Surface-Enhanced Counter Electrode Materials for the Fabrication of Ultradurable Electrochromic Devices

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.