Uses
of Graphene: From the Present to the Future

In
1947 P.R. Wallace’s paper 1 was published describing the band structure of
graphene. It was useful then in understanding the properties of graphite
(stacked layers of graphene) but nowadays represents the first step in
understanding a material whose incredible properties have led to its use in a
range of technologies reaching from energy storage via supercapacitors to more
durable bicycle tyres. Furthermore considering the intense research currently
surrounding the material, it is all but guaranteed to be utilised in the
future.

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Considering
applications of the substance that are current, possibly realised in a few year
and more hypothetical, should underline just how vital graphene is to many
parts of our lives both now and in the future as well as provide a better
understanding of the relevant scientific principles.

What
is It?

Carbon is present in the universe
as a large number of allotropes, graphene being one of them. Its structure is
deceptively simple, being a single large hexagonal lattice, thus making it a 2D
material.

Many
of its physical properties reside at the extremes, it being an excellent
conductor and possessing a large specific surface area as well as incredible
mechanical strength. These unusual properties go some way to explaining why it
is utilised in so many applications as will be shown in more detail later on.

 

The
Present-Graphene and Supercapacitors

With
conventional capacitors rarely possessing a capacitance exceeding the ?F range
, a supercapacitor can easily be rated up to several kF. With them possessing a
very low equivalent series resistance (ESR: if the supercapacitor was modelled
as an ideal capacitor in series with a resistor the ESR would be the value of
said resistor) and high power output, withstand thousands of charge and
discharge cycles without significant degradation in performance and having an
energy density many times higher than a conventional capacitor 3 makes them
ideal devices in certain situations. Limitations are of course present. Low
specific energies compared to that of a battery, low maximum voltage and a high
self-discharge rate plague supercapacitors reducing the applicability in other
areas 4.

Whilst
the range of applicability of supercapacitors ranges from defibrillators in the
medical industry to regenerative braking systems in automobiles, and thus too
large to go into detail here, it is worthwhile to give one example for
illustrative purposes.

Supercapacitors
can be utilised as auxiliary backup power supplies. In the case of the primary
power supply failing or being interrupted, a supercapacitor can provide the
necessary power to the system until functionality is restored. Low ESR
(especially than batteries), a long lifetime and their high specific energy
densities (compared to capacitors) make them ideal to be used for systems where
high power input is required and only limited space available. Backup of SSD
computer memory would be a specific example of this.

 

 

 

How
do they work?

A
conventional capacitor consists of 2 electrodes separated by an insulating
dielectric. Applying a voltage to a capacitor causes opposite charges to
accumulate on the surfaces of each electrode. The electric field thus developed
across the dielectric allows the device to store energy.

The
same principles apply to the supercapacitor. However the way that the charge is
stored is fundamentally different and gives rise to the massive increase in
capacitance. In fact there are 3 mechanisms for charge storage giving rise to 3
general classes: hybrid, pseudo and electrochemical double-layer capacitors.

Electrochemical
double-layer capacitors

These
capacitors rely on the double layer phenomenon for their increased capacitance.
Here an ion-permeable membrane separates 2 electrodes with an electrolyte (a
mixture of negative and positive ions dissolved in a solvent) electrically
connecting both electrodes. When a voltage is applied to the capacitor, as
before charge accumulates in  both
electrode surfaces as layers of opposing polarity. Furthermore due to the
adhesive forces between solvent and electrode a single layer of solvent
particles (called the inner Helmholtz plane, IHP)is attached to the surface of
the electrode.

Coulombic
attraction between the charged internal layer of particles and the dissolved
ions distributed in the electrolyte then cause a second layer of opposite
polarity to form fitted against the IHP (now acting as a molecular dielectric).
The separation of 2 layers of oppositely charged ions by the IHP stores
electrical charges as in a regular capacitor, with an extremely strong static
electric field forming across the IHP. Fig. 1 illustrates this. The capacitance
arising due to this double layer phenomenon is thus dependent on the area of
the layers (and thus dependent on the surface area of the electrode) as well as
the inter layer separation (in this case the thickness of a single molecule) in
a fashion similar to the usual parallel plate capacitor. Hence due to the
extremely thin nature of molecules and use of electrode materials with an
extremely large surface areas, incredibly large capacitances arise.

Several
different materials exist that are normally utilised for the electrode,
including activated carbon (a processed from of carbon designed to have small
low volume pores), carbon aerogels, and carbon nanotubes6. Each bring their
own advantages and disadvantages. Activated carbon for example is less
expensive whilst the aerogels possess a lower ESR thus yielding higher power
outputs7.

Pseudocapacitors

A
pseudocapacitor is also constructed in a similar fashion to the electrochemical
double-layer capacitors. Again when a voltage is applied to the device a double
layer forms at each of the electrodes. However it is possible for some of the
electrolyte ions that are part of the ion layer to penetrate the IHP to become
de-solvated (the electrolyte ion, the solute, no longer interacts with the
solvent) and adsorb onto the electrode’s surface. Note that no chemical
reaction is occurring currently between the atoms of the electrode and the
adsorbed ion i.e no chemical bonds arise. Then a charge transfer process occurs
(in these processes a charge-transfer complex is created, which are 2 or more
molecules that are associated by transfer of charge from one to the other)
between the 2. The result is a faradaic current (a current that flows across
the electrode-solution interface) caused by one of three processes between the
adsorbed electrolyte and electrode. Fig.2 illustrates this. Hence charge is now
stored via the electrolyte and electrode interaction ready to release it by
reversing the process. The size of the resulting capacitance is dependent on
the surface area of the electrode, its material and structure of said
electrodes.

It
is worth noting that whilst here the 2 types of capacitance were described as 2
different phenomena, in reality pseudocapacitance cannot occur without the
static double-layer capacitance as the former is inextricably dependent on the
latter’s existence. Hence the total capacitance of a supercapacitor is in fact
a sum of the contributions from each type.  

Hybrid
Capacitors

These
capacitors are designed to utilise both double-layer capacitance and pseudocapacitance
in order to mitigate the disadvantages of one type with the advantages of the
other. Specifically EDLCs usually suffer from lower energy and power densities
whilst possessing greater cycling stability and affordability.

Use
of Graphene

So in what aspect of
supercapacitors could graphene be utilised? The electrodes in all 3 types of
capacitors require high surface area per unit mass and volume to maximise the
possible capacitance (both pseudo and double layer capacitance increase with
increasing surface area), good conductivity in order to reduce the ESR and be
inert to ensure long lifetimes. Graphene fulfils all of these requirements
excellently which is why it is such an ideal candidate for the electrode
material. Furthermore its excellent mechanical strength makes it well suited
for next generation flexible thin film supercapacitors.

As with all cutting edge
technologies however there are problems that considerably reduce the efficacy
of said technology. In this case processing of the graphene material is made
more difficult by the restacking and agglomeration processes (caused by van der
Waals forces) that occur when handling of the substance occurs. As a direct
consequence of this is a reduction in diffusion of electrolyte ions between
graphene layers and available surface area. However by crumpling the graphene
sheets, using spacers and template assisted growth has allowed this problem to
be resolved.

Whilst
graphene supercapacitors may seem a thing of the future they already exist
commercially. The company Skeleton Technologies offers their patented “curved
graphene” products that according to their website is utilised in a large range
of applications such as in hybrid city buses 9.

So
we’ve seen that currently graphene is utilised in supercapacitors, which in
turn are used in multiple important applications. Yet research is already being
done on implementing graphene in many more technologies. For example according
to the International Technology Roadmap for Semiconductors graphene utilised as
the material for transistor construction is only 10-15 years away, resulting in
a massive leap forward in computing ability.

The
Future-Graphene, the New Silicon?

The
transistor is one the most important technological advancements of the 20th
century. It forms the building block of the integrated circuit, devices that
are virtually omnipresent in all modern electronic equipment ,such as our
smartphones, and thus a foundation on which our society rests. Nearly all of
said transistors are made from silicon and have been for over 60 years, and for
that time have been subject to Moore’s Law, predicting a doubling in transistor
density every 2 years. This was achieved by continually decreasing transistor
size down to just 5nm in June 2017 with the technology giant IMB’s latest
design10. However decreasing transistor size is and has been for years a less
and less efficient way of improving the performance of transistors due to the
increasing cost of firstly designing devices that small that function well at
that size and producing such minute constructs. Furthermore at such scales
quantum mechanical effects such as tunnelling could cause huge problems. Due to
the ever present need for more powerful computers etc. in our society a
pressure exists on the electronics industry to continue to improve transistors.
Whilst some solutions involve changing the chip and transistor architecture or
instead moving to more specialised chips instead of more powerful all-rounder
designs, one possibility would be instead to move to a totally different
substance, graphene, as stated by the International Technology Roadmap for
Semiconductors 11, a document outlining the most likely directions of
research and the probable time line for the semiconductor industry.

Graphene
Transistors

In
June 2017 assistant professor Ryan M. Gelfand and the research team he was part
of developed a graphene based transistor in the University of Central Florida
12 that with further study and refinement could lead to computers a hundred
times as efficient and a thousand times as powerful (in the terahertz operating
speed range). But how do they work? To answer this we have to consider
spintronics.

Spintronics
and Graphene

The
study of the spin of the electron in solid-state devices is known as
spintronics. By manipulating the spin-degree of freedom in a system it is
possible to design switching devices based on this phenomenon. However
cascading such devices in order to construct logic gates (which are what
computers are based upon) has long been a major challenge, yet an all-carbon
design might be feasible.

The
switching device (transistor) proposed by the team working in Florida consists
of a graphene nanoribbon (GNR, a thin strips of graphene) created by unzipping
a carbon nanotube (CNT) with 2 parallel CNT control wires on either side. There
exists a constant voltage across all of the 3 components.  With current flowing in the CNT a magnetic
field is thus generated. Fig.3 illustrates this.

The
important phenomenon here is the negative magnetoresistance of  the GNR, that is to say its resistivity
decreases with increasing external magnetic field. This results due to spin
interaction in the material with the magnetic field that will not be explained
in detail here. Hence the magnitude of the GNR current acts as the binary
output of the transistor. Specifically binary 1 is represented by the large
current when the CNT magnetic fields are present and binary 0 by the much
smaller current when the magnetic field is not present. The current from the
GNR can now act as the binary input to further cascaded GNR gates and thus be
used to form the complex set of logic gates that are used to perform the
desired function.  The exceptionally high
computational ability of computers designed on such transistors (a clock speed
of 2 THz is proposed) is a product of the low switching delay, which are in
turn caused by the low times required to switch the magnetic field on and off.

Now
whilst other materials exhibiting negative magnetoresistance and high
conductivity could be utilised instead of graphene no other material currently
fits the requirements as well as  CNTs
and GNRs. 14

Whilst
graphene is set to be utilised in a number of applications in the relatively
near future (10-15 years), some research shows possibilities exist for graphene
to be utilised in a generation from now, such as the solar sail.

The
Far Future-Solar Sails in Interstellar Spacecraft

Today’s
spacecraft all rely exclusively on chemical rocket engines as their primary
means of propulsion in order to escape earth’s gravity well and also make
extensive use of them for travel and manoeuvring in space. Yet for the purpose
of a interstellar spacecraft, rocket engines simply do not generate enough
thrust over a long enough period of time in order to achieve speeds that would
allow manned craft to reach the nearest solar system in an acceptable time
frame i.e less than the thousands of years currently required. The question of
how exactly space exploration should be tackled is one that has received
significant consideration since the idea of space travel has even existed.
Current answers range from slow to fast, manned and unmanned projects each with
their own advantages and disadvantages. Propulsion systems utilised on said
projects are just as varied, with ideas ranging from the more mundane such as
ion engines, to the very theoretical such as faster than light travel using an
Alcubierre drive. Even a cursory description would warrant an entire article on
its own yet one propulsion system is worth going into some detail, the solar
sail.

What
are they?

The
phenomenon of radiation pressure (electromagnetic radiation incident on a
surface exerts pressure) can be utilised to design a propulsion system for a
spacecraft called a solar sail. Whilst the actual forces exerted on solar sail
are minute, they are constant and thus provide a significant acceleration over
a large enough time frame. Furthermore beam sailing, the concept of using high
intensity laser beams focused on the solar sail to provide a much greater thrust,
provide a way of significantly increasing usability. 

Sail
Material-Graphene

Graphene
is a potential candidate for the material solar sails are constructed from. Not
only does graphene possess a very low density, ensuring minimal mass for the
space craft and thus a large as possible acceleration as well as a high
mechanical strength that ensures the sail is able to survive the rigours of
interstellar travel. However there is another feature that distinguishes
graphene. In May 2016, a Chinese team of researcher was investigating graphene
sponges 15 (layers of graphene fused together), noticed how the laser being
used to cut the material was actually propelling the centimetre size sample
forwards. Radiation pressure was soon discounted as a mechanism for the seen
propulsion as it was estimated to be around 10-9 N, so much too
small to explain the seen motion. Similarly, the laser burning off some of the
material to provide thrust was similarly non-feasible due to the much too small
laser intensity.

Instead
a third mechanism was suggested. Graphene absorbs all wavelengths of light
well, and thus under constant illumination by a laser causes its electrons
population to become excited into a higher energy state. Further study showed
that some electrons obtained enough energy to be ejected out and to become free
electrons, thus providing extra thrust ,several orders larger than the
radiation pressure, in the direction of the laser beam. Emphasis was placed in
the article on the fact that the macroscale propulsion follows due to the
unique optoelectronic properties of the graphene sheets.

Hence
graphene seems to be ideally suited to be used as solar sail material.

Breakthrough
Starshot Initiative

Whilst
solar sails already have been constructed and tested in orbit around earth,
such as the IKAROS spacecraft (a Japanese experiment that demonstrated solar
sails functionality in interplanetary space), the major current proponent of
solar sail technology as a propulsion system for interstellar flight, that aims
to launch within the next generation, is the Breakthrough Starshot intiative.
The objective is to develop a solar sail spacecraft capable of reaching Alpha
Centauri, our closest neighbouring star system. Currently graphene
based-materials are being considered as the solar sail substance.

So
whilst the idea of an interstellar spacecraft powered by graphene may seem like
something out of a sci-fi novel, rest assured that work is already being done
to implement such a futuristic technology within their lifetime. Of course many
problems still face the program and the technology as a whole, for example how
will the extremely thin sail stand up to extremely high velocity (up to
possibly 20% the speed of light) impacts over its 20 year journey?

Conclusion

The
aim of this article was to illustrate with a few examples just how integral the
substance graphene is to our modern technology both presently and in the
future. With supercapacitors based on graphene offering a device ideal for use
in a plethora of applications (such as the medical industry or regenerative braking
systems), graphene nanoribbon transistors potentially revolutionizing computing
ability in the near future and graphene solar sails providing a possible propulsion
system that will allow mankind to make the first interstellar journey within out
lifetime, shows this more than adequately.

Bibliography 

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