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How to reconcile energy and power for electrochemical storage: The pseudocapacitors

Whereas batteries provide energy over several hours, supercapacitors deliver energy peaks in seconds, and provide strong power. Balancing energy and power under constrained volumes is the key challenge for the growth of many industrial applications including the electric vehicle where autonomy and speed have to be merged as well as the field of mobile (charging time and power of flash a mobile phone) and of micro-electronics.

Published on 14 January 2014
Since 2010, two teams from our institute we have developed, in collaboration wwith other researchers a our institute (SP2M/SINAPS and SPrAM/LEMOH), the concept of micro-supercapacitor which electrodes are made of nanostructured silicon substrate with silicon nanowires. This structuration at the nano-scale allows a substantial increase of the developed surface of the silicon electrode; electric capacities of the order of 200μF/cm2 were obtained, more than 30 times the electric capacity of the original surface. The capacitive behavior of these electrodes is almost ideal, with exceptional stability during cycling, up to 4 million cycles for a 25% loss.

In order to add a "battery element", a redox material must be added to the silicon electrode, in a thin layer and in a manner consistent with the nanostructure to maintain the high surface area developed without losing kinetic limitation thus power. To do this, we use electropolymerization of conjugated polymers which allows to access to the entire active surface of the electrode and to deposit an electroactive polymer. 300 nm thin films of PEDOT (Poly(EthyleneDiOxyThiophene)) were electrodeposited on a substrate of n doped silicon nanowires (SiNWs) (≈ 50 nm diameter and length 20 μm). The monitoring of deposit by SEM shows that the integrity of the nanowires and their attachment to the substrate are preserved (See Figure 1).

Figure 1: SEM image of the silicon nanowires before (a) and after (b) the electrodeposition of a 300nm thin PEDOT film.

Galvanostatic charge / discharge curves retain the quasi- perfect aspect obtained for non-functionalized SiNWs (Figure 2), whereas the electrical capacity reaches 11 mF/cm2, a gain of more than 2 orders of magnitude.
The optimization of the electropolymerization kinetics and of the parameters of the doped silicon nanowires are under study in order to still improve stability under electrochemical cycling. This work is performed in collaboration with IMN (Nantes).

Figure 2: Galvanostatic charge / discharge curves (under constant current, 0.25 mA/cm²) for non-functionnalized (a) and for functionnalized (b) Silicon nanowires.

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