## 3 Results and Discussion
### Long-Term Stability of Switching Characteristics
The direct fast application of \(-1.0\) V leads to higher current densities and chances of local overcharging at the beginning of the coloration process as opposed to a slow, well-defined change of the potential during CV. Overcharging or undesired side reactions can result in lower \(CE_{710\mathrm{nm}}\). Comparing the different kinds of films, however, \(CE_{710\mathrm{nm}}\) follows the same trend as observed before during CV. From CV, \(CE_{710\mathrm{nm}}\) even higher for WO\({}_{31}\)up400, WO\({}_{31}\)up600 and WO\({}_{31}\)upm compared to commWO\({}_{3}\) were reached as opposed to similar \(CE_{710\mathrm{nm}}\) for WO\({}_{31}\)comp, WO\({}_{31}\)mp and commWO\({}_{3}\). Reversibility of the switching process deserves further attention. Films of WO\({}_{31}\)comp showed a monotonous upward shift of \(T_{\lambda}\) when the films were switched back and forth between \(-1.0\) V and \(1.0\) V for \(300\) s each over three cycles (Figure 9a). This trend continued when the films were studied over \(50\) more cycles (Figure 10a). The compact film obviously contained a considerable concentration of trapped ions [57]. It has been found earlier [63; 64] that the concentration of intercalated ions \(x\) in Li\({}_{x}\)WO\({}_{3}\) in the films, clearly influenced the equilibria of W\({}^{6+}\rightleftharpoons\) W\({}^{5+}\) and W\({}^{5+}\rightleftharpoons\) W\({}^{4+}\) leading to the observed changes in the optical absorption of WO\({}_{3}\). It was observed that up to \(x\) = 0.3-0.5, the transmittance first decreased and then shifted back to higher transmittance for larger \(x\) whereby after exceeding \(x\approx 0.7\) the films became optically irreversible showing a light-brownish color. The range of \(x\) < 0.3-0.7 in the present experiments was chosen to provide a good compromise between intense coloration and high reversibility. However, in Figure S14a it can be seen that after the first intercalation of WO\({}_{31}\)comp for \(300\) s at \(-1.0\) V (1. Cycle) ions were trapped within the films leading to a bluish film even after reversal to \(1.0\) V (Figure S14c). The color impressions of the film after \(53\) cycles indicate that the concentration of trapped ions must have increased subsequently until the films became light-brownish with \(T_{\lambda}\) for both \(-1.0\) V and \(1.0\) V shifted to higher values (Figure S14a,c). For WO\({}_{31}\)mp, despite mesoporous morphology, the remaining polymer in the film also led to trapped ions and, thus, a similar trend towards increasing \(T_{\lambda}\) (Figure S13a) as obtained for WO\({}_{31}\)comp. Hence, long coloration times are disadvantageous for such films and lead to the observed irreversibilities. For the WO\({}_{31}\)up400, WO\({}_{31}\)up600 and WO\({}_{31}\)upm films, however, \(T_{\lambda}\) in the bleached as well as in the colored states remained widely constant at \(1.0\) V and \(-1.0\) V, respectively (Figures 10b, S13b,c and S14b,c). These films point out the possibility to endure multiple reversible switching at a high stability of the films. The porous network in these films without significant amounts of residual polymer fragments enables a facile intercalation and deintercalation of the Li\({}^{+}\) ions allowing the observed reversible switching of these films. The reversibility dependent on the applied negative potential is compared for WO\({}_{31}\)comp and WO\({}_{31}\)up600 in Figure S15. It could be observed that independent of the negative potential, WO\({}_{31}\)up600 could be fully bleached again at \(1.0\) V reaching a transmittance equal to the one in the initial state. For WO\({}_{31}\)comp, however, the transmittance was lower than in the initial state already after prior polarization at \(-0.25\) V only, indicating trapped ions already for weakly reduced WO\({}_{31}\)comp. For both films it can be observed that the spectral shape also changed with increasing negative potential.