Electrochromic properties of nickel oxide in different electrolytes

Abstract: A half cell of an electrochromic (EC) device has been used to determine theelectrochromic response of a nickel oxide film in nine different electrolytes. Six of thenine electrolytes were 0.1 M non-aqueous salts dissolved in equal weight % ofpropylene carbonate and ethylene carbonate. Three of them were lithium-based andthree of them tetrabutylammonium (TBA)-based. The last three electrolytes wereproton-based aqueous solutions of 1 M KOH, 0.1 M propionic acid and 0.1 Mphosphoric acid, respectively. The electrolytes were subjected to electrochemical measurements of cyclicvoltammetry and square wave voltammetry, both with simultaneous in-situ opticaltransmittance measurements in the visible region. Ex-situ optical measurements wereperformed in the UV-VIS-NIR (300-2500 nm) range and IR-spectroscopymeasurements in the 600 – 4000 cm-1range.To determine the performance of the nickel oxide films, the coloration efficiency (CE)is used as a figure of merit. The desired value is to achieve a high optical modulationwith as little amount of charge inserted/extracted as possible.The results show that neither lithium nor TBA has a significant impact on theelectrochromic (EC) response, compared with the protonic electrolytes. Anargument can be made that the intercalation of neither cation (lithium or TBA) is thereason behind the electrochromic behaviour of the nickel oxide. In KOH it is ratherthe OH- that transfer to the surface and attracts protons (H+) from the bulk nickeloxide that enhances the EC response. In both propionic and phosphoric acid, it is thereversible intercalation of protons (H+) into the porous nickel oxide that gives theelectrochromic response.