In some cases the very viscous ionic liquids but in most cases the composite organic/inorganic materials are referred as quasi-solid electrolytes. Nanocomposite organic/inorganic materials are constituted of two interpenetrating subphases which are mixed in nanoscale. The organic subphase is usually consisted of few surfactants or polyether chains and the inorganic subphase is made of an inorganic network which typically is silicon dioxide or titanium dioxide. Such nanocomposite gels can accommodate appropriate solvents within the organic subphase (that is within the pores left by the inorganic network) so that ionic conductivity can be raised to a satisfactory degree. The design and synthesis of such materials makes for fascinating research with numerous scientific and technological implications in iono-electronics, mechanics and optics. There are two prospects of making organic/inorganic blends which are depended on the specific interactions between the two subphases (scheme 1). Such blends which were obtained by simply mixing of the two components together, characterized as materials of Class I corresponding no covalent or iono-covalent bonds. In these materials the various components only exchange weak interactions such as hydrogen bonding Van Der Waals interactions or electrostatic forces. On the other hand, materials which are formed by chemically bonding between the two subphases are characterized as Class II (hybrid materials). Class II materials organic/inorganic components are linked through strong chemical bonds e. g. covalent, iono-covalent or Lewis acid-base bonds. Usually materials of Class II have better mechanical properties than Class I as they present rubbery behavior (Stathatos, 2005).
Sol-Gel chemistry allows the combination at the nanosize level of inorganic and organic since solubility of most organic substances, especially, hydrophobic ones, is limited in pure oxides (e. g. SiO2) causing migration and aggregation with subsequent decrease of their functionality. Nanocomposite gels made of the two different subphases the oxide network, as the inorganic subphase and the polymer or surfactant as the organic subphase mixed in nanoscale providing in this way an access to an immense new area of materials science. In principal, in sol-gel chemistry a metal alkoxide (including silicon alkoxides) is hydrolyzed and the subsequent inorganic polymerization leads to the formation of the corresponding oxide with an oncoming condensation of the material. The whole process is carried out at ambient conditions. The process can be summarized to reactions 7 mentioned before.
An alternative route to the oxide synthesis is the slow water release in the solution with no initial water addition into the solution. In this case the existence of an organic acid in the sol is indispensable, typically acetic or formic acid as initially referred in reactions 8.
(In reactions 7a and b, as well as in the above reaction 8, only one of usually four reacting alkoxy groups is taken into account, for reasons of simplicity). Reactions 8 show that the end product of the sol-gel process is – M-O-M-, which can be obtained by successive chemical reactions. Metal ester (M-OAc) as a result of (2) can react with the metal alkoxide forming – M-O-M-, additionally; water released through esterification reaction 3 can yield oxide by the hydrolysis route. When ethanol is introduced in the sol, which is a common recipe in many works, even more water can be released by direct esterification reaction between the ethanol (EtOH) and the acetic acid (AcOH). Furthermore, intermediate M-OAc ester or -M-O-M – oligomers may create entities which offer polymorphism to the sol-gel evolution. Thus the presence of a self-organizing agent, e. g. a surfactant, plays a crucial role in organizing the structure of the material and in creating well defined and reproducible nanophases. Slow
Scheme 1. (a) Class I of composite organic-inorganic electrolyte (b) Class II of hybrid organic-inorganic electrolyte
water release, organic acid solvolysis and surfactant organization are then the key factors that dictate the structure and the quality of the nanocomposite organic/inorganic gel. A different approach to the gel process of quasi-solid electrolytes is the use of modified materials (Jovanovski et al., 2006) with silicon alkoxy-groups which may easily hydrolyzed and finally lead to a gel formation (e. g. scheme 2). The modified materials could be a series of additives usually employed in liquid electrolytes e. g. benzyl-imidazoles for open circuit voltage enhancement which are now bearing alkoxy-groups for jellifying process.
As a consequence, gel electrolytes are roughly distinguished into three categories: (1) One way to make a gel electrolyte is to add organic or inorganic (or both) thickeners. Such materials may be long-chain polymers like poly (ethylene oxide) or inorganic nanoparticles like titania or silica; (2) A second way is to introduce a polymerizable precursor into the electrolyte solution and polymerize the mixture in situ; (3) a third route is to produce a gel incorporating the I-/I3- redox couple through the sol-gel process by using a sol-gel precursor, like a titanium or silicium alkoxide. This precursor may be a functionalized derivative of one of the components of the electrolyte. This last method has been very successful since the sol-gel process leads to the formation of nanocomposite organic-
inorganic materials. Such materials are composed of an inorganic subphase, which binds and holds the two electrodes together and seals cell and an organic subphase, which assures dispersion of ionic species and supports ionic conductivity. The whole composition is compatible with titania nanocrystalline electrode and provides good electrical conduct and finally satisfactory ionic conductivity. Such cells are easy to make. After dye-adsorption on titania electrode, it suffices to place a small drop of the sol on the surface of the electrode and then press the counter electrode on the top by hand under ambient conditions. The two electrodes bind together while the fluid sol enters into titania nanoporous structure and achieves extensive electrical conduct.
Scheme 2. Example of a hybrid organic/inorganic material used as quasi-solid electrolyte. (Jovanovski et al., 2006)