In the past two decades, a lot of attention has been put on Electrochemical Capacitors (ECs), also known as supercapacitors, since they are one of the most promising electrochemical energy storage devices for high power delivery or energy harvesting applications. The charge storage mechanism in supercapacitor electrodes is achieved through electrostatic attraction between the ions of an electrolyte and the charges present at the electrode surface, leading to a charge separation at the electrolyte/electrode interface that works as a dielectric capacitor. Since no faradaic reaction is involved in the charge storage mechanism, supercapacitors hold higher power density (15kW/kg) and much better cyclability (>106) as compared with batteries.
Figure: Schematics of a supercapacitors cell assembled with two electrodes containing high surface area porous carbons. Under polarization, ions are adsorbed to balance the charge present on the carbon surface (anions at the positive electrode and cations at the negative). The charge is released during discharge. Inset: zoom of a porous carbon grain of the electrode, showing a schematic of ions adsorbed inside the pores of the carbon.
In summary, batteries keep our devices working throughout the day—that is, they have a high energy density - but they can take hours to recharge when they run down. For rapid power delivery and recharging (i.e., high power density), supercapacitors are used. One such application for ECs is regenerative braking, used to recover power in cars and electric mass transit vehicles (trams, buses…) that would otherwise lose braking energy as heat.
The most important challenge ECs are facing today is to increase the device energy density to reach 10 Wh per kg and more, moving ECs closer to batteries in terms of energy density and cutting the cost at the same time. The energy density (Wh) of a supercapacitor changes according to
where W is the energy (Wh), C the capacitance (F) and V the nominal voltage (V). The energy (and power as well) is usually normalized by the weight or volume of the device or the electrode (it is very important to distinguish between those) to obtain energy and power densities. A high cell voltage (V), which is mainly limited by the electrolyte stability, is needed to reach high energy (eq.1). However, the development of a high-voltage electrolyte (>4.5V) that preserves high ionic conductivity and low viscosity - the Holy Grail for the battery and supercapacitor communities – appears really complex. Increasing the energy density (eq. 1) can be also achieved by increasing the capacitance C of the carbon, which is controlled by the carbon / electrolyte interface. Basically, it requires designing the carbon--electrolyte interface for optimizing the adsorption of ions from the electrolyte to maximize the charge stored (F) per gram or per cubic centimeter of carbon.
At Université Paul Sabatier, P. Simon and his co-workers (P.L. Taberna and B. Daffos, http://www.energie-rs2e.com/fr) are studying the dynamics and adsorption of ions of an electrolyte inside nanoporous carbons, in the aim of designing high performance carbons for supercapacitors applications. They have developed for several years collaborations with other groups, and have shown that the charge storage was maximum when the pore size of the carbon was in the same range of the ion size (less than 1 nm), defying the conventional wisdom that pores larger than the ion size were needed. Such a behavior was explained by a change in the organization of the ion and solvent molecules in these confined pores alloying ions to get closer to carbon surface. Electrochemical Quartz-Crystal Microbalance (EQCM) experiments have shown that ions were entering the pores partially desolvated from solvent molecules, that could explain the fast ion dynamics preserved in the confined pores. Moving from large-size supercapacitors to micro-supercapacitors, the preparation of bulk films of porous carbon with pore size less than 1 nm has led to the preparation of high energy density micro-devices that could complement, or even sometimes replace, micro-batteries.
More generally, the use of nanoporous carbons has led to the development of supercapacitors with improved energy density. These devices are currently used today in many applications including transportation. Examples can be found with Mazda and Citroën cars (stop and start function), tramways for braking energy recovery (Bombardier, Alstom with Maxwell and Blue Solutions), electric buses or electric boats (where the autonomy is limited to few km but with an added value of fast charging in about 30 s during passenger exchange). Aside, harbor cranes (to recover kinetic energy), airplanes (A380 for emergency door opening), cordless tools and toys as well as power electronics are other applications of supercapacitors currently available.
P. Simon has received several awards in the sole 2015 year: Rusnano prize, CNRS silver medal, SF2M Charles EICHNER medal. He was also invited to present these activities at SF2M (member of FEMS) annual meeting focused on Materials for Energy Generation http://www.sf2m.fr/JA2015/JA2015.htm.
Patrice Simon, Professor, Université Paul Sabatier Toulouse 3,
Laboratoire CIRIMAT UMR CNRS 5085,
118 route de Narbonne, 31062 Toulouse Cedex