In this work the optical and electrochemical properties of sputtered nickel oxide thin films have been investigated. Nickel oxide is a widely used material for electrochromic applications, in for example smart windows. Electrochromic materials change their optical properties when ions together with charge compensating electrons are inserted or extracted. Nickel oxide colours upon charge extraction and bleaches upon charge insertion.
The ions and charge compensating electrons were intercalated by cyclic voltammetry. In cyclic voltammetry the film is immersed in an electrolyte and the film potential is cycled which gives rise to intercalation/deintercalation. Different kinds of ions can be intercalated into nickel oxide and in this work the films have been investigated by using different electrolytes for the cyclic voltammetry. The electrolytes under investigation were potassium hydroxide (KOH), propionic acid and lithium perchlorate in propylene carbonate (Li-PC). The change of the optical properties in the visible range was observed while cycling between the dark and the bleached state. The optical modulation depends on charge capacity, so the coloration efficiency was also calculated and compared for the different electrolytes to get a charge independent parameter. By changing the intercalation rate and using the so called Randles-Sevèik equation, the diffusion coefficient was investigated.
It was observed that the difference in optical modulation and charge capacity was small between immersing the film in propionic acid and in Li-PC. If KOH was used however, the charge capacity was almost ten times as large compared to the other electrolytes, and the optical modulation was about three times as large. The coloration efficiency though was lowest when using KOH. The difference in diffusion coefficients was small and of the size 10-13 cm2/s. The conclusion is that nickel oxide in KOH is superior when it comes to optical modulation and charge capacity. In order to be used as an electrochromic material with Li ions the charge capacity needs to be improved.
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