Iodopropyl-branched polysiloxane gel electrolytes with improved ionic conductivity upon cross-linking

Materials. 3-chloropropylmethyldimethoxysilane, hexamethyldisiloxane and sodium iodide were purchased from ABCR. Dimethyldiethoxysilane, p-toluenesulfonic acid monohydrate, poly(dimethylsiloxane) bis(3aminopropyl) terminated Mn≈2500, 1-methylimidazole, lithium iodide and 3-methoxypropionitrile were purchased from Sigma Aldrich. All the reagents were used as received. MilliQ water (resistivity 18 MΩ) was obtained by a Millipore filtering system unit.

Such obtained polymers are however characterized by a too much high molecular weights (in the order of 10 5 Da) and needed to be subjected to and equilibration reaction in order set the molecular weights to an approximate valued of 20 kDa.In a typical equilibration 10 g of silicone polymer were taken up in 20 ml of toluene, 150 mg of p-toluenesulfonic acid and 30 µl of hexamethyldisiloxane are added to the very viscous solution, with stirring until the high molecular weight material has disappeared.50 µl of water are then added and stirring is continued for two hours.The solution is finally washed with water in a separating funnel and toluene distilled off, leaving a low molecular weight silicone oil.Obtained polymers are finally washed with warm methanol to remove cyclic oligomers and dried in oven at 60°C.
Quaternization procedure.The prepared poly(3-iodopropylmethylsiloxane-co-dimethylsiloxane)s were mixed with four parts of acetonitrile in a flask connected with a reflux condenser and purged with argon for 15 minutes.In order to partially quaternarize pending iodopropyl unities, a molar defect of 1methylimidazole was then added via a syringe while continuing purging.The solution was refluxed for 15 hours in dark conditions, provided by an aluminum foil covering the whole reactor.Products are finally recovered by precipitation in diethylether from concentrated solution in acetonitrile and dried at 70°C under reduced pressure.To avoid hydration of imidazolium moieties, polymers were stored under controlled atmosphere prior the use.
1 H NMR spectra of unquaternized GL11 and GL14 polymers are reported in Fig. S2a while 55%-quaternized products (namely GL11Q55 and GL14Q55) are reported in Fig. S2b.Molecular weight distributions of GL11 and GL14 are reported respectively in Fig. S3a) and S3b).After the polymerization step, obtained products presented as mixtures of high molecular weight polymer and low molecular weight compounds (Figure S3, black squares) which can be indeed identified as cyclic oligomers.
Their amount seems to be dependent on the quantity of 3-iodopropyldimethoxymethylsilane employed in the initial mixture of reactants: we have in fact experimentally found that the increasing concentration of 3halogenopropyl monomer resulted in a higher amount of cyclic intermediates, which were subsequently removed through a series of washing cycles in warm methanol.The equilibration procedure allowed us to tailor the molecular weight of synthesized polymers to a 20 kDa value (Figure S3, red circles).The electrodes were gradually heated under an air flow in oven and sintered at 480 °C for 30 min.They were then sensitized with a recently disclosed organic dye, which has been referred as G2 [9]: TiO 2 films were immersed into a 0.2 mM solution THF which also contained a proper amount of chenodeoxycholic acid (CDCA, 80 mM) as co-adsorbent which reduces/prevents the detrimental aggregation effects between neighboring dye molecules.A batch of reference electrodes was also provided by using a solution 0.5 mM of bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N′-bis(4-carboxylato-4′-carboxylic acid-2,2-bipyridine) ruthenium(II) (N719) in a mixture of acetonitrile and tert-butyl alcohol (v/v, 1:1).In both of the cases, dye loading was performed by keeping the electrodes immersed into the dye solution for 14h.DSSCs were finally assembled by using a Platinum-coated glass (counter electrode) and sealed with a 60μm-thick Surlyn gasket.Electrolytes were introduced into the cell through a hole, which had been predrilled on the back of the counter electrode and was connected to a vacuum pump.Two batches of reference cells have been provided by filling both of G2-and N719-sensitized cells with a liquid electrolyte having the following composition: 0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide and 0.5 M tertbutylpyridine in 3-methoxypropionitrile.The holes were then sealed up with a Surlyn hot-melt film attached to a glass-based cap.
J-V (current density−voltage) curves detected from the measurement of the here referred dye solar cells under 1 sun illumination are shown in Figure S8 and their performances are summarized in Table S2.EIS analysis on DSSCs.Electrochemical impedance spectroscopy (EIS) measurements were performed within 72 hours also on the dye solar cells.The resulting spectra were fitted with the ZView software and analyzed through the well-known equivalent circuit [10].
Figure S9a and Figure S9b show the charge-transfer resistance (R CT ) and the diffusion resistance (R diff ) as a function of applied voltage.From the analysis of figure S9a, it can be noticed that the values of Rct, in the range of applied bias, slightly decrease during the first 20 hours and then is quite constant during the remaining hours of monitoring.
This trend confirms that of open circuit voltage variation (Fig 3a).The trend in Rct during the 72 hours is partially related to the increase of iodine concentration, which is delivered upon the curing process but it can be also ascribed to the better permeation of the viscous liquid into the TiO 2 mesoporous electrode.
The diffusion resistance (Fig. S9b) resulted indeed in a noticeable reduction upon the completion of the curing test, thanks to increased iodine concentration which seems to overcome the detrimental effects on ion mobility associated to the higher viscosity of the x-linked polymer.

CH 2
CH 2 ), 3.20-3.22(t, 2H, CH 2 I), 3.52 (s, 6H, OCH 3 ). in FigureS1.The anionic exchange reaction is confirmed by the shift from 3.53 ppm to 3.19 ppm of 3-halogenopropylic hydrogens.The shift to lower frequencies clearly indicates that electron density of halo-methylene group has risen as a consequence of the anionic exchange with the less electron negative iodine.Dimerization of alkoxysilanes by hydrolysis/condensation mechanism also occurs, but its extent is very limited, moreover, the following polymerization and equilibration processes reshuffle/recombine the structure of the polymer.

Figure S2. 1 H
Figure S2. 1 H NMR spectra of a) GL11 and GL14 and b) of the same polymers upon partial quaternization process with 1-methylimidazole.

Figure S3 .
Figure S3.Molecular weight distribution of polymers GL11 a) and GL14 b) before (black squares) and after the equilibration procedure (red circles) Electrolytes formulation.Partially quaternized polymers were dissolved in 3-methoxypropionitrile-based electrolytes.The calculated amount of poly(dimethylsiloxane) bis(3-aminopropyl) terminated cross-linker was added to the mixture to fully quaternarize residual 3-iodopropyl pending units.Samples were vigorously shaken for five minutes prior to be used and characterized.A schematic representation of cross-linking process is shown in Figure S4.

Figure S4 .
Figure S4.Cross linking reaction between aminopropyl terminals of cross-linker and pending iodopropyl groups on the polymer

Figure S5 .
Figure S5.Evolution G' and G" during the x-linking process as a function of the quaternization rate and the curing temperature: a) GL14 cured at 50°C, b) GL14 cured at 75°C, c) GL11 cured at 50°C, d) GL11 cured at 75°C

Figure S8 .
Figure S8.J-V curves of GL11-based dye solar cells compared with two reference cells filled with a liquid electrolyte

Figure S9 .
Figure S9.Fitted values of R ct a) and R diff b) extrapolated from EIS measurements on a GL11_Q80-based DSSC during the x-linking process.

Table S1 .
Chemical composition, molecular weights and cross-linking ratio for synthesized GL11 and GL14

Table S2 .
Photovoltaic performances of GL11-based dye solar cells compared with two reference cells filled with liquid electrolyte * data measured upon completion of cross-linking process