Tetraethylammonium Hydroxide with Polyacrylamide As Hydroxide Conducting Polymer Electrolytes for Electrochemical Capacitors
Authors: Li Jak , Lian Keryn 
Published: February 2020

Introduction

Alkaline electrolytes, especially KOH, have been extensively used for energy storage devices such as batteries and electrochemical capacitors (ECs) in symmetric and asymmetric configurations [1-3]. An alkaline polymer electrolyte enables safe, thin, and flexible devices. Due to the limitation of crystallization with KOH, quaternary ammonium hydroxide based polymer electrolytes, specifically, tetraethylammonium hydroxide (TEAOH) in polyvinyl alcohol (PVA) was studied and shown to be a suitable alternative to KOH-PVA [4]. Solid AC-line filtering devices were enabled using this TEAOH-PVA polymer electrolyte [5]. However, PVA is a semi-crystalline polymer with limitations in water retention which deters the electrochemical performance of these alkaline polymer electrolytes.

In this study, our objectives are to (a) develop an alkaline polymer electrolyte using TEAOH with polyacrylamide (PAM) as an alternative polymer matrix to PVA, (b) characterize the polymer electrolyte in terms of its long term ionic conductivity and material properties, and (c) fabricate solid EC devices and compare them with a liquid analogue.

Experimental

Aqueous TEAOH-PAM polymer electrolytes were prepared in different precursor solutions. Smooth metallic nickel (Ni) foils were used as electrodes with 1 cm2 areas. Two-electrode test vehicles were made by sandwiching TEAOH-PAM between the Ni electrodes and sealed with a chemically inert tape. Solid EC devices were made by sandwiching TEAOH-PAM between CNT-graphite electrodes on a Ni substrate with 1 cm2geometric surface areas. All cell characterizations were performed under ambient conditions unless otherwise specified.

Results

The ionic conductivity of the polymer electrolytes with different TEAOH:PAM ratios were measured and demonstrated remarkable ionic conductivities up to ca. 20 mScm-1 in the pristine condition. The trend revealed an optimal composition with 75-25 weight ratio of TEAOH-PAM. As shown in Figure 1, the TEAOH-PAM cells were tracked over a period of 50 days under ambient conditions and demonstrated good environmental stability. Further conditioning the cells under 45% RH showed an appropriate representation of the cells under steady-state. Details regarding ionic conductivity and long term stability will be discussed and related to the structural and chemical characterizations of these TEAOH-PAM polymer electrolytes. Finally, solid EC device performances will be compared to liquid TEAOH devices.

References

[1] K.-W. Nam and K.-B. Kim, "A study of the preparation of NiO x electrode via electrochemical route for supercapacitor applications and their charge storage mechanism," Journal of the Electrochemical Society, vol. 149, pp. A346-A354, 2002.

[2] http://www.elton-cap.com/products/

[3] Y.-G. Wang, Z.-D. Wang, and Y.-Y. Xia, "An asymmetric supercapacitor using RuO 2/TiO 2 nanotube composite and activated carbon electrodes," Electrochimica Acta, vol. 50, pp. 5641-5646, 2005.

[4] H. Gao, J. Li, K. Lian., "Alkaline quaternary ammonium hydroxides and their polymer electrolytes for electrochemical capacitors," RCS Advances, vol. 4, pp. 21332-21339, 2014.

[5] H. Gao, J. Li, J. R. Miller, R. A. Outlaw, S. Butler, and K. Lian, "Solid-state electric double layer capacitors for ac line-filtering," Energy Storage Materials, vol. 4, pp. 66-70, 2016.

Figure 1