Recent Advances in Carbon-Based Metal-Free Electrocatalysts

High-Yield Continuous-Flow Synthesis of Spheroidal C60@Graphene Composites as Supercapacitors

Graphene spheres confining fullerene C60 are quantitatively formed under high-shear and continuous-flow processing using a vortex fluidic device (VFD). This involves intense micromixing a colloidal suspension of graphite in DMF and an o-xylene solution of C60 at room temperature in the absence of surfactants and other auxiliary substances. The diameters of the composite spheres, C60@graphene, can be controlled with size distributions ranging from 1.5 to 3.5 μm, depending on the VFD operating parameters, including rotational speed, flow rate, relative ratio of C60 to graphite, and the concentration of fullerene. An electrode of the composite material has high cycle stability, with a high areal capacitance of 103.4 mF cm-2, maintaining its capacitances to 24.7 F g-1 and 86.4 mF cm-2 (83.5%) at a high scan rate of 100 mV s-1.

Silicon Carbide as a Protective Layer to Stabilize Si-Based Anodes by Inhibiting Chemical Reactions

Developing a practical silicon-based (Si-based) anode is a precondition for high-performance lithium-ion batteries. However, the chemical reactivity of the Si renders it liable to be consumed, which must be completely understood for it to be used in practical battery systems. Here, a fresh and fundamental mechanism is proposed for the rapid failure of Si-based materials. Silicon can chemically react with lithium hexafluorophosphate (LiPF₆) to constantly generate lithium hexafluorosilicate (Li₂SiF₆) aggregates during cycling. In addition, nanocarbon coated on silicon acts as a catalyst to accelerate such detrimental reactions. By taking advantage of the high strength and toughness of silicon carbide (SiC), a SiC layer is introduced between the inner silicon and outer carbon layers to inhibit the formation of Li₂SiF₆. The side reaction rate decreases significantly due to the increase in the activation energy of the reaction. Si@SiC@C maintains a specific capacity of 980 mAh g^-1 at a current density of 1 A g^-1 after 800 cycles with an initial Coulombic efficiency over 88.5%. This study will contribute to improved design of Si-based anode for high-performance Li-ion batteries.

Self-Sacrificial Template-Directed Vapor-Phase Growth of MOF Assemblies and Surface Vulcanization for Efficient Water Splitting

Direct use of metal-organic frameworks (MOFs) with robust pore structures, large surface areas, and high density of coordinatively unsaturated metal sites as electrochemical active materials is highly desirable (rather than using as templates and/or precursors for high-temperature calcination), but this is practically hindered by the poor conductivity and low accessibility of active sites in the bulk form. Herein, a universal vapor-phase method is reported to grow well-aligned MOFs on conductive carbon cloth (CC) by using metal hydroxyl fluorides with diverse morphologies as self-sacrificial templates. Specifically, by further partially on-site generating active Co₃ S₄ species from Co ions in the echinops-like Co-based MOF (EC-MOF) through a controlled vulcanization approach, the resulting Co₃ S₄ /EC-MOF hybrid exhibits much enhanced electrocatalytic performance toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with overpotentials of 84 and 226 mV required to reach a current density of 10 mA cm^-2 , respectively. Density functional theory (DFT) calculations and experimental results reveal that the electron transfer between Co₃ S₄ species and EC-MOF can decrease the electron density of the Co d-orbital, resulting in more electrocatalytically optimized adsorption properties for Co. This study will open up a new avenue for designing highly ordered MOF-based surface active materials for various electrochemical energy applications.

Versatile Nanoemulsion Assembly Approach to Synthesize Functional Mesoporous Carbon Nanospheres with Tunable Pore Sizes and Architectures

Functional mesoporous carbons have attracted significant scientific and technological interest owning to their fascinating and excellent properties. However, controlled synthesis of functional mesoporous carbons with large tunable pore sizes, small particle size, well-designed functionalities, and uniform morphology is still a great challenge. Herein, we report a versatile nanoemulsion assembly approach to prepare N-doped mesoporous carbon nanospheres with high uniformity and large tunable pore sizes (5-37 nm). We show that the organic molecules (e.g., 1,3,5-trimethylbenzene, TMB) not only play an important role in the evolution of pore sizes but also significantly affect the interfacial interaction between soft templates and carbon precursors. As a result, a well-defined Pluronic F127/TMB/dopamine nanoemulsion can be facilely obtained in the ethanol/water system, which directs the polymerization of dopamine into highly uniform polymer nanospheres and their derived N-doped carbon nanospheres with diversely novel structures such as smooth, golf ball, multichambered, and dendritic nanospheres. The resultant uniform dendritic mesoporous carbon nanospheres show an ultralarge pore size (∼37 nm), small particle size (∼128 nm), high surface area (∼635 m² g^-1), and abundant N content (∼6.8 wt %), which deliver high current density and excellent durability toward oxygen reduction reaction in alkaline solution.

Strongly coupled ultrasmall-Fe7C3/N-doped porous carbon hybrids for highly efficient Zn–air batteries

This report presents a simple method to produce an ultrasmall-Fe₇C₃/N-doped porous carbon hybrid (u-Fe₇C₃@NC) as an excellent catalyst for oxygen reduction reaction and zinc–air batteries.