## 3 Results and Discussion
### Charge Transport within Microporous vs. Compact Tungsten Oxide  
The direct influence of the microporous network on electrode kinetics of the electrochromic reaction was independently analyzed by impedance spectroscopy in different redox states of WO\({}_{31}\)comp and WO\({}_{31}\)up600 films. The resulting Nyquist plots (Figure 12) can be fitted well with the equivalent circuit shown in Figure 12a. This circuit is typically used for the description of intercalation processes in electrochromic films [68; 69]. \(R_{1}\) corresponds to a series resistance caused by the electrolyte and the substrate [69; 70; 71; 72]. \(R_{\rm ct}\) is assigned to the resistance of the charge transfer from the WO\({}_{3}\) surface to the electrolyte and describes the intercalation of the ions into the WO\({}_{3}\) film as well as the interfacial redox processes [68; 69; 71; 72; 73]. The electrochemical double layer consisting of the electrons at the electrode surface and ions on the electrolyte side can be modelled by a constant phase element or a capacitance [68; 69; 71; 72; 73]. A more accurate fit of the present data was obtained by using a constant phase element \(CPE_{\rm dl}\). A semi-infinite type Warburg impedance \(Z_{\rm w}\) represents the diffusion of Li\({}^{+}\) ions in the WO\({}_{3}\) films [68; 69; 71; 73] but had to be replaced by a finite-length-type Warburg element \(Z_{\rm w,s}\)[72; 73] for the impedance spectra measured at a potential of \(-1.0\) V, which led to more reliable fits of the data. \(C_{\rm L}\) refers to a limiting capacitance caused by the finite length of the WO\({}_{3}\) film and considers a diffusion limited by charge accumulation in the film at low frequencies [68; 69; 74]. As expected, the series resistance \(R_{1}\) in the high frequency range of the Nyquist plot (Figure 12) was found quite constant around 30-35 \(\Omega\) for compact or microporous WO\({}_{3}\) and at different applied potentials. Similarly, \(Q_{\rm CPE}\) and \(\alpha\) representing the double layer capacitance and \(C_{\rm L}\) representing the limiting capacitance of WO\({}_{3}\) showed no significant changes either with potential or among the films speaking in favor of a rather constant arrangement of ions and electrons at the electrode surface and of a rather constant maximum charge uptake. The resistance \(R_{\rm ct}\), however, for WO\({}_{31}\)comp decreased from about 51 \(\Omega\) at \(-0.25\) V to 9 \(\Omega\) at \(-1.0\) V, a trend directly seen by a decreasing radius of the semicircle in the moderate frequency range (Figure 12a). At negative potentials, a driving force is established to insert Li\({}^{+}\) into the films and the barrier to enter is decreased. The higher value of \(R_{\rm ct}\) at \(-0.25\) V compared to the value at \(-1.0\) V points out that a higher barrier has still to be overcome to insert Li\({}^{+}\) at \(-0.25\) V, possibly caused by slow transport of ions and/or  
Figure 11: Depth profiles obtained from ToF-SIMS analysis of the WO\({}_{3}\)– (\(\star\)\(\star\)), Li\({}^{-}\) (-o-) and SnO\({}^{-}\) (-\(\square\)-) signals for thin films of WO\({}_{31}\)comp (green), WO\({}_{31}\)up400 (blue) and WO\({}_{31}\)up600 (red) films intercalated at \(-1.0\) V for 300 s. Dashed lines mark the approximate position of the interface between WO\({}_{3}\) and FTO estimated from the WO\({}_{3}\)– and SnO\({}^{-}\) signals. electrons within the WO\({}_{3\,|}\)comp structure. For WO\({}_{3\,|}\)up600, \(R_{\rm ct}\) of about 248 \(\Omega\) at \(-\)0.25 V was found considerably higher than for WO\({}_{3\,|}\)comp. Since ion transport should not be hindered in a microporous compared to a compact material, the reason should be sought in an attenuated electron transport. The porous morphology and a thicker film can both lead to a larger number of grain boundaries posing additional barriers for electron transport.