Electrochromic devices – are devices that can change their optical properties under applied current. They find application in various fields: glazing of energy-efficient building, marketing, automobile industry etc..
Nickel hydroxide electrochromic film (NHEF) are perspective for application in electrochromic devices. High coloration degree, potentially long cycle life, low cost and large number of deposition methods are main advantage of NHEF.
Glass coated with a transparent conductive layer is used as a substrate for NHEF and other electrochromic films. Currently, fluorine-doped SnO2 (FTO) coating the most perspective. It is cheaper than its competitor – indium-tin oxide (ITO) with comparable conductivity and transparency.
Uniform deposition of NHEF from aqueous onto FTO-coated glass is difficult, owing to low wettability and almost flat surface on microlevel of FTO layer.
In order to alter surface properties of FTO layer, we have proposed a soft etching using alternating current [1, 2]. The electrochemical etching has been carried according to following regime: –1mA/cm2 10 s, +1mA/cm2 100 s, –1mA/cm2 40 s, +1mA/cm2 100 с в 1М HCl. The initial resistivity of FTO-layer was 7.5 Ω∙cm. Nickel foil was used as counter electrode.
Figure 1 – Electrochemically etched FTO-coated glass (left – dry, right –wetted with water)
As a result of etching, the properties of FTO layer have been changed drastically: reflection hue of etched region has changed (Fig. 1), transparency has increased and wettability increased significantly . Qualitative and quantitative changes of FTO layer are presented in Table .1
|Reflection hue||Contact angle, °||Resistivity, Ω∙cm|
*-after washing and drying
After that, a film of nickel hydroxide has been deposited on two pieces of FTO-coated glass (etched and non-etched), using cathodic template method. Deposition parameters:-0.1 mA/cm2 10 min, electrolyte 0.01 М Ni(NO3)2 and 5% of polyvinyl alcohol.
The samples prepared were tested using cyclic voltamperometry in 0.1 КОН in potential range from +200 mV to + 750 mV. During cycling, coloration-bleaching curves have been recorded.
Figure 2 – Coloration-bleaching curves of NHEF deposited on etched (red) and non-etched (black) FTO-coated glass.
Films deposited on etched FTO-coated glass were more uniform in both bleached and colored state, and had better coloration degree (about 4 % higher). Thus soft electrochemical etching can be used to improved properties of electrochromic film and for other application, where pretreatment of FTO-coated glass is required.
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