Low Temperature Performance of Solid Electrochemical Capacitors: Effects of Electrolytes Additives and Electrode Design

Abstract

Solid-state, thin, and flexible electrochemical capacitors (ECs) can provide energy storage for next-generation wearable electronics such as smart textiles and medical sensors. One of the enablers for high performance and safe electrical double layer capacitors ECs is neutral pH polymer electrolytes due to their wide operating potentials, good ionic conductivity and benign chemical nature [1-3]. Nonetheless, the operation temperature is often limited to above the freezing point of water. For example, a polymer electrolyte based on Na2SO4 ion conductor and polyacrylamide (PAM) host has shown a promising performance at ambient (good shelf-life >30 days and wide 1.8 V window with activated carbon [4]), but its performance degraded drastically at sub-zero conditions [5]. Furthermore, it is of practical importance to understand the effect of high surface area porous carbon materials (e.g. carbon nanotube (CNT) [6-7] or activated carbon (AC) [7-9]) on solid EC at low temperatures, which has not been extensively characterized. To ensure robustness of EC devices operating under winter/sub-arctic and outer space conditions, it is necessary to develop proper polymer electrolytes and the right match of electrode/electrolyte for low temperature ECs.In this study, the performances of solid EC cells constructed with carbon electrodes and Na2SO4-PAM-based electrolytes were assessed at ambient and low temperatures. The objectives were to (i) improve the performance of Na2SO4-PAM electrolytes using anti-icing additives and (ii) elucidate the key characteristics of electrodes (i.e. pore structure and electrode loading/thickness) affecting cell performance at low temperatures.To improve the low temperature performance of the Na2SO4-PAM electrolyte, several anti-icing additives (e.g. dimethyl sulfoxide/DMSO or ethylene glycol/EGly) have been investigated and shown promising improvement to the ionic conductivity of the electrolyte at low temperatures. Leveraging the polymer electrolyte with these additives, capacitive CV profiles were attained at -10 ˚C (Fig. 1a), much more pronounced than their binary baseline. The cell performance with the DMSO-containing ternary electrolyte was then evaluated using either CNT or AC electrodes at various carbon loadings (Fig. 1b). Although solid cells with AC electrodes demonstrated higher capacitance at ambient, those with CNT electrodes retained more capacitance at low temperature. This is likely due to better ion accessibility into CNT electrodes with relatively more open pores. Meanwhile, AC electrodes with lower carbon loading have higher capacitance retention at low temperature due to better electrode utilization in a thinner electrode. These observations will provide the guideline for designing the next generation of ECs that are suitable for low temperature applications.