Electrochromic components and electrical energy storage systems are becoming widespread, the first predominantly in real estate, the second finding both stationary and mobile uses. Electrochromic devices give us electrically controllable features such as intrinsic color changes and privacy effects, for example to electrically adjust transparency or darkness levels in windows. Rechargeable batteries and electrochemical capacitors (also called super capacitors) are growing in importance for storing large energy quantities, for household energy or in vehicles powered solely by electricity for example. Even if these technologies look very different at first sight, they have more in common than might be thought.
Since these components are very similar in functional principle, reaction kinetics, material properties and design principle, research in recent years has increasingly looked at whether and how electrochromatic glazing - as found in modern, energy-efficient office buildings (keyword “Intelligent Windows” or “Smart Privacy Glass) or other electrochromatic components (as in newer flat screens) - can also be used to store energy. Conversely, research is also looking at whether and where there are practical uses for electrical storage systems that also offer electrically induced color changes, for example in batteries, that make charge levels or the amount of energy remaining visible.
Electrochromatic devices are multilayer constructions. They consist of an active electrochromic electrode, an electrolyte layer, a counter electrode, two flat transparent conductor tracks - each being outside the electrodes - as well as the mechanical support structure made of glass or plastic. This structure can be seen as a rechargeable thin-film battery whose charge level is shown in optical absorption. However, the choice of this “battery’s” electrode material is clearly guided by its reversible color change ability. In order to integrate the two aspects of electrochromism and electrical energy storage into one component in the future, electrode material investigation is currently focusing on three material groups: metal oxides, conductive polymers and inorganic non-oxides.
In metal oxides, we distinguish between “cathodic” and “anodic” electrochromics. In the first case, ion deposits lead to color changes; in the second, it is ion emission. Conductive polymers can be tailor-made either for electrochromism or for energy storage. Various polyanilines and polypyrroles show good combination properties. In combination with gold and WO 3, significant electrochromic energy stores were recently realized for the first time. Currently outstanding in the case of inorganic oxides are the dye Prussian blue and the carbon modification graphene. For both substances, research into electrochromic energy storage is at an early stage.