Nanocellulose-based porous lightweight materials with flame retardant properties: A review

This review discusses the development and application of nanocellulose (NC)-aerogels, a sustainable and biodegradable biomaterial, with enhanced flame retardant (FR) properties. NC-aerogels combine the excellent physical and mechanical properties of NC with the low density and thermal conductivity of aerogels, making them promising for thermal insulation and other fields. However, the flammability of NC-aerogels limits their use in some applications, such as electromagnetic interference shielding, oil/water separation, and flame-resistant textiles. The review covers the design, fabrication, modification, and working mechanism of NC porous materials, focusing on how advanced technologies can impart FR properties into them. The review also evaluates the FR performance of NC-aerogels by employing widely recognized tests, such as the limited oxygen index, cone calorimeter, and UL-94. The review also explores the integration of innovative and eco-friendly materials, such as MXene, metal-organic frameworks, dopamine, lignin, and alginate, into NC-aerogels, to improve their FR performance and functionality. The review concludes by outlining the potential, challenges, and limitations of future research on FR NC-aerogels, identifying the obstacles and potential solutions, and understanding the current progress and gaps in the field.

Revisiting the phosphonium salt chemistry for P-doped carbon synthesis: toward high phosphorus contents and beyond the phosphate environment

The introduction of phosphorus and nitrogen atoms in carbo-catalysts is a common way to tune the electronic density, and thereby the reactivity, of the material, as well as to introduce surface reactive sites. Numerous environments are reported for the N atoms, but the P-doping chemistry is less explored and focuses on surface POx groups. A one-step synthesis of P/N-doped carbonaceous materials is presented here, using affordable and industrially available urea and tetrakis(hydroxymethyl)phosphonium chloride (THPC) as the N and P sources, respectively. In contrast to most of the synthetic pathways toward P-doped carbonaceous materials, the THPC precursor only displays P-C bonds along the carbon backbone. This resulted in unusual phosphorus environments for the materials obtained from direct thermal treatment of THPC-urea, presumably of type C-P-N according to 31P NMR and XPS. Alternatively, the in situ polymerization and calcination of the precursors were run in calcium chloride hydrate, used as a combined reaction medium and porogen agent. Following this salt-templating strategy led to particularly high phosphorus contents (up to 18 wt%), associated with porosities up to 600 m2 g-1. The so-formed P/N-doped porous materials were employed as metal-free catalysts for the mild oxidative dehydrogenation of N-heterocycles to N-heteroarenes at room temperature and in air.

A Superior Two-Dimensional Phosphorus Flame Retardant: Few-Layer Black Phosphorus

The usage of flame retardants in flammable polymers has been an effective way to protect both lives and material goods from accidental fires. Phosphorus flame retardants have the potential to be follow-on flame retardants after halogenated variants, because of their low toxicity, high efficiency and compatibility. Recently, the emerging allotrope of phosphorus, two-dimensional black phosphorus, as a flame retardant has been developed. To further understand its performance in flame-retardant efficiency among phosphorus flame retardants, in this work, we built model materials to compare the flame-retardant performances of few-layer black phosphorus, red phosphorus nanoparticles, and triphenyl phosphate as flame-retardant additives in cellulose and polyacrylonitrile. Aside from the superior flame retardancy in polyacrylonitrile, few-layer black phosphorus in cellulose showed the superior flame-retardant efficiency in self-extinguishing, ~1.8 and ~4.4 times that of red phosphorus nanoparticles and triphenyl phosphate with similar lateral size and mass load (2.5~4.8 wt%), respectively. The char layer in cellulose coated with the few-layer black phosphorus after combustion was more continuous and smoother than that with red phosphorus nanoparticles, triphenyl phosphate and blank, and the amount of residues of cellulose coated with the few-layer black phosphorus in thermogravimetric analysis were 10 wt%, 14 wt% and 14 wt% more than that with red phosphorus nanoparticles, triphenyl phosphate and blank, respectively. In addition, although exothermic reactions, the combustion enthalpy changes in the few-layer black phosphorus (-127.1 kJ mol^-1) are one third of that of red phosphorus nanoparticles (-381.3 kJ mol^-1). Based on a joint thermodynamic, spectroscopic, and microscopic analysis, the superior flame retardancy of the few-layer black phosphorus was attributed to superior combustion reaction suppression from the two-dimensional structure and thermal nature of the few-layer black phosphorus.

High-Efficiency Lithium-Ion Transport in a Porous Coordination Chain-Based Hydrogen-Bonded Framework

Fast and selective Li⁺ transport in solid plays a key role for the development of high-performance solid-state electrolytes (SSEs) of lithium metal batteries. Porous compounds with tunable Li⁺ transport pathways are promising SSEs, but the comprehensive performances in terms of Li⁺ transport kinetics, electrochemical stability window, and interfacial compatibility are difficult to be achieved simultaneously. Herein, we report a porous coordination chain-based hydrogen-bonded framework (NKU-1000) containing arrayed electronegative sites for Li⁺ transport, exhibiting a superior Li⁺ conductivity of 1.13 × 10^-3 S cm^-1, a high Li⁺ transfer number of 0.87, and a wide electrochemical window of 5.0 V. The assembled solid-state battery with NKU-1000-based SSE shows a high discharge capacity with 94.4% retention after 500 cycles and can work over a wide temperature range without formation of lithium dendrites, which derives from the linear hopping sites that promote a uniformly high-rate Li⁺ flux and the flexible structure that can buffer the structural variation during Li⁺ transport.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O₂ ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O₂  reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m²  g^-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg²⁺ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 10¹⁹  sites g_FeNC ^-1  and a record 52% FeN_x electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

In Situ Constructing a Catalytic Shell for Sulfur Cathode via Electrochemical Oxidative Polymerization

Sluggish multiphase reaction kinetics and severe shuttle effect of lithium polysulfides (LiPSs) are two major challenges facing lithium-sulfur (Li-S) batteries, which largely prevent them from becoming a reality. Herein, a shell with catalytic function for sulfur cathode is in situ constructed through an ingenious electrochemical oxidative polymerization strategy by introducing hexafluorocyclotriphosphazene (HFPN) as additives, which suppresses the shuttle effect and promotes efficient sulfur conversion. The shell with abundant heteroatoms effectively confines polysulfides to the cathode matrix by chemically interacting with them to eliminate capacity degradation. Moreover, the shell exhibits high catalytic activities, which turns Li₂S₍₂₎ into an activated state and facilitates its dissociation. The functionalized shell substantially advances the performance of Li-S batteries, thanks to efficient lithium-ion transportation and abundant adsorption-catalytic sites. As a result, Li-S batteries demonstrate superb resistance to self-discharge, ultrastable cycle performance, and greatly enhanced rate capability. Impressively, the batteries show an ultralow capacity decay rate of 0.034% throughout 700 cycles at 2C. They deliver a capacity of 517 mAh g^-1 even at a 4C rate, exhibiting relieved electrochemical polarization and excellent sulfur utilization. This work provides an ingenious strategy to construct adsorption-catalytic nets for next-generation Li-S batteries with enhanced lifespan and electrochemical performance.

"Robust-Soft" Anisotropic Nanofibrillated Cellulose Aerogels with Superior Mechanical, Flame-Retardant, and Thermal Insulating Properties

Advanced thermal insulation materials with low thermal conductivity and robustness derived from regenerative resources are badly needed for building energy conservation. Among them, nanofibrillated cellulose aerogels have huge application potential in the field of thermal insulation materials, but it is still a challenge to prepare cellulose aerogels of excellent comprehensive properties in a simple way. Herein, we demonstrate a unidirectional freeze-drying strategy to develop a novel "robust-soft" anisotropic nanofibrillated cellulose aerogel (NFC-Si-T) by integrating nanofibrillated cellulose (NFC) and Si-O-Si bonding networks under the catalytic dehydration of p-toluenesulfonic acid (TsOH). The anisotropic structure endows the NFC-Si-T with high flexibility that can be easily bent or even tied with a knot, and in addition, it possesses high Young's modulus (1-3.66 MPa) that can resist the compression weight of 10,000 times of its own weight without deformation. Furthermore, the NFC-Si-T aerogels exhibit anisotropic thermal insulation performances with a low average thermal conductivity (0.028-0.049 W m^-1 K^-1). More importantly, the limited oxygen index of the NFC-Si-T reaches up to 42.6-51%, showing excellent flame-retardant performance. Therefore, the "robust-soft" anisotropic NFC-Si-T aerogels can be used as an advanced thermal insulation material for building thermal insulation applications.

Strong Chemical Interaction between Lithium Polysulfides and Flame-Retardant Polyphosphazene for Lithium-Sulfur Batteries with Enhanced Safety and Electrochemical Performance

The shuttle effect of lithium polysulfides (LiPS) and potential safety hazard caused by the burning of flammable organic electrolytes, sulfur cathode, and lithium anode seriously limit the practical application of lithium-sulfur (Li-S) batteries. Here, a flame-retardant polyphosphazene (PPZ) covalently modified holey graphene/carbonized cellulose paper is reported as a multifunctional interlayer in Li-S batteries. During the discharge/charge process, once the LiPS are generated, the as-obtained flame-retardant interlayer traps them immediately through the nucleophilic substitution reaction between PPZ and LiPS, effectively inhibiting the shuttling effect of LiPS to enhance the cycle stability of Li-S batteries. Meanwhile, this strong chemical interaction increases the diffusion coefficient for lithium ions, accelerating the lithiation reaction with complete inversion. Moreover, the as-obtained interlayer can be used as a fresh 3D current collector to establish a flame-retardant "vice-electrode," which can trap dissolved sulfur and absorb a large amount of electrolyte, prominently bringing down the flammability of the sulfur cathode and electrolyte to improve the safety of Li-S batteries. This work provides a viable strategy for using PPZ-based materials as strong chemical scavengers for LiPS and a flame-retardant interlayer toward next-generation Li-S batteries with enhanced safety and electrochemical performance.