An Electrochemical Impedance Spectroscopy Study of Hydroxide Ion Conduction in Polymer Electrolytes

Abstract

Solid alkaline polymer electrolytes are important enablers for safe, lightweight, electrochemical energy storage devices. Tetraethylammonium hydroxide (TEAOH) is an OH- ion conductor, compatible with various water soluble polymers[1,2]. Polyacrylamide (PAM), a hygroscopic amorphous polymer, was combined with TEAOH to form a solid electrolyte. The resulting TEAOH-PAM electrolyte exhibited good ionic conductivity (> 10 mS cm-1) at room temperature. However, its performance can vary with the environmental relative humidity (RH). It was reported that TEAOH is prone to bond with water, forming different hydrate compounds that may impact OH- ion-conduction[3]. In addition, the role of ion content and ion motion on OH- ion-conduction in a polymer matrix needs to be further clarified by using dielectric analysis[4]. Therefore, it is important to elucidate the influence of these effects on OH- ion-conduction in the solid-state under different environmental conditions (i.e. RH, temperature). In this study, we investigated the ionic conduction mechanism at various temperatures using electrochemical impedance spectroscopy (EIS). The capacitances of the enabled solid cells and conductivities of the electrolytes were correlated to ion content and ion motion by dielectric analyses from the EIS data. The hydration structure of ions in TEAOH-PAM was quantified under three levels of RH[3]. Through analyzing these properties at elevated temperatures, an interplay of conduction mechanisms between ion transport via segmental motion of the polymer chains and OH- ion hopping was discovered (Fig. 1). High temperature studies were performed on 2D solid-state electrochemical double layer capacitors (EDLCs), leveraging the TEAOH-PAM electrolyte with vertically-oriented graphene nanosheet (VOGN). The super high rate performance of the solid-state EDLC was demonstrated at and above 100 °C. While the device operates optimally at 100 °C, it is also capable of withstanding short-term temperature exposure up to 120 °C. The excellent thermal resilience was further evidenced by the repeatable performance after subjecting to thermal cycles. From thermogravimetric analysis and Raman spectroscopy experiments at different temperatures, we found that dehydration of TEAOH-PAM is linked to rearrangement of ion conformations. This change in chemistry with higher thermal energy are key attributes responsible for the device’s temperature dependent performance. These results provide guidelines and strategies to design polymer electrolytes for high performance solid-state energy storage devices.