Although electricity can be produced in multiple ways, it remains very difficult to store it efficiently. At the moment most of our storage capacity relies on pumped-storage hydroelectricity, in which the energy is stored in the form of gravitational potential energy of water. A huge amount of efforts are currently devoted to the development of electrochemical storage devices. There are two important characteristics which have to be optimized: the specific energy and specific power. In practice, when used in an electric vehicle, the former measures how far one can go on a single charge, while the latter shows how fast one can go. The two main families of electrochemical energy storage devices are supercapacitors and batteries. Their respective ranges of performance are shown on Fig. 1. It appears clearly that from batteries one can obtain much larger specific energies, with the best performances corresponding to approximately 500 Wh kg⁻¹ for the most recent Li-ion technologies. In comparison, supercapacitors can only reach 5 Wh kg⁻¹, but they show much better specific powers, i.e. 10 kW kg⁻¹. This difference is better understood by comparing the characteristic charge/discharge times, which are given by the diagonal lines on Fig. 1: Typical values of 1 h and 1 s are respectively obtained for batteries and supercapacitors. They have therefore different applications, but they may also complement one another in a system which needs both high power and energy density. While batteries are more and more used as the primary energy storage for electric cars, supercapacitors are used in their stop & start systems. Nevertheless supercapacitors are also used alone in new applications, such as trolley buses. Indeed, such vehicles can be charged when they board/unboard passengers, with a typical charging time of 1 minute, and the needed autonomy is the distance between two bus stops, i.e. less than 1 km.