Electrochemical double-layer capacitors (EDLCs) represent the most important family of today’s commercially available Electrochemical Capacitors (ECs). Such systems can store charges electrostatically in the electrochemical double-layer that arises from the separation of charges at the electrode / organic electrolyte interfaces when polarizing the electrodes, and exhibit an excellent cycling stability combined with fair gravimetric and volumetric energy densities. Other types of ECs use “pseudocapacitive” materials, i.e. materials that use fast and reversible surface redox (faradaic) reactions to store energy. To combine the advantages of both organic EDLCs and aqueous symmetric systems, the design of aqueous asymmetric supercapacitors was proposed. These devices comprise either two pseudocapacitive materials, or a pseudocapacitive and a capacitive carbon-based material which exhibit complementary electroactive windows. As a result, operating cell voltages approaching 2 V can be obtained, leading to competitive energy densities without the disadvantages of using an organic electrolyte.
Both specific energy and power densities have to be increased and improving the volumetric capacitance of supercapacitors, which is one of the limiting factors of today’s stationary applications, is essential. In order to meet these requirements, solid state chemistry is an powerful tool to design new materials. Our strategy was based on the study of polycationic oxide materials. They can combine transition metals such as Fe or Mn, that are known to be electrochemically active in aqueous media, with dense elements such as W, Bi,… The Fe/W/O has a peculiar interest since our recent studies have shown that FeWO4 exhibit excellent electrochemical properties [1,2]. Its pseudocapacitive behaviour has been demonstrated by operando XAS experiments with the use of a special cell designed in our lab [3].
The use of low-temperature synthesis methods in order to get nanosized particles with high specific surface areas is also required. Synthesis conditions and materials characterizations of the electrodes and also of full devices will be detailed in the presentation, highlighting the crucial role of the electroactive elements, the crystallographic structure, and the morphology of the synthesized materials on their electrochemical performance.
[1] N. Goubard-Bretesché, O. Crosnier, C. Payen, F. Favier, T. Brousse, Electrochem. Commun. 57 (2015) 61.
[2] N. Goubard-Bretesché, O. Crosnier, F. Favier, T. Brousse, Electrochim. Acta 206 (2016), 458.
[3] N. Goubard-Bretesché, O. Crosnier, C. Douard, A. Iadecola, R. Retoux, C. Payen, F. Favier, M.-L. Doublet, T. Brousse, in preparation.