Improving the energy storage in graphene with defects

(Nanowerk Spotlight) The ability to store and retrieve energy when and where needed is becoming a crucial component of the renewable energy sector. Both batteries and supercapacitors store electrical energy in materials. The former usually have high energy density (i.e. they can hold a lot of energy) and the latter have high-rate discharge capability (i.e. they can release their energy in fast bursts).

To overcome their relative shortcomings (low power density and short cycle life for batteries; lower energy density for supercapacitors), over the past few years researchers have increasingly made use of nanostructured materials for both types of energy storage systems.

For instance, one of the greatest challenges in using conventional batteries for electric cars is that they are not optimized to deal with the rapid energy requirements posed by fast acceleration and deceleration (i.e. braking). To resolve this, automobile manufacturers are closely working with researchers to develop nanocarbon-based supercapacitors that are suitable to provide or captured large amounts of electrical energy within short durations.

"Carbon supercapacitors, however, are not yet ready for driving cars on the open road due to another primary bottleneck: the microscopic distribution of electrons in nanocarbons," Ramakrishna Podila, an Assistant Professor in the Department of Physics and Astronomy at Clemson University, tells Nanowerk. "This electron distribution in nanocarbons limits the total amount of energy through a property called 'quantum capacitance'. Although a lot of charge could be stored in the pores on nanocarbons due to their high surface area, their inherently low quantum capacitance reduces the net energy that could be drawn from supercapacitors."

Finding ways to improve quantum capacitance therefore is a major research focus for scientists working with nanocarbon materials, especially graphene for batteries.

In new work – counter intuitive to our idea of 'perfection equals best performance' – a team of researchers at Clemson University and the University of California at San Diego has shown that defects in nanocarbons could provide a breakthrough for increasing the quantum capacitance.

They report their findings in Nano Letters ("Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing").

Reactive-ion etching with Ar+ ions can induce nanopores in graphene, some of which (particularly ones with zig-zag type edges) increase the electronic density of states in graphene and thereby enhance the quantum capacitance by at least 2-6 fold. (Image: Achyut J. Raghavendra, Clemson University)

"Defects are often not wanted in many fields including art and medicine; but here defects provide an advantage," says Apparao Rao, Director of Clemson Nanomaterials Center. "Indeed, defect is an extra element that our periodic table does not show. Removing a carbon atom from where it is supposed to be can open numerous possibilities to improve a material's properties."

By subjecting graphene layers to a reactive-ion etching process, the team has poked holes into graphene to create holey graphene, which can change the microscopic distribution of electrons and thereby increase the quantum capacitance of graphene by at least fourfold.

"We have been engineering defects in nanocarbons to go beyond what the traditional materials can offer," says Podila, who instigated the study in collaboration with Rao and the UC San Diego team. "We already realized superconductivity in carbon nanotubes by doping them with boron and demonstrated improved thermoelectric performance in spark plasma sintered Bi 2 Te 3 due to the creation of charged grain boundaries. In this study, we brought Raman spectroscopy and electrochemistry together to go beyond what is already known in graphene science."

"Our ability to engineer defects coupled with scalable manufacturing methods can result in next-generation supercapacitors that can enable electric cars," hopes Mehmet Karakaya, a PhD candidate at the Clemson Nanomaterials Center and a co-author of the paper.

Fate it seems is not without a sense of irony. Defects, always written off in striving for perfection, appear to be the hol(e)y grail that researchers have been searching for all this time.