Electrochromic (EC) materials change their optical transparency when voltage is applied. A particularly promising category of EC materials are metal-organic coordination complexes. Given the impressive EC characteristics of these complexes, there's much potential to develop processes for preparing high-performance EC materials and films containing them. Furthermore, the full potential of electrochromic molecular- and polymer-based devices remains largely untapped. This is partly due to challenges in production, cost and issues with stability and device integration that limit widespread use.

Combining different functions in organic devices and coupling them with conventional technologies is still at an early stage. A key objective in energy device development is to amalgamate energy storage and distribution with extra capabilities and the market is increasingly seeking advanced electronic devices that leverage the electrochromic properties of their components for enhanced and integrated functionality.

Metal-coordinated organic complexes form an interesting category of electrochromic (EC) materials, characterized by a metal ion coordinately bonded to an organic molecule (ligand). In the newly developed energy storage devices, the metal-ion coordinated organic complex serves as a redox-active material on at least one electrode. The color or absorption spectrum of the EC material shifts in response to the oxidation or reduction state of the metal ions. The heart of the device is an electrochromic metallo–organic layer that functions as both the battery-type electrode and the charge indicator. The capacitive electrode is a layered composite of carbon nanotubes (CNTs) and a conductive polymer. The device operates under low potentials (−0.6 to 2 V), displays high energy and power densities, a high coulomb efficiency (99%), a short charging time (≈2 s), and a charge retention of ≈60 min. The electrochromic–hybrid supercapacitor is wired with a conventional circuit board to be charged and subsequently to operate a diode. The results demonstrate the potential of metallo–organic assemblies for usage in these types of supercapacitors.

Prof. van der Boom and his team have developed smart energy storage devices that enable monitoring of its charging status by the naked eye. The team has successfully demonstrated film fabrication and characterization of EC materials on both rigid and flexible supports. They have also demonstrated the incorporation of EC films in solid configurations, for example - in the basic structure of an electrochromic device and explored various EC deposition methods. Furthermore, the researchers have shown an integrated optoelectronic device where the level of charge is indicated through color change and have developed a unique composite capacitive electrode to obtain efficient hybrid supercapacitors. Stability, both in color and energy, for more than 1000 consecutive charging–discharging cycles was demonstrated and no significant changes in device temperature were indicated under the operating conditions. Moreover, the team has shown that their devices can be used to operate in widespread used printed circuit boards (PCBs).