Jitong Wang

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.

Free-Standing T-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 (T-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 T-Nb2O5/graphene composite papers exhibit a nanoporous layer-stacked structure with good ionic-electric conductive pathways, high T-Nb2O5 loading of 74.2%, and high bulk density of 1.55 g cm(-3). Such T-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 T-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).

A high-rate lithium–sulfur battery assisted by nitrogen-enriched mesoporous carbons decorated with ultrafine La2O3 nanoparticles

Nitrogen-enriched mesoporous carbons (NMCs) were decorated with ultrafine La2O3 nanoparticles via a simple wet impregnation method. The resulting composites with well developed mesoporous structures, high nitrogen content and uniform dispersions of La2O3 nanoparticles served as scaffolds to house sulfur for high rate lithium–sulfur batteries. Apart from their on-site trapping of polysulfides, the La2O3 nanoparticles decorated on the mesoporous carbon framework were also found to have a strong catalytic effect on sulfur reduction, offering high discharge voltages and fast electrochemical reaction kinetics. Combining the multiple effects of the well developed mesopores, nitrogen doping and La2O3 nanoparticles, the resulting ternary NMC/La2O3/S nanocomposites can deliver an initial capacity of 1043 mA h g−1 at 1 C, which remains at 799 mA h g−1 after 100 cycles. Moreover, they still maintain ultra-high rate capacities of 579 and 475 mA h g−1 at 3 C and 5 C, respectively, after 100 cycles. These encouraging results suggest that other metal oxides with suitable adsorption and catalytic abilities can be widely applied to decorate carbon frameworks for use in high rate lithium–sulfur systems.

High-power and high-energy asymmetric supercapacitors based on Li+-intercalation into a T-Nb2O5/graphene pseudocapacitive electrode

The intercalation pseudocapacitance which leads to the extraordinary charge storage properties has been confirmed as an intrinsic capacitive property of orthorhombic Nb2O5 (T-Nb2O5) nanocrystals. However, the poor electronic conductivity of T-Nb2O5 nanocrystals may limit their electrochemical utilization and high-rate performance especially for thick electrodes with high mass loadings. To address this issue, we herein reported a hydrothermal-heat treatment method to anchor T-Nb2O5 nanocrystals on conductive graphene sheets, which form a layer-by-layer integrated electrode with much shortened ion transport paths and results in excellent electrochemical capacitive properties, including high capacitance (626 C g−1), excellent rate handling and cyclic stability. Furthermore, asymmetric supercapacitors were constructed by using the high-rate response T-Nb2O5/graphene nanocomposite and mesoporous carbon as the negative and positive electrode, respectively. The asymmetric supercapacitor could deliver a high energy density of 16 W h kg−1 at an unprecedented power density of 45 kW kg−1 (discharge time of 1.2 s). The outstanding power properties of the supercapacitors are mainly attributed to the improved high-rate Li-insertion/extraction capability of the T-Nb2O5/graphene electrode and appropriate pairing of the mesoporous carbon electrode.

Surfactant promoted solid amine sorbents for CO2 capture

A new strategy to improve the CO2 capture performance of solid amine sorbents has been developed based on the balance of CO2 kinetic diffusion and thermodynamic sorption. The CO2-neutral surfactant was introduced into polyethyleneimine (PEI) to create extra CO2 transfer pathways, facilitating CO2 diffusion into the deeper PEI films. Consequently, the sorbents offered increased amount of reactive sites and higher utilization efficiency of amine groups, leading to a dramatically enhanced CO2 dynamic capacity and total capacity. Due to the facilitating diffusion, the sorbents could work at room temperature with very good performance. At 30 °C the surfactant-promoted sorbents had the CO2 capture capacity as high as 142 mg g−1 and their amine utilization was over 50%, which are the highest values ever reported for the PEI loaded sorbents working at this temperature. The surfactant-promoted sorbents also exhibited much better sorption kinetics and regeneration performance. In addition to advancing the support or amine, the present study provides another cost-efficient and general approach to design high performance CO2 solid sorbents and may have a major impact on the advance of current carbon capture and storage technologies.

Kinetically-enhanced polysulfide redox reactions by Nb2O5 nanocrystals for high-rate lithium–sulfur battery

To combat the shuttle of soluble polysulfides, physical confinement approaches aiming to trap sulfur within porous hosts have been pursued, but their success in terms of capacity decay and kinetic sluggishness is still limited. Herein, we update the confinement approach by integrating ultra-fine Nb2O5 nanocrystals onto a well-developed mesoporous carbon framework, which could primitively combine the advantages of the physical entrapment and the chemical interaction of sulfur species. An electrochemical kinetic study further reveals that the Nb2O5 nanocrystals could serve as an electrocatalyst, significantly accelerating the kinetics of polysulfide redox reactions, especially for the reduction of soluble Li2S6/Li2S4 to insoluble Li2S2/Li2S. By overcoming the kinetic barriers of the redox reactions, the resulting mesoporous carbon microspheres/Nb2O5/S composites have a much improved electrochemical performance in terms of lower polarization, higher Coulombic efficiency, and much improved rate capability. They could deliver a high initial capacity of 1289 mA h g−1 at 0.5C with a reversible capacity of 913 mA h g−1 after 200 cycles. And more importantly, the ternary cathodes demonstrate an unprecedented rate capability with 887 mA h g−1 at 5C, and a reversible capacity of 650 mA h g−1 at 2C after 500 cycles. The concept of accelerating the kinetics of polysulfide redox reactions will pave the way for future development of high-rate and long-cycle Li–S batteries.

Carbon dioxide capture using polyethylenimine-loaded mesoporous carbons

A high efficiency sorbent for CO2 capture was developed by loading polyethylenimine (PEI) on mesoporous carbons which possessed well-developed mesoporous structures and large pore volume. The physicochemical properties of the sorbent were characterized by N2 adsorption/desorption, scanning electron microscopy (SEM), thermal gravimetric analysis (TG) and Fourier transform infrared spectroscopy (FT-IR) techniques followed by testing for CO2 capture. Factors that affected the sorption capacity of the sorbent were studied. The sorbent exhibited extraordinary capture capacity with CO2 concentration ranging from 5% to 80%. The optimal PEI loading was determined to be 65 wt.% with a CO2 sorption capacity of 4.82 mmol-CO2/g-sorbent in 15% CO2/N2 at 75 degrees C, owing to low mass-transfer resistance and a high utilization ratio of the amine compound (63%). Moisture had a promoting effect on the sorption separation of CO2. In addition, the developed sorbent could be regenerated easily at 100 degrees C, and it exhibited excellent regenerability and stability. These results indicate that this PEI-loaded mesoporous carbon sorbent should have a good potential for CO2 capture in the future.

Effective removal of hexavalent chromium from aqueous solutions by adsorption on mesoporous carbon microspheres

High-surface-area mesoporous carbon microspheres were successfully synthesized by a spraying method with the purpose of removing Cr(VI) from waste water. Various factors influencing the adsorption of Cr(VI), including pH, adsorption temperature, and contact time were studied. As the adsorption process was pH dependent, it showed maximum removal efficiency of Cr(VI) at pH 3.0. Pseudo-second-order model was found to best represent the kinetics of Cr(VI) adsorption. The adsorption parameters were determined using both Langmuir and Freundlich isotherm models, and Qm value was as high as 165.3mg/g. The thermodynamic parameters including standard Gibbs free energy (ΔG(0)), standard enthalpy (ΔH(0)) and standard entropy (ΔS(0)) were investigated for predicting the nature of adsorption, which suggested the adsorption was an endothermic and a spontaneous thermodynamically process. Furthermore, Fe3O4-loaded MCMs were prepared to rapidly separate the adsorbent from the solution by a simple magnetic process. Fe3O4-loaded MCMs had a high adsorption capacity of 156.3mg/g, and a good regeneration ability with a capacity of 123.9mg/g for the fifth adsorption-desorption cycle.

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.

Nanoarchitectured Nb2O5 hollow, Nb2O5@carbon and NbO2@carbon Core-Shell Microspheres for Ultrahigh-Rate Intercalation Pseudocapacitors

Li-ion intercalation materials with extremely high rate capability will blur the distinction between batteries and supercapacitors. We construct a series of nanoarchitectured intercalation materials including orthorhombic (o-) Nb2O5 hollow microspheres, o-Nb2O5@carbon core-shell microspheres and tetragonal (t-) NbO2@carbon core-shell microspheres, through a one-pot hydrothermal method with different post-treatments. These nanoarchitectured materials consist of small nanocrystals with highly exposed active surface, and all of them demonstrate good Li+ intercalation pseudocapacitive properties. In particular, o-Nb2O5 hollow microspheres can deliver the specific capacitance of 488.3 F g−1, and good rate performance of 126.7 F g−1 at 50 A g−1. The o-Nb2O5@carbon core-shell microspheres show enhanced specific capacitance of 502.2 F g−1 and much improved rate performance (213.4 F g−1 at 50 A g−1). Furthermore, we demonstrate for the first time, t-NbO2 exhibits much higher rate capability than o-Nb2O5. For discharging time as fast as 5.9 s (50 A g−1), it still exhibits a very high specific capacitance of 245.8 F g−1, which is 65.2% retention of the initial capacitance (377.0 F g−1 at 1 A g−1). The unprecedented rate capability is an intrinsic feature of t-NbO2, which may be due to the conductive lithiated compounds.