Dynamic Phase Transformations of Prussian Blue Analogue Crystals in Hydrotherms.

Prussian blue analogue (PBA)/metal-organic frameworks (MOFs) are multifunctional precursors for the synthesis of metal/metal compounds, carbon, and their derived composites (P/MDCs) in chemical, medical, energy, and other applications. P/MDCs combine the advantages of both the high specific surface area of PBA/MOF and the electronic conductivity of metal compound/carbon. Although the calcination under different atmospheres has been extensively studied, the transformation mechanism of PBA/MOF under hydrothermal conditions remains unclear. The qualitative preparation of P/MDCs in hydrothermal conditions remains a challenge. Here, we select PBA to construct a machine-learning model and measure its hydrothermal phase diagram. The architecture-activity relationship of substances among nine parameters was analyzed for the hydrothermal phase transformation of PBA. Excitingly, we established a universal qualitative model to accurately fabricate 31 PBA derivates. Additionally, we performed three-dimensional reconstructed transmission electron microscopy, X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, in situ X-ray powder diffraction, and theoretical calculation to analyze the advantages of hydrothermal derivatives in the oxygen evolution reaction and clarify their reaction mechanisms. We uncover the unified principles of the hydrothermal phase transformation of PBA, and we expect to guide the design for a wide range of composites.

Ion Exchange-mediated 3D Cross-linked ZIF-L Superstructure for Flexible Electrochemical Energy Storage.

Metal-organic frameworks (MOFs) are considered as a promising candidate for advancing energy storage owing to their intrinsic multi-channel architecture, high theoretical capacity, and precise adjustability. However, the low conductivity and poor structural stability lead to unsatisfactory rate and cycling performance, greatly hindering their practical application. Herein, we propose a sea urchin-like Co-ZIF-L superstructure using molecular template to induce self-assembly followed by ion exchange method, which shows improved conductivity, successive channels, and high stability. The ion exchange can gradually etch the superstructure, leading to the reconstruction of Co-ZIF-L with three-dimensional (3D) cross-linked ultrathin porous nanosheets. Moreover, the precise control of Co to Ni ratios can construct effective micro-electric field and synergistically enhance the rapid transfer of electrons and electrolyte ions, improving the conductivity and stability of CoNi-ZIF-L. The Co6.53Ni-ZIF-L electrode exhibits a high specific capacity (602 F g-1 at 1 A g-1) and long cycling stability (95.3% retention after 4,000 cycles at 5 A g-1). The Co6.53Ni-ZIF-L//AC asymmetric flexible supercapacitor employing gel electrolyte also exhibits excellent cycling stability (93.3% retention after 4000 cycles at 5 A g-1). This discovery provides valuable insights for electrode material selection and energy storage efficiency improvement.

Thiosalicylic-Acid-Mediated Coordination Structure of Nickel Center via Thermodynamic Modulation for Aqueous Ni-Zn Batteries.

Uniquely functional nanocomplexes with rich coordination environments are critical in energy storage. However, the construction of structurally versatile nanocomplexes remains challenging. In this study, a nickel-based complex with structural variations is designed via thermodynamic modulation using a dual-ligand synthesis strategy. A nickel-based nanomaterial (NiSA-SSA-160) with a large specific surface area is synthesized around the competing coordination of the host and guest molecules that differ in terms of the chemical properties of the O and S elements. Concurrently, the coordination environment of NiSA-SSA-160 is investigated via X-ray absorption fine structure spectroscopy. The thiol functional groups synergistically induced an electron-rich Ni structure, thus increasing the electron density of the central atom. The electrochemical performance of an assembled NiSA-SSA-160//Zn@CC battery is shown to improve significantly, with a maximum energy density of 0.54 mWh cm-2 and a peak power density of 49.49 mW cm-2. This study provides a new perspective regarding coordination transformations and offers an idea for the design of functionally rich nanomaterials.

Stabilizing Ni2+ in Hollow Nano MOF/Polymetallic Phosphides Composites for Enhanced Electrochemical Performance in 3D-Printed Micro-Supercapacitors.

Polymetallic phosphides exhibit favorable conductivities. A reasonable design of nano-metal-organic frame (MOF) composite morphologies and in situ introduction of polymetallic phosphides into the framework can effectively improve electrolyte penetration and rapid electron transfer. To address existing challenges, Ni, with a strong coordination ability with N, is introduced to partially replace Co in nano-Co-MOF composite. The hollow nanostructure is stabilized through CoNi bimetallic coordination and low-temperature controllable polymetallic phosphide generation rate. The Ni, Co, and P atoms, generated during reduction, effectively enhance electron transfer rate within the framework. X-ray absorption fine structure (XAFS) characterization results further confirm the existence of Ni-N, Ni-Ni, and Co-Co structures in the nanocomposite. The changes in each component during the charge-discharge process of the electrochemical reactions are investigated using in situ X-ray diffraction (XRD). Theoretical calculations further confirm that P can effectively improve conductivity. VZNPGC//MXene MSCs, constructed with active materials derived from the hollow nano MOF composites synthesized through the Ni2+ stabilization strategy, demonstrate a specific capacitance of 1184 mF cm-2, along with an energy density of 236.75 µWh cm-2 (power density of 0.14 mW cm-2). This approach introduces a new direction for the synthesis of highly conductive nano-MOF composites.

Layered (AlO)2OH·VO3 composite superstructures for ultralong lifespan aqueous zinc-ion batteries.

The unique superstructures electrode materials are of dominant significance for improving the performance of aqueous zinc-ion batteries (AZIBs). In this work, using nano MIL-96 (Al) as the precursor, a series of the layered (AlO)2OH·VO3 composite superstructures with different morphologies and V-oxide contents were prepared by combining calcination and hydrothermal synthesis. Among which, the HBC650·V4 superstructure is composed of the amorphous Al2O3/C, V-oxide, and the fluffy structure of (AlO)2OH, thus the superstructure can enhance the stability, increase the active center, and shorten Zn2+ diffusion, respectively. It is commendable that, the HBC650·V4 superstructure exhibits a high specific capacity of 180.1 mAh·g-1 after 300 cycles at 0.5 A·g-1. Furthermore, the capacity retention can be as high as 99.6 % after 5000 cycles at a high current density of 5.0 A·g-1, showing superior long cycling stability. Importantly, the in-situ XRD patterns and ex-situ analysis revealed the structural changes and reaction mechanisms of the HBC650·V4 superstructure during Zn2+ insertion/extraction. Therefore, the HBC650·V4 superstructure prepared using Al-MOF exhibits the advanced AZIBs performance. The preparation of nano-MOF into multifunctional superstructures through innovative strategies will be development trend in this field, which opens a new way to design AZIBs cathode materials.

MXene-mediated Interfacial Growth of 2D-2D Heterostructured Nanomaterials as Cathodes for Zn-based Aqueous Batteries.

In this study, we introduce a novel approach for synthesizing two-dimensional (2D) MXene heterostructures featuring a sandwiched and cross-linked network structure. This method addresses the common issue of activity degradation in 2D nanomaterials caused by inevitable aggregation. By utilizing the distinct surface characteristics of MXene, we successfully induced the growth of various 2D nanomaterials on MXene substrates. This strategy effectively mitigates self-stacking defects and augments the exposure of surface areas. In particular, the obtained 2D-2D MXene@NiCo-layered double hydroxide (MH-NiCo) heterostructures exhibit enhanced structural stability, improved chemical reversibility, and heightened charge transfer efficiency, outperforming pure NiCo LDH. The aqueous MH-Ni4Co1//Zn@carbon cloth (MH-Ni4Co1//Zn@CC) battery demonstrates exceptional performance with a remarkable specific capacity of 0.61 mAh cm-2, maintaining 96.6 % capacitance after 2300 cycles. Additionally, it achieves an energy density of 1.047 mWh cm-2 and a power density of 32.899 mW cm-2. This research not only paves the way for new design paradigms in energy-related nanomaterials but also offers invaluable insights for the application and optimization of 2D-2D heterostructures in advanced electrochemical devices.

Recent progress of MOF-functionalized nanocomposites: From structure to properties.

Metal-organic frameworks (MOFs) are novel crystalline porous materials assembled from metal ions and organic ligands. The adaptability of their design and the fine-tuning of the pore structures make them stand out in porous materials. Furthermore, by integrating MOF guest functional materials with other hosts, the novel composites have synergistic benefits in numerous fields such as batteries, supercapacitors, catalysis, gas storage and separation, sensors, and drug delivery. This article starts by examining the structural relationship between the host and guest materials, providing a comprehensive overview of the research advancements in various types of MOF-functionalized composites reported to date. The review focuses specifically on four types of spatial structures, including MOFs being (1) embedded in nanopores, (2) immobilized on surface, (3) coated as shells and (4) assembled into hybrids. In addition, specific design ideas for these four MOF-based composites are presented. Some of them involve in situ synthesis method, solvothermal method, etc. The specific properties and applications of these materials are also mentioned. Finally, a brief summary of the advantages of these four types of MOF composites is given. Hopefully, this article will help researchers in the design of MOF composite structures.

Synergistic Effect of Oxygen Vacancy and High Porosity of Nano MIL-125(Ti) for Enhanced Photocatalytic Nitrogen Fixation.

This work reports that a low-temperature thermal calcination strategy was adopted to modulate the electronic structure and attain an abundance of surface-active sites while maintaining the crystal morphology. All the experiments demonstrate that the new photocatalyst nano MIL-125(Ti)-250 obtained by thermal calcination strategy has abundant Ti3+ induced by oxygen vacancies and high specific surface area. This facilitates the adsorption and activation of N2 molecules on the active sites in the photocatalytic nitrogen fixation. The photocatalytic NH3 yield over MIL-125(Ti)-250 is enhanced to 156.9 µmol g-1 h-1 , over twice higher than that of the parent MIL-125(Ti) (76.2 µmol g-1 h-1 ). Combined with density function theory (DFT), it shows that the N2 adsorption pattern on the active sites tends to be from "end-on" to "side-on" mode, which is thermodynamically favourable. Moreover, the electrochemical tests demonstrate that the high atomic ratio of Ti3+ /Ti4+ can enhance carrier separation, which also promotes the efficiency of photocatalytic N2 fixation. This work may offer new insights into the design of innovative photocatalysts for various chemical reduction reactions.

Synthesis of Conductive MOFs and Their Electrochemical Application.

Conductive metal-organic frameworks (MOFs) are a type of porous material. It consists of metal ions coordinated with highly conjugated organic ligands. The high density of carriers and orbital overlap contribute to the amazing conductivity. Additionally, conductive MOFs inherit the advantages of large specific surface area, structural diversity, and adjustable pore size from MOFs. These excellent properties have attracted many researchers to explore controllable synthesis and electrochemical applications over the past decade. This work provides an overview of the recent advances in the synthesis strategies of conductive MOFs and highlights their applications in electrocatalysis, supercapacitors, sensors, and batteries. Finally, the challenges faced by the synthesis and application of conductive MOFs are discussed, as well as the views on promising solutions for them are presented.

Metal Ion-controlled Growth of Different Metal-Organic Framework Micro/nanostructures for Enhanced Supercapacitor Performance.

We report a metal ion-modulated effective strategy to achieve different metal-organic framework (MOF) micro/nanostructures using different metal precursors like CoCl2 ⋅ 6H2 O, CoCl2 ⋅ 6H2 O and NiCl2 ⋅ 6H2 O, and NiCl2 ⋅ 6H2 O with pyridine-3,5-dicarboxylate (3,5-pdc). The structural characterizations confirm that different morphological structures, hollow microsphere, hierarchical nanoflower, and solid nanosphere are for Co-(3,5-pdc), Co0.19 Ni0.81 -(3,5-pdc), and Ni-(3,5-pdc), respectively. These different MOF micro/nanostructures correlate with the coordination ability of Co and Ni with 3,5-pdc. Benefitting from the synergistic effect of the alloying metal nodes of Co and Ni producing rapid and rich redox reactions and the hierarchical nanoflower with higher surface area enabling excellent ion kinetics, the Co0.19 Ni0.81 -(3,5-pdc) exhibits higher specific capacitance of 515 F g-1 /273 C g-1 at 0.5 A g-1 than that of Ni-(3,5-pdc) (290 F g-1 /153.7 C g-1 ) and Co-(3,5-pdc) (132 F g-1 /67 C g-1 ), good rate capability and cycling stability. Moreover, the asymmetric supercapacitor device (Co0.19 Ni0.81 -(3,5-pdc)//AC) assembled from Co0.19 Ni0.81 -(3,5-pdc) and activated carbon (AC) achieves a maximum energy density of 42.6 Wh kg-1 at a power density of 277.3 W kg-1 .

Mesoporous Iron Family Element (Fe, Co, Ni) Molybdenum Disulfide/Carbon Nanohybrids for High-Performance Supercapacitors.

As the demand for fuel continues to increase, the development of energy devices with excellent performance is crucial. Supercapacitors (SCs) are attracting attention for their advantages of high specific energy and a long cycle life. At present, the development of high-performance electrode materials is the main point for research and development of SCs. Transition metal sulfides have the advantages of a large interlayer space and high theoretical capacity, making them promising electrode materials. Herein, we reported a series of ultrathin mesoporous iron family element (Fe, Co, Ni) molybdenum disulfide (MxMo1-xS2/C, M = Fe, Co, and Ni) by a template method. The original monolayer mesoporous structure of MoS2/C was maintained, and accumulation and agglomeration of MoS2/C were avoided. Based on our investigations, the best performance was that of CoxMo1-xS2/C nanohybrids. Furthermore, the concentrations of Co and Mo ions were modulated to obtain the best performance, in which Mo and Co ions were released at 1:1, 1:2, and 1:3 ratios and they were named CoxMo1-xS2/C-1, CoxMo1-xS2/C-2, and CoxMo1-xS2/C-3, respectively. Overall, these materials represent a significant improvement and show promise as high-performance SC electrode materials due to their enhanced capacitance and stability. At a current density of 0.5 A g-1, CoxMo1-xS2/C-2 has the optimal specific capacitance of 184 F g-1. CoxMo1-xS2/C-2 as an SC electrode exhibited better reversible capacity and cycling stability than MoS2/C, which is an improvement over MoS2/C regarding reversible capacity and cycling stability.

Regulating Intramolecular Electron Transfer of Nickel-Based Coordinations through Ligand Engineering for Aqueous Batteries.

The integration of electronic effects into complexes for the construction of novel materials has not yet attracted significant attention in the field of energy storage. In the current study, eight one-dimensional (1D) nickel-based salicylic acid  complexes (Ni-XSAs, X = pH, pMe, pMeO, mMe, pBr, pCl, pF, and pCF3 ), are prepared by ligand engineering. The coordination environments in the Ni-XSAs are explored using X-ray absorption fine structure spectroscopy. The charge transfer of the complexes is modulated according to the difference in the electron-donating ability of the substituents, in combination with frontier orbital theory. Furthermore, density functional theory is used to investigate the effect of the substituent position on the electronic properties of the complexes. Ni-mMeSA exhibits better electrical conductivity than Ni-pMeSA. The electrochemical performance of Ni-mMeSA as an aqueous battery cathode is remarkably improved with a maximum energy density of 0.30 mWh cm-2 (125 Wh kg-1 ) and a peak power density of 33.72 mW cm-2 (14.03 kW kg-1 ). This study provides ideas for the application of new coordination chemistry in the field of energy materials science.

Enhanced Active Sites and Stability in Nano-MOFs for Electrochemical Energy Storage through Dual Regulation by Tannic Acid.

The limited active sites and poor acid-alkaline solution stability of metal-organic frameworks (MOFs), significantly limit their wider application. In this study, the acid property of tannic acid (TA) was used as an etchant to etch the surface-active sites. Subsequently, the further chelation of the protonated TA with the exposed metal active site can effectively protect the metal ions. Meanwhile, the TA provided a large amount of phenolic hydroxyl groups, which can greatly improve the stability of imidazolate-coordinated MOFs. The electrochemical test results indicated that the MOFs composite materials synthesized using this scheme had high specific capacitance and stability. And the mechanism of its electrochemical reaction process was explored through in situ X-ray diffraction (XRD) and theoretical calculations. In addition, the same treatment was carried out through a series of carboxyl-coordinated MOFs, which further confirmed the principle of this scheme to obtain a higher active site and stability. This paper explains the mechanism of functionalization of nano-MOFs by polyphenolic compounds, providing new ideas for the research of nano-MOFs.

Synthesis of 2D Co-MOF Nanosheets and Low-Temperature Calcination Activation for Lithium-Ion Batteries.

Metal-organic frameworks (MOFs), a novel type of porous crystalline material, have aroused widespread interest in lithium-ion batteries (LIBs). The design and preparation of MOF electrodes with a stable structure and excellent electrochemical performance are primary concerns for improving the capacity of LIBs. Among them, two-dimensional (2D) materials with larger specific surface areas, richer active sites, and higher aspect ratios have great potential. We adopted a facile approach to synthesize unique Co-MOF nanosheets with a 2D flaky morphology and a mesoporous structure. In addition, low-temperature calcination increases the specific surface area and improves the porosity to achieve mass transfer. Sample M2 delivers high specific capacities and long lives (1402.0 mA h g-1 after 100 cycles at 500 mA g-1 and 462.4 mA h g-1 after 300 cycles at 1.0 A g-1) when the calcination temperature is 200 °C. Significant improvements in the cycle life and stability are attributed to the 2D flaky structure of the M2 sample and the available low-temperature calcination activation, which provide a simple strategy for the fabrication of inexpensive and excellent anodes for LIBs.

Space-Confined Metal Ion Strategy for Carbon Materials Derived from Cobalt Benzimidazole Frameworks with High Desalination Performance in Simulated Seawater.

Various metal ions with different valence states (Mg2+ , Al3+ , Ca2+ , Ti4+ , Mn2+ , Fe3+ , Ni2+ , Zn2+ , Pb2+ , Ba2+ , Ce4+ ) are successfully confined in quasi-microcube shaped cobalt benzimidazole frameworks using a space-confined synthesis strategy. More importantly, a series of derived carbon materials that confine metal ions are obtained by high-temperature pyrolysis. Interestingly, the derived carbon materials exhibited electric double-layer and pseudocapacitance properties because of the presence of metal ions with various valence states. Moreover, the presence of additional metal ions within carbon materials may create new phases, which can accelerate Na+ insertion/extraction and thus increase electrochemical adsorption. Density functional theory results showed that carbon materials in which Ti ions are confined exhibit enhanced insertion/extraction of Na+ resulting from the presence of the characteristic anatase crystalline phases of TiO2 . The Ti-containing materials have an impressive desalination capacity (62.8 mg g-1 ) in capacitive deionization (CDI) applications with high cycling stability. This work provides a facile synthetic strategy for the confinement of metal ions in metal-organic frameworks and thus supports the further development of derived carbon materials for seawater desalination by CDI.

How About Vanadium-Based Compounds as Cathode Materials for Aqueous Zinc Ion Batteries?

Aqueous zinc-ion batteries (AZIBs) stand out among many monovalent/multivalent metal-ion batteries as promising new energy storage devices because of their good safety, low cost, and environmental friendliness. Nevertheless, there are still many great challenges to exploring new-type cathode materials that are suitable for Zn2+ intercalation. Vanadium-based compounds with various structures, large layer spacing, and different oxidation states are considered suitable cathode candidates for AZIBs. Herein, the research advances in vanadium-based compounds in recent years are systematically reviewed. The preparation methods, crystal structures, electrochemical performances, and energy storage mechanisms of vanadium-based compounds (e.g., vanadium phosphates, vanadium oxides, vanadates, vanadium sulfides, and vanadium nitrides) are mainly introduced. Finally, the limitations and development prospects of vanadium-based compounds are pointed out. Vanadium-based compounds as cathode materials for AZIBs are hoped to flourish in the coming years and attract more and more researchers' attention.

Co-Intercalation of Dual Charge Carriers in Metal-Ion-Confining Layered Vanadium Oxide Nanobelts for Aqueous Zinc-Ion Batteries.

Vanadium-based oxides with high theoretical specific capacities and open crystal structures are promising cathodes for aqueous zinc-ion batteries (AZIBs). In this work, the confined synthesis can insert metal ions into the interlayer spacing of layered vanadium oxide nanobelts without changing the original morphology. Furthermore, we obtain a series of nanomaterials based on metal-confined nanobelts, and describe the effect of interlayer spacing on the electrochemical performance. The electrochemical properties of the obtained Al2.65 V6 O13 ⋅ 2.07H2 O as cathodes for AZIBs are remarkably improved with a high initial capacity of 571.7 mAh ⋅ g-1 at 1.0 A g-1 . Even at a high current density of 5.0 A g-1 , the initial capacity can still reach 205.7 mAh g-1 , with a high capacity retention of 89.2 % after 2000 cycles. This study demonstrates that nanobelts confined with metal ions can significantly improve energy storage applications, revealing new avenues for enhancing the electrochemical performance of AZIBs.

Directional Growth of Conductive Metal-Organic Framework Nanoarrays along [001] on Metal Hydroxides for Aqueous Asymmetric Supercapacitors.

Metal-organic frameworks (MOFs) are promising electrochemical materials that possess large specific surface areas, high porosities, good adjustability, and high activities. However, many conventional MOFs exhibit poor conductivity, which hinders their application in electrochemistry. In recent years, conductive MOFs (cMOFs) have attracted a considerable attention. As an important transition metal hydroxide, Ni(OH)2 nanosheets exhibit a high theoretical specific capacitance and a high energy density but a poor electrical conductivity. In this study, we combined a typical cMOF(Ni-HHTP, HHTP = 2,3,6,7,10,11-hexahydroxybenzene) with Ni(OH)2 nanosheets and synthesized a series of Ni-HHTP@Ni(OH)2 nanoarrays. The composite materials exhibit a high electrical conductivity and ionic transfer efficiency and a good stability. Most importantly, our study reveals the chemical interaction between cMOFs and metal hydroxide composites and the relationship between facet exposure and the growth orientation of cMOFs. When Ni-HHTP@Ni(OH)2-2 was assembled as a positive electrode material in an aqueous asymmetric supercapacitor, 98% of the initial capacitance was maintained after 5000 cycles at a high current density of 3 A g-1. The findings of this study will provide meaningful insights into the design of cMOF composites combining other metal hydroxides.

In Situ Growth of Three-Dimensional MXene/Metal-Organic Framework Composites for High-Performance Supercapacitors.

In this study, we propose a versatile method for synthesizing uniform three-dimensional (3D) metal carbides, nitrides, and carbonitrides (MXenes)/metal-organic frameworks (MOFs) composites (Ti3 C2 TX /Cu-BTC, Ti3 C2 TX /Fe,Co-PBA, Ti3 C2 TX /ZIF-8, and Ti3 C2 TX /ZIF-67) that combine the advantages of MOFs and MXenes to enhance stability and improve conductivity. Subsequently, 3D hollow Ti3 C2 TX /ZIF-67/CoV2 O6 composites with excellent electron- and ion-transport properties derived from Ti3 C2 TX /ZIF-67 were synthesized. The specific capacitance of the Ti3 C2 TX /ZIF-67/CoV2 O6 electrode was 285.5 F g-1 , which is much higher than that of the ZIF-67 and Ti3 C2 TX /ZIF-67 electrode. This study opens a new avenue for the design and synthesis of MXene/MOF composites and complex hollow structures with tailorable structures and compositions for various applications.

Applications of Metal-Organic Frameworks in Water Treatment: A Review.

The ever-expanding scale of industry and agriculture has led to the gradual increase of pollutants (e.g., heavy metal ions, synthetic dyes, and antibiotics) in water resources, and the ecology and wastewater are grave problems that need to be solved urgently and has attracted widespread attention from the research community and industry in recent years. Metal-organic frameworks (MOFs) are a type of organic-inorganic hybrid material with a distinctive 3D network crystal structure. Lately, MOFs have made striking progress in the fields of adsorption, catalytic degradation, and biomedicine on account of their large specific surface and well-developed pore structure. This review summarizes the latest research achievements in the preparation of pristine MOFs, MOF composites, and MOF derivatives for various applications including the removal of heavy metal ions, organic dyes, and other harmful substances in sewage. Furthermore, the working mechanisms of utilizing adsorption, photocatalytic degradation, and membrane separation technologies are also briefly described for specific pollutants removal from sewage. It is expected that this review will provide inspiration and references for the synthesis of pristine MOFs as well as their composites and derivatives with excellent water treatment performance.