New formulations extend operation into lower temperatures.
A previous NASA Tech Brief [“Low-Temperature Supercapacitors” (NPO-44386) NASA Tech Briefs, Vol. 32, No 7 (July 2008), page 32] detailed ongoing efforts to develop non-aqueous supercapacitor electrolytes capable of supporting operation at temperatures below commercially available cells (which are typically limited to charging and discharging at ≥40 °C). These electrolyte systems may enable energy storage and power delivery for systems operating in extreme environments, such as those encountered in the Polar regions on Earth or in the exploration of space. Supercapacitors using these electrolytes may also offer improved power delivery performance at moderately low temperatures (e.g., –40 to 0 °C) relative to currently available cells, offering improved cold-cranking and cold-weather acceleration capabilities for electrical or hybrid vehicles.
Supercapacitors store charge at the electrochemical double-layer, formed at the interface between a high surface area electrode material and a liquid electrolyte. The current approach to extending the low-temperature limit of the electrolyte focuses on using binary solvent systems comprising a high-dielectric-constant component (such as acetonitrile) in conjunction with a low-melting-point co-solvent (such as organic formates, esters, and ethers) to depress the freezing point of the system, while maintaining sufficient solubility of the salt.
Recent efforts in this area have led to the identification of an electrolyte solvent formulation with a freezing point of –85.7 °C, which is achieved by using a 1:1 by volume ratio of acetonitrile to 1,3-dioxolane (as determined by differential scanning calorimetry). This is in contrast to a freezing point of –45.7 °C for the pure acetonitrile solvent used in typical supercapacitor cells. This solvent system readily solubilizes salts commonly used in supercapacitor electrolytes, such as tetraethylammonium tetrafluoroborate (TEATFB) and lithium hexafluorophosphate.
Full electrolyte systems were formulated through the addition of TEATFB to the 1:1 solvent blend, over a range of salt concentrations. Coin cells were then filled with the various electrolytes for low-temperature electrical testing. Commercially available high surface area carbon-based materials were used as the electrode material, in conjunction with a polyethylenebased separator material. Representative DC discharge data for the 0.50 M concentration system have shown a highly linear discharge over a wide range of temperatures (with little fade in capacitance at the lowest measured temperatures).
This work was done by Erik Brandon, Marshall Smart, and William West of Caltech for NASA’s Jet Propulsion Laboratory.
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