Use of a Cobalt-Based Redox Electrolyte in Hybrid Electrochromic Devices

In the present study, highly transparent evaporated tungsten oxide films with improved charge storage properties were used in battery-like (b-ECDs) and hybrid electrochromic devices (h-ECDs). A Co2+/3+ redox couple was added to the electrolyte as an alternative to other redox couples that have been already used in h-ECDs. The as-prepared h-ECDs, colored homogeneously, exhibited a contrast ratio of up to 7:1 in the visible spectrum, at a cathodic voltage of −2.5 V for only 10 s, compared to 3.5:1 at a cathodic voltage of −3 V for 180 s for a b-ECD. Moreover, when the redox couple was present in the electrolyte, almost a 50% higher areal capacitance and a 55% lower charge transfer resistance at the electrochromic layer/electrolyte interface were achieved. Also, the results show that the optical performance depends strongly on the coloration procedure (potentiostatic or galvanostatic), that self-bleaching is not so intense, and especially that the energy density consumed during bleaching is reduced in the presence of the redox couple. Overall, the findings of this study highlight the benefits of using a cobalt redox electrolyte in h-ECDs, allowing a direct comparison with b-ECDs, to dynamically control incoming solar irradiation in a building, thus improving buildings’ energy efficiency.