## 3 Results and Discussion
### Electrochromic and Electrochemical Behavior
The coloration state is attributed to the formation of the compound Li\({}_{x}\)TiO\({}_{2}\) according to the cathodic Equation (1) [63; 64]:  
\[TiO_{2}+xLi^{+}+x\,e^{-}\leftrightarrow Li_{x}TiO_{2} \tag{1}\]  
where \(x\) is the total of Li\({}^{+}\) intercalation and e\({}^{-}\) represents the charge-compensating electrons [65]. Cyclic voltammetry measurements were carried out for all the TiO\({}_{2}\) nanostructured films synthesized (Figure 6). Moreover, in Figure S2, it is presented all the voltammograms (1500 cycles) obtained during the CV experiments for the three types of TiO\({}_{2}\) films studied. The films' anodic scan oxidation leads to the bleaching state, and the cathodic scan reduction is associated to their coloring states. The nitric-based film demonstrated to be highly stable during cycles, maintaining the consistence during measurements without significant loss of its EC performance with the increase of the number cycles. In fact, the nitric-based material continued to work as an EC device after the cycles limit imposed (results not shown). In the cases of the hydrochloric and acetic-based films, it is clear that the EC performance has decreased with the increase of cycles, until the deficient EC device operation, with possible loss of optical modulation (Figure S2). Despite the increase in anodic and cathodic currents observed for the material synthetized with hydrochloric acid, its EC performance was substantially lower than the nitric one. Generally, the increase of anodic and cathodic currents indicates that the amount of ions and electrons inserted into the film also increases resulting in enhanced EC reaction activities [62], however, this effect has not been observed, as it can be seen by the instability observed on the CV voltammograms and lower durability of the device, with intercalation and deintercalation up to 850 cycles. The TiO\({}_{2}\) film synthetized with acetic acid demonstrated the poorest EC performance, reaching maximum of 800 cycles and lower currents (\(\rightarrow\)\(\pm\)1 V), in agreement to what was observed in the optical measurements (Figure 5c). The CV measurements attested the durability of TiO\({}_{2}\) nanostructured films, and as previously mentioned, the porous and thick structure of the nitric-based material guaranteed an improved EC performance, when compared with the other acids. Considering this, lifetime cycles measurements were carried out for this material (Figure 7). In order to stabilize the reflectance, the switching time was fixed at 240 s. The switching between both states was observed, with coloration of 71 s and bleaching times of 58 s, calculated as the time required to reach 10% to 90% of the initial and final reflectance values, respectively. Despite the long bleaching and coloring times, from the video presented in supplementary information, the visual color turning is faster than what was measured. Other studies also reported the switch behavior of paper EC devices. [PERSON] et al. [30] demonstrated EC devices with paper-based substrates and printed poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes showing slower switching time when produced with a lateral device architecture. [PERSON] et al. [32] reported PEDOT:PSS-based EC displays with an extra layer of dihexyl-substituted poly(3,4-propylenedioxythiophene) (PProDOT-Hx2) and using flexible paper substrates in roll-to-roll printing. It has been demonstrated slow switch to the OFF-state of the produced devices, which was attributed to the poor ionic conductivity of the PProDOT-Hx2 film in the aqueous electrolyte. Taking in consideration paper-based UV sensors from an analogous study [10], the contribution from the paper structure shown to be imperative to the final performance of the electronic devices, so following this line, it can be suggested that a similar effect can occur with the EC devices.