Nitrogen-Doped Carbon Aerogels Prepared by Direct Pyrolysis of Cellulose Aerogels Derived from Coir Fibers Using an Ammonia–Urea System and Their Electrocatalytic Performance toward the Oxygen Reduction Reaction

One-Pot Synthesis of Cellulose-Based Carbon Aerogel Loaded with TiO2 and g-C3N4 and Its Photocatalytic Degradation of Rhodamine B

A cellulose-based carbon aerogel (CTN) loaded with titanium dioxide (TiO2) and graphitic carbon nitride (g-C3N4) was prepared using sol-gel, freeze-drying, and high-temperature carbonization methods. The formation of the sol-gel was carried out through a one-pot method using refining papermaking pulp, tetrabutyl titanate, and urea as raw materials and hectorite as a cross-linking and reinforcing agent. Due to the cross-linking ability of hectorite, the carbonized aerogel maintained a porous structure and had a large specific surface area with low density (0.0209 g/cm3). The analysis of XRD, XPS, and Raman spectra revealed that the titanium dioxide (TiO2) and graphitic carbon nitride (g-C3N4) were uniformly distributed in the CTN, while TEM and SEM observations demonstrated the uniformly distributed three-dimensional porous structure of CTN. The photocatalytic activity of the CTN was determined according to its ability to degrade rhodamine B. The removal rate reached 89% under visible light after 120 min. In addition, the CTN was still stable after five reuse cycles. The proposed catalyst exhibits excellent photocatalytic performance under visible light conditions.

Controlling N-Doping Nature at Carbon Aerogels from Biomass for Enhanced Oxygen Reduction in Seawater Batteries

Pyridinic N-type doped at carbon has been known to have better electrocatalytic activity toward the oxygen reduction reaction (ORR) than the others. Herein, we proposed to prepare pyridinic N doped at carbon aerogels (CaA) derived from biomass, i.e., coir fiber (CF) and palm empty fruit bunches (PEFBs), by adjusting the pyrolysis temperature during carbonization of the biomass-based-cellulose aerogels. The cellulose aerogels were prepared by the ammonia-urea system as the cellulose solvent, in which ammonia also acted as a N source for doping and urea as the cellulose cross-linker. The as-prepared cellulose aerogels were directly pyrolyzed to produce N-doped CaA. It was found that the type of N doping is dominated by pyrrolic N at pyrolysis temperature of 600 °C, pyridinic N at 700 °C, and graphitic N at 800 °C. The pyridinic N exhibited better performance as an electrocatalyst for the ORR than pyrrolic N and graphitic N. The ORR using pyridinic N follows the four-electron pathway, which quantitatively implies a more electrochemically stable process. When used as a cathode for the Mg-air battery using a 3.5% NaCl electrolyte, the pyridinic N CaA exhibited excellent performance by giving a cell voltage of approximately 1.1 V and delivered a high discharge capacity of 411.64 mA h g-1 for CF and 492.64 mA h g-1 for PEFB corresponding to an energy density of 464.23 and 529.49 mW h g-1, respectively.

Hydrophobic Modification of Sulfonated Carbon Aerogels from Coir Fibers To Enhance Their Catalytic Performance for Esterification

The hydrophilicity of sulfonic acid-functionalized solid catalysts tends to accelerate the deactivation of the catalyst for chemical reactions where water is produced during the process. In this work, we proposed a hydrophobic carbon aerogel acid catalyst derived from coir fibers by a sulfonation-hydrophobization route via the diazo reduction method. Sulfonation using the diazo reduction method offers some advantages such as the process takes only a few minutes and the modified surface can be easily modified further to be hydrophobic. The carbon aerogel was produced by direct pyrolysis of cellulose aerogels derived from coir fibers using an NH4OH-urea method and then sulfonated and hydrophobized using sulfanilic acid and 4-tert-butylaniline (TBA), respectively. The carbon aerogel exhibited a very high surface area (2624.93-3911.05 m2 g-1), which provides a lot of number of sites for sulfonate groups (2.30-2.70 mmol g-1). The water contact angle of the sulfonated catalyst after hydrophobization ranged from 70 to 115°, depending on the mass ratio of the TBA-to-solid catalyst. The hydrophobic catalyst exhibited better catalytic performance toward esterification of acetic acid with ethanol. A conversion of 65-74% could be achieved in a brief time using the hydrophobic catalyst. The conversions were much higher than that obtained by the unmodified hydrophilic catalyst. Our study offers a strategy to tune the surface hydrophobicity of the sulfonated solid acid catalyst to match for specific chemical reactions.

A Hybrid of the Fe4N-Fe Heterojunction Supported on N-Doped Carbon Nanobelts and Ketjen Black Carbon as a Robust High-Performance Electrocatalyst

Herein, an extremely simple l-alanine-assisted pyrolysis method was proposed for the construction of a novel hierarchically porous hybrid of Fe₄N-Fe supported on N-doped carbon nanobelts and Ketjen black carbon (denoted as Fe₄N-Fe@N-C/N-KB). It has been found that the participation of l-alanine in pyrolysis can dramatically increase the total pyridinic-N/graphitic-N content in Fe₄N-Fe@N-C/N-KB, which is peculiarly conducive to the enhancement of ORR performance. The in-site formation of the Fe₄N-Fe heterojunction via the thermal reduction and decomposition of Fe₃N as well as the introduction of cheap KB can significantly improve the ORR performance. As a result, the activity, durability, and methanol tolerance of this hybrid are comprehensively better than those of commercial 20 wt % Pt/C, promising future applications in practical devices. Density functional theory calculations disclose that the highly improved ORR activity of Fe₄N-Fe@N-C/N-KB also benefits from the favorable electron penetration and excellent electronic conductivity between the Fe₄N nanoparticles and the N-incorporated carbon frameworks.

Urea-Mediated Monoliths Made of Nitrogen-Enriched Mesoporous Carbon Nanosheets for High-Performance Aqueous Zinc Ion Hybrid Capacitors

Aqueous zinc ion hybrid capacitors (aZHCs) are of great potential for large-scale energy storage and flexible wearable devices, of which the specific capacity and energy density need to be further enhanced for practical applications. Herein, a urea-mediated foaming strategy is reported for the efficient synthesis of monoliths consisting of nitrogen-enriched mesoporous carbon nanosheets (NPCNs) by prefoaming drying a solution made of polyvinylpyrrolidone, zinc nitrate, and urea at low temperatures, foaming and annealing at high temperatures, and subsequent acid etching. NPCNs have a large lateral size of ≈40 µm, thin thickness of ≈55 nm, abundant micropores and mesopores (≈3.8 nm), and a high N-doping value of 9.7 at.%. The NPCNs as the cathode in aZHCs provide abundant zinc storage sites involving both physical and chemical adsorption/desorption of Zn²⁺  ions, and deliver high specific capacities of 262 and 115 mAh g^-1 at 0.2 and 10 A g^-1 , and a remarkable areal capacity of ≈0.5 mAh cm^-2  with a mass loading of 5.3 mg cm^-2 , outperforming most carbon cathodes reported thus far. Moreover, safe and flexible NPCNs based quasi-solid-state devices are fabricated, which can withstand drilling and mechanical bending, suggesting their potential applications in wearable devices.

Preparation of rGO/MnO2 Composites through Simultaneous Graphene Oxide Reduction by Electrophoretic Deposition

We report the preparation of manganese dioxide (MnO₂) nanoparticles and graphene oxide (GO) composites reduced by an electrophoretic deposition (EPD) process. The MnO₂ nanoparticles were prepared by the electrolysis of an acidic KMnO₄ solution using an alternating monopolar arrangement of a multiple-electrode system. The particles produced were γ-MnO₂ with a rod-like morphology and a surface area of approximately 647.2 m²/g. The GO particles were produced by the oxidation of activated coconut shell charcoal using a modified Hummers method. The surface area of the GO produced was very high, with a value of approximately 2525.9 m²/g. Fourier transform infrared spectra indicate that a significant portion of the oxygen-containing functional groups was removed from the GO by electrochemical reduction during the EPD process after sufficient time following deposition of the GO. The composite obtained by the EPD process was composed of reduced graphene oxide (rGO) and γ-MnO₂ and exhibited excellent electrocatalytic activity toward the oxygen reduction reaction following a two-electron transfer mechanism. This approach opens the possibility for assembling rGO composites in an efficient and effective manner for electrocatalysis.

Recent Developments in Carbon-Based Nanocomposites for Fuel Cell Applications: A Review

Carbon-based nanocomposites have developed as the most promising and emerging materials in nanoscience and technology during the last several years. They are microscopic materials that range in size from 1 to 100 nanometers. They may be distinguished from bulk materials by their size, shape, increased surface-to-volume ratio, and unique physical and chemical characteristics. Carbon nanocomposite matrixes are often created by combining more than two distinct solid phase types. The nanocomposites that were constructed exhibit unique properties, such as significantly enhanced toughness, mechanical strength, and thermal/electrochemical conductivity. As a result of these advantages, nanocomposites have been used in a variety of applications, including catalysts, electrochemical sensors, biosensors, and energy storage devices, among others. This study focuses on the usage of several forms of carbon nanomaterials, such as carbon aerogels, carbon nanofibers, graphene, carbon nanotubes, and fullerenes, in the development of hydrogen fuel cells. These fuel cells have been successfully employed in numerous commercial sectors in recent years, notably in the car industry, due to their cost-effectiveness, eco-friendliness, and long-cyclic durability. Further; we discuss the principles, reaction mechanisms, and cyclic stability of the fuel cells and also new strategies and future challenges related to the development of viable fuel cells.