Experimental Results on Single Materials

In this section we present some examples of impedance spectra, in order to illustrate the concepts introduced above. We show how the methods in Section 4.3.4.3 may be used to characterize electrochromic materials and inorganic ion conductors. Data on titanium dioxide and tantalum oxide are emphasized, because they illustrate especially well several of the concepts treated above. Of conrse, we also review the application of impedance spectroscopy to the widely nsed electrochromic materials tungsten oxide and nickel oxide. 

Tungsten trioxide is a very widely smdied electrochronuc material and the impedance response has been measured several times. The results are consistent in that they can invariably be fitted to a Randles circuit, but details in the spectra vary. 

In particular, values of the ion diffusion coefficient and the other circuit parameters depend not only on the crystallinity, but also on the deposition conditions of the films. Since tungsten oxide easily becomes non-stoichiometric when deposited as a thin film, the stoichiometry probably also plays a role. Data have been presented for films produced by thermal evaporation (Ho et al. [1980], Bohnke and Bohnke [1990], Lee et al. [2000,2001]), electron-beam evaporation (Yoshiike et al. [1984]), sputtering (Pyun and Bae [1996]), sol-gel (Wang and Bell [1999], Sharma et al. [2002]), and various other techniques (Bohnke and Bohnke [1988]). Our data presented here are in general agreement with other studies of sputtered films, as well as with the recent detailed study of Lee et al. [2000, 2001]. 

There are a few questions, however, that deserve a further study. Sometimes the Warburg element is not of the finite-length type, but exhibits a power-law response instead. The reason for this behavior is not clear it has been attributed to the WOs/substrate interface (Bohnke et al. [1993]), or to an additional kinetic step in the bulk of the film (Fabregat-Santiago et al. [2001]). 

Counter Electrode Materials. The anodic electrochromic materials and ion storage materials have not been so widely studied as the cathodic electrochromic materials discussed above. In general, the main features of the impedance spectra are similar to those shown above. The impedance response of nickel oxide films with 

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