Graphene, first produced in a lab at the University of Manchester in 2004, is a single layer of carbon atoms, arranged in a honeycomb lattice. The material has been long sought-after, having been predicted to possess excellent charge carrier mobility and mechanical strength greater than steel, accompanied by a collection of exotic physical phenomena unprecedented in the world of materials.
For years, the difficulty lay in finding a method of producing such a thin film of carbon. The breakthrough came, after years of research by professors Geim and Novoselov, by the way of a rather modest technology later dubbed “the scotch tape technique”. As the name would suggest, the Manchester group used scotch tape to successively peel off layers from a chunk of graphite until only a single layer of atoms was left. It may sound trivial, but it took a genius idea and lots of practice to isolate the first piece of graphene, which was then used to showcase all kinds of interesting physics, like the quantum Hall effect. In fact, the richness of the physics loot was so full that it earned Geim and Novoselov a Nobel prize in physics in 2010. The remarkably short time between discovery and Nobel prize foreshadowed the quick arrival of the potential applications of graphene. It is coming as a bit of a surprise that graphene can also be used on many fronts in the energy sector.
Graphene Batteries and Electrodes
The potential of graphene for batteries becomes more apparent each day, with headlines touting new graphene electrodes and battery materials. Graphene electrodes are ideal for applications where transparent, highly conductive and flexible surfaces are desired, such as flexible solar cells or flexible mobile devices.
Already some years ago, engineers at Northwestern University have shown that graphene anodes hold energy better than graphite anodes, with 10x faster charging. In lithium ion batteries, the charge-carrying lithium ions circulate from the lithium fuel cell through the anode and cathode, giving away their charge to power the battery. Charging reverses the process, leading to freshly charged lithium ions. The performance of the battery depends on the ability of the anode to hold lithium ions.
Traditional graphite anodes are solid, with lithium ions accumulating around the outer surface of the anode. Engineering the anode to produce pathways for lithium is possible, however graphene offers a much more elegant solution. Researchers at Northwestern, and others after them, have punctured tiny holes in graphene sheets. The holes are 10-20 nm big, allowing for lithium ions to pass through. In an anode that consists of multiple graphene sheets, the lithium ions migrate through the sheets and permeate the anode, providing for optimal use of storage area and ease of extraction of electricity. The resulting battery can store 10 times more power than ones that utilize graphite anodes.
In April this year, Rice University featured in the graphene spotlight. This time, researchers have shown that graphene mixed with vanadium oxide in an industrially scalable process leads to cost-efficient superb-performance cathodes. The batteries made with such cathodes recharge in 20 seconds and retain more than 90 percent of their capacity even after 1000 cycles of use. The process involves cooking vanadium pentoxide, an inexpensive material, together with graphene oxide nanosheets in water. The nanostructures required for the cathode spontaneously form during the cooking process, bringing scalability and mass production within reach.
Most recently, researchers came out with a prediction that adding some boron atoms to the graphene structure would result in an ultrathin efficient flexible anode for lithium ion batteries. The addition of boron helps the lithium ions of the battery stick better to the graphene anode, which was a problem with some earlier designs of graphene anodes.
Graphene anodes result in faster battery charging and discharging compared to conventional anodes. The research on the boron-graphene mixture was performed together with Honda, who aim to use graphene in batteries for electric cars. Kia and Hyundai have also shown interest in graphene for electric vehicles, all showing that industrial energy applications of graphene are not very far away.
Graphene is not only being used for the electrodes of batteries, but for the active material itself. Graphene makes such a good battery material that the devices are called "supercapacitors," batteries which hold enormous power and charge within a few seconds. Already two years ago graphene supercapacitors that store as much energy as nickel metal hydride (Ni:MH) batteries were demonstrated. And in February this year, UCLA researchers caused shock waves when they showed a graphene mixture that can be coated onto the surface of a regular DVD. An ordinary DVD burner is then used to inscribe millions of supercapacitor circuits into the graphene layer. The layer can later be simply peeled off and transferred anywhere where a superpowered battery is desired.
With such development, we might soon see ultrathin flexible batteries which charge in less than a minute that could be integrated into clothes, paper, car dashboards or just about anywhere you want them.
Graphene Solar Cells
Graphene's use for energy does not stop at battery materials, but reaches towards solar cells as well. Researchers in India have shown that graphene stacks can be made into good antireflection coatings for solar cells. Reflection of sunlight at the surface of solar cells is one of the big issues facing the solar industry, alongside the efficiency of coupling light to the active layer of the cell. With graphene stacks, reflectance near the ultraviolet part of the solar spectrum is reduced from 35 percent to just 15 percent, saving much energy. The graphene stacks operate like traditional anti-reflection coatings, effectively pushing light that would otherwise be reflected into the active layer of the cell. Stacking graphene and traditional semiconductors leads to more efficient solar cells and LEDs. Substitute the semiconductor with atomic monolayers of other materials, such as transition metal dichalcogenides, and you get very sensitive photovoltaic devices.
Mass Production of Graphene
Graphene is only a few years young, and is already living up to its promise of changing our world. What we need now is to get away from the lab-only scotch tape technique towards methods of mass manufacturing, maintaining a high quality of graphene, similar to what one can get with the scotch tape technique, while progressing towards larger surface areas, which are more suitable for mass production with established methods.
One chief process is chemical vapor deposition, during which precursor gases are introduced into a chamber under a controlled environment. The gases react with a thin metal film and monolayer high quality graphene forms at the surface of the metal. The graphene is subsequently transferred onto standard SOI wafers, retaining high quality at sizes up to 4 inches in diameter. Significant progress towards even larger areas is being made through the European project GRAFOL – “Roll to roll production of graphene”.
Another direction is graphene oxide in solution. Graphene oxide does not retain carrier mobilities desired for high-tech applications such as transparent flexible displays, however it is still good enough for printed conductive inks, for example, or for the mentioned supercapacitor.
Fresh ideas for the use of graphene materialize every day, and mass production is one of the key issues to be solved on the road to full exploitation of the material's potential. Energy applications will take up a large portion of the graphene market.
Lead image: graphene by Dr Thomas Szkopek via Wikimedia Commons