Solid electrolytes based on i-carrageenan and different ionic liquids for sustainable electrochromic devices

The increase in electronic waste (e-waste) is a significant global concern due to fast technological development in which products rapidly become obsolete. To mitigate this problem in the field of electrochromic devices, four different types of solid polymer electrolytes (SPEs) were developed based on iota-carrageenan, a water-soluble biopolymer, and 40 wt% concentration of different ionic liquids (ILs): 1-butyl-3-methyl-imidazolium thiocyanate ([BMIM][SCN]), 1-ethyl-3-methyl-imidazolium thiocyanate ([EMIM][SCN]), 1-butyl-3-methyl-imidazolium dicyanamide ([BMIM][N(CN)₂)]), and 1-ethyl-3-methyl-imidazolium dicyanamide ([EMIM][N(CN)₂)]). The resulting composites present a uniform and compact morphology, with a good distribution of the ILs within the polymer matrix, thermal stability up to ∼100 °C, and suitable mechanical properties. Their ionic conductivity at room temperature is in the range of ∼10⁻⁴ S.cm⁻¹ in the solid state and around ∼10⁻³ S.cm⁻¹ in the liquid state. Each of the developed electrolytes was integrated on a printed electrochromic device (ECD) fabricated with poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) as a working electrode and their performance was evaluated using spectroelectrochemical techniques. All four ECDs operate at voltages between -1 V and 1 V, providing coloration efficiencies between 253 and 571 cm².C⁻¹ for the oxidation process and between −435 and − 847 cm².C⁻¹ for the reduction process at 98% full contrast, and presenting switching times between the bleached and colored states around 3.2–4.8 s at 98% full contrast. These low-cost SPEs provide a suitable approach for the development of high-performance sustainable ECDs.