Many solution processing methods of exfoliation of layered materials have been studied during the last few years; most of them are based on organic solvents or rely on surfactants and other funtionalization agents. Pure water should be an ideal solvent, however, it is generally believed, based on solubility theories that stable dispersions of water could not be achieved and systematic studies are lacking. Here we describe the use of water as a solvent and the stabilization process involved therein. We introduce an exfoliation method of molybdenum disulfide (MoS2) in pure water at high concentration (i.e., 0.14 ± 0.01 g L−1). This was achieved by thinning the bulk MoS2 by mechanical exfoliation between sand papers and dispersing it by liquid exfoliation through probe sonication in water. We observed thin MoS2 nanosheets in water characterized by TEM, AFM and SEM images. The dimensions of the nanosheets were around 200 nm, the same range obtained in organic solvents. Electrophoretic mobility measurements indicated that electrical charges may be responsible for the stabilization of the dispersions. A probability decay equation was proposed to compare the stability of these dispersions with the ones reported in the literature. Water can be used as a solvent to disperse nanosheets and although the stability of the dispersions may not be as high as in organic solvents, the present method could be employed for a number of applications where the dispersions can be produced on site and organic solvents are not desirable.

Electric double-layer capacitors (EDLCs) or supercapacitors (SCs) are fast energy storage devices with high pulse efficiency and superior cyclability, which makes them useful in various applications including electronics, vehicles and grids. Aqueous SCs are considered to be more environmentally friendly than those based on organic electrolytes. Because of the corrosive nature of the aqueous environment, however, expensive electrochemically stable materials are needed for the current collectors and electrodes in aqueous SCs. This results in high costs for a given energy-storage capacity. To address this, we developed a novel low-cost aqueous SC using graphite foil as the current collector and a mix of graphene, nanographite, simple water-purification carbons and nanocellulose as electrodes. The electrodes were coated directly onto the graphite foil by using casting frames and the SCs were assembled in a pouch cell design. With this approach, we achieved a material cost reduction of greater than 90% while maintaining approximately one-half of the specific capacitance of a commercial unit, thus demonstrating that the proposed SC can be an environmentally friendly, low-cost alternative to conventional SCs.

The number of applications based on graphene, few-layer graphene, and nanographite is rapidly increasing. A large-scale process for production of these materials is critically needed to achieve cost-effective commercial products. Here, we present a novel process to mechanically exfoliate industrial quantities of nanographite from graphite in an aqueous environment with low energy consumption and at controlled shear conditions. This process, based on hydrodynamic tube shearing, produced nanometer-thick and micrometer-wide flakes of nanographite with a production rate exceeding 500 gh-1 with an energy consumption about 10 Whg-1. In addition, to facilitate large-area coating, we show that the nanographite can be mixed with nanofibrillated cellulose in the process to form highly conductive, robust and environmentally friendly composites. This composite has a sheet resistance below 1.75 Ω/sq and an electrical resistivity of 1.39×10-4 Ωm and may find use in several applications, from supercapacitors and batteries to printed electronics and solar cells. A batch of 100 liter was processed in less than 4 hours. The design of the process allow scaling to even larger volumes and the low energy consumption indicates a low-cost process.

Graphene and porous carbon materials are widely used as electrodes in supercapacitors. In order to form mechanically stable electrodes, binders can be added to the conducting electrode material. However, most binders degrade the electrical performance of the electrodes. Here we show that by using nanofibrillated cellulose (NFC) as a binder the electrical properties, such as capacitance, were enhanced. The highest capacitance was measured at an NFC content of approximately 10 % in ratio to the total amount of active material. The NFC improved the ion transport in the electrodes. Thus, electrodes made of a mixture of nanographite and NFC achieved larger capacitances in supercapacitors than electrodes with nanographite only. In addition to electrical properties, NFC enhanced the mechanical stability and wet strength of the electrodes significantly. Furthermore, NFC stabilized the aqueous nanographite dispersions, which improved the processability. Galvanostatic cycling was performed and an initial transient behaviour of the supercapacitors during the first cycles was observed. However, stabilized supercapacitors showed efficiencies of 98–100%.

Inkjet-printed metal films are important within the emerging field of printed electronics. For large-scale manufacturing, low-cost flexible substrates and low temperature sintering is desired. Tailored coated substrates are interesting for roll-to-roll fabrication of printed electronics, since a suitable tailoring of the ink-substrate system may reduce, or remove, the need for explicit sintering. Here we utilize specially designed coated papers, containing chloride as an active sintering agent. The built-in sintering agent greatly assists low-temperature sintering of inkjet-printed AgNP films. Further, we examine the effect of variations in coating pore size and precoating type. Interestingly, we find that the sintering is substantially affected by these parameters. IntroductionThe interest in printed electronics and other printed functionalities is considerable worldwide. In contrast to traditional subtractive photolithography-and etching processes, the printing process is purely additive, which means less material waste and a reduced need for processing chemicals. This results in cost savings and environmental advantages, in particular when large area coverage is desired. Additionally, printing processes have good compatibility with flexible substrates such as plastic films and paper. Applying electronic-or other functionality on flexible, low-cost substrates, across large areas in roll-to-roll processes, enables novel applications with large potential. Among the many important applications for printed electronics are photovoltaic devices . It is capable of covering large areas with simple and cost-effective setups. Furthermore, the non-contact nature of deposition allows it to be used for delicate and pressure-sensitive surfaces. The ink formulation is relatively challenging. To be reliably jetted, properties such as viscosity, surface tension, volatility and particle size need to be well controlled. To avoid aggregation, the particles are typically stabilized using polymers that adhere to the surfaces. Depending on the specific polymers and electrical charges in the system, the stabilization may be either steric, electrostatic or a combination of both (electrosteric) 13 . Some of the most common stabilizing polymers are polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) 14 .Electrically conducting layers are important since they are included in most applications of printed electronics. Although conductive inkjet inks may be formulated from several classes of materials, including conductive polymers 15 , carbon-based materials 16,17 , and metal nanoparticles (NPs) [18][19][20] . The metal NP inks are particularly suitable when high conductivity is required, for example when printing antennas 21 and circuit board conductors 22,23 .Paper-based substrates for printed electronics are interesting for several reasons. Environmental friendliness, flexibility and low cost are the key benefits. The physical and chemical properties may be altered by chemical additives or changes in materials and processes. Therefor...