Solid-State Planar Edlc Design Enabled By Hydroxide-Conducting Polymer
Authors: Lian Keryn , Gao Han , Li Jak , Miller John R. , Outlaw Ronald A , Butler Sue M. 
Published: February 2020

Electrolytic capacitors are the current solution for 120 Hz power filtering. However such devices are bulky, which limits power electronics miniaturization. Electric double layer capacitors (EDLCs), often referred to by the product name Supercapacitor, have much higher volumetric charge storage and potentially should allow for a size reduction in the power electronics. For an EDLC to be capable of efficient AC line filtering, its impedance phase angle at 120 Hz must reach or be close to -90 degree. The very first EDLC that met this requirement was fabricated using vertically-oriented graphene with a liquid KOH electrolyte [1]. Both series resistance and distributed charge storage were minimized to reach this level of performance.
Further development has led to a planar interdigitated cell design, which offers volumetric advantages and a simple approach for series-connecting cells [2]. With this design, polymer electrolytes are preferred since they can cover each planar cell without flowing to an adjacent cell. In the past, we have demonstrated a tetraethylammonium hydroxide (TEAOH)-based polymer electrolyte system that outperformed the KOH-based polymer electrolytes [3]. In this study, we leveraged the TEAOH polymer electrolyte and the vertically-oriented graphene to demonstrate solid-state planar EDLC cells. Impedance behavior of the cells at both room temperature and elevated temperatures was investigated.
Two polymer electrolyte systems based on TEAOH were studied: (a) TEAOH-polyvinyl alcohol (TEAOH-XLPVA); and (b) TEAOH- polyacrylamide (TEAOH-PAM). Utilizing these polymer electrolytes, we assembled solid-state EDLC cells using vertically-oriented graphene electrodes. These solid-state devices were first tested at room temperature for aging stability and then at higher temperature for thermal stability.
Figure 1 shows three plots of capacitance versus frequency for TEAOH-based electrolyte solid-state EDLCs. The capacitance values were calculated assuming a series-RC circuit model. While both TEAOH-PAM-based and TEAOH-XLPVA-based electrolyte cells showed capacitive behavior, the former exhibited higher initial capacitance than the latter (38 vs. 32 μF at 120 Hz). Although both capacitors showed slightly reduced capacitance after ca.25 days storage without packaging (Fig. 1a), both capacitors demonstrated good shelf life at room temperature.
Further evaluations of the thermal stability of these capacitors at elevated temperatures were performed at temperatures up to 110 oC (Fig. 1b and 1c). Capacitance increased with increasing temperature for both solid-state electrolytes. A detailed analysis including comparisons will be presented. Capacitor equivalent series resistance and characteristic response times will be discussed.

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Fig. 1: Capacitance versus frequency of solid-state ELDCs made with TEAOH-based polymer electrolytes demonstrating (a) the effect of ca. 25-day shelf-storage at room temperature; and elevated-temperature performance of (b) the TEAOH-XLPVA-based electrolyte solid-state capacitor; and (c) the TEAOH-PAM-based electrolyte solid-state capacitor.

Figure 1