Investigation of hydroxide ion-conduction in solid polymer electrolytes via electrochemical impedance spectroscopy

Abstract

This study explores the conduction of hydroxide (OH−) ions in solid alkaline polymer electrolytes, and their contribution to the performance of solid-state electric double layer capacitors (EDLCs). The conductivity of tetraethylammonium hydroxide (TEAOH) polyacrylamide (PAM) and the capacitance of a solid-state EDLC based on this solid polymer electrolyte were investigated under three controlled relative humidity (RH) conditions (15, 45, and 75% RH). These properties are found to be highly dependent on the hydration structure of OH− ions that evolved in the different environments. To identify the role of hydration and ion structure in solid alkaline polymer electrolytes on device performance, dielectric analyses on the impedance spectra of TEAOH-PAM were utilized and revealed an interesting connection between the frequency dependent capacitive behaviour of the device and the ion response in the polymer electrolyte. The capacitance of EDLCs is closely related to the accumulation of ions at electrode/electrolyte interface, while the high frequency time constant for EDLCs corresponds to the transition of ion motion from vibration (energy dissipating) to translation (energy storing). Based on dielectric analysis and correlating the hydration of TEAOH-PAM with the activation energies of ionic conductivity in the three environments, we propose a framework to explain the changes of OH− ion transport in the solid-state. Under 15% RH, the segmental motion of the polymer chains facilitates the transport of TEAOH ions bound as 4:1 H2O:TEAOH crystal hydrates in the solid phase. As hydration rises under 45% RH, the segmental motion coordinates the conduction of 7.5:1 H2O:TEAOH crystal hydrates in the liquid phase. Under 75% RH, TEAOH-PAM is well hydrated and the conduction mechanism is dominated by liquid-like OH− ion-hopping of fully dissociated ions (hydration number 16.6:1). These results demonstrate the interplay of segmental motion and OH− ion-hopping in solid alkaline polymer electrolytes. The methods and findings of this study can be used to design solid aqueous polymer electrolytes with predictable long-term behaviour for inherently safe, thin, and lightweight electrochemical devices.