Comparison of electrochromic properties of Ni1−xO in lithium and lithium-free aprotic electrolytes: From Ni1−xO pigment coatings to flexible electrochromic devices

Abstract Electrochromic nickel oxide (Ni 1− x O) pigment powder which was made from nickel acetate reacting with H 2 O 2 /urea solution followed by heat treatment of xerogels at 400 °C (24 h) was milled with zirconia beads in pure water or water with added nickel oxyhydroxy precipitate acting simultaneously as the dispersant and the coating binder. The corresponding pigment particle suspensions were spin coated on FTO glass and flexible ITO-PET foils, respectively, and cured at 150 °C. The optical properties of the deposited pigment coatings on FTO glass substrates were determined with an UV VIS spectrometer, providing total direct transmittance (TT), total diffuse transmittance (DT) and haze (in %). Haze increased with the coating thickness but did not exceed 4%. SEM micrographs of the deposited pigment coatings revealed that they consisted of agglomerated Ni 1− x O pigment with voids. Potential cycling of Ni 1− x O pigment coatings and Ni 1− x O thin films was performed in 1 M LiClO 4 /PC and 0.1 M TBA + triflate/PC electrolytes, revealing that the pigment coatings exhibited electrochemical and electrochromic activity also in lithium free (aprotic) electrolyte, while variations of the oxidation and reduction reaction peaks at various scan rates confirmed the presence of surface electrochemical reactions. The near-grazing incidence angle reflection–absorption spectroscopic technique (IR RA) was employed for identifying surface and inner-grain Ni–O stretching modes, the former being responsible for observed electrochemical properties, which was also demonstrated from the ex situ IR RA measurements performed for coatings charged/discharged in 1 M LiClO 4 /PC and 0.1 M TBA + triflate/PC electrolytes. The results disclosed in this study suggest that the electrochromic effect was not in direct correlation with the de-insertion/insertion of small ions into the NiO (bunsenite) grains but rather the pigment coatings behave similarly to nickel oxide electrochemical capacitors. The electrochromic response of a flexible EC devices made either of PEDOT (self-standing foil) or WO 3 and Ni 1− x O pigment coatings glued with PMMA based electrolytes without and with lithium salt were recorded for the demonstration of the possible practical application of wet deposited Ni 1− x O pigment coatings made as “electrochromic” paints.