Batteries or solar cells: where graphene application would be more effective?
Batteries or solar cells: where graphene application would be more effective?

Like diamond or graphite, graphene is a structural modification (an allotrope) of carbon, that has many special properties that make it a very useful material with great potential for application in technology. In essence, graphene is an isolated atomic plane of graphite, which is very light (1-square-meter sheet weighing only 0.77 milligrams) and at the same time very strong (graphene has a breaking strength over 100 times greater than a hypothetical steel film of the same thickness). The electrical properties of this novel material are being extensively researched for the wide range of potential graphene applications. In fact, graphene may even provide a basis for the new definition of the SI unit of electric current, the ampere.

The International System of Units (a modern form of the metric system) is built around seven base units: meter (length), kilogram (mass), kelvin (temperature), candela (luminous intensity), mole (amount of substance), second (time) and ampere (electric current). All other units of measurement are derived or expressed in terms of these base units. As the International System of Units (SI) is an evolving system, the unit definitions are periodically modified as the technology of measurement progresses and as the precision of measurements improves.

Ampère’s force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere, which states that it is “the constant current that will produce an attractive force of 2 × 10–7 newton per meter of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one meter apart in a vacuum”.

This definition of the ampere, however, is vulnerable to drift and instability. This is not sufficient to meet the accuracy needs of present and certainly future electrical measurement. Therefore, it has been proposed to redefine the ampere in terms of the electron charge, and the world’s first graphene single-electron pump (SEP) recently developed by the National Physical Laboratory (NPL) and the University of Cambridge may provide the means to do so.

SEPs create a flow of individual electrons by shuttling them into a quantum dot—a particle holding pen—and emitting them one at a time and at a well-defined rate. The paper in Nature Nanotechnology describes how a graphene SEP has been successfully produced and characterized for the first time, and confirms its properties are extremely well suited to define the unit of electrical current.

Electron microscope images show a new material for transparent electrodes that might find uses in solar cells, flexible displays for computers and consumer electronics, and future "optoelectronic" circuits for sensors and information processing. The electrodes are made of silver nanowires covered with a material called graphene. At bottom is a model depicting the "co-percolating" network of graphene and silver nanowires. (Credit: Purdue University / Birck Nanotechnology Center)
Electron microscope images show a new material for transparent electrodes—silver nanowires covered with graphene. (Credit: Purdue University / Birck Nanotechnology Center)

Another recent publication describes how graphene can be applied for creation of batteries that are more stable, have longer lifetime and, most importantly, can be charged in a matter of minutes. Scientists at the Pacific Northwest National Laboratory have shown that the small quantities of graphene, when used with the batteries based on titanium oxide and carbon structures, can dramatically improve the performance of the batteries.

Further advancement of this technology may give electric cars the same indefinite range that most cars with internal combustion engines already have, thus solving the problem of running out of energy from the battery before reaching a destination. This will no doubt make electric cars much more attractive to future potential buyers.

Graphene also finds its application in the production of solar cells. A study by the University of Manchester and the National University of Singapore has shown how a combination of graphene with other similar 2D crystals will allow to significantly increase the efficiency of solar cells and create the next generation of optoelectronic devices.

According to the authors of a paper, published in Science, their breakthrough could lead to electric energy that runs entire buildings generated by sunlight absorbed by its exposed walls; the energy can be used at will to change the transparency and reflectivity of fixtures and windows depending on environmental conditions, such as temperature and brightness.

Unique properties of graphene also make it possible to create flexible solar cells. In a recent article published in journal Advanced Functional Materials, researchers describe a new graphene-coated transparent electrode made of silver nanowires. Because of its ability to bend without breaking, the new invention can be used to create flexible solar cells, computer and consumer electronics displays and future “optoelectronic” circuits for sensors and information processing.

Two last articles show that there is a huge untapped potential for graphene applications in combination with other advanced materials. This approach often allows scientists to obtain materials with new properties not observed in the constituents.

[notification type=”help”]Chen, R., Das, S., Jeong, C., Khan, M., Janes, D., & Alam, M. (2013). Co-Percolating Graphene-Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes Advanced Functional Materials DOI: 10.1002/adfm.201300124[/notification] [notification type=”help”]Connolly, M., Chiu, K., Giblin, S., Kataoka, M., Fletcher, J., Chua, C., Griffiths, J., Jones, G., Fal’ko, V., Smith, C., & Janssen, T. (2013). Gigahertz quantized charge pumping in graphene quantum dots Nature Nanotechnology DOI: 10.1038/nnano.2013.73[/notification] [notification type=”help”]Britnell, L., Ribeiro, R., Eckmann, A., Jalil, R., Belle, B., Mishchenko, A., Kim, Y., Gorbachev, R., Georgiou, T., Morozov, S., Grigorenko, A., Geim, A., Casiraghi, C., Neto, A., & Novoselov, K. (2013). Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films Science DOI: 10.1126/science.1235547[/notification]