Supercapacitors are energy storage devices that can deliver or harvest high power during few tens of seconds, thus complementing batteries in a broad range of applications . Carbon-based supercapacitors store the charge in an electrostatic way, by reversible adsorption of ions from an electrolyte onto the surface of high specific surface area carbons; there is thus no redox reactions. Although their power performance is really impressive (> 10 kW/kg), the energy density of EDLCs (up to 8 Wh/kg) is still limiting their discharge time, thus narrowing their range of applications. Improving the energy density of supercapacitors would then result in broadening their range of applications.
The energy density of supercapacitors is given by (equation 1)
E=1/2.C.V² (eq. 1)
where E is the energy (J), C the capacitance (F) and V the cell voltage (V). From equation 1, the solution the address the key challenge ECs are facing is to increase the capacitance C or the cell voltage V. Accordingly, we focus our work in the following main directions:
i) Improving the capacitance of porous carbons
Following our discovery of the capacitance increase in carbon nanopores, we developed fundamental work on the understanding of ion transfer and adsorption in confined pores. We combine modelling and in-situ experimental techniques to get further information on the charge storage mechanisms where ions needs to partially desolvate to access these narrow pores. Model materials like Carbide-derived Carbons with controlled pore size are of great interest in that aim.
ii) Improving the capacitance: moving from carbon to pseudo-capacitive materials
Another way to increase the capacitance of supercapacitors is to use pseudo-capacitive materials that, differently from porous carbons (in adsorption), store the charge through fast, surface redox reactions. The goal here is to design materials exhibiting high (pseudo)capacitance in organic or aqueous electrolytes. We work on the nanostructured oxides (such as MnO2 or Nb2O5) and on MXenes 2D materials.
iii) Developing high voltage and solid state electrolytes (ionogel) electrolytes for solid-state supercapacitors and micro-supercapacitors
We try to develop electrolytes in conjunction with the carbon material, that is to say we try to adapt the carbon structure to the electrolyte properties. We work on eutectic ionic liquid mixtures showing high voltage (3.5 and beyond) and can be used in a large operation temperature range (-40°C - + 100°C). We also prepare solid state electrolyte-based supercapacitors by developing gel or ionogel electrodes. Ionogels are ionic liquid electrolytes entrapped inside an inorganic matrix lie SiO2.
We also develop activities on the design of materials and electrodes for micro-supercapacitors , which are currently focusing a lot of interest for applications such as for Radio frequency identification (RFID) tags for the development of smart environments (internet of things), or for powering sensor networks. We work on fully integrated micro-devices on Si chips from chlorination of sputtered TiC films but also on metal oxide-based flexible micro-supercapacitors obtained from laser scribing techniques.
A last part of our research activity deals with the design of nanostructured interfaces for Li-ion micro-batteries electrode.
This research work is done within the Electrochemical Energy Sources French network RS2E, FR CNRS3459 as well as the ALISTORE European Research Institute, FR CNRS n°3104
The RS2E aims at developing innovation and industrial network on electrochemical energy storage.