Polymer electrolytes are important enablers for safe, thin, future flexible electronics. Alkaline electrolytes such as KOH have been extensively used in energy storage devices such as batteries and electrochemical capacitors (ECs). Although aqueous KOH is useful, it has limitations when serving in a polymer electrolyte due to crystallization that compromises hydroxide (OH^-) ion conductivity. An alternative alkaline electrolyte, tetraethylammonium hydroxide (TEAOH) was shown to be more stable with polymers compared to KOH.[1,2] Polyacrylamide (PAM) is a promising polymer host material due to its hygroscopic nature, amorphous structure, and useful functional groups that support ion conduction. When combined with TEAOH, a good ionic conductivity (> 10 mS cm^-1) was achieved under ambient conditions (room temperature and 45% relative humidity (RH)). Thin, lightweight, electrochemical double layer capacitors (EDLC) were also demonstrated with TEAOH-PAM that outperformed their liquid analogues.[3]

One of the important properties of TEAOH-PAM is its OH^- ion-conduction dependence on hydration. As its degree of hydration changes with environmental factors such as RH and temperature, so does the conductivity of TEAOH-PAM as shown in Fig. 1. An additional consequence of this environmental vulnerability is poor dimensional stability of the polymer electrolyte which can lead to premature failure. To address these issues, the use of inert inorganic fillers as additives is explored. Inorganic fillers such as SiO₂, TiO₂, and Al₂O₃ have been commonly used to enhance the ionic conductivity, strengthen the mechanical properties, and improve the thermal stability of polymer electrolytes.[4–6] Here, we leverage different inorganic fillers to improve the environmental stability, expand the working temperature range, and provide better structural support for TEAOH-PAM polymer electrolytes.

In this talk, the effects of different inorganic fillers such as SiO₂ and TiO₂ on OH^- ion-conduction will be presented. For example, SiO₂ showed both advantageous and detrimental effects on OH^- ion conductivity depending on the RH condition and the size of SiO₂. The possibility to enhance the OH^- ion conductivity of TEAOH-PAM in low and high temperatures using SiO₂ and TiO₂ will be discussed. A combination of electrochemical and spectroscopy experiments is used to study the interaction of these inorganic fillers with the TEAOH-PAM alkaline polymer electrolyte. These results will help further clarify OH^- ion-conduction in polymer electrolytes.

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[2] J. Li, K. Lian, A comparative study of tetraethylammonium hydroxide polymer electrolytes for solid electrochemical capacitors, Polymer. 99 (2016) 140–146. doi:10.1016/j.polymer.2016.07.001.

[3] J. Li, J. Qiao, K. Lian, Investigation of polyacrylamide based hydroxide ion-conducting electrolyte and its application in all-solid electrochemical capacitors, Sustain. Energy Fuels. 1 (2017) 1580–1587. doi:10.1039/C7SE00266A.

[4] S. Ketabi, K. Lian, Effect of SiO2 on conductivity and structural properties of PEO-EMIHSO4 polymer electrolyte and enabled solid electrochemical capacitors, Electrochim. Acta. 103 (2013) 174–178. doi:10.1016/j.electacta.2013.04.053.

[5] J.E. Weston, B.C.H. Steele, Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly(ethylene oxide) polymer electrolytes, Solid State Ionics. 7 (1982) 75–79. doi:10.1016/0167-2738(82)90072-8.

[6] J. Przyluski, M. Siekierski, W. Wieczorek, Effective medium theory in studies of conductivity of composite polymeric electrolytes, Electrochim. Acta. 40 (1995) 2101–2108. doi:10.1016/0013-4686(95)00147-7.