## 3 Applications of IL-Based Gels
### Electrochromic Device  
Electochromic devices (ECDs) are finding widespread architectural applications in energy-saving smart windows, automotive mirrors, privacy glass, displays, electronic paper, military camouflage and so on [135; 136; 137]. ECDs undergo reversible color change due to electrochemical oxidation reduction in response to electric field. Typically, an ECD consists of a soft substrate, conductive electrode, electrochromic material layer, ion storage layer, and electrolyte. The electrolyte plays a crucial role in the coloration/bleaching efficiency of the ECDs. In modern ECDs applications, compared to liquid electrolytes, ionic liquid based gel electrolytes or ionogels have attracted immense attention owing to their outstanding ionic conductivity and improved mechanical and electrochemical stability [135]. In this section we introduce some of the latest developments in ionic liquid based gel electrolytes for ECDs application.  
In 2021, [PERSON] et al. used a sol-gel derived ionogel comprised of trialkoxysily-functionalized 1,14-bis(3-(3-(3-(methoxysilyl)propyl)imidazolium 1-il)-3,6,9 trioxa undecan diiodide ((EO)\({}_{3}\)[TMSPIm+1]\({}_{2}\)) and 1-methyl-3-propyl imidazolium iodide (MPIm+1-) ionic liquids in acetic acid [138]. These redox electrolytes having various iodine contents with ionic conductivity in the range of about 1 mS cm\({}^{-1}\) were tested in hybrid ECD for 11,000 operational cycles. The ECDs exhibited similar optical modulation and was approximately 30-35% at 634 nm. Previously in 2017, [PERSON] and coworkers prepared a gel electrolyte by dissolving PMMA, 1 M LiClO\({}_{4}\) in propylene carbonate and EMIMBF\({}_{4}\)[139]. The ECD assembled as glass/fluorine doped tin oxide (FTO)/WO\({}_{3}\)/PMMA-EMIMBF\({}_{4}\) composite electrolyte/FTO/glass showed an enhanced electrochromic performance with a  
Figure 8: (**a**) Synthetic route of the polymeric skeleton, (**b**) the illustration of PG-DHS and PG-MHS IL-based gels, and (**c**) polarization curves of the H\({}_{2}\)/O\({}_{2}\) fuel cell prepared from PG-MHS at temperatures from 90 to 120 \({}^{\circ}\)C. Reprinted with permission from ref. [134].  
reduced anodic reaction potential. The as designed ECD was therefore found to be bleached easily at a smaller external voltage. Meanwhile, [PERSON] et al. designed flexible ECD displays using P(VDF-HFP) with butyl-methylammonium bis(trifluoromenthanesulfonyl)imide (N\({}_{1114}\)TFSI) (79% wt.) which showed high transparency with 83% transmittance and ionic conductivity of 1.06 mS cm\({}^{-1}\)[140]. In 2018, [PERSON] et al. fabricated an ECD by sandwiching a ionic gel containing P(VDF-HFP) and BMPTFSI or BMIMBF\({}_{4}\) between ITO-coated glasses and the devices were tested with two electrochromic materials, diheptyl viologen bis(hexafluorophosphate) [DHV(PF\({}_{6}\))\({}_{2}\)] and monoheptyl viologen hexafluorophosphate [MHV(PF\({}_{6}\))] [141]. The MHV\({}^{+}\)-based devices exhibited high color purity, large transmittance, and good coloration efficiency compared to that of DHV\({}^{2+}\) -based ECDs. It was expected that MHV+ can act as a solid state ECD electrolyte while providing magenta (reddish) pixels for display applications. Recently in 2021, [PERSON] et al. reported construction of an ECD utilizing a metal-organic frameworks (MOFs) and 1-hexyl-[4,4\({}^{\prime}\) -bipyridin]-1-ium bis(trifluoromethane sulfonyl)imide [MHV][TFSI]-based hybrid ionogel electrolyte [142]. The prepared ECD showed a remarkable stability upto 2000 cycles with high coloration efficiency of 99.14 cm\({}^{2}\)/C and fast coloration time of 4.35 s and bleaching time of 7.72 s.