Microemulsion electrolyte for electric double layer supercapacitor

<p dir="ltr">The development of efficient, sustainable, and high-performance energy storage solutions is vital to addressing global energy challenges, particularly in the context of the transition to renewable energy sources. Electric double-layer supercapacitors (EDLCs), because of their high power density, fast charge-discharge capabilities, and exceptional cycle life, are gaining prominence in the energy storage landscape. However, their performance is constrained by conventional electrolytes, which have narrow electrochemical stability, modest ionic conductivity, thermal instability, environmental concerns, and cost inefficiencies. Microemulsion electrolytes, which are thermodynamically stable mixtures of oil, water, and surfactants, have shown promising performance in supercapacitors by offering an extended electrochemical potential window (5V), comparable ionic conductivity to aqueous electrolytes, and additional benefits such as non-toxicity, environmental compatibility, and lower cost compared to other commercially available electrolytes. Despite validation, there remains a significant knowledge gap in understanding the physicochemical properties, microstructure, and electrochemical behavior of microemulsion electrolytes in electric double-layer supercapacitors.</p><p dir="ltr">This thesis systematically investigates the formulation and optimization of the ionic conductivity in microemulsion electrolytes to enhance their performance in supercapacitors. Pseudoternary phase diagrams for the water/cyclohexane/sodium dodecyl sulfate/butanol system were constructed to identify stable single-phase regions for different surfactant to co-surfactant ratios. The ionic conductivity was measured, focusing on the percolation behavior of the aqueous domains and the influence of surfactant to co-surfactant ratios. Electrical percolation thresholds were identified, marking the transition from isolated droplets to continuous conductive pathways, which was further confirmed through cryo-transmission electron microscopy imaging, which provides direct visualization of the associated morphological changes. In addition, advanced statistical tools including design-of-experiments and predictive modeling through artificial neural networks were used to further optimize microemulsion electrolyte formulations. This approach successfully identified compositions achieving maximum ionic conductivity, experimentally validating the predictive model results. Salt uptake by the microemulsion systems assisted in further enhancing the ionic conductivity. The compatibility of commercial separators with the microemulsion electrolyte was assessed using a custom-designed 3D printed conductivity cell and electron microscopy, identifying cellulose and polyethylene separators as the most suitable.</p><p dir="ltr">Finally, optimized microemulsion electrolyte formulations were translated into commercial grade supercapacitor prototypes and benchmarked against a 25 F Maxwell supercapacitor. Microemulsion electrolyte-based supercapacitors demonstrated a stable operating voltage of 2.0V and an equivalent series resistance of 34mΩ, compared to 26mΩ for the commercially available device. In addition, fabrication protocols were standardized, ensuring reproducibility and scalability for future commercialization. This work bridges fundamental laboratory insights with practical industrial applicability and takes microemulsion-based supercapacitors one step closer to commercialization.</p>