Andreas Lab
Learn More About Supercapacitors

Supercapacitors are similar to batteries in that they are charge storage systems and they are charged and discharged like batteries. The difference between supercapacitors and batteries which gives supercapacitors their main advantage is their method of storing charge. Batteries store their charge by actually changing the material on the battery electrode (in a lead-acid car battery the two electrodes when charged are composed of lead and lead-oxide, but during discharge these electrodes are converted to lead-sulfate). Supercapacitors, however, store their charge at the interface between the electrode and the solution inside the supercapacitor (called the electrolyte). In contrast to batteries, no species are changed during charging or discharging of a supercapacitors, and instead, the supercapacitor electrode is charged up (either positively or negatively), and ions from the electrolyte spontaneously move towards the surface to balance that charge. In this way, the supercapacitor acts like a typical parallel plate capacitor, but because the supercapacitor stores charge on a very large surface, it can store about a million times more charge than a regular capacitor.

The benefits of this charge storage mechanism are two-fold. The first benefit is that, in theory, the charge can be removed from a supercapacitor much faster than from a battery. This leads to enhanced high power performance.  For an electric vehicle application, high power situations are when you accelerate or drive up a hill. In these situations, batteries (and fuel cells) cannot provide the power required. Supercapacitors, however, can provide this power. Conversely, supercapacitors do not have enough energy to allow us to drive very far, and batteries and fuel cells are excellent for this.  Thus, in the future, a hybrid between supercapacitors and batteries (or fuel cells) could meet all of the requirements for an electric vehicle.

The second benefit that derives from the special manner of charge storage is a significantly higher cycle life for the supercapacitors. In fact, supercapacitors can be charged and discharged about a million times, whereas in most commercial batteries only a couple of hundred or thousand charges are possible. This advantage makes supercapacitors ideal for coupling with solar- and wind- power energy production sources, particularly those in remote or hard-to-reach locations where replacing the energy storage system is difficult. Because of the high cycle life, these supercapacitors do not need to be replaced as often as batteries in the same systems.

Unfortunately, supercapacitors undergo a process called ‘self-discharge’. This is where the supercapacitor loses charge when it has been charged but not immediately used. For example, if a supercapacitor replaces the lead-acid battery in your vehicle and you drive to the airport and park your vehicle, by the time you come back from vacation two weeks later, the supercapacitor would have lost all of its charge, and you would be unable to start your vehicle. Obviously, therefore, self-discharge may be a significant hindrance to the commercialization of supercapacitors.

Supercapacitors come in many different types, using different electrodes and different electrolytes. The systems currently under study in the Andreas laboratory use a very large area carbon as the electrodes and a water-based electrolyte such as 1 to 5 mol/L sulfuric acid.