The molecular organization and functioning of supercapacitors have for the first time been explored by researchers from France’s Centre National de la Recherche Scientifique and the Université d’Orléans. The French scientists devised a technique which provides a new tool for optimizing and improving tomorrow’s supercapacitors.
Despite the daunting their name, supercapacitors are not science fiction, but part of everyday life. For instance, these electricity storage devices installed on buses are charged up during braking and provide electricity to open the doors to let you off at your stop. They are also used in trains, hybrid cars, and even for opening emergency exits on the Airbus A380.
Supercapacitors are electricity storage devices but are quite different from batteries. They are charged much faster, typically in seconds, and they do not suffer as fast wearing due to charging/discharging. Conversely, supercapacitors of equivalent size offering greater power, are unable store as much electrical energy as batteries.
Carbon-based supercapacitors provide energy densities of around 5 Wh/kg compared to around 100 Wh/kg for lithium-ion batteries. Compare that to dynamite, with an energy density of 5 MJ/kg and gasoline’s 46 MJ/kg.
Storage of electricity in a supercapacitor works via the interaction between nanoporous carbon electrodes and ions, which carry positive and negative charges, and move about in a liquid known as an electrolyte. When charging, the negatively charged ions (anions) are replaced by positively charged ions (cations) in the negative electrode and vice versa. The greater this exchange and the higher the available carbon surface area, the greater the capacity of the supercapacitor.
Nuclear Magnetic Resonance Spectroscopy
Using Nuclear Magnetic Resonance, the researchers delved deeper into this phenomenon and were able to quantify the ratio in which charge exchanges take place in two supercapacitors using commercially available carbons. This had never been done before.
Through comparison of two nanoporous carbon materials, they were able to show that the supercapacitor containing the carbon with the most disordered structure had superior capacitance and improved high-voltage tolerance. This may be a result of better electronic charge distribution upon contact with the electrolyte molecules.
Exploring electrolyte organization in supercapacitor electrodes with solid-state NMR
M. Deschamps, E. Gilbert, P. Azais, E. Raymundo-Pinero, M.R. Ammar, P. Simon, D. Massiot, F. Béguin, Nature Materials. DOI: 10.1038/NMAT3567