Donghui Long

Preparation of Nitrogen-Doped Graphene Sheets by a Combined Chemical and Hydrothermal Reduction of Graphene Oxide

Nitrogen-doped graphene sheets were prepared through a hydrothermal reduction of colloidal dispersions of graphite oxide in the presence of hydrazine and ammonia at pH of 10. The effect of hydrothermal temperature on the structure, morphology, and surface chemistry of as-prepared graphene sheets were investigated though XRD, N(2) adsorption, solid-state (13)C NMR, SEM, TEM, and XPS characterizations. Oxygen reduction and nitrogen doping were achieved simultaneously under the hydrothermal reaction. Up to 5% nitrogen-doped graphene sheets with slightly wrinkled and folded feature were obtained at the relative low hydrothermal temperature. With the increase of hydrothermal temperature, the nitrogen content decreased slightly and more pyridinic N incorporated into the graphene network. Meanwhile, a jellyfish-like graphene structure was formed by self-organization of graphene sheets at the hydrothermal temperature of 160 °C. Further increase of the temperature to 200 °C, graphene sheets could self-aggregate into agglomerate particles but still contained doping level of 4 wt % N. The unique hydrothermal environment should play an important role in the nitrogen doping and the jellyfish-like graphene formation. This simple hydrothermal method could provide the synthesis of nitrogen-doped graphene sheets in large scale for various practical applications.

Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%

The enhancement of power conversion efficiency (PCE) and the development of toxic Cd-, Pb-free quantum dots (QDs) are critical for the prosperity of QD-based solar cells. It is known that the properties (such as light harvesting range, band gap alignment, density of trap state defects, etc.) of QD light harvesters play a crucial effect on the photovoltaic performance of QD based solar cells. Herein, high quality ∼4 nm Cd-, Pb-free Zn-Cu-In-Se alloyed QDs with an absorption onset extending to ∼1000 nm were developed as effective light harvesters to construct quantum dot sensitized solar cells (QDSCs). Due to the small particle size, the developed QD sensitizer can be efficiently immobilized on TiO2 film electrode in less than 0.5 h. An average PCE of 11.66% and a certified PCE of 11.61% have been demonstrated in the QDSCs based on these Zn-Cu-In-Se QDs. The remarkably improved photovoltaic performance for Zn-Cu-In-Se QDSCs vs Cu-In-Se QDSCs (11.66% vs 9.54% in PCE) is mainly derived from the higher conduction band edge, which favors the photogenerated electron extraction and results in higher photocurrent, and the alloyed structure of Zn-Cu-In-Se QD light harvester, which benefits the suppression of charge recombination at photoanode/electrolyte interfaces and thus improves the photovoltage.

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.

Facile preparation and ultra-microporous structure of melamine–resorcinol–formaldehyde polymeric microspheres

A facile hydrothermal method was developed to synthesize nitrogen-rich phenolic microspheres with a tunable ultra-microporous structure for CO(2) adsorption. The results highlighted that chemical composition and ultramicroporous size, much more than surface area, dictated the CO(2) uptake in a microporous organic polymer.

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 CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 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.

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.