Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage
Authors: Bonaccorso Francesco , Colombo Luigi , Yu Guihua , Stoller Meryl , Tozzini Valentina , Ferrari Andrea C. , Ruoff Rodney S. , Pellegrini Vittorio 
Published: January 2015

Background

The integration of graphene in photovoltaic modules, fuel cells, batteries, supercapacitors, and devices for hydrogen generation offers opportunities to tackle challenges driven by the increasing global energy demand. Graphene’s two-dimensional (2D) nature leads to a theoretical surface-to-mass ratio of ~2600 m
2
/g, which combined with its high electrical conductivity and flexibility, gives it the potential to store electric charge, ions, or hydrogen. Other 2D crystals, such as transition metal chalcogenides (TMDs) and transition metal oxides, are also promising and are now gaining increasing attention for energy applications. The advantage of using such 2D crystals is linked to the possibility of creating and designing layered artificial structures with “on-demand” properties by means of spin-on processes, or layer-by-layer assembly. This approach exploits the availability of materials with metallic, semiconducting, and insulating properties.

Advances
The success of graphene and related materials (GRMs) for energy applications crucially depends on the development and optimization of production methods. High-volume liquid-phase exfoliation is being developed for a wide variety of layered materials. This technique is being optimized to control the flake size and to increase the edge-to-surface ratio, which is crucial for optimizing electrode performance in fuel cells and batteries. Micro- or nanocrystal or flake edge control can also be achieved through chemical synthesis. This is an ideal route for functionalization, in order to improve storage capacity. Large-area growth via chemical vapor deposition (CVD) has been demonstrated, producing material with high structural and electronic quality for the preparation of transparent conducting electrodes for displays and touch-screens, and is being evaluated for photovoltaic applications. CVD growth of other multicomponent layered materials is less mature and needs further development. Although many transfer techniques have been developed successfully, further improvement of high-volume manufacturing and transfer processes for multilayered heterostructures is needed. In this context, layer-by-layer assembly may enable the realization of devices with on-demand properties for targeted applications, such as photovoltaic devices in which photon absorption in TMDs is combined with charge transport in graphene.

Outlook

Substantial progress has been made on the preparation of GRMs at the laboratory level. However, cost-effective production of GRMs on an industrial scale is needed to create the future energy value chain. Applications that could benefit the most from GRMs include flexible electronics, batteries with efficient anodes and cathodes, supercapacitors with high energy density, and solar cells. The realization of GRMs with specific transport and insulating properties on demand is an important goal. Additional energy applications of GRMs comprise water splitting and hydrogen production. As an example, the edges of MoS
2
single layers can oxidize fuels—such as hydrogen, methanol, and ethanol—in fuel cells, and GRM membranes can be used in fuel cells to improve proton exchange. Functionalized graphene can be exploited for water splitting and hydrogen production. Flexible and wearable devices and membranes incorporating GRMs can also generate electricity from motion, as well as from water and gas flows.

Tailored GRMs for energy applications.

The ability to produce GRMs with desired specific properties paves the way to their integration in a variety of energy devices. Solution processing and chemical vapor deposition are the ideal means to produce thin films that can be used as electrodes in energy devices (such as

solar panels, batteries, fuel cells, or in hydrogen storage). Chemical synthesis is an attractive route to produce “active” elements in solar cell or thermoelectric devices.