Hybrid Carbon Nanotubes-Graphene Nanostructures: Modeling, Formation, Characterization

Cold Cathodes with Two-Dimensional van der Waals Materials

Two-dimensional van der Waals materials could be used as electron emitters alone or stacked in a heterostructure. Many significant phenomena of two-dimensional van der Waals field emitters have been observed and predicted since the landmark discovery of graphene. Due to the wide variety of heterostructures that integrate an atomic monolayer or multilayers with insulator nanofilms or metallic cathodes by van der Waals force, the diversity of van der Waals materials is large to be chosen from, which are appealing for further investigation. Until now, increasing the efficiency, stability, and uniformity in electron emission of cold cathodes with two-dimensional materials is still of interest in research. Some novel behaviors in electron emission, such as coherence and directionality, have been revealed by the theoretical study down to the atomic scale and could lead to innovative applications. Although intensive emission in the direction normal to two-dimensional emitters has been observed in experiments, the theoretical mechanism is still incomplete. In this paper, we will review some late progresses related to the cold cathodes with two-dimensional van der Waals materials, both in experiments and in the theoretical study, emphasizing the phenomena which are absent in the conventional cold cathodes. The review will cover the fabrication of several kinds of emitter structures for field emission applications, the state of the art of their field emission properties and the existing field emission model. In the end, some perspectives on their future research trend will also be given.

Renewable Carbon Materials as Electrodes for High-Performance Supercapacitors: From Marine Biowaste to High Specific Surface Area Porous Biocarbons

Waste, in particular, biowaste, can be a valuable source of novel carbon materials. Renewable carbon materials, such as biomass-derived carbons, have gained significant attention recently as potential electrode materials for various electrochemical devices, including batteries and supercapacitors. The importance of renewable carbon materials as electrodes can be attributed to their sustainability, low cost, high purity, high surface area, and tailored properties. Fish waste recovered from the fish processing industry can be used for energy applications and prioritizing the circular economy principles. Herein, a method is proposed to prepare a high surface area biocarbon from glycogen extracted from mussel cooking wastewater. The biocarbon materials were characterized using a Brunauer-Emmett-Teller surface area analyzer to determine the specific surface area and pore size and by scanning electron microscopy coupled with energy-dispersive X-ray analysis, Raman analysis, attenuated total reflectance Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrochemical characterization was performed using a three-electrode system, utilizing a choline chloride-based deep eutectic solvent (DES) as an eco-friendly and sustainable electrolyte. Optimal time and temperature allowed the preparation of glycogen-based carbon materials, with a specific surface area of 1526 m² g^-1, a pore volume of 0.38 cm³ g^-1, and an associated specific capacitance of 657 F g^-1 at a current density of 1 A g^-1, at 30 °C. The optimal material was scaled up to a two-electrode supercapacitor using a DES-based solid-state electrolyte (SSE@DES). This prototype delivered a maximum capacitance of 703 F g^-1 at a 1 A g^-1 of current density, showing 75% capacitance retention over 1000 cycles, delivering the highest energy density of 0.335 W h kg^-1 and power density of 1341 W kg^-1. Marine waste can be a sustainable source for producing nanoporous carbon materials to be incorporated as electrode materials in energy storage devices.

Tapered Optical Fiber Sensor Coated with Single-Walled Carbon Nanotubes for Dye Sensing Application

The present study aimed to improve the optical sensing performance of tapered optical fiber sensors toward aqueous Rhodamine B solution of different concentrations by applying single-walled carbon nanotubes (SWCNTs). The functional coating was formed on the surface of the tapered optical fiber sensor using an aerosol layer-by-layer deposition method. Before deposition, the SWCNTs were processed with multistage liquid-phase treatment in order to form a stable dispersion. The effect of SWCNT treatment was investigated through Raman spectroscopy. The deposition of 220 layers caused a reduction of up to 60% of the initial optical power of radiation propagating through the optical fiber core. The optical fiber sensor coated with SWCNTs demonstrated significantly higher sensitivity compared to a non-coated sensor in the range of 2-32 mg/L of Rhodamine B concentration in an aqueous solution. The experimental results demonstrated that the sensitivity was increased 10 times from 32 (mg/L)^-1, for the non-coated sensor, up to 317 (mg/L)^-1 after SWCNT coating deposition. Moreover, the SWCNT-coated sensor demonstrated high repeatability that allowed for the evaluation of the concentration regardless of the previously analyzed dye concentration.

Chemically Induced Compatible Interface in Pyrolyzed Bacterial Cellulose/Graphene Sandwich for Electrochemical Energy Storage

Herein, a three-step approach toward a multi-layered porous PBC/graphene sandwich has been developed, in which the chemical bonding interactions have been successfully enhanced via esterification between the layers of pyrolyzed bacterial cellulose (PBC) and graphene. Such a chemically induced compatible interface has been demonstrated to contribute significantly to the mass transfer efficiency when the PBC/graphene sandwich is deployed as electrode material for both supercapacitors and lithium-sulfur batteries. The high specific capacitance of the supercapacitors has been increased by three times, to 393 F g^-1 at 0.1 A g^-1. A high initial discharge specific capacity (~1100 mAhg^-1) and high coulombic efficiency (99% after 300 cycles) of the rPG/S-based lithium-sulfur batteries have been achieved.