Construction of Bimetallic ZIF-Derived Co-Ni LDHs on the Surfaces of GO or CNTs with a Recyclable Method: Toward Reduced Toxicity of Gaseous Thermal Decomposition Products of Unsaturated Polyester Resin

Enhancing the Performance of 2D Ni-Fe Layered Double Hydroxides by Cabbage-Inspired Carbon Conjunction for Oxygen Evolution Reactions

Layered double hydroxide (LDH) nanosheets as one type of two-dimensional materials have garnered increasing attention in the field of oxygen evolution reaction (OER) in recent decades. To address the challenges associated with poor conductivity and limited electron and charge transfer capability in LDH materials, we have developed a straightforward one-pot synthesis method to successfully fabricate a composite material with a microstructure resembling cabbage, which encompasses NiFe-LDH and nanocarbon (referred as NiFe-LDH@C). Atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) revealed that the monolayer NiFe-LDH with a height of ~0.5-0.8 nm is uniformly distributed and closely bonded to the carbon support, leading to a significant enhancement in conductivity and facilitating faster electron and charge transfer. Moreover, the NiFe-LDH@C exhibits a substantial number of surface defect sites, which enhances the interaction with oxygen species. This dual enhancement in charge transfer and oxygen species-mediated transfer greatly improves the catalytic OER performance, which is further corroborated by theoretical calculations. Notably, the Ni10Fe6-LDH@C with the highest concentration of surface oxygen vacancies demonstrated superior water oxidation performance, surpassing commercially available RuO2 catalysts; an OER overpotential of 231 mV@10 mA cm-2 with a Tafel slope of 71 mV dec-1 was achieved.

Bio-based P-N flame retardant with ZIF-67 in-situ growth on flexible polyurethane foam with excellent fire safety performance

The wide application of flexible polyurethane foam (FPUF) poses a giant challenge to human society in terms of fire prevention and environmental pollution. To solve this problem, the lignocellulose-based P-N flame retardant (LFPN) has been developed using mechanochemical methods. It was found that FPUF treated using LFPN exhibited good flame retardancy, but suffered from high smoke generation and toxicity. The hollow dodecahedral ZIF-67 has been used for smoke suppression catalysis, but the agglomeration phenomenon makes it inefficient. Hence, in this study, the adhesive properties of polydopamine (PDA) were utilized to assist the in-situ growth of ZIF-67. The results showed that the total smoke release rate of the treated FPUF was reduced by 40.5%. The toxic gases, such as carbon monoxide (CO), hydrogen cyanide, etc., also showed the same decreasing trend. What's more, the catalytic effect of ZIF-67 itself and the synergistic effect with LFPN gave FPUF great flame retardant and smoke inhibition properties. This novel FPUF provides a new reference for achieving smoke suppression and toxicity reduction.

Enhancing char formation of flame retardant epoxy composites: Onigiri-like ZIF-67 modification with carboxymethyl β-cyclodextrin crosslinking

To enhance char formation of flame retardant epoxy (EP) composites, carboxymethyl β-cyclodextrin (CM-β-CD) is employed as an etchant for or ZIF-67 derivatives. In the early stage, etching plays a dominant role. The mismatch in size between CM-β-CD opening and ZIF-67 pore leads to the stacking of carboxyl cobalt complexes on the shell. When the reaction time is prolonged, crosslinking occurs between carboxyl and hydroxyl groups. Crosslinked CM-β-CD weakens and eventually stops the etching process. Triethyl phosphate (TEP), an additive to improve flame retardancy, is also absorbed on the shell in this one-pot synthesis. Herin, the synthesis of metal-organic framework (MOF) derivatives can impart multiple functions to MOF. This novel nanohybrid significantly improved flame retardancy of EP composites with only 2.0 wt% loading. The peak heat release rate (pHRR) and total smoke production (TSP) were reduced by 54.8 and 46.9%, respectively. The integrated multi-element system resulted in an expanded and reinforced char layer. This study proposes a simple and precise method for controlling the structure of MOF-carbohydrate hybrids through competition between chemical reactions.

Metal-organic framework-derived bird's nest-like capsules for phosphorous small molecules towards flame retardant polyurea composites

The loading treatment of phosphorus flame retardants can mitigate their migration and plasticization effect. However, designing suitable carriers has remained a great challenge. Herein, two kinds of Co-based isomers, namely cobalt-cobalt layered double hydroxides (CoCo-LDH) and cobalt basic carbonate (CBC), were synthesized by employing ZIF-67 as a self-template, assemblied into two different nanostructures namely multi-yolk@shell CBC@CoCo-LDH (m-CBC@LDH) and solid CBC nanoparticles by facilely tuning the reaction time, which were employed as carriers, respectively. Subsequently, triphenyl phosphate (TPP)-loaded m-CBC@LDH (m-CBC-P@LDH) was prepared using TPP as the guest. The m-CBC@LDH with high specific surface area and hollow structure exhibited up to more than 30% of TPP loading. The peak of heat release rate and total heat release of polyurea composite blended with 5 wt% m-CBC-P@LDH reduced by 41.7% and 20.6% respectively, and the mechanical properties were less damaged. This work complements a feasible approach for preparation of metal-organic frameworks-derived flame retardant carriers.

Composites Filled with Metal Organic Frameworks and Their Derivatives: Recent Developments in Flame Retardants

Polymer matrix is vulnerable to fire hazards and needs to add flame retardants to enhance its performance and make its application scenarios more extensive. At this stage, it is more necessary to add multiple flame-retardant elements and build a multi-component synergistic system. Metal organic frameworks (MOFs) have been studied for nearly three decades since their introduction. MOFs are known for their structural advantages but have only been applied to flame-retardant polymers for a relatively short period of time. In this paper, we review the development of MOFs utilized as flame retardants and analyze the flame-retardant mechanisms in the gas phase and condensed phase from the original MOF materials, modified MOF composites, and MOF-derived composites as flame retardants, respectively. The effects of carbon-based materials, phosphorus-based materials, nitrogen-based materials, and biomass on the flame-retardant properties of polymers are discussed in the context of MOFs. The construction of MOF multi-structured flame retardants is also introduced, and a variety of MOF-based flame retardants with different morphologies are shown to broaden the ideas for subsequent research.

Design of ZIF-67 nanoflake derived NiCo-LDH/rGO hybrid nanostructures for aqueous symmetric supercapattery application under alkaline condition

Well-defined polyhedral ZIF-67 metal-organic frameworks (MOFs) are usually synthesized using methanol as solvent. In this work, methanol is replaced with deionized water as a solvent to synthesize ZIF-67 MOFs with unique nanoflake morphology. The ZIF-67 nanoflakes are synthesized directly byin situmethod on reduced graphene oxide (rGO) to obtain ZIF-67/rGO-xprecursors which are further transformed into NiCo-layered double hydroxide nanocomposites (NiCo-LDH/rGO-x,x = 10, 30, 50 and 90 mg of rGO). The NiCo-LDH/rGO-xnanostructured composites are found to be excellent materials for battery type supercapacitor (supercapattery) applications. Among these samples, the NiCo-LDH/rGO-30 composite gives maximum specific capacity of 829 C g^-1(1658 F g^-1) at a current density of 1 A g^-1and high rate capability. The as fabricated 2-electrode symmetric Swagelok deviceNiCo-LDH/rGO-30NiCo-LDH/rGO-30delivered a high energy density of 49.2 Wh kg^-1and a power density of 4511 W kg^-1, and enabled us to glow red, blue and white LED bulbs using three coin cells. The device can show good capacity retention even after 3000 continuous charge-discharge cycles. The NiCo-LDH/rGO-30 composite,in situderived from ZIF-67 MOF in combination with optimal amount of rGO, is an excellent material to deliver both high energy density and high power density in supercapattery devices.

Core-Shell ZIF67@ZIF8 Modified with Phytic Acid as an Effective Flame Retardant for Improving the Fire Safety of Epoxy Resins

Despite many important industrial applications, epoxy resin (EP) suffers from high flammability and toxicity emission, extremely hampering their applications. To circumvent the problem, core-shell structured ZIF67@ZIF8 is successfully synthesized and further functionalized with phytic acid (PA) to obtain PA-ZIF67@ZIF8 hybrids. Then, it is used as an efficient flame retardant to reduce the fire risk of EP. The fire test results show a significant reduction in heat and smoke production. Compared with EP, incorporating 5.0 wt % PA-ZIF67@ZIF8 into EP, the peak heat release rate, total heat release, and peak carbon monoxide production are dramatically reduced by 42.2, 33.0, and 41.5%, respectively. Moreover, the EP/PA-ZIF67@ZIF8 composites achieve the UL-94 V-0 rating and the limiting oxygen index increases by 29.3%. These superior fire safety properties are mainly attributed to the excellent dispersion and the catalytic effect of metal oxide and phosphorus-containing compounds. This work provides an efficient strategy for preparing a promising ZIF-based flame retardant for enhancing flame retardancy and smoke toxicity suppression of EP.

MOF-derived 3D petal-like CoNi-LDH array cooperates with MXene to effectively inhibit fire and toxic smoke hazards of FPUF

The toxic smoke produced by the combustion of flexible polyurethane foam (FPUF) may not only caused casualties, but also polluted the environment. Here, double metal hydroxide derived from ZIF-67 (MOF-LDH) modified Ti₃C₂T_X (Ti₃C₂T_X@MOF-LDH) was innovatively designed to solve the serious smoke and fire hazards of FPUF. The FPUF nanocomposite containing 6 wt% Ti₃C₂Tx@MOF-LDH achieved a 16.1% reduction in total smoke production (TSP) along with 22.2% reduction in peak smoke production rate (PSPR), which greatly reduced the hazard of smoke. At the same time, toxic gases, such as carbon monoxide (CO), carbon dioxide (CO₂), and aromatic compounds, showed the same reduction pattern. In addition, the heat release of FPUF nanomaterials was also suppressed. In particular, the FPUF/Ti₃C₂Tx@MOF-LDH 3.0 achieved 110.4% and 76.1% increase in compressive strength and tensile strength, respectively, confirming the effective mechanical enhancement. Therefore, this work provided a new reference for the preparation of high-performance FPUF nanocomposites with low smoke, low fire hazard and excellent mechanical properties.

Cooperative Effect of ZIF-67-Derived Hollow NiCo-LDH and MoS2 on Enhancing the Flame Retardancy of Thermoplastic Polyurethane

In this work, a novel three-dimensional (3D) hollow nickel-cobalt layered double hydroxide (NiCo-LDH) was synthesized using zeolitic imidazole framework-67 (ZIF-67) as a template, and then utilized to functionalize molybdenum disulfide (NiCo-LDH/MoS2) via electrostatic force. Flame retardant thermoplastic polyurethane (TPU) composites were prepared by the melt blending method. Compared to pure TPU, NiCo-LDH/MoS2 filled TPU composite was endowed with a decrease of 30.9% and 55.7% of the peak heat release rate (PHRR) and the peak smoke production rate (PSPR), respectively. Furthermore, the addition of NiCo-LDH/MoS2 can significantly improve the thermal stability and char yield of the TPU composite. The catalytic carbonization effect and dilution effect of NiCo-LDH, and the barrier effect of MoS2 nanosheets enable TPU composites with excellent flame retardancy and toxic gas suppression ability.

Exploration on structural rules of highly efficient flame retardant unsaturated polyester resins

Owing to the lack of research on structure-activity relationship and interaction mechanism between unsaturated polyester resins (UPR) and flame retardants, it has been a big challenge to prepare high-efficiency flame retardants for UPR in industry. In this research, to explore structural rules of high-efficiency flame retardants, several polymeric flame retardants were synthesized with varied main-chain, side-chain, phosphorus valence states and contents of flame retardant elements. The thermal stabilities of flame retardants and UPR composites were firstly assessed. It has been found the interaction existed between flame retardants and UPR, through transesterification reaction and β scission pathway in polyester and polystyrene chains. With only 15 wt% of PCH₃-S, UPR composites can reach V0 rating in UL-94. The PHRR and THR values can be maximumly decreased by 71.66 % and 77.67 %, with 20 wt% of PB-S. It has been found flame retardants with sulfone group and + 3 valence state of phosphorus in molecular backbone can release SO₂ and phosphorus containing compounds in gaseous phase, which diluted fuel fragments and catalyzed H⋅ and HO⋅ radical removal. The mechanism for improved flame retardancy of UPR composites with various polymeric flame retardants were discussed in detail. Some general rules for highly efficient flame retardant UPR can be summarized: First, gaseous phase flame retardant mechanism plays the major role in improvement of flame retardant performance of UPR composites; Second, the combination of + 3 valence state of phosphorus structures, higher phosphorus contents and sulfone groups effectively improves the flame retardant efficiency of flame retardants.

Hierarchical porous Ni, Fe-codoped Co-hydroxide arrays derived from metal–organic-frameworks for enhanced oxygen evolution

The multi metal organic frameworks can be successfully transformed into hierarchical porous Ni,Fe-codoped Co-hydroxide nanowire array catalysts with excellent electrocatalytic performance for the OER in alkaline media.

Construction of hierarchical layered hydroxide grown in situ on carbon tubes derived from a metal-organic framework for asymmetric supercapacitors

Electrode materials are very important for the performance of supercapacitors (SCs). Therefore, preparation of hybrid electrode materials is an effective way to develop high-performance SCs. We firstly design and prepare metal organic framework (MOF) derived carbon nanotubes as the core skeleton to support the shell of a nickel gallium layered hydroxide nanosheet (NiGa-LDH). MOF derived carbon nanomaterials have high conductivity and a large specific surface area, which can promote electron transfer and improve the agglomeration of LDH. The deposited LDH can provide high specific capacitance and the layered structure can further enhance the reaction site. The NiGa-LDH@CNT-500@CC has an excellent specific capacitance of 2580 F g-1 at 1 A g-1 and a high capacitance retention rate of 83.3% at 5 A g-1 due to the synergistic effect of two materials. The assembled NiGa-LDH@CNT-500@CC//carbon NS asymmetric supercapacitor (ASC) has an operating voltage of 1.6 V and a high energy density of 52 W h kg-1 at a power density of 952 W kg-1. Therefore, the core-shell structure composed of LDH and carbon nanomaterials provides an effective way for the design of high-performance electrodes.

Recent Progress on Metal-Organic Framework and Its Derivatives as Novel Fire Retardants to Polymeric Materials

High flammability of polymers has become a major issue which has restricted its applications. Recently, highly crystalline materials and metal-organic frameworks (MOFs), which consisted of metal ions and organic linkers, have been intensively employed as novel fire retardants (FRs) for a variety of polymers (MOF/polymer). The MOFs possessed abundant transition metal species, fire-retardant elements and potential carbon source accompanied with the facile tuning of the structure and property, making MOF, its derivatives and MOF hybrids promising for fire retardancy research. The recent progress and strategies to prepare MOF-based FRs are emphasized and summarized. The fire retardancy mechanisms of MOF/polymer composites are explained, which may guide the future design for efficient MOF-based FRs. Finally, the challenges and prospects related to different MOF-based FRs are also discussed and aim to provide a fast and holistic overview, which is beneficial for researchers to quickly get up to speed with the latest development in this field.

Fire-safe unsaturated polyester resin nanocomposites based on MAX and MXene: a comparative investigation of their properties and mechanism of fire retardancy

Recently, MXene, as a novel graphene-like nanomaterial, has been found to bestow good flame-retardant and smoke-suppression properties to polymers mainly due to the physical barrier effect of its 2D nanosheets. However, a comprehensive investigation of its chemical components as an important factor for these properties has not been conducted to date. To address this issue, herein, MXene (Ti₃C₂T_x) and MAX (Ti₃AlC₂) were introduced into unsaturated polyester resin (UPR) at same amounts (2.0 wt%). Their structures are different (multilayer for MXene and bulk for MAX), but the chemical components are similar; therefore, it is important to study the influence of the chemical components of MXene on the fire-safety properties of polymers. In this study, 2 wt% MAX was added to the UPR, and the peak heat release rate (PHRR), the total smoke production (TSP), and carbon monoxide production (COP) of the resulting material were reduced by 11.04%, 19.08%, and 15.79%, respectively; these findings demonstrate the important role of the chemical components of MAX: Ti exerts a catalytic attenuation effect on the UPR nanocomposites during combustion. Moreover, a better fire-safety property of the MXene/UPR nanocomposites (reduction of PHRR by 29.56%, TSP by 25.26%, and COP by 31.58%) than that of the MAX/UPR nanocomposites was achieved, which was due to the physical barrier effect of the MXene nanosheets. This study verifies that in addition to the physical barrier effect, the chemical components play a very important role in the fire safety enhancement of MXene-based nanocomposites.

Nanoreinforcements of Two-Dimensional Nanomaterials for Flame Retardant Polymeric Composites: An Overview

Polymer materials are ubiquitous in daily life. While polymers are often convenient and helpful, their properties often obscure the fire hazards they may pose. Therefore, it is of great significance in terms of safety to study the flame retardant properties of polymers while still maintaining their optimal performance. Current literature shows that although traditional flame retardants can satisfy the requirements of polymer flame retardancy, due to increases in product requirements in industry, including requirements for durability, mechanical properties, and environmental friendliness, it is imperative to develop a new generation of flame retardants. In recent years, the preparation of modified two-dimensional nanomaterials as flame retardants has attracted wide attention in the field. Due to their unique layered structures, two-dimensional nanomaterials can generally improve the mechanical properties of polymers via uniform dispersion, and they can form effective physical barriers in a matrix to improve the thermal stability of polymers. For polymer applications in specialized fields, different two-dimensional nanomaterials have potential conductivity, high thermal conductivity, catalytic activity, and antiultraviolet abilities, which can meet the flame retardant requirements of polymers and allow their use in specific applications. In this review, the current research status of two-dimensional nanomaterials as flame retardants is discussed, as well as a mechanism of how they can be applied for reducing the flammability of polymers.

Hierarchical Structure: An effective Strategy to Enhance the Mechanical Performance and Fire Safety of Unsaturated Polyester Resin

It is still a big challenge to prepare polymer/layered double hydroxide (LDH) composites with high performance, due to the strong agglomeration tendency of LDHs in the polymeric matrix. In this study, to avoid the agglomerated situation, the orientated LDH nanosheets were vertically grown on a ramie fabric surface, which was then embedded in unsaturated polyester resin (UPR) through the combination method of hand lay-up and vacuum bag. Due to the increased contact area and the restricted interfacial slip in the in-plane direction, the hierarchically LDH-functionalized ramie fabrics (denoted as Textile@LDH) significantly enhanced the mechanical performance of UPR composites. Then, the phosphorus- and silicon-containing coating (PSi) was used for the further improvement of the interfacial adhesion. The tensile strength of UPR/Textile@LDH@PSi composites increased by 121.67%, compared to that of neat UPR. The reinforcement mechanism was studied through analyzing the surface nano/microstructure and wetting properties of the raw and modified textiles, as well as the interfacial interaction between the ramie fabrics and UPR. Meanwhile, the thermal stability, thermal conductivity, and flame-retardant performance of ramie-reinforced UPR composites were improved. Particularly, as-prepared hierarchical Textile@LDH@PSi inhibited the heat release during the combustion process of fabric-reinforced UPR composites, and the peak heat release rate and total heat release values decreased by 36.56 and 47.57%, respectively, compared with the neat UPR/Textile composites. The suppression mechanism was further explored by analyzing the microstructure and chemical compositions of char residues. This research paved a feasible solution to improve the poor dispersion of LDHs in polymers and prepared the high-performance UPR composites with multifunctional applications.

Rapid Synthesis of Oxygen-Rich Covalent C2N (CNO) Nanosheets by Sacrifice of HKUST-1: Advanced Metal-Free Nanofillers for Polymers

A covalent oxygen-rich C₂N (CNO) network derived from metal-organic framework (HKUST-1) was innovatively synthesized by a rapid and green microwave irradiation method. This method can produce CNO multilayers efficiently, which paves a way for practical application of the nanosheets. Structural characterization and synthesis processes of CNO nanosheets were investigated to further understand the key role of HKUST-1. The as-prepared CNO has a layered feature, which theoretically favors to improve flame retardancy and mechanical performance of polymers. Desirable results were obtained as expected: the fire safety, antitensile, and impact resistance of polylactic acid (PLA) were prominently enhanced after adding CNO nanosheets, which can be attributed to the excellent dispersion and compatibility. PLA/CNO nanocomposite was self-distinguished at 2 wt % content of CNO, whereas the tensile strength was increased more than 36% compared with that of pure PLA, as well as the impact strength. This work broadens the application fields of CNO and endows it a possibility of actual application.