## 3 Results and Discussion
### Redox Characteristics and Intercalation of Li\({}^{+}\)  
In order to analyze the electrochromic reduction reaction and the diffusion of Li\({}^{+}\) ions in the films, cyclic voltammetry (CV) was performed at different scan rates as shown for a WO\({}_{31}\,\text{\upmu}\)mp film in Figure 5 and for all other types of films in Figure S10. Following a CV curve with a just slightly different shape during a first conditioning cycle (Figure 6a), reproducible CV curves were obtained at each scan rate representing a reversible intercalation and deintercalation of Li\({}^{+}\) upon reduction and reoxidation as expected for amorphous WO\({}_{3}\) films [7]. The reversibility \(K\) of the bleaching and coloration process can be determined from the ratio \(q_{\text{out}}/q_{\text{in}}\) of the extracted and inserted charge densities [56]. During the first conditioning cycle (Table 6) and especially for the WO\({}_{31}\,\text{comp}\) and WO\({}_{31}\,\text{mp}\) films, the reversibility is rather low indicating that not all inserted ions can be deintercalated but are trapped within the films. Slow or irreversible movement in compact WO\({}_{3}\) can be expected. For the porous WO\({}_{31}\,\text{mp}\) films, however, this originally came unexpected and points at  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Film Type** & \(d\)**/nm** & \(d\)**/nm** & \(\rho\)**/g cm\({}^{-3}\)** & \(P_{\text{chem}}\)/\(\psi\)** & \(P_{\text{E}}\)/\(\psi\)** \\ \hline WO\({}_{31}\,\text{comp}\) & 98 & 95 & 7.15 & - & - \\ WO\({}_{31}\,\text{up}400\) & 138 & 163 & 6.31 & 11.8 & 12.3 \\ WO\({}_{partial pore filling by residual polymer or polymer fragments, as also concluded from a brownish color after annealing during film preparation and relevant mass losses at higher temperature during TG analysis. For the WO\({}_{31}\)up400, WO\({}_{31}\)up600 and WO\({}_{31}\)up films, the reversibility is close to 100% confirming that only a few ions are trapped within the films [39; 57]. Hence, diffusion in the microporous network and in the absence of residual polymer fragments assures complete extraction of Li\({}^{+}\) even during the first bleaching process. However, from the second cycle on, all the films showed a high reversibility which means that less ions were trapped inside the films and that the sites where ions might be trapped were already occupied during the first coloration. In this respect, the films are similar to the commercially sputtered commWO\({}_{3}\) film. Figure 5: Cyclic voltammograms at the second cycle at different scan rates of a WO\({}_{31}\)up film in LiClO\({}_{4}\) in PC. Figure 6: CV of the first cycle at a scan rate of 5 mV s\({}^{-1}\) (**a**) and transmittance at 710 nm (**b**) in dependence of the applied potential during the CV of thin films of WO\({}_{31}\)comp (black), WO\({}_{31}\)up400 (yellow), WO\({}_{31}\)up600 (green), WO\({}_{31}\)up (blue) and WO\({}_{31}\)up (red) as well as commWO\({}_{3}\) (grey) in LiClO\({}_{4}\) in PC. The initial states of each sample are marked by vertical lines and the arrows indicate the direction of cycling. The anodic peak current densities \(j_{\rm p,a}\) and the cathodic current densities at negative scan reversal \(j_{\rm r,c}\) (Figures 5 and S10) were plotted against the square root of the scan rate \(v^{1/2}\) following the Randles Sevcik equation at room temperature  
\[j=2.69\times 10^{5}\cdot n^{3/2}\cdot D^{1/2}\cdot c\cdot v^{1/2} \tag{1}\]  
where \(n\) is the number of electrons transferred in the reaction (1 in the present case [14]) and \(c\) is the concentration of Li\({}^{+}\) in the bulk of the electrolyte [14; 56] to determine two approximations of the effective diffusion coefficient of Li\({}^{+}\) and electrons in the films, \(D=D_{\rm a,CV}\) in the anodic branch and \(D=D_{\rm c,CV}\) in the cathodic branch. Diffusion of Li\({}^{+}\) in the electrolyte can be considered much faster than that in the films, and it has not to be considered separately.