Abstract
Aqueous supercapacitors offer a safe alternative for intermediate energy storage in energy harvesting applications, but their performance is limited to relatively warm temperatures. We report the performance of glycerol as a non-toxic anti-freeze for a water-based electrolyte from room temperature to −30 °C at various concentrations. The supercapacitors are manufactured with graphite and activated carbon as current collector and electrode on a flexible polyester (PET) substrate by stencil printing, with a sodium chloride solution as the electrolyte. The devices are characterized at various constant temperatures for electrical performance, as well as in room temperature for mass loss and development of performance over time. It is shown that supercapacitors with glycerol function well in the decreased temperatures compared to water: the capacitance experiences only a slight decrease and the leakage current is significantly reduced. The equivalent series resistance is affected the most by the reduced temperatures, and should be considered the primary limiting factor in low-temperature applications. Electrolytes with 30–40% glycerol perform the best in commercial freezer temperatures, but below −20 °C a higher concentration of 45% glycerol retains better function. The results show great promise for a non-toxic alternative for improving the temperature range of printed supercapacitors.
Highlights
The Internet of Things (IoT) is one of the most studied topics in recent years, and potential applications range from domestic appliances to industrial equipment and integrated services in our everyday surroundings
For sub-zero conditions, glycerol is a good candidate for an anti-freeze of water because it reduces the freezing point of water significantly[17], and because it has a tendency to decrease ice crystal formation, which potentially could be of advantage in activated carbon supercapacitors[18]
Capacitance, equivalent series resistance (ESR) and leakage current are the key performance measures for supercapacitors and these characteristics were measured for the printed devices over the temperature range of +23–−30 °C
Summary
Capacitance, ESR and leakage current are the key performance measures for supercapacitors and these characteristics were measured for the printed devices over the temperature range of +23–−30 °C. Capacitance (Fig. 2a) decreases only moderately, likely due to decreased mobility, and different concentrations are affected quite It appears that at the coldest temperatures there are two mechanisms at play considering the increasing ESR: one in the dominantly water based solutions and another in the dominantly glycerol ones. Increase in the ESR is quite steady for most samples as is the decrease in capacitance This is probably largely related to the changing glycerol concentration in these samples, since most of the mass loss is likely due to water evaporation, leaving behind a higher concentration of glycerol, and leading to the weakening of the performance. Adding glycerol to the electrolyte solutions appears to be a good option for safely increasing the service life of printed supercapacitors, some compromising in performance or foot print area might be required
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