Electrochromic properties and coloration mechanisms of sol-gel NiO-TiO2 layers and devices built with them

Abstract

Electrochromic films of NiO-TiO2 with Ni concentration of 100, 90, 87, 83, 75, 66, 50 and 33 mol % have been obtained via the sol-gel route by dip coating technique using ethanolic sols of nickel acetate tetrahydrate (Ni(CH3COO)2•4H2O) and titanium n—propoxide Ti(C3H7O)4 precursors and sintered in air between 250 and 500 °C. Xerogels obtained by drying the sols have been studied up to 900 °C by thermal analysis (DTA/TG) coupled to mass and IR spectroscopy. The crystalline structure and morphology of the layers in the as deposited, bleached and colored states was determined by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. Their electrochromic properties have been studied in 1 M KOH aqueous electrolyte as a function of the layer composition, thickness and sintering temperature. Deep brown color with reversible transmittance changes have been obtained using cycling voltammetry and chronoamperometry processes. The best composition to get stable sols, a high reversible transmittance change and fast switching times (10 s) was obtained with double NiO-TiO2 layers 160 nm thick having 75 % Ni molar concentration, and sintered between 300 and 350 °C. The electrochromism of the layer was also studied in LiClO4-PC electrolyte. The mechanism of coloration and morphology transformation of the layer during cycling in 1 M KOH electrolyte are discussed in terms of an activation and degradation period. The nature of the ions involved in the coloration process has been studied using an Electrochemical Quartz Crystal Microbalance (EQCM). It was found that the activation period was associated with an increase of the mass of the layer after each cycle due to a gradual incorporation of OH- groups and the transformation of Ni(OH)2 into hydrated NiOOH giving the brown coloration as well as the formation of lattice water (OH- + H+→ H2O). The gradual change of the layer composition led to a much more open and fragile morphology that eventually allowed the incorporation of K+ ions and more water molecules associated with unusual large increase of the mass of the layer after each cycle and responsible for the degradation period. Finally, devices have been mounted and tested using either NiO-TiO2 layers or NiO-TiO2 layers covered with a thin anticorrosion dielectric layers as a working electrode together with CeO2-TiO2 layer or with a WO3 or a Nb2O5 cathodic layer acting as an active counter electrode. Also a new type of electrolyte based on KOH mixed with starch has been also developed and tested with complete windows.