Porous Ti₃C₂T_<i>x</i> MXene for Ultrahigh-Rate Sodium-Ion Storage with Long Cycle Life

The development of anode materials remains a challenge to satisfy the requirements of sodium-ion storage for large-scale energy-storage applications, which is ascribed to the low kinetics of ionic/electron transfer of electrode materials. Here we show that the controlled anisotropic assembly of highly conductive Ti₃C₂T_<i>x</i> MXene nanosheets to form a porous structure can enhance the sodium-ion storage kinetics. At high current densities of 1 and 10 A g^–1, the porous Ti₃C₂T_<i>x</i> electrode delivered capacities of 166 and 124 mA h g^–1, respectively. Even at an extremely high current density of 100 A g^–1, a capacity of 24 mA h g^–1 could be achieved. The porous Ti₃C₂T_<i>x</i> electrode also exhibited a long cycle life that can be extended to 1000 cycles with no capacity decay at a current density of 1 A g^–1. This work demonstrates successful control of the Ti₃C₂T_<i>x</i> architecture to push electrochemical sodium-ion storage closer to large-scale applications and is expected to shed light on the rational utilization of the outstanding properties of MXenes by controlling their microscopic assembly

Nitrogen-Doped Graphene Nanosheets as Metal-Free Catalysts for Aerobic Selective Oxidation of Benzylic Alcohols

This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene latticepyridinic N, pyrrolic N, and graphitic Nthe graphitic sp² N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol^–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol^–1) proceeding on the catalyst Ru/Al₂O₃ reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp² N–O₂ adduct transition state, which can oxidize alcohols directly to aldehydes without any byproduct, including H₂O₂ and carboxylic acids, may be a key element step. Our results advance graphene chemistry and open a window to study the graphitic sp² nitrogen catalysis

Microwave-assisted Synthesis of Mesoporous Co₃O₄ Nanoflakes for Applications in Lithium Ion Batteries and Oxygen Evolution Reactions

Mesoporous Co₃O₄ nanoflakes with an interconnected architecture were successfully synthesized using a microwave-assisted hydrothermal and low-temperature conversion method, which exhibited excellent electrochemical performances as anode materials in lithium ion batteries and as catalysts in the oxygen evolution reaction (OER). Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) observations showed the unique interconnected and mesoporous structure. When employed as anode materials for lithium ion batteries, mesoporous Co₃O₄ nanoflakes delivered a high specific capacity of 883 mAh/g at 0.1C current rate and stable cycling performances even at higher current rates. Post-mortem analysis of <i>ex situ</i> FESEM images revealed that the mesoporous and interconnected structure had been well maintained after long-term cycling. The mesoporous Co₃O₄ nanoflakes also showed both OER active properties and good catalytic stability. This could be attributed to both the stability of unique mesoporous structure and highly reactive facets