Most of us are familiar with rechargeable batteries. So much so, in fact, that many non-experts can convincingly argue the merits of particular battery technologies at a fairly technical level. Today’s mobile electronic equipment is powered by low-cost rechargeable cells, and their energy capacity and performance is increasing rapidly with our understanding of the materials science and device engineering involved. On a larger scale we are also seeing rapid developments in heavy duty batteries that can power hybrid and all-electric road vehicles.
A much less well-known but promising way of storing and releasing electrical energy is with double layer capacitors. Commonly known as ultracapacitors (or supercapacitors), these devices are currently under development, and are used either on their own in large arrays as a primary power source for energy-intensive applications, or in combination with batteries or fuel cells. Ultracapacitors are particularly well-suited to applications with a short load cycle such as cranes, forklift trucks and electric vehicles. Advantages of ultracapacitors over traditional batteries include higher capacities, longer lifetimes, a wider operational temperature range, lighter, more flexible packaging and lower maintenance.
Each ultracapacitor unit cell consists of two porous carbon electrodes electrically isolated from each other by a porous separator material. Devices based on electrochemical double layer capacitance (EDLC) store and release energy by nanoscopic charge separation at the interface between an electrode and electrolyte. Metal foils or carbon-impregnated polymers are used to collect the current from each electrode.
The amount of energy stored in an ultracapacitor is inversely proportional to the thickness of the double layer, and this can result in energy densities thousands of times greater than those of conventional dielectric capacitors. However, the highest densities achieved to date remain significantly lower than those of the best batteries. There is much work being done to rectify this, stimulated in part by a US Department of Energy challenge. Researchers are in particular looking for materials that perform better than the activated charcoals traditionally used in ultracapacitors.
Graphene – a single atom-thick sheet of carbon – is one such material being considered for ultracapacitors. And, given the almost ‘wonder material’ status of graphene, the work of physical chemist Rod Ruoff and his colleagues at the University of Texas at Austin is attracting considerable attention. The researchers, who include postgraduate student Meryl Stoller and postdocs Sungjin Park, Yanwu Zhu and Jinho An, have claimed a breakthrough in the use of chemically modified graphene (CMG) that could double the capacity of existing ultracapacitors. CMG materials can be synthesised in a number of ways and into various forms. They may, for example, be kept suspended in solution, or embedded in paper-like materials, polymers or ceramic nanocomposites.
The graphene used in Ruoff’s experiments was derived from graphite via graphite oxide. The oxide exfoliates readily in water, forming platelets of electrically insulating graphene oxide in a colloidal suspension which is then exposed to a reductant through the addition of hydrazine. “Chemistry occurs, the oxygen content of the graphene oxide platelets is greatly reduced, and the now-electrically conductive graphene-like platelets are collected, dried and weighed,” says Ruoff. “A known weight is added to the electrolyte, a paste is made, and a test ultracapacitor cell is configured. The amounts needed for testing are of the order of a gram.”
In terms of hard numbers, the Austin team reports on capacitors made with graphene, each single gramme of which has a surface area of over 2,500 m2, that display specific capacitance values of 135 and 99 farads per gramme in aqueous and organic electrolytes, respectively. And that with only a portion of the graphene sheets exposed to the electrolyte. The variation in specific capacitance is also found to remain relatively linear at higher voltages. Based on their experimental results, the researchers argue that CMGs with good electrical conductivity and very large surface areas are promising candidates for double-layer ultracapacitors.
“The values we publish for specific capacitance rival the values of carbons used in commercial ultracapacitors,” says Ruoff. “We suggest that higher values might be achievable, and also higher values of energy density. However, this is not a situation of knowing the answer in advance, and simply needing clever reverse engineering to make the answer come true. We are hypothesising, based on reasonable assumptions, but the science is not done yet. More science is called for to see if higher values of specific capacitance will be achieved.”
When it comes to practicalities and scalability, graphite is an abundant and low-cost material, and ultracapacitors based on graphene could in principle be cost-effective. University research labs and a few commercial enterprises are beginning to churn out graphene and graphene-like materials of varying qualities and in considerable amounts. Whether this graphene is any good depends, of course, on the demands of the applications for which the material is being produced. Graphene is so often described in superlative terms, and the tendency is to think that the material must be 100% pure, and in single atom layers only. Reality is somewhat more flexible.
“There is a great deal of interest and thus R&D in graphene,” says Ruoff. “It does not matter where the graphene comes from, and it need not be perfect monolayer-only material. As long as it is graphene-like (i.e., quite thin platelets), with good electrical conductivity and a high surface area, it is a candidate for ultracapacitor technology. Whether the specific route we used is going to be economically viable is an open question.”
These are cautious words, and rightly so given the strength of feeling in the science community surrounding the sometimes extravagant claims made for novel nanomaterials such as graphene. Take, for example, the criticism of Andre Geim, the Manchester University physicist who led the group that first isolated graphene back in 2004. Geim is not one to mince words, and in this case he is rather sceptical about the potential for graphene in ultracapacitors.
“The large surface area and high conductivity of graphene has obviously been attracting interest from a community dealing with batteries, and I am aware of other groups working in this direction,” says Geim. “Ruoff’s paper is a step in the right direction, supporting in practice the same basic idea (rather than saying it in words). However, the large area and conductivity are not directly transferable into real products. The major problem is to make graphene powder stable and robust so that it can withstand numerous charge-discharge cycles. You do not want to have a capacitor that can be charged only a few times, do you? Some revolutionary idea is required to make this graphene idea into the reality.”
To be fair to Ruoff and his colleagues, the same could be said of any novel material proposed for use in ultracapacitors. And though the Nano Letters paper in which the Austin scientists report their findings does not address the issue of device lifetime, Ruoff is already on the case.
“That is indeed an important issue,” says Ruoff. “We are working on it now, and I have little doubt that others who read our paper will also.” Apart from that, Ruoff is reluctant to say any more at this time: “When we do, we will submit, likely, to a peer-reviewed journal, and take our lumps from the reviewers, so that we end up with the best science through the standard process (at least for me, as a scientist!).”
If the objections of Geim and Ruoff’s other peers can be met, then it might not take long before graphene finds its way out of the lab and into real-world ultracapacitors. The EDLC principle is well established, and it is now a case of finding the best materials for this application. If graphene oxide is shown to be structurally stable, producing material of the required quality and in sufficient quantities should not be too much of a challenge.
Rod Ruoff is a physical chemist in the Department of Mechanical Engineering at the University of Texas at Austin, US. A former Fullbright scholar at the Max Planck Institute for Dynamics and Self-Organisation in Göttingen, Germany, Ruoff is a specialist in novel carbon nanomaterials including graphene and nanotubes. His current research focuses on nanocomposites, the use of manipulation tools for measuring the mechanics and electro-mechanics of nanoscale specimens, and the synthesis of novel nanostructures.
Andre Geim is a condensed matter physicist at the University of Manchester in northern England, and director of the Manchester Centre for Mesoscience and Nanotechnology. In 2004 Geim led the research team that first isolated the single atom-thick sheets of carbon known as graphene. This Fellow of the Royal Society is arguably more famous for levitating a frog. For that stupendous achievement he won the 2000 IgNobel Prize in Physics.
“Graphene-Based Ultracapacitors”, Stoller et al., Nano Lett. 8, 3498 (2008).
Ragone chart showing energy density vs. power density for various electrical energy storage devices (source: Stan Zurek – Wiki Commons).
Scanning Electron Microscope image of a chemically-modified graphene agglomerate surface. This material shows promise as the basis of ultracapacitor energy storage devices. The scale bar at the bottom left represents 100 nm. (source: Meryl Stoller/University of Texas).
This is a revised version of an article first published in Nanomaterials World.