The article describes novel electrochromic materials (ECMs) that are based on a monolayer consisting of two or three isostructural metal complexes of 4′-(pyridin-4-yl)-2,2’:6′,2’’-terpyridine simultaneously deposited on surface-enhanced support. The support was made by screen printing of indium tin oxide (ITO) nanoparticles on ITO-glass and has a surface area sufficient for a monolayer to give color visible to the naked eye. The ability to separately electrochemically address the oxidation state of the metal centers on the surface (i.e., Co2+/Co3+, Os2+/Os3+, and Fe2+/Fe3+) provides an opportunity to achieve several distinct color-to-color transitions, thus opening the door for constructing monolayer-based multicolor ECMs.

The paper reports a general methodology for the rational tuning and optimization of electrochromic devices (ECDs) that are based on a monolayer of Fe (II) (4'-(4-pyridyl)-2,2':6',2"terpyridine) 2 complex (Fe 4'T) covalently embedded onto a screen-printed high-surface area indium tin oxide working electrode. We demonstrate that the nature of the counter electrode and the resulting device configuration could drastically improve the long-term stability of the ECD. We show that the replacement of the flat ITO glass electrode with a high surface area ITO electrode or the use of symmetrical working and counter electrode architecture leads to a much longer performance of the device. We propose a methodology to determine optimal operating conditions for ECDs by fine-tuning the lower and upper operation potentials. In addition to using traditional cyclic voltammetry (CV), we utilize electrochemical impedance spectroscopy (EIS) to study the intrinsic properties of the devices and understand the factors that define the longterm cycling stability, which is further described through the use of equivalent circuit models. The main reasoning behind decomposition processes in ECDs and ways to suppress them are discussed.

No abstract

Electrochromic devices (ECDs) and especially electrochromic supercapacitors, where the real-time state of charge is indicated by the color, have a wide range of applications. However, to meet modern challenges, these devices should demonstrate exceptional charge–discharge durability. In this work, we demonstrate that electrochemical cycling stability of ECDs based on a monolayer of 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine–iron­(II) complex that was covalently linked to the working electrode (WE) can be drastically enhanced by a proper design of the counter electrode (CE). Enhancing the surface area of the flat indium tin oxide (ITO) CE by a layer of screen-printed ITO nanoparticles results in an ECD that upon continuous spectro-electrochemical switching for 600 cycles demonstrates negligible deterioration of the change in optical density. The enhanced surface area of the CE significantly diminishes the electrode and gel electrolyte degradation and allows us to gain fundamental insight into the pathway of degradation of the electrochromic molecules at the WE. During prolonged cycling (20,000 and 50,000 cycles), the overall total resistance of ECDs remains fairly unchanged, while the capacitance decreases due to a loss of the pseudocapacitive component associated with electrochromic molecules. X-ray photoelectron spectroscopy (XPS) suggests that this loss is likely due to the cleavage of the linkage C–N+ bond followed by the dissolution of the entire metal complex molecule into the electrolyte.

Sequential embedding of metal complexes of 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine to a surface-enhanced supports pre-functionalized with a templating layer results in hetero-bimetallic (Os–Fe, Co–Fe) and hetero-trimetallic (Co–Os–Fe) monolayer materials.