In Operando Probing of Lithium-Ion Storage on Single-Layer Graphene.

Despite high-surface area carbons, e.g., graphene-based materials, being investigated as anodes for lithium (Li)-ion batteries, the fundamental mechanism of Li-ion storage on such carbons is insufficiently understood. In this work, the evolution of the electrode/electrolyte interface is probed on a single-layer graphene (SLG) film by performing Raman spectroscopy and Fourier transform infrared spectroscopy when the SLG film is electrochemically cycled as the anode in a half cell. The utilization of SLG eliminates the inevitable intercalation of Li ions in graphite or few-layer graphene, which may have complicated the discussion in previous work. Combining the in situ studies with ex situ observations and ab initio simulations, the formation of solid electrolyte interphase and the structural evolution of SLG are discussed when the SLG is biased in an electrolyte. This study provides new insights into the understanding of Li-ion storage on SLG and suggests how high-surface-area carbons could play proper roles in anodes for Li-ion batteries.

Direct Laser Writing of Graphene Made from Chemical Vapor Deposition for Flexible, Integratable Micro-Supercapacitors with Ultrahigh Power Output.

High-performance yet flexible micro-supercapacitors (MSCs) hold great promise as miniaturized power sources for increasing demand of integrated electronic devices. Herein, this study demonstrates a scalable fabrication of multilayered graphene-based MSCs (MG-MSCs), by direct laser writing (DLW) of stacked graphene films made from industry-scale chemical vapor deposition (CVD). Combining the dry transfer of multilayered CVD graphene films, DLW allows a highly efficient fabrication of large-areal MSCs with exceptional flexibility, diverse planar geometry, and capability of customer-designed integration. The MG-MSCs exhibit simultaneously ultrahigh energy density of 23 mWh cm-3 and power density of 1860 W cm-3 in an ionogel electrolyte. Notably, such MG-MSCs demonstrate an outstanding flexible alternating current line-filtering performance in poly(vinyl alcohol) (PVA)/H2 SO4 hydrogel electrolyte, indicated by a phase angle of -76.2° at 120 Hz and a resistance-capacitance constant of 0.54 ms, due to the efficient ion transport coupled with the excellent electric conductance of the planar MG microelectrodes. MG-polyaniline (MG-PANI) hybrid MSCs fabricated by DLW of MG-PANI hybrid films show an optimized capacitance of 3.8 mF cm-2 in PVA/H2 SO4 hydrogel electrolyte; an integrated device comprising MG-MSCs line filtering, MG-PANI MSCs, and pressure/gas sensors is demonstrated.

Enhanced physical properties of γ-Al2O3-rGO hybrids prepared by solvothermal and hot-press processing.

In this study, a solvothermal method was employed for the first time to fabricate hybrids composed of cross-linked γ-Al2O3 nanorods and reduced graphite oxide (rGO) platelets. After calcination and hot-press processing, monoliths of Al2O3-rGO hybrids were obtained with improved physical properties. It was found that the oxygen-containing groups on graphene oxide were beneficial for the adsorption of aluminum isopropoxide, leading to a uniform dispersion of rGO with Al2O3, which was obtained by hydrolysis of aluminum isopropoxide during the solvothermal reaction. The hybrid, which was subsequently calcinated for 3 h showed electrical conductivity of 6.7 × 101 S m-1 together with 90% higher mechanical tensile strength and 80% higher thermal conductivity as compared to the bare Al2O3. In addition, the dielectric constant of the hybrid was 12 times higher than that of the bare Al2O3. In this study, the highest values of electrical conductivity (8.2 × 101 S m-1), thermal conductivity (2.53 W m-1 K-1), dielectric constant (104) and Young's modulus (3.7 GPa) were obtained for the alumina-rGO hybrid calcinated for 1 h. XRD characterization showed that an increase in calcination temperature and further hot-press processing at 900 °C led to enhanced crystallinity in the γ-Al2O3 nanorods in the hybrid, resulting in enhanced physical properties in the hybrids.

Diameter-Sensitive Breakdown of Single-Walled Carbon Nanotubes upon KOH Activation.

While potassium hydroxide (KOH) activation has been used to create pores in carbon nanotubes (CNTs) for improved energy-storage performance, the KOH activation mechanism of CNTs has been rarely investigated. In this work, the reaction between single-walled CNTs (SWCNTs) and KOH is studied in situ by thermogravimetric analysis coupled to infrared (IR) spectroscopy and gas chromatography/mass spectrometry (MS). The IR and MS results clearly demonstrate the sequential evolution of CO, hydrocarbons, CO2 , and H2 O in the activation process. By using the radial breathing mode of Raman spectroscopy, a diameter-sensitive selectivity is observed in the reaction between SWCNTs and KOH, leading to a preferential distribution of SWCNTs with diameters larger than 1 nm after activation at 900 °C and a preferential removal of SWCNTs with diameters below 1 nm upon activation.

Incorporating Pyrrolic and Pyridinic Nitrogen into a Porous Carbon made from C60 Molecules to Obtain Superior Energy Storage.

Nitrogen-doped porous carbon is obtained by KOH activation of C60 in an ammonia atmosphere. As an anode for Li-ion batteries, it shows a reversible capacity of up to ≈1900 mA h g-1 at 100 mA g-1 . Simulations suggest that the superior Li-ion storage may be related to the curvature of the graphenes and the presence of pyrrolic/pyridinic group dopants.

Supercapacitors: A Hierarchical Carbon Derived from Sponge-Templated Activation of Graphene Oxide for High-Performance Supercapacitor Electrodes (Adv. Mater. 26/2016).

H. Ji, Y. Zhu, and co-workers demonstrate a 3D hierarchically porous carbon by introducing a polyurethane sponge to template graphene oxide into a 3D interconnected structure while KOH activation generates abundant micropores in its backbone. As described on page 5222, a supercapacitor assembled with this carbon material achieves a high energy density of 89 W h kg(-1) (64 W h L(-1) ) and outstanding power density due to its shortened ion transport distance in three dimensions.

A Hierarchical Carbon Derived from Sponge-Templated Activation of Graphene Oxide for High-Performance Supercapacitor Electrodes.

A hierarchical porous carbon is fabricated by introducing a polyurethane sponge to a template graphene oxide into a 3D interconnected structure, while KOH activation generates abundant micropores in its backbone. Supercapacitors assembled with this carbon achieve a high energy density of 89 W h kg(-1) (64 W h L(-1) ) and outstanding power density due to the shortened ion-transport distance in 3D.

Creating Pores on Graphene Platelets by Low-Temperature KOH Activation for Enhanced Electrochemical Performance.

KOH activation of microwave exfoliated graphite oxide (MEGO) is investigated in detail at temperatures of 450-550 °C. Out of the activation temperature range conventionally used for the preparation of activated carbons (>600 °C), the reaction between KOH and MEGO platelets at relatively low temperatures allows one to trace the structural transition from quasi-two-dimensional graphene platelets to three-dimensional porous carbon. In addition, it is found that nanometer-sized pores are created in the graphene platelets at the activation temperature of around 450 °C, leading to a carbon that maintains the platelet-like morphology, yet with a specific surface area much higher than MEGO (e.g., increased from 156 to 937 m(2) g(-1) ). Such a porous yet highly conducting carbon shows a largely enhanced electrochemical activity and thus improved electrochemical performance when being used as electrodes in supercapacitors. A specific capacitance of 265 F g(-1) (185 F cm(-3) ) is obtained at a current density of 1 A g(-1) in 6 m KOH electrolyte, which remains 223 F g(-1) (156 F cm(-3) ) at the current density of 10 A g(-1) .

Assembling carbon quantum dots to a layered carbon for high-density supercapacitor electrodes.

It is found that carbon quantum dots (CQDs) self-assemble to a layer structure at ice crystals-water interface with freeze- drying. Such layers interconnect with each other, forming a free-standing CQD assembly, which has an interlayer distance of about 0.366 nm, due to the existence of curved carbon rings other than hexagons in the assembly. CQDs are fabricated by rupturing C60 by KOH activation with a production yield of ~15 wt.%. The CQDs obtained have an average height of 1.14 nm and an average lateral size of 7.48 nm, and are highly soluble in water. By packaging annealed CQD assembly to high density (1.23 g cm(-3)) electrodes in supercapacitors, a high volumetric capacitance of 157.4 F cm(-3) and a high areal capacitance of 0.66 F cm(-2) (normalized to the loading area of electrodes) are demonstrated in 6 M KOH aqueous electrolyte with a good rate capability.

Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au-graphene-Ag hybrids.

We report an efficient surface-enhanced Raman scattering (SERS) substrate by utilizing the multi-dimensional plasmonic coupling in Au nanoparticle (NP)-graphene-Ag NP hybrid structures. An ultrasensitive SERS detection with a limit of down to 10(-13) M has been achieved when the sandwiched hybrid film is fabricated on an Ag substrate.