Design and Testing of Supercapacitors for High Temperature Operation

Supercapacitors combine efficient energy storage and delivery of power under high pulse current conditions, while retaining a high capacitance and low equivalent series resistance (ESR) during extended cycling. A high specific power is achieved via charge storage at a high surface area electrode/electrolyte interface, supported by a very low cell-level ESR.  

Another advantage of this type of electrochemical storage is the relatively wide operating range of the technology. This is due to the nature of the charge storage mechanism, which is dominated by non-Faradaic processes. Concerns over solid electrolyte interphase stability and ionic electrode diffusivity at wide temperature limits (associated with lithium ion batteries) are not present.

Commercially available supercapacitors are typically limited in operation to ~-40 C to +65ºC. These limits are fixed by the liquid range of the non-aqueous acetonitrile solvent used in most supercapacitor electrolytes. Propylene carbonate possesses a wider liquid range, but limits low temperature power delivery due to its higher viscosity. Wider temperature operation is of interest for numerous applications, such as energy storage for remote distributed sensing and electric vehicle power trains.

Previous work in wide temperature supercapacitors has demonstrated that operation to <-70ºC is possible, through the proper design and use of solvent blends (which extend the melting point of the electrolyte solution).^1-4Primary concerns include preventing precipitation of the salt onto the high surface area carbon electrodes and maintaining a sufficiently low ESR. This is achieved by designing high dielectric/low viscosity solvent blends.

Higher temperature operation is limited in large part by the boiling point of traditional non-aqueous solvents, as well as increased decomposition rates. ^5-6Supercapacitors for elevated temperature operation have been designed and tested to +160ºC, as part of ongoing efforts to develop wider temperature energy storage technologies. To enable operation at these temperatures, ionic liquids have been utilized as the electrolyte. New electrode formulations, designed for long duration elevated temperature exposure, have also been designed and tested. Initial test data over several thousand cycles will be presented.

ACKNOWLEDGEMENT

The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration (NASA).

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