Unlike batteries, supercapacitors and fuel cells, which convert chemical energy into electrical energy, or photovoltaic systems, which convert light energy into electricity, electrochromic devices, which are nothing more than thin optical batteries, can modulate optical behavior in the visible range by applying a potential. These devices can thus be used to manage the transmission/absorption of light through a pane of glass, finding their place in building architecture, in the form of " intelligent glazing ", to control interior luminosity at different times of the day. On the same principle, infrared electrochromic devices are also important for controlling emissivity, leading to essential applications in energy-saving thermal management of homes. The aim of this lecture was therefore to i) retrace the history of electrochromic materials from the accidental discovery of Prussian blue in 1704 to the realization of the first devices in 1970, ii) explain the fundamental operating principles, linked to the modification of the optical gap of electrode materials by redox reactions accompanied by charge transfer, iii) review the current state of the art, mentioning existing automotive applications (roofs and rear-view mirrors) and future applications linked to their manufacture on plastic substrates, and iv) present future prospects, in both the visible and IR domains.
As electrochromic systems are nothing other than optical batteries, their commercialization involves solving classic and complex electrochemical problems (electrode/electrolyte interface, material stability) while meeting demanding optical criteria (transparency, contrast, color neutrality). These criteria must also be adapted to the specific demands of each type of application. Despite the success achieved with small-scale commercial applications, the transition to large-scale building applications is problematic due to the many difficulties that persist both in terms of materials and the operation of the devices, whose performance at high temperatures remains somewhat limited. The difficulties with materials are exacerbated for electrochromic devices in the less mature IR range, for which the ideal emissive insert materials have yet to be isolated, so for want of anything better, we're still using WO3.
Finally, a few figures bear witness to the need for research in this field: i) intelligent glazing offers energy savings of 40% compared with traditional glazing; ii) over 7% of our annual energy consumption is linked to the insulation quality of our windows. This means we can save a number of GWs, and even more, if we consider that for every joule saved, 4 to 5 joules of primary energy are saved.
