g-Meng Chen

Low-Temperature Exfoliated Graphenes: Vacuum-Promoted Exfoliation and Electrochemical Energy Storage

A preheated high-temperature environment is believed to be critical for a chemical-exfoliation-based production of graphenes starting from graphite oxide, a belief that is based on not only experimental but also theoretical viewpoints. A novel exfoliation approach is reported in this study, and the exfoliation process is realized at a very low temperature, which is far below the proposed critical exfoliation temperature, by introducing a high vacuum to the exfoliation process. Owing to unique surface chemistry, low-temperature exfoliated graphenes demonstrate an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the high-temperature exfoliated ones. The low-temperature exfoliation approach presents us with a possibility for a mass production of graphenes at low cost and great potentials in energy storage applications of graphene-based materials.

Powering Lithium–Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts

Lithium-sulfur (Li-S) battery system is endowed with tremendous energy density, resulting from the complex sulfur electrochemistry involving multielectron redox reactions and phase transformations. Originated from the slow redox kinetics of polysulfide intermediates, the flood of polysulfides in the batteries during cycling induced low sulfur utilization, severe polarization, low energy efficiency, deteriorated polysulfide shuttle, and short cycling life. Herein, sulfiphilic cobalt disulfide (CoS2) was incorporated into carbon/sulfur cathodes, introducing strong interaction between lithium polysulfides and CoS2 under working conditions. The interfaces between CoS2 and electrolyte served as strong adsorption and activation sites for polar polysulfides and therefore accelerated redox reactions of polysulfides. The high polysulfide reactivity not only guaranteed effective polarization mitigation and promoted energy efficiency by 10% but also promised high discharge capacity and stable cycling performance during 2000 cycles. A slow capacity decay rate of 0.034%/cycle at 2.0 C and a high initial capacity of 1368 mAh g(-1) at 0.5 C were achieved. Since the propelling redox reaction is not limited to Li-S system, we foresee the reported strategy herein can be applied in other high-power devices through the systems with controllable redox reactions.

Permselective Graphene Oxide Membrane for Highly Stable and Anti-Self-Discharge Lithium–Sulfur Batteries

Lithium-sulfur batteries hold great promise for serving as next generation high energy density batteries. However, the shuttle of polysulfide induces rapid capacity degradation and poor cycling stability of lithium-sulfur cells. Herein, we proposed a unique lithium-sulfur battery configuration with an ultrathin graphene oxide (GO) membrane for high stability. The oxygen electronegative atoms modified GO into a polar plane, and the carboxyl groups acted as ion-hopping sites of positively charged species (Li(+)) and rejected the transportation of negatively charged species (Sn(2-)) due to the electrostatic interactions. Such electrostatic repulsion and physical inhibition largely decreased the transference of polysulfides across the GO membrane in the lithium-sulfur system. Consequently, the GO membrane with highly tunable functionalization properties, high mechanical strength, low electric conductivity, and facile fabrication procedure is an effective permselective separator system in lithium-sulfur batteries. By the incorporation of a permselective GO membrane, the cyclic capacity decay rate is also reduced from 0.49 to 0.23%/cycle. As the GO membrane blocks the diffusion of polysulfides through the membrane, it is also with advantages of anti-self-discharge properties.

Macroporous ‘bubble’ graphene film via template-directed ordered-assembly for high rate supercapacitors

A three-dimensional bubble graphene film, with controllable and uniform macropores and tailorable microstructure, was fabricated by a facile hard templating strategy and exhibit extraordinary electrochemical capacitance with high rate capability (1.0 V s(-1)).

Three-Dimensional Hierarchically Ordered Porous Carbons with Partially Graphitic Nanostructures for Electrochemical Capacitive Energy Storage

Three-dimensional, hierarchically ordered, porous carbon (HOPC) with designed porous textures, serving as an ion-buffering reservoir, an ion-transport channel, and a charge-storage material, is expected to be advanced an energy material for high-rate supercapacitors. Herein, HOPC without/with partially graphitic nanostructures have been directly synthesized by means of a simple one-pot synthesis procedure. The designed porous textures of the 3D HOPC materials are composed of highly ordered, fcc macroporous (300 nm), interconnected porous structures, including macroporous windows (170 nm), hexagonally ordered mesopores (5.0 nm), and useful micropores (1.2 nm). 3D HOPC-g-1000 (g=graphitic, 1000=pyrolysis temperature of 1000 °C) with partially graphitic nanostructures has a low specific surface area (296 m(2) g(-1)) and a low gravimetric specific capacitance (73.4 F g(-1) at 3 mV s(-1)), but improved electrical conductivity, better rate performance, higher electrolyte accessibility (24.8 μF cm(-2) at 3 mV s(-1)), faster frequency response (≈1 Hz), and excellent cycling performance (>5400 cycles). The specific capacitance per surface area is higher than that of conventional porous carbons, carbon nanotubes, and modified graphene (10-19 μF cm(-2)).

Rational Integration of Polypropylene/Graphene Oxide/Nafion as Ternary-Layered Separator to Retard the Shuttle of Polysulfides for Lithium-Sulfur Batteries.

The reversible electrochemical transformation from lithium (Li) and sulfur (S) into Li2 S through multielectron reactions can be utilized in secondary Li-S batteries with very high energy density. However, both the low Coulombic efficiency and severe capacity degradation limits the full utilization of active sulfur, which hinders the practical applications of Li-S battery system. The present study reports a ternary-layered separator with a macroporous polypropylene (PP) matrix layer, graphene oxide (GO) barrier layer, and Nafion retarding layer as the separator for Li-S batteries with high Coulombic efficiency and superior cyclic stability. In the ternary-layered separator, ultrathin layer of GO (0.0032 mg cm(-2) , estimated to be around 40 layers) blocks the macropores of PP matrix, and a dense ion selective Nafion layer with a very low loading amount of 0.05 mg cm(-2) is attached as a retarding layer to suppress the crossover of sulfur-containing species. The ternary-layered separators are effective in improving the initial capacity and the Coulombic efficiency of Li-S cells from 969 to 1057 mAh g(-1) , and from 80% to over 95% with an LiNO3 -free electrolyte, respectively. The capacity degradation is reduced from 0.34% to 0.18% per cycle within 200 cycles when the PP separator is replaced by the ternary-layered separators. This work provides the rational design strategy for multifunctional separators at cell scale to effective utilizing of active sulfur and retarding of polysulfides, which offers the possibility of high energy density Li-S cells with long cycling life.

Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications

With willow catkins as highly accessible carbon sources, hierarchical porous carbon microtubes (denoted as HPNCTs) have been successfully prepared by a facile carbonization and subsequent KOH activation process. The resulting materials not only inherited the natural tubular morphology of willow catkins, but also developed a hierarchical porous structure by activation, with nitrogen from the biomass being self-doped in the resulting carbon. A maximum specific surface area of 1775.7 m2 g−1 with a pore volume of 0.8516 cm3 g−1 was achieved for HPNCT-800. When evaluated as an electrode by a three-electrode system in 6 M KOH aqueous solution, the material exhibited a high gravimetric capacitance of 292 F g−1 at a current density of 1 A g−1, with a good rate capability of 83.5% retention at 10 A g−1. HPNCT-800 was further employed in a coin-type symmetric device with 1 M LiPF6 electrolyte, and exhibited a high energy density of 37.9 W h kg−1 at a power density of 700 W kg−1, with excellent cycling stability with 90.6% retention after 4000 cycles. By taking advantage of the unique structure of abundant biomass from nature, this work sheds light on the creation of advanced porous carbon materials towards energy storage applications.

Thermally reduced graphene oxide films as flexible lateral heat spreaders

A thermally reduced graphene oxide film (r-GOF), with tailorable micro-structures and macro-properties, is fabricated by annealing a filtrated graphene oxide film (GOF) in a confined space. The structural evolution of the film at different annealing temperatures is systematically investigated, and further correlated to the thermal conductivity and mechanical performances. With the increase of temperature, more oxygen-containing functional groups are removed from the film by a simultaneous conversion from sp3 to sp2 carbon in the graphitic lattice. As the temperature reached 1200 °C, the r-GOF achieves an ultrahigh thermal conductivity of ca. 1043.5 W m−1 K−1, while 1000 °C is a critical temperature in enhancing the thermal conductivity. Moreover, G1200 exhibits excellent mechanical stiffness and flexibility with a high tensile strength (13.62 MPa) and Youngs modulus (2.31 GPa). The combined conductivity and mechanical performances render the r-GOFs promising materials as flexible lateral heat spreaders for electronics.

Graphene-supported Au-Pd bimetallic nanoparticles with excellent catalytic performance in selective oxidation of methanol to methyl formate.

Graphene supported Au-Pd bimetallic nanoparticles exhibit high catalytic activity in methanol selective oxidation, with a methanol conversion of 90.2% and selectivity of 100%, to methyl formate at 70 °C, owing to the synergism of Au and Pd particles as well as the strong interaction between graphene and Au-Pd nanoparticles.

Dual-heteroatom-modified ordered mesoporous carbon: Hydrothermal functionalization, structure, and its electrochemical performance

The diverse applications of ordered mesoporous carbons (OMCs) are not only bonded to their superior structural properties, but also to their chemical properties. The termination of graphene sheets in OMCs provides abundant sites for heteroatom decoration to mediate their chemical properties. In this contribution, boron and phosphorus were co-incorporated into OMCs via a facile aqueous self-assembly taking advantage of a hydrothermal doping strategy. The as-obtained B/P-modified OMCs process a large surface area of ca. 600 m2 g−1, and a uniform pore size of ca. 6.3 nm, as well as long range ordering. By varying the hydrothermal synthesis temperature, the concentration of B and P introduced can be controlled from 0.8 to 1.6 wt% and from 2.3 to 3.6 wt%, respectively. The interaction of heteroatom B and P was enhanced when the hydrothermal temperature is above 100 °C. The heteroatom-containing groups were firmly embedded and homogeneously distributed on the carbon frameworks. When the B/P co-modified OMCs were applied as electrodes in supercapacitors, they presented promising performance compared with B- or/and P-modified OMC obtained without hydrothermal treatment.