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Graphene materials for energy storage applications

(Nanowerk Spotlight) Graphene-based nanomaterials have many promising applications in energy-related areas. For instance, graphene improves both energy capacity and charge rate in rechargeable batteries; activated graphene makes superior supercapacitors for energy storage; graphene electrodes may lead to a promising approach for making solar cells that are inexpensive, lightweight and flexible; crumpled graphene boosts performance of high-energy lithium storage materials; and multifunctional graphene mats are promising substrates for catalytic systems.
These examples highlight the four major energy-related areas where graphene will have an impact: solar cells, supercapacitors, lithium-ion batteries, and catalysis for fuel cells. Read more about these areas in our Nanowerk summary: Graphene Nanotechnology in Energy.
Today we are looking at two of these fields: energy storage applications for batteries and supercapacitors.
Graphene and graphene-based materials have attracted great attention in this area owing to their unique properties of high mechanical flexibility, large surface area, chemical stability, superior electric and thermal conductivities that render them great choices as alternative electrode materials for electrochemical energy storage systems.
A recent review article in Small ("Graphene and Graphene-Based Materials for Energy Storage Applications") summarizes the progress in graphene and graphene-based materials for four energy storage systems, i.e., lithium-ion batteries, supercapacitors, lithium-sulfur batteries and lithium-air batteries.
Morphologies of a graphene-based air electrode
Morphologies of the graphene-based air electrode. a, b) SEM images of as-prepared functionalized graphene sheets (FGS) (carbon/ oxygen (C/O) = 14) air electrodes at different magnifications. c, d) Discharged air electrode using FGS with C/O = 14 and C/O = 100, respectively. (© American Chemical Society)

Graphene for batteries

The key to create high-power LIBs is developing new materials with high electrical conductivity for fast electron transport and a large surface area and well-developed nanostructures with shortened diffusion length for Li ions.
In this aspect, graphene, due to its superior electrical conductivity, excellent mechanical flexibility, good chemical stability, and high surface area (2630 m2 g-1), is expected to be a good candidate. However, it is reported that LIBs with pristine graphene anodes cannot provide stable potential outputs, which sets obstacle for its practical applications.
To circumvent such problem and further improve the performance of graphene electrodes, variable strategies have been developed, the advantages and challenges of which – for both anodes and cathodes – are discussed in the review.

Graphene-Based Materials for Supercapacitors

As another fast-rising class of energy capture and storage devices, electrochemical capacitors (ECs) – also known as supercapacitors or ultracapacitors – are more advantageous than batteries in terms of their higher power density and more excellent cyclability. Besides, they are essentially maintenance-free, require a very simple charging circuit, and will experience no memory effect and thus are considered as very promising candidates for emerging renewable energy applications.
While so far, the energy density for commercialized ECs can only achieve 510 Wh kg-1 , which is significantly lower than that of batteries, i.e., 120170 Wh kg-1 of lithium-ion cells.
In this section of the review, the authors discuss the latest progress and challenges in reaching ECs with high power and high energy densities based on graphene and graphene-based composites.

Graphene and Graphene-Based Composites for Lithium Sulfur Batteries

Since the early study of lithium-sulfur (Li-S) batteries back in the 1940s, numerous efforts have been invested to its commercialization. However, even after all these years, significant challenges remain that hinders the commercialization of this technology. One major problem is the inherent low electrical conductivity of sulfur, which results in limited active material utilization efficiency and rate capability. Another issue is the polysulfide anions formed as reaction intermediates in the charge-discharge process are highly soluble.
So far, the possible solution to the above issues is to encapsulate sulfur in a carbon matrix, which is supposed to enhance the electrical conductivity of sulfur cathode, trap soluble Li2Sn intermediates, and accommodate volume variation of electrode during cycling. Among these carbonaceous materials, application of graphene in Li-S battery is very promising due to its unique 2D structure, high conductivity and superior mechanical flexibility. Besides, the surface functional groups of graphene can be tuned flexibly to immobilize S/Li2Sx on the graphene surface during the cycling process.
Various strategies for fabricating graphene-based electrode materials for Li-S batteries are discussed in the article.

Graphene and Graphene-Based Composites for Lithium Air/Oxygen Batteries

As potential next-generation energy storage devices, the lithium air/oxygen (Li-O2) batteries abandon the intercalation electrodes in traditional Li-ion batteries. The Li ions react directly with the oxygen in a porous electrode.
Such unique battery chemistry and electrode architecture provide a greatly increased theoretical specific energy (∼3500 Wh kg-1), holding significant potential to meet the targets set for batteries in automotive applications (∼1700 Wh kg-1).
Concluding their review, the authors note that incorporation of graphene and its derivatives into the traditional active materials have brought about many remarkable advances in the frontier energy storage systems, that is, Li-ion battery, supercapacitor, Li-S battery and Li-O2 battery. For the future design and optimization of supercapacitors, 3D structures based on the self-assembly behavior of 2D graphene sheets and their further hybridization with active materials (e.g., metal oxide, metal sulfides) may be very promising.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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