Electrogravimetric and stress measurements of ion intercalation in NiO x thin films

Abstract

The electrochromic properties of transition metal oxides are a consequence of an ion/electron intercalation process. In nickel oxide electrodes, the electrochemical/electrochromic reaction has been pointed out to be a proton/electron intercalation process. However, recent works showed that the mechanism of the reaction could be much more complex. In this work, we studied the electrochemical reactions of r.f. sputtered thin nickel oxide films in aqueous electrolytes containing different cations.An electrochemical quartz crystal microbalance has been used in order to follow the mass changes during the experiments. Also, the mechanical stress changes has been measured in situ by an optical technique. The results showed that the incorporation of the cations in the film has to be considered; possible reaction mechanisms are discussed in this work.

Mass changes accompanying the electrochemical reaction of sol-gel formed 50:50 NiCo oxide films

“…As was stated in the introduction, various reaction schemes have been proposed to describe the redox behavior of Ni, Co and Ni-Co oxide films, including the exchange of protons, hydroxyl ions, alkali metal cations and water [20,28,30]. Stress measurements [31,32] have indicated a decrease in the film volume during Ni oxide oxidation, which the authors related to the expulsion of a large number of protons simultaneously with the intercalation of a few heavy cations (Na + ), and vice versa during reduction. In the present work, the observed mass changes have been attributed to the injection of OH and hydrated Na + ions during the anodic cycle and their expulsion during oxide reduction, consistent with the proposed reaction scheme (reaction 6).…”

“…This is similar in magnitude to the results reported in the literature for Ni, Co and Ni-Co oxide films formed by various other techniques. Faria et al [31] reported a 5 g mol -1 e~ ratio, calculated during the reduction of rf sputtered Ni oxide films in 0.1 M NaOH, while Mo et al [35] reported a value of 7 g mol ~1 e~ for electrodeposited Ni oxide films. However, the proposed stoichiometry of the 50:50 Ni-Co oxide film redox reaction (reaction 6), obtained from the pH dependence of the reaction and involving 1.45 OH" and 0.45 Na + per each electron, predicts a mass change of 35 g mol -1 e".…”