Applied Power Electronics Conference 2019

An introduction to graphene supercapacitors

14 February 2019
SkelGrid Omni, an ultracapacitor energy storage system | Source: Skeleton Technologies

Graphene may have found a use case as a commodity in the manufacture of next-generation energy storage solutions. Supercapacitors provide unmatched power density, and while they have been limited by discharge potential and energy density, the inclusion of graphene has enabled the device to increase its value as an energy storage solution.

What is a supercapacitor?

Traditional capacitors are composed of two conducting surfaces separated by a dielectric medium. In a charged state, an electrostatic field is established between the two opposing conductive surfaces and is quantified by a value of capacitance. In a charged state, potential energy is stored electrostatically, and as there are no electrochemical reactions taking place, capacitors rapidly accept or release energy. They are also capable of undergoing exponentially more charge cycles when compared to other energy storage devices, like lithium-ion batteries.

The amount of energy stored in a capacitor is a function of the conductor surface area, the distance between conducting surfaces and the dielectric constant of the insulating medium. Capacitance is then proportional to the conductor surface area and inversely proportional to the distance between them.

In order to vastly improve the amount of energy stored, supercapacitors have been developed with a slightly different arrangement. Supercapacitors differ from standard capacitors in that there is no solid dielectric medium existing between charged plates. Instead, electrostatic energy is commonly stored by a phenomenon known as electrostatic double-layer capacitance (EDLC).

In EDLC technology, two electrodes are separated by a membrane and an electrolyte acts as the dielectric. In a charged state, ion absorption occurs on the surface of each electrode. This is where energy is stored and the absorbed ions remain within a few ångströms (0.3 to 0.8 nm) of the electrodes. The device acts as a stacked capacitor and energy is stored in what is known as the Helmholtz double layer on the surface of the asymmetrical electrodes.

Graphene supercapacitors

EDLC vastly increases the amount of stored energy by greatly reducing the distance between charged surfaces and increasing the surface area of conducting surfaces. In order to further improve upon the design, the conductive surfaces have been coated with activated carbon, a highly porous medium that further increases the available surface area.

Graphene not only offers greater surface area than active carbon, but also acts to reduce the weight of the device. The largest fallback of graphene supercapacitors lies within the economics of producing a functional device. However, several manufacturers have brought graphene supercapacitors to market, and, while supercapacitors were originally seen as a power storage device best suited as a complementary device to other energy storage mediums, graphene-based energy storage devices are now considered a viable energy storage device themselves.

Patented curved-graphene technology

Skeleton Technologies is one of the front-runners behind graphene supercapacitors. They have received a patent for their curved graphene ultracapacitors. While their products are first seen as a power storage device, their energy storage solutions are also beginning to find use cases.

Skeleton Technologies claims to be the only manufacturer using an inorganic precursor for their curved graphene substrate. They also claim their technology provides for one of the lowest ESR (equivalent series resistance) levels on the market, which directly impacts operational efficiency. Considerable energy is lost through heat when storing energy in a conventional capacitor due to ESR, but as ESR is lowered, efficiency gains are realized.

Skeleton’s energy storage solution, Skelgrid Omni, is based on their ultracapacitors modules. Each module is composed of several cells connected in series, increasing the potential difference. This allows the energy storage device to become suitable for powering electrical circuits, whereas the electrical potential of a single ultracapacitor remains insufficient.

Skelgrid Omni functions as a high power, uninterruptible power source. It can be deployed in off-grid or grid-tied systems. The system also includes a multi-port inverter, which eliminates the need for dedicated solar inverters or wind turbine inverters when the system is integrated with renewable energy systems and micro-grids.

Conclusion

Supercapacitors will experience a period of rapid growth from 2018 through 2023 as stated by a recent report on Market Research Future. Growth is primarily fueled by their use in hybrid automobiles and renewable energy technologies. Use of supercapacitors in these applications has helped manufacturers overcome the limitations of other energy storage devices, like lithium batteries, which are limited by charge lifecycle expectancy and their ability to quickly absorb power spikes.

Market growth may open new possibilities, and with next-generation energy storage solutions like Skelgrid Omni gaining market share as an energy storage solution, manufacturers will be able to address market restraints including cost and energy density. This will allow supercapacitors to compete with lithium titanite batteries as an energy storage device in the automotive market and in other energy harvesting applications.

Related: For our coverage of the Applied Power Electronics Conference that runs March 17-21 in Anahemin, California, visit our dedicated section Electronics360 covers APEC 2019.



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