Controllable Nitrogen Doping of High-Surface-Area Microporous Carbons Synthesized from an Organic–Inorganic Sol–Gel Approach for Li–S Cathodes

High-surface-area microporous carbons with controllable nitrogen doping were prepared via a novel organic–inorganic sol–gel approach, using phenolic resol and hexamethoxymethyl melamine (HMMM) as carbon precursors, and partially hydrolyzed tetraethoxysilane as silica template. The pore structures of microporous carbons were completely replicated from a thin silica framework and could be tailored greatly by changing the organic/inorganic ratio. The nitrogen atoms doped into the carbon framework were issued from high-nitrogen-content HMMM precursors, and the nitrogen content could be adjusted in a wide range by changing the phenolic resol/HMMM ratio. Moreover, the porous structure and nitrogen content could be simultaneously controlled, allowing the preparation of a series of microporous carbons with almost the same microstructures (BET surface areas of ca.1900 m²·g^–1and pore volumes of ca. 1.2 cm³·g^–1, and the same pore size distributions) but with different nitrogen contents (0–12 wt %). These results provided a general method to synthesize nitrogen-doped microporous carbons and allowed these materials to serve as a model system to illustrate the role of nitrogen content on the performance of the carbons. When used as the supports for sulfur cathodes, only an appropriate nitrogen content of ca. 6.3 wt % was found to effectively improve sulfur utilization and cycle life of the sulfur cathodes. The resulting sulfur cathodes could deliver an outstanding reversible discharge capacity of 1054 mAh·g^–1 at 0.5 C after 100 cycles

Rational Design of High-Surface-Area Carbon Nanotube/Microporous Carbon Core–Shell Nanocomposites for Supercapacitor Electrodes

All-carbon-based carbon nanotube (CNT)/microporous carbon core–shell nanocomposites, in which a CNT as the core and high-surface-area microporous carbon as the shell, have been prepared by in situ resorcinol–formaldehyde resin coating of CNTs, followed by carbonization and controlled KOH activation. The obtained nanocomposites have very high Brunauer–Emmett–Teller surface areas (up to 1700 m²/g), narrow pore size distribution (<2 nm), and 1D tubular structure within a 3D entangled network. The thickness of the microporous carbon shell can be easily tuned from 20 to 215 nm by changing the carbon precursor/CNT mass ratio. In such a unique core–shell structure, the CNT core could mitigate the key issue related to the low electronic conductivity of microporous carbons. On the other hand, the 1D tubular structure with a short pore-pathway micropore as well as a 3D entangled network could increase the utilization degree of the overall porosity and improve the electrode kinetics. Thus, these CNT/microporous carbon core–shell nanocomposites exhibit a great potential as an electrode material for supercapacitors, which could deliver high specific capacitance of 237 F/g, excellent rate performance with 75% maintenance from 0.1 to 50 A/g, and high cyclability in H₂SO₄ electrolyte. Moreover, the precisely controlled microporous carbon shells may allow them to serve as excellent model systems for microporous carbons, in general, to illustrate the role of the pore length on the diffusion and kinetics inside the micropores

Free-Standing <i>T</i>‑Nb₂O₅/Graphene Composite Papers with Ultrahigh Gravimetric/Volumetric Capacitance for Li-Ion Intercalation Pseudocapacitor

Free-standing electrodes with high gravimetric/volumetric capacitance will open up potential applications in miniaturized consumer electronics. Herein, we report a simple synthesis technology of free-standing orthorhombic Nb₂O₅ (<i>T</i>-Nb₂O₅)/graphene composite papers for Li-intercalating pseudocapacitive electrodes. Through a facile polyol-mediated solvothermal reaction, the Nb₂O₅ nanodots are homogeneously decorated onto the surface of reduced graphite oxide (rGO), which can form a homogeneous Nb₂O₅/rGO colloidal suspension that can be easily fabricated into flexible composite papers. The heat-treated <i>T</i>-Nb₂O₅/graphene composite papers exhibit a nanoporous layer-stacked structure with good ionic–electric conductive pathways, high <i>T</i>-Nb₂O₅ loading of 74.2%, and high bulk density of 1.55 g cm^–3. Such <i>T</i>-Nb₂O₅/graphene composite papers show a superior pseudocapacitor performance as free-standing electrodes, as evidenced by an ultrahigh gravimetric/volumetric capacitance (620.5 F g^–1 and 961.8 F cm^–3 at 1 mV s^–1) and excellent rate capability. Furthermore, an organic electrolyte-based asymmetric supercapacitor is assembled based on <i>T</i>-Nb₂O₅/graphene composite papers, which can deliver a high energy density of 47 W h kg^–1 and power density of 18 kW kg^–1

Highly Selective Upgrading of Polyethylene into Light Aromatics via a Low-Temperature Melting-Catalysis Strategy

The selective upgrading of polyethylene waste into light aromatics is hampered by relatively high C–C bond cleavage temperatures and low product selectivity. Herein, we report a low-temperature melting-catalysis strategy that directly upgrades low-density polyethylene (LDPE) into light aromatics over commercial ZSM-5 zeolite under mild conditions, eliminating the need for precious metals, solvent, or external H2. Experimental results combined with DFT calculations and molecular dynamics simulations revealed that the molten LDPE microenvironment facilitates intimate LDPE-catalyst contact, promoting primary C–C cleavage while suppressing olefin intermediates diffusion out of pores. This feature increases the residence time for subsequent direct olefin cyclization within the confined micropores. Moreover, online mass spectra confirmed that the in situ generated hydrogen from cyclization and dehydroaromatization reactions plays a vital role in C–C bond scission. By optimizing the reaction conditions, a light aromatic yield of 50.6 wt % with an impressive selectivity of 90.9% toward benzene, toluene, and xylenes was achieved at 280 °C for 1 h. This strategy is not limited to the model polyethylene but also demonstrates remarkable efficiency in the depolymerization of various widely used polyethylene-rich plastics, enabling an economically viable and environmentally benign chemical recycling path for plastic wastes

Organic Amine-Mediated Synthesis of Polymer and Carbon Microspheres: Mechanism Insight and Energy-Related Applications

A general organic amine-mediated synthesis of polymer microspheres is developed based on the copolymerization of resorcinol, formaldehyde, and various organic amines at room temperature. Structure formation and evolution of colloidal microspheres in the presence of polyethylenimine are monitored by dynamic light scattering measurements. It is found that the colloidal clusters are formed instantaneously and then experience an anomalous shrinkage–growth process. This should be caused by two different reaction pathways: cross-linking inside the microspheres and step-growth polymerization of substituted resorcinol on the microsphere surface, leading to the formation of core–shell heterogeneous structures as confirmed by TEM observation and XPS analysis. A formation mechanism of polymer microspheres is provided based on the aggregation of polyethylenimine/resorcinol–formaldehyde (PEI-RF) self-assembled nuclei, which is apparently different from the conventional Stöber process. Furthermore, nitrogen-doped carbon microspheres are prepared by the direct carbonization of these polymer microspheres, which exhibit microporous BET surface areas of 400–500 m² g^–1, high nitrogen contents of 5–6 wt %, and a good CO₂ adsorption capacity up to 3.6 mmol g^–1 at 0 °C. KOH activation is further employed to develop the porous texture of carbon microspheres without sacrificing the spherical morphology. The resultant activated carbon microspheres exhibit small particle size (<80 nm), high BET surface areas of 1500–2000 m² g^–1, and considerable nitrogen content of 2.2–2.0 wt %. When used as the electrode materials for supercapacitors, these activated carbon microspheres demonstrate a high capacitance up to 240 F g^–1, an unprecedented rate performance and good cycling performance

Nitrogen Doping Effects on the Physical and Chemical Properties of Mesoporous Carbons

Nitrogen-doped mesoporous carbons (NMCs) with controllable nitrogen doping and similar mesoporous structures are prepared by a facile colloidal silica nanocasting method using melamine, phenol, and formaldehyde as precursors. Various physicochemical properties, such as the oxidation stability, the conductivity and the electrochemical capacitive performance, the CO₂ adsorption, the basicity, and the metal-free catalytic activity of the NMCs, are studied extensively in relation to the incorporation amount of nitrogen in the carbon backbone. The dependence of the oxidation stability and the conductivity of the NMCs on the nitrogen content are similar; both of the biggest improvements are achieved at a low nitrogen content of ca. 4.2 wt %. While used as the supercapacitor electrodes, the NMCs with a mediate nitrogen content of ca. 8 wt % can take full advantage of the nitrogen-induced pseudocapacitance and the nitrogen-enhanced conductivity, delivering an excellent high-rate capacitive performance. The nitrogen content does not play an important role in the CO₂ physical adsorption, where the effect of microporosity prevails over the nitrogen-doped carbon surface. However, the nitrogen content determines the basicity of the NMCs, which governs their CO₂ chemical adsorption ability and the metal-free catalytic activity for direct oxidation of H₂S. The higher the nitrogen content, the higher the basicity and the catalytic activity. Our studies give a reliable relationship between nitrogen doping and the physicochemical properties of mesoporous carbons, which should provide a useful guide to their practical applications

High Efficiency Immobilization of Sulfur on Nitrogen-Enriched Mesoporous Carbons for Li–S Batteries

Nitrogen-enriched mesoporous carbons with tunable nitrogen content and similar mesoporous structures have been prepared by a facile colloid silica nanocasting to house sulfur for lithium–sulfur batteries. The results give unequivocal proof that nitrogen doping could assist mesoporous carbon to suppress the shuttling phenomenon, possibly via an enhanced surface interaction between the basic nitrogen functionalities and polysulfide species. However, nitrogen doping only within an appropriate level can improve the electronic conductivity of the carbon matrix. Thus, the dependence of total electrochemical performance on the nitrogen content is nonmonotone. At an optimal nitrogen content of 8.1 wt %, the carbon/sulfur composites deliver a highest reversible discharge capacity of 758 mA h g^–1 at a 0.2 C rate and 620 mA h g^–1 at a 1 C rate after 100 cycles. Furthermore, with the assistance of PPy/PEG hybrid coating, the composites could further increase the reversible capacity to 891 mA h g^–1 after 100 cycles. These encouraging results suggest nitrogen doping and surface coating of the carbon hosts are good strategies to improve the performance carbon/sulfur-based cathodes for lithium–sulfur batteries

Nitrogen-Rich Mesoporous Carbons: Highly Efficient, Regenerable Metal-Free Catalysts for Low-Temperature Oxidation of H₂S

We demonstrate that it is possible to transform traditional mesoporous carbons into a superior metal-free catalyst for low-temperature H₂S removal via doping a high concentration of nitrogen atoms into the carbon. The nitrogen doping level is important for the activity of mesoporous carbons as a metal-free catalyst. Although carbons doped with only an intermediate amount of nitrogen (e.g., 4.3 wt %), show little aptitude for the catalytic oxidation of H₂S when nitrogen doping reaches a certain high level (e.g., 8.5 wt %), the nitrogen-rich mesoporous carbons (NMC) exhibit high catalytic activity and selectivity toward H₂S oxidation at low temperature. Further study suggests that the pyridinic nitrogen atoms are responsible for the catalytic activity in H₂S oxidation. Owing to the metal-free nature of the NMC catalyst, it can be easily regenerated by CS₂ scrubbing, and the product sulfur can be recovered. Our desulfurization results suggest that such metal-free carbons could, indeed, overcome the limitations of the conventional H₂S catalysts and provide suitable, sustainable, and inexpensive solutions for technological development in H₂S removal

Enhanced Electrochemical Performance of Hydrous RuO₂/Mesoporous Carbon Nanocomposites via Nitrogen Doping

Hydrous RuO₂ nanoparticles have been uniformly deposited onto nitrogen-enriched mesoporous carbons (NMCs) via a facile hydrothermal method. The nitrogen doping in the carbon framework not only provides reversible pseudocapacitance but also guides uniform deposition of RuO₂ nanoparticles. As a result, an extremely high specific capacitance of 1733 F/g per RuO₂, comparable to the theoretic capacitance of RuO₂, is reached when 4.3 wt % of RuO₂·1.25H₂O is loaded onto the NMCs. Systematic studies show that either nitrogen-free or excess nitrogen doping result in RuO₂ clusters formation and worsen the electrochemical performances. With intermediate nitrogen and RuO₂ content (8.1 wt % N, 29.6 wt % of RuO₂·1.25H₂O), the composites deliver excellent power performance and high specific capacitance (402 F/g) with reversible capacitive response at 500 mV/s. The unique properties of nitrogen in textual, morphological, and electrochemical aspects may also provide further understanding about the effects of nitrogen doping and metal oxide deposition on supercapacitor performance

Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%

The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu₂S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe_0.65Te_0.35 QDSCs to a certified record of 11.16% (<i>J</i>_sc = 20.68 mA/cm², <i>V</i>_oc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs