POSS based ionic liquid as an electrolyte for hybrid electrochromic devices M. ˇ Colovic ´ a , I. Jerman a,b , M. Gaberˇ sc ˇek a,b , B. Orel a,b,n a National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia b CO-NOT, Hajdrihova 19, Ljubljana, Slovenia article info Article history: Received 1 June 2011 Accepted 8 August 2011 Available online 6 September 2011 Keywords: POSS ionic liquids Hybrid electrochromic device abstract The main objective of this study was to broaden the assortment of I  /I 3  redox ionic liquids using polyhedral oligomeric silsesquioxanes (POSS) acting as nanobuilding blocks for the construction of functionalized 1,3-alkylimidazolium iodide solid (melting temperature 150–200 1C) and room tem- perature (RT) ionic liquids. The structural characteristics of the synthesised final ionic liquids and the corresponding inter- mediates were determined using 1 H, 29 Si NMR and infrared spectroscopic measurements. Raman spectra were next reported, in order to demonstrate the presence of polyiodides formed after the addition of iodine and the formation of redox electrolytes. Ionic conductivity values obtained from the impedance (EIS) spectra were determined in the temperature interval from room temperature up to 100 1C. Finally, a hybrid electrochromic cell was constructed from room temperature MePrIm þ I x  IO 7 T 8 POSS (x ¼1, 1.2, 3 and 5) ionic liquids encapsulated between a lithiated WO 3 working and Pt counter- electrode, and colouring–bleaching changes assessed for cells cycled up to 1000 repetitive cycles. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids [1] have already found applications in various electrochemical systems as robust and non-volatile electrolytes. For specific applications, such as dye sensitised photoelectro- chemical (DSPEC) [2], hybrid electrochromic (hybrid EC) [3–5] and photoelectrochromic (PEC) cells [6–8], in which room tem- perature ionic liquids create sealing problems, gel [9] or, even better, solid or condensed ionic liquid-based electrolytes [10] that combine the non-volatility, temperature stability and high con- ductivity of room temperature ionic liquids with the mechanical integrity of solid electrolytes, are preferable [11]. Hybrid EC cells exhibit some special features, such as simple construction (ion-storage layer is not needed [4]) and self-erasing of their colour state [5], the latter being important for aircraft EC windows for safety reasons. The first relatively large hybrid EC window, constructed from 30  30 cm 2 large segments, has only recently been reported [12]. Semi-solid electrolyte based on hydro- phobic room temperature dialkylimidazolium iodide ionic liquid condensed with nano silica particles has been used. For ‘‘all-solid state’’ hybrid EC cells, by analogy with standard battery type EC cells [3,13] it would be best to have a redox electrolyte with thermo- plastic properties [14], for example those employed in some commercial EC systems such as helmet visors [15]. Accordingly, the main objective of this study was to broaden the assortment of I  /I 3  redox solid (melting temperature 150–200 1C) and room temperature (RT) ionic liquids suitable as electrolytes for the hybrid EC cells. We focused here on dialkylimidazolium iodide ionic liquid, which served as a basic compound for functionalization with polyhedral oligomeric silsesquioxane (POSS). POSS have already been used by Maitra and Wunder [16–19] for the preparation of Li þ conductors based on polyethylene oxide (PEO)—functionalized with eight POSS units with incorporated lithium salt. Similarly, Tanaka et al. [20], recently reported the synthesis of a room temperature ionic liquid consisting of an octacarboxy POSS anion [POSS-(COO  ) 8 ] and imidazolium cation [i.e., 1-butyl-3-methyl imidazolium], but an imidazolium based iodide ionic liquid with imidazolium cation functionalized on just a single corner of POSS, as shown in Fig. 1, has not yet been reported. POSS are specific compounds characterised by a cage-like structure (1  3 nm in size) resembling in this respect organically functionalized nanosized particles of SiO 2 [21,22]. POSS are the most ordered product of hydrolytic condensation of alkyltrialk- oxysilanes and consist of a silica core (SiO 3/2 ) and organic corona (R). The variety of organic groups (R 0 ,R 00 ,R 000 ), which are located at the corners of the silsesquioxane polyhedra, gives an enormous number of heteroleptic POSS (R 0 x R 00 y R 000 z SiO 1.5 ) 8 ,(x þ y þ z ¼ 8) with multifunctional properties. Specifically, in this study we decided to prepare monofunctio- nalized (Im þ I  R 7 (SiO 1.5 ) 8 ) instead of octameric imidazolium iodide POSS ionic liquids (Im þ I  ) 8 (SiO 1.5 ) 8 , by analogy with the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.08.009 n Corresponding author at: National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia. Tel.: þ386 1 4760 276; fax: þ386 1 4259 244. E-mail address: boris.orel@ki.si (B. Orel). Solar Energy Materials & Solar Cells 95 (2011) 3472–3481