Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

Deciphering the Thermal and Electrochemical Behaviors of Dual Redox-Active Iron Croconate Violet Coordination Complexes

Interest in coordination compounds based on non-innocent ligands (NILs) for electrochemical energy storage has risen in the last few years. We have focused our attention on an overlooked redox active linker, croconate violet, which has not yet been addressed in this field although closely related to standard NILs such as catecholate and tetracyanoquinodimethane. Two anionic complexes consisting of Fe(II) and croconate violet (-2) with balancing potassium cations were isolated and structurally characterized. By a combination of in situ and ex situ techniques (powder and single-crystal X-ray diffraction, infrared, and ⁵⁷Fe Mössbauer spectroscopies), we have shown that their dehydration occurs through complex patterns, whose reversibility depends on the initial crystal structure but that the structural rearrangements around the iron cations occur without any oxidation. While electrochemical studies performed in solution clearly show that both the organic and inorganic parts can be reversibly addressed, in the solid state, poor charge storage capacities were initially measured, mainly due to the solubilization of the solids in the electrolyte. By optimizing the formulation of the electrode and the composition of the electrolyte, a capacity of >100 mA h g^-1 after 10 cycles could be achieved. This suggests that this family of redox active linkers deserves to be investigated for solid-state electrochemical energy storage, although it requires the solving of the issues related to the solubilization of the derived coordination compounds.

Polysulfide Catalytic Materials for Fast-Kinetic Metal-Sulfur Batteries: Principles and Active Centers

Benefiting from the merits of low cost, ultrahigh-energy densities, and environmentally friendliness, metal-sulfur batteries (M-S batteries) have drawn massive attention recently. However, their practical utilization is impeded by the shuttle effect and slow redox process of polysulfide. To solve these problems, enormous creative approaches have been employed to engineer new electrocatalytic materials to relieve the shuttle effect and promote the catalytic kinetics of polysulfides. In this review, recent advances on designing principles and active centers for polysulfide catalytic materials are systematically summarized. At first, the currently reported chemistries and mechanisms for the catalytic conversion of polysulfides are presented in detail. Subsequently, the rational design of polysulfide catalytic materials from catalytic polymers and frameworks to active sites loaded carbons for polysulfide catalysis to accelerate the reaction kinetics is comprehensively discussed. Current breakthroughs are highlighted and directions to guide future primary challenges, perspectives, and innovations are identified. Computational methods serve an ever-increasing part in pushing forward the active center design. In summary, a cutting-edge understanding to engineer different polysulfide catalysts is provided, and both experimental and theoretical guidance for optimizing future M-S batteries and many related battery systems are offered.

Electron-Conductive Metal-Organic Framework, Fe(dhbq)(dhbq = 2,5-Dihydroxy-1,4-benzoquinone): Coexistence of Microporosity and Solid-State Redox Activity

Redox-active metal-organic frameworks (MOFs) have great potential for use as cathode materials in lithium-ion batteries (LIBs) with large capacities because the organic ligands can undergo multiple-electron redox processes. However, most MOFs are electrical insulators, limiting their application as electrode materials. Here, we report an electron-conductive MOF with a 2,5-dihydroxy-1,4-benzoquinone (dhbq) ligand, Fe(dhbq). This compound had an electrical conductivity of 5 × 10^-6 S cm^-1 at room temperature due to d-π interactions between the Fe ion and the ligand and the permanent microporosity. Fe(dhbq) had an initial discharge capacity of 264 mA h g^-1, corresponding to the two-electron redox process of dhbq.

An Electrically Conducting Li-Ion Metal-Organic Framework

Metal-organic frameworks (MOFs) have emerged as an important, yet highly challenging class of electrochemical energy storage materials. The chemical principles for electroactive MOFs remain, however, poorly explored because precise chemical and structural control is mandatory. For instance, no anionic MOF with a lithium cation reservoir and reversible redox (like a conventional Li-ion cathode) has been synthesized to date. Herein, we report on electrically conducting Li-ion MOF cathodes with the generic formula Li₂-M-DOBDC (wherein M = Mg²⁺ or Mn²⁺; DOBDC^4- = 2,5-dioxido-1,4-benzenedicarboxylate), by rational control of the ligand to transition metal stoichiometry and secondary building unit (SBU) topology in the archetypal CPO-27. The accurate chemical and structural changes not only enable reversible redox but also induce a million-fold electrical conductivity increase by virtue of efficient electronic self-exchange facilitated by mix-in redox: 10^-7 S/cm for Li₂-Mn-DOBDC vs 10^-13 S/cm for the isoreticular H₂-Mn-DOBDC and Li₂-Mg-DOBDC, or the Mn-CPO-27 compositional analogues. This particular SBU topology also considerably augments the redox potential of the DOBDC^4- linker (from 2.4 V up to 3.2 V, vs Li⁺/Li⁰), a highly practical feature for Li-ion battery assembly and energy evaluation. As a particular cathode material, Li₂-Mn-DOBDC displays an average discharge potential of 3.2 V vs Li⁺/Li⁰, demonstrates excellent capacity retention over 100 cycles, while also handling fast cycling rates, inherent to the intrinsic electronic conductivity. The Li₂-M-DOBDC material validates the concept of reversible redox activity and electronic conductivity in MOFs by accommodating the ligand's noncoordinating redox center through composition and SBU design.

Metal-Organic Frameworks and Their Derivatives: Designing Principles and Advances toward Advanced Cathode Materials for Alkali Metal Ion Batteries

Metal-organic frameworks (MOFs) and their derivatives have attracted enormous attention in the field of energy storage, due to their high specific surface area, tunable structure, highly ordered pores, and uniform metal sites. Compared with the wide research of MOFs and their related materials on anode materials for alkali metal ion batteries, few works are on cathode materials. In this review, design principles for promoting the electrochemical performance of MOF-related materials in terms of component/structure design, composite fabrication, and morphology engineering are presented. By summarizing the advancement of MOFs and their derivatives, Prussian blue and its analogs, and MOF surface coating, challenges and opportunities for future outlooks of MOF-related cathode materials are discussed.

Tetrathiafulvalene-Based Metal-Organic Framework as a High-Performance Anode for Lithium-Ion Batteries

Metal-organic frameworks (MOFs) have aroused great interest as lithium-ion battery (LIB) electrode materials. In this work, we first report that a pristine three-dimensional tetrathiafulvalene derivatives (TTFs)-based zinc MOF, formulated [Zn₂(py-TTF-py)₂(BDC)₂]·2DMF·H₂O (1) (py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene and H₂BDC = terephthalic acid), can work as a high-performance electrode material for rechargeable LIBs. The TTFs-Zn-MOF 1 electrode displayed a high discharge specific capacity of 1117.4 mA h g^-1 at a current density of 200 mA g^-1 after 150 cycles along with good reversibility. After undergoing elevated discharging/charging rates, the electrode showed superior lithium storage performance in the extreme case of 20 A g^-1 and could finally recover the capability when the current rate was back to 200 mA g^-1. Particularly, specific capacities of 884.2, 513.8, and 327.8 mA h g^-1 were reached at high current densities of 5, 10, and 20 A g^-1 after 180, 175, and 300 cycles along with good reversibility, respectively. Such an excellent performance is first reported for the LIB anode materials. TTFs-Zn-MOF 2, namely, [Zn₂(py-TTF-py) (BDC)₂]·DMF·2H₂O (2), was prepared as a contrast to explore the relationship between the structures of the electrode materials and the electrochemical properties. Based on the structural analysis of 1 and 2 and ex situ X-ray photoelectron spectroscopy, the TTF moiety and the twofold TTF pillar play a key role in the excellent electrochemical performance. The full cell of MOF 1 with NMC 622 delivered the capacity of 131.9 mA h g^-1 at 100 mA g^-1 with the Coulombic efficiency of 99.45% after 70 cycles and exhibited the tolerance to high-current operation.

Na3V2(PO4)3/Porous Carbon Skeleton Embellished with ZIF-67 for Sodium-Ion Storage

Sodium super ionic conductor (NASICON) Na₃V₂(PO₄)₃ (NVP) is a type of promising cathode material for sodium-ion batteries (SIBs) with superior structural integrity, high energy density, and fast diffusion of sodium ions. However, NVP suffers from intrinsically low electrical conductivity, which results in poor rate performance. Herein, we report on the outstanding cathode performance of Na₃V₂(PO₄)₃ (NVP@C-ZIF67) wrapped by the ZIF-67-derived carbon, which was prepared by sol-gel method and solid-phase method. Electrochemical measurements show an initial discharge-specific capacity of 135 mA h g^-1 at 1 C, and the discharge capacity maintains 82 mA h g^-1 after 1000 cycles at 10 C. The results indicate the improvement in electrical conductivity and electrochemical performance due to the Co doping from ZIF-67. Moreover, we calculate the diffusion coefficient of sodium ions by the cyclic voltammetry (CV, D_Na⁺ = 1.521 × 10^-11 and 2.3484 × 10^-11 cm² s^-1 for charging and discharging, respectively) and the galvanostatic intermittent titration technique (GITT, D_Na⁺ ranges from 10^-11-10^-15 cm² s^-1). The exceptional performance is ascribed to the excellent structural stability and outstanding electrical conductivity of NVP modified by porous carbon skeleton and ZIF-67-derived carbon.