Electrochemical performance and microstructure of microemulsion electrolytes for supercapacitors

In this study, the structural and electrochemical properties of microemulsions (MEs) with varying hydrophobicity and water content, controlled through the amount of surfactant ( S ) and oil ( O ) relative to water ( W ), x = ( S + O ) / W , were investigated to understand their potential as electrolytes for energy storage applications. The ellipsoidal and Teubner-Strey (T–S) structural models were applied to the MEs’ small-angle neutron scattering (SANS) data, revealing a morphological transition from droplets to bicontinuous-like structures with increasing x . Cryo-TEM imaging and SANS data analysis via Porod analysis confirmed this structural evolution and highlighted the associated changes in flexibility, compactness, and nanoscale organisation. Electrochemical studies using ferrocene (Fc) as a probe demonstrated that the Fc diffusion increased with x , reflecting the enhanced structural heterogeneity and diffusional pathways of the bicontinuous MEs. Higher conductivity and lower resistance at the lower x = 0.2 suggested superior supercapacitor performance. In contrast, higher x = 1.0 and 1.5 MEs showed promise for application in redox flow batteries with differently polar charged states due to their well-defined biphasicity, which facilitates fast diffusion and electron transfer. Capacitance measurements and Ragone plots further corroborated the correlation between structure and performance, with the x = 0.2 ME outperforming its counterparts in energy and power density. These findings highlight the critical influence of morphology on ME functionality, with future studies using neutron reflectometry (NR) planned to probe the lamellar interfacial structures believed to be responsible for high-voltage activity. • Microstructure–performance correlation established for SDS-based microemulsions (MEs) as electrolytes in energy storage devices. • SANS and cryo-TEM reveal composition-dependent transitions from ellipsoidal droplets to bicontinuous domains, directly impacting charge transport. • ME0.2 exhibits highest capacitance and superior performance as a supercapacitor electrolyte due to water-rich domains and low resistance. • ME1.0 and ME1.5 show enhanced diffusion and kinetics, making them better suited for redox flow batteries with well-defined bicontinuous structures. • Design principles proposed for ME-based electrolytes by tuning morphology and interfacial characteristics to target specific energy storage applications.