The large specific surface area (SSA)—i.e., the surface-to-mass ratio—of graphene, combined with its high electrical conductivity, high mechanical strength, ease of functionalization, and potential for mass production, makes it an extremely attractive platform for energy applications, such as a transparent conductive electrode for solar cells or as flexible high-capacity electrode in lithium-ion batteries and supercapacitors, notes a team of researchers from Europe, the US and Korea, in a paper reviewing the role of graphene and related systems for energy conversion and storage published in the journal Science. The combination of chemical functionalization and curvature control also opens new opportunities for hydrogen storage.
In addition to graphene, they note, other two-dimensional crystals such as the transition metal dichalcogenides (TMDs) display insulating, semiconducting (with band gaps in the visible region of the spectrum), and metallic behavior and can enable novel device architectures also in combination with graphene. As with graphene, these materials can be integrated on flexible surfaces and can be mass-produced. Yet another class of 2D crystals is the MXenes (e.g., earlier post)—layered, hexagonal carbides and nitrides that can accommodate various ions and molecules between their layers by intercalation. MXene sheets are promising for energy applications, such as lithium-ion batteries, supercapacitors, and hydrogen storage.
Further, the authors note, some 2D crystals are also promising for fuel cells and in water-splitting applications because of the large photocatalytic properties of their edges. The creation of hybrids with graphene and other nanomaterials, such as CNTs, can find applications in energy storage devices, such as supercapacitors, but also in photovoltaics. In their paper, Bonaccorso et al. refer to graphene, other 2D crystals, and hybrid systems as graphene and related materials (GRMs).
In the paper in Science, Bonaccorso et al. review the use of GRMs for energy conversion in solar cells, thermoelectric devices, and fuel cells; and for energy storage in batteries, supercapacitors, and hydrogen production and storage.
Graphene, related 2D crystals, and hybrid systems might play a major role in future energy conversion and storage technologies. The ability to produce these GRMs, and control their properties, might enable a range of device characteristics, with optimized energy/power densities, lifetime, safety, and potentially reducing cost while minimizing environmental impact. To be commercially viable, GRMs must substantially surpass the performance of existing materials at comparable manufacturing costs. For example, GRMs have been reported with specific capacitances of ~300 F/cm3, which is much higher than chemically activated state-of-the-art carbons. The ability to create stacked hetero-structures of metallic, semiconducting, and insulating 2D crystals might enable an even broader spectrum of device structures, perhaps with tunable properties. This might enable bulk thermoelectric materials with on-demand band structures and transport properties, or photosensitizers with broadband photon absorption. Owing to the GRMs intrinsic flexibility, we also envision applications such as wearable energy devices and energy harvesting from water or gas flows.
Additionally, because GRMs can perform different functions, they may enable the realization of affordable energy systems with integrated conversion, storage, and sensing modules. In the future, it might be possible to target flexible photovoltaic cells with efficiencies of 12% and cost of ~0.5€/Wpeak (peak power output), fuel cells with 10 kW per gram of platinum, and energy storage devices with an energy density of at least 250 Wh/kg and cyclability up to 5000 cycles for batteries and a power density of 100kW/kg for supercapacitors. For hydrogen storage, the challenge is to achieve a gravimetric storage of 5.5%.—Bonaccorso et al.
Francesco Bonaccorso, Luigi Colombo, Guihua Yu, Meryl Stoller, Valentina Tozzini, Andrea C. Ferrari, Rodney S. Ruoff, and Vittorio Pellegrini (2015) “Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage,” Science 347 (6217), 1246501 doi: 10.1126/science.1246501