For the last decade, interest in graphene for energy storage applications has risen dramatically. In the first half of this year alone, nearly 500 papers were published on the topic, with much more sure to come. One of the first graphene-based ultracapacitors (also known as supercapacitors) was announced in 2008, with interest growing exponentially ever since. The 2008 work, published in NanoLetters by Meryl D. Stoller and colleagues, demonstrated the performance of chemically modified graphene in an ultracapacitor cell. As the authors said, the high electrical conductivity and strong performance over a wide range of voltage scan rates illustrated the “exciting potential for high performance, electrical energy storage devices based on this new class of carbon material”
Why graphene?
Among graphene’s many popular features, its high surface area (up to 2630 m2 g-1) is of particular interest for supercapacitors. Recently, researchers from Arizona State University and Tsinghua University, Beijing, reported the intrinsic capacitance of single layer graphene to be 21 µF cm-2. This sets the upper limit for electric double layer (EDL) capacitance for all carbon-based materials. These valuable properties of graphene mean a supercapacitor based on graphene could theoretically achieve an EDL capacity as high as 550 F g-1, if the entire surface area can be fully utilised. This combination of surface area and capacitance promises a great future for energy storage, which is already beginning to be realised.
Since the groundwork of the early years, the performance of graphene supercapacitors has been impressive. Most recently, a researcher at Australia’s Swinburne University has developed a battery that charges up in just seconds, and can be used repeatedly. Dr. Han Lin, of the Centre for Micro-Photonics, has developed a supercapacitor that not only overcomes the limitations of long charging times but also costs less than a traditional lithium ion battery over the course of its lifetime.
On the other side of the world, researchers from Lawrence Livermore National University and UC Santa Cruz recently doubled the performance of 3D printed graphene-based supercapacitors. Their method is based on sandwiching lithium ion and perchlorate ion between layers of graphene in aerogel electrodes. This allowed the researchers to enhance the 3D printed graphene aerogel by “exfoliating the stacked graphene layers and functionalizing their surface, without damaging structural integrity”, said Yat Li, UCSC professor and corresponding author.
As well as supercapacitors, graphene is also useful for a wide range of batteries – redox flow, metal-air, lithium-sulphur, and most importantly, lithium-ion batteries. In a recent example featured in the Journal of Materials Chemistry, first principles calculations indicate that lithium ions can be stored on both sides of graphene to form C3Li. Graphene can accordingly deliver a theoretical capacity of 744 mAh g-1, about twice the capacity of conventional graphite electrodes.
With all this and much more going on in the world of graphene supercapacitors, the upcoming months promise yet more exciting developments.
