There is a huge demand to find renewable alternatives to our traditional, finite energy sources. The elements are already playing an important role – only this year wind and solar energy generated more energy over a three-month period than coal – but other substitutes are still required.
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice. It is a unique material with a range of desirable properties including high carrier mobility – meaning electrons move quickly through the material - optical transparency and extraordinary mechanical and electronic properties. Ever since its isolation using sticky tape by Andre Geim and Konstantin Novoselov in Manchester in 2004, graphene has become a research hot topic. The versatile substance is capable of revolutionizing a number of technologically important areas - including the energy sector.
This novel material could be used for electrical components, sensors, batteries, composites and ion-exchange membranes to give just a few examples. Graphene has been shown to make excellent battery electrodes thanks to its high surface area capable of storing lithium ions, while polymer matrices laced with graphene flakes make strong but workable composites.
In 2011, engineers at Northwestern University discovered that graphene anodes could hold up to 10 times more power than graphite ones, charged 10 times faster and lasted longer. The anode contains sheets of graphene riddled with millions of holes 10-20nm in diameter which allow the lithium ions to jump through the nanoholes instead of traveling around the edge of the graphene layer.
Such batteries could have potential use in electric vehicles: by reducing the weight of batteries, graphene could be used to boost the efficiency of such vehicles, helping to extend their range and overcome a major concern preventing their wider uptake.
Two years later in 2013, researchers from Rice University found graphene laced with boron could be used to produce an ultrathin flexible anode for lithium-ion batteries. The boron – which replaces a quarter of the carbon atoms - helps the lithium ions stick to the graphene, which results in faster charging. The same team also discovered that graphene mixed with vanadium oxide could be utilized in high-performance, cost-effective cathodes that can be recharged in 20 seconds and hold more than 90% of their capacity after extensive use.
In addition to being used in electric vehicles, graphene in batteries could also boost the energy storage sector. The University of Manchester, the home of graphene, has already trialed a grid-scale battery and converter system on campus. The aim is to develop high capacity electrical storage capable of supplying the National Grid. The system is being used to study methods to control the flow of electricity and settle differences between power generation and local demand.
Researchers at the University of Manchester have recently discovered that graphene membranes, when illuminated with sunlight, can conduct protons at an increased rate. This photo-proton effect could be employed to artificially mimic photosynthesis and directly harvest solar energy to produce hydrogen gas. This gas, could then be used in fuel cells in electric vehicles. Such a move would help lower the cost of renewable hydrogen fuel and make hydrogen fuel stations a more common sight along the roads as the cost of processing drops.
Eight thousand times more solar energy is produced each year than is consumed worldwide; while some of the power reaching the Earth is harvested and utilized, a massive percentage is not. As energy needs increase, solar power is becoming an attractive alternative, but its efficiency is still lagging. Furthermore, commercially available silicon-based photovoltaic cells are expensive to produce and install.
Graphene could have an important role to play here too, in anti-reflection coatings for solar cells. Researchers in India found that such cells lower reflectance near the ultraviolet part of the spectrum from 35% to 15%.
Graphene incorporated within the photovoltaic cells can, say a team at MIT, deliver a higher power conversion efficiency. Hybrid cells based on graphene are less expensive and more effective as a conducting electrode than the traditionally used indium tin oxide. It also provided flexibility, is light weight, and imparts mechanical strength and a chemical robustness to the cell.
While graphene has the potential to revolutionize the energy sector, there is still much work to be done before graphene-based batteries and hydrogen fuel cells are powering cars on our roads.
References and Further Reading