Bio-based tetraalkylammonium iodide salt as electrolyte for dye-sensitized solar cell

Abstract

Tetraalkylammonium ion is one of the cations used in preparing ionic liquids (ILs), which are commonly constructed from petrochemical products. Due to the salt dissociation and resource depletion issue, we explored the functionality of renewable materials extracted from vegetable oil in synthesizing a bio-based tetraalkylammonium salt (TAS) via two-step chemical reactions, (1) amidation and (2) quaternization. The objectives of this study are to determine the optimum parameter of amidation process, the physicochemical and electrochemical properties of TAS from bio-based fatty amide (FA) as well as to determine the performance of bio-based TAS as electrolyte in dye-sensitized solar cell (DSSC). In this work, bio-based FA compound was synthesized between tris(3-aminopropyl)amine (TPA) and selected fatty acids such as lauric acid (LA) and palmitic acid, and various parameters such as reaction time, type of solvent, reaction temperature, and mole ratio of reactants were studied. It is found that the mole ratio of (1:4) of LA:TPA which reacted at 110 ° C at 12 h stand out as the parameter producing the highest percentage of yield which is 76 %. The quaternization step was proceeded on the LA-based FA to produce bio-based TAS by reacting with iodopropane via Menschutkin reaction. The chemical structure of FA was confirmed by the shifting of wavenumber in infrared (FTIR) spectrum from 3357 and 3280 cm-1 to 3282 cm-1 corresponding to the changes from primary amine stretching to secondary amide. The peak of C=O in LA, also shifted to a lower wavenumber from 1694 cm-1 to 1638 cm-1 indicating the formation of amide (N-C=O). Both of FA and TAS were also further analyzed by using 1D and 2D Nuclear Magnetic Resonance spectroscopy (NMR). From 2D spectrum, the correlation of carbonyl carbon of LA with hydrogen of TPA was clearly seen indicating the amide bond formation. For TAS, the correlation between proton and its carbon proved the addition of propyl chain into FA. The molecular weight of the compounds were obtained using mass spectroscopy analysis proved that the experimental values is in agreement with theoretical value. The TAS compound showed a solid-solid transition before it reached the complete melting temperature indicating that behaviors are reflecting an ionic liquid crystal characteristic. Similar pattern on thermal behaviour showed by the electrolyte of TAS 1-6 and TAS 1-9. The ionic conductivity value of TAS 1-4 obtained is 1.83×10-6 S cm-1 at 0 °C and increased up to 8.15×10-5 S cm-1 at 60 °C showing a stable thermal-conductivity relation. The TAS electrolytes was fabricated in DSSC using FTO/TiO2-dye/electrolyte/Pt configuration. The cell efficiency of TAS 1-4 electrolyte obtained was 0.6 % with the cell parameter of Jsc, Voc, and FF are 3.4 mA cm-2, 0.67 V and 25.57, respectively. The efficiency obtained was higher than conventional tetrabutylammonium iodide salt (TBAI) which proved the presence of iodide ions in TAS is outnumbered than TBAI at the same concentration. The addition of 1.0 mol% iodine as redox couple mediator improved further the photovoltaic response of TAS electrolytes with Jsc of 2.9 mA cm-2, Voc of 0.57 V, FF of 58.04 and power conversion efficiency of 1.0 % under light intensity of 100 mW cm-2. Other than as an electrolyte, the bio-based TAS have potential to be used in medicine, household products as well as in industrial plants.