You know that a material is important when researchers who work with it start to address humanity's great challenges, such as energy and combating disease. And although graphene initially caused excitement in the microelectronics industry due to its great carrier mobility, the more imminent (and perhaps even more pertinent) applications seem to blossom in drug release, prosthetics, and energy.
Image 1: Graphene is an enabling technology for healthcare and energy solutions. Source: freeimages.com
Supercapacitors are a technology on the rise, with a high likelihood of becoming the competing, or at least complementary technology with lithium ion batteries, today's dominant energy storage device. Batteries pack a lot of energy density, enabling them to push demanding consumers, like for example electric vehicles. At the same time, batteries charge very slowly, because their operation relies on an electrochemical process. The charging and discharging also degrades the chemical compounds over time, leading to a rather short battery lifetime. On the other hand, supercapacitors store their energy on the surface of electrodes, allowing for a quick charge and discharge. Yet, to date supercapacitors have been plagued by low energy densities, basically failing to deliver the power needed for demanding workhorse applications. To give some numbers (see image 2), supercapacitors (also known as ultracapacitors) can charge in less than one second, yet they hold 100 times less energy per unit weight than their smartphone battery counterparts.
Recent years and months brought a surge of research into graphene supercapacitors. Due to its large surface-to-volume ratio, graphene immediately emerges as a candidate for a supercapacitor electrode material. It's only a logical choice, given that most modern supercapacitors use some form of carbon for their electrode.
Lately, researchers have been experimenting with networks of hollow graphene-based materials, like for example the holey graphene network reported recently at the California NanoSystems Institute (CNSI) at UCLA. That particular research, published in Nature Communications, used the novel material as a supercapacitor electrode. The results indicate that the new electrode material led to an increase in energy density, while still preserving all the desirable supercapacitor properties, such as a high power density and cycle life. The energy density soared to 35 watt hours per kilogram, which is on par with lead acid batteries (and still 3-4 times smaller than that of lithium ion batteries).
The rapid progress of graphene-based supercapacitors prompted a research review paper by a group of scientists from China and Germany. The article, titled "Recent advances in graphene-based planar micro-supercapacitors for on-chip energy storage”, was published in the Beijing-based National Science Review. The paper reviews the various graphene-based materials, fabrication methods, and geometries used in graphene supercapacitors. The small volume of graphene materials is likely to lead the use of these devices as power sources for portable microelectronics, such as micro-electromechanical systems, microrobots and implantable medical devices.
In the latest twist in graphene-aided drug delivery, researchers from Monash University have shown that graphene oxide sheets can change shape into liquid crystal spherical droplets under an applied magnetic field. The research is supposed to lead to a new drug delivery scheme, in which medicinal molecules are delivered to a diseased site in a graphene droplet, which is opened up at the precise moment when the drug reaches the target.
Targeted drug delivery is not the only graphene researchers' dream aimed at healing people. Researchers led by the Walter Schottky Institut in Munich have hit the headlines recently when their project to develop an artificial retina using graphene was granted EU funding within the billion-euro Graphene Flagship. Retinal implants convert incident light into charge-carrying electrons which are guided to the brain through an optical nerve. Graphene photoreceptors are an important topic of investigation in the graphene research space, and graphene optoelectronics is one of only eleven work packages in the Graphene Flagship. Graphene is one of two ten-year billion-euro projects financed by the European Commission, the other being The Human Brain Project.
With these new developments in applications to energy and human health, graphene is starting to live up to its potential as a disruptive technology.