Aqueous Activated Graphene Dispersions for Deposition of High-Surface Area Supercapacitor Electrodes.

High-surface area activated graphene has a three-dimensional porous structure that makes it difficult to prepare dispersions. Here we report a general approach that allows the preparatioon of stable water-based dispersions/inks at concentrations of ≲20 mg/mL based on activated graphene using environmentally friendly formulations. Simple drying of the dispersion on the substrate allows the preparation of electrodes that maintain the high specific surface area of the precursor material (∼1700 m2/g). The electrodes are flexible because of the structure that consists of micrometer-sized activated graphene grains interconnected by carbon nanotubes (CNTs). The electrodes prepared using activated graphene demonstrate performance superior to that of reduced graphene oxide in supercapacitors with KOH and TEA BF4/acetonitrile electrolytes providing specific capacitance values of 180 and 137 F/g, respectively, at a specific current of 1 A/g. The high surface area of activated graphene in combination with the good conductivity of CNTs allows an energy density of 35.6 Wh/kg and a power density of 42.2 kW/kg to be achieved. The activated graphene dispersions were prepared in liter amounts and are compatible with most industrial deposition methods.

Activated graphene as a material for supercapacitor electrodes: effects of surface area, pore size distribution and hydrophilicity.

Activated reduced graphene oxide (a-rGO) is a material with a rigid 3D porous structure and high specific surface area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ∼1000-3000 m2 g-1, and significant differences in pore size distribution and oxygen content. The performance of activated graphene as an electrode in a supercapacitor with KOH electrolyte was correlated with the structural parameters of the materials and water sorption properties. a-rGO is a hydrophobic material as evidenced by the negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, the total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide an improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). A decrease in the activation temperature from 850 °C to 550 °C is found to result in materials with a narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or a lower activation temperature provided materials with better specific capacitance despite a significant decrease (by ∼30%) of the BET (N2) SSA. The best gravimetric and volumetric capacitances in KOH electrolyte were achieved not for samples with the highest value of the BET (N2) SSA but for materials with 80-90% of the total pore volume in micropores and an increased BET (H2O) SSA. Comparing the performance of electrodes prepared using rGO and a-rGO shows that a more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte.

Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment.

A method to produce highly efficient and long-range vertical charge transport is demonstrated in an undoped polythiophene thin film, with average mobilities above 3.1 cm(2) V(-1) s(-1) . These record high mobilities are achieved by controlled orientation of the polymer crystallites enabling the most efficient and fastest charge transport along the chain backbones and across multiple chains. The significant increase in mobility shown here may present a new route to producing faster and more efficient optoelectronic devices based on organic materials.

Graphene oxide hydration and solvation: an in situ neutron reflectivity study.

Graphene oxide membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Using isotope contrast, neutron reflectivity was applied to evaluate the amounts of solvents intercalated into a membrane from pure and binary vapors and to evaluate the selectivity of the membrane permeation. Particularly, the effect of D2O, ethanol and D2O-ethanol vapours on graphene oxide (GO) thin films (∼25 nm) was studied. The interlayer spacing of GO and the amount of intercalated solvents were evaluated simultaneously as a function of vapour exposure duration. The significant difference in neutron scattering length density between D2O and ethanol allows distinguishing insertion of each component of the binary mixture into the GO structure. The amount of intercalated solvent at saturation corresponds to 1.4 molecules per formula unit for pure D2O (∼1.4 monolayers) and 0.45 molecules per formula unit (one monolayer) for pure ethanol. This amount is in addition to H2O absorbed at ambient humidity. Exposure of the GO film to ethanol-D2O vapours results in intercalation of GO with both solvents even for high ethanol concentration. A mixed D2O-ethanol layer inserted into the GO structure is water enriched compared to the composition of vapours due to slower ethanol diffusion into GO interlayers.