Electrochromic Behavior of Ni1-xO Thin Films in Various Electrolytes

NiO is a well known anodic electrochromic (EC) material showing optical modulation under electrochemical oxidation and reduction [1]. Despite the simplicity of this oxide, electrochromism in NiO thin films is rather complicated although it is generally accepted that the optical modulation is related to a charge-transfer process between Ni(II) and Ni(III). The NiO is commonly associated to WO₃ in inorganic EC devices, a combination leading to a neutral color due to the simultaneous brownish and blue color of NiO in oxidized state and WO₃ reduced state, respectively. Nevertheless, their association is complicated owing to their different stabilities in acidic/basic media, a factor which also impinges on the device stability. In our group, an unusual reactivity for non-stoichiometric Ni_1-xO thin films cycled in various electrolytes was recently reported opening the choice of materials for complete devices working in both lithium and lithium free electrolyte [2].

      Ni_1-xO thin films were grown at room temperature by sputtering deposition on ITO-glass substrate. The influence of various partial pressures of O₂ (a 4 Pa total pressure was maintained during deposition) on the stoichiometry, morphology and cristallinity of Ni_1-xO thin films was studied and correlated to their EC behavior. Unsurprisingly, an increase in oxygen partial pressure is related to a stronger brownish appearance associated with to an increase in the Ni³⁺ amount as well as a decrease in film porosity and roughness. Moreover, the comparison of the electrochemical properties of Ni_1-xO films cycled in aqueous (KOH), lithium based and lithium free ionic liquid electrolytes, shows an evolution from a faradic to a pseudo-capacitive behavior, while optical contrast from colorless to brownish remains in all cases. This suggests that electrochromism is not only the result of of small cations (de)insertion but also depends on surface phenomena including possible participation of anions.

      Herein, the relationship between the physical-chemical properties and EC performances of non-stoichiometry Ni_1-xO thin films will be investigated. Finally, the combination of Ni_1-xO with WO₃ and PEDOT in full devices will be discussed.

[1] C.G. Granqvist, Handbook of inorganic electrochromic materials, Elsevier, Amsterdam, Netherlands, 1995

[2]  Moulki, H., Faure, C., Mihelčič, M., Vuk, A.S., Švegl, F., Orel, B., Campet, G., Rougier, A. Electrochromic performances of non-stoichiometric NiO thin films, Thin Solid Films, 2014, 553, 63-66.