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Posted: Feb 03, 2015
Looking at graphene and other 2D crystals in energy
conversion and storage
(Nanowerk News) The single-atom thick allotrope of carbon known as graphene has many potenƟal
applicaƟons, among them energy conversion and storage. Graphene and related two-dimensional
crystals combine high electrical conducƟvity with physical flexibility and a huge surface to weight raƟo.
Such qualiƟes make them suitable for storing electric charge in baƩeries and supercapacitors, and as
catalysts in solar and fuel-cell electrodes.
A number of energy applicaƟons for 2D crystals are under development worldwide, and Europe’s
Graphene Flagship has invested significant resources in this area. In an arƟcle for the journal Science
("Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage"),
flagship scienƟsts, together with colleagues in the US and Korea, have reviewed the potenƟal for
graphene and related materials in the energy sector. The authors hope that their review will guide
researchers in academia and industry in drawing up a strategy for energy applicaƟons and their
implementaƟon.
In the Science review, the researchers – led by physicist Francesco Bonaccorso, who is based at the
Graphene Labs of the InsƟtuto Italiano di Tecnologia in Genova, and is a Royal Society Newton Fellow at
the Cambridge Graphene Centre – note the substanƟal progress made in material preparaƟon at the
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laboratory level. They also highlight the challenge of producing the materials on an industrial scale in a
cost-effecƟve manner.
Surface, funcƟon and producƟon
Graphene, the best-known of the hundreds of two-dimensional crystals invesƟgated to date, has a very
high surface-to-mass raƟo. With around 2,600 square metres for every gramme, graphene is all surface
and no bulk, and it is this 2D nature which gives graphene its unique electrical, thermal and mechanical
properƟes.
Other two-dimensional materials, including the transiƟon metal dichalcogenide molybdenum disulphide
(MoS2), various transiƟon metal oxides, and the MAX-phase class of 2D crystals, display properƟes
complementary to those of graphene. In all cases, the potenƟal for mass producƟon and chemical
funcƟonalisaƟon make graphene and related materials an ideal plaƞorm for energy applicaƟons. The
challenge is to produce them on a large scale with properƟes tailored for specific purposes.
There are various ways of producing graphene in bulk, but, when it comes to industrial scale
manufacture of the volumes required for energy applicaƟons, liquid-phase exfoliaƟon looks very
promising. This involves exploiƟng the agitaƟng effect of high-frequency sound waves to separate
graphene flakes from graphite held in suspension. Electronic-grade graphene can be produced through
chemical vapour deposiƟon, but the process is less suitable for energy applicaƟons. A third possibility is
the chemical synthesis of graphene flakes, but there are quesƟons of scalability.
Solar cells
Barely a week goes by without reports of new developments in solar cell technology. Efficiencies are on
the rise, and various nanomaterials are employed in the manufacture of photovoltaic films and
electrodes. Silicon remains the most widely used photon absorber, and this established semiconductor
material dominates the solar cell market. Second and third-generaƟon photovoltaic cells, including those
based on organic materials and quantum dots, have lower photocurrent efficiency than those fabricated
from silicon, and there are issues with material stability and durability.
Graphene and related materials are aƩracƟng serious interest in solar energy conversion, on the grounds
that they could improve efficiency, reduce producƟon costs, and deal with a number of environmental
issues. In addiƟon, 2D crystals have the advantages of transparency and flexibility, and in Ɵme they could
replace indium Ɵn oxide films, which are relaƟvely briƩle. They can also act as catalysts in dye-sensiƟsed
solar cells, in place of plaƟnum. This may result in a four orders-of-magnitude reducƟon in device cost.
Graphene can be engineered to match the low sheet resistance and high transmiƩance of transparent
conducƟng films made from established materials, and graphene-based materials have with varying
degrees of success been implemented in solar energy systems, including inorganic, organic,
dye-sensiƟsed and hybrid organic/inorganic cells. This is an especially acƟve area of research and
development, with chemically-doped graphene a parƟcular focus of aƩenƟon.
As well as the photo-sensiƟvity of solar cells, charge collecƟon and transport are important issues to
consider, and this applies both to dye-sensiƟsed cells and organic photovoltaics. For example, graphene
and related materials can be used to lessen the negaƟve effects of charge carrier recombinaƟon. In the
case of dye-sensiƟsed cells, reduced graphene oxide is employed in combinaƟon with Ɵtanium dioxide
nanoparƟcles, whilst graphene quantum dots are efficient electron hole transport layers in organic
photovoltaics.
There is a need to replace or at least reduce the amount of plaƟnum in solar cell counter-electrodes,
owing to the cost of the precious metal, and its tendency to degrade when in contact with iodide
electrolytes, which reduces device efficiency. FuncƟonalised graphene flakes and other 2D crystals have
been used in place of plaƟnum in counter-electrodes, someƟmes outperforming the plaƟnum.
The efficiency of photovoltaic devices based on graphene and related materials is improving at a pace
superior to those based on established materials, with the highest reported efficiency of 13% for
dye-sensiƟsed cells achieved using graphene nanoplatelets as a counter-electrode. With graphene-based
perovskite solar cells we have seen an efficiency of 15.6%, and that achieved with relaƟvely
low-temperature processing, yielding a significant cost reducƟon.
Thermoelectric devices
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Recovering waste heat is clearly a good thing, and this may be done with solid-state devices which
generate electricity from temperature gradients. Thermoelectric devices can also convert heat produced
by sunlight into electricity, and here they can enhance solar cells by capturing heat produced by photons
with energies below the band gaps of the photosensiƟve materials employed.
The effecƟveness of thermoelectric devices is assessed by the fracƟon of absorbed heat converted into
electricity, along with a figure of merit which depends on the material’s electrical and thermal
conducƟviƟes. TradiƟonal thermoelectric devices have conversion efficiencies of around 5%, and this low
value limits their widespread use.
In thermoelectric materials one is looking for high electrical and low thermal conducƟvity. On the face of
it this rules out graphene, in which the electrical and thermal conducƟviƟes are both high. However, it is
possible to tailor the thermal transport properƟes of graphene by introducing defects, edge roughness,
and periodic holes into the material. This can reduce thermal conducƟvity by up to 100 Ɵmes when
compared with prisƟne graphene. It can also raise the figure of merit by a factor of three over that
obtained with other materials.
The problem here is the difficulty in scaling tailored graphene nanoribbons via chemical synthesis. SƟll,
we can blend two-dimensional materials with carbon nanotubes to increase electrical conducƟvity
without reducing thermoelectric sensiƟvity.
Fuel cells
A fuel cell converts chemical energy from a fuel such as hydrogen gas into electricity via a reacƟon with
oxygen or another oxidising agent. Much of the aƩenƟon devoted to fuel cells focuses on high power
applicaƟons.
When it comes to improving and beƩer exploiƟng fuel cells, the principal difficulty is one of cost, and
specifically the need to replace expensive noble metals such as plaƟnum and gold as catalysts in the
chemical reacƟon. Fuel cell electrodes must also be chemically stable over the long term, and for some
applicaƟons physically flexible.
Graphene and related materials are promising candidates for fuel cell electrodes, and also as membranes
in proton exchange fuel cells. Where they cannot replace precious metal catalysts, the addiƟon of
two-dimensional materials may lower the amount of expensive catalyst materials required. For example,
reduced graphene oxide modifies the properƟes of plaƟnum electro-catalysts supported on it, leading to
improved methanol oxidaƟon when compared with commercial plaƟnum/carbon black mixtures.
BaƩeries
Today’s state-of-the-art rechargeable baƩeries are based on lithium-based cathodes and graphite
anodes, and crucial to their performance is the capacity of the anode material to store lithium ions.
Graphene has a larger gravimetric capacity than graphite, and the flexible nature of the material has
advantages when it comes to certain applicaƟons and environments. Graphene and related materials are
also appealing as fuel cell cathodes, owing to their high electrical capacity per unit weight.
Graphene-based hybrid electrodes may also be employed in rechargeable baƩeries to increase electron
transport, capacity, discharge current and device longevity. Where graphene is not actually incorporated
into baƩery electrodes, it may be used as a substrate for the growth of high-performance anode/cathode
nanoparƟcles. For example, lithium-based nanorods grown on reduced graphene oxide flakes show a
significantly lower degradaƟon over a fixed number of cycles than reduced graphene oxide or graphite.
Wrapping electrochemically-acƟve parƟcles within graphene or MoS2 flakes is another possibility.
Supercapacitors
Electrochemical capacitors store energy in an electric field set up between conducƟng plates separated
by an insulaƟng material. Supercapacitors, which divide into electrostaƟc double-layer capacitors, hybrid
and pseudocapacitors, are ideal for high power applicaƟons in which the required energy density is an
order of magnitude greater than is possible with lithium-ion baƩeries. Such applicaƟons include electric
and hybrid motor vehicles, heavy liŌing, load levelling and backup power for electric uƟliƟes and
industrial plant.
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Supercapacitors are necessarily large devices, and the materials of which they are made are produced in
bulk into electrodes 100 to 200 microns thick. Most double-layer capacitors have carbon electrodes
impregnated with organic electrolytes. Another type of supercapacitor is based on lithium-ion hybrid
cells, in which a graphite lithium-ion anode is coupled with an acƟvated carbon cathode. In this case the
energy density is around twice that of double-layer capacitors.
The performance of supercapacitors is dependent on the electrode surface area accessible to the
electrolyte, and this is where 2D crystals such as graphene have the advantage over graphite and
acƟvated carbon. In pracƟce this can include graphene-based platelets with spacer materials such as
carbon nanotubes, mesoporous carbon spheres, water and ionic liquids, and resins chemically acƟvated
to create a porous structure.
Note that large material surface areas do not always translate into high-performance supercapacitors,
especially if the material packing density is low. It is possible to increase the packing density through
evaporaƟon drying of graphene hydrogels, and by capillary compression of reduced graphene oxide.
Raising the operaƟng voltage is another way of increasing the energy storage capacity.
Double-layer capacitors are marked by their rapid charge/discharge and long cycle life, and lithium-ion
baƩeries by a high energy storage capacity. Combine the two qualiƟes, and the result is a lithium-ion
hybrid supercapacitor, albeit with performance trade-offs common to both types of device.
Electrodes for double-layer capacitors made from microwave-expanded graphite oxide and lithium-ion
baƩery electrodes comprising graphite, lithium and iron oxides have been studied, as have electrodes
containing metal oxides based on ruthenium, manganese and molybdenum, with conducƟng polymers
to increase the specific capacitance via chemical reducƟon and oxidaƟon reacƟons. Graphene is included
in these systems as a conducƟve support.
Hydrogen producƟon and storage
Hydrogen has an energy density more than three Ɵmes that of petrol, and its combusƟon by-product is
water. The principal challenges in using hydrogen as a green fuel are to produce and store the gas.
Electrolysis is the key mechanism for hydrogen gas producƟon, and the edges of 2D crystals including
MoS2 and reduced graphene oxide are acƟve catalyƟc sites in the reacƟon. Whilst simple in outline, the
detail of the hydrogen evoluƟon reacƟon varies from material to material, with resisƟve losses a
significant issue with non-metallic electrodes. Combining graphene and related materials with carbon
nanotubes can enhance the reacƟon by improving electron transport efficiency.
When it comes to storing hydrogen, carbon-based structures are aƩracƟve owing to their high
gravimetric capacity, and graphene parƟcularly so. CalculaƟons show that the gravimetric density may in
principal be up to 8% in graphene mulƟ-layers spaced with pillar structures such as carbon nanotubes.
This applies to cryogenic temperature and/or high pressure regimes. With ambient condiƟons the
maximum theoreƟcal density is 4%. In pracƟce the numbers are lower, with up to 1% density at room
temperature and pressure.
Doping graphene with alkaline or transiƟon metals can increase the gravimetric density to 10%. This can
be achieved by funcƟonalising graphene with metals such as palladium, which catalyse the dissociaƟon
of hydrogen molecules into ions on the graphene surface.
PerspecƟves
Graphene and related two-dimensional crystals may play a major role in future energy conversion and
storage technologies, and this is an acƟve area of research and development for Graphene Flagship
partners, both academic and industrial.
“The huge interest in two-dimensional crystals for energy applicaƟons comes both from their physicochemical properƟes, and the possibility of producing and processing them in large quanƟƟes, in a
cost-effecƟve manner,” says Bonaccorso. “In this context, the development of funcƟonal inks based on
two-dimensional crystals is the gateway for the realisaƟon of new generaƟon electrodes in energy
storage and conversion devices.” Bonaccorso adds that the challenge ahead is to demonstrate a
disrupƟve technology in which two-dimensional materials not only replace tradiƟonal electrodes, but
more importantly enable whole new device concepts.
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Review co-author Andrea Ferrari, who chairs the ExecuƟve Board of the Graphene Flagship, and is
director of the Cambridge Graphene Centre, offers a soberly opƟmisƟc view of the potenƟal for
graphene in this area: “Graphene and related materials have great promise in these areas, and the
Graphene Flagship has idenƟfied energy applicaƟons as a key area of investment. We hope that our
criƟcal overview will guide researchers in academia and industry in idenƟfying opƟmal pathways toward
applicaƟons and implementaƟon, with an eventual benefit for society as a whole.”
Source: By Francis Sedgemore, Graphene Flagship
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