Electrochromism is the principal technology in Smart Windows that combine energy

efficient and comfort enhancing in buildings [1], and are part in green technology for

buildings [2]. The most extensively studied electrochromic material is Tungsten oxide

[1], together with the effects of aging in thin films. However, recently a new method

of restoring aged films close to initial conditions was discovered [3, 4, 5, 6] and labeled

’Rejuvenation’. This method is very new and thus little can be said with certainty.

It is not clear what mechanisms are behind the film restoration and to what extent.

Hence, a large amount of research needs to be focused on this method in order to

fully grasp the mechanisms behind the process, and this thesis tries to provide some

answers. The goal was to try to expose the effects the rejuvenation process had on

an aged electrochromic film, and provide further information on the method. This

study focused on electrochemical impedance spectroscopy measurements to study

the film state of an amorphous tungsten oxide thin film that underwent multiple

aging and rejuvenation processes, using Cyclic Voltammetry and Galvanostatic

treatment, respectively. The amorphous tungsten oxide thin film was deposited on

an ITO glass substrate by a dc-magnetron sputtering deposition system and used in

a three-electrode cell system consisting of the film substrate as working electrode

together with two Li-foils used as counter and reference electrode. This was in turn

immersed in a liquid electrolyte composed of LiClO4 and Propylene Carbonate (PC).

Two anomalous diffusion (AD) models labeled "AD1a" and "AD1b" were employed to

analyze the impedance data, and the circuit model containing the AD1b component

demonstrated the best fit to experimental data and was thusly used to describe the

diffusion kinetic development. The results obtained showed that at high frequencies

an unknown electrochemical process showed up after a first rejuvenation process, and

was present throughout the rest of the impedance measurements. This may be related

to an adsorbed intermediate and was modeled by modifying the Randles circuit such

that it included that process. Both models could to some degree describe the film

state when the film was not aged, but neither would with confidence when the film

state was aged, and is most probable due to incorrect models. The high-frequency

resistances rose with each rejuvenation process, increasing in our case by 20-30 %

the first time and around 85 % the second time, and may be an indication that the

electrolyte was being affected by the rejuvenation process. Also, the charge-transfer

and the additional high-frequency resistances experienced increment. The effective

diffusion coefficient was unreliable at potentials above 3.0 V when the film was not

aged, and highly unreliable for all potentials above 2.0 V when the film was aged,

and may be due to truncation of the frequency range of the impedance results. This

study exposed an unidentified electrochemical process occurring at high frequencies

as a result of the rejuvenation process, and the models used where not the most

adequate ones.