 and 33.2% for Co-MOF and Ni-MOF, respectively, which are occupied by water molecules in Co-MOF and dimethylformamide (DMF) molecules in Ni-MOF. Electrochemical studies of these pristine MOFs infer that both MOFs display efficient OER activity in alkaline solutions. These MOFs demand overpotentials of 470 and 450 mV to generate a current density of 10 mA/cm2, which is nearly equivalent to 10% solar energy conversion efficiency. The 2D layers with nitrogen-rich ligands and oxygen-rich linkers are expected to promote better diffusion of electrons and protons in the course of the electrocatalytic process. The presence of hydrophilic channels further promoted to adsorb considerable amounts of CO2 gas (46 and 15 cm3/g, respectively) at 273 K and 1 bar pressure.

Role of Counterions in the Structural Stabilisation of Redox-Active Metal-Organic Frameworks.

The crystal structures of metal-organic frameworks (MOFs) are typically determined by the strong chemical bonds formed between the organic and inorganic building units. However, the latest generation of redox-active frameworks often rely on counterions in the pores to access specific charge states of the components. Here, we model the crystal structures of three layered MOFs based on the redox-active ligand 2,5-dihydroxybenzoquinone (dhbq): Ti2 (Cl2 dhbq)3 , V2 (Cl2 dhbq)3 and Fe2 (Cl2 dhbq)3 with implicit and explicit counterions. Our full-potential first-principles calculations indicate that while the reported hexagonal structure is readily obtained for Ti and V, the Fe framework is stabilised only by the presence of explicit counterions. For high counterion concentrations, we observe the formation of an electride-like pocket in the pore center. An outlook is provided on the implications of solvent and counterion control for engineering the structures and properties of porous solids.

Efficient Photothermal Energy Conversion Triggered by near-Infrared Light in a Dithiolene-Based Metal–Organic Framework

A Ni-dithiolene ligand, {Ni(pedt)2}− (pedt = 1-(pyridine-4-yl)ethylene-1,2-dithiolate), was used to prepare a novel interpenetrated redox-active Metal–Organic Framework (MOF), which exhibited an efficient photothermal conversion under low-power-density near-infrared (NIR) laser illumination. Solid-state UV–vis-NIR spectroscopy on bulk crystals indicated strong absorbance in the NIR region, attributed to the radical monoanion of the pedt ligands, which was also evident in the solid-state Electron Paramagnetic Resonance (EPR) spectrum. DFT computations on a model system provided new insights into the structural factors underpinning the photothermal properties. First, the NIR absorption corresponds to a high-energy excited state (D4), while the lowest-lying excited state (D1) is close in energy to the energies of the vibrational motions, facilitating the conversion of electronic to thermal energy. Second, close contacts between interpenetrated nets of the MOF may further enhance the propagation of thermal motion. The new photothermal MOF was then doped into a self-healing polymer, PDU (polydimethylsiloxane-urea), leading to composites which could recover from dynamic damage using the heat generated from the embedded MOF by photostimulus. This process led by the photothermal effect offers a highly efficient and energy-saving approach for self-healing materials design.

Enhanced Catalytic Activity of TEMPO-Mediated Aerobic Oxidation of Alcohols via Redox-Active Metal-Organic Framework Nodes.

Metal-organic frameworks (MOFs) are outstanding platforms for heterogeneous catalysis due to their tunable pore size, huge surface area, large porosity, and potential active sites. The design and synthesis of MOF/organocatalyst co-catalytic systems have attracted considerable interest owing to their high catalytic activity, low toxicity, and mild reaction conditions. Herein, we reported the synthesis of a bifunctional TEMPO-IsoNTA organocatalyst featuring a pyridyl group as an anchoring site and a TEMPO radical as a catalytic active site. By using the topologically isomorphic structures of MIL-101(Fe) and MIL-101(Cr) as co-catalysts, these MOF/TEMPO-IsoNTA systems enable the efficient aerobic oxidation of various alcohols to their corresponding aldehydes or ketones under mild conditions. Notably, the MIL-101(Fe)/TEMPO-IsoNTA system exhibits superior catalytic activity, thanks to their redox-active FeIII-oxo nodes, which facilitate the regeneration of TEMPO-IsoNTA. Our research not only solves the problem of potential heavy metal contamination in the TEMPO-based homogeneous catalytic system, but also enriches the understanding of synergism of MOFs/organocatalysts.

Constructing a Redox-Active Cu(I)-Pyridyltriazine Framework for Catalytic Photoreduction of Nitrobenzenes and Carboxylic Cyclization of Alkynol with CO2.

A redox-active metal-organic framework, Cu(I)-TPT, was synthesized by combination of Cu(I), the halogenoid cyanide group (CN), and redox-active organic bridging ligand 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) into one single framework. Cu(I)-TPT displays a two-dimensional (2D) plane structure by 1D -Cu(I)-CN- chains connected with TPT ligands. Cu(I)-TPT exhibits intrinsic semiconductive features with a moderate bandgap energy (1.97 eV). Under irradiation, Cu(I)-TPT has an electrical conductivity of 2 × 10-7 S cm-1 in the presence of the sacrificial electron donor ethanol under the ambient test conditions, which is owing to the π-π stacking interactions between TPT moieties, the d-π conjugation between the Cu(I) ion and the CN ligands, and the permanent microporosity. Cu(I)-TPT displayed highly efficient hole-electron separation and ordered electron transfer, which is beneficial for the photoreduction of nitrobenzene. In addition, Cu(I)-TPT displays high efficiency in carboxylic cyclization of alkynol with CO2 because it possesses highly decentralized Cu(I) catalytic sites to the active C≡C bond of alkynol and affluent N atoms on the 2D sheets to facilitate the trapping and activation of CO2.

Mechanochemical synthesis of ZIF(Zn, Co)/NixCoyAlz-LDH composite by solid state ion exchange as advanced materials for pseudocapacitor electrode

Metal-organic frameworks (MOFs), and layered double hydroxides (LDHs) as worthy hot-spot materials have gained substantial attention for electrochemical energy storage and conversion applications as a result of their structural diversities, excellent physical properties, and promising electrochemical performance. In this work, we synthesized composite materials comprised of ZIF-8 and NiCoAl-LDHs via a mechanochemical method and in-situ solid state ion exchange. The findings have shown that the synergetic effects between ZIF-8 and LDHs in composite materials and tuning of the percentages of Ni and Co in LDHs have a positive contribution to enhancing electrochemical performance in supercapacitors. In this regard, among pure LDHs and ZIF-8\\NixCoyAl0.33 LDHs, ZIF-8\\Ni(17%)Co(50%)Al(33%)-LDH indicated higher specific capacitance of 256 F g−1 and higher rate of performance. In ZIF-8/Ni(17%)Co(50%)Al(33%)-LDH, LDH nanosheets’ structure improves the conductivity of ZIF-8, and optimized bimetal Ni-Co provides more active sites. In addition, the asymmetric supercapacitor (ASC) assembled by ZIF-8/Ni(17%)Co(50%)Al(33%)-LDH as the cathode and activated carbon (AC) as the anode achieved a good performance in terms of specific capacity, energy density, and cyclic stability. For instance, the ASC delivered a maximum energy density of 184 Wh kg−1 at 1291 W kg−1 and outstanding capacitance retention of 91% after 5000 cycles. This study demonstrates that the interaction between different element ratios in LDHs and the composition of LDHs with ZIFs (porous and redox-active MOFs) has significant effects on using ZIFs/LDHs electrode materials in advanced supercapacitor applications.

Ni/Co-MOF@aminated MXene hierarchical electrodes for high-stability supercapacitors

Owing to the instability and poor electronic conductivity of redox-active metal–organic frameworks (MOFs), the redox-active MOFs as a direct electrode for achieving fast and high-stability supercapacitors is a significant challenge. Herein, highly redox-active bimetallic Ni/Co metal–organic frameworks (Ni/Co-MOF) is in-situ stabilizied by 2D thin-layer aminated Ti3C2Tx to construct promising hierarchical heterostructural electrodes with superior wettability for high-performance supercapacitors. Benefitting from high conductivity and the good wettability with electrolyte of aminated MXene, the bimetallic Ni/Co-MOF can be uniformly in-situ stabilizied on the aminated MXene surface, thus guaranteeing the superior electronic and ionic transmission from the redox active center of Ni/Co-MOF, and good interfacial compatibility and large contact area. The modified MXene is beneficial to slow down the van der Waals interactions between functional groups to exfoliate the sheet-like structures. For the rapid redox reactions whereas the synergy of highly redox-active Ni/Co in bimetallic Ni/Co-MOF provides a fast redox reaction for Faradaic capacitance. Therefore, as-obtained Ni/Co-MOF@TCT-NH2 presents an ultra-high specific capacitance of 1924 F·g−1 at 0.5 A·g−1 and an ultra-long cycling stability with 10,000 cycles at 10 A·g-1 in a three-electrode system. Besides, the assembled Ni/Co-MOF@TCT-NH2//AC asymmetrical devices exhibit a maximum specific energy density of 98.1 Wh·kg−1 at 600 W·kg−1 and superior rate stability with 15,600 cycles. Such superior electrochemical performance demonstrates that the heterojunction construction of thin-layer aminated MXene to stabilize highly redox-active bimetallic MOFs is an effective strategy for MOFs as a direct electrode to enhance the energy storage.

Interfacial Reduction Nucleation of Noble Metal Nanodots on Redox-Active Metal-Organic Frameworks for High-Efficiency Electrocatalytic Conversion of Nitrate to Ammonia.

Electrochemically converting nitrate to ammonia is a promising route to realize artificial nitrogen recycling. However, developing highly efficient electrocatalysts is an ongoing challenge. Herein, we report the construction of stable and redox-active zirconium metal-organic frameworks (Zr-MOFs) based on Zr6 nanoclusters and redox-reversible tetrathiafulvalene (TTF) derivatives as inorganic nodes and organic linkers, respectively. The redox-active Zr-MOF can facilitate the in situ reduction of noble metal precursors free of external reductants and realize the uniform nucleation of noble metal nanodots (NDs) on Zr-MOF, achieving the preparation of M-NDs/Zr-MOF (M = Pd, Ag, or Au). The highly porous Zr-MOF with good conductivity can facilitate the mass transfer process. Among the M-NDs/Zr-MOF catalysts, Pd-NDs/Zr-MOF exhibits the highest electrocatalytic activity, delivering a NH3 yield of 287.31 mmol·h-1·g-1cat. and a Faradaic efficiency of 58.1%. The proposed interfacial reduction nucleation strategy for anchoring M NDs on Zr-MOFs can be applied to other challenging energy conversion reactions.

A Host-Guest Electron Transfer Mechanism for Magnetic and Electronic Modifications in a Redox-Active Metal-Organic Framework.

Host-guest electron transfer (HGET) in molecular framework systems is a critical trigger for drastic functional changes in both host framework and guest. A reversible magnetic phase transition was achieved via HGET in a layered framework, [{Ru2 (2,6-F2 PhCO2 )4 }2 (BTDA-TCNQ)] (1), where 2,6-F2 PhCO2 - and BTDA-TCNQ represent 2,6-difluorobenzoate and bis[1,2,5]dithiazolotetracyanoquinodimethane, respectively. The guest-free 1 with an antiferromagnetic ground state transformed into a paramagnet, [{Ru2 (2,6-F2 PhCO2 )4 }2 (BTDA-TCNQ)]I3 (1-I3 ), by adsorbing iodine (I2 ). The local charge distribution of [{Ru2 II,III }+ -(BTDA-TCNQ).- -{Ru2 II,II }] in 1 was reversibly modified to [{Ru2 II,III }+ -(BTDA-TCNQ)0 -{Ru2 II,II }](I3 - ) in 1-I3 through HGET. Theoretical calculations of 1-I3 indicated a partial charge delocalization as [{Ru2 }(1-δ)+ -(BTDA-TCNQ)0 -{Ru2 }δ+ ](I3 - ) with δ≈0.2, aided by weak ferromagnetic coupling. 1-I3 exhibited a hundred-fold enhancement in electrical conductivity compared to that of 1.

Redox-active metal-organic frameworks for the removal of contaminants of emerging concern

The pollution due to the presence of contaminants of emerging concern (CECs) is a major cause for concern because of the serious threat it supposes to human health and ecosystem functions. Many efforts have been geared toward their removal to guarantee safer freshwater. Metal-organic frameworks (MOFs) are crystalline hybrid materials with high surface area and flexible rational design, which allows the incorporation of different active sites into a particular framework, thereby emerging as a potentially excellent candidate for water and wastewater treatment. Benefiting from the unique redox-active properties of MOFs, this review surveys literature update on their application for the removal of CECs. The underlaying electron transfer mechanism and strategies for incorporating redox-active sites into MOFs are comprehensively discussed. Different components of MOFs that are redox-active are further highlighted. This study elaborates the application of MOFs for Fenton-type and other advanced oxidation processes (AOPs) for removing emerging contaminants. AOPs generate highly reactive strong oxidants like hydroxyl and sulfate radicals that are efficient for degrading emerging pollutants with high mineralization rates. MOFs display semiconductor-like properties. Their photocatalytic use for the removal of dissolved emerging pollutants is detailed in the discussion. This review also provides an overview of the most promising directions for future research.

Enhancing the photothermal conversion of tetrathiafulvalene-based MOFs by redox doping and plasmon resonance.

Near-infrared (NIR) photothermal materials hold great promise for use in several applications, particularly in photothermal therapy, diagnosis, and imaging. However, current NIR responsive materials often show narrow absorption bands and low absorption efficiency, and have long response times. Herein, we demonstrate that the NIR absorption of tetrathiafulvalene-based metal–organic frameworks (MOFs) can be tuned by redox doping and using plasmonic nanoparticles. In this work, a MOF containing redox-active tetrathiafulvalene (TTF) units and Dy-carboxylate chains was constructed, Dy-m-TTFTB. The NIR absorption of the as-synthesized Dy-m-TTFTB was further enhanced by Ag+ or I2 oxidation, transforming the neutral TTF into a TTF˙+ radical state. Interestingly, treatment with Ag+ not only generated TTF˙+ radicals, but it also formed Ag nanoparticles (NPs) in situ within the MOF pores. With both TTF˙+ radicals and Ag NPs, Ag NPs@Dy-m-TTFTB was shown to exhibit a wide range of absorption wavelengths (200–1000 nm) and also a high NIR photothermal conversion. When the system was irradiated with an 808 nm laser (energy power of 0.7 W cm−2), Ag NPs@Dy-m-TTFTB showed a sharp temperature increase of 239.8 °C. This increase was higher than that of pristine Dy-m-TTFTB (90.1 °C) or I2 treated I3−@Dy-m-TTFTB (213.0 °C).

Probing the electronic and ionic transport in topologically distinct redox-active metal–organic frameworks in aqueous electrolytes

The redox-hopping-based electrochemical behaviours in aqueous electrolytes are studied in various concentrations of Na2SO4 to separately probe the electronic and ionic transports within the thin films of three Mn-decorated Zr-MOFs.