Synthesis and characterisation of sol- gel based ionogels as electrolyte for supercapacitors

Abstract

With growing public demand for consumer safety regarding electronic devices, ionogels have recently been suggested as a safe and solid replacement for the conventional flammable and toxic organic electrolytes used for various energy storage devices including batteries and supercapacitors. Ionogels are porous solid scaffolds that encapsulate ionic liquids and prevent electrolyte leakage which can otherwise be a potential hazard to the consumer or the surrounding circuitry. In this work, sol-gel was utilised as a manufacturing technique for 1- ethyl-3-methylimidazolium trifluoromethanesulfonate based-ionogels. Sol-gel chemistry is a facile and industrially relevant technology that allows the formation of solid scaffolds from homogenous precursors including a variety of silicon alkoxides. Previous studies have used a single or a mixture of silicon alkoxides to fabricate ionogels. In this work, four different precursors (two tetraalkoxysilanes and two alkyltrialkoxysilanes) were utilised to explore the influence of the type of silicon alkoxide on the reaction kinetics, thermal stability, microstructure and the electrochemical performance of the ionogels as electrolyte for electric double-layer capacitors (EDLCs). Furthermore, thermally-cured 1-ethyl-3-methylimidazolium trifluoromethanesulfonate- based ionogels have been realised for the first time in this work. The influence of curing temperature on the structure of the ionogels and their electrochemical performance as the electrolyte for EDLCs have been investigated. Ionogels were synthesised via a non-hydrolytic sol-gel route and were fully gelled post heat-treating at 125, 150, 175 and 200°C for 60 minutes with minimal shrinkage. Charge transfer resistance (a rate-limiting parameter in cell kinetics during charge/discharge cycles) was reduced by ∼80% via increasing the heat-treatment temperature; this was partially attributed to the interlocking effect at the electrode-electrolyte interface facilitated by high curing temperature. The fast-cure fabrication process for ionogels removes one of the major hurdles in their industrial application, their long curing and aging time, while the improved room temperature ion transport kinetics expands the potential application of ionic liquid-based electrochemical systems. Finally, an exploratory investigation on the long-term stability of the fabricated EDLCs indicated that cells with 150°C cured ionogels show an improved electrochemical performance compared to the EDLCs with ionic liquid electrolyte without the gel network. The results gathered in this work provide an insight into sol-gel processed ionogels as a replacement for the conventional electrolyte formulations and further proves that, this class of materials has the potential to be a safe, solid and durable electrolyte for EDLCs.