Graphene: Friend or Foe?

October 14, 2016

Introduction

Graphene has been around now for some 12 years. For those in the minor metals industry, should it be seen as a friend or a foe – a substitute or complementary? This short piece will look at graphene: what it is and just some of what it does. And how it may be regarded by those involved in minor metals.

What is Graphene?

First isolated in 2004 by two professors, Andre Geim and Kostya Novoselov, from the University of Manchester in England, graphene is the first two-dimensional material known to man. (For this achievement the two of them were, deservedly, awarded the Nobel Prize for Physics in 2010.)

In stark contrast to most materials that are made up of a 3D structure of atoms, graphene consists of just a single, 2D, layer of carbon atoms that forms a honeycomb (hexagonal lattice) structure. The graphite, used, for example, in pencils, is made out of millions of layers of graphene.

Some Amazing Properties

As a consequence, graphene displays some truly amazing characteristics, many of them seemingly contradictory or paradoxical:

It is 200 times stronger than steel;
It is immensely tough, but ultra-light;
It is incredibly flexible and can be stretched;
It is the world’s thinnest material – one million times thinner than a human hair;
It is transparent;
It can act as a perfect barrier – it is impermeable even to helium; and,
It is a superb conductor (indeed, the world’s most conductive).

And as research into the material continues, the list above will soon probably only constitute a small sub-set of the advantages it offers.

Three Applications

One of the most interesting areas of potential use for graphene is in electronics. A number of uses have already been identified and are under development. Here are three:

Touch Screens

Exploiting graphene’s properties of being transparent, tough, flexible, and a stunning conductor, it should be possible to produce touch screens that are not only flexible (and perhaps foldable), but also thin and strong – physical electronic newspapers, anyone?

As such, graphene may soon start to give indium (in ITO) a run for its money; whilst ITO may be a good conductor, it remains brittle.

At the end of 2015, researchers at GRAFOL (a European project aiming at roll-to-roll production of graphene films on silicon wafers) announced that they had been able to demonstrate a cost-effective production tool that could make large sheets of graphene on an industrial scale. They believe that graphene could substitute for ITO electrodes in OLEDs. They have, in addition, shown that it may be possible to “integrate graphene in silicon photonics platforms, as well as flexible thin-film solar cells with transparent electrodes (like perovskite PVs, for example) .”

But we certainly shouldn’t be in hurry to give up on ITO in this context. If Chinese researchers at the Ocean University of China in Qingdao are successful in their research, graphene may, on occasion, usefully complement, rather than just substitute for, the indium in PV cells. That occasion is when it rains! Observing that the water in raindrops is not pure, but contains salts that separate into negative and positive ions, they have been experimenting with adding a layer of graphene to dye-sensitized solar cells and using it to generate electricity by separating the positively-charged ions. Initial results have been promising.

And as this wasn’t inventive enough, researchers in the UK have created a material based on graphene that has proved very effective at absorbing both ambient heat and light.

Batteries

Another area in which scientists have great hope for graphene is in batteries. The current generation of batteries works on the principal of storing energy in controlled chemical reactions. These batteries are usually named after different chemicals and/or metals you find inside them: Alkaline-manganese, zinc-carbon, nickel metal hydride (NiMH), nickel cadmium (Ni-Cd), silver-zinc (Ag-Zn), nickel-hydrogen (NiH2) and lead acid (Pb-acid).

And then, of course, there are the many different lithium-based batteries, some of which, apart from lithium itself, also contain different minor metals. These include: lithium iron disulfide (Li- FeS2), lithium-thionyl chloride (LiSOCl2 or LTC), lithium sulfur dioxide (LiSO2), lithium manganese oxide LiMn2O4 or LMO) amongst others. And those that contain more minor metals: Lithium cobalt oxide – LiCoC2 (LCO), lithium nickel manganese – LiNiMnCoO2 (NMC), lithium iron phosphate – LiFePo4 (LFP), lithium nickel cobalt aluminium oxide – LiNiCoAlO2 (NCA), and lithium titanate – Li2TiO3 (LTO).

Much of the competition in the battery world currently revolves around which of the lithium technologies is going to prevail in the world of electric vehicles. Will it be the cheapest, “pouch cell”, battery – LFP? Or the cylindrical battery – NCA – the most expensive? Or the cylindrical NMC battery?

Whilst it is probably not safe yet to open a book on the winner in the battle of the batteries, it is safe to say that for whoever wins, graphene could help increase battery lifespan significantly.

The anodes of lithium batteries currently use 99.99% purity graphite. It is expensive and its production process is wasteful. It is here that graphene comes in. Initial research has shown that anodes made of graphene can hold energy very much better than those made of graphite. In addition, it appears that charge time can be shortened significantly, perhaps to one tenth of the time it takes to charge a traditional lithium-ion battery. Add to this better longevity, and the prospect for major performance improvements looks quite exciting.

Rather than with a traditional graphite anode where lithium ions accumulate around its outer surface, with a graphene anode, the lithium ions can pass through the graphene sheets. This leads to both easy extraction and an optimal storage area.

It appears, too, that performance can be improved even further with the addition of vanadium oxide to the cathode, with experimental batteries so adapted recharging in 20 seconds and, even after 1,000 cycles, retaining 90% capacity.

Finally, researchers are also looking at the use of graphene in supercapacitors. As mentioned above, the current generation of lithium-ion, and, indeed, other traditional batteries, store energy by means of a controlled chemical, or electrochemical, reaction. Capacitors store energy by means of a static charge. They are charged by applying a voltage differential between their positive and negative plates.

Image Source: Intercalactionrp.png: Antonderivative work: Mattman723 (talk) – Intercalactionrp.png, CC BY-SA 2.5

Supercapacitors or EDLCs (electric double-layer capacitors), as the name suggests, have very high capacitance, or ability to store a great deal of energy. Because of the relatively greater surface area it can offer, graphene is already being used as a substitute for the activated carbon that is currently used in supercapacitors. This higher surface area leads not only to considerable improvements in charge storage, but also, because it is so light, it could lead to significant reductions in weight – useful in both airplanes and automobiles. And because graphene is so resilient, supercapacitors’ longevity could also see huge improvements.

So in the context of both batteries and supercapacitors, graphene should, perhaps, not be seen as a foe, but a friend offering a helping hand in making improvements.

Semiconductors

With both its conductivity and thinness, graphene is an ideal candidate to become a semiconductor. Unfortunately, it normally lacks a bandgap, a sine qua non of semiconductors. However, various ways are already being devised to introduce one. It is, variously, being doped, fabricated in the shape of ribbons, grown in certain ways with certain other materials, and having its “wrinkles” manipulated through “graphene engineering”. If any one or all of these proves successful, we could find ourselves using graphene semiconductors. It appears that graphene chips have already been proven to be much faster than silicon ones.

Conclusion

Whilst it is still early days, the potential of graphene in so many applications is already becoming evident. But it is going to take a long time to nurture, grow and realize that potential, turning visions for its use into reality. Much of what it appears it is currently being made to do is improve upon technology that we already have, for example, being used to improve radically the performance of batteries and capacitors. I have seen little about it replacing, out and out, other materials and, in particular, individual minor metals: lithium batteries look set to be using lithium for some time yet.

We should, therefore, treat it, for the time being anyway, very much as a friend. Yes, it may, at some stage, become a foe, but probably only in certain applications. If we think of just how new (relatively) some of the most significant uses (for example, LEDs) are for some minor metals, in contrast graphene really is only in its infancy.

And then, as always, there are bound to be instances where uses will be found for a minor metal that will still further enhance performance already improved by graphene. You only have to look at the use of vanadium in cathodes mentioned above for an example.

So, on the whole, I think we should answer Friend, not Foe.