Brown coloring electrochromic devices based on NiO–TiO2 layers

Brown coloring electrochromic 5×10 cm2 windows with the configuration K-glass/NiO–TiO2/electrolyte/CeO2–TiO2/K-glass have been prepared and characterized by optoelectrochemical techniques (cyclic voltammetry, chronoamperometry and galvanostatic measurements). The electrochromic layers have been prepared by the sol–gel technique. As electrolyte either a 1 M aqueous KOH solution or a newly developed starch-based gel impregnated with KOH have been used. The CeO2–TiO2 sol–gel layers sintered at 550 °C have been previously characterized in 1 M aqueous KOH electrolyte as a function of the thickness up to 2000 cycles and showed a highly reversible behavior without any corrosion effect. The NiO–TiO2 sol–gel layers sintered at 300 °C have been extensively characterized in the same electrolyte up to about 7000 cycles. All windows present a deep brown color characteristic of the presence of Ni3+ (NiOOH) species, that is fully reversible for several thousands of cycles with a rather-fast kinetics (<30 s). The transmittance of the bleached state however slowly decreases with cycling (permanent coloration). The full-bleached condition can be nevertheless recovered by applying a negative potential for a long duration. Deeper coloration is usually obtained by cycling the windows galvanostatically with a current density of 20 μA/cm2. The lifetime of the windows is however limited because of the degradation of the NiO-based layers due to the not fully reversible exchange of OH− that turns the layers mechanically fragile and leads eventually to their complete removal from the substrate. Windows working satisfactorily up to 7000 and 17 000 cycles have been obtained using aqueous KOH electrolyte and starch KOH gel electrolyte, respectively. Memory tests showed that the devices bleach at the open circuit potential from T=39% (colored state) to about T=50% in 60 min.