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1.
Chem Soc Rev ; 2019 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-31573024

RESUMO

Electrocatalysis plays an essential role in diverse electrochemical energy conversion processes that are vital for improving energy utilization efficiency and mitigating the aggravating global warming challenge. The noble metals such as platinum are generally the most frequently used electrocatalysts to drive these reactions and facilitate the relevant energy conversion processes. The high cost and scarcity of these materials pose a serious challenge for the wide-spread adoption and the sustainability of these technologies in the long run, which have motivated considerable efforts in searching for alternative electrocatalysts with reduced loading of precious metals or based entirely on earth-abundant metals. Of particular interest are graphene-supported single atom catalysts (G-SACs) that integrate the merits of heterogeneous catalysts and homogeneous catalysts, such as high activity, selectivity, stability, maximized atom utilization efficiency and easy separation from reactants/products. The graphene support features a large surface area, high conductivity and excellent (electro)-chemical stability, making it a highly attractive substrate for supporting single atom electrocatalysts for various electrochemical energy conversion processes. In this review, we highlight the recent advancements in G-SACs for electrochemical energy conversion, from the synthetic strategies and identification of the atomistic structure to electrocatalytic applications in a variety of reactions, and finally conclude with a brief prospect on future challenges and opportunities.

2.
Science ; 363(6428): 723-727, 2019 02 15.
Artigo em Inglês | MEDLINE | ID: mdl-30765563

RESUMO

Ceramic aerogels are attractive for thermal insulation but plagued by poor mechanical stability and degradation under thermal shock. In this study, we designed and synthesized hyperbolic architectured ceramic aerogels with nanolayered double-pane walls with a negative Poisson's ratio (-0.25) and a negative linear thermal expansion coefficient (-1.8 × 10-6 per °C). Our aerogels display robust mechanical and thermal stability and feature ultralow densities down to ~0.1 milligram per cubic centimeter, superelasticity up to 95%, and near-zero strength loss after sharp thermal shocks (275°C per second) or intense thermal stress at 1400°C, as well as ultralow thermal conductivity in vacuum [~2.4 milliwatts per meter-kelvin (mW/m·K)] and in air (~20 mW/m·K). This robust material system is ideal for thermal superinsulation under extreme conditions, such as those encountered by spacecraft.

3.
ACS Appl Mater Interfaces ; 11(2): 2218-2224, 2019 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-30582695

RESUMO

Graphene oxide (GO) is not only a unique class of two-dimensional (2D) materials but also an important precursor for scalable preparation of graphene. The efficient size fractionation of GO is of great importance to the fundamental and applied studies of chemically modified graphene, but remains a great challenge. Herein, we report an efficient and scalable fractionation method of GO employing reversible adsorption/desorption of temperature-responsive poly( N-isopropylacrylamide) on GO to amplify its mass difference and significantly improve the fractionation efficiency. Furthermore, size-dependent sodium ion storage of the resulting fractionated reduced GO (RGO) is revealed for the first time with high sodium storage performance achieved for the smallest RGO because of its largest d-spacing and most defect sites. This work provides valuable insights into the size fractionation and size-dependent electrochemical performance of graphene, which can be potentially extended to other 2D materials.

4.
ACS Nano ; 12(12): 12879-12887, 2018 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-30525431

RESUMO

Deliberate design of advantageous nanostructures holds great promise for developing high-performance electrode materials for electrochemical energy storage. However, it remains a tremendous challenge to simultaneously gain high gravimetric, areal, and volumetric capacities as well as high rate performance and cyclability to meet practical requirements mainly due to the intractable insufficient ion diffusion and limited active sites for dense electrodes with high areal mass loadings. Herein we report a double-holey-heterostructure framework, in which holey Fe2O3 nanosheets (H-Fe2O3) are tightly and conformably grown on the holey reduced graphene oxide (H-RGO). This hierarchical nanostructure allows for rapid ion and electron transport and sufficient utilization of active sites throughout a highly compact and thick electrode. Therefore, the free-standing flexible H-Fe2O3/H-RGO heterostructure anode can simultaneously deliver ultrahigh gravimetric, areal, and volumetric capacities of 1524 mAh g-1, 4.72 mAh cm-2, and 2621 mAh cm-3, respectively, at 0.2 A g-1 after 120 cycles, and extraordinary rate performance with a capacity of 487 mAh g-1 (1.51 mAh cm-2) at a high current density of 30 A g-1 (93 mA cm-2) as well as excellent cycling stability with a capacity retention of 96.3% after 1600 cycles, which has rarely been achieved before.

5.
Chem Sci ; 9(34): 7009-7016, 2018 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-30210776

RESUMO

Herein a novel and general microwave-assisted chemical vapor deposition (CVD)-like synthetic strategy was developed to realize the ultrafast synthesis of a series of well-dispersed monolayer/few-layer N-doped graphene shell encapsulated metal nanocrystals (M@NC) by using a metal-organic framework (MOF) on graphene as precursors for the first time. Unlike traditional programmed heat treatment, this microwave-assisted method decomposed the MOF into separated metal and carbon- and nitrogen-containing gases rather than aggregated metal and carbon composites during the initial thermal transformation stages. This change ensured the effective control of the subsequent formation process of carbon on the surface of metal and led to the formation of well-dispersed M@NC with monolayer/few-layer NC. Moreover, the graphene substrate promoted the full exposure of all active monolayer/few-layer NC, and thus the obtained FeNi@NC/graphene displays the best electrocatalytic properties for the oxygen evolution reaction of all of the previously reported M@NC based catalysts, including the lowest overpotential (261 mV) at 10 mA cm-2 in alkaline electrolyte (1 M KOH), the smallest Tafel slope (40 mV dec-1) and excellent durability for at least 120 h.

6.
ACS Appl Mater Interfaces ; 10(39): 33269-33275, 2018 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-30199222

RESUMO

In the past years, considerable efforts have been devoted to the deliberate synthesis of nanosulfur in various hosts with sophisticated structures to improve the performance of lithium-sulfur batteries (LSBs) and reveal the structure-property relationship. It is taken for granted that these elaborate sulfur nanostructures are well maintained in the ultimate electrode after the traditional mixing and coating method. Herein, we, for the first time, reveal the unexpected sulfur structure deterioration in nanosulfur/graphene composites during the electrode preparation using the traditional method because of the long-term neglected dissolution-recrystallization effect of sulfur in solvents. Consequently, compared with binder-free three-dimensional graphene/sulfur electrodes, the milled graphene/sulfur electrodes exhibit much worse electrochemical performance. On the basis of this, we further propose a facile and universal graphene oxide-assisted assembly method to avoid the dissolution-recrystallization of sulfur, by which binder-free three-dimensional ethylenediamine-functionalized graphene/sulfur (3DEFGS) electrodes have been successfully prepared. The 3DEFGS electrodes with a high areal sulfur loading of ∼6 mg cm-2 exhibit an ultrahigh initial capacity of 1394 mA h g-1 at 0.1 C, an excellent rate performance with a capacity of 796 mA h g-1 at 4 C, and superior long-term cycling stability (885 mA h g-1 after 500 cycles at 1 C), which are among the best performances achieved by all reported LSB cathodes with high areal sulfur loadings.

7.
Adv Mater ; 30(35): e1802146, 2018 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-30016001

RESUMO

Graphene-supported single atomic metals (G-SAMs) have recently attracted considerable research interest for their intriguing catalytic, electronic, and magnetic properties. The development of effective synthetic methodologies toward G-SAMs with monodispersed metal atoms is vital for exploring their fundamental properties and potential applications. A convenient, rapid, and general strategy to synthesize a series of monodispersed atomic transition metals (for example, Co, Ni, Cu) embedded in nitrogen-doped graphene by two-second microwave (MW) heating the mixture of amine-functionalized graphene oxide and metal salts is reported here. The MW heating is able to simultaneously induce the reduction of graphene oxide, the doping of nitrogen, and the incorporation of metal atoms into the graphene lattices in one simple step. The rapid MW process minimizes metal diffusion and aggregation to ensure exclusive single metal atom dispersion in graphene lattices. Electrochemical studies demonstrate that graphene-supported Co atoms can function as highly active electrocatalysts toward the hydrogen evolution reaction. This MW-assisted method provides a rapid and efficient avenue to supported metal atoms for wide ranges of applications.

8.
Nature ; 557(7707): 696-700, 2018 05.
Artigo em Inglês | MEDLINE | ID: mdl-29769729

RESUMO

The junctions formed at the contact between metallic electrodes and semiconductor materials are crucial components of electronic and optoelectronic devices 1 . Metal-semiconductor junctions are characterized by an energy barrier known as the Schottky barrier, whose height can, in the ideal case, be predicted by the Schottky-Mott rule2-4 on the basis of the relative alignment of energy levels. Such ideal physics has rarely been experimentally realized, however, because of the inevitable chemical disorder and Fermi-level pinning at typical metal-semiconductor interfaces2,5-12. Here we report the creation of van der Waals metal-semiconductor junctions in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning. The Schottky barrier height, which approaches the Schottky-Mott limit, is dictated by the work function of the metal and is thus highly tunable. By transferring metal films (silver or platinum) with a work function that matches the conduction band or valence band edges of molybdenum sulfide, we achieve transistors with a two-terminal electron mobility at room temperature of 260 centimetres squared per volt per second and a hole mobility of 175 centimetres squared per volt per second. Furthermore, by using asymmetric contact pairs with different work functions, we demonstrate a silver/molybdenum sulfide/platinum photodiode with an open-circuit voltage of 1.02 volts. Our study not only experimentally validates the fundamental limit of ideal metal-semiconductor junctions but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics.

9.
ACS Cent Sci ; 4(5): 590-599, 2018 May 23.
Artigo em Inglês | MEDLINE | ID: mdl-29806005

RESUMO

The development of future sustainable energy technologies relies critically on our understanding of electrocatalytic reactions occurring at the electrode-electrolyte interfaces, and the identification of key reaction promoters and inhibitors. Here we present a systematic in situ nanoelectronic measurement of anionic surface adsorptions (sulfates, halides, and cyanides) on ultrathin platinum nanowires during active electrochemical processes, probing their competitive adsorption behavior with oxygenated species and correlating them to the electrokinetics of the oxygen reduction reaction (ORR). The competitive anionic adsorption features obtained from our studies provide fundamental insight into the surface poisoning of Pt-catalyzed ORR kinetics by various anionic species. Particularly, the unique nanoelectronic approach enables highly sensitive characterization of anionic adsorption and opens an efficient pathway to address the practical poisoning issue (at trace level contaminations) from a fundamental perspective. Through the identified nanoelectronic indicators, we further demonstrate that rationally designed competitive anionic adsorption may provide improved poisoning resistance, leading to performance (activity and lifetime) enhancement of energy conversion devices.

10.
Nature ; 555(7695): 231-236, 2018 03 07.
Artigo em Inglês | MEDLINE | ID: mdl-29517002

RESUMO

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

11.
ACS Nano ; 12(4): 3947-3953, 2018 Apr 24.
Artigo em Inglês | MEDLINE | ID: mdl-29558111

RESUMO

Synthesis of ultrasmall metal-organic framework (MOF) nanoparticles has been widely recognized as a promising route to greatly enhance their properties but remains a considerable challenge. Herein, we report one facile and effective spatially confined thermal pulverization strategy to successfully transform bulk Co-MOF particles into sub-5 nm nanocrystals encapsulated within N-doped carbon/graphene (NC/G) by using conducting polymer coated Co-MOFs/graphene oxide as precursors. This strategy involves a feasible mechanism: calcination of Co-MOFs at proper temperature in air induces the partial thermal collapse/distortion of the framework, while the uniform coating of a conducting polymer can significantly improve the decomposition temperature and maintain the component stability of Co-MOFs, thus leading to the pulverization of bulk Co-MOF particles into ultrasmall nanocrystals without oxidation. The pulverization of Co-MOFs significantly increases the contact area between Co-MOFs with electrolyte and shortens the electron and ion transport pathway. Therefore, the sub-5 nm ultrasmall MOF nanocrystals-based composites deliver an ultrahigh reversible capacity (1301 mAh g-1 at 0.1 A g-1), extraordinary rate performance (494 mAh g-1 at 40 A g-1), and outstanding cycling stability (98.6% capacity retention at 10 A g-1 after 2000 cycles), which is the best performance achieved in all reported MOF-based anodes for lithium-ion batteries.

12.
Small ; 14(13): e1703969, 2018 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-29363874

RESUMO

The designable structure with 3D structure, ultrathin 2D nanosheets, and heteroatom doping are considered as highly promising routes to improve the electrochemical performance of carbon materials as anodes for lithium-ion batteries. However, it remains a significant challenge to efficiently integrate 3D interconnected porous frameworks with 2D tunable heteroatom-doped ultrathin carbon layers to further boost the performance. Herein, a novel nanostructure consisting of a uniform ultrathin N-doped carbon layer in situ coated on a 3D graphene framework (NC@GF) through solvothermal self-assembly/polymerization and pyrolysis is reported. The NC@GF with the nanosheets thickness of 4.0 nm and N content of 4.13 at% exhibits an ultrahigh reversible capacity of 2018 mA h g-1 at 0.5 A g-1 and an ultrafast charge-discharge feature with a remarkable capacity of 340 mA h g-1 at an ultrahigh current density of 40 A g-1 and a superlong cycle life with a capacity retention of 93% after 10 000 cycles at 40 A g-1 . More importantly, when coupled with LiFePO4 cathode, the fabricated lithium-ion full cells also exhibit high capacity and excellent rate and cycling performances, highlighting the practicability of this NC@GF.

13.
Nano Lett ; 17(9): 5495-5501, 2017 09 13.
Artigo em Inglês | MEDLINE | ID: mdl-28823157

RESUMO

Negative transconductance (NTC) devices have been heavily investigated for their potential in low power logical circuit, memory, oscillating, and high-speed switching applications. Previous NTC devices are largely attributed to two working mechanisms: quantum mechanical tunneling, and mobility degradation at high electrical field. Herein we report a systematic investigation of charge transport in multilayer two-dimensional semiconductors (2DSCs) with optimized van der Waals contact and for the first time demonstrate NTC and antibipolar characteristics in multilayer 2DSCs (such as MoS2, WSe2). By varying the measurement temperature, bias voltage, and body thickness, we found the NTC behavior can be attributed to a vertical potential barrier in the multilayer 2DSCs and the competing mechanisms between intralayer lateral transport and interlayer vertical transport, thus representing a new working mechanism for NTC operation. Importantly, this vertical potential barrier arises from inhomogeneous carrier distribution in 2DSC from the near-substrate region to the bulk region, which is in contrast to conventional semiconductors with homogeneous doping defined by bulk dopants. We further show that the unique NTC behavior can be explored for creating frequency doublers and phase shift keying circuits with only one transistor, greatly simplifying the circuit design compared to conventional technology.

14.
ChemSusChem ; 10(17): 3419-3426, 2017 09 11.
Artigo em Inglês | MEDLINE | ID: mdl-28722277

RESUMO

Small conjugated carbonyl compounds are intriguing candidates for organic electrode materials because of their abundance, high theoretical capacity, and adjustable molecular structure. However, their dissolution in aprotic electrolytes and poor conductivity eclipse them in terms of practical capacity, cycle life, and rate capability. Herein, we report a foldable and binder-free nanocomposite electrode consisting of 2-aminoanthraquinone (AAQ) nanowires wrapped within the 3 D graphene framework, which is prepared through antisolvent crystallization followed by a facile chemical reduction and selfassembly process. The nanocomposite exhibited a very high capacity of 265 mA h g-1 at 0.1 C for AAQ, realizing 100 % utilization of active material. Furthermore, the nanocomposite shows superior cycling stability (82 % capacity retention after 200 cycles at 0.2 C and 76 % capacity retention after 1000 cycles at 0.4 C) and excellent rate performance (153 mA h g-1 at 5 C). Particularly, the nanocomposite can deliver the highest capacity of 165 mA h g-1 among all reported anthraquinone- and anthraquinone-analogues-based electrodes per mass of the whole electrode, which is essential for practical application. Such outstanding electrochemical performance could be largely attributed to the wrapping structure of the flexible composite, which provides both conductivity and structural integrity.


Assuntos
Antraquinonas/química , Fontes de Energia Elétrica , Grafite/química , Lítio/química , Nanotecnologia/métodos , Nanofios/química , Eletrodos , Modelos Moleculares , Conformação Molecular
15.
Chem Commun (Camb) ; 53(54): 7481-7484, 2017 Jul 04.
Artigo em Inglês | MEDLINE | ID: mdl-28597899

RESUMO

Herein we report a facile mechanochemical synthesis of 2D aromatic polyamides (2DAPAs) under solvent-free and room temperature conditions for the first time. The solvent-free conditions are found to be key to the successful synthesis of 2DAPAs. These micrometer-size 2DAPAs have ultrathin graphene-like structures and are highly crystalline, solvent dispersible and thermally stable up to 400 °C.

16.
Science ; 356(6338): 599-604, 2017 05 12.
Artigo em Inglês | MEDLINE | ID: mdl-28495745

RESUMO

Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb2O5) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.

17.
ACS Nano ; 11(5): 5140-5147, 2017 05 23.
Artigo em Inglês | MEDLINE | ID: mdl-28457124

RESUMO

Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage.

18.
ACS Appl Mater Interfaces ; 9(18): 15549-15556, 2017 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-28425698

RESUMO

Polymer cathode materials are promising alternatives to inorganic counterparts for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their high theoretical capacity, adjustable molecular structure, and strong adaptability to different counterions in batteries, etc. However, they suffer from poor practical capacity and low rate capability because of their intrinsically poor conductivity. Herein, we report the synthesis of self-assembled graphene/poly(anthraquinonyl sufide) (PAQS) composite aerogel (GPA) with efficient integration of a three-dimensional (3D) graphene framework with electroactive PAQS particles via a novel dispersion-assembly strategy which can be used as a free-standing flexible cathode upon mechanical pressing. The entire GPA cathode can deliver the highest capacity of 156 mAh g-1 at 0.1 C (1 C = 225 mAh g-1) with an ultrahigh utilization (94.9%) of PAQS and exhibits an excellent rate performance with 102 mAh g-1 at 20 C in LIBs. Furthermore, the flexible GPA film was also tested as cathode for SIBs and demonstrated a high-rate capability with 72 mAh g-1 at 5 C and an ultralong cycling stability (71.4% capacity retention after 1000 cycles at 0.5 C) which has rarely been achieved before. Such excellent electrochemical performance of GPA as cathode for both LIBs and SIBs could be ascribed to the fast redox kinetics and electron transportation within GPA, resulting from the interconnected conductive framework of graphene and the intimate interaction between graphene and PAQS through an efficient wrapping structure. This approach opens a universal way to develop cathode materials for powerful batteries with different metal-based counter electrodes.

19.
Chemistry ; 23(35): 8358-8363, 2017 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-28349610

RESUMO

Facile and controllable integration of metal cyanides (MCs) into three-dimensional graphene (3DG) with advantageous structures is of fundamental importance for the development of superior MC-based electrode materials for electrochemical energy storage and catalysis. Here a facile and versatile spatially-confined Ostwald ripening strategy was developed to synthesize a series of 3DG wrapped MC aerogels with different compositions, size, and structure based on the chemical instability of MC in the reaction system. Remarkably, the integration of Prussian blue (PB) into 3DG, with such unique architecture, largely improves the rate performance and long-term cycling stability of PB as a cathode material for sodium ion batteries.

20.
Nat Commun ; 7: 13432, 2016 11 17.
Artigo em Inglês | MEDLINE | ID: mdl-27853174

RESUMO

Natural plants consist of a hierarchical architecture featuring an intricate network of highly interconnected struts and channels that not only ensure extraordinary structural stability, but also allow efficient transport of nutrients and electrolytes throughout the entire plants. Here we show that a hyperaccumulation effect can allow efficient enrichment of selected metal ions (for example, Sn2+, Mn2+) in the halophytic plants, which can then be converted into three-dimensional carbon/metal oxide (3DC/MOx) nanocomposites with both the composition and structure hierarchy. The nanocomposites retain the 3D hierarchical porous network structure, with ultrafine MOx nanoparticles uniformly distributed in multi-layers of carbon derived from the cell wall, cytomembrane and tonoplast. It can simultaneously ensure efficient electron and ion transport and help withstand the mechanical stress during the repeated electrochemical cycles, enabling the active material to combine high specific capacities typical of batteries and the cycling stability of supercapacitors.

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