Double-layer capacitors are unique energy storage devices, capable of supporting large current pulses as well as a very high number of charging and discharging cycles. The performance of double-layer capacitors is highly dependent on the nature of the electrolyte system used. Many applications, including for electric and fuel cell vehicles, back-up diesel generators, wind generator pitch control back-up power systems, environmental and structural distributed sensors, and spacecraft avionics, can potentially benefit from the use of double-layer capacitors with lower equivalent series resistances (ESRs) over wider temperature limits. Higher ESRs result in decreased power output, which is a particular problem at lower temperatures. Commercially available cells are typically rated for operation down to only –40 °C.
Previous briefs [for example, “Low Temperature Supercapacitors” (NPO-44386), NASA Tech Briefs, Vol. 32, No. 7 (July 2008), p. 32, and “Supercapacitor Electrolyte Solvents With Liquid Range Below –80 °C” (NPO-44855), NASA Tech Briefs, Vol. 34, No. 1 (January 2010), p. 44] discussed the use of electrolytes that employed low-melting-point co-solvents to depress the freezing point of traditional acetonitrile-based electrolytes. Using these modified electrolyte formulations can extend the low-temperature operational limit of double-layer capacitors beyond that of commercially available cells. This previous work has shown that although the measured capacitance is relatively insensitive to temperature, the ESR can rise rapidly at low temperatures, due to decreased electrolyte conductance within the pores of the high-surface-area carbon electrodes. Most of these advanced electrolyte systems featured tetraethylammonium tetrafluoroborate (TEATFB) as the salt. More recent work at JPL indicates the use of the asymmetric quaternary ammonium salt triethylmethylammonium tetrafluoroborate (TEMATFB) or spiro-(l,l’)-bipyrrolidium tetrafluoroborate (SBPBF4) in a 1:1 by volume solvent mixture of acetonitrile (AN) and methyl formate (MF) enables double- layer capacitor cells to operate well below –40 °C with a relatively low ESR. Typically, a less than two-fold increase in ESR is observed at –65 °C relative to room-temperature values, when these modified electrolyte blends are used in prototype cells. Double-layer capacitor coin cells filled with these electrolytes have displayed the lowest measured ESR for an organic electrolyte to date at low temperature (based on a wide range of electrolyte screening studies at JPL). The cells featured high-surface-area (≈2,500 m/g) carbon electrodes that were 0.50 mm thick and 1.6 cm in diameter, and coated with a thin layer of platinum to reduce cell resistance. A polyethylene separator was used to electrically isolate the electrodes.
This work was done by Erik J. Brandon, Marshall C. Smart, and William C. West of Caltech for NASA’s Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
NPO-47327
This Brief includes a Technical Support Package (TSP).
Substituted Quaternary Ammonium Salts Improve Low-Temperature Performance of Double-Layer
(reference NPO-47327) is currently available for download from the TSP library.
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Overview
The document is a technical report from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in the performance of double-layer capacitors, particularly at low temperatures. Double-layer capacitors are energy storage devices known for their ability to support large current pulses and endure numerous charging and discharging cycles. However, traditional capacitors typically operate effectively only down to -40°C, which limits their applications in extreme environments.
The report discusses the development of new electrolyte systems that utilize substituted quaternary ammonium salts, specifically triethylmethylammonium tetrafluoroborate (TEMATFB) and spiro-(1,1’)-bipyrrolidium tetrafluoroborate (SBPBF₄). These salts are combined with a solvent mixture of acetonitrile (AN) and methyl formate (MF) in a 1:1 ratio. This innovative approach allows the capacitors to maintain low equivalent series resistance (ESR) at temperatures as low as -65°C, significantly improving their operational limits compared to conventional systems.
The report highlights that cells filled with a baseline electrolyte of 1 M TEMATFB in AN exhibit low ESR only down to -45°C, after which performance declines sharply. In contrast, the modified electrolyte systems demonstrate stable performance and low ESR at much lower temperatures. The findings indicate that the use of these new electrolyte formulations can enhance the efficiency and reliability of double-layer capacitors in various applications, including electric vehicles, backup power systems, and spacecraft avionics.
Additionally, the document references previous research and patent applications related to low-temperature capacitor technology, emphasizing the ongoing efforts to improve energy storage solutions. The report concludes by acknowledging the collaborative work conducted at JPL under NASA's sponsorship, which aims to extend the operational capabilities of double-layer capacitors and explore their potential in diverse technological fields.
Overall, this report serves as a significant contribution to the field of energy storage, showcasing how innovative materials and formulations can lead to enhanced performance in extreme conditions, thereby broadening the scope of applications for double-layer capacitors.
