Charge coloration dynamics of electrochromic amorphous tungsten oxide studied by simultaneous electrochemical and color impedance measurements

Abstract

The coloration mechanisms in electrochromic systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques-that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy. This was done at different bias potentials and their associated intercalation levels. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components from the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron-ion couple in the film. A quantity denoted complex optical capacitance was compared to the complex electrical capacitance-particularly, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. The complex spectra of perfectly synchronized electrical and optical responses are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented here, can help to elucidate the dynamics of the coloration mechanisms in electrochromic materials at various conditions-for example, at different intercalation levels and optical wavelengths.

Impedance Spectroscopy of Electrochromic Hydrous Tungsten Oxide Films

“…The impedance response is modeled by a circuit containing various circuit elements, which subsequently should be interpreted in terms of the electrochemical and electrical transport processes occurring in the sample under study. A variety of physical processes should be taken into consideration when formulating an equivalent-circuit model for WO 3 -based films [11,23]: double-layer capacitance, charge transfer of ions from electrolyte to film, an intermediate adsorption step in which an ion combines with an electron from the conduction band of the film [34], ion diffusion in the film, as well as possible effects occurring at the back contact to the film. In some cases, constant-phase elements (CPEs) have to be used when fitting in order to describe the capacitive effects.…”

“…where τ is a generalized capacitance related to the amplitude of the CPE, ω is angular frequency, and n is a power-law exponent. It should be noted that an equivalent-circuit model is never unique, but in the present work we build on extensive previous experience of EIS studies of WO 3 thin films [11,22,23]. We first consider the potential region below 3.5 V vs. Li/Li + and employ a generalized Randles model, as in our previous works [22,23].…”

“…It should be noted that an equivalent-circuit model is never unique, but in the present work we build on extensive previous experience of EIS studies of WO 3 thin films [11,22,23]. We first consider the potential region below 3.5 V vs. Li/Li + and employ a generalized Randles model, as in our previous works [22,23]. Figure 1a shows an equivalent circuit; it includes interfacial CPE and resistance elements as well as an anomalous diffusion element, which is used instead of the ordinary diffusion impedance.…”

“…The Z d element describes diffusion in the film as well as back interface (EC film/ITO) effects, since diffusion is assumed to be blocked at the back interface. It is common to use a distributed element representation (generally a transmission line) for Z d , as shown in the figure where q and c denote the impedance per unit length of the constant-phase element and the capacitance per unit length, respectively [21][22][23].…”

“…Extensions to the model have involved effects of ion trapping [19,20] and anomalous diffusion phenomena [21]. In particular, anomalous diffusion models provide a versatile and accurate description of the frequency-dependent impedance of EC materials such as amorphous WO 3 [22,23] and polycrystalline IrO 2 [24]. As already remarked above, having information about diffusion kinetics is vital to controlling the kinetics of EC devices.…”