Controlling the BET Surface Area of Porous Carbon by Using the Cd/C Ratio of a Cd-MOF Precursor and Enhancing the Capacitance by Activation with KOH

Out-of-Plane Single-Copper-Site Catalysts for Room-Temperature Benzene Oxidation

Crafting single-atom catalysts (SACs) that possess "just right" modulated electronic and geometric structures, granting accessible active sites for direct room-temperature benzene oxidation is a coveted objective. However, achieving this goal remains a formidable challenge. Here, we introduce an innovative in situ phosphorus-immitting strategy using a new phosphorus source (phosphorus nitride, P3N5) to construct the phosphorus-rich copper (Cu) SACs, designated as Cu/NPC. These catalysts feature locally protruding metal sites on a nitrogen (N)-phosphorus (P)-carbon (C) support (NPC). Rigorous analyses, including X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), validate the coordinated bonding of nitrogen and phosphorus with atomically dispersed Cu sites on NPC. Crucially, systematic first-principles calculations, coupled with the climbing image nudged-elastic-band (CI-NEB) method, provide a comprehensive understanding of the structure-property-activity relationship of the distorted Cu-N2P2 centers in Cu/NPC for selective oxidation of benzene to phenol production. Interestingly, Cu/NPC has shown more energetically favorable C-H bond activation compared to the benchmark Cu/NC SACs in the direct oxidation of benzene, resulting in outstanding benzene conversion (50.3 %) and phenol selectivity (99.3 %) at room temperature. Furthermore, Cu/NPC achieves a remarkable turnover frequency of 263 h-1 and mass-specific activity of 35.2 mmol g-1 h-1, surpassing the state-of-the-art benzene-to-phenol conversion catalysts to date.

A highly sensitive and selective Cd-MOF fluorescent probe for the detection of His, NB, TC and PTH and its applications in real samples

A novel metal-organic framework (MOF) with the formula of [Cd(CIA)(4,4'-BB)·H2O]·H2O (Cd-MOF) (H2CIA = 5-methoxyisophthalic acid, 4,4'-BB: 4,4'-bis((1H-imidazol-1-yl)methyl)-1,1'-biphenyl) has been designed and synthesized through solvothermal reaction. The structure was characterized by TG, IR, PXRD, elemental analysis, single-crystal X-ray diffraction and XPS. The coordination pattern of the fully deprotonated ligand (CIA) in Cd-MOF is μ2-к1: к0: к2. The complex is found to be a three-dimensional reticular material with good thermal stability. Further study revealed that Cd-MOF has good fluorescence performance and can be used as a fluorescent probe for sensitive detection of histidine (His), nitrobenzene (NB), tetracycline (TC) and pyrimethanil (PTH) in water. Four cycles experiments can be achieved with a low detection limit. The detection limits were 0.3 µM (His), 0.05 µM (NB), 0.2 µM (TC) and 0.01 µM (PTH), respectively. To verify the feasibility of detecting PTH in real samples, we further explored it in water samples and apple peels. The spiked recoveries were 95.3-99.4 % and 97.2-101.2 %, respectively. Finally, the fluorescence quenching mechanism of Cd-MOF in His, NB, TC and PTH detection is discussed in detail.

Effect of Electrolytic Cations on a 3D Cd-MOF for Supercapacitive Electrodes

A cadmium-based metal-organic framework (Cd-MOF) is synthesized in a facile manner at ambient temperature by an easy slow diffusion process. The three-dimensional (3D) structure of Cd-MOF is authenticated by single-crystal X-ray diffraction studies and exhibits a cuboid-shaped morphology with an average edge length of ∼1.13 μm. The prepared Cd-MOF was found to be electroactive in nature, which resulted in a specific capacitance of 647 F g^-1 at 4 A g^-1 by maintaining a retention of ∼78% over 10,000 successive cycles in the absence of any binder. Further, to distinguish the efficiency of Cd-MOF electrodes, different electrolytes (NaOH, KOH, and LiOH) were explored, wherein NaOH revealed a higher capacitive response due to its combined effect of ionic and hydrated ionic radii. To investigate the practical applicability, an asymmetric supercapacitor (ASC) device is fabricated by employing Cd-MOF as the positive electrode and activated carbon (AC) as the negative electrode, enabling it to light a commercial light-emitting diode (LED) bulb (∼1.8 V). The as-fabricated ASC device delivers comparable energy density and power density.

Tunable Capacitive Behavior in Metallopolymer-based Electrochromic Thin Film Supercapacitors

Volumetric capacitance is a more critical performance parameter for rechargeable power supply in lightweight and microelectronic devices as compared to gravimetric capacitance in larger devices. To this end, we report three electrochromic metallopolymer-based electrode materials containing Fe²⁺ as the coordinating metal ion with high volumetric capacitance and energy densities in a symmetric two-electrode supercapacitor setup. These metallopolymers exhibited volumetric capacitance up to 866.2 F cm^-3 at a constant current density of 0.25 A g^-1. The volumetric capacitance (poly-Fe-L2: 544.6 F cm^-3 > poly-Fe-L1: 313.8 F cm^-3 > poly-Fe-L3: 230.8 F cm^-3 at 1 A g^-1) and energy densities (poly-Fe-L2: 75.5 mWh cm^-3 > poly-Fe-L1: 43.6 mWh cm^-3 > poly-Fe-L3: 31.2 mWh cm^-3) followed the order of the electrical conductivity of the metallopolymers and are among the best values reported for metal-organic systems. The variation in the ligand structure was key toward achieving different electrical conductivities in these metallopolymers with excellent operational stability under continuous cycling. High volumetric capacitances and energy densities combined with tunable electro-optical properties and electrochromic behavior of these metallopolymers are expected to contribute to high performance and compact microenergy storage systems. We envision that the integration of smart functionalities with thin film supercapacitors would warrant the surge of miniaturized on-chip microsupercapacitors integrated in-plane with other microelectronic devices for wearable applications.

Facile Strategy for Efficient Charge Separation and High Photoactivity of Mixed-Linker MOFs

Two new sets of UiO-Zr metal-organic framework (MOF) bearing mixed linkers BDC-(SCH₃)₂ and BDC-(SOCH₃)₂ that have different band gaps and edges were prepared through post oxidation and direct methods, namely, UiO-66-(SCH₃)₂-xh (x = 4, 9, 12 oxidation hours) and UiO-66-(SOCH₃)_x(SCH₃)_2-x (x = 0, 0.4, 0.6, 2), respectively. These composites with stoichiometric components were fully characterized via proton nuclear magnetic resonance (¹H NMR) spectroscopy, powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectra, Brunauer-Emmett-Teller (BET), photo electrochemical measurements, and femtosecond transient absorption (fs-TA) spectroscopy. The structure, electronic property, and photoresponsive and catalytic ability as the functions of the molar ratio of linkers and the synthetic protocol were first investigated. The mixed-linker UiO-66-(SCH₃)₂-xh and UiO-66-(SOCH₃)_x(SCH₃)_2-x exhibited improved performances as compared to the UiO-66-(SCH₃)₂ and UiO-66-(SOCH₃)₂ possessing neat linkers only. Their photo response and catalytic activity varied with different linker ratios. For UiO-66-(SCH₃)₂-xh, the performance increased with the increasing linker BDC-(SOCH₃)₂ ratio upon oxidation but reached the highest as the BDC-(SOCH₃)₂ being of 24.4% in UiO-66-(SCH₃)₂-9h. In comparison, the best photocurrent (80.74 uA/cm^-2) and the highest H₂ generation rate (2018.8 μmol g^-1 h^-1) (λ > 400 nm) in UiO-66-(SCH₃)₂-9h are about twice those of UiO-66-(SOCH₃)_0.4(SCH₃)_1.6 obtained by direct synthesis, although the linker BDC-(SOCH₃)₂ ratio of those two composites is almost the same (24.4% vs 23.9%). Recorded shorter lifetime and higher charge separation efficiency of the former than those of the latter suggest the postsynthetic protocol as the efficient method for achieving the mixed-liner-MOF-based photocatalyst with high performance. A new type-II tailored homojunction is proposed in these mixed-linker MOFs for their efficient charge separation and improved activity.

Metal organic framework derived porous carbon materials excel as an excellent platform for high-performance packaged supercapacitors

Designing and synthesizing new materials with special physical and chemical properties are the key steps to assembling high performance supercapacitors. Metal organic framework (MOF) derived porous carbon materials have drawn great attention in supercapacitors because of their large specific surface area, high chemical/thermal stability and tunable pore structure. Thus, the recent development of porous carbon as an electrode material for supercapacitors is reviewed. The types, design and synthesis strategies of porous carbon are systematically summarized. This review will be divided into three main parts: (1) the design and synthesis of MOF precursors and templates for MOF-derived porous carbon materials; (2) the application of different types of MOF-derived carbon in supercapacitors; and (3) the design of typical structures of porous carbon composites for supercapacitors. Finally, the problems and challenges confronted when using porous carbon are assessed and elaborated, and some suggestions on future research directions are proposed.

Single Alkali Metal Ion-Activated Porous Carbon With Heteroatom Doping for Supercapacitor Electrode

A single alkali metal ion activation method was used to prepare sulfur-doped microporous carbons. A series of alkali metal ions such as Li⁺, Na⁺, K⁺, and Cs⁺ was used in the polymerization process of 3-hydroxythiophenol and formaldehyde to obtain metal ion anchored in the sulfur-containing resin, which was further treated to obtain xerogel and carbonized to obtain microporous carbon with sulfur doping. In this case, the monodispersed alkali metal ions could realize highly effective activation with low activating agent dosage. Intensive material characterizations show that the alkali metal ions determine the pore structure and surface properties of as-prepared carbons. C-Cs prepared by Cs⁺ ion possesses a high Brunauer-Emmett-Teller specific surface area of 1,037 m² g^-1 with interconnected microporosity and sulfur doping. The specific capacitance of C-Cs can reach up to 270.9 F g^-1 in a two-cell electrode measurement system, whereas C-Cs-based supercapacitors can deliver an energy density of 7.6 Wh kg^-1, which is much larger than that of other samples due to its surface functionalities and well-interconnected porosities.

Porous Carbon-Based Supercapacitors Directly Derived from Metal-Organic Frameworks

Numerously different porous carbons have been prepared and used in a wide range of practical applications. Porous carbons are also ideal electrode materials for efficient energy storage devices due to their large surface areas, capacious pore spaces, and superior chemical stability compared to other porous materials. Not only the electrical double-layer capacitance (EDLC)-based charge storage but also the pseudocapacitance driven by various dopants in the carbon matrix plays a significant role in enhancing the electrochemical supercapacitive performance of porous carbons. Since the electrochemical capacitive activities are primarily based on EDLC and further enhanced by pseudocapacitance, high-surface carbons are desirable for these applications. The porosity of carbons plays a crucial role in enhancing the performance as well. We have recently witnessed that metal-organic frameworks (MOFs) could be very effective self-sacrificing templates, or precursors, for new high-surface carbons for supercapacitors, or ultracapacitors. Many MOFs can be self-sacrificing precursors for carbonaceous porous materials in a simple yet effective direct carbonization to produce porous carbons. The constituent metal ions can be either completely removed during the carbonization or transformed into valuable redox-active centers for additional faradaic reactions to enhance the electrochemical performance of carbon electrodes. Some heteroatoms of the bridging ligands and solvate molecules can be easily incorporated into carbon matrices to generate heteroatom-doped carbons with pseudocapacitive behavior and good surface wettability. We categorized these MOF-derived porous carbons into three main types: (i) pure and heteroatom-doped carbons, (ii) metallic nanoparticle-containing carbons, and (iii) carbon-based composites with other carbon-based materials or redox-active metal species. Based on these cases summarized in this review, new MOF-derived porous carbons with much enhanced capacitive performance and stability will be envisioned.

Atomic- and Molecular-Level Design of Functional Metal-Organic Frameworks (MOFs) and Derivatives for Energy and Environmental Applications

Continuing population growth and accelerated fossil-fuel consumption with recent technological advancements have engendered energy and environmental concerns, urging researchers to develop advanced functional materials to overcome the associated problems. Metal-organic frameworks (MOFs) have emerged as frontier materials due to their unique porous organic-inorganic hybrid periodic assembly and exceptional diversity in structural properties and chemical functionalities. In particular, the modular nature and modularity-dependent activity of MOFs and MOF derivatives have accentuated the delicate atomic- and molecular design and synthesis of MOFs, and their meticulous conversion into carbons and transition-metal-based materials. Synthetic control over framework architecture, content, and reactivity has led to unprecedented merits relevant to various energy and environmental applications. Herein, an overview of the atomic- and molecular-design strategies of MOFs to realize application-targeted properties is provided. Recent progress on the development of MOFs and MOF derivatives based on these strategies, along with their performance, is summarized with a special emphasis on design-structure and functionality-activity relationships. Next, the respective energy- and environmental-related applications of catalysis and energy storage, as well as gas storage-separation and water harvesting with close association to the energy-water-environment nexus are highlighted. Last, perspectives on current challenges and recommendations for further development of MOF-based materials are also discussed.

Synthesis, Molecular and Supramolecular Structures of New Cd(II) Pincer-Type Complexes with s-TriazineCore Ligand

The manuscript described the synthesis and characterization of the new [Cd(BDMPT)2](ClO4)2; 1 and [Cd2(MBPT)2(H2O)2Cl](ClO4)3.4H2O ; 2s-triazine pincer-type complexes, where BDMPT and MBPT are 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine and 2-methoxy-4,6-bis(2-(pyridin-2-ylmsethylene)hydrazinyl)-1,3,5-triazinerespectively.The synthesized complexes were characterized using Fourier-transform infrared spectroscopy (FTIR), 1H and 13C NMR spectroscopy, and the single-crystal X-ray diffraction technique.The homoleptic mononuclear complex (1)contains a hexa-coordinated Cd(II) center with two tridentate N-pincer ligand (BDMPT) with a highly distorted octahedral coordination environment located as an intermediate case between the octahedron and trigonal prism. The heteroleptic dinuclear complex (2) contains two hepta-coordinated Cd(II) coordination spheres where each Cd(II) is coordinated with one pentadentate pincer N-chelate (MBPT), one water, and one bridged chloride ligand connecting the two metal ions. The different intermolecular interactions in the studied complexes were quantified using Hirshfeld analysis. Their thermal stabilities and FTIR spectra were compared with the corresponding free ligands. The strength and nature of Cd–N, Cd–O, and Cd–Cl coordination interactions were discussed in light of atoms in molecules calculations (AIM). The M(II)–BDMPT and M(II)–MBPT interaction energies revealed that such sterically hindered ligands have higher affinity toward large-size metal ions (M =Cd) compared to smaller ones (M= Ni or Mn).

Six Isomorphous Window-Beam MOFs: Explore the Effects of Metal Ions on MOF-Derived Carbon for Supercapacitors

Six isomorphous metal-organic frameworks (MOFs) with a 3D window-beam architecture have been synthesized from solvothermal reactions, and are named Zn, Cd, Ni, Co, Mn and Cu-MOF, respectively. The series of MOFs was utilized as precursors to synthesize MOF-derived carbon with different morphologies. Zn and Cd-MOFs lead to the derivation of porous carbons (PCs), which exhibit remarkable BET specific surface areas. For derivates of Ni, Co and Mn-MOFs, graphitized carbons (GCs) show some carbon graphitization, but their BET specific surface areas are relatively small. C-Cu has the smallest BET specific surface area, and there is no carbon graphitization. Therefore, the metal ion of the parent MOF exerts a crucial effect on the preparation of MOF-derived carbon, such as the pore-forming effect of Zn and Cd species, and catalytic graphitization of Ni, Co, and Mn species. The capacitances of MOF-derived carbon follow the sequence of PCs>GCs>C-Cu, which reveals that the specific surface area plays a dominant role in the capacitive performance of electrical double layer capacitors (EDLCs), and that the graphitization could improve the capacitance. Significantly, PC-Zn exhibits the best specific capacitance (138 F g^-1 at 0.5 Ag^-1 ), and excellent life cycle, which can be applied as an electrode material in supercapacitors.

A sulfur vacancy rich CdS based composite photocatalyst with g-C3N4 as a matrix derived from a Cd-S cluster assembled supramolecular network for H2 production and VOC removal

By calcination, a sulfur vacancy rich CdS based composite photocatalyst with graphitic carbon nitride (g-C₃N₄) as a matrix has been synthesized successfully from a tetranuclear Cd-S cluster assembled supramolecular network. In this photocatalyst (CdS@g-C₃N₄), CdS nanoparticles with a size of about 5 to 8 nm disperse homogenously in the g-C₃N₄ matrix. During calcination, some coordinated nitrogen atoms dope in the lattice of CdS and replace sulfur atoms, which generates a large number of sulfur vacancies. Under visible light irradiation, CdS@g-C₃N₄ exhibits excellent H₂ production activity with a rate achieving as high as 19.88 mmol g^-1 h^-1 in the absence of a Pt cocatalyst. Its H₂ production ability remains stable for 30 h, which does not decay. Besides H₂ production, CdS@g-C₃N₄ also shows excellent photocatalytic activity for Volatile Organic Compound (VOC) degradation. For a photocatalyst, chemical content plays an important role in its performance. Here, the influence of sulfur vacancies on H₂ production and VOC degradation is discussed in detail. We expect that the sulfur vacancy rich CdS@g-C₃N₄ can act as an efficient material for H₂ production and indoor air purification.

Solvent-Induced Cadmium(II) Metal-Organic Frameworks with Adjustable Guest-Evacuated Porosity: Application in the Controllable Assembly of MOF-Derived Porous Carbon Materials for Supercapacitors

In this work, five new cadmium metal-organic frameworks (Cd-MOFs 1-5) have been synthesized from solvothermal reactions of Cd(NO₃ )₂ ⋅4 H₂ O with isophthalic acid and 1,4-bis(imidazol-1-yl)-benzene under different solvent systems of CH₃ OH, C₂ H₅ OH, (CH₃ )₂ CHOH, DMF, and N-methyl-2-pyrrolidone (NMP), respectively. Cd-MOF 1 shows a 3D diamondoid framework with 1D rhombic and hexagonal channels, and the porosity is 12.9 %. Cd-MOF 2 exhibits a 2D (4,4) layer with a 1D parallelogram channel and porosity of 23.6 %. Cd-MOF 3 has an 8-connected dense network with the Schäfli symbol of [4²⁴ ⋅6⁴ ] based on the Cd₆ cluster. Cd-MOFs 4-5 are isomorphous, and display an absolutely double-bridging 2D (4,4) layer with 1D tetragonal channels and porosities of 29.2 and 28.2 %, which are occupied by DMF and NMP molecules, respectively. Followed by the calcination-thermolysis procedure, Cd-MOFs 1-5 are employed as precursors to prepare MOF-derived porous carbon materials (labeled as PC-me, PC-eth, PC-ipr, PC-dmf and PC-nmp), which have the BET specific surface area of 23, 51, 10, 122, and 96 m²  g^-1 , respectively. The results demonstrate that the specific surface area of PCs is tuned by the porosity of Cd-MOFs, where the later is highly dependent on the solvent. Thereby, the specific surface area of PCs could be adjusted by the solvent used in the synthese of MOF precusors. Significantly, PCs have been further activated by KOH to obtain activated carbon materials (APCs), which possess even higher specific surface area and larger porosity. After a series of characterization and electrochemical investigations, the APC-dmf electrode exhibits the best porous properties and largest specific capacitances (153 F g^-1 at 5 mV s^-1 and 156 F g^-1 at 0.5 Ag^-1 ). Meanwhile, the APC-dmf electrode shows excellent cycling stability (ca. 84.2 % after 5000 cycles at 1 Ag^-1 ), which can be applied as a suitable electrode material for supercapacitors.

Benzoate Acid-Dependent Lattice Dimension of Co-MOFs and MOF-Derived CoS2@CNTs with Tunable Pore Diameters for Supercapacitors

Herein three novel cobalt metal-organic frameworks (Co-MOFs) with similar ingredients, [Co(bib)(o-bdc)]_∞ (1), [Co₂(bib)₂(m-bdc)₂]_∞ (2), and {[Co(bib)(p-bdc)(H₂O)](H₂O)_0.5}_∞ (3), have been synthesized from the reaction of cobalt nitrate with 1,4-bis(imidazol-1-yl)benzene (bib) and structure-related aromatic acids (1,2-benzenedicarboxylic acid = o-bdc, 1,3-benzenedicarboxylic acid = m-bdc, and 1,4-benzenedicarboxylic acid = p-bdc) by the solvothermal method. It is aimed to perform systematic research on the relationship among the conformation of benzoate acid, lattice dimension of Co-MOF, and pore diameter of MOF-derived carbon composite. Through the precursor strategy, Co-MOFs 1-3 have been utilized to synthesize porous cobalt@carbon nanotube composites (Co@CNTs). After the in situ gas-sulfurization, secondary composites CoS₂@CNTs were successfully obtained, which kept similar morphologies of corresponding Co@CNTs without destroying previous highly dispersed structures. Co-MOFs and two series of composites (Co@CNTs and CoS₂@CNTs) have been well characterized. Topology and Brunauer-Emmett-Teller analyses elucidate that the bdc^2- ion could control the pore diameters of MOF-derived carbon composites by adjusting the lattice dimension of Co-MOFs. The systematic studies on electrochemical properties demonstrate that (p)-CoS₂@CNT possesses hierarchical morphology, moderate specific surface area, proper pore diameter distribution, and high graphitization, which lead to remarkable specific capacitances (839 F g^-1 at 5 mV s^-1 and 825 F g^-1 at 0.5 A g^-1) in 2 M potassium hydroxide solution. In addition, the (p)-CoS₂@CNT electrode exhibits good electrochemical stability and still retains 82.9% of initial specific capacitance at the current density of 1 A g^-1 after 5000 cycles.