Electrochromic Properties of Nickel Oxide Electrodes
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ELECTROCHROMIC PROPERTIES OF NICKEL OXIDE ELECTRODES R. PILEGGI, B. SCROSATI AND S. PASSERINI Dipartimento di Chimica,UniversitA di Roma' La Sapienza' Rome, Italy ENIRICERCHE, Monterotondo, Italy. ABSTRACT Nickel oxide sputtered electrodes can undergo a reversible lithium intercalationdeintercalation process which is accompanied by a net electrochromic effect. The characteristics of this type of electrode and the mechanism of the related electrochromic process are discussed on the basis of results obtained by cyclic voltammetry and optical transmittance. The results show that nickel oxide is electrochromically activated by insertion of lithium in the oxide structure. The lithium intercalation process may be promoted by cyclic voltammetry or, more extensively, by polarizing cathodically the nickel oxide electrode in a non-aqueous electrolyte containing a lithium salt. Activated nickel oxide electrodes are of considerable interest for the realization of new types of efficient solid state electrochromic windows..
INTRODUCTION A great interest has been devoted to proton intercalation process in hydrated nickel oxide from alkaline solutions because of its importance in electrochemical systems such as batteries and electrochromic devices [1-4]. Two different mechanisms have been proposed to account for the electrochemical process: the first as double extraction of protons and electrons, the second as extraction of electrons and simultaneous injection of hydroxiles. More recently, J. Scarminio et al. [5] have found that electrochromic effect in sputtered thin film nickel oxide samples may be promoted by intercalation-deintercalation process in water-free oxide materials. If water is unnecessary for protons injection it is reasonable that other small cations may as well undergo an intercalation process in nickel oxide. In fact, it has been demonstrated in our laboratory [6] that under rigorously anhydrous conditions lithium ions are reversibly inserted and extracted into nickel oxide thin films, the process being accompanied by a net electrochromic effect. In this paper we report further study on the electrochemical, optical and mechanical measurements on lithiumintercalated nickel oxide films. EXPERIMENTAL• Thin film electrodes on glass support have been obtained by dc-reactive sputtering for nickel oxide samples and by thermal evaporation for tungsten oxide samples [7]. Glass supports of two different thicknesses coated with a layer of ITO to provide electronic conduction have been chosen. Coming cover glass plates of 150 iitm thick were used for stress measurements and boro-silicate glass windows of 1 mm thick as support for all other tests. Purified reagent-grade chemicals, lithium perchlorate and propylene carbonate, were stored in a
water-free atmosphere. All the procedures have been performed in a dry-box under a controlled
Argon atmosphere (water content lower than 10 ppm). In this way rigorously anhydrous conditions have been approached. Electrochemical systems using liquid electrolytes have been studied in thr
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