Advanced processing and characterization of Nafion electrolyte films for solid-state electrochromic devices fabricated at room temperature on single substrate

The design and development of effective solid-state electrolytes represent an emerging challenge towards the fabrication of electrochromic (EC) and multifunctional devices, such as photoelectrochromics, with benefits in terms of reduced cost, environmental impact as well as safety. Here, we present the processing and the characterization of a suitable Nafion film with the certain thickness of 8 μm used as electrolyte layer in a novel solid-state EC device fabricated at room temperature (RT) on a single substrate (both glass and plastics) with an architecture based on a substrate/ITO/WO3/Nafion-film/ITO configuration. In particular, we focused on the morphological characteristics, proton conductivity and water molecular dynamics of Nafion film in order to provide insight into the EC behavior of these novel devices. EIS analysis performed over a wide range of RH and temperature, showed good proton conductivity values (e.g. 4.42 × 10− 3 S cm− 1 at 30 °C and 50% RH), suitable for practical EC operation. At the same time, high values of water self-diffusion coefficients (D) and the spin-lattice relaxation times (T1) were measured by NMR spectroscopy, proving a similar behavior as well as the same proton conduction mechanism with that observed for thicker Nafion membrane (50 μm thick) prepared by solution casting. These findings were confirmed by the water uptake measurements since both the film and the membrane showed a water uptake value of about 24 wt%. Furthermore, a homogenous, uniform and very smooth surface (Ra of 0.94 nm) with small grain size (ca. 50 nm) was observed by SEM and AFM analysis. Noteworthy, the Nafion film ensured high optical properties, interfacial robustness and electrochemical stability to EC device: cyclability (300 CV cycles), long-term durability of at least 1000 chronoamperometric cycles.