Graphene synthesis and applications
Graphene is a two-dimensional material, containing carbon atoms arranged in a hexagonal (or 'honeycomb') lattice. Each carbon atom in graphene is sp2 hybridised and covalently bonded to 3 neighbouring atoms. Each atom has four valence electrons that become involved in bonding: 3 in single, sigma bonds, and a fourth in a pi bond. This arrangement means that carbon-carbon bonds in graphene are extremely strong. Furthermore, the pi electrons are delocalised and free to move throughout a conjugated network that extends throughout the lattice.
Thanks to this structure, graphene presents peculiar properties, such as:
- Huge surface area to weight ratio, up to 2630 m2/g
- Electron mobility up to 200,000 cm2/Vs
- High electrical conductivity
- Ballistic conduction over micron length-scales
- Thermal conductivity of about 5000 W/mK
- Exceptional mechanical properties: Young's modulus about 1 TPa, and fracture strength approximately 130 GPa
- 97.7% transmittance across the optical and IR regions
In our labs we developed methods for the deposition of high quality single layer graphene sheets on copper substrate and technological steps for its transfer to other substrates. Among the possible application graphene has in the field of nanotechnology, our research activity is currently focused on the technological processes for the fabrication of devices for biological analysis, exploiting the favorable characteristics of the interface between graphene and living cells and its interaction with metallic NPs for the development of sensors based on SERS technique. Moreover, since graphene is a very promising material for the controlled separation processes, we are developing innovative systems for the fabrication of selective membranes for water purification.
Graphene-based aerogel synthesis
Graphene-based materials with hierarchical structures, interconnected in three-dimensional (3D) network, are easily prepared by a one-step hydrothermal reduction of aqueous graphene oxide (GO) dispersion. After the hydrothermal, process performed in autoclave below 200°C, overnight, a hydrogel is obtained. A fast freezing in liquid N2 and a subsequent freeze-drying process in a lyophilizer, allows the removal of the water trapped in the hydrogel structure without affecting the pores size and shape. An aerogel is so produced.
Graphene hydrogel by hydrothermal synthesis
Graphene aerogel by lyophilization
The addition to the GO dispersion of liquid precursors of transition metal oxides (MOx) or sulphides (MSx) allow the production of hybrid aerogels. The MOx nanoparticles or MSx nanosheets of these oxidation-reduction performing materials are so in situ synthesized together with graphene 3D network and homogenously decorate the graphene aerogel surface. These hybrid materials are highly performing as electrodes for supercapacitors.
Graphene Aerogel Structure
MoO2-graphene Aerogel structure
Supercapacitors are robust energy storage devices that bridge the gap between conventional capacitors and standard batteries, offering high power density, long life-time (> 100000 cycles), fast charge/discharge rates, and wide operational temperature range.
In general, a supercapacitor consists of two porous electrodes and an electrolyte sandwiched between them.
The nanostructuration of the electrode materials will bring considerable advantages to the performance of supercapacitors, in particular allowing to increase their energy density without losing the high power density. With the aim to reduce the manufacturing cost, while keeping a good operation window, carbon-based materials have been widely favored for the electrodes fabrication. In particular, different kinds of nanostructured carbon-based materials have been studied as active material in electrochemical cells, including graphite powder and carbon black, glassy carbon, carbon nanotubes, and graphene. The graphene and graphene-based materials can be considered ideal materials for supercapacitor (SC) electrodes. In fact, the graphene exhibits excellent chemical and physical properties, such as high electrical conductivity, high surface area, chemical and thermal stability, high flexibility and mechanical strength, large electrochemical window and both its exterior surfaces can be readily accessible by an electrolyte.
The synthesis of graphene-based electrodes exhibiting a high surface area usually takes place by hydrothermal methods, because of the easy implementation, high cleanliness of the process, environmental-friendly and scalable on an industrial scale. One strategy lead in the fabrication of 3D graphene network allowing to further improve the specific capacitance by acting of the available surface area.
Standard planar supercapacitor consists of two porous electrodes deposited by screen printing onto conductive substrates and a separator swelled into electrolytic solution sandwiched between them.
The whole structure was irreversibly sealed exploiting thermoplastic polymer as adhesion layer.
Alternatively we have developed an housing system, previously employed for dye sensitized solar cells, that allows a reversible sealing of the device and the testing of flexible electrode such as metal grids.
The rapid development of the wearable electronics or e-textiles has increased the need of innovative energy storage systems in order to overcome the integration problems of old fashioned bulky capacitors into these devices. To date a fully developed textile energy storage device does not exist, nor does a streamlined manufacturing process integrating the various components.
We are studying the fabrication of wire-shaped supercapacitors exploiting innovative synthesis approaches. Graphene aerogel self-assembly procedure on copper wire was obtained by easy hydrothermal reaction of GO dispersion. Precursor reduction and winding self-assembly around the copper wire take place during the process. Polymeric electrolyte-coated wires were used as symmetric electrodes into flexible wearable supercapacitors revealing superior energy storage behavior.
The growing demand of new temporary energy storage system push the use of new material and method to obtain high performance micro devices to be integrated in mobile devices (smartphones, cameras,...). The micro supercapacitors exploit the MEMS technology to embed a Graphene based material to obtain high capacitance in a miniaturized device. In our labs, new structures of micro devices so called Micro-supercapacitors are studied following all the steps from the design, through the fabrication, to the final electrochemical tests. Interdigitated electrode patterns are designed with particular attention for the increase of the surface area exposed in order to maximize the ionic interaction with the surface of the active material. The fabrication of the devices is conducted in our facilities through the use of conventional semiconductor fabrication processes in a clean room environment. The electrochemical tests are then performed to confirm the capacitive behaviour of the device and from them have been proved the superior performance of the graphene devices compared to conventional graphite based ones. Besides more traditional Si based devices, polymer microsupercapacitors take advantages from replication techniques that are able to lower the costs of the single device and to exploit the flexibility of the substrate. The main goal of this study is the optimization of a fabrication process for the development of a low cost flexible device with high capacitance per area.
Micro-supercapacitor device on a silicon substrate.
Micro-supercapacitor device on a PDMS substrate.
Cyclic Voltammetry measurement
- "A novel graphene based nanocomposite for application in 3D flexible micro-supercapacitors", S.L. Marasso, P. Rivolo, R. Giardi, D. Mombello, A. Gigot, M. Serrapede, S. Benetto, A. Enrico, M. Cocuzza, E. Tresso, C.F. Pirri, Materials Research Express, 2016, Vol. 3(6), doi:10.1088/2053-1591/3/6/065001