Flexible Supercapacitor for High Temperature Applications

Abstract

Introduction During the last decade there have been numerous advances in the field of electrochemical storage. This has created huge opportunities for devices with much increased energy and power densities, but new innovative ideas on thermally stable energy storage systems are still dormant. Despite their lower energy density compared to batteries, for temperatures above 120°C supercapacitors are the optimal choice of energy storage device. Supercapacitors have cycle lives far exceeding that of batteries, and in high temperature environments reliable service is more important than energy density [1]. This study presents a flexible supercapacitor with high temperature stability. Supercapacitor components are chosen mainly based on their material properties with emphasis on thermal stability. Few supercapacitor prototypes can operate above 140°C (200°C is the highest possible temperature so far) [2], because the requirements on all the components are high. For example, glass-fiber separator is dielectric, ion-permeable and will withstand high temperatures, but has been reported to have a tendency to shrink [2] and thus short circuiting the device, though this is not observed in our devices. Our measurements show increased energy density with increased temperature and no sign of degradation at 190°C. Method The 1.6 µm pore size Whatman® glass-fiber mat separators used in our study are ca. 100 µm thick, have circular shape with 13 mm diameter, slightly larger than the electrode material at 10 mm diameter. Activated carbon pellets (with 10 wt.% PTFE and 10 wt.% carbon black) are used as electrodes since they are well characterized and reproducible [3]. The liquid electrolyte used, 1-Ethyl-3-methylimidazolium acetate (EMIMAc), needs to be properly sealed to avoid contact with air and water vapor, and this is done by encapsulating the entire supercapacitor in thin polyimide films glued together with a high temperature silicone adhesive. EMIMAc degradation has been noted to be slightly above 200°C [4], so our stability tests are limited to this temperature. The first test was performed to examine the thermal degradation. The sealed supercapacitor was placed in an oven at 190°C for 2 h. Measurements were conducted after the device was cooled down to see if there was any degradation of the cell performance. During the second test capacitance was measured in situ at different temperatures to examine the gradient effect on the capacitance. The temperature was ramped in distinct steps until it reached 178°C where it was held for 2 h. To examine the degradation, we ramped the temperature to 250°C for a 10 min and cooled the sample back down to 178°C. Results The first tests at 190°C show no visible degradation of the components, rather a small increase in capacitance. The same results could be seen in the second test where the capacitance, at least for a while, slowly increased during the test and later remained enhanced after it was cooled down. The cyclic voltammetry in Figure 1 shows a capacitance of 57 mF at 37°C (body temperature) and when increasing the temperature to 178°C this capacitance was increased to 92 mF. When increasing the temperature to 250°C the slope on the curve changes drastically which possibly indicates non-electrostatic behavior, i.e. Faradaic reactions. However, when cooled down again to 178°C the curve is almost back to its previous values. Based on the results, it would be interesting to test other electrolytes and other carbon allotropes such as CNT on CNF which previously has shown good results at room temperature [5]. Conclusion The aim of this study was to examine the temperature stability of a supercapacitor with reproducible and flexible materials suitable for high temperatures applications. The results show promise with no sign of degradation at 190°C and a large gain in capacitance from 57 mF at 37°C to 92 mF at 178°C. However, the slightly increased capacitance at room temperature after heat treatment could be a sign of Faradaic reactions. More extensive tests are needed to see if there is long term stability of this increased performance.