A nitrogen boost for graphene-based supercapacitors

As a promising basis for low-cost, high-performance supercapacitors, researchers, from Lomonosov Moscow State University in Russia, have demonstrated a way to multiply the charge-storing capacity of graphene carbon nanoflakes¹.

Supercapacitors are long-lasting energy-storage devices that can charge and discharge electricity almost instantaneously. Previously used in niche power applications, supercapacitors are now attracting considerable interest as a superior solution to batteries for smoothing out short-term fluctuations in electrical supply from solar panels and wind turbines. Graphene – flakes of crystalline carbon just a single atomic layer thick – is a promising electrode material for supercapacitors, but on its own doesn’t achieve the charge-storing capacity needed for commercial applications.

“A supercapacitor consists of two electrodes with high surface areas, a separator between them, and an electrolyte that provides the charge transfer between the electrodes,” explains lead researcher, Ekaterina Arkhipova. “Graphene-based materials are very promising electrode materials because of their high specific surface area, as well as being cheap, chemically stable, and easily producible in large quantities.”

Reporting in the Russian Journal of Physical Chemistry, Arkhipova’s team prepared graphene nanoflakes by a pyrolysis heating method in the presence of nitrogen-bearing vapours. The resulting nanoflakes, obtained on a magnesium oxide substrate as a crystal template, retained the beneficial properties of graphene, while having nitrogen groups attached to the surface.

“The most effective electrode material for supercapacitors should combine both electrostatic and electrochemical charge accumulation mechanisms, as well as having good wettability with the electrolyte,” says Arkhipova. “Our challenge was to dope enough nitrogen into the graphene nanoflakes to boost capacitance via electrochemical charge storage, while retaining the optimal mesoporous structure and high specific surface area of the graphene which gives it its fast and effective electrostatic charge accumulation.”

By progressively optimising the synthesis conditions, such as pyrolysis temperature, time and the nitrogen source, the researchers found that a nitrogen content of about 6 atomic percent gave the biggest boost in capacitance.

“We found that the incorporation of nitrogen into the graphene nanoflakes under optimal synthesis conditions nearly tripled the specific capacitance of a supercapacitor using the modified graphene electrode,” says Arkhipova.

This work is part of a broader research programme at the Chemistry Department of Lomonosov Moscow State University investigating carbon nanomaterials for use in energy storage devices and catalysis, and represents a major step forward in functionalising an already promising material.

“Our results show not only that nitrogen significantly increases the specific capacitance of the electrode material, but also that the specific surface area is not the only essential parameter that affects supercapacitor capacitance,” Arkhipova says.