Hierarchical Carbon Network Composites Derived from ZIF-8 for High-Efficiency Microwave Absorption.

Metal-organic framework (MOF)-derived composites have gained wide attention due to their specific structures and enhanced performance. In this work, we prepared carbon nanotubes with Fe nanoparticles connected to two-dimensional (2D) hierarchical carbon network composites via a low-pressure gas-solid reaction strategy. Specifically, the three-dimensional (3D) networks derived from ZIF-8 exploited the carbon nanotubes with the function of charge modulation. Meanwhile, we utilized the interconnected 2D nanostructures to optimize impedance matching and facilitate multiple scattering, ultimately improving the overall microwave absorption performance. Furthermore, based on the well-designed structures, the composites prepared at 800 °C (Fe-N-C@CNTs-800) achieved the best reflection loss (RL) of -58.5 dB, thereby obtaining superior microwave absorption performance. Overall, this study provides a good groundwork for further investigation into the modification and dimension design of novel hierarchical microwave absorbers.

Graphite-Based Composite Anodes with C-O-Nb Heterointerfaces Enable Fast Lithium Storage.

To better satisfy the increasing demands for electric vehicles, it is crucial to develop fast-charging lithium-ion batteries (LIBs). However, the fast-charging capability of commercial graphite anodes is limited by the sluggish Li+ insertion kinetics. Herein, we report a synergistic engineering of uniform nano-sized T-Nb2 O5 particles on graphite (Gr@Nb2 O5 ) with C-O-Nb heterointerfaces, which prevents the growth and aggregation of T-Nb2 O5 nanoparticles. Through detailed theoretical calculations and pair distribution function analysis, the stable existence of the heterointerfaces is proved, which can accelerate the electron/ion transport. These heterointerfaces endow Gr@Nb2 O5 anodes with high ionic conductivity and excellent structural stability. Consequently, Gr@10-Nb2 O5 anode, where the mass ratio of T-Nb2 O5 /graphite=10/100, exhibits excellent cyclic stability and incredible rate capabilities, with 100.5 mAh g-1 after 10000 stable cycles at an ultrahigh rate of 20 C. In addition, the synergistic Li+ storage mechanism is revealed by systematic electrochemical characterizations and in situ X-ray diffraction. This work offers new insights to the reasonable design of fast-charging graphite-based anodes for the next generation of LIBs.

Dual Single-Atom Moieties Anchored on N-Doped Multilayer Graphene As a Catalytic Host for Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are promising for energy storage, especially in the era of carbon neutrality. Nonetheless, the sluggish kinetics of converting soluble lithium polysulfides into solid lithium sulfide impedes its development. In this work, we design Fe and Co dual single-atom moieties anchored on N-doped multilayer graphene (FeCoNGr) as a catalytic sulfur cathode host for Li-S batteries. With an efficient catalytic role in converting soluble lithium polysulfides into solid Li2S, the FeCoNGr-based Li-S cell demonstrates a capacity of 878.7 mA h g-1 at 0.2 C and retains 77.4% of the initial value after 100 cycles. The first and retained capacities are ∼1.7 and ∼1.8 times those of the NGr (without single atoms)-based cell, respectively. Theoretical calculations reveal that the Fe-N4 moiety has a higher binding energy toward low-order lithium polysulfides, while the Co-N4 moiety has a higher binding energy toward high-order lithium polysulfides. The efficient catalytic conversion of soluble lithium polysulfides into solid lithium sulfides of FeCoNGr plays important roles in outperforming NGr. This work enhances our knowledge on the tandem role of dual single-atom moieties and confirmed the high catalytic efficiency of single-atom catalysts in Li-S batteries.

A "MOFs plus ZIFs" Strategy toward Ultrafine Co Nanodots Confined into Superficial N-Doped Carbon Nanowires for Efficient Oxygen Reduction.

N-doped carbon-confined transition metal nanocatalysts display efficient oxygen reduction reaction (ORR) performance comparable to commercial Pt/C electrocatalysts because of their efficient charge transfer from metal atoms to active N sites. However, the sheathed active sites inside the electrocatalysts and relatively large-size confined metal particles greatly restrict their activity improvement. Here, we develop a facile and efficient "MOFs plus ZIFs" synthesis strategy to successfully construct ultrafine sub-5 nm Co nanodots confined into superficial N-doped carbon nanowires (Co@C@NC) via a well-designed synthesis process. The unique synthesis mechanism is based on low-pressure vapor superassembly of thin zeolitic imidazolate framework (ZIF) coatings on metal-organic framework substrates. During the successive pyrolysis, the preferential formation of the robust N-doped carbon shell from the ZIF-67 shell keeps the core morphology without shrinkage and limits the growth of Co nanodots. Benefiting from this architecture with accessible and rich active N sites on the surface, stable carbon confined architecture, and large surface area, the Co@C@NC exhibits excellent ORR performance, catching up to commercial Pt/C. Density functional theory demonstrates that the confined Co nanodots efficiently enhance the charge density of superficial active N sites by interfacial charge transfer, thus accelerating the ORR process.

Universal Approach to Fabricating Graphene-Supported Single-Atom Catalysts from Doped ZnO Solid Solutions.

Single-atom catalysts (SACs) have attracted widespread interest for many catalytic applications because of their distinguishing properties. However, general and scalable synthesis of efficient SACs remains significantly challenging, which limits their applications. Here we report an efficient and universal approach to fabricating a series of high-content metal atoms anchored into hollow nitrogen-doped graphene frameworks (M-N-Grs; M represents Fe, Co, Ni, Cu, etc.) at gram-scale. The highly compatible doped ZnO templates, acting as the dispersants of targeted metal heteroatoms, can react with the incoming gaseous organic ligands to form doped metal-organic framework thin shells, whose composition determines the heteroatom species and contents in M-N-Grs. We achieved over 1.2 atom % (5.85 wt %) metal loading content, superior oxygen reduction activity over commercial Pt/C catalyst, and a very high diffusion-limiting current (6.82 mA cm-2). Both experimental analyses and theoretical calculations reveal the oxygen reduction activity sequence of M-N-Grs. Additionally, the superior performance in Fe-N-Gr is mainly attributed to its unique electron structure, rich exposed active sites, and robust hollow framework. This synthesis strategy will stimulate the rapid development of SACs for diverse energy-related fields.

Solvent-Free Synthesis of Uniform MOF Shell-Derived Carbon Confined SnO2 /Co Nanocubes for Highly Reversible Lithium Storage.

Tin dioxide (SnO2 ) has attracted much attention in lithium-ion batteries (LIBs) due to its abundant source, low cost, and high theoretical capacity. However, the large volume variation, irreversible conversion reaction limit its further practical application in next-generation LIBs. Here, a novel solvent-free approach to construct uniform metal-organic framework (MOF) shell-derived carbon confined SnO2 /Co (SnO2 /Co@C) nanocubes via a two-step heat treatment is developed. In particular, MOF-coated CoSnO3 hollow nanocubes are for the first time synthesized as the intermediate product by an extremely simple thermal solid-phase reaction, which is further developed as a general strategy to successfully obtain other uniform MOF-coated metal oxides. The as-synthesized SnO2 /Co@C nanocubes, when tested as LIB anodes, exhibit a highly reversible discharge capacity of 800 mAh g-1 after 100 cycles at 200 mA g-1 and excellent cycling stability with a retained capacity of 400 mAh g-1 after 1800 cycles at 5 A g-1 . The experimental analyses demonstrate that these excellent performances are mainly ascribed to the delicate structure and a synergistic effect between Co and SnO2 . This facile synthetic approach will greatly contribute to the development of functional metal oxide-based and MOF-assisted nanostructures in many frontier applications.