3D Porous VOx/N-Doped Carbon Nanosheet Hybrids Derived from Cross-Linked Dicyandiamide-Chitosan Hydrogels for Superior Supercapacitor Electrode Materials

Three-dimensional porous carbon materials with moderate heteroatom-doping have been extensively investigated as promising electrode materials for energy storage. In this study, we fabricated a 3D cross-linked chitosan-dicyandiamide-VOSO4 hydrogel using a polymerization process. After pyrolysis at high temperature, 3D porous VOx/N-doped carbon nanosheet hybrids (3D VNCN) were obtained. The unique 3D porous skeleton, abundant doping elements, and presence of VOx 3D VNCN pyrolyzed at 800 °C (3D VNCN-800) ensured excellent electrochemical performance. The 3D VNCN-800 electrode exhibits a maximum specific capacitance of 408.1 F·g-1 at 1 A·g-1 current density and an admirable cycling stability with 96.8% capacitance retention after 5000 cycles. Moreover, an assembled symmetrical supercapacitor based on the 3D VNCN-800 electrode delivers a maximum energy density of 15.6 Wh·Kg-1 at a power density of 600 W·Kg-1. Our study demonstrates a potential guideline for the fabrication of porous carbon materials with 3D structure and abundant heteroatom-doping.

Metal-Organic Framework-Derived Fe-Doped Ni3Se4/NiSe2 Heterostructure-Embedded Mesoporous Tubes for Boosting Oxygen Evolution Reaction

It is crucial but challenging to promote sluggish kinetics of oxygen evolution reaction (OER) for water splitting via finely tuning the hierarchical nanoarchitecture and electronic structure of the catalyst. To address such issues, herein we present iron-doped Ni₃Se₄/NiSe₂ heterostructure-embedded metal-organic framework-derived mesoporous tubes (Ni-MOF-Fe-Se-400) realized by an interfacial engineering strategy. Due to the hierarchical nanoarchitecture of conductive two-dimensional nanosheet-constructed MOF-derived mesoporous tubes, coupled with fine tuning of the electronic structure via Fe-doping and interactions between Ni₃Se₄/NiSe₂ heterostructures, the Ni-MOF-Fe-Se-400 catalyst delivers superior OER activity: it requires only a low overpotential of 242 mV to achieve 10 mA cm^-2 (E_j=10), surpassing the benchmark RuO₂ (E_j=10 = 286 mV) and displays exceptional durability in the chronoamperometric i-t test with a small current decay (6.2%) after 72 h. Furthermore, the water splitting system comprises a Ni-MOF-Fe-Se-400 anode and a Pt/C cathode requires a low cell voltage of 1.576 V to achieve E_j=10 with an excellent Faradic efficiency (∼100%), outperforming the RuO₂-Pt/C combination. This work presents a novel interfacial engineering strategy to finely adjust the morphology and electronic structure of the non-noble metal-based OER catalyst via a facile fabrication method.

Heteroatom-Doped Flash Graphene

Heteroatom doping can effectively tailor the local structures and electronic states of intrinsic two-dimensional materials, and endow them with modified optical, electrical, and mechanical properties. Recent studies have shown the feasibility of preparing doped graphene from graphene oxide and its derivatives via some post-treatments, including solid-state and solvothermal methods, but they require reactive and harsh reagents. However, direct synthesis of various heteroatom-doped graphene in larger quantities and high purity through bottom-up methods remains challenging. Here, we report catalyst-free and solvent-free direct synthesis of graphene doped with various heteroatoms in bulk via flash Joule heating (FJH). Seven types of heteroatom-doped flash graphene (FG) are synthesized through millisecond flashing, including single-element-doped FG (boron, nitrogen, oxygen, phosphorus, sulfur), two-element-co-doped FG (boron and nitrogen), as well as three-element-co-doped FG (boron, nitrogen, and sulfur). A variety of low-cost dopants, such as elements, oxides, and organic compounds are used. The graphene quality of heteroatom-doped FG is high, and similar to intrinsic FG, the material exhibits turbostraticity, increased interlayer spacing, and superior dispersibility. Electrochemical oxygen reduction reaction of different heteroatom-doped FG is tested, and sulfur-doped FG shows the best performance. Lithium metal battery tests demonstrate that nitrogen-doped FG exhibits a smaller nucleation overpotential compared to Cu or undoped FG. The electrical energy cost for the synthesis of heteroatom-doped FG synthesis is only 1.2 to 10.7 kJ g^-1, which could render the FJH method suitable for low-cost mass production of heteroatom-doped graphene.

Studies on Kinetics, Isotherms, Thermodynamics and Adsorption Mechanism of Methylene Blue by N and S Co-Doped Porous Carbon Spheres

Heteroatom-doped carbon is widely used in the fields of adsorbents, electrode materials and catalysts due to its excellent physicochemical properties. N and S co-doped porous carbon spheres (N,S-PCSs) were synthesized using glucose and L-cysteine as carbon and heteroatom sources using a combined hydrothermal and KOH activation process. The physicochemical structures and single-factor methylene blue (MB) adsorption properties of the N,S-PCSs were then studied. The optimized N,S-PCSs-1 possessed a perfect spherical morphology with a 2-8-μm diameter and a large specific area of 1769.41 m2 g-1, in which the N and S contents were 2.97 at% and 0.88 at%, respectively. In the single-factor adsorption experiment for MB, the MB adsorption rate increased with an increase in carbon dosage and MB initial concentration, and the adsorption reached equilibrium within 2-3 h. The pseudo-second-order kinetic model could excellently fit the experimental data with a high R2 (0.9999). The Langmuir isothermal adsorption equation fitted well with the experimental results with an R2 value of 0.9618, and the MB maximum adsorption quantity was 909.10 mg g-1. The adsorption of MB by N,S-PCSs-1 was a spontaneous, endothermic, and random process based on the thermodynamics analyses. The adsorption mechanism mainly involved Van der Waals force adsorption, π-π stacking, hydrogen bonds and Lewis acid-base interactions.

Interface Heteroatom-doping: Emerging Solutions to Silicon-based Anodes

Silicon-based composites have been recognized as a promising anode material for high-energy lithium-ion batteries (LIBs). However, the intrinsically low conductivity and the huge volume expansion during lithiation/delithiation progresses impede its further practical applications. In the past decades, numerous efforts have been made for surface and interface modification of Si-based anodes. Among these, doping of active materials with heteroatoms is one promising method to endow silicon many unmatched electrochemical properties. In this review, we focus on the effects of heteroatom doping on the interfacial properties of Si-based anodes, and some typical strategies for the interface doping are highlighted. We aim to give some reference for interfacial doping of Si-based anodes in LIBs.

Sustainable Carbonaceous Materials Derived from Biomass as Metal-Free Electrocatalysts

Although carbon is the second most abundant element in the biosphere, a large proportion of the available carbon resources in biomass from agriculture, stock farming, ocean fisheries, and other human activities is currently wasted. The use of sustainable carbonaceous materials as an alternative to precious metals in electrocatalysis is a promising pathway for transforming sustainable biomass resources into sustainable energy-conversion systems. The development of rational syntheses of metal-free carbonaceous catalysts derived from sustainable biomass has therefore become a topic of significant interest in materials chemistry. However, great efforts are still required to develop methods that are low cost, scalable, and environmentally friendly and which afford carbonaceous materials having an electrocatalytic performance comparable to, or even better than, existing precious metal catalysts. Herein, recent achievements in developing metal-free carbonaceous catalysts based on biomass are reviewed and discussed and the critical issues which still need to be addressed are highlighted. The focus is on representative synthesis and optimization strategies applicable to different kinds of biomass, as well as studies of the physicochemical structure and electrochemical performance of the resulting metal-free carbonaceous catalysts. Finally, some guidelines for the future development of this important area are provided.