Holey Ti3C2 MXene-Derived Anode Enables Boosted Kinetics in Lithium-Ion Capacitors.

Lithium-ion capacitors (LICs) attract enormous attention because of the urgent demands for high power and energy density devices. However, the intrinsic imbalance between anodes and cathodes with different charge-storage mechanisms blocks the further improvement in energy and power density. MXenes, novel two-dimensional materials with metallic conductivity, accordion-like structure, and regulable interlayer spacing, are widely employed in electrochemical energy storage devices. Herein, we propose a holey Ti3C2 MXene-derived composite (pTi3C2/C) with enhanced kinetics for LICs. This strategy effectively decreases the surface groups (-F and -O) and generates expanded interplanar spacing. The in-plane pores of Ti3C2Tx lead to increased active sites and accelerated lithium-ion diffusion kinetics. Benefiting from the expanded interplanar spacing and accelerated lithium-ion diffusion, the pTi3C2/C as an anode implements excellent electrochemical property (capacity retention about 80% after 2000 cycles). Furthermore, the LIC fabricated with a pTi3C2/C anode and an activated carbon cathode displays a maximum energy density of 110 Wh kg-1 and a considerable energy density of 71 Wh kg-1 at 4673 W kg-1. This work provides an effective strategy to achieve high antioxidant capability and boosted electrochemical properties, which represents a new exploration of structural design and tuneable surface chemistry for MXene in LICs.

A nanostructured porous carbon/MoO2 composite with efficient catalysis in polysulfide conversion for lithium-sulfur batteries.

Lithium-sulfur batteries are considered as the next generation of energy storage systems because of their high theoretical specific capacity and energy density. Unfortunately, the sluggish reaction kinetics, weak adsorption toward to lithium polysulfides, and slow lithium ion diffusion impede the smooth electrochemical process, resulting in the lithium-sulfur batteries with the unsatisfactory cycling stability and rate performance. Since it is recognized that polar metal oxides and doped nitrogen in carbon materials have chemical interaction with lithium polysulfides, a nanostructured nitrogen-doped porous carbon/MoO2 composite is synthesized through a simple hydrothermal method by using graphene oxide nanoribbon and phosphomolybdic acid hydrate as precursors. The porous nanostructure promotes the charge and mass transport, while MoO2 nanoparticles immobilize lithium polysulfides via strong chemisorption and enhance the redox kinetics of polysulfides owing to the efficient catalytic activity in liquid-solid boundary. Consequently, the as-obtained nanostructured porous carbon/MoO2-based sulfur cathode exhibits low polarization, high initial discharge capacity (1403 mAh g-1 at 0.1 C), good rate capabilities (584 mAh g-1 at 4 C), and impressive cycling performance at 1 C (503 mAh g-1 after 500 cycles with capacity fade rate of 0.07% per cycle).

Investigating the Electrocatalysis of a Ti3C2/Carbon Hybrid in Polysulfide Conversion of Lithium-Sulfur Batteries.

Despite the fact that lithium-sulfur batteries are regarded as promising next-generation rechargeable battery systems owning to high theoretical specific capacity (1675 mA h g-1) and energy density (2600 W h kg-1), several issues such as poor electrical conductivity, sluggish redox kinetics, and severe "shuttle effect" in electrodes still hinder their practical application. MXenes, novel two-dimensional materials with high conductivity, regulable interlayer spacing, and abundant functional groups, are widely applied in energy storage and conversion fields. In this work, a Ti3C2/carbon hybrid with expanded interlayer spacing is synthesized by one-step heat treatment in molten potassium hydroxide. The subsequent experiments indicate that the as-prepared Ti3C2/carbon hybrid can effectively regulate polysulfide redox conversion and has strong chemisorption interaction to polysulfides. Consequently, the Ti3C2/carbon-based sulfur cathode boosts the performance in working lithium-sulfur batteries, in terms of an ultrahigh initial discharge capacity (1668 mA h g-1 at 0.1 C), an excellent rate performance (520 mA h g-1 at 5 C), and an outstanding capacity retention of 530 mA h g-1 after 500 cycles at 1 C with a low capacity fade rate of 0.05% per cycle and stable Coulombic efficiency (nearly 99%). The above results indicate that this composite with high catalytic activity is a potential host material for further high-performance lithium-sulfur batteries.

Tuning Both Surface Chemistry and Porous Properties of Polymer-Derived Porous Carbons for High-Performance Gas Adsorption.

In this study, we have prepared novel pyrrole-formaldehyde polymers through polymerizing pyrrole and formaldehyde in the mixture solvent of water and ethanol by using hydrochloric acid as a catalyst. The as-synthesized polymers possess a nitrogen content of 6.7 atom % and are composed of spherical particles with the diameter of approximately 1-3 µm. A series of nitrogen-doped porous carbons with high specific surface areas (680-2340 m2 g-1) were successfully obtained through the activation treatment of the polymer spheres. The porous properties and surface chemistry of the as-prepared porous carbons are tuned by choosing different activating agents and changing the activation temperature. The morphology, porous properties, and chemical composition of the obtained nitrogen-doped porous carbons are revealed by various characterization methods, such as scanning electron microscopy, nitrogen sorption measurement, and X-ray photoelectron spectroscopy. The as-prepared nitrogen-doped porous carbons as gas adsorbents display high carbon dioxide uptake capacities of 3.80-5.81 mmol g-1 at 273 K and 1.0 bar. They also show excellent carbon dioxide adsorption capacities (2.40-3.37 mmol g-1 at 1.0 bar) and good gas selectivities (CO2/N2 selectivities of 16.9-70.2) at 298 K.

Nanostructured porous carbons derived from nitrogen-doped graphene nanoribbon aerogels for lithium-sulfur batteries.

A nanostructured porous carbon (NPC) is prepared by using a facile physical activation method, with nitrogen-doped graphene nanoribbon aerogel and carbon dioxide as a precursor and an activating agent, respectively. The morphology, porosity parameters, and chemical properties of the as-prepared NPC have been revealed by using various characterization methods, including scanning electron microscopy, nitrogen sorption analysis, and X-ray photoelectron spectroscopy (XPS). The NPC with a moderate nitrogen content (5.1 atom % on the basis of XPS analysis) retains the sponge-like morphology of nitrogen-doped graphene nanoribbon aerogel, shows a high Brunauer-Emmett-Teller specific surface area (1380 m2 g-1), and possesses hierarchically porous structures. Based on its excellent properties such as high porosity, conductive network, and nitrogen-doping, NPC as a superior host is used to fabricate a sulfur-based cathode for lithium-sulfur batteries. The high specific surface area and the pore volume of NPC not only allow uniform distribution of sulfur in an amorphous form, but also help to alleviate the shuttle effect of polysulfides. As a result, the as-obtained cathode exhibits a good rate capability and cycling stability.

Effect of Porosity Parameters and Surface Chemistry on Carbon Dioxide Adsorption in Sulfur-Doped Porous Carbons.

In this work, a series of highly porous sulfur-doped carbons are prepared through physical activation methods by using polythiophene as a precursor. The morphology, structure, and physicochemical properties are revealed by a variety of characterization methods, such as scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and nitrogen sorption measurement. Their porosity parameters and chemical compositions can be well-tuned by changing the activating agents (steam and carbon dioxide) and reaction temperature. These sulfur-doped porous carbons possess specific surface area of 670-2210 m2 g-1, total pore volume of 0.31-1.26 cm3 g-1, and sulfur content of 0.6-4.9 atom %. The effect of porosity parameters and surface chemistry on carbon dioxide adsorption in sulfur-doped porous carbons is studied in detail. After a careful analysis of carbon dioxide uptake at different temperatures (273 and 293 K), pore volumes from small pore size (less than 1 nm) play an important role in carbon dioxide adsorption at 273 K, whereas surface chemistry is the key factor at a higher adsorption temperature or lower relative pressure. Furthermore, sulfur-doped porous carbons also possess good gas adsorption selectivity and excellent recyclability for regeneration.

Metal-Organic Framework-Derived Metal Oxide Embedded in Nitrogen-Doped Graphene Network for High-Performance Lithium-Ion Batteries.

Metal-organic frameworks (MOFs) are hybrid inorganic-organic materials that can be used as effective precursors to prepare various functional nanomaterials for energy-related applications. Nevertheless, most MOF-derived metal oxides exhibit low electrical conductivity and mechanical strain. These characteristics limit their electrochemical performance and hamper their practical application. Herein, we report a rational strategy for enhancing the lithium storage performance of MOF-derived metal oxide. The hierarchically porous Co3O4@NGN is successfully prepared by embedding ZIF-67-derived Co3O4 particles in a nitrogen-doped graphene network (NGN). The high electrical conductivity and porous structure of the NGN accelerates the diffusion of electrolyte ions and buffers stress resulting from the volume changes of Co3O4. As an anode material, the Co3O4@NGN shows high capacity (1030 mA h g-1 at 100 mA g-1), outstanding rate performance (681 mA h g-1 at 1000 mA g-1), and good cycling stability (676 mA h g-1 at 1000 mA g-1 after 400 cycles).

Nitrogen-doped graphene aerogel as both a sulfur host and an effective interlayer for high-performance lithium-sulfur batteries.

Lithium-sulfur batteries have attracted great concern because of the high theoretical capacity of sulfur (1675 mA h g-1). However, the poor electrical conductivity and volumetric expansion of sulfur along with the dissolution of lithium polysulfides largely limit their practical application. In this study, nitrogen-doped graphene aerogel (NGA) with high nitrogen content and porosity is used as a host for the impregnation of sulfur. The effects of sulfur impregnation on the specific surface area, pore volume, and microstructure of NGA supported sulfur composite (S@NGA) are well investigated. Furthermore, NGA is also processed into a NGA film, which is sandwiched between a separator and S@NGA cathode. The lithium-sulfur battery with such a configuration delivers a high reversible capacity of 1514 mA h g-1 at 0.1 C, excellent rate performance (822 mA h g-1 at 2.0 C), and good cycling stability (946 mA h g-1 at 0.5 C even after 100 cycles). The enhanced electrochemical performance can be ascribed to the introduction of the NGA interlayer, the unique interconnected porous structure, and strong interaction between the three-dimensional nitrogen-doped graphene network and the homogeneously dispersed sulfur and/or lithium polysulfides.

Synthesis of Core-Shell Structured Porous Nitrogen-Doped Carbon@Silica Material via a Sol-Gel Method.

Core-shell structured nitrogen-doped porous carbon@silica material with uniform structure and morphology was synthesized via a sol-gel method. During this process, a commercial triblock copolymer and the in situ formed pyrrole-formaldehyde polymer acted as cotemplates, while tetraethyl orthosilicate acted as silica precursor. The synergetic effect of the triblock copolymer and the pyrrole-formaldehyde polymer enables the formation of the core-shell structure. Herein, the pyrrole-formaldehyde polymer acted as not only the template, but also the nitrogen-doped carbon precursor of the core. The obtained core-shell structured porous material possesses moderate Brunauer-Emmett-Teller specific surface area (410 m2 g-1) and pore volume (0.53 cm3 g-1). Moreover, corresponding hollow silica spheres or nitrogen-doped porous carbon spheres can be synthesized by calcining the core-shell structured material in air or etching it with HF. The X-ray photoelectron spectroscopy results reveal that the nitrogen states of the obtained material are mainly pyridinic-N and pyridonic-N/pyrrolic-N, which are beneficial for carbon dioxide adsorption. The carbon dioxide uptake capacity of the nitrogen-doped carbon spheres can reach 12.3 wt % at 273 K and 1.0 bar, meanwhile, the material shows good gas adsorption selectivities for CO2/CH4 and CO2/N2.

Preparation and characterization of a composite hydrogel with graphene oxide as an acid catalyst.

In this study, a facile method for synthesizing a novel graphene oxide/pyrrole-formaldehyde (GOP-1) composite hydrogel was developed via in situ polymerization of pyrrole and formaldehyde in the presence of graphene oxide sheets without any additional catalyst. During the polymerization, graphene oxide can act as a two-dimensional template to regulate the aggregation state of polymer and as an acid catalyst to accelerate the reaction rate of pyrrole and formaldehyde. The morphology and microstructure were investigated by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, respectively. The chemical properties were analyzed via X-ray photoelectron spectroscopy, infrared spectroscopy, and Raman spectroscopy. The freeze-dried GOP-1 composite hydrogel exhibited a large specific surface area, high nitrogen content, and three-dimensional network structure. Based on the above features, the freeze-dried GOP-1 composite hydrogel used as a gas adsorbent showed a high carbon dioxide uptake capacity at 1.0 bar and 273 K (11.1 wt%), in sharp contrast to that of graphene oxide (7.4 wt%). Furthermore, the as-prepared composite hydrogel may possess attractive potential in the fields of electrode material, tissue engineering, and water treatment.

Nitrogen-doped graphene aerogels as efficient supercapacitor electrodes and gas adsorbents.

Nitrogen-doped graphene has been demonstrated to be an excellent multifunctional material due to its intriguing features such as outstanding electrocatalytic activity, high electrical conductivity, and good chemical stability as well as wettability. However, synthesizing the nitrogen-doped graphene with a high nitrogen content and large specific surface area is still a challenge. In this study, we prepared a nitrogen-doped graphene aerogel (NGA) with high porosity by means of a simple hydrothermal reaction, in which graphene oxide and ammonia are adopted as carbon and nitrogen source, respectively. The microstructure, morphology, porous properties, and chemical composition of NGA were well-disclosed by a variety of characterization methods, such as scanning electron microscopy, nitrogen adsorption-desorption measurements, X-ray photoelectron spectroscopy, and Raman spectroscopy. The as-made NGA displays a large Brunauer-Emmett-Teller specific surface area (830 m(2) g(-1)), high nitrogen content (8.4 atom %), and excellent electrical conductivity and wettability. On the basis of these features, the as-made NGA shows superior capacitive behavior (223 F g(-1) at 0.2 A g(-1)) and long-term cycling performance in 1.0 mol L(-1) H2SO4 electrolyte. Furthermore, the NGA also possesses a high carbon dioxide uptake capacity at 1.0 bar and 273 K (11.3 wt %).

Preparation of three-dimensional graphene oxide-polyethylenimine porous materials as dye and gas adsorbents.

We report a facile method for the fabrication of three-dimensional (3D) porous materials via the interaction between graphene oxide (GO) sheets and polyethylenimine (PEI) with high amine density at room temperature under atmospheric pressure without stirring. The structural and physical properties of GO-PEI porous materials (GEPMs) are investigated by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and nitrogen adsorption-desorption measurement and their chemical properties are analyzed by X-ray photoelectron spectroscopy, infrared spectroscopy, and Raman spectroscopy. GEPMs possess low density and hierarchical morphology with large specific surface area, and big pore volume. Furthermore, the as-prepared 3D porous materials show an excellent adsorption capacity for acidic dyes on the basis of the pore-rich and amine-rich graphene structure. GEPMs exhibit an extremely high adsorption capacity for amaranth (800 mg g(-1)), which are superior to other carbon materials. In addition, GEPMs also exhibit good adsorption capacity for carbon dioxide (11.2 wt % at 1.0 bar and 273 K).